WO2009132396A1

WO2009132396A1 - Method and apparatus for determining the pressure of a fluid

Info

Publication number: WO2009132396A1
Application number: PCT/AU2009/000543
Authority: WO
Inventors: Kurt Liffman; Ilija Denis Sutalo; Richard Manasseh; Michael Lawrence-Brown; James Semmens
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2008-05-02
Filing date: 2009-05-01
Publication date: 2009-11-05

Abstract

An implantable medical device for measuring pressure of a fluid within a patient comprising an enclosed volume of gas connected to the implantable member, wherein the enclosed volume of gas is positioned within the fluid so that it will resonate when subjected to acoustic energy and wherein the frequency at which the enclosed volume of gas resonates corresponds to a pressure of the fluid. Also described is a method of determining a difference in pressure between a first space and a second space, including positioning a barrier between the first and second spaces wherein the barrier will deflect in response to a pressure differential between the first and second spaces, imaging the deflection of the barrier and determining the difference in pressure by measuring the size of deflection of the barrier.

Description

FIELD

The present invention relates to methods and apparatus for determining the pressure of a fluid. The invention is particularly suitable for determining [he pressure of fluid, such as blood in vivo, however the invention is not limited to this application.

The measurement of pressure is important in the assessment, diagnosis and management of many medical conditions. For example, measurement of blood pressure is central to the management of cardiovascular disease.

Blood pressure (strictly speaking: vascular pressure) refers to the force exerted by circulating blood on the walls of blood vessels, and constitutes one of the principal vital signs. The pressure of the circulating blood decreases as blood moves through arteries, arterioles, capillaries, and veins; the term blood pressure generally refers to arterial pressure, i.e.. the pressure in the larger arteries, arteries being the blood vessels which take blood away from the heart. Arterial pressure is most commonly measured via a sphygmomanometer, which uses the height of a column of mercury to reflect the circulating pressure. Although many modern vascular pressure devices no longer use mercury, vascular pressure values are still universally reported in millimetres of mercury (mm Hg).

Methods and devices for measuring arterial pressure that do not involve access to the blood stream, such as those that are auscultatory (from the Latin for listening) are referred to as "non-invasive". The auscultatory method uses a stethoscope and a sphygmomanometer. This comprises an inflatable cuff placed around the upper arm at roughly the same vertical height as the heart. attached to a mercury or aneroid manometer. The mercury manometer measures the height of a column of mercury, giving an absolute result without need for calibration, and consequently not subject to the errors and drift of 1 calibration which affect other methods. The use of mercury manometers is often required in clinical trials and for [he clinical measurement of hypertension,

Oscillometry methods are sometimes used in the long-term measurement of arterial pressure and sometimes in general practice. The equipment is functionally similar to that of the auscultatory method, but with an electronic pressure sensor (transducer) used to detect blood flow, instead of using the stethoscope and the medical practitioner's ear. in practice, the pressure sensor is a calibrated electronic device with a numerical readout of blood pressure. To maintain accuracy, calibration must be checked periodically, unlike the inherently accurate mercury manometer. Oscillometry measurement requires less skill than the auscultatory technique, and may be suitable for use by untrained staff and for automated patient home monitoring.

Arterial blood pressure is most accurately measured invasively through an arterial line. Invasive arterial pressure measurement with intravascular cannulae involves direct measurement of arterial pressure by placing a cannula needle in an artery (usually radial, femoral, dorsalis pedis or brachial). This is usually done by an anesthesiologist or surgeon in a hospital.

The cannula is connected to a sterile, fluid-filled system, which is connected to an electronic pressure transducer. The advantage of this system is that pressure is constantly monitored beat-by-beat, and a waveform (a graph of pressure against time) can be displayed. Cannulation for invasive vascular pressure monitoring can sometimes result in complications such as thrombosis, infection, and bleeding. Patients with invasive arterial monitoring require very close supervision, as there is a danger of severe bleeding if the line becomes disconnected.

The non-invasive auscultatory and oscillometry measurements are simpler and quicker than invasive measurements, require less expertise in fitting, have virtually no complications, and are less unpleasant and painful for the patient. However, non-invasive measures may yield somewhat lower accuracy and small systematic differences in numerical results. Non-invasive measurement methods are more commonly used for routine examinations and monitoring. Invasive techniques are regularly employed in human and veterinary intensive care medicine, anesthesiology, and for research purposes where rapid variations in arteriai pressure are anticipated.

Apart from vascular pressure within blood vessels, it is aiso useful to be able to measure pressure in other compartments within the body. For example, it is useful to be able to measure: intra-cranial pressure in patients suffering from head injury; bladder pressure in patient^'s suffering from urinary tract disorders such as urinary retention and neurogenic bladders; and intraluminal pressure in patient^'s suffering from bowel obstruction.

It would be advantageous to be able to measure pressure in a compartment of the human body without requiring an invasive arterial line connected to fluid- filled system and an external electronic pressure transducer.

An echocardiogram is a test that uses high-frequency sound waves (ultrasound) to create an image of the heart. A Doppler test uses sound waves to measure the speed and direction of blood flow. By using Bernoulli^'s equation, the pressure of the blood can be estimated. Although this technique has the benefit of being non-invasive it is not ideal as it involves large errors.

It would be advantageous to be able to directly and accurately measure pressure in a compartment within the body.

It would also be advantageous to be able to directly measure pressure in a compartment within the body, such as the vascular pressure within blood vessels, without requiring an invasive arterial line connected via a fluid-filled system to an external electronic pressure transducer.

It would also be advantageous to provide a method and apparatus for directly measuring pressure in a compartment of the body without requiring cannulation or some other form of invasive access to the compartment for the duration over which pressure in the compartment is measured.

A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Accordingly, in one aspect, the present invention provides a method for determining a pressure of a fluid, the method including: positioning an enclosed volume of a gas within a fluid; subjecting the enclosed volume of gas to an acoustic energy; and determining a pressure of the fluid by determining a frequency of the acoustic energy at which the enclosed volume of gas resonates when positioned within the fluid.

The invention enables the direct measurement of the pressure of the fluid without requiring direct access to the fluid for the duration of time over which the pressure of the fluid is measured. Where the fluid is enclosed within a compartment, the invention enables the direct measurement of the pressure of the fluid within the enclosed compartment by the application of acoustic energy from outside the compartment. Thus, the invention does not require direct access to the fluid within the compartment for the duration of time over which the pressure of the fluid is measured.

In one form, determining the pressure of the fluid includes comparing the frequency of the acoustic energy at which the enclosed volume of gas resonates when positioned within the fluid with frequencies of the acoustic energy at which the enclosed volume of gas is known to resonate when positioned within the same type of fluid at known pressures.

In another form, determining the frequency of the acoustic energy at which the enclosed volume of gas resonates when positioned within the fluid includes subjecting the enclosed volume of gas to acoustic energy at different frequencies and determining the frequency at which the enclosed volume of gas resonates.

In yet another form, detecting the frequency at which the enclosed volume of gas resonates includes determining an intensity of acoustic energy from the enclosed volume of gas induced by subjecting the enclosed volume of gas to acoustic energy.

The method may further include partially enclosing the volume of gas within a deformable material. In another form, the method may further include fully enclosing the volume of gas within a deformable material. In one form, the deformable material is flexible. In another form, the deformable material is elastic, in yet another form, the deformable material is resilient. The deformable material may be formed out of any one or more of the following classes of materials: proteinaceous, lipid, saccharide, carbohydrate, elastomeric, polymer, inorganic, organic, or biological. The material structure may be homogeneous, heterogeneous or multiphase combinations of the above or others. The materials may exhibit isotropic or anoisotropic properties.

The method may include positioning at least two of the enclosed volumes of gas within the fluid, wherein the elastic material enclosing one of the enclosed volumes of gas has a different thickness to the elastic material enclosing another one of the enclosed volumes of gas.

in one form, the method includes positioning at least two enclosed volumes of gas having different volumes within the fluid so that when the enclosed volumes of gas are subjected to the acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

In another form, the method includes positioning at least two enclosed volumes of gas having different types of gas within the fluid so that when the enclosed volumes of gas are subjected to the acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

In yet another form, the method includes positioning the enclosed volume of gas within an internal bodily fluid.

Yet another form further includes connecting the enclosed volume of gas to an implantable medical device. This form of the invention enables direct measurement of the pressure of fluid, in vivo, within a patient^'s body at any desired time as long as the enclosed volume of gas and the implantable medical device to which it is connected remain implanted within the patient's body.

In one form, the implantable medical device is a stent graft.

The method may also further include positioning the implantable medical device within an internal bodily fluid vessel.

The method may further include connecting the enclosed volume of gas to an internal surface of a wall of the stent graft and connecting another of the enclosed volumes of gas to an external surface of the wall of the stent graft opposite to the internal surface.

This form of the invention is advantageous in that it enables the measurement of a difference in pressure between fluid on opposite sides of the wall of the stent graft. In particular, the difference in pressure between fluid on the inside and on the outside of the stent graft can be measured.

in another form, positioning the enclosed volume of gas within a fluid includes positioning the enclosed volume of gas within blood in an internal blood vessel.

This form of the invention may enable direct measurement of the pressure of blood, in vivo, in a blood vessel without requiring direct access to the blood while the pressure of the blood is being measured. Thus, the pressure of blood can be directly measured, in vivo, in a blood vessel without the risk of infection or other complications associated with direct percutaneous access to [he blood in the blood vessel. This form of the invention achieves this by enabling direct measurement of the pressure of blood, in vivo, in a blood vessel within a patient's body by the external application of acoustic energy from outside the patient's body and by detecting returned acoustic energy.

Accordingly, the invention enables direct measurement of the pressure of blood within the blood vessel without the need for a probe to be inserted into the blood vessel via a cannula or via some other direct means involving percutaneous access to the blood vessel while the pressure of the blood is being measured.

In yet another form, determining the pressure of the fluid includes determining the pressure of the blood at two or more times during a cardiac cycle when the blood is under a pulsatile flow.

The method may further include a transducer for generating the acoustic energy to which the enclosed volume of gas is subjected and for generating electrical signals from acoustic energy from the enclosed volume of gas, and a processor for receiving the electrical signals and determining whether the acoustic energy to which the enclosed volume of gas is subjected is causing the enclosed volume of gas to resonate.

In the method, the enclosed volume of gas may be a bubble.

In another aspect, the invention includes an implantable medical device capable of determining a pressure of a fluid within a patient, the device including:

an implantable member configured to be positioned within a patient;

an enclosed volume of gas connected to the implantable member and adapted to be positioned in contact with a fluid within the patient; and

wherein the enclosed volume of gas is configured so that when positioned within the fluid the enclosed volume of gas will resonate when subjected to acoustic energy and that a frequency of the acoustic energy at which the enclosed volume of gas resonates corresponds to a pressure of the fluid.

In one form, the enclosed volume of gas is partially enclosed within a deformabie material. In another form, the enclosed volume of gas is fully enclosed within a deformabie material. In one form, the deformabie material is flexible, in another form, the deformabie materia! is elastic. \n yet another form, the deformabie materia! is resilient. The deformabie material may be formed out of any one or more of the following classes of materials: proteinaceous, lipid, saccharide, carbohydrate, elastomeric, polymer, inorganic, organic, or biological. The material structure may be homogeneous, heterogeneous or multiphase combinations of the above or others. The materials may exhibit isotropic or anoisotropic properties.

Another form may include at least two of the enclosed volumes of gas wherein the elastic materia! enclosing one of the enclosed volumes of gas has a different thickness to the elastic material enclosing another one of the enclosed volumes of gas.

Yet another form may include at least two of the enclosed volumes of gas wherein the elastic material enclosing one of the enclosed volumes of gas is different the elastic material enclosing another one of the enclosed volumes of gas.

In one form the device includes at least two of the enclosed volumes of gas having different sized enclosed volumes of gas so that when subjected to acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

In another form the device includes at least two of the enclosed volumes of gas having different types of gas so that when subjected to acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

In a form of the device, the implantable medical device is configured to be positioned within an internal bodily fluid vessel.

In another form, the implantable medical device is a stent graft.

In one form, one of the enclosed volumes of gas is positioned on an internal surface of a wall of the stent graft and one of the enclosed volumes of gas is positioned on an external surface of the wall of the stent graft opposite to the internal surface.

in one form of the device, the enclosed volume of gas is a bubble. In another aspect, the invention provides an apparatus for use with a medical device positioned within a patient, wherein the medical device includes an enclosed volume of gas in contact with a fluid within the patient that will resonate when subjected to acoustic energy and that a frequency of the acoustic energy at which the enclosed volume of gas resonates corresponds to a pressure of the fluid, the apparatus including:

a transducer for generating acoustic energy and subjecting the enclosed volume of gas to the acoustic energy and for generating electrical signals from acoustic energy returning from the enclosed volume of gas;

a processor for receiving the electrical signals from the transducer and for determining a frequency of the acoustic energy generated by the transducer at which the enclosed volume of gas resonates and thereby determines the pressure of the fluid.

One form of the apparatus includes a second receiving transducer for generating a second set of electrical signals that are received by the processor for eliminating a background reflection of acoustic energy from the tissue of the patient's body.

In another form, the processor is configured so that when the enclosed volume of gas is in contact with blood within a patient the processor determines the pressure of the blood at two or more times during a cardiac cycle when the blood is under pulsatile flow.

In yet another form, the apparatus is an ultrasound device.

In another aspect, the invention provides a method of determining a pressure of a fluid, the method including:

positioning an enclosed volume of gas within a fluid wherein the enclosed volume of gas is configured to change in size as a function of the pressure of the fluid;

imaging the enclosed volume of gas within the fluid; and determining the pressure of the fluid by measuring the size of the enclosed volume of gas within the fluid.

In one form, imaging the enclosed volume of gas includes directing radiation at the enclosed volume of gas and detecting reflected radiation from the enclosed volume of gas.

In another form, imaging the enclosed volume of gas includes directing acoustic energy at the enclosed volume of gas and detecting reflected acoustic energy from the enclosed volume of gas.

in yet another form, imaging the enclosed volume of gas includes using one or more imaging techniques including: magnetic resonance imaging, positron emission tomography or computed tomography, x-ray imaging, radiography, photoacoustic imaging, ultrasound and digital subtracton angiography.

In a form of the method, determining the pressure of the fluid includes comparing the size of the volume of the enclosed volume of gas within the fluid with a known size of the volume of the enclosed volume of gas within a fluid at a known pressure.

Positioning the enclosed volume of gas within a fluid may include positioning the enclosed volume of gas within blood in an internal blood vessel.

Determining the pressure of the fluid may include determining the pressure of the blood at [wo or more times during a cardiac cycle when the blood is under a pulsatile flow.

In another aspect, the invention provides a method of determining a difference in pressure between a first space and a second space, the method including:

positioning a barrier between a first space and a second space wherein the barrier is configured to deflect in response to a difference in pressure between the first space and the second space:

imaging the deflection of the barrier; and determining the difference in pressure between the first space and the second space by measuring a size of the deflection of the barrier.

In one form, imaging [he barrier includes directing radiation at the barrier and detecting reflected radiation from the barrier.

In another form, imaging the barrier includes directing acoustic energy at the barrier and detecting reflected acoustic energy from the barrier.

In yet another form, determining the difference in pressure between the first space and the second space includes comparing the measured size of the deflection of the barrier with a known size of the deflection of the barrier when the barrier is positioned between another two spaces having a known pressure difference.

The barrier may be formed from a materia! whereby the size of the deflection of the barrier reduces due to a reduction in the difference in pressure between the first space and the second space.

In one form, the barrier may be formed out of an elastic material.

In another form, the barrier may be formed out of an inelastic material.

The barrier may be positioned within an internal bodily cavity adjacent to a wall surrounding the cavity and the first space is an inner portion of the cavity and the second space is an outer portion of the cavity between the barrier and the wail surrounding the cavity.

In a form of the method, the barrier is connected to an implantable medical device.

in another form of the method, the implantable medical device is a stent graft.

in one form of the method, the stent graft includes a wall and the barrier is positioned within the wall of the stent graft and the wall and the barrier are positioned to occlude an aneurysm from an internal volume within a blood vessel In the method, the effectiveness of the stent for occluding the aneurysm may be determined by measuring the deflection of [he barrier and thereby determining the difference in pressure between a space defined within the aneurysm and the internal volume within the blood vessel.

In one form of the method, [he deflection of [he barrier is measured iwo or more times during a cardiac cycle when the blood is under a pulsatile flow.

In another aspect, the invention provides a device for determining a difference in pressure between a first space and a second space, the device including:

a barrier configured to be positioned between a first space and a second space and to deflect in response to a difference in pressure between the first space and the second space; and

wherein the deflection of the barrier is proportional to the difference in pressure between the first space and the second space.

The barrier may be calibrated so that a size of the deflection of the barrier corresponds with a size of the difference in pressure between the first space and the second space.

The barrier may be configured so that the deflection of the barrier is reversed in response to an equalization of pressures in the first space and the second space.

In one form, the barrier is formed out of an elastic material. In another form, the barrier is formed out of an inelastic material.

In yet another form, the device is an implantable medical device and the barrier is connected to a wall of the implantable medical device. The implantable medical device may be a stent graft.

in one form of the device, the enclosed volume of gas is a bubble.

in another aspect, the invention provides an apparatus for use with a medical device positioned within a patient, wherein the medical device includes a barrier configured to be positioned between a first space and a second space within a patient and to deflect in response to a difference in pressure between the first space and the second space and wherein the deflection of the barrier is proportional to the difference in pressure between the first space and the second space, the apparatus including an imaging device for creating an image of the barrier and a processor for determining the difference in pressure between the first space and the second space by measuring the deflection of the barrier from the image of the barrier.

In a form of the apparatus, the imaging device may include one or more imaging devices including: magnetic resonance imaging, positron emission tomography or computed tomography, x-ray imaging, radiography, photoacoustic imaging, ultrasound and digital subtracton angiography.

In another form of the apparatus, when the medical device is a stent graft and the barrier is positioned within a wall of the stent graft and the wall and the barrier are positioned within a blood vessel to occlude an aneurysm from an internal volume within the blood vessel, the imaging device is operable for creating an image of the barrier, and the processor is operable for determining a difference in pressure between fluid located between the aneurysm and the barrier and blood located between the barrier and the internal volume within the blood vessel by measuring the deflection of the barrier from the image of the barrier.

In another form of the apparatus, the processor is configured to determine the difference in pressure between fluid located between the aneurysm and the barrier and blood located between the barrier and the internal volume within the blood vessel at two or more times during a cardiac cycle when the blood is under pulsatile flow.

The present invention will now be described with reference to the accompanying Figures illustrating preferred embodiments of the present Figure 1 is a schematic illustration of a method and an apparatus in accordance with a form of the invention for measuring a pressure of blood within a human body including an implantable medical device to which is attached a plurality of enclosed volumes of gas in the form of bubbles and an ultrasound scanner for applying acoustic energy in the form of ultrasound energy to the bubbles without requiring direct access to the bubbles.

Figure 2 is a schematic illustration of an implantable medical device in the form of a stent graft positioned within a diseased blood vessel within a patient, the implantable medical device including a plurality of bubbles connected to a side wall of the stent graft wherein each bubble is configured so that when subjected to acoustic energy at a given frequency each bubble will resonate within fluid at different pressures, or each bubble will only resonate in a respective range of pressures of the fluid.

Figure 3A illustrates a section view of a portion of the side wall of the stent graft of Figure 2 in which the bubbles are connected to the wall of the stent graft by encapsulation within a block of an elastic material attached to the wall of the stent graft.

Figure 3B illustrates a section view of a portion of the side wall of an alternative form of the stent graft of Figure 2 in which the bubbles are connected to the wall of the stent graft by being encapsulated within a thin layer of an elastic material attached to the wall of the stent graft.

Figure 3C illustrates a section view of a portion of the side wall of another alternative form of the stent graft of Figure 2 in which bubbles of smaller diameter than those of the embodiments of Figures 3A and 3B, or microbubbles. are connected to the wall of the stent graft by being encapsulated within a block of an elastic material attached to the wall of the stent graft.

Figure 3D illustrates a section view of a portion of the side wall of another alternative form of the stent graft of Figure 2 in which bubbles of differing diameters are connected to the wall of the stent graft by being encapsulated within a block of an elastic material attached to the wall of the stent graft. Figures 4a and 4b are graphs illustrating the results of a laboratory experiment showing frequencies at which resonance of an enclosed volume of gas was detected when the enclosed volume of gas was located in water at a range of pressures and subjected to acoustic energy.

Figure 5 is a graph illustrating the results of a laboratory experiment showing a variation in size of an image of an enclosed volume of gas when the enclosed volume of gas was positioned in water at a range of pressures.

Figure 6 is a schematic illustration of an implantable medical device in the form of a stent graft positioned within a diseased blood vessel within a patient, the implantable medical device including a means for measuring a differential pressure between fluid within the blood vessel and within an aneurysm that includes a pair of expandable balloons attached at respective ends of a tube passing through a wall of the stent graft.

Figure 7 is a schematic illustration of a portion of the wall of the stent graft of Figure 8 showing in more detail the means for measuring a differential pressure between fluid within the blood vessel and within an aneurysm separated by the wall.

Figure 8 is a schematic illustration of the apparatus of Figure 7 in which a reversal of the pressure difference between the spaces either side of the wall have resulted in the balloon which was inflated in Figure 7 becoming deflated and the balloon which was deflated in Figure 7 becoming inflated.

Figure 9 is a schematic illustration of an implantable medical device in the form of a stent graft having a side wall defining an internal space and separating the internal space from an external space and having a plurality of elastic barriers. in the form of elastic membranes, connected to the side wall of the stent graft wherein each elastic barrier is configured to deflect in response to a difference in pressure between the internal space and the external space.

Figure 10 is a series of photographs labeled A to F illustrating the results of a laboratory experiment in which an elastic barrier, in the form of an elastic membrane in accordance with a form of the invention, is shown deflecting to varying degrees in response to varying differences in pressure between spaces separated by the elastic barrier.

Figure 11 is a graph illustrating the resuits of [he laboratory experiment of Figure 10 in which the measured size of deflection of the elastic barrier is plotted against the difference in pressure between [he spaces separated by the barrier.

Figure 12 is a series of photographs labeled A to F illustrating the results of a laboratory experiment in which an inelastic barrier, in the form of an inelastic membrane in accordance with a form of the invention, is shown deflecting to varying degrees in response to varying pressures of fluid into which the inelastic barrier is positioned.

Referring to Figure 1 , a preferred form of the invention includes an implantable medical device 10 positioned internally within a patient's body 20. The implantable medical device 10 has fixed to it one or more enclosed volumes of gas 30. When the implantable medical device 10 is positioned within a compartment in the patient's body 20 the enclosed volume of gas 30 may be positioned within a fluid contained within the compartment. For example, as illustrated in Figure 2, the enclosed volume of gas 30 may be positioned within a compartment in the form of a blood vessel 60 and thereby in the patient^'s blood stream.

The enclosed volume of gas 30 is configured so that it will resonate within the fluid at a particular pressure when subjected to acoustic energy at a particular frequency. By configuring the enclosed volume of gas 30 so that for a given pressure of the fluid the frequency at which the enclosed volume of gas 30 will resonate is known the pressure of the fluid can be determined. Thus, in the example where the fluid is blood within the patient's body 20 and the enclosed volume of gas 30 is appropriately configured and positioned within the blood the pressure of the blood can be determined by subjecting the enclosed volume of gas 30 to acoustic energy at a range of frequencies and detecting the frequency at which the enclosed volume of gas 30 resonates. The enclosed volume of gas 30 may be subjected to acoustic energy by any suitable means. For example, in Figure 1 , the enclosed volume of gas 30 is subjected to acoustic energy by an acoustic energy generating device 40. The acoustic energy generating device 40 includes a transducer 45 encased in a probe 44. The transducer 45, which may be a piezoelectric transducer, is electrically connected via a cord 47 to a machine 42. The machine 42 generates strong, short electrical pulses which are conveyed to the transducer 45 via the cord 47. The transducer 45 generates acoustic energy in response to the electrical pulses from the machine 42. The machine 42 is configured such that it can generate electrical pulses that cause the transducer 45 to generate acoustic energy at desired frequencies. The frequencies can be in the Hz, kHz or MHz ranges. The acoustic energy generated by the transducer 45 is focused by either the shape of the transducer 45 or by a lens (not shown) positioned in front of the transducer 45 or by a predetermined set of controlled electrical pulses from the machine 42. This focusing of the acoustic energy produces an arc-shaped acoustic energy wave from the face (not shown) of the transducer 45. The wave of acoustic energy penetrates the tissue of the patient's body 20 and comes into focus at a predetermined depth within the patient's body 20.

The acoustic energy waves generated by the transducer 45 are partially reflected by layers between different tissues within the patient's body 20. in particular, acoustic energy is reflected anywhere where there is a density change between tissues in the patient's body 20. Some of the reflections of acoustic energy return to the transducer 45. The return of the reflected acoustic energy waves to the transducer 45 causes the transducer 45 to vibrate. The transducer 45 converts the vibrations into electrical pulses that return to the machine 42 via the cord 47. The machine 42 processes the electrical pulses generated by the transducer 45 and produces an output which may be digital data, a visual image produced on a screen or some other form of output.

The enclosed volume of gas 30 is surrounded by a material of different density to that of the gas forming the enclosed volume of gas 30. As illustrated in Figures 3A to 3D, the enclosed volume of gas 30 is surrounded by either an elastic materia! 15 or a deformable membrane 38 or both. The deformable membrane 38 may be flexible and/or elastic and/or resilient. The elastic materia! 15 and the deformabie membrane 36 each have densities that are different to the density of the gas within the enclosed volume of gas 30. Thus, when acoustic energy generated by the transducer 45 is directed at the enclosed volume of gas 30 it wil! reflect some acoustic energy back towards the transducer 45. Acoustic energy reflected by the enclosed volume of gas 30 to the transducer 45 wil! result in the generation of electrical pulses which are processed by the machine 42 into an output. The output generated by the machine 42 may be digital data and/or a visual image of the enclosed volume of gas 30.

The elastic material 15 and/or the deformable membrane 36 that surrounds the enclosed volume of gas 30 may be formed out of any one or more of the following classes of materials: proteinaceous, lipid, saccharide, carbohydrate, elastomeric, polymer, inorganic, organic, or biological. The material structure may be homogeneous, heterogeneous or multiphase combinations of the above or others. The materials may exhibit isotropic or anoisotropic properties.

It is to be appreciated that the acoustic energy generating device 40 may be any apparatus that is capable of generating acoustic energy at one or more frequencies which results in the enclosed volume of gas 30 exhibiting resonance whilst positioned within a fluid whose pressure is to be determined. One form of the acoustic energy generating device 40 may be an ultrasound device. However, other forms of the acoustic energy generating device 40 may also fall within the scope of the invention.

As illustrated in Figures 3A to 3D, the enclosed volume of gas 30 may be surrounded by the deformable membrane 36. The deformable membrane 36 may be completely or partially surrounded or replaced by the elastic material 15. The elastic materia! 15 and/or the deformable membrane 38 that surrounds the enclosed volume of gas 30 may be formed out of any one or more of the following classes of materials: proteinaceous, lipid, saccharide, carbohydrate, elastomeric, polymer, inorganic, organic, or biological. The material structure may be homogeneous, heterogeneous or multiphase combinations of the above or others. The materials may exhibit isotropic or anoisotropic properties.

In the embodiments illustrated in Figures 3A, 3C and 3D the enclosed volume of gas 30 is surrounded by [he deformable membrane 38 which is surrounded completely by the elastic material 15, In the embodiment illustrated in Figure 3B the elastic materia! 15 only partially surrounds the deformable membrane 36 and the enclosed volume of gas 30. in circumstances where the elastic materia! 15 completely surrounds the enclosed volume of gas 30 there need not necessarily be a distinct deformable membrane 36 surrounding the enclosed volume of gas 30. Instead, the enclosed volume of gas 30 may be surrounded only by the elastic materia! 15. The elastic material 15 and/or the deformable membrane 36 that surrounds the enclosed volume of gas 30 may be formed out of any one or more of the following classes of materials: proteinaceous, lipid, saccharide, carbohydrate, elastomeric, polymer, inorganic, organic, or biological. The materia! structure may be homogeneous, heterogeneous or multiphase combinations of the above or others. The materials may exhibit isotropic or anoisotropic properties.

In order for the enclosed volume of gas 30 to reflect acoustic energy the enclosed volume of gas 30 must be at least partially surrounded by materia! of differing density to the gas forming the enclosed volume of gas 30. Also, in order for the enclosed volume of gas 30 to resonate when subjected to acoustic energy the enclosed volume of gas 30 must be at least partially surrounded by an elastic material. The enclosed volume of gas 30 in the embodiments described herein are surrounded by an elastic materia! in the form of the deformable membrane 36 or the elastic material 15 or both. However, it is to be appreciated that any materia! surrounding the enclosed volume of gas 30 that will enable the enclosed volume of gas 30 to resonate when subjected to acoustic energy when positioned within fluid at a pressure may fall within the scope of forms of the invention defined herein. Also, any material surrounding the enclosed volume of gas 30 that will assist in reflection of acoustic energy directed towards the enclosed volume of gas 30 may fall within the scope of forms of the invention defined herein. Referring to Figure 2, when the implantable medical device 10 is positioned within a blood vessel 80 the pressure of blood within the blood vessel 80 can be determined by subjecting the enclosed volume of gas 30 to acoustic energy at a range of frequencies. At a certain pressure of the blood within the blood vessel 80 the enclosed volume of gas 30 will resonate when subjected to acoustic energy at a certain frequency. Resonance of the enclosed volume of gas will be detected by the ultrasound, or other appropriate, acoustic energy generating machine 42 as an increase in intensity of acoustic energy reflected by the enclosed volume of gas towards the transducer 45. An increase in intensity of acoustic energy reflected by the enclosed volume of gas 30 towards the transducer 45 is reflected in the electrical pulse being transmitted via the cord 47 to the machine 42. The resonance of the enclosed volume of gas 30 will be identified by the machine 42 as a relatively high intensity of reflected acoustic energy from the enclosed volume of gas 30 compared with the intensity of reflected acoustic energy at other frequencies of acoustic energy at which the enclosed volume of gas 30 does not resonate. If the machine 42 is configured to produce an output in the form of an image then the resonance of the enclosed volume of gas 30 will be identified by the machine 42 as a relatively high intensity image of the enclosed volume of gas 30 compared with the intensity of the image of the enclosed volume of gas 30 at other frequencies of acoustic energy at which the enclosed volume of gas 30 does not resonate.

Accordingly, the invention enables the direct measurement of the pressure of blood, or other substances, in a blood vessel 60 within a patient's body 20 by positioning the enclosed volume of gas 30 within the blood, or other substances, in the blood vessel 60 and applying acoustic energy from outside the patient^'s body 20 at a range of frequencies and determining the frequency at which the enclosed volume of gas 30 resonates. Furthermore, the invention enables direct measurement of the pressure of blood, or other substances, within the blood vessel 60 without requiring direct access to the blood, or other substances, within the blood vessel 60 while the pressure of the blood is being measured. In other words, [he invention enables direct measurement of the pressure of blood within the blood vessel 60 without the need for a probe to be inserted into the blood vessel 60, such as via a cannula or some other direct means involving percutaneous access to the blood vessel 60, for the duration of time over which the pressure of the blood is being measured. Accordingly, the invention also enables direct measurement of the pressure of blood, or other substances, within a blood vessel 60 within a patient's body 20 without the risk of infection or other complications associated with direct percutaneous access to the blood within the blood vessel 60, Furthermore, the invention enables direct measurement of the pressure of the blood, or other substances, within the blood vessel 60 at any desired time as long as the enclosed volume of gas 30, or the implantable medical device 10 to which the enclosed volume of gas 30 is attached, remains implanted within [he blood vessel 80.

In one form, the acoustic energy generating machine 42 includes a second transducer (not shown) for generating a second set of electrical signals in response to the return of the reflected acoustic energy waves from the enclosed volume of gas 30. The signal amplitude of acoustic energy from the enclosed gas volume 30 falls off rapidly with distance, whereas background resonance due to acoustic properties of tissue of the patient's body 20 would not change so rapidly with distance. Accordingly, the second transducer is positioned further from the enclosed volume of gas 30 than the first transducer 45 but sufficiently close to the first transducer 45 so that the first transducer 45 and the second transducer receive substantially the same intensity of acoustic energy from tissue of the patient^'s body but different intensities of acoustic energy from the enclosed volume of gas 30. Thus, the second transducer generates a second set of electrical signals from acoustic energy from the enclosed volume of gas 30 and from the tissue of the patient's body 20 that are received and processed by the machine 42 and compared with the electrical signals from the first transducer 45. By comparing the electrical signals from the first transducer 45 and the second transducer the background resonance due to acoustic properties of tissue of the patient's body 20 can be eliminated.

Referring to Figure 2, the implantable medical device 10 is in the form of a stent graft 13 that has been inserted and positioned within a vessel or conduit within the patient's body 20. In the exemplified embodiment, the vessel or conduit is a blood vessel 60. The stent graft 13 may be positioned within the blood vessel 60 to treat a disease or weakening of a wall 64 of the blood vessei 60. Figure 2 illustrates a diseased blood vessel 60 including a wall 64 that has an aneurysm 62 represented by an expanded portion of [he wall 64 of the blood vessei 60. The stent graft 13 is positioned to occlude the aneurysm 62 from the remainder of the internal volume 66 of the blood vessel 60 through which blood travels in the direction indicated by the arrow A.

The stent graft 13 may be used for the treatment of abdominal aortic aneurysm (AAA). This may involve a surgeon repairing the AAA by delivering the stent graft 13 via a catheter inserted into a groin artery and positioning the stent graft 13 and securing it in place by inflating a balloon to expand the stent graft 13 to prevent blood flow into the aneurysm 62 in [he wall 64 of the blood vessel 60.

Alternatively, the stent graft 13 may take a variety of different forms for the treatment of a variety of different diseases and conditions. The stent graft 13 may take any of a variety of forms including, but not limited to, a coronary heart disease stent, a rectal stent, and oesophageal stent, thoracic stent graft, renal stent, a biliary stent or a pancreatic stent.

The stent graft 13 has a plurality of enclosed volumes of gas 30 in the form of bubbles 35 attached to a side wail 17 of the stent graft 13. As illustrated in Figure 2, the bubbles 35 are attached to the side wall 17 in a linear fashion from a first end 12 to a second end 14 of the side wall 17 of the stent graft 13. The side wall 17 has a cylindrical shape, an external surface 18 and an internal surface 19. The stent graft 13 has a first row 32 of the bubbles 35 attached to the external surface 18 of the side wall 17 and a second row 34 of the bubbles 35 attached to the internal surface 19 of the side wall 17.

The first row 32 of bubbles 35 is intended to directly measure the pressure of fluid such as blood, if any, within a space 63 defined between the aneurysm 62 and the external surface 18 of the side wall 17 of the stent graft 13. instead of containing blood the space 63 defined between the aneurysm and the external surface 18 of the stent graft 13 may contain fluid and a thrombus, such as a laminated thrombus which is a jelly-like form of congealed blood and other materials. Accordingly, the first row 32 of bubbles 35 is also intended to directly measure the pressure within the space 83 defined between [he aneurysm 62 and the external surface 18 of the stent graft 13. The second row 34 of bubbles 35 is intended to directly measure [he pressure of blood within an internal volume 16 defined within the internal surface 19 of the side wall 17 of the stent graft 13. Directly determining the respective pressures of the internal space 83 within the aneurysm 62 and the interna! volume 18 of the stent graft 13 enables a medical practitioner to determine whether or not the stent graft 13 has been positioned properly so as to occlude the internal space 63 of the aneurysm 82 from the internal volume 66 of the blood vessel 80.

If the directly measured pressure of fluid such as blood or other material, if any, in the internal space 63 of the aneurysm 62 is less than the directly measured pressure of blood within the internal volume 16 of the stent graft 13 then this suggests that the stent graft 13 has been positioned correctly and is functioning so as to occlude the interna! space 63 of the aneurysm 62 from the interna! volume 66 of the blood vessel 60. On the other hand, if the directly measured pressure of fluid such as blood or other material, if any, within the interna! space 83 of the aneurysm 62 is the same or relatively similar to the pressure of the blood within the internal volume 16 of the stent graft 13 then this indicates that the stent graft 13 has not been positioned correctly and is not functioning so as to occlude the internal space 63 of the aneurysm 62 from the internal volume 66 of the blood vessel 60. Accordingly, the invention enables a medical practitioner to diagnose whether or not the stent graft 13 is positioned correctly or is functioning properly so as to occlude the interna! space 63 of the aneurysm 82 from the internal volume 86 of the blood vessel 60 and whether or not further surgery will be required to reposition or replace the stent graft 13 within the blood vessel 80.

Resonance of an Enclosed Volume of Gas

In one form, the bubbles 35 forming the first row 32 and the second row 34 may each be configured or tuned such that each successive bubble 35 within the first row 32 or the second row 34 will resonate within a different range of fluid pressures when acoustic energy within a given range of frequencies is applied to the bubbles 35. For example, when acoustic energy within a given range of frequencies is applied to the first row 32 of bubbles 35 in Figure 2 the upper-most bubble 35a of [he first row 32 may be tuned so as to resonate within a range of pressures from 100 to 120 mm Hg. For the same range of frequencies of acoustic energy the subsequent bubble 35b in descending order may be tuned to resonate within a range of pressures from 80 to 100 mm Hg₁ the next bubble 35c in descending order may be tuned to resonate within a range of pressures from 80 to 80 mm Hg. the next bubble 35d in descending order may be tuned to resonate within a range of pressures from 40 to 60 mm Hg and the lower-most bubble 3Se may be tuned to resonate within a range of pressures from 0 to 40 mm Hg.

Similarly, the bubbles 35 in the second row 34 may also be tuned so that under the application of acoustic energy within a given range of frequencies the bubbles 35 will resonate within respective pressure ranges identical to those of the bubbles 35 of the first row 32.

By determining the frequency of acoustic energy at which a bubble 35 of either the first row 32 or the second row 34 resonates the pressure of the blood, or other substances, in which the bubble 35 is positioned can be determined. For example, say the upper most bubble 35a of the second row 34 resonates when subjected to acoustic energy at a frequency which indicates that the pressure of the blood, or other substances, within the internal volume 16 of the stent graft 13 is at a pressure of between 100 to 120 mm Hg. And say that the lower most bubble 35 of the first row 32 resonates at a frequency of acoustic energy that indicates that the pressure of blood within the internal space 83 of the aneurysm 62 is at a pressure of approximately 0 mm Hg. This indicates to a medical practitioner that the blood within the internal volume 16 of the stent graft 13 is not in pressure communication with the blood, or other substances, within the interna! space 63 of the aneurysm 82. A reason for this may be that the stent graft 13 has been correctly positioned and is effectively occluding the interna! space 63 of the aneurysm 62 from the internal volume 68 of the blood vessel 60. As will be appreciated, the positioning of the bubbles 35 on the interna! surface

19 and the external surface 18 of the side wall 17 of the stent graft 13 can be reconfigured as required. For example, there may be a plurality of the first row 32 of bubbles 35 attached to the external surface 18 about the external circumference of the side wall 17. Similarly, there may be a plurality of second rows 34 of bubbles 35 attached to the internal surface 19 of the side wall 17 about the internal circumference of the side wall 17. Also, the bubbles 35 within the first row 32 and the second row 34 need not necessarily be positioned such that they are tuned to resonate at higher frequency ranges to lower frequency ranges in descending order. On the contrary, the bubbles 35 may be arranged within the first row 32 and the second row 34 whereby [he bubbles 35 are tuned to resonate at higher frequency ranges to lower frequency ranges in ascending order or in any other suitable order. The purpose of having the bubbles 35 tuned to decreasing pressure ranges in descending order, or any other order, is so that the bubble 35 of the respective row 32. 34 that resonates due to the application of acoustic energy at a given frequency can be identified. Any other means of identifying which bubble 35 of the respective row 32. 34 is resonating may be used instead of the means described above.

Referring to Figures 3A to 3D, each Figure represents a section of a portion of the side wall 17 of the stent graft 13 of Figure 2. The portions of the side wall 17 represented in Figures 3A to 3D include a plurality of the bubbles 35 connected to the internal surface 19 of the side wall 17 by any suitable means. Although Figures 3A to 3D represent connection of the bubbles 35 to the internal surface 19 of the side wall 17 the same applies in relation to the connection of the bubbles 35 to the external surface 18 of the side wall 17.

In the arrangements illustrated in Figures 3A to 3D each bubble 35 is connected to the internal surface 19 of the side wall 17 by elastic material 15. Furthermore, each bubble 35 includes the enclosed volume of gas 30 being surrounded by the deformable membrane 38. However, it is to be appreciated that the deformable membrane 36 need not be separate from the elastic materia! 15. For example, the deformable membrane 36 and the elastic materia! 15 may be integral with each other. Thus, each bubble 35 may simply be an enclosed volume of gas 30 enclosed by the elastic material 15. Although not shown in the Figures, one of the enclosed volumes of gas 30 may be enclosed by elastic material, such as the deformabie membrane 36 and/or the elastic material 15, of differing thickness to the elastic material, such as the deformabie membrane 38 and/or the elastic material 15, enciosing another one of the enclosed volumes of gas 30.

in Figure 3A, each of the plurality of bubbles 35 is identical in size and is represented as a spherical bubble 35. However, it is to be appreciated that the bubbles 35 need not necessarily be spherical but could be, for example, a cube, a trapezoidal prism, or any other shaped prism with varying numbers of facets and sides. The bubbles 35 are completely encased within a block of the elastic material 15. in turn, the elastic material 15 is connected to the internal surface 19 of the side wall 17 of the stent graft 13 by any suitable means.

The deformabie membrane 38 and/or the elastic material 15 encasing the bubble 35 is deformabie in order that the bubble 35 may be able to resonate when positioned within a fluid at a pressure and subjected to acoustic energy. The bubble 35 may also be surrounded by material that is elastic in order that the pressure of the fluid in which the bubble 35 and the elastic material 15 are positioned might have an influence on the frequency of acoustic energy at which the bubble 35 will resonate. The bubble 35 may also be surrounded by materia! that is elastic in order that the pressure of the fluid in which the bubble 35 and the elastic material 15 are positioned might have an influence on the physical characteristics of the bubble 35, such as the size of enclosed volume of gas 30 contained therein.

In another form, the pressure of the fluid in which the bubble 35 and the elastic material 15 are positioned can be determined by measuring the size of the enclosed volume of gas 30. For example, the enclosed volume of gas 30 may have a known size when positioned within a fluid at a known pressure. Furthermore, the enclosed volume of gas 30 may change in size by a known magnitude when the pressure of the fluid in which the enclosed volume of gas 30 is positioned changes by a certain amount, Thus, by knowing the size that the enclosed volume of gas 30 will be when positioned within fluid of a given pressure, the pressure of a fluid can be determined by measuring the size of the enclosed volume of gas 30 when positioned within the fluid. The size of the enclosed volume of gas 30 when positioned within a fluid, such as blood in vivo in a blood vessel can be determined by any suitable means. One suitable means includes by imaging the enclosed volume of gas 30 when positioned within the blood in the blood vessel. Such imaging techniques include one or more of the imaging techniques including Magnetic Resonance Imaging (MRI). Positron Emission Tomography (PET), X-ray imaging, Computed Tomography (CT), Radiography, Photoacoustic imaging, Ultrasound and digital subtracton angiography. However, it is to be appreciated that any other means by which the size of the enclosed volume of gas 30 can be determined when positioned within a fluid contained in an enclosed compartment may be used.

Encasing the enclosed volume of gas 30 within the elastic material 15 allows the pressure of the fluid in which the enclosed volume of gas 30 and the elastic material 15 are positioned to affect the size of the enclosed volume of gas 30. For example, as the pressure of the fluid in which the enclosed volume of gas 30 and the elastic material 15 are positioned increases the pressure on the enclosed volume of gas 30 also increases which results in a reduction in the size of the enclosed volume of gas 30. Conversely, as the pressure of the fluid in which the enclosed volume of gas 30 and the elastic material 15 are positioned decreases the pressure on the enclosed volume of gas 30 also decreases which in turn results in an increase in the size of the enclosed volume of gas 30. As the size of the enclosed volume of gas 30 changes so does the frequency of acoustic energy applied to the enclosed volume of gas 30 at which the enclosed volume of gas 30 resonates.

Figure 3B illustrates a portion of the side wall 17 of an alternative form of the stent graft 13. The side wall 17 includes the bubble 35 enclosed by the deformable membrane 36 and either partially enclosed by the elastic material 15 or completely enclosed by a very thin layer of the elastic material 15 in comparison to the relatively thick layer of the elastic material 15 of the embodiment illustrated in Figure 3A. By having a relatively thin layer of elastic materia! 15 surrounding the enclosed volume of gas 30 the enclosed volume of gas 30 of the arrangement illustrated in Figure 3B could resonate in response to different frequencies of acoustic energy compared to the enclosed volume of gas 30 of the arrangement illustrated in Figure 3A. The frequencies of acoustic energy under which the enclosed volume of gas 30 of the arrangement disclosed in Figure 3B, which has a thin layer of elastic material 15, will resonate may be orders of magnitude different to the frequencies of acoustic energy under which the enclosed volume of gas 30 of the arrangement disclosed in Figure 3A will resonate.

Figure 3C illustrates a portion of [he side wall 17 of an alternative form of the stent graft 13, The side wall 17 includes enclosed volumes of gas 30 that are of order of magnitude smaller than those of the arrangements illustrated in Figure 3A and Figure 3B. The arrangement illustrated in Figure 3C includes microbubbies 34 containing the enclosed volumes of gas 30. Each of the microbubbles 34 includes an enclosed volume of gas 30 surrounded by a deformable membrane 36 and/or an elastic material 15. By utilising smaller microbubbles 34, the arrangement illustrated in Figure 3C is suitable for exhibiting resonance under the application of acoustic energy at frequencies that are in orders of magnitude different to the frequencies at which the bubbles 35 of the arrangements disclosed in Figure 3A and Figure 3B will exhibit resonance. For example, the smaller microbubbles 34 of the arrangement illustrated in Figure 3C may resonate when subjected to acoustic energy in the ultrasound range of frequencies and when positioned within fluid at physiologically relevant pressures. \n contrast, the larger bubbles 35 of the arrangements illustrated in Figures 3A and 3B may resonate when subjected to acoustic energy in the audible range of frequencies and when positioned within fluid at physiologically relevant pressures.

The inclusion of microbubbles 34 in the arrangement disclosed in Figure 3C is advantageous in that the layer of elastic material 15 enclosing the microbubbles 34 is relatively thin compared with the arrangement disclosed in Figure 3A and Figure 3B. Also, the overall thickness of the side wall 17 and the elastic material 15 attached to the internal surface 19 is thinner than the arrangements disclosed in Figure 3A and Figure 3B. Thus, the side wall 17, the elastic materia! 15 and the microbubbie 34 of the arrangement illustrated in

Figure 3C may enable stent grafts 15 to be manufactured having a relatively small wail thickness, interna! and external diameter which is particularly advantageous where the stent graft 13 is to be implanted into a blood vessel 60 having a relatively small internal diameter.

Referring to Figure 3D, there is shown a section view of a portion of the side wall 17 of another form of the stent graft 13 in which the internal surface 19 has attached to it a layer of elastic material 15 enclosing bubbles 35 of differing sizes. Enclosed within the elastic material 15 is a bubble 35 of similar diameter to those which are incorporated in the arrangements illustrated in Figures 3A and 3B. Also enclosed within the elastic material 15 is a microbubbie 34 of comparable size to those that are contained in [he arrangement disclosed in Figure 3C. Another bubble 33 is enclosed within the elastic material 15 which has a size between that of the microbubbie 34 contained in the arrangement disclosed in Figure 3C and of the bubble 35 contained in the arrangement disclosed in Figure 3A and 3B. Thus, under the application of acoustic energy over a given frequency range each of the bubbles 35. 33 and the microbubbie 34 may be configured to resonate when positioned within fluid at different pressure ranges. Alternatively, each of the bubbles 35, 33 and the microbubbie 34 may be configured to resonate under the application of acoustic energy over different frequency ranges when positioned within fluid at the same pressure.

Accordingly, the stent graft 13 incorporating the side wall 17 with a layer of elastic material 15 attached to the internal surface 19 enclosing bubbles 35, 33 and microbubbies 34 containing enclosed volumes of gas 30 of different sizes enables the measurement of the pressure of fluid, such as blood, over a plurality of pressure ranges or with a plurality of frequency ranges of acoustic energy.

Although in the arrangements illustrated in Figure 3A to 3D the bubbles 35, 33 and microbubbies 34 are enclosed in elastic material 15 attached to the internal surface 19 of the side wall 17 it is to be appreciated that the bubbles 35, 33 and microbubbies 34 may be enclosed within elastic material 15 attached to the externa! surface 18 of the side wall 17 in addition to or instead of the internal surface 19.

Examples

Referring to Figures 4a and 4b, there is shown a graph illustrating the results of a laboratory experiment showing the frequencies of acoustic energy at which resonance of an enclosed volume of gas 30 was detected by an ultrasound machine 42 when the enclosed volume of gas 30 was located in water at a range of pressures. Figures 4a and 4b also illustrate a comparison of experimental data with theory, In Figure 4a all data experimental data points are compared with coupled-oscillator theory, fos- In Figure 4b the average of the data of Figure 4a is shown on an expanded frequency scale and compared with both coupled-oscillator theory, fos, (solid line) and classical Minnaert, fo, theory (dashed line).

Figure 5 is a graph illustrating the results of a laboratory experiment showing the variation in size of an image of an enclosed volume of gas 30 contained within a bubble 35 when the bubble 35 was located in water at a range of pressures. The correlation between the variations in size of an enclosed volume of gas 30 when positioned in water at a range of pressures is discussed below.

Gases are compressible whereas liquids are essentially incompressible. Thus, a gas bubble will decrease in volume as the pressure of the surrounding liquid increases. If the compression is isothermal, the variation with pressure of the radius, R₀, of a bubble is given by the ideal-gas law,

where P_∞ and P^ are the absolute pressures at a standard and a different liquid pressure respectively and Ro-? is the radius of the bubble under the different pressure. The blood pressure Pu of clinical relevance is given by

Pu ^~ P∞ D1 ^" ' aim where P_ato is the atmospheric pressure. The surface area of a bubble is proportional to the square of its radius. The ultrasound energy scattered from it might be expected to vary provided its surface area, is significantly bigger than the wavelength of the sound. Thus, the scattered ultrasound intensity at the different pressure, LZ_0?. is given by

Uoi = U₀ ( (P/. + Patm) I (PiI + Patm))²"

Accordingly, the measurement of the blood pressure Pu would be given by

Pu = (Pi + Patndi U₀ ! Uoif² - P_atm

Figure 6 schematically illustrates another form of the stent graft 13. Similar to the arrangement disclosed in Figure 2, the stent graft 13 in Figure 6 is positioned to occlude the aneurysm 82 within the diseased blood vessel 60 from the remainder of the internal volume 66 of the blood vessel 60 through which blood travels in the direction indicated by the arrow A, The stent graft 13 includes a means for measuring a differential pressure between fluid within the internal volume 66 of the blood vessel 60 and within the internal space 63 within the aneurysm 62. Figures 7 and 8 illustrate an enlarged view of a portion of the side wall 17 of the stent graft 13 of Figure 6 containing the means for measuring a differential pressure between fluid within the internal volume 66 of the blood vessel 60 and within the internal space 63 within the aneurysm 82.

The side wall 17 of the stent graft 13 contains a gas filled tube 70 which passes between the external surface 18 and the internal surface 19 of the side wall 17. The tube 70 has a cylindrical side wall 74 defining an internal volume 76 containing a gas and extending between a first open end 72 and a second open end 78. Although in the embodiment illustrated in Figure 7 the tube 70 has a cylindrical side wall 74 it is to be appreciated that the tube 70 may take any suitable form or configuration as long as the tube 70 includes an internal volume 78 of at least some magnitude and a first open end 72 on the external side of the side wall 17 and a second open end 78 on the interna! side of the side wall 17.

Attached to the first open end 72 of the tube 70 is an expandable enclosed volume in the form of a first inflatable bailoon 71. The first inflatable balloon 71 is attached to the first open end 72 such [hat a seal is provided between the first balloon 71 and the first open end 72 of the tube 70. The first inflatabie balloon 71 has a column shaped portion 81 that is located within the internal volume 76 of the tube 70,

Attached to the second open end 78 is an expandable enclosed volume in the form of a second inflatable balloon 73 similar to the first inflatable balloon 71. The second inflatable balloon 73 is attached to [he second open end 78 such that there is a seal in between. Furthermore, the second inflatable balloon 73 has an opening 82 in communication with the internal volume 76 of the tube 70.

The first inflatable balloon 71 has an internal volume 75 containing an incompressible liquid. The internal volume 75 of the first inflatable balloon 71 is sealed so that the total volume of the internal volume 75 of the first inflatable balloon 71 remains constant. The second inflatable balloon 73 has an internal volume 79. The internal volume 79 of the second inflatable balloon 73 and the internal volume 76 of the tube 70 contains a gas. The gas freely moves between the interna! volume 79 of the second inflatable balloon and the internal volume 78 of the tube 70 via the opening 82 in the second balloon 73.

In other forms, either the first balloon 71 or the second balloon 73 or both may be sealed and may include no such opening. Also, either the first balloon 71 or the second balloon 73 or both may include a gas or a fluid or both which may or may not be incompressible. Also, the internal volume 76 of the tube 70 may contain a gas or fluid which may or may not be compressible.

Although the first inflatable balloon 71 illustrated in Figure 7 has a substantially spherical shape it is to be appreciated that the shape of the first inflatable balloon 71 may take any suitable form and may be irregular in shape. The only requirement of the first inflatable balloon 71 is that it have an internal volume 75 defined by a wall 77, The wall 77 may be formed out of resilient material elastic material or inelastic material such that the shape of [he first inflatable balloon 71 and the internal volume 75 may change in the manner described herein. The second inflatable balloon 73 is similar to the first inflatable balloon 71 in that the second inflatable balloon 73 also has an internal volume 79 defined by a wall 80. The wall 80 of the second inflatable balloon 73 is formed out of resilient material or elastic material that is capable of enabling the size of the internal volume 79 to change in the manner described herein.

Referring to Figure 8, the side wall 17 is configured to act as a barrier between a space [hat is external to the stent graft 13 and a space that is internal of the stent graft 13, When the stent graft 13 is positioned within a blood vessel 60 for the purpose of occluding the internal space 63 of [he aneurysm 82 from [he internal volume 86 of the blood vessel 80 the side wall 17 may be configured to act as a barrier between the internal space 63 of the aneurysm 62 and the internal volume 16 defined within the internal surface 19 of the stent graft 13. When the stent graft 13 has been properly positioned to occlude the internal space 63 of the aneurysm 62 then this will result in a pressure difference between the internal volume 68 of the blood vessel 60 and the internal space 63 of the aneurysm 82.

Referring to Figures 8 to 8, the first open end 72 and the first inflatable balloon 71 will be positioned within the internal volume 16 of the stent graft 13 whereas the second inflatable balloon 73 and the second open end 78 of the tube 70 will be positioned within the internal space 83 of the aneurysm. The first inflatable balloon 71 will be subjected to a relatively high external pressure from blood flowing at pressure through the internal volume 68 of the blood vessel 80 compared with the second inflatable balloon 73 which will be subjected to a relatively low external pressure within the internal space 63 of the aneurysm 62.

Referring to Figure 7, the external pressure encountered by the first inflatable balloon 71 is at a relatively low level in comparison to the external pressure encountered by the second inflatable balloon 73, Because of the resilience or elasticity of the wall 79 of [he second inflatable balloon 73 gas or liquid is expelled from within the second inflatable balloon 73 through the opening 82 and into the tube 70. As a result of the increase in the amount of gas of liquid being forced into the tube 70 the fluid within the column shaped portion 81 of the first inflatable balloon 71 is forced out of the column shaped portion 81 and into the main body of the interna! volume 75 of the first inflatable balloon 71. Thus, the first inflatable balloon 71 expands while the second inflatable balloon 73 contracts.

Assuming the blood vessel 60 is an artery, the blood will flow through the blood vessel 60 under pulsatile flow. This means that the pressure of the blood flowing through [he internal volume 66 of the blood vessel 60 will vary between a pressure associated with systole and another pressure associated with diastole. The systolic arterial pressure is defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle, whereas the diastolic arterial pressure is the lowest pressure, which occurs at the resting phase of the cardiac cycle.

In Figure 7 the second inflatable balloon 73 is deflated and the first inflatable balloon 71 is inflated. Thus, Figure 7 may represent the case of blood flowing through blood vessel 60 at the lowest pressure associated with diastole, in Figure 8 the first inflatable balloon 71 is deflated and the second inflatable balloon 73 is inflated. Thus, Figure 8 may represent the case of the blood flow through the blood vessel 60 at the peak arterial pressure associated with systole. Accordingly, in each cardiac cycle, the first inflatable balloon and the second inflatable balloon 73 may, respectively, inflate and deflate at least once.

The extent to which the first inflatable balloon 71 inflates and the column shaped portion 81 reduces in size during diastole, and to which the first inflatable balloon 71 deflates and the column shaped portion 81 increases in size during systole can be measured by any suitable imaging technique including, but not limited to, MRI, PET, CT, X-ray imaging, Radiography, Photoacoustic Imaging, Ultrasound or digital subtraction angiography. Accordingly, by measuring the extent to which the first inflatable balloon 71 inflates or deflates and the column shaped portion 81 reduces or increases in size during systole and diastole, respectively, can be used to determine whether the side wall 17 of [he stent graft 13 has effectively occluded [he internal space 83 of the aneurysm 62 from the interna! volume 86 of the blood vessel 60.

For example, if the first inflatable balloon 71 deflates and the column shaped portion 81 increases in size during diastole and the first inflatable balloon 71 deflates and the column shaped portion 81 increases in size significantly during systole then this may indicate that the side wall 17 of the stent graft 13 is effectively occluding the internal space 63 of the aneurysm 82 from the internal volume 68 of the blood vessel 60. This is because only a significant difference In pressure between the internal volume 66 of the blood vessel 60 and the internal space 63 of the aneurysm 82 will result in the first inflatable balloon 71 deflating and the column shaped portion 81 increasing in size during diastole and the first inflatable balloon 71 deflating and the column shaped portion 81 increasing in size significantly during systole.

In contrast, if the first inflatable balloon 71 does not deflate significantly and the column shaped portion 81 does not increase in size during diastole and the first inflatable balloon 71 does not deflate significantly and the column shaped portion 81 does not increase in size significantly during systole then this may indicate that the interna! space 83 of the aneurysm 62 and the internal volume 66 of the blood vessel 60 are in pressure communication. Thus, the side wall 17 of the stent graft 13 may not have effectively occluded the internal space 63 of the aneurysm 62 from the internal volume 66 of the blood vessel 60. This may be an indication that the stent graft 13 requires repositioning or replacement with a new stent graft 13 that effectively occludes the internal space 63 of the aneurysm 82 from the internal volume 86 of the blood vessel 60,

Any one of the imaging techniques set out above may be used to generate an image of the first inflatable balloon 71 in order to enable the size of inflation, and deflation, if any of the first inflatable balloon 71 and the change in size of the column shaped portion 81 during systole and diastole over the cardiac cycle to be measured. The imaging technique that is utilised may incorporate a means of processing the image of the first inflatable baiioon 71 or the column shaped portion 81 to determine the extent of any inflation, deflation or change in size and thereby determine the extent of the difference in pressure, if any, between blood flowing through the interna! volume 16 of the stent graft 13 compared to blood or other fluid contained in the internal space 63 of the aneurysm 82. This arrangement may enable a medical practitioner to determine the extent of the effectiveness of the stent graft 13 in occluding the internal space 83 of the aneurysm 62 from the internal volume 86 of the blood vessel 60,

Referring to Figures 9 and 10, there is shown another form of the stent graft 13 including an elastic barrier 90, in the form of an elastic membrane, connected to the side wall 17 of the stent graft 13. The barrier 90 may be formed out of any suitable elastic material. In another form, the barrier 90 may be formed out of any suitable resilient material. The barrier 90 has an internal surface 92 that faces the internal volume 16 of the stent graft 13 in the same way that the internal surface 19 of the side wall 17 faces the internal volume 16 of the stent graft 13. The internal surface 92 of the barrier 90 may be substantially continuous with the internal surface 19 of the side wall 17. The barrier 90 also has an external surface 94 that faces outwardly and away from the internal volume 16 of the stent graft 13 in the same way that the external surface 18 of the side wall 17 of the stent graft 13 faces outwardly and away from the internal volume 16 of the stent graft 13. The external surface 94 of the barrier 90 may be substantially continuous with the internal surface 18 of the side wall 17.

When the stent graft 13 is positioned within the blood vessel 60, as illustrated in Figure 9, for the purpose of occluding the internal space 63 of the aneurysm 62 from the internal volume 66 of the blood vessel 60 the barrier 90 deflects in response to a difference in pressure between fluid, such as blood, within the internal volume 16 of the stent graft 13 and fluid within the internal space 63 of the aneurysm 62. In particular, where the pressure of the blood within the interna! volume 16 of the stent graft 13 is greater than the pressure of the blood or other fluid, if any, in the internal space 63 of the aneurysm 62 then the barrier 90 will tend to deflect outwardly and away from the internal volume 16 of the stent graft 13 and into the internal space 63 of the aneurysm 62. The extent of the difference between the pressure of the blood in the interna! volume 16 within the stent graft 13 and the pressure of the blood or other fluid in the internal space 63 within the aneurysm 62 is reflected in the extent to which the barrier 90 deflects outwardly into the internal space 63 of the aneurysm 62, if at all. By measuring the extent of deflection, if any, of the barrier 90 the extent of any pressure difference between blood in the interna! volume 16 within the stent graft 13 and blood or oiher fluid in the interna! space 63 within the aneurysm 62 can be determined.

In Figure 10, there is shown a series of images represented by the letters A to F showing [he results of experiments conducted on one of the barriers 90. The barrier 90 and a portion of the side wall 17 of the stent graft 13 is shown in each image of Figure 10. The image represented by the letter A shows the barrier 90 in a relaxed state with no deflection outwardly from the externa! surface 18 of the side wall 17. The subsequent images represented by letters B to F show the barrier 90 progressively deflecting outwardly from the externa! surface 18 of the side wall 17 as a result of a progressively increasing difference in pressure between fluid contacting the interna! surface 92 of the barrier 90 and fluid contacting the external surface 94 of the barrier 90. The image represented by the letter F illustrates the barrier 90 in a state of greatest deflection compared to the states of deflection of the elastic barrier 90 illustrated in the images represented by the letters A to E. The differential pressure in mm Hg to which the barrier 90 is subjected in the images represented by the letters A to F in Figure 10 are: A = 0.0, B= 18.4, C = 29.4, D = 35.0, E = 49.6, F = 55.1.

When the stent graft 13 is positioned within the blood vessel 60, the external surface 94 of the barrier 90 is in contact with blood, or other fluid, within the interna! space 63 of the aneurysm 62 whereas the internal surface 92 of the barrier 90 is in contact with blood within the internal volume 16 of the stent graft 13. If the side wall 17 of the stent graft 13 effectively occludes the interna! space 63 of the aneurysm 62 from the interna! volume 66 of the blood vessel 60 then during systole the extent of the deflection of the barrier 90 will be greatest whereas during diastole the extent of the deflection of the barrier 90, if any, will be least. On the other hand, if the side wall 17 of the stent graft 13 does not effectively occlude the internal space 63 of the aneurysm 62 from the internal volume 66 of the blood vessel 60, then even during systole the barrier 90 may not deflect outwardly from the internal volume 16 of the stent graft 13 and into the internal space 63 of the aneurysm 62 to any substantial degree. This is because where the side wall 17 of the stent graft 13 does not effectively occlude the internal space 63 of the aneurysm 62 from the internal volume 66 of the blood vessel 60 the internal space 63 of the aneurysm 62 and the internal volume 66 of the blood vessel 60 are in pressure communication and there will not be a pressure difference, or there will be a reduced pressure difference, therebetween.

The extent of deflection, if any, of the barrier 90, as discussed above, can be imaged and measured by the use of any suitable imaging technique. For example, imaging techniques such as MR!, PET, CT, X-ray imaging, Radiography, Photoacoustic Imaging, Ultrasound, digital subtraction angiography, or any other suitable imaging technique, may be used to create an image of the barrier 90 when the stent graft 13 is located in situ within the blood vessel 60. The extent of the deflection, if any, of the barrier 90 can be assessed either manually or by electronic means of processing the image data. Thus, a medical practitioner can determine the extent to which the stent graft 13 is effectively occluding the internal space 63 of the aneurysm 62 from the internal volume 66 of the blood vessel 60 and whether or not repositioning or replacement of the stent graft 13 is required.

In Figure 11 there is shown a graph illustrating the results of experiments conducted on a prototype barrier 90 incorporated into the side wall 17 wherein the magnitude of deflection is represented as extension measured in millimetres as a function of the difference in pressure between fluid contacting the internal surface 92 of the barrier 90 and fluid contacting the externa! surface 94 of the barrier 90, Referring to Figure 12 there is shown a series of images labelled A to F illustrating another form of the side wall 17 of the stent graft 13 in which an inelastic or non-resilient barrier 100, in the form of an inelastic membrane, is attached to the side wall 17. The inelastic barrier 100 is in the form of a substantially half spherical body projecting outwardly from the external surface of the side wail 17. The material forming the ineiastic barrier 100 will not stretch when subjected to pressures within ranges of physiological significance. The inelastic barrier 100 is responsive to differences in pressure between fluid in contact with an external surface 104 and an internal surface 102 to deflect, collapse or buckle once the threshold difference in pressure is reached. Accordingly, in contrast to the form of the invention that is the subject of Figures 9 to 11 the form of the invention iilustrated in Figure 12 is suitable for determining critical pressure differences between fluid in contact with the internal surface 104 of the inelastic barrier 100 and fluid in contact with the internal surface 102 of the inelastic barrier 100.

Referring to Figure 12, in the image labelled F the inelastic barrier 100 is in a substantially half spherical form, in this instance, the pressure of the fluid in contact with the external surface 104 relative to the pressure of the fluid in contact with the internal surface 102 of the inelastic barrier 100 is at a magnitude that is insufficient to cause the inelastic barrier 100 to reach its buckling threshold. Referring to the image labelled E, although the pressure of the fluid in contact with the internal surface 104 is increased relative to the pressure of the fluid in contact with the internal surface 102 the relative increase in pressure is still not sufficient to cause the inelastic barrier 100 to buckle. Similarly in the image labelled D, despite a relative increase in the pressure of fluid in contact with the external surface 104 compared to the internal surface 102, the inelastic barrier 100 has still not buckled. The images represented by the letters C, B and A illustrate progressively higher increases in the pressure of the fluid in contact with the external surface 104 compared to the pressure of the fluid in contact with the internal surface 102. In the image represented by the letter C the inelastic barrier 100 has reached a critical threshold difference in pressure between the fluid in contact with the external surface 104 and the fluid in contact with the internal surface 102 such that the inelastic barrier 100 has begun to buckle. In the images represented by the letters B and A progressive increases in the pressure difference have resulted in progressively more extensive buckling of the inelastic barrier 100. However, it is apparent that the threshold at which the inelastic barrier initially buckled was achieved during the increase in pressure of the fluid in contact with the external surface 104 compared to the fluid pressure in contact with the internal surface 102 to the point represented by the image labelled with the letter C.

In each of the images labelled A to F in Figure 12. the difference in the pressure, in mm Hg, of the fluid in contact with the external surface 104 relative to the pressure of the fluid in contact with the internal surface 102 of the inelastic barrier 100 is: A = 0.0, B = 0.7, C =1.8, D = 2.9, E = 9.2, F= 12.8

Buckling of the inelastic barrier 100 can be ascertained when the stent graft 13 is positioned in situ within the blood vessel 60 by any suitable imaging technique including, but not iimited to, MRI, PET, CT. X-ray imaging. Radiography, Photoacoustic Imaging, Ultrasound and digital subtraction angiography.

Some of the forms of the invention described in detail above and claimed below are advantageous in thai they enable measurement of absolute pressure of fluid within an enclosed compartment such as the absolute pressure of blood within an enclosed blood vessel. Other forms of the invention described in detail above and claimed below are advantageous in that they enable measurement of differential pressure between fluid within one portion of an enclosed compartment and fluid within another portion of the enclosed compartment. All forms of the invention are advantageous in that they do not require direct access to the fluid within the enclosed compartment during the time in which the pressure measurements are being made. Thus, the invention is advantageous over direct pressure measurement techniques that require direct access to fluid within an enclosed compartment whilst pressure measurements are being taken. Furthermore, the invention does not pose risks of infection and other complications associated with repeated or extended percutaneous and direct access to blood vessels for at least the period of time over which direct measurements of blood pressure are taken.

The invention is advantageous in that once an implantable medical device incorporating the invention is implanted in situ within the human body, such as when the medical device is a stent graft positioned within a blood vessel for the purpose of treating an aneurysm, the invention enables direct and continuous measurement of the absolute and/or differential pressure of blood within the blood vessel. Also, forms of the invention enable direct and continuous measurement of differential pressure between blood within an internal volume defined within the stent graft and the pressure of blood within an aneurysm outside of the stent graft. Accordingly, forms of the invention enable a medical practitioner to determine the effectiveness of the stent graft in occluding the aneurysm from the internal volume of the blood vessel and for ascertaining the necessity for repositioning or replacing the stent graft in order to effectively occlude the aneurysm and thereby effectively treat the diseased blood vessel.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described, it is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the invention.

Future Patent applications may be filed on the basis of, or claiming priority from, the present Application. It is to be understood that the following provisional claims are provided by way of example only, and are not intended to limit the scope of what may be claimed in any such future Application. Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method for determining a pressure of a fluid, the method including:

positioning an enclosed volume of a gas within a fluid;

subjecting the enclosed volume of gas to an acoustic energy; and

determining a pressure of the fluid by determining a frequency of the acoustic energy at which the enclosed volume of gas resonates when positioned within the fluid.

2. The method of claim 1 , wherein determining the pressure of the fluid includes comparing the frequency of the acoustic energy at which the enclosed volume of gas resonates when positioned within the fluid with frequencies of the acoustic energy at which the enclosed volume of gas is known to resonate when positioned within the fluid at known pressures.

3. The method of claim 1 or claim 2, wherein determining the frequency of the acoustic energy at which the enclosed volume of gas resonates when positioned within the fluid includes subjecting the enclosed volume of gas to acoustic energy at different frequencies and determining the frequency at which the enclosed volume of gas resonates.

4. The method of claim 3, wherein determining the frequency at which the enclosed volume of gas resonates includes determining an intensity of the acoustic energy from the enclosed volume of gas induced by subjecting the enclosed volume of gas to acoustic energy.

5. The method of any preceding claim, further including partially enclosing the volume of gas within a deformable material.

6. The method of any one of claims 1 to 4, further including fully enclosing the volume of gas within a deformable material.

7. The method of any preceding claim, including positioning at least two enclosed volumes of gas having different volumes within the fluid so that when the enclosed volumes of gas are subjected to the acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

8. The method of any preceding claim, including positioning at least two enclosed volumes of gas having different types of gas within the fluid so that when the enclosed volumes of gas are subjected to the acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

9. The method of any preceding claim, further including positioning the enclosed volume of gas within an internal bodily fluid.

10. The method of any preceding claim, further including connecting the enclosed volume of gas to an implantable medical device.

1 1. The method of claim 10, wherein the implantable medical device is a stent graft

12. The method of claim 10 or claim 1 1 , further including positioning the implantable medical device within an internal bodily fluid vessel.

13. The method of claim 12, further including connecting the enclosed volume of gas to an internal surface of a wall of the stent graft and connecting another of the enclosed volumes of gas to an external surface of the wall of the stent graft opposite to the internal surface

14. The method of any one of the preceding claims, wherein positioning the enclosed volume of gas within a fluid includes positioning the enclosed volume of gas within blood in an internal blood vessel.

15. The method of claim 14, wherein determining the pressure of the fluid includes determining the pressure of the blood at two or more times during a cardiac cycle when the blood is under a pulsatile flow.

16. The method of any one of the preceding claims, further including a transducer for generating the acoustic energy to which the enclosed volume of gas is subjected and for generating electrical signals from acoustic energy from the enclosed volume of gas, and a processor for receiving the electrical signals and determining whether the acoustic energy to which the enclosed volume of gas is subjected is causing the enclosed volume of gas to resonate.

17. The method of any one of the preceding claims, wherein the enclosed volume of gas is a bubble.

18 An implantable medical device capable of determining a pressure of a fluid within a patient, the device including:

a implantable member configured to be positioned within a patient;

19. The device of claim 18, wherein the volume of gas is enclosed by a deformable material.

20. The device of claim 18, wherein the volume of gas is fully enclosed with a deformable material.

21. The device of claim 19 or claim 20, wherein the deformable material includes a material selected from any one of the following classes of materials: proteinaceous, lipid, saccharide, carbohydrate, elastomeric, polymer, inorganic, organic, or biological.

22. The device of any one of claims 18 to 21 , wherein the device includes at least two of the enclosed volumes of gas having different sized enclosed volumes of gas so that when subjected to acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

23. The device of any one of claims 18 to 22, wherein the device includes at least two of the enclosed volumes of gas having different types of gas so that when subjected to acoustic energy at the same frequency the two volumes resonate within the fluid at different pressures.

24. The device of any one of claims 18 to 23, wherein the implantable medical device is configured to be positioned within an internal bodily fluid vessel.

25. The device of any one of claims 18 to 24, wherein the implantable medical device is a stent graft.

26. The device of claim 25, wherein one of the enclosed volumes of gas is positioned on an internal surface of a wall of the stent graft and one of the enclosed volumes of gas is positioned on an external surface of the wall of the stent graft opposite to the internal surface.

27. The device of any one of claims 18 to 26, wherein the enclosed volume of gas is a bubble.

28. An apparatus for use with a medical device positioned within a patient, wherein the medical device includes an enclosed volume of gas in contact with a fluid within the patient that will resonate when subjected to acoustic energy and that a frequency of the acoustic energy at which the enclosed volume of gas resonates corresponds to a pressure of the fluid, the apparatus including:

29. The apparatus of claim 28, further including a second receiving transducer for generating a second set of electrical signals that are received by the processor for eliminating a background reflection of acoustic energy from the tissue of the patient's body.

30. The apparatus of claim 28 or claim 29, wherein the processor is configured so that when the enclosed volume of gas is in contact with blood within a patient the processor determines the pressure of the blood at two or more times during a cardiac cycle when the blood is under pulsatile flow.

31. The apparatus of any one of claims 28 to claim 30, wherein the apparatus is an ultrasound device.

32. A method of determining a pressure of a fluid, the method including:

imaging the enclosed volume of gas within the fluid; and

determining the pressure of the fluid by measuring the size of the enclosed volume of gas within the fluid.

33. The method of claim 32, wherein imaging the enclosed volume of gas includes directing radiation at the enclosed volume of gas and detecting reflected radiation from the enclosed volume of gas.

34. The method of claim 32, wherein imaging the enclosed volume of gas includes directing acoustic energy at the enclosed volume of gas and detecting reflected acoustic energy from the enclosed volume of gas.

35. The method of any one of claims 32 to 34, wherein imaging the enclosed volume of gas includes using one or more imaging techniques including: magnetic resonance imaging, positron emission tomography or computed tomography, x-ray imaging, radiography, photoacoustic imaging, digital subtraction angiography and ultrasound.

36. The method of any one of claims 32 to 35, wherein determining the pressure of the fluid includes comparing the size of the volume of the enclosed volume of gas within the fluid with a known size of the volume of the enclosed volume of gas within a fluid at a known pressure.

37. The method of any one of claims 32 to 36, wherein positioning the enclosed volume of gas within a fluid includes positioning the enclosed volume of gas within blood in an internal blood vessel.

38. The method of claim 37, wherein the pressure of the blood is determined at two or more times during a cardiac cycle when the blood is under a pulsatile flow.

39. A method of determining a difference in pressure between a first space and a second space, the method including:

positioning a barrier between a first space and a second space wherein the barrier is configured to deflect in response to a difference in pressure between the first space and the second space;

imaging the deflection of the barrier; and

determining the difference in pressure between the first space and the second space by measuring a size of the deflection of the barrier.

40. The method of claim 39, wherein imaging the barrier includes directing radiation at the barrier and detecting reflected radiation from the barrier.

41. The method of claim 39, wherein imaging the barrier includes directing acoustic energy at the barrier and detecting reflected acoustic energy from the barrier.

42. The method of any one of claims 39 to 41 , wherein the difference in pressure between the first space and the second space is determined by comparing the measured size of the deflection of the barrier with a known size of deflection of the barrier when the barrier is positioned between two spaces having a known pressure difference.

43. The method of any one of claims 39 to 42, wherein the barrier is formed from a material whereby the size of the deflection of the barrier reduces due to a reduction in the difference in pressure between the first space and the second space.

44. The method of any one of claims 39 to 43, wherein the barrier is formed out of an elastic material.

45. The method of any one of claims 39 to 44, wherein the barrier is formed out of an inelastic material.

46. The method of any one of claims 39 to 45, wherein the barrier is positioned within an internal bodily cavity adjacent to a wall surrounding the cavity and the first space is an inner portion of the cavity and the second space is an outer portion of the cavity between the barrier and the wall surrounding the cavity.

47. The method of any one of claims 39 to 46, wherein the barrier is connected to an implantable medical device.

48. The method of claim 47, wherein the implantable medical device is a stent graft.

49. The method of claim 48, wherein the stent graft includes a wall and the barrier is positioned within the wall of the stent graft and the wall and the barrier are positioned to occlude an aneurysm from an internal volume within a blood vessel.

50. The method of claim 49, wherein the effectiveness of the stent for occluding the aneurysm is determined by measuring the deflection of the barrier and thereby determining the difference in pressure between a space defined within the aneurysm and the internal volume within the blood vessel.

51. The method of claim 49 or claim 50, wherein the deflection of the barrier is measured two or more times during a cardiac cycle when the blood is under a pulsatile flow.

52. A device for determining a difference in pressure between a first space and a second space, the device including: a barrier configured to be positioned between a first space and a second space and to deflect in response to a difference in pressure between the first space and the second space; and

53. The device of claim 52, wherein the barrier is calibrated so that a size of the deflection of the barrier corresponds with a size of the difference in pressure between the first space and the second space.

54. The device of claim 52 or claim 53, wherein the barrier is configured so that the deflection of the barrier is reversed in response to an equalization of the pressures in the first space and the second space.

55 The device of any one of claims 52 to 54, wherein the barrier is formed out of an elastic material.

56. The device of any one of claims 52 to 54, wherein the barrier is formed out of an inelastic material.

57. The device of any one of claims 52 to 56, wherein the device is an implantable medical device and the barrier is connected to a wall of the implantable medical device.

58. The device of claim 57, wherein the implantable medical device is a stent graft.

59. The device of any one of claims 52 to 58, wherein the enclosed volume of gas is a bubble.

60. An apparatus for use with a medical device positioned within a patient, wherein the medical device includes a barrier configured to be positioned between a first space and a second space within a patient and to deflect in response to a difference in pressure between the first space and the second space and wherein the deflection of the barrier is proportional to the difference in pressure between the first space and the second space, the apparatus including an imaging device for creating an image of the barrier and a processor for determining the difference in pressure between the first space and the second space by measuring the deflection of the barrier from the image of the barrier.

61. The apparatus of claim 60, wherein the imaging device includes one or more imaging devices including: magnetic resonance imaging, positron emission tomography or computed tomography, x-ray imaging, radiography, photoacoustic imaging, ultrasound and digital subtraction angiography.

62. The apparatus of claim 60 or claim 61 , wherein when the medical device is a stent graft and the barrier is positioned within a wall of the stent graft and the wall and the barrier are positioned within a blood vessel to occlude an aneurysm from an internal volume within the blood vessel, the imaging device is operable for creating an image of the barrier, and the processor is operable for determining a difference in pressure between fluid located between the aneurysm and the barrier and blood located between the barrier and the internal volume within the blood vessel by measuring the deflection of the barrier from the image of the barrier.

63. The apparatus of claim 62, wherein the processor is configured to determine the difference in pressure between fluid located between the aneurysm and the barrier and blood located between the barrier and the internal volume within the blood vessel at two or more times during a cardiac cycle when the blood is under pulsatile flow.