GB2246201A - RF applicator for magnetic resonance imaging - Google Patents

Abstract

An applicator for coupling an rf field to or from a sample during magnetic resonance imaging comprises an electrical periodic structure which oscillates with substantially the same phase over its whole length at its operating frequency (NMR or ESR). The simplified version shown has two sections 2, 3 twisted relative to each other by 90 DEG to give a helical field pattern and connected to quadrature oscillators to give a rotating field. The latter effect may also be obtained using pairs of coils tuned to slightly different frequencies. The field may be focussed to reduce external noise. The arrangement is particularly useful in apparatus employing dynamic polarisation, i.e. for exciting ESR coupled to NMR spins (Overhauser effect) to enhance the NMR signal (ESREMRI). <IMAGE>

Description

VHF APPLICATOR FOR MAGNETIC RESONANCE IMAGING The present invention relates to improvements in and relating to methods of magnetic resonance imaging and apparatus therefor.

Magnetic resonance imaging (MRI) is a non-invasive imaging technique which is achieving progressively wider acceptance by physicians for use in medical diagnoses.

The technique was developed by Lauterbur who published the first magnetic resonance (MR) images in 1973. By 1985, at least 500 MR imagers had been installed for clinical use around the world (see for example Lauterbur, Nature 242: 190-191 (1973), Steinberg, A.J.R.

147: 453-454 (1986) and Steiner, A.J.R. 145: 883-893 (1985).

MR images are generated by manipulation of the MR signals detected from a sample, for example a human or animal body, placed in a magnetic field and exposed to pulses of radiation, typically radio frequency (RF) radiation, of a frequency selected to excite MR transitions in selected non-zero spin nuclei (the "imaging nuclei") in the sample.

In order to encode spatial information into the-MR signals during the imaging procedure, the magnetic field experienced by the sample is modified by the imposition onto a primary uniform magnetic field B of magnetic field gradients. Thus, for example, since the resonating frequency of an imaging nucleus is dependent on the strength of the magnetic field (B) in which it lies as well of course as on factors such as the chemical environment and the isotopic nature of the nucleus (which may for example be 1H, 13C, 19F), by imposing a field gradient in the z direction on the sample during periods during which the sample is exposed to pulses of the MR transition exciting radiation, the position and width in the z direction of the slice through the sample in the xy plane from which the MR signals are emitted is defined by the strength of the primary field B,, the applied field gradient dh/dz, and the frequency and bandwidth of the exciting pulses.

Subsequent imposition of further field gradients in the period between the initial pulse of the MR transition exciting radiation and the period during which the MR signal is detected and also during that period of detection can similarly encode x and y spatial information into the MR signal.

There are several different encoding techniques known in the art, but all rely upon the imposition onto the primary field of field gradients of different magnitudes and/or in different directions, in particular sequences within the MR-transition-excitation/MR-signal-detection cycles.

Moreover, using different pulse and detection sequences, for example spin echo, spin inversion, spin recovery, etc., different types of MR images can be generated from the detected signals, for example images in which the pixel intensity is proportional to the density of the imaging nuclei in the corresponding volume element of the sample (e.g. proton density images), T1 and T2 images.

For a general discussion of the principles of MRI, the reader is referred to the articles by Bottomley, Rev.

Sci. Instrum. 53:1319-1337 (1982), Hinshaw et al., Proc.

IEEE 71: 338-350 (9183), House, IEEE Trans. Nucl. Sci.

NS-27: 1220-1226 (1980), Koutcher et al., J. Nucl. Med.

25: 371-382 (1984), Mansfield et al. in "Advances in Magnetic Resonance" edited by Waugh, Academic Press, new York (1982), Pykett, Sci. Am. 146: 54-64 (1982), Twieg, Med. Phys. 10:610-621 (1983) and Kean et al. "Magnetic Resonance Imaging", Heinemann, London (1986).

In an MR imager, the primary magnetic field B is conventionally generated by a superconducting magnet, a resistive magnet or a permanent magnet. The choice of the primary magnetic field strength used in MRI affects the quality and characteristics of the images that can be generated and also affects the image acquisition time and the manufacturing and running costs of the MR imager. Thus, for example, for a given image acquisition procedure, the use of higher strength primary fields results in improved signal to noise (S/N) ratios. As a result, the best MR images that have so far been obtained have been produced using the large primary magnets. This is because such magnets give very strong, stable and homogeneous fields while at the same time providing some shielding against external perturbing magnetic fields.The disadvantages, however, are that such magnets are very expensive and are very difficult to service and maintain and also that it is now recognized that there are dangers associated with the use of high fields.

At lower fields, for example 2000 gauss, resistive magnets may be used, and at fields of 200 gauss or less such magnets are quite inexpensive and simple to operate and install. At low fields, however, technical problems arise, in particular the poor S/N ratio which results from the low MR signal amplitude and frequency.

This technical problem has been addressed in a variety of ways. Thus, for example Hafslund Nycomed in PCT/GB88/00479 (published as WO-A-88/10419) and Lurie et al. in J. Magn. Reson. 76:360-370 (1988) have described the use of dynamic polarization, produced by stimulation of coupled ESR (electron spin resonance) transitions to increase signal strength, and Stepisnik et al. in Society of Magnetic Resonance in Medicine, Seventh Annual Meeting, 20-26 August 1988, page 1060, have suggested that polarization may be enhanced by pulsing the imaging nuclei with a higher magnetic field prior to detection of the MR signals. It is also possible to increase the S/N ratio by decreasing the noise of the MR signal detector, the RF receiver coil, by cooling it with liquid nitrogen and/or by making it of a superconductive material.

This invention relates to improvements in methods relating to said use of dynamic polarization, also referred to as the Overhauser effect. Overhauser effect magnetic resonance imaging, or ESR enhanced MRI, uses ESR to enhance the nuclear polarization, and thus the image signal strength. The enhancement factor E can be quite big, up to several hundred. During the scan the patient to be imaged is, for instance, injected with the paramagnetic agent used for the Overhauser effect and as an end result an image is obtained in which the regions affected by the agent are highlighted.

The advantages of ESR enhanced MRI are thus a greatly improved signal-to-noise ratio and good outlining or contrast of the enhancement medium. The method needs, however, in addition to the equipment used for ordinary MRI, means for stimulating the ESR.

The range of ESR frequencies of interest in ESR enhanced MRI is typically in the range of hundreds of megahertz, often designated as the VHF region, though UHF to a few gigahertz might be used in special cases. Corresponding wavelengths inside the human body are typically 3-30 cm, which is comparable to or shorter than the region of interest to be imaged. The wave nature of the VHF signal thus has to be taken into account when designing the means for applying it. Said means we will henceforth call "applicator", conforming with the terminology in hyperthermia, where similar problems arise (hyperthermia is a method whereby selective heating is applied to the human body for therapy, for instance using electromagnetic radiation).

Alternatively we use the word "antenna".

In the design and use of VHF applicators for hyperthermia some general rules have been found (ref. to Proc. of the NATO Advance Study Institute on "Physics and Technology of Hyperthermia", Urbino, Italy, July August 1986, Martinus Nijhoff, Publ. 1987, especially pp. 159-187).

The applicator active region or aperture can be modelled by a distribution of electric and magnetic dipoles, oscillating at the VHF signal frequency and thus radiating an electromagnetic wave. The electric and magnetic fields are grouped into a "far-field" part, which travels with the wave, and "near-fields", which remain local near the aperture. In strongly attenuating media, such as the body, the distinction becomes blurred. The electric fields produce heat in the patient tissues, which usually becomes a problem for ESR enhanced MRI and at best is unnecessary. The magnetic field gives the wanted ESR effect. The two far-fields are simply interconnected via the Maxwell equations and always accompany each other. In the near-field region the interdependence is more complicated and less stringent. Therefore the electric near field can and should be minimized, being an unnecessary nuisance.

The individual dipoles of the model have strong electric near fields. Those of the electric dipoles are especially strong and depend on distance as l/r3.

Magnetic dipoles have weaker near fields with dependence 1/r2. When the dipoles are combined to a continuous aperture the near fields tend to cancel and for a large optimized aperture the field becomes progressively more like a plane wave. Near fields from accumulated charges in systems where they are unevenly distributed are especially strong, causing local "hot spots" in the imaging region. Therefore the applicator should be designed to have an even distribution of charges and also of currents.

In practice the optimal electromagnetic field for ESR in the human body at the frequencies of interest, with applicator optimized for minimum electric near field is close to a transverse electromagnetic (TEM) field. In this case the ratio of electric to magnetic field in the body is determined by the properties of tissue and equal to the so-called intrinsic impedance of the tissues, which has a magnitude of, very roughly, 50 ohms. In vacuum the ratio is 377 ohms. To keep heating down at the surface of the body the applicator should be "impedance matched" to the body to give a ratio of fields equal to or less than 50 ohms.

A good VHF applicator (or antenna) should satisfy the following requirements: - it should give an even field distribution in the region of interest and not produce any "hot spots".

This is very important.

- It should give a circularly polarized field in the sense (i.e. right handed or left handed) of the precessing electron spins to minimize rf heating effects. Linearly polarized fields contain a circulating polarization component in the opposite sense which gives extra heat without any ESR effect.

- It should have a good efficiency, meaning that most of the VHF power should pass into the patient to be imaged rather than be dissipated in the antenna itself.

It is, furthermore, an advantage that the applicator has a simple mechanical design in the sense of being inexpensive to build and having a small influence on the losses of the surrounding NMR coil. To this end it helps if the VHF power can be fed to the applicator in one or two regions only, for instance at one end of the applicator, a multiplicity of feed lines entering at different points being expensive and difficult to combine with the NMR coil. The present invention shows how this can be done and simultaneously a good VHF field profile be obtained.

To further explain the difficulties involved we study the fictional head applicator shown in Fig. 1. The VHF signal is fed into the helmet-shaped system from the left and we imagine that it progresses through the applicator with some wavelength L,. The "aperture" of the applicator is formed by the inner surface of the helmet.

In systems designed for frequencies around and below 100 MHz, like the NMR signal in ordinary MRI scanners, the applicator has traditionally been some simple coil arrangement, the corresponding La is then much longer than the length of the helmet. We are presently concerned with higher frequencies, typically 300 MHz, with correspondingly shorter wavelengths. For tissues the wavelength at 300 MHz is Lb = 11 cm. Impedance matching, in the sense explained above as needed to lower electric power dissipation in tissues, usually requires that La is about equal to just somewhat bigger than 4. The assumptions for La and La in the figure are thus valid.

We assume further that the VHF wave-fronts in the helmet lie approximately in planes which are perpendicular to the dc field direction (Bo) and that the wave having reached the end of the helmet is reflected back.

Incident wave-fronts are drawn with solid and reflected ones with dashed lines. We finally assume that the fields of the waves in the helmet couple to the head so as to induce a circularly polarized wave in the head.

The wave-fronts of the induced wave have been indicated in the figure.

This applicator fulfils some of the listed criteria for a good system but it has a drawback: The reflected wave in the helmet induces a corresponding one in the head which will form an interference pattern with the original wave. In Fig. 1 the dotted line indicates one region where the two waves in the head will interfere either constructively to form a maximum or destructively for a minium. This type of uncontrollable interference is not desirable.

According to the invention we provide a method of magnetic resonance imaging comprising applying a magnetic field to a sample to be imaged, applying pulses of MR transition-exciting electromagnetic radiation to the sample, applying pulses of electromagnetic radiation to the sample for stimulating coupled ESR transitions and detecting the resulting radiation from the sample with an NMR-signal detector, characterised by applying the ESR pulses through an electrical periodic structure constructed so as to oscillate with substantially the same phase over its whole length.

Viewed from another aspect, the invention provides magnetic resonance imaging apparatus comprising means for applying a magnetic field to a sample to be imaged, means for applying pulses of MR transition-exciting electromagnetic radiation to the sample, an NMR signal detector for detecting the resulting radiation from the sample, and applicator means for applying pulses of electromagnetic radiation to the sample for stimulating coupled ESR transitions, characterised in that the applicator means comprises an electrical periodic structure constructed so as to oscillate with substantially the same phase over its whole length.

Viewed from yet another aspect the invention comprises an applicator for applying an electromagnetic field to a sample to be imaged by magnetic resonance imaging, comprising an electrical periodic structure constructed so as to oscillate with substantially the same phase over its whole length.

Periodic structures are described for instance in "Fields and Waves in Communication Electronics" by S.

Ranno, J.R. Whinney and T. Van Duzer (John Wiley & sons 1965) pp. 474-479. Such structures are formed by cascading electromagnetically identical components to a chain. It can be shown that such a chain will transmit waves, somewhat like transmission lines. Usually they transmit only within certain allowed frequency bands with "forbidden" regions inbetween. The limiting frequency between the bands are called cutoff frequencies. At some of these the wavelength is infinite. Using this feature a periodic structure can be designed that oscillates with the same phase over its whole length without any standing wave nodes. The phase velocity along this structure is infinite.

Preferably the periodic structure is constructed so as to produce a rotating magnetic field in the sample.

Preferably there is provided a first linear periodic structure comprising series inductances alternating with shunt capacitances, and a second similar linear periodic structure, the magnetic axes of the inductances of the first and second structures being perpendicular and the two structures being fed in quadrature. Alternatively the periodic structure may comprise coupled resonance circuits comprising pairs of coils tuned to slightly different frequencies.

Preferably the periodic structure is arranged to produce a plane wave with circular polarisation. In use, such wave will be arranged to travel in the direction of the d.c. field. This can be achieved by arranging the direction of the local magnetic fields of successive sections of the applicator to be twisted by a certain angle around the lengthwise axis of the structure, so that the field forms a helix. The pitch of the helix is preferably substantially equal to the wavelength in the sample of the ESR pulses so as to produce said plane wave. Alternatively, however, the pitch can be somewhat longer to produce a focusing effect.

Certain embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a VHF applicator for the head, illustrating the undesired interference effects; Fig. 2 is a circuit diagram of an electrical periodic structure; Fig. 3 is a schematic view of an applicator for the head according to the invention; and Fig. 4 is a schematic view of an applicator for the spine according to the invention.

A simple example of a periodic structure is shown in Fig. 2. The oscillator feeds the structure and the elements, capacitors and inductors, are chosen so as to give a resonance with the same current at any moment of time in inductors 1,3 and so on and the opposite current in inductors 2,4...; at this frequency the phase velocity along the structure is infinite and we are rid of standing-wave nodes. This is important to the invention.

The invention can be further varied to give it beneficial properties for different uses. One important property is to produce a rotating magnetic field at any point at least in most of space to keep down unnecessary heating. This can be accomplished in at least two ways: One method is to start by designing all the coils of Fig. 2 to produce, at some specific moment, a field in, say, the x-direction. A second similar linear structure is subsequently integrated with the first one but turned 90C around the z-axis, which is defined to be along the structure, so as to let the coils produce fields in the y-direction. This structure is fed from a signal source oscillating in quadrature (90 ) with respect to the one feeding the original structure. The field at each pair of coils will then rotate. This is the first method.

The other method is to take pairs of coils, tuned with their own capacitors to slightly different frequencies.

These pairs are connected in a chain using capacitive or/and inductive coupling. Each such pair or section gives a local rotating magnetic field provided that the coupling to each coil in the pair is similar and of suitable strength. For more information about the use of such coupled resonance circuits giving rotating fields see M. Savelainen's "Magnetic Resonance Imaging at 0.02T: Design and Evaluation of Radio Frequency Coils with Wave Winding", Acta Polytechnica Scandinavica, Ph 158, Helsinki 1988.

The final variation of the invention is to tailor it to couple to some particular motional mode of waves in the object to be imaged. A simple such mode which has the advantage of providing a uniform VHF field which couples well to the precessing spins is plane wave with circular polarization progressing in the direction of B,, the dc magnetic field. The applicator can be shaped to couple to such a wave in the following way: Each section of the linear structure is formed so as to give a direction of the local rotating magnetic field which at some specific moment of time is twisted around the lengthwise direction of the structure by a certain angle with respect to the previous section.The VHF field produced by the complete structure in the region of interest thus forms a helix twisting around the lengthwise direction of the linear structure, which should, in turn, be made to coincide with the direction of B,.

The rotating helical field couples efficiently to circularly polarized waves travelling in a'specific direction in the object being imaged. If the pitch of the helix is chosen to be equal to the VHF wavelength in the body we will couple to plane waves. If the pitch is chosen to be longer we can bend the wave-front to produce focusing of the wave. This is all seen from Fig. 1, where now La can be interpreted to be the pitch.

The effect of using the invention is, firstly, that either what used to be the reflected wave, or the incident one, is eliminated. This eliminates the interference. The second effect, focusing, has been obtained by choosing La to be bigger than Lb' which gives a bend in the wavefronts.

We will next present two preferred embodiments of the invention. Fig. 3 shows a somewhat simplified applicator 1 for the head. It contains two sections, 2, 3 each with four lengthwise wires 4 connected by four capacitors 5. The current directions for a certain moment of time are indicated. The sections each give a rotating magnetic field perpendicular to B0 inside and they are twisted by 90O to each other to give a total helical field pattern in the shape of a quarter turn helix. More sections can be added, each adds another quarter turn.

A more practical applicator would have more lengthwise wires for a more even field distribution and less near field effects (hot spots). The next step in this direction is taken by adding another four wires to each section, at equal angular distance from the existing ones, together with four more capacitors to each ring shaped arrangement between sections.

The applicator can also be combined with the RF coil to have some parts in common. The S-shaped lines in Fig.3 can each form a part of one of the turns in the RF coil.

The remaining parts of said turns will now tend to short the indicated feed points. The applicator can instead be fed by placing another similar periodic structure around it. This feed applicator can be made simple with only a few wires, like in Fig. 3, and optimized for a good matching to the signal source. The inner structure, having many wires, will distribute currents and fields evenly around the object to be imaged and thus eliminate hot spots.

Another version of the invention is the spine applicator 7 shown in Fig. 4. This applicator contains three sections 8 in a plane. The magnetic field in the region above the plane makes a 180 helical twist. Signal current phases are indicated by numbers and field directions by arrows.

Another use of the VHF applicator is for use as an RF coil for nuclear magnetic resonance in high-field MRI machines. The advantage is that the phase of the protons can be arranged in a controlled way because the proton resonance signal can be made to form a plane wave in the object to be imaged. In a spin-warp imaging sequence, which. is the most common type in use, the phase-code direction can be chosen along the direction of propagation of this wave. The result is that phase coding of the image can be made in the usual manner.

The phase distortion caused by the finite phase velocity of the electromagnetic wave which communicates the excitation and subsequent response of the proton resonance is now eliminated because the wave fronts are along the equal phase lines in the object and the phase velocity along wave fronts is infinite.

Claims (20)

Claims

1. An applicator for coupling an electromagnetic field to a sample to be imaged by magnetic resonance imaging, comprising an electrical periodic structure constructed so as to oscillate with substantially the same phase over its whole length.

2. An applicator as claimed in claim 1 adapted to couple the ESR resonance signal to a sample in an MRI apparatus employing dynamic polarization.

3. An applicator as claimed in claim 1 adapted to couple the resonance signal to a sample in an MRI apparatus employing either nuclear or electron spin resonance.

4. An applicator as claimed in claim 3 adapted to receive the spin resonance signal.

5. An applicator as claimed in claim 3 adapted to transmit the spin resonance signal.

6. An applicator as claimed in claim 3 adapted to both receive and transmit the spin resonance signal.

7. An applicator as claimed in any preceding claim adapted to couple an approximately helically shaped electromagnetic field to said sample.

8. An applicator as claimed in any preceding claim adapted to couple electric field and magnetic field in a ratio approximately equal to or less than 50 Ohms.

9. An applicator as claimed in any preceding claim adapted to couple an electromagnetic field into the sample with a spatial variation approximately a plane wave in said sample for the VHF frequency used so as to be able to excite said wave.

10. An applicator as claimed in claim 9 in which the direction of the local magnetic fields of successive sections of the applicator are twisted by a certain angle around the lengthwise axis of the structure, so that the field forms a helix.

11. An applicator as claimed in claim 10 in which the pitch of the helix is substantially equal to the wavelength in the sample of the ESR pulses so as to produce said plane wave.

12. An applicator as claimed in any of claims 1 to 8 adapted to couple an electromagnetic field into the sample which is focussed into some region of interest.

13. An applicator as claimed in claim 12 wherein said focussing effect is used to counteract the attenuation of the electromagnetic radiation in the sample so as to produce an approximately homogenous VHF field in at least part of said sample.

14. An applicator as claimed in claim 12 adapted to couple the resonance signal to a sample in an MRI apparatus employing either nuclear or electron spin resonance wherein said focussing effect is used to decrease the noise coupling to the applicator from outside said region of interest.

15. An applicator as claimed in any preceding claim having a first linear periodic structure comprising series inductances alternating with shunt capacitances, and a second similar linear periodic structure, the magnetic axes of the inductances of the first and second structures being perpendicular and the two structures being fed in quadrature.

16. An applicator as claimed in any of claims 1 to 14 having coupled resonance circuits comprising pairs of coils tuned to slightly different frequencies.

17. A method of magnetic resonance imaging comprising applying a magnetic field to a sample to be imaged, applying pulses of MR transition-exciting electromagnetic. radiation to the sample, applying pulses of electromagnetic radiation to the sample for stimulating coupled ESR transitions and detecting the resulting radiation from the sample with an NMR-signal detector, characterised by applying the ESR pulses through an electrical periodic structure constructed so as to oscillate with substantially the same phase over its whole length.

18. Magnetic resonance imaging apparatus comprising means for applying a magnetic field to a sample to be imaged, means for applying pulses of MR transitionexciting electromagnetic radiation to the sample, an NMR signal detector for detecting the resulting radiation from the sample, and applicator means for applying pulses of electromagnetic radiation to the sample for stimulating coupled ESR transitions, characterised in that the applicator means comprises an electrical periodic structure constructed so as to oscillate with substantially the same phase over its whole length.

19. An applicator substantially as hereinbefore described with reference to the accompanying drawings.

20. A method of magnetic resonance imaging substantially as hereinbefore described with reference to the accompanying drawings.