GB2253909A - Coil arrangements in nuclear magnetic resonance apparatus - Google Patents

Abstract

A coil set (27 or 29) in a magnetic resonance apparatus for imposing on a cylindrical volume (3) (where an object to be examined is placed) a gradient in a direction transverse to the axis of the volume. The coil set effectively comprises a pair of coils (27A, C and 27B, D or 29A, C and 29B, D) positioned around the volume diametrically opposite one another, each of the coils having an axial end portion where its conductors are axially spaced, which end portions are radially aligned with the cylindrical volume. <IMAGE>

Description

Coil Arrangements in Magnetic Resonance Apparatus This invention relates to coil arrangements in magnetic resonance (MR) apparatus.

More particularly the invention relates to coil arrangements in MR apparatus of the kind suitable for use in medical examination of patients, for example, to provide an image representing the distribution in a selected region of a patient of a chosen quantity, e.g. the density of chosen nucleons, such as hydrogen protons, or MR spin relaxation time constants.

Such apparatus operates by the application to the selected region of the patient of a radio frequency (RF) magnetic excitation field in the presence of a main uniform static magnetic field of high strength on which magnetic field gradients may be imposed, and the sensing and analysis of the resulting magnetic resonance produced in the patient's body.

Current developments in MR imaging techniques, such as diffusion weighted imaging, require relatively steep gradients.

Such steep gradients can be imposed on the main magnetic field by means of a small gradient coil set inserted inside the main field coils, but since such a coil set normally needs to surround the patient closely, patient access problems can arise if a gradient coil set of conventional form is used. For example, where the gradient coil set is carried on a tubular former disposed with its axis parallel to the direction of the main magnetic field, a Maxwell coil pair is conventionally used to impose a gradient in the direction of the field (i.e. the z-direction), a set of four saddle coils is used to impose a gradient in the x-direction, and a further set of four saddle coils is used to impose a gradient in the y-direction. Each set of four saddle coils comprises two pairs of diametrically opposite coils, the two pairs being spaced axially from one another.Thus the tubular former has an axial length greater than twice the axial length of the saddle coils. The saddle coils typically have dimensions and positions such that the axial length of the coil arrangement is about 2times the radius of the coils. As a result a coil set of a diameter such as to fit closely around the head of a patient is prevented by the patient's shoulders from being positioned with the centre of the coil set, where field linearity is best, centrally around the patient's head.

It is an object of the present invention to provide a gradient coil arrangement for a magnetic resonance apparatus wherein the above described problem is alleviated.

According to the present invention there is provided a coil set in a magnetic resonance apparatus for imposing on a magnetic field in a cylindrical volume in which an object to be examined is placed in use of the apparatus a gradient in a direction transverse to the axis of the volume the coil set effectively comprising a pair of coils correspondingly positioned around said volume diametrically opposite one another, each of said coils having an axial end portion in which the conductors of the coil are spaced apart axially, and which is substantially radially aligned with said volume.

In one particular embodiment of the invention said conductors in said axial end portion of each coil are positioned in two groups spaced apart axially. Such an embodiment suitably comprises a first pair of identical coils positioned around said volume diametrically opposite one another and a second pair of identical coils positioned around said volume diametrically opposite one another and overlapping said first pair of coils axially.

The invention will now be further explained and one coil arrangement in accordance with the invention described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a magnetic resonance imaging apparatus; Figure 2 is a diagram illustrating a conventional gradient coil set for use in the apparatus of Figure 1; Figure 3 is a diagram illustrating how the coil arrangement of Figure 2 may be modified to produce a coil arrangement according to the invention; and Figure 4 is a diagram of a coil arrangement according to the invention incorporating a so-called bucking coil arrangement.

Referring to Figure 1, the apparatus includes a tubular electromagnet 1 which produces a strong uniform static main axial magnetic field in a cylindrical volume 3 in which a body to be imaged is placed in use of the apparatus.

The strength of the field in the volume 3, and hence in the body being imaged, is controlled by a main magnet control means 7 which controls the supply of energising current to the electromagnet energising coil (not shown).

The apparatus further includes a gradient coil arrangement 9 whereby a gradient may be imposed on the static magnetic field in the volume 3 in any one or more of three orthogonal directions. The coil arrangement 9 is energised by a gradient field control means 11 under control of the computer 5.

The apparatus further includes an r.f. coil system 13 energised by an r.f. transmitter 15 under control of the computer 5 to apply an r.f. field to the body being imaged.

The r.f. coil system 13 is also arranged to detect r.f.

signals resulting from magnetic resonance excited in the body being imaged. The detected signals are passed via a receiver 19 to an imager 21 which under control of the computer 5 processes the signals to produce signals representing an image of the body.

These signals are, in turn, passed to a display device 23 to provide a visual display of the image.

In operation of the apparatus the strong magnetic field provided by the electromagnet 1 defines an equilibrium axis of magnetic alignment in the body being imaged.

To obtain an image of a selected region, e.g. a cross-sectional slice of the body, an r.f. field pulse is first applied to the body by means of the coil system 13 to excite magnetic resonance in the selected region. To this end the coil system 13 produces a field in a direction orthogonal to the static field direction so as to tip the spins of nuclei in the selected region from the direction of the static field into a plane orthogonal to the static field direction. To restrict excitation to the selected region the r.f. field pulse is applied in conjunction with magnetic field gradients imposed by the coil arrangement 9, the frequency of the r.f. field being chosen in conjunction with the magnitudes and directions of the imposed gradients so that the Larmor frequency of chosen protons in the body, e.g. hydrogen protons, is equal to the r.f. field frequency only in the selected region.

The r.f. signals resulting from excitation are then spatially encoded by application of one or more further gradient magnetic fields in known manner, detected by the r.f. coil system 13 and processed to produce an image.

Normally a number of excitation and signal detection sequences are required to produce sufficient data to produce a satisfactory image.

Referring to Figure 2, for imposing a gradient in the axial (z-direction) on the magnetic field in the volume 3 the coil arrangement 9 includes a Maxwell circular coil pair 25A, 25B positioned one at each end of the volume 3. When energised with current passing in opposite directions as indicated by arrows in Figure 2, one of the coils 25A, 25B locally increases the high strength main magnetic field in the volume 3 whilst the other one of the coils 25A, 25B locally decreases the high strength main magnetic field thereby imposing a gradient centred on an x - y plane midway between the coils 25A, 25B.

For imposing a gradient in each of the x and y directions the coil arrangement 9 includes four identical rectangular saddle coils 27A, 27B, 27C, 27D or 29A, 29B, 29C, 29D. The coils 27A, 27B, 27C, 27D for the x-direction comprise two pairs of coils 27A, 27B and 27C, 27D the coils of each pair 27A, 27B or 27C, 27D being positioned diametrically opposite one another in the x-direction around the curved surface of the volume 3 with the two pairs axially spaced, i.e. with their inner axial arcuate ends spaced apart in the z-direction. The four coils 27 are normally energised in series and connected so that current flows in the directions indicated by arrow heads in Figure 2. As a result the main field is locally increased in strength at one side of the volume 3 in the x-direction and reduced at the opposite side of the volume 3.

The four coils 29 are similarly positioned on oppposite sides of the volume 3 in the y-direction.

In Figure 2 coils 27 and 29 are shown, for clarity, as not overlapping in the circumferential direction but in practice they typically do so, each typically subtending an angle of about 1200 at the axis of the volume 3.

It will be appreciated that the various gradient coils 25, 27, 29 are supported in position with respect to one another and to the volume 3 on a former (not shown) e.g. of tubular form.

With an arrangement as shown in Figure 2 the region where satisfactorily linear gradients are imposed, i.e. the volume 3, extends between the adjacent inner axial arcuate ends of the coils 27 and 29 only. As a result, the region of a body to be imaged has to be positioned between the inner ends of the coils 27, 29. This presents problems due to the fact that the above-mentioned x-y plane, at the centre of volume 3 has to be at a distance from the outer axial arcuate ends of the coils 27, 29 of not less than times the radius R of the coils 27, 29 if the outer axial arcuate ends of the coils 27, 29 are to be sufficiently far from their inner axial ends not to reduce significantly gradient linearity and strength.Hence, it is difficult, for example, to image the central and lower parts of an adult humans head with such a coil arrangement since the subject's shoulders prevent the subject's head entering sufficiently far into the coil arrangement.

In accordance with the present invention this problem is alleviated for each set of four saddle coils 27 or 29 by folding back one set of coils A, B over the other pair of coils C, D, as illustrated in Figure 3. With such an arrangement the region of satisfactory gradient linearity is positioned toward one end of the coils 27, 29. The position of the centre of the region of satisfactory gradient linearity in the axial directions is central between the Maxwell coils 25A, 25B, which are positioned at a distance apart equal to about 3 of the coil radius R. Thus the distance between the centre of the region of satisfactory gradient linearity, i.e. volume 3, is ff3R/2 from the nearer end of the coil arrangement compared with F2 R in the arrangement of Figure 2.It is pointed out in this connection that the coils 25, 27 and 29 although shown in Figures 2 and 3 as being of different radii for clarity, are normally of very similar radius.

It will be seen from Figure 3 that the four coils 27 now effectively comprise a single pair of coils 27A, C and 273, D correspondingly positioned on opposite sides of the volume 3 with the conductors of each coil 27A, C or 27B, D spaced axially at each end, more particularly in two groups, and with the left hand end portions of each coil 27A, C or 27B, D radially aligned with the volume 3. It will be appreciated in this connection that the spacing of the conductors at the right hand ends of the coils 27, 29 in Figure 3 is not essential in respect of gradient linearity in the volume 3 but arises from the desirability of making all the coils 27 identical. The same remarks apply, of course, to the four coils 29.

With the particular arrangement shown in Figure 3 it is found that an improvement in gradient linearity can be effected by arranging for the coils 27C and 27D to produce a larger number of ampere turns than the coils 27A and 27B. In one particular arrangement with coils of internal radius 320 mms and arranged to carry identical currents the coils 27A and 27B each have 12 turns and the coils 27C and 27D 15 turns. The same remarks apply, of course, to the four coils 29.

One difficulty that arises with a coil set according to the invention is that the magnetic field produced when it is energised reacts with the static magnetic field to produce a net torque such as to tend to rotate the coils and hence the former on which the coil set is mounted. This is due to the non-symmetrical form of the coil set, and consequently this difficulty does not arise with symmetrical prior art coil sets, as shown in Figure 2 where the forces produced cancel one another out.

In order to overcome this problem so-called bucking coils may be used. As shown in Figure 4 these coils 31, 33 are positioned beyond the ends of the coils 27, 29 remote from the volume 3 and are arranged so as to produce, when energised, forces equal and opposite to those produced by the coils 27, 29 and thereby counteract such forces. To this end a pair of identical diametrically oppositely positioned coils 31A, B or 33A, B are suitably used for each gradient coil set 27 or 29 each pair of bucking coils 31A, B or 33A, B being axially aligned with the coils of the corresponding gradient coil set 27 or 29.

The bucking coils 31, 33 are conveniently connected for energisation in series with the gradient coils 27, 29. However they may be energised separately if desired, for example, to avoid increase of the inductance presented to the gradient coil energising currents due to the bucking coils 31, 33 and thereby avoid increase of gradient current pulse rise and fall times due to the bucking coils 31, 33.

The coils of an arrangement according to the invention are suitably wound with a conductor of rectangular cross-section with the turns lying alongside one another in a direction parallel to the axis of the cylindrical volume they surround.

It will be appreciated that whilst in the embodiment of the invention described above by way of example with reference to Figure 3 the cylindrical volume is of circular cross-section this is not necessarily the case in other embodiments of the invention and the volume may for example be of elliptical cross-section.

Claims (10)

1. A coil set in a magnetic resonance apparatus for imposing on a magnetic field in a cylindrical volume in which an object to be examined is placed in use of the apparatus a gradient in a direction transverse to the axis of the volume, the coil set effectively comprising a pair of coils correspondingly positioned around said volume diametrically opposite one another, each of said coils having an axial end portion in which the conductors of the coil are spaced apart axially, and which is substantially radially aligned with said volume.

2. A coil set according to Claim 1 wherein said conductors in said axial end portion of each coil are positioned in two groups spaced apart axially.

3. A coil set according to Claim 2 comprising a first pair of identical coils positioned around said volume diametrically opposite one another and a second pair of identical coils positioned around said volume diametrically opposite one another and overlapping said first pair of coils axially.

4. A coil set according to Claim 3 wherein said first pair of coils have more turns than said second pair of coils.

5. A coil set according to Claim 4 wherein said volume has a radius of substantially 320 mms and the coils of said first pair have 15 turns and the coils of said second pair have 12 turns.

6. A coil set according to any one of the preceding claims wherein each said coil is a saddle coil of rectangular shape.

7. A coil set according to any one of the preceding claims in combination with a second coil set for imposing a gradient on said magnetic field in the direction of the axis of said volume comprising a Maxwell coil pair disposed coaxially with said volume in axially spaced relationship so as to lie one on either side of said axial end portions.

8. A coil set according to any one of the preceding claims in combination with a bucking coil arrangement which when energised produces forces which counteract forces arising from energisation of said coil set.

9. A coil set according to Claim 8 wherein said bucking coil arrangement comprises a pair of coils positioned at the end of said coil set remote from said volume and each axially aligned with a respective one of the coils of said coil set.

10. A coil set according to Claim 1 substantially as hereinbefore described with reference to Figure 3 or 4 of the accompanying drawings.