WO1991014947A1

WO1991014947A1 - Nuclear magnetic resonance apparatus

Info

Publication number: WO1991014947A1
Application number: PCT/GB1991/000415
Authority: WO
Inventors: Robert Lambert
Original assignee: The General Electric Company, P.L.C.
Priority date: 1990-03-21
Filing date: 1991-03-18
Publication date: 1991-10-03
Also published as: GB9105693D0; JPH04506263A; EP0473761A1; GB9006320D0; GB2242275A

Abstract

In order to overcome the effect of a magnetic object in a subject (2) under investigation, the invention provides NMR apparatus which is operable at more than one RF frequency. The apparatus follows normal practice as far as obtaining an NMR response or image from a given nuclear species is concerned, but, in addition, interrogates the nuclear spin system at a frequency which is different from the resonance frequency normally used for the given nuclear species, as determined from the applied magnetic field Bo. This additional frequency is chosen so that the nuclei of interest, which would normally give a resonance signal or an image at a frequency f1 in the applied magnetic field Bo, when located in the modified magnetic field close to a magnetised or magnetisable object give a response at a different frequency f2. Detection of a signal at the frequency f2 then indicates the presence of the chosen nuclei close to the magnetised or magnetisable object.

Description

Nuclear Magnetic Resonance Apparatus

This invention relates to detection apparatus which operates by virtue of nuclear magnetic resonance in nuclei of a given isotope of a particular element in a substance which is being investigated. The apparatus may also produce an image of the substance.

Nuclear magnetic resonance (NMR) has been employed as a high resolution spectroscopic technique for several decades, while its extension to medical imaging is now being increasingly used as a clinical tool for anatomical imaging. In standard medical NMR imaging, considerable efforts are made to avoid the introduction of ferromagnetic objects or magnets into the vicinity of the magnet which provides the high-intensity field. This is done both to avoid imaging problems caused by effects of the additional magnetic fields, and for safety reasons.

In contrast, in some new developing fields of wider application, it has to be expected that some ferromagnetic objects or magnets may be present in the subjects being studied or examined. The presence of such magnetisable or magnetised objects within the NMR magnet will affect the extent to which clear images of the object or material being imaged can be obtained. For nuclei which are very close to the ferromagnetic object or magnet, the magnetic field experienced by the nuclear spins will be significantly changed from that imposed by the magnet of the NMR system, and for many systems no significant image of the object or material will be obtained. In such circumstances it may still be useful to obtain an NMR signal from the material near the extraneous magnet or magnetisable material, and it is an object of the present invention to provide such signal.

Within the field of an NMR magnet, many ferromagnetic materials will become magnetically saturated, having a magnetisation of approximately 1.5 Tesla, aligned along the direction of the imposed magnetic field Bo. In general, the field around a magnetised object will be a complex function of position, whereas further away from the object the field will approach that of a magnetic dipole. Permanent magnets possess similar flux densities to magnetised ferromagnetic objects, and can be considered to behave in a similar manner, though their fields may not be aligned to the direction of the main field Bo.

In low-field NMR systems, for example those employing magnetic fields where Bo is less than 0.5 Tesla, close to a magnetised object the total magnetic field will be greater than Bo and consequently in this region the extraneous field will be the main external field experienced by the nuclear spins. In high-field NMR systems, for example with magnetic fields in excess of 2 Tesla, the imposed field will be as strong as, or stronger than, that from a magnetic object but, even in this case, close to the magnetic object the field intensity will undergo considerable variation in strength.

The effects of the perturbing magnetic field will be several, and are here described for systems designed to detect NMR signals as well as those designed to provide images from NMR responses.

Firstly, there will be a change in the effective value of Bo as the magnetisable object or the magnet is approached. Depending upon position, this may be either a positive or a negative change. This change in effective Bo will change the resonant NMR frequency of the nuclei under consideration. If the change is considerable, the modified frequency may fall outside the range to which the system is sensitive. When known NMR slice selection techniques are employed, the change in Bo value will also affect both the position and the shape of the object slice selected for imaging. In many cases it will be found that objects within a region where the magnetic field varies by more than lmT from the Bo value will not normally be i ageabl e.

Secondly, a magnetised object will produce field gradients both across and within the slice of an object being imaged. These gradient fields will act in addition to the imposed mapping field gradients which are used to provide the imaging capability, and will give rise to image shape and position distortion and to variations in signal intensity. If the additonal extraneous gradient field approaches the value of the imaging field gradient strength within the selected slice, the image of the object will be distorted considerably.

Finally, the field gradients from the magnetised object will produce a dephasing of the spins within each imaged pixel or volume element of the object, resulting in a much shorter effective T (the spin-spin relaxation time) value. However, given the use of a 'spin-echo' sequence, as is possible for materials with T values of at least several ms, such dephasing presents few practical problems.

It is an object of the present invention to provide an improved NMR system.

According to the invention there is provided nuclear magnetic resonance apparatus, comprising first magnet means to produce a static magnetic field for acting on nuclei of a given isotope of a particular element in a target to be investigated; means operable to produce a radio-frequency (RF) magnetic field substantially perpendicular to the static magnetic field; transmitter means to apply RF signals to the RF field producing means: receiver means for producing output signals in response to nuclear magnetic resonance in said nuclei; and means to vary the frequency of said RF signals to cause nuclear magnetic resonance in said nuclei despite local variation of said static magnetic field caused by the presence of a magnetised or magnetisable object therein.

The present invention therefore provides NMR apparatus which is operable at more than one RF frequency. The apparatus can follow known practice as far as the purpose of obtaining an NMR response or image from a given nuclear species is concerned. In addition, however, the apparatus can interrogate the nuclear spin system at a frequency which is different from that of the resonance frequency nor ally used for the given nuclear species , as determined from the applied magnetic field Bo. This additional frequency is chosen so that the nuclei which would normally give a resonance signal or an image at a frequency fl in the applied magnetic field Bo, when located in the modified magnetic field close to a magnetised or magnetisable object, give a response at a different frequency f2.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, which is a block schematic diagram of NMR apparatus in accordance with the invention.

Referring to the drawing, an NMR imaging system comprises a magnet 1 which is shown in dotted outline and which provides a uniform magnetic field Bo in the region of a target or sample 2 which is to be investigated, the field being in a direction perpendicular to the plane of the paper. The magnet 1 may be a permanent magnet, an electromagnet, or a superconducting magnet, together with any necessary power supplies (not shown). Within the magnet and encircling the sample region are sets of gradient coils 3 which are controlled by a gradient field control circuit 4 to provide an additional field the strength of which is position dependent. In consequence, the total magnetic field, and hence the NMR frequency, has a positional dependence, enabling spatial information to be encoded within the NMR signal. Also within the magnet are an RF transmit antenna 5 and its associated impedance matching and tuning network 6. This antenna, when energised by an RF transmitter 7, provides a vertical RF magnetic field Bla at the Larmor frequency fl appropriate to the nucleus under consideration and the magnetic field strength Bo (fl = x Bo, whereof is the magnetogyric ratio for the nucleus). It is assumed that the usual NMR relaxation times T^ and T₂ of material in the sample 2 are such that the apparatus is normally capable of detecting such NMR signals. A receiver antenna 8, orientated to be sensitive to RF signals from the sample and orthogonal to the main field Bo and to the RF transmit direction is connected to a preamplifier 9. The output from the preamplifier is fed to a receiver 10 and from there, under control of a computer 11, to an imaging and display system 12. In this respect the system is in accordance with known practice.

In addition, there is provided a second RF antenna 13 within the magnet 1 and connected to the feed from the RF transmitter 7 by an impedance matching and tuning network 14. When energised by the RF transmitter 7, the antenna 13 provides a vertical RF magnetic field Bib at a frequency f2 which is different from the Larmor frequency of the nucleus under consideration at the magnetic field strength Bo. Alternatively the RF transmission system may be so designed that onl one antenna with a doubly-tuned matching network providing simultaneous tuning to both frequencies fl and f2 is required in order to cause resonance at the two frequencies of interest.

The preamplifier 9 includes a circuit 15 the function of which is to tune the preamplifier to either the frequency fl or the frequency f2, under control from the computer 11.

In operation of the apparatus, the control computer 11 sets the frequency of a reference frequency generator 16 to the value appropriate to that for the Larmor frequency fl and provides the necessary pulse sequences to the RF transmitter 7 to obtain a response to the nuclear magnetism of the nucleus of interest, or to build up sets of responses that can be subsequently transformed to produce images of the sample or objects within the sample in a known manner.

If the sample contains a magnetised or magnetisable object, then the images of the chosen nucleus close to that object will be disturbed, and if the material is very close to the object, no image will be obtained. The presence of such material close to any such object is therefore determined as follows.

Under the control of the computer 11 the reference frequency of the system as set by the reference frequency generator 16 is altered to change the output frequency of the RF transmitter 7 from fl to f2. The tuning of the preamplifier 9 and the receiver 10 are similarly adjusted so that the apparatus is now sensitive to the frequency f2. Suitable RF pulses are applied by the RF transmitter 7 to the antenna 13 via the matching network 14. These pulses will typically be a pulse to rotate the nuclear magnetism by 90 degrees fro the Bo direction into the perpendicular plane. At a time t after this pulse a 180 degree RF pulse is applied so that an echo of the RF signal is received by the receiver antenna 8 at a time 2t after the 90 degree RF pulse. This 'spin-echo' pulse sequence is used to overcome rapid dephasing of the nuclear spin signal due to the large magnetic field gradients in the vicinity of the magnetised object.

The output signal from the receiver 10 can be used directly to indicate the presence of the chosen nuclei close to a magnetised or magnetisable object, or the data, which are recorded as a function of time, can be Fourier transformed to provide a frequency distribution of the nuclear spin density. This, in turn, is directly related to the magnetic field strength distribution in which the chosen nuclei are located. This information can then be displayed, either on a computer console or on the imaging and display system 12.

The extent to which the RF transmit and receive sections of the apparatus are tuned for frequency f2 will depend upon the operational needs of the apparatus. For high sensitivity, a highly-tuned, narrow frequency band configuration may be preferred, whereas to interrogate as much as possible of the space close to a magnetised or magnetisable object, a wide-band system will be favoured.

By way of illustration, the magnetic field close to a uniformly magnetised sphere (chosen for simplicity) is shown in Table 1. Spot values of Bz(in mT), the z-component of the field, are shown for one quadrant of the field in the X-Z plane around the sphere with a saturation magnetisation Br of 1.5 Tesla.

-7-

Table 1

z= 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

The origin of the co-ordinates in Table 1 and in the following Tables 2 and 3 is at the centre of the sphere, and points within the sphere are shown hashed. The distances Z and X are given in units of a, where a is the radius of the sphere.

If the magnetisable sphere is inserted into a uniform field of 0.15 Tesla, oriented along the z axis, the combined Bo field will be as shown in Table 2. Spot values of Bz(in mT) are shown for one quadrant of the field in the X-Z plane around the uniformly magnetised sphere with a saturation magnetisation Br of 1.5 Tesla. PCI7GB91/00415

-8-

Tabl e 2

z= 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Considering the detection of proton NMR signals, y= 42.57 MHz/T_* and hence the frequency fl will be 6.4 MHz. To detect proton signals from regions close to the magnetised object, a target field strength of 0.20 Tesla (200mT) might be chosen. This then determines the frequency f2 to be 8.5 MHz. At this frequency, signals from some regions close to the magnetised sphere, as well as other regions a little removed therefrom, can be obtained.

As a second illustration, for an NMR system operating with a field of 2.0 Tesla (fl=85MHz), the relative effect of a magnetised object is less, as shown in Table 3. Spot values of Bz(in mT) are shown for one quadrant of the field in the X-Z plane around the uniformly magnetised sphere with a saturation magnetisation Br of 1.5 Tesla. Tabl e 3

z= 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Here an f2 value of 89 MHz (for example) gives signals from protons located in field strength regions of 2.1 Tesla, close to the magnetised object.

It should be noted that as well as signals from protons, other nuclei which are capable of giving detectable NMR signals, and which are present close to the magnetised or magnetisable object, may also contribute to the signal detected at the frequency f2. Clearly, the frequency f2 should be chosen so as not to correspond to the Larmor frequency, at the chosen operating magnetic field Bo, of any other nuclei that might be present within the sample and capable of -10-

providing a detectable signal. For that reason it is suggested that in proton detection or imaging, as the proton has the second highest agnetogyric ratio of all nuclei, f2 is preferably higher than fl, though the method is equally applicable for f2 less than fl.

Some examples of applications of the invention will now be described briefly.

In a- first example, the sample is an object to be imaged as part of an automatic inspection system, and the image shape is to be compared with a stored 'ideal' shape. Here the presence of any magnetised or magnetisable object alters the shape and size of the image which could then lead to the sample being incorrectly rejected. NMR interrogation at the second frequency f2 will indicate whether the image distortion is due to magnetic contamination, for which alternative corrective action is appropriate.

In a second example, the samples normally provide no NMR signal or image unless an item to be detected is present within the sample. In this case, the presence of a magnetised or magnetisable object could affect the ability of the system to obtain an acceptable image. Routine use of the second frequency f2 then provides a guard against failure to detect or image such an item using the normal operating frequency fl.

In a third example, the sample is a liquid or a liquid mixture flowing along a pipe coaxial with the direction of the magnetic field Bo. The possible presence of magnetic particles in the liquid affecting the validity of the normal signals or images is checked by continuously monitoring at the frequency f2 as well as acquiring signals at the normal frequency fl.

In each case, the RF system, for operation at the frequency f2, may be tuned to cover a narrow band centred around f2. Alternatively, it may be tuned to cover a wide band centred around f2, but not including fl.

Claims

1. Nuclear magnetic resonance apparatus, characterised by first magnet means (1) to produce a static magnetic field for acting on nuclei of a given isotope of a particular element in a target to be investigated; means (5,6;13,14) operable to produce a radio-frequency (RF) magnetic field substantially perpendicular to the static magnetic field; transmitter means (7) to apply RF signals to the RF field producing means: receiver means (8,9,10,15)for producing output signals in response to nuclear magnetic resonance in said nuclei; and means (16) to vary the frequency of said RF signals to cause nuclear magnetic resonance in said nuclei despite local variation of said static magnetic field caused by the presence of a magnetised or magnetisable object therein.

2. Apparatus as claimed in Claim 1, characterised in that the means (16) to vary the RF signal frequency is operable to switch said frequency between a first predetermined frequency and a second predetermined frequency.

3. Apparatus as claimed in Claim 2, characterised in that the RF field producing means comprises two RF antennas (5,13) operable respectively at said first and second frequencies and two matching and tuning networks (6,14) coupling the respective antennas to the transmitter means (7).

4. Apparatus as claimed in Claim 2, characterised in that the RF field producing means comprises an RF antenna; and a matching and tuning network coupling the antenna to the transmitter means (7) to provide for tuning at both said first and second frequencies.

5. Apparatus as claimed in Claim 4, characterised in that the receiver means (8,9,10,15) is tunable between said first and second frequencies.

6. Apparatus as claimed in Claim 5, characterised in that the receiver means (8,9,10,15) includes a preamplifier (9) which is tunable between said first and second frequencies.

7. Apparatus as claimed in any one of Claims 3-6, characterised in that the tuning of the RF field producing means (13), the transmitter means (7), the receiver means (8,9,10,15) and the network (14), when operative at said second frequency, extends over a narrow band centred on said second frequency.

8. Apparatus as claimed in any one of Claims 3-6, characterised in that the tuning of the RF field producing means (13), the

- transmitter means (7),the receiver (8,19,10,15) and the network (14), when operative at said second frequency, extends over a wide band centred on said second frequency, but not including said first frequency.

9. Apparatus as claimed in any one of Claims 2-8, characterised by means (11) to provide a Fourier transform of the receiver output signal about said second frequency to indicate the distribution of NMR nuclei in the vicinity of a said magnetised or magnetisable object.

10. Apparatus as claimed in any preceding claim, characterised in that varying of the RF signal frequency is computer controlled.

11. Apparatus as claimed in any preceding claim, characterised by gradient coil (3) means for providing an additional magnetic field which is position dependent; and means (4) to control the operation of said coils.

12. Apparatus as claimed in any preceding claim, characterised by means (12) responsive to the receiver output signals to produce an image of a target region.

13. Apparatus as claimed in any preceding claim, operative to detect proton NMR signals.