WO2012120498A1

WO2012120498A1 - Non-invasive thermal treatment monitoring

Info

Publication number: WO2012120498A1
Application number: PCT/IL2012/000104
Authority: WO
Inventors: Noam WEISS; Haim Azhari; Jacob Sosna
Original assignee: Technion Research & Development; Hadasit Medical Research Services And Development Ltd.
Priority date: 2011-03-04
Filing date: 2012-03-04
Publication date: 2012-09-13
Also published as: WO2012120498A8

Abstract

A method for thermal monitoring of HIFU and other thermal treatments, utilizing electro-magnetic radiation signal detection or image processing. The images or detectors output signals show the attenuation at the target tissue, and changes in the attenuation level may be correlated to changes in the tissue temperature and condition during treatment and the subsequent cooling down period. Additionally, the point at which denaturation and coagulation of the tissue occurs can be determined. Analysis of the functional change in attenuation with time of treatment enables the accumulated tissue damage to be quantified. The advantage of this method is that it indicates the level of damage in the tissue directly, instead of displaying tissue temperatures only. The method provides a cost effective non-invasive method for thermal monitoring during ablation, as an alternative to MRI monitoring or to CT monitoring based on temperature determination.

Description

NON-INVASIVE THERMAL TREATMENT MONITORING

FIELD OF THE INVENTION

The present invention relates to the field of monitoring tissues undergoing thermal treatment, especially by measurements of the changes in attenuation of the tissue being treated, these measurements being conducted using Electro-Magnetic radiation such as X-rays or Gamma rays directed through the tissue.

BACKGROUND OF THE INVENTION

In recent years, the non-invasive uses of High Intensity Focused Ultrasound (HIFU) treatments for tumor therapy and other clinical applications have gradually became more prevalent. This is mainly due to the ability of Ultrasound (US) waves to penetrate the body, and to be focused onto the region to be treated, without essentially affecting other organs located in positions other than the target focus. The focusing capabilities of the ultrasound beam, allow high precision and localizing effect of the acoustic wave energy. The effects of the ultrasonic wave are mostly induced by heating of the treated tissue. By focusing an energetic ultrasonic beam onto a small focal area, the entire beam's energy is concentrated at that point, causing a local increase of the temperature at the focus. For efficient treatment, and also for safety reasons, thermal monitoring is mandatory in these kinds of procedures. This monitoring is necessary not only for determining the temperature at the point of treatment, but also to ensure that the treatment is being executed at the correct location. Since the heating at the focus could take a few seconds until reaching temperatures above the desired denaturation temperature, the thermal monitoring must have a high temporal resolution as well as high spatial resolution to detect the exact position of the small focal point. Furthermore it is beneficial to be able to track changes the tissue undergoes in order to optimize treatment time and energy, avoiding situations where, on the one hand, the tissue did not yet become sufficiently damaged by the time the treatment was terminated, or, on the other hand, where the tissue became too hot, and healthy tissues around it were unnecessary injured. Other non-invasive thermal treatment methods are also known, such as microwave ablation, laser ablation, thermal balloon ablation, and radiofrequency (RF) treatment, and these too require temperature monitoring of the treated region. Another type of thermal treatment, known as cryogenic treatment, involves freezing the tissue. In such a procedure, temperature and process monitoring is also essential.

Currently, Magnetic Resonance Imaging (MRI) is the leading modality for this kind of thermal monitoring, using proton resonance frequency shift to monitor the treatment. The MRI imaging is utilized, at least for HIFU use, to locate the initial target position which requires treatment, to monitor the temperature at the target, and to ensure that the energy is focused at this target, which, because of refraction, may not be at the geometrical focus of the focusing or beam directing element. However, MRI installations are very costly, both in capital expense, and in consequent running costs, and hence have comparatively low availability. In addition, they can impose a very restrictive magnetic and RF environment on the treatment heads being used, limiting the structure of the equipment that can be used for performing the thermal treatment. In addition, the limited space available for the insertion of the treatment apparatus, and its required position, may prevent the treatment of some body parts. Furthermore, temporal resolution may be low because of the time taken to process the images and temperature data, and since it is important to monitor the treated area temperature in real time, this may be a serious drawback. Additionally, research, as described in the article entitled "Temperature monitoring in fat with MRI" by K. Hynynen, et al, published in Journal of Magnetic Resonance Imaging, Vol. 43, pages 901-904, (2000), and in more recent articles, has demonstrated that MRI thermometry based on proton resonance frequency shift (PRFS), especially in fat tissues, is not sufficiently accurate to ensure a high level of safety in thermal treatments. This is especially important in breast tumor HIFU treatments, where there is a lot of fat tissue in close vicinity to the tumor. The same applies to ultrasonic cosmetic fat reduction treatments, which are becoming increasingly popular.

In US Patent Application Publication No. 2008/0033418 for Methods for Monitoring Thermal Ablation" to M.W. Nields et al, there is described a thermal ablation system using an X-ray CT system to measure temperature changes throughout a volume of interest in a patient. Image data sets captured by the CT system during a thermal ablation procedure provide temperature change information for the volume being subjected to the thermal ablation. Methods of performing thermal ablation using CT system temperature measurements as a feedback source are also provided. In general, in this publication the attenuation, in terms of Hounsfield Units, is used in order to determine the temperatures and temperature changes utilized in the methods described in this publication. The CT imaging system may be operable to measure radiodensity or HU properties within a patient. This ability may then be used in determining temperature changes within the Volume of Interest (VOI) that may take place during a thermal ablation procedure. Additionally, a method for performing thermal ablation within a VOI in a patient is described, that includes adjusting a thermal ablation plan based at least in part on differences between a baseline digital image and a temperature differential digital image. The temperature differential digital image may be calibrated by measuring temperature of at least a first spatial location (corresponding to the same locations measured when calibrating the baseline digital image) within the VOI at or near the time the temperature differential is being captured and correlating the measured temperature to the temperature differential digital image at that location. This correlation between temperature differential digital image and temperature may then be combined with the correlation between the baseline digital image and temperature to develop a mathematical relationship between the values obtained from the imaging process (e.g. Hounsfield units measured at a particular location) and actual temperature. This relationship may then be applied across the VOI to yield calibrated temperatures across the VOI.

This method may have a disadvantage in that decisions and feedback mechanisms used to monitor and control the thermal ablation procedure come from temperatures determined from the Hounsfield units measurements made on the tissue being ablated. Since such measurements may be dependent on the immediate past history of the tissue being measured, in other words, on changes the tissue may have undergone as a result of the extent and intensity of the thermal treatment applied so far, a large calibration data bank is required in order to provide conversion between the measured Hounsfield units and the temperature of a specific tissue.

There therefore exists a need for another method of non-invasive tissue treatment monitoring which overcomes at least some of the disadvantages of prior art systems and methods.

The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety. SUMMARY OF THE INVENTION

The present disclosure describes new exemplary systems and methods for monitoring the thermal treatment of a tissue in real time by means of Electro-Magnetic (EM) imaging, or by signal detection of electromagnetic radiation passing through a tissue. The electromagnetic radiation system can be an existing system, for example an X-ray imaging system, either by conventional digital radiography, or by computerized tomography imaging. Alternatively, it can be a dedicated system, such as a pair comprising of a radioactive source and an opposing detector located on the opposite side of the tissue being treated, such that it detects the attenuation of the beam passing through the tissue. The thermal treatment changes the attenuation of the tissue, due to processes that range from simple thermal expansion, and extend to denaturation of protein tissue and coagulation, and eventual cell necrosis, once the tissue temperature has reached the level at which such processes occur. These changes in the nature of the tissue result in changes in the EM radiation attenuation, changing the signal level outcome of the detectors. This detector signal measurement can be performed continuously during the treatment and thus can provide real-time monitoring of the state of the tissue being treated.

In the first place, absorption measurements can provide a determination of the temperature of the tissue being imaged, though such determination may be subject to inaccuracies because of the need for careful pre-calibration against tissue samples having the similar treatment history to the tissue being treated. However, according to novel exemplary methods described in this disclosure, further information relating to the progress of the thermal treatment, beyond just the tissue temperature, can be obtained from the absorption measurements performed.

According to further analyses of the obtained attenuation data from the detected signal, changes therein can be correlated to the point in time of the treatment where denaturation and coagulation of the tissue begins to occur. At that point the attenuation reaches a minimum level, and as the coagulation proceeds, the attenuation of the tissue increases again resulting in a change in the behavior of the detector outcome signal, until termination of the application of the thermal treatment. After termination of the application of the treatment, the attenuation returns towards higher values (not necessarily its initial value), generally linearly with temperature. Because of the change in the structure of the coagulated tissue, the attenuation level at the end of the cooling down period may not return to its exact starting level, since in some cases the tissue can become denser than its original density. This cycle of change in the attenuation level as the treatment application and cooling down period proceeds, can be quantitatively used by one of several different calculation methods, to enable the accumulated thermal damage to the tissue to be assessed. This assessment is based on analysis of the trend of temporal changes in detector signal (in correlation with the changes in attenuation) especially relative to a minimum level which occurs at the point of denaturation and coagulation onset, in some implementations, without direct use of any temperature measurements or temperatures determined from the measurements.

Since all thermal treatment methods, whether by laser, microwave, RF ablation or High Intensity Focused Ultrasound (HIFU), damage and kill cells using the same basic mechanisms, such monitoring for evaluating thermal ablation treatments may be useful for any of these methods. Furthermore, this system and method may also be useful to monitor the progress of cryogenic ablative treatments, involving freezing the tissue in order to destroy it, since the temperature of the treated tissue also needs to be closely monitored to ensure optimal treatment without excessive collateral tissue damage.

The system and methods of the present disclosure may have a significant advantage over prior art MR! monitoring, both because of the significantly decreased response time, enabling true real-time monitoring to be achieved, and because of the increased spatial resolution possible, thereby increasing the positional accuracy of the treatment and its monitoring. Furthermore, the system and methods of the present disclosure may have a significant advantage over prior art X-ray CT based ablation monitoring methods, because information about the physical processes occurring in the tissue during treatment are available, rather than just the tissue temperature. There are additional self evident advantages because of the more advantageous environment engendered by use of a conventional digital radiography system or a CT system, rather than the prior art MRI system, such advantages include lower-cost, higher availability, easier access to the body part being imaged, and the lack of the need for any precautions for the thermal treatment apparatus because of the presence of high magnetic fields.

One exemplary implementation involves a method of monitoring thermal treatment of a tissue, comprising:

(i) passing electromagnetic radiation through the tissue in at least the region where the thermal treatment is being performed, (ii) measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed during at least the duration of the application of the thermal treatment, and

(iii) utilizing the functional variation of changes in the measured radiation attenuation as a function of time, directly to monitor the progress of the thermal treatment.

Such a method may further comprise determining when a minimum in the radiation attenuation of the tissue is achieved, and continuing to apply the thermal treatment for a predetermined time beyond the minimum, according to the dosage of thermal treatment desired. In such a case, the minimum in the radiation attenuation of the tissue may be determined from the time derivative of the radiation attenuation data.

Further implementations of these methods may involve utilizing predetermined calibration data to relate the measured radiation attenuation to the temperature of the tissue in the region where the thermal treatment is being performed, and using changes in the determined temperatures as a function of time to monitor the treatment.

In any of these methods, the radiation attenuation of the tissue may also be measured during the cooling down period after application of the thermal treatment.

Additional implementations can include the further step of determining when the measured radiation attenuation of the tissue reaches a minimum level, such that the point in time of commencement of at least one of denaturation and coagulation processes can be determined.

Additionally, such methods may further comprise the step of determining the difference in radiation attenuation between the point of minimum attenuation, and the point at which the application of the thermal treatment is ceased, and utilizing predetermined calibration data of this difference to determine the extent of thermal damage achieved by the treatment.

Alternatively, any of the above described methods may further comprise the step of determining the difference in radiation attenuation between the point of minimum attenuation, and the point of maximum attenuation, and utilizing predetermined calibration data of this difference to determine the extent of thermal damage achieved by the treatment. In such a case, the point of maximum attenuation is taken as being the point of completion of the cooling down process after thermal treatment. Monitoring of the treatment may be achieved while the treatment is being performed, and the monitoring of the treatment may comprise the determination of the duration of the treatment, or even the determination of when the treatment has achieved its aim.

Additionally, alternative implementations of the above-described methods may further involve the step of determining the maximum difference in radiation attenuation between any point during the application of the thermal treatment and the cooling down period, and utilizing predetermined calibration data for this difference to determine the extent of thermal damage achieved by the treatment.

In any of the above described methods, the electromagnetic radiation may be any one of X-rays, gamma rays or laser radiation, and the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, may be achieved by analysis of pixels from a 2- dimensional or 3-dimensional image. Alternatively, the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, may be achieved by using a detector to measure the transmitted intensity of a collimated beam of the electromagnetic radiation.

Another example implementation can involve a method of monitoring thermal treatment of a tissue, the treatment having a thermal application period and a cooling down period, the method comprising:

(i) passing electromagnetic radiation through the tissue in at least the region where the thermal treatment is being performed,

(ii) measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, during the thermal application period and at least part of the cooling down period,

(iii) determining the temperature of the tissue at those times when values of radiation attenuation are measured,

(iv) establishing the functional relationship between the measured values of radiation attenuation and the determined temperatures,

(v) determining the area contained within any hysteresis found in the functional relationship, between the thermal application period and the cooling down period, and

(vi) utilizing this determined area to evaluate the extent of thermal damage achieved by the thermal treatment.

In such a method, the temperature of the tissue may be determined from the measured values of radiation attenuation, using predetermined calibration data. In such a case, the predetermined calibration data may include analysis of the characteristic behavior of radiation attenuation data obtained by thermal treatment of samples of tissue similar to that being treated by the present method.

In any of the previously mentioned methods, the electromagnetic radiation may be X-rays, and the radiation attenuation is measured in Hounsfield Units, and the thermal treatment of the tissue may be performed by use of any one of High Intensity Focused Ultrasound, radio frequency ablation, microwave ablation and laser ablation. The thermal treatment of the tissue may also be performed cryogenicaily.

Monitoring of the treatment may be achieved while the treatment is being performed, and the monitoring of the treatment may comprise the determination of the duration of the treatment, or even the determination of when the treatment has achieved its aim. The electromagnetic radiation may be any one of X-rays, gamma rays or laser radiation.

Yet other implementations perform such methods in which the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, is achieved by analysis of pixels from a 2-dimensional or 3-dimensional image. Alternatively, the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment ma be performed, is achieved by using a detector to measure the transmitted intensity of a collimated beam of the electromagnetic radiation.

Another exemplary implementation can involve a system for monitoring thermal treatment of tissues, comprising:

(i) a source of electromagnetic radiation, adapted to direct the radiation through a tissue in at least the region undergoing thermal treatment,

(ii) a detection unit adapted to measure the radiation attenuation of the tissue in the region where the thermal treatment is being performed, and to generate radiation attenuation data of that region as a function of time, and

(iii) a control unit adapted to discern functional variation of changes in the radiation attenuation data as a function of time, and to utilize the discerned functional variation directly to monitor the progress of the thermal treatment.

Such a system may further comprise a calibration data unit for accessing predetermined calibration data relating the measured radiation attenuation to the temperature of the tissue, the control unit utilizing the temperatures obtained by the calibration data unit as a function of time to monitor the progress of the treatment. Additionally, in such a system the control unit may be adapted to determine from the time derivative of the radiation attenuation data, when a minimum in the attenuation of the tissue is achieved, so that the thermal treatment can be applied for a predetermined time beyond the minimum, according to the dosage of thermal treatment desired.

In any such systems, the thermal treatment of the tissue may be generated by use of any one of High Intensity Focused Ultrasound, radio frequency ablation, microwave ablation and laser ablation. The thermal treatment of the tissue may also be performed cryogenically.

In additional implementations of such systems, the detection unit may be either an image analysis unit or a detector signal processing unit. Furthermore, the electromagnetic radiation may be X-rays, and the source and detection unit be part of either one of a digital radiography system or a computerized tomography system. Finally, the electromagnetic radiation may be any one of Gamma-rays, infra red radiation or laser radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently claimed invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

Fig.1 shows a prior art HIFU treatment system using MR! position and temperature monitoring;

Fig. 2 illustrates schematically an X-ray CT imaging system implementing the methods described in this disclosure, for monitoring the thermal treatment of target tissue in a patient;

Figs. 3A and 3B illustrate how CT images of a block of fat tissue during HIFU treatment enable detection of the heated area at and around the focus of the HIFU, while Fig. 3C illustrates how the attenuation, in this case measured by HU value changes as a function of time during a thermal treatment session on such a sample of fat tissue;

Figs. 4A to 4E illustrate how the attenuation (in this case measured by Hounsfield Units) measured on pixels at the focus of an HIFU application to a porcine fat sample, such as that in Fig. 3A, change during the course of the thermal treatment for different accumulations of thermal dose; Figs. 5A to 5I show attenuation graphs measured in HU, for three different thermal treatment times on porcine fat tissue sample, and for each of these different treatments, three different parameters of measuring the accumulated tissue damage are superimposed graphically;

Fig. 6 is a table summarizing the correlation between the maximum measured temperature measurements and four parameters used for measurement of the accumulated tissue damage in a thermally treated fat sample;

Fig. 7 is a contour map of estimated tissue coagulation in the treated region graphically imposed on the CT image of a tissue undergoing thermal treatment; and

Fig. 8 illustrates a further implementation of the present systems using a C-arm system irradiating a target being thermally treated using a narrow electromagnetic beam which traverses the target area and is detected in a single detector.

DETAILED DESCRIPTION

Reference is now made to Fig. 1 , which illustrates schematically a HIFU system shown treating a breast tumor, using an MRI system for monitoring the position of the treatment and the temperature status of the treated region, as used in the prior art. The system shown is similar to the ExAblate^® system, as supplied by Insightec, a GE company, of Tirat HaCarmel, Israel. In Fig. 1 , the patient 10 is seen lying on the treatment table 11 , within the confines of the MRI system magnet tunnel 12, with the ultrasound transducer 13 directing focused US energy onto the tumor 14 being heated. The MRI system monitors the position and temperature of the treated area. As is observed, the space available within the tunnel confines of the MRI system tunnel is very limited, and the tunnel needs to be of significant length, as indicated in the partial isometric drawing of Fig. 1 , so this places spatial restrictions on the placement of the patient and the ultrasound heating equipment.

Reference is now made to Fig. 2, which illustrates schematically an exemplary Computerized Tomography (CT) imaging system 20, used for implementing the methods described in this disclosure, for monitoring the thermal treatment of a target region in a patient (not shown). The apparatus for generating the thermal treatment energy can be located within the patient table 21 , or more preferably attached to an external device such as a robotic arm 25 attached to the bed or gantry. As is observed, the diameter of the patient aperture 23 is generally larger than that of the prior art MRI system shown in Fig. 1 , but more significantly, the length of this "imaging tunnel" is much shorter, such that it is considerably simpler to position the thermal treatment equipment without interfering with the CT imaging procedure. In the exemplary system of Fig. 2, the thermal treatment source 26 is located on the end of the robotic arm 25. Almost any body part can thus be treated. Furthermore, since there are no high magnetic field or RF fields present, the thermal treatment apparatus can be of normal construction without any need for the use of special materials, structures or motors. Additionally, the CT image acquisition time of typically 0.5 sec. or less, implies that the thermal heating process can be monitored in essentially real time. The spatial resolution of a CT imaging system, typically of the order of 300pm, is generally better than that of an MR I imager, enabling the achievement of more accurate monitoring of the position and extent of the treated area.

The CT control and image processing system 20 is similar to a conventionally used CT system, but with the addition of a number of control units, to optimally use the methods described in the present disclosure. An output of the conventional CT image generation unit 25 is input to an image analysis unit 26 for quantitative monitoring of the radiographic attenuation of the tissue in the region where the thermal treatment is being performed. In addition, a calibration data unit 27 may be used providing access to a library of predetermined calibration data relating the measured radiographic attenuation performed on this particular type of CT system to the temperature and physiological nature of the tissue, and a control unit 28 utilizing the temperatures obtained by the calibration data unit as a function of time, to monitor the treatment. A temperature and or coagulation map generator 29 can also be provided, whose output can be graphically depicted on the conventional CT image of the region being treated, as displayed in real time on the system screen 30. In alternative implementations, the control unit 28 can utilize parameters based on the form of the attenuation data as a function of time to output information relating to the progress and monitoring of the thermal treatment, without the need for relating the attenuation data to specific temperature-related calibration.

Although the X-ray imaging system shown in Fig. 2 is a CT system, it is to be understood that radiographic attenuation of the treated tissue can also be determined using a conventional digital radiography X-ray system, such as in a C-arm configuration or a simple system with an electro-magnetic radiation emission unit (such as a laser or radioactive source in a shielded box with a pinhole tunnel for emission, and a opposed detector, as will be shown hereinbelow in connection with the implementation of Fig. 8. Although the CT implementation is used as the exemplary system throughout this disclosure, it is to be understood that the invention is not intended to be limited to use of a CT-based X-ray system.

The method used to perform the thermal treatment monitoring uses a novel correlation of the X-ray opacity of the various regions of the image to the temperature of that region and more importantly the tissue condition. The images of an X-ray CT show differences in the attenuation of the X-rays passing through that region, and these attenuation differences are due to changes in density and structure of the material. As materials get hotter, they become less dense due to thermal expansion, hence their CT attenuation is reduced, and they appear as darker areas in the image. According to the Lambert-Beer e uation,

where

l_x is the intensity measured after passage through a mass thickness x,

lo is the incident intensity, and

μ is the mass attenuation coefficient,

the signal that reaches the CT detectors is dependent on the material attenuation, where μ is the mass attenuation coefficient and x, the mass thickness, defined as the mass per unit area. The mass attenuation coefficient μ depends on the material density. In the case, for instance, of HIFU treatment of fat tissues, as the fat become hotter, its density changes, initially in accordance with its thermal expansion coefficient, but later also as a result of denaturation and coagulation processes, and so its X-ray attenuation coefficient will change accordingly.

In CT technology, grey levels in the images, which are equivalent to the radiographic opacity or attenuation of the pixel being imaged, are conventionally measured in Hounsfield units. The Hounsfield Unit (HU) scale is a scale where the radio-density of distilled water at STP is defined as zero Hounsfield units, while the radio-density of air at STP is defined as -1000 HU. The radio-density of tissues radiographically denser than water then have positive HU values, of for instance, up to 400 or more for cortical bone. It can be shown that a change of one Hounsfield Unit in an imaged pixel represents a change of 0.1 % of the attenuation coefficient of water.

The definition for the HU value for a material m at temperature T is given by:

where

HUm (T) is the dimensionless Hounsfield unit for the material m at a temperature T; Mim is the linear attenuation coefficient of the material m;

p_pw is the mass attenuation coefficient of water (set only during calibration at STP); and

p_m(T) is the density of the material m at temperature T.

Since _pw remains constant after calibration of the machine, and the linear attenuation coefficient Mi_m is not temperature dependent, the value of the element Mi_m/u_pw is constant during image reconstruction, while only p_m(T) changes with change in temperature.

In general for a given energy, materials with higher density have higher attenuation. Heated materials tend to expand according to their thermal expansion coefficient, reducing their electron density, hence lowering their attenuation. Consequently, this can be seen as a lower HU and a darker area in the CT image. On the other hand, above some threshold temperature, denaturation and coagulation of tissues occurs. This could make the tissue again denser, which also changes the HU value of the tissue, this time to higher values. Since the thermal treatment is generally intended to cause coagulation of the tissue, the CT images are capable of offering this information beyond the simple thermometry information. The manner in which this information is obtained and analyzed as an integral body of data, to provide the doctor with a competent monitor of the thermal treatment process is explained hereinbelow. This aspect is important to the attending physician, since he/she is generally interested in the overall effect of the thermal treatment, rather than in obtaining information about the temperature achieved in the treated region of the subject's tissue. Although the temperature reached may provide information about whether and for how long the tissue being treated has been at or above the coagulation temperature, the doctor then needs to insert those temperatures and times into a predetermined model of the thermal treatment in order to monitor the progress of the treatment. Even though this may be done in a dedicated computer routine, the question still arises as to the accuracy of relating the CT absorption level to a specific tissue temperature, because of the change in the tissue nature and its absorptivity with change in structure during and after tissue coagulation, a temperature measurement based on HU level may therefore be dependent on the history of the sample being treated, and calibration of the temperature reading may be problematic. Therefore it is of importance to attempt to relate values of the X-ray attenuation measured directly to the integrated effect of the treatment, rather than simply providing equivalent temperatures for consequent use in treatment models. Reference is now made to Figs. 3A and 3B, which illustrate how the CT images of a block of fat tissue during and after HIFU treatment enable the detection of the heated area at and around the focus of the HiFU. Fig. 3A shows a 25cm x 25cm CT image of piece of fat tissue, submerged in a bowl of water so that the HIFU will be able to operate. The water appears as the white area in the CT image. The darker area with an even darker center shows the effect of the heating, which increases towards the center, where the HIFU focus is situated. Fig. 3B represents a 3D diagram of the differences of the HU values of a slice of the fat tissue shown in Fig. 3A, during the HIFU treatment.

In order to provide a basis for the origin of useful information regarding the relationship between the HU values and the thermal effect of the HIFU or other thermal treatment, reference is now made to Fig. 3C, which illustrates how the HU value changes as a function of time during a thermal treatment session on a sample of porcine fat. This curve is typical of the time dependence of the HU values during thermal treatment, and one possible explanation of the significance of the various parts of the curve will be given in the drawings following. The measured temperature is also plotted, so that the relationship can be obtained between HU values and the temperature. In this experimentally obtained graph, the temperature was measured using a fine thermocouple inserted into the fat sample in the focal region. In Fig. 3C, the sample underwent thermal treatment for 1200 seconds, after which it was allowed to cool down again to ambient temperature. The temperature was measured near the HIFU focus, and is shown by the dashed line. The CT attenuation measurements showed a very distinct non-monotonic behavior. Initially the HU value fell, until the point during the treatment when the temperature at the focus approached 40°C, after which a distinct minimum was formed, at approximately 43° to 44°C after which the HU level begins to increase again. After this minimum, the HU value began to increase again at a decreasing rate until it reached the point at 1200 sec. at which time the thermal treatment was ceased. The attenuation curve then changed character drastically, with the HU value rising rapidly at first, and then asymptotically towards a value slightly above its initial value.

Reference is now made to Figs. 4A to 4E, which illustrate experimental results showing the change during the course of the thermal treatment, of the CT Hounsfield Units, as a function of temperature, measured on a group of pixels at the focus of HIFU applied to a fat sample such as that in Fig. 3A. Each of the graphs shows the HU value measured during the application time of the treatment and during the cooling down period after cessation of the treatment. Since the temperatures in these experimental graphs are temperatures measured by means of a thin thermocouple probe, like those shown in the dashed curve of Fig. 3C, the relationship of the HU values with temperature is reliable, and not dependent on the history of the treatment of the sample. The arrows on the curves show the direction of change of temperature, to differentiate between the heating up and the cooling down segments of the curves. In each of the curves, the HIFU intensity was maintained constant, whereas the application time was varied, from the shortest heating time in Fig. 4A to the longest heating time in Fig. 4E, yielding increasingly high maximal measured temperatures.

In Fig. 4A, with the shortest treatment time, and lowest maximum temperature, which is below denaturation temperature, the HU level decreased almost linearly during the heating and then returned to its original value following cessation of the treatment along almost the same plotted line. The lack of any significant difference between the heating up and cooling down curves is attributed to the reversibility of the internal process the tissue undergoes.

Referring now to Fig. 4B, taken for a slightly longer treatment time until the temperature reached 42°C, it is observed that during the heating up period, the plot of the HU value followed a characteristic quadratic curved path towards a minimum, until the temperature reached 42°C at which point the heating was stopped. HU values then followed a linear return towards the starting value during the cooling down period. As can be noted the HU curve pattern depicts a substantial hysteresis. This can be attributed to irreversible changes which occurred in the thermally treated fat tissue.

Referring now to Fig. 4C, taken for an even longer treatment time, it is observed that during the heating up period, the plot of the HU value follows the characteristic quadratic curved path shown in Fig. 4B, but continues past a real minimum value at about 43°C, and increases towards higher values after that minimum (the pixels becoming radiographically more dense) until the end of the heating period, followed by a linear return towards slightly higher value that the original value.

Figs. 4D to 4E show the effects of increasingly longer treatment times and higher maximal measured temperature respectively. All of these graphs show the same characteristics - (i) a minimum at around 43°C, (ii) an increase in attenuation values after this minimum value, until the thermal application is terminated, and (iii) a linear behavior during the cooling generally linear return to the starting region. Step (ii) occurs as the temperature of the treated region exceeds 43°C and goes to higher and higher levels going from Fig. 4D to Fig. 4F. Additionally, the area enclosed by the "round-trip trajectory", or the curve hysteresis increases with increase in the treatment duration.

Minor differences between the starting and finishing points on the different graphs may be attributed to differences in the constitution of the fat samples following heating past the tissue coagulation temperature.

The use of the graphs shown in Figs. 4A to 4F provides a method of pre- calibrating the CT system, such that the change in HU level is correlated not only to the temperature of the region of the tissue whose pixel or pixels are being imaged but also to the total integrated thermal damage produced. This can be obtained by using two features: (i) the coefficients of the polynomial fit to the heat up curve for each tissue type and (ii) the point of minima which is expected to occur at about 43°C and is attributed to the starting point of the coagulation process. This point is expected to be common for all tissue types. It is thus possible to determine the temperature at those pixels by a simple measurement of the radiographic attenuation at those pixels. This thus provides a method of measuring temperature noninvasively at the point of thermal treatment, by image processing of the CT images in real time to determine the HU level at the point of thermal treatment.

However, there is considerably more information than simple temperature measurement that can be obtained from the variations in HU attenuation values associated with these heating and cooling curves. The methods described in this disclosure attempt to determine the total accumulated effect of the thermal treatment at those pixels, from the curves of the radiographic attenuation. The gradual reduction in CT attenuation, as shown by the decreasing level of the HU reading, from the commencement of the thermal treatment up to the region approaching the minimum attenuation point at around 43°C, is attributed to conventional thermal expansion of the tissue in the focus region as it is heated up. It is further understood that this point of minimum HU can be attributed, as stated above, to the commencement of protein denaturation and tissue coagulation. The measured temperature at which this minimum occurs is corroborated by knowledge of protein denaturation and tissue coagulation from other fields. This minimum point is common for all tissue types, and even though the exact quadratic polynomial equation around the curve minimum for each tissue type may be different for different types of tissue, the characteristic minimum at around 43 °C occurs for all types. It is to be noted at this point of the disclosure, that although the physical mechanisms by which the various aspects of the invention operate are believed to be correctly interpreted, for instance as suggested in the previous two paragraphs, the invention is not meant to be limited thereby, and is operative regardless of the true physical mechanisms responsible for the effects observed and used.

Consequently, according to this exemplary method of the present disclosure, by monitoring in the CT images, the temporal changes in Hounsfield units of the radiographic attenuation of the thermally treated region during the course of the thermal treatment procedure, the point of coagulation can be readily and noninvasivefy detected without the need to make a measurement of the temperature of the tissue, or to estimate the temperature based on the values of the HU obtained at the tissue being treated. This point is of importance to the attending physician or the surgeon, since it indicates that the treatment has reached its effective point. As is evident from the graphs shown in Figs. 4A to 4E, the accuracy of the method is good, possibly enabling the point of coagulation to be determined to within 1 degree C. Furthermore, from the way in which the graph continues beyond the minimum HU value, an estimate can be made of the total integrated ablative damage to the tissue, this translating the length of the treatment into a measure of the effects of the treatment. This aspect will be treated more rigorously in relation to Figs. 5A to 5I below.

In order to provide a quantitative basis against which to measure the total integrated damage to the tissue, a number of different methods can be used. What is sought by this measurement is the quantification of the total coagulation level of the target tissue over the whole length of the treatment, and not simply the point at which protein denaturation and tissue coagulation commences, as would be obtained from a temperature measurement, since the total effect and tissue condition is the actual data which the doctor or surgeon needs to know during treatment. The total damage may, to some extent, become apparent only after completion of the procedure, or at least, after the heating stage has been completed. However it would be useful to the surgeon to obtain this information as early as possible during the procedure, so that decisions on the procedure can be made closer to the time when the effects are occurring. Each of the different methods of measurement provides the information on the total coagulation and hence ablation level of the tissue at different points in the treatment cycle. Reference is now made to Fig. 5A to 5i, which show experimentally obtained CT attenuation curves measured in HU, as a function of measured temperature, for three different thermal treatment times and three maximal measured temperatures on different samples of porcine fat tissue, and for each of these different treatments, for three different methods of measuring the accumulated tissue damage. A first method is implicit in all of the graphs, as well as in the graphs of Figs. 4A to 4F. In addition to this first method, Figs. 5A to 5C show a second method of measurement for three different treatment regimes on the different tissue samples. Likewise, for Figs. 5D to 5F for a third method and for Figs. 5G to 5I for a fourth method. The curves shown are the results of fitting a best quadratic function through the measured points for the heating stage, and a linear function for the cooling down stage of the treatment.

In all of the graphs, according to the first measurement method, in order to quantify the accumulated denaturation and coagulation effect, the area enclosed by the heating and cooling curves is calculated between the two intercept points, i.e. between the intersect point of the beginning and end of the treatment, and the intersect point at the hottest point in the treatment. The curves are defined by means of fitted functions for the heating and cooling down curves. The calculated enclosed areas are shaded gray. Referring back to Figs. 4A to 4F, the calculated areas are denoted above each of the individual graphs. As is observed, the area increases with increased duration of the thermal treatment, and hence, with increase in the coagulation level which the sample underwent, and with the maximum temperature achieved. This method requires some additional measurements during the cooling stage, to enable an extrapolation of the linear line, in order to calculate the intercept point and the total area.

Because of the non-monotonic behavior of the attenuation curves, three other exemplary methods of calculating the accumulated effect of the heat treatment on the tissue are proposed. All of these approaches utilize differences in HU values measured either between points on the heating and cooling segments of the attenuation curves, or on the heating segment only. The advantages of these methods over the previously described area measurement method are in that they involve measurements from specific points on the curves, which can be defined more uniquely, and for some of the measurement methods, sooner than the area measurement.

Reference is first made to Figs. 5A to 5C, which show the basis for a first one of these additional point measurement methods. In this method, the total difference in HU values between minimal and maximal values is measured, during both heating and cooling segments, and this total range of HU values is used as the metric for assessing total tissue damage during the complete procedure. This HU range is indicated on the graphs by the vertical black line, which always runs between the minimum point, close to where coagulation commences, and the intersection of the starting and finishing points of the treatment cycle curves. The results of using this method show the least variation between different treatment regimes, such that calibration of this method is the most difficult.

Reference is now made to Figs. 5D to 5F, which show a second one of these additional point measurement methods. In this method, a measurement is made of the maximum disparity obtained between HU values for any specific temperature measured once during the heating segment, and once during the cooling segment. The larger the hysteresis in the curves, the larger the disparity measured, indicating the presence of more coagulation and tissue damage. Both this method and the previous method provide an estimation of the total coagulative level accumulated by the tissue. This method also requires measurements during the cooling stage, to enable an extrapolation of the linear line.

Reference is now made to Figs. 5G to 5I, which show a third one of these additional point measurement methods. In this method, the HU values are measured only during the heating stage. The metric used is the difference between the HU value at the minimum of the quadratic heating curve and the HU value at the maximum temperature point when the heating is ceased. This measurement defines the "depth" of the indentation of the heating quadratic curve. A large difference in HU values would indicate that both the finally reached temperature, and consequently, the total coagulative level are high. An advantage of this method is that for clinical use, it avoids the need to wait until the tissue has cooled down, providing an earlier result and hence potentially speeding up the time required for the complete treatment. As is observed, this type of measurement shows the largest difference between the three different treatment conditions shown. Thus besides providing the quickest result, it should also be the simplest to calibrate and the most sensitive.

In order to determine which of the four methods provides the most accurate measurement of total thermal effect, reference is now made to Fig. 6, which is a table summarizing the experimental results of the correlation between the maximum measured temperature measurements and the four above-described parameters used for measurement of the amassment of denaturation level in the fat sample. The correlation is strong for all of the methods, including the included area measurement method, such that all may be used in the systems described in this disclosure. The best correlation between temperature and coagulation amassment, though not by a significant margin, would appear to be obtained with the maximum disparity approach of Figs. 5D to 5F.

In order to implement any of the above described methods, it is necessary to process the obtained HU data as a function of elapsed time, in order to generate the virtual curves for extracting clinically useful information from the HU data. With the exception of the depth of indentation method of Figs, 5G to 51, which provides a dosage metric by the end of the heating period of the treatment, the clinical decision as to how long to apply the thermal treatment has to be based on predetermined calibrations of the method for various tissues and input powers, since the dosage measurement parameter is only known in the course of or on completion of the cooling-down segment, which means that the application time has already been completed.

For the "total difference" method of Figs. 5A to 5C, and for the "indentation depth" method of Figs. 5G to 51, the HU data required can be obtained directly to determine the parameters for assessing the treatment effect, since the values of the HU level at the defined points required, whether starting or finishing HU values, or minimum HU value at the point of coagulation, or the HU value at the termination of the heating stage, can all be determined from the HU plot as a function of elapsed time of treatment, such as is plotted in the full curve of Fig. 3C. However, in order to perform an assessment based on the total area metric, as shown in Figs. 4A to 4F, or on the "maximum disparity" method of Figs. 5D to 5E, it is necessary to generate complete hysteresis curves of the HU values as a function of temperature at the treated tissue, rather than only as a function of elapsed time. This involves use of preliminary calibration of such HU plots to ascertain the temperature level for each HU level, preferably taking into account the heating cycle history of the sample. Thus, although the "maximum disparity" method of Figs. 5D to 5F may have been found to provide the most accurate parameter to use of the methods described above, since this method involves use of the estimated temperature, which may be subject to inaccuracies, there may be no advantage in its use over the other methods described.

The utilization of the HU data and "HU versus time" curves may, according to one possible implementation of these methods, be achieved using the following steps: (i) Determine from the accumulating data, the time derivative of the measured radiographic attenuation of the treated target region in HU.

(ii) Search for a minimum in the HU values obtained. This minimum represents the point at which thermal ablation begins, generally around 43°C.

(iii) Continue application of the thermal energy beyond this minimum for a predetermined time defined by the extent of thermal ablation required at the target. This can be determined by estimates based on preliminary calibration of the ensuing ablation dose. As previously explained, if the "total difference" method of Figs. 5A to 5C, or the "depth of indentation" method of Figs. 5G to 5l is used to quantify the ablative dose desired for the treatment, the termination of the applied heating can be based not on any predetermined estimates, but rather on the real-time HU reading reached, to provide the required difference between the HU reading at the minimum point, and the HU reading reached.

(iv) Continue to record the HU readings as a function of time during the cooling down period, if needed, until the tissue is considered by the doctor to have returned to its ambient temperature and the procedure is completed.

While step (iv) of the above procedure of curve plotting is being completed, the other methods of determining the total ablation dose can be performed, at a point in time depending on the method used. Thus, the maximum disparity method of Figs. 5D to 5F to determine the total dose given, can be performed as soon as that difference in HU readings is reached during the cooling down period, and the HU values have been correlated to the estimated temperature at each point of time required. On the other hand, both the total HU range and the area measurements can only provide a total dosage measure after completion of the cooling down period. if, according to any of the above methods providing only retroactive information of the total tissue damage delivered, it becomes apparent that insufficient ablation had been achieved, the treatment can be continued after the first session to complete the required dose.

In an exemplary operating system, such as that shown in Fig. 2, using any of the above-described methods, it would be useful if the surgeon or doctor were able to correlate the temperature at the target tissue achieved by the thermal treatment, with an image of the target tissue itself. This could be achieved by superimposing a virtual temperature image, or coagulation map, as calculated by any of the above described methods from HU level measurements on the target tissue, onto the actual CT image of the target tissue. The virtual temperature image then becomes a map from which the level of coagulation can be shown directly on the CT image of the treated region. A change in HU greater than a certain predetermined level may then indicate that the tissue being measured is dead tissue, thus portraying a contour map of tissue condition in the treated region. Reference is now made to Fig. 7 which illustrates such a contour map imposed on the CT image of a tissue undergoing thermal treatment. This map is based on calculations using the "indentation depth" method. The coagulation contours can be clearly seen in this image. Since the resolution of the CT system is sub-millimetric, the temperature rise can also be pinpointed with high precision. This is apparent from the marker bar in Fig. 7, which is only 5 mm in length.

Although the systems have been described hereinabove using conventional X- ray fluoroscopy or CT imaging, it is to understand that it is possible to relate the absorption of other electromagnetic radiation to the status of the tissue through which that radiation is passing. The changes in tissue properties at the onset of structural changes resulting from thermal treatment will be reflected in changes in the transmission properties through that tissue of other electromagnetic radiations. Thus for instance it is possible to use gamma radiation or laser radiation besides the X-ray radiation hitherto discussed. Furthermore, although the systems have been described hitherto as utilizing images in order to track changes in tissue properties, and thereby to monitor thermal treatment, it is not essential that a complete image be obtained in order to perform the absorption analysis needed to monitor the thermal treatment. Simpler systems can be proposed in which a single collimated beam is directed at the tissue being thermally ablated or treated, and the attenuation of this beam is determined in one dimension only, in order to ascertain the attenuation in the region through which the beam passes, and hence the progress of the thermal treatment.

Reference is now made to Fig. 8, which illustrates such further implementations of the present system and method. In Fig. 8 there is shown a C-arm system 80, irradiating a target 81 being thermally treated within a body 82. The radiation source 83 is shown emitting a narrow electromagnetic beam 84, which passes through the target area 81 and is detected in the detector 85. The attenuation measured on this detector provides an indication of the progress of the thermal treatment, as for the imaging implementations described hereinabove. The radiation can be X-radiation, gamma radiation, or laser radiation, or any other electromagnetic radiation which can be readily generated, collimated and detected. Such a system has the added advantage that a collimated beam has specific directivity, and thus reduces the undesired ambient radiation level in the surroundings of the treatment area. Additionally, fans of rays may be used for wider range monitoring. Furthermore, such an apparatus implemented using gamma rays may be substantially less costly than an X-ray system, even if a simple fluoroscopic system. Additionally, a simple source- detector linear beam combination may be incorporated into a more costly CT or MRI system, to provide the additional diagnostic capabilities for use in thermal treatment of tumors, for instance, as mentioned hereinabove.

It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

CLAIMS We claim:

1. A method of monitoring thermal treatment of a tissue, comprising:

passing electromagnetic radiation through the tissue in at least the region where the thermal treatment is being performed;

measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed during at least the duration of the application of the thermal treatment; and

utilizing the functional variation of changes in the measured radiation attenuation as a function of time, directly to monitor the progress of the thermal treatment.

2. A method according to claim 1 , further comprising determining when a minimum in the radiation attenuation of the tissue is achieved; and

continuing to apply the thermal treatment for a predetermined time beyond said minimum, according to the dosage of thermal treatment desired.

3. A method according to claim 2, wherein the minimum in the radiation attenuation of the tissue is determined from the time derivative of the radiation attenuation data.

4. A method according to claim 1 , further comprising:

utilizing predetermined calibration data to relate the measured radiation attenuation to the temperature of the tissue in the region where the thermal treatment is being performed; and

using changes in the determined temperatures as a function of time to monitor the treatment.

5 A method according to any of the previous claims wherein the radiation attenuation of the tissue is measured also during the cooling down period after application of the thermal treatment.

6. A method according to any of the previous claims, further comprising the step of determining when the measured radiation attenuation of the tissue reaches a minimum level, such that the point in time of commencement of at least one of denaturation and coagulation processes can be determined.

7. A method according to any of the previous claims, further comprising the step of determining the difference in radiation attenuation between the point of minimum attenuation, and the point at which the application of the thermal treatment is ceased, and utilizing predetermined calibration data of this difference to determine the extent of thermal damage achieved by the treatment.

8. A method according to any of claims 1 to 6, further comprising the step of determining the difference in radiation attenuation between the point of minimum attenuation, and the point of maximum attenuation, and utilizing predetermined calibration data of this difference to determine the extent of thermal damage achieved by the treatment.

9. A method according to claim8 wherein the point of maximum attenuation is taken as being the point of completion of the cooling down process after thermal treatment.

10. A method according to any of the previous claims wherein monitoring of the treatment is achieved while the treatment is being performed.

11. A method according to any of the previous claims wherein the monitoring of the treatment comprises the determination of the duration of the treatment.

12. A method according any of the previous claims, wherein the monitoring of the treatment comprises the determination of when the treatment has achieved its aim.

13. A method according to any of claims 1 to 6, further comprising the step of determining the maximum difference in radiation attenuation between any point during the application of the thermal treatment and the cooling down period, and utilizing predetermined calibration data for this difference to determine the extent of thermal damage achieved by the treatment.

14. A method according any of the previous claims, wherein the electromagnetic radiation is any one of X-rays, gamma rays or laser radiation.

15. A method according any of the previous claims, wherein the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, is achieved by analysis of pixels from a 2- dimensional or 3-dimensiona! image.

16. A method according any of claims 1 to 14, wherein the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, is achieved by using a detector to measure the transmitted intensity of a collimated beam of the electromagnetic radiation.

17. A method of monitoring thermal treatment of a tissue, said treatment having a thermal application period and a cooling down period, the method comprising:

measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, during the thermal application period and at least part of the cooling down period;

determining the temperature of the tissue at those times when values of radiation attenuation are measured;

establishing the functional relationship between the measured values of radiation attenuation and the determined temperatures;

determining the area contained within any hysteresis found in the functional relationship, between the thermal application period and the cooling down period; and

utilizing this determined area to evaluate the extent of thermal damage achieved by the thermal treatment.

18. A method according to claim 17, wherein the temperature of the tissue is determined from the measured values of radiation attenuation, using predetermined calibration data.

19. A method according to claim 18, wherein the predetermined calibration data includes analysis of the characteristic behavior of radiation attenuation data obtained by thermal treatment of samples of tissue similar to that being treated by the present method.

20. A method according to any of claims 17 to 19, wherein the electromagnetic radiation is X-rays, and the radiation attenuation is measured in Hounsfield Units.

21. A method according to any of claims 7 to 20, wherein the thermal treatment of the tissue is performed by use of any one of High Intensity Focused Ultrasound, radio frequency ablation, microwave ablation and laser ablation.

22. A method according to claim 17, wherein the thermal treatment of the tissue is performed cryogenically.

23. A method according to any of claims 17 to 22 wherein monitoring of the treatment is achieved while the treatment is being performed.

24. A method according any of the claims 17 to 23, wherein the monitoring of the treatment comprises the determination of when the treatment has achieved its aim.

25. A method according any of claims 17 to 24, wherein the electromagnetic radiation is any one of X-rays, gamma rays or laser radiation.

26. A method according any of claims 17 to 24, wherein the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, is achieved by analysis of pixels from a 2-dimensional or 3-dimensional image.

27. A method according any of claims 17 to 24, wherein the step of measuring as a function of time, the radiation attenuation of the tissue in the region where the thermal treatment is being performed, is achieved by using a detector to measure the transmitted intensity of a collimated beam of the electromagnetic radiation.

28. A system for monitoring thermal treatment of tissues, comprising: a source of electromagnetic radiation, adapted to direct the radiation through a tissue in at least the region undergoing thermal treatment;

a detection unit adapted to measure the radiation attenuation of the tissue in the region where the thermal treatment is being performed, and to generate radiation attenuation data of that region as a function of time; and

a control unit adapted to discern functional variation of changes in the radiation attenuation data as a function of time, and to utilize the discerned functional variation directly to monitor the progress of the thermal treatment.

29. A system according to claim 28, further comprising a calibration data unit for accessing predetermined calibration data relating the measured radiation attenuation to the temperature of the tissue,

wherein the control unit utilizes the temperatures obtained by the calibration data unit as a function of time to monitor the progress of the treatment.

30. A system according to claim 28, wherein the control unit is adapted to determine from the time derivative of the radiation attenuation data, when a minimum in the attenuation of the tissue is achieved, so that the thermal treatment can be applied for a predetermined time beyond said minimum, according to the dosage of thermal treatment desired.

31. A system according to any of claims 28 to 30, wherein the thermal treatment of the tissue is performed by use of any one of High Intensity Focused Ultrasound, radio frequency ablation, microwave ablation and laser ablation.

32. A system according to any of claims 28 to 30, wherein the thermal treatment of the tissue is performed cryogenically.

33. A system according to any of claims 28 to 32, wherein the detection unit is either of an image analysis unit or detector signal processing unit

34. A system according to any of claims 28 to 33, wherein the electromagnetic radiation is X-rays, and the source and detection unit comprise either one of a digital radiography system or a computerized tomography system.

35. A system according to any of claims 28 to 33, wherein the electromagnetic radiation is any one of Gamma-rays, infra red radiation or laser radiation.