GB2327762A - MRI magnet with shuttling HTSC prepolarising block - Google Patents

Abstract

Interventional MRI has led to the use of open H or C magnets 1, but these have relatively low main fields. To overcome this, a prepolarising unit 7 is provided in which an HTSC magnet, such as a YBCO or BSCCO block 8, is brought adjacent to the patient 5 and then withdrawn to position 8' by a linear motor (figure 3), which may use an HTSC rotor. The NMR signals then evolve in the homogeneous field of the main magnet 1. There may be prepolarising units on each side of the patient (figure 2).

Description

MAGNETIC RESONANCE IMAGING APPARATUS The present invention relates to magnetic resonance imaging (MRI) apparatus and more particularly is concerned with a technique known as pre-polarisation by which a low magnetic field may be temporarily increased for the purpose of enhancing the sensitivity of a relatively low field NMR (Nuclear Magnetic Resonance) experiment.

The aforementioned pre-polarisation technique will now be described by way of background information.

This is a technique in which a large low quality magnetic field is temporarily applied to an object to be studied by NMR particularly in an NMR environment where there is a low, or even zero magnetic field. With this technique a large field is first applied for a period to cause the target object to become magnetised to a level related to that field. The magnetisation produced by the high field develops with the time constant T1 characteristic of the high field.

The polarising field is then very rapidly removed so that the target object magnetisation decays back towards that which was applied originally with the value of the time constant T1 which is characteristic of the lower field.

This allows quite a long period (typically less than 0.5T1 at the low field) during which data can be recovered but during which the magnetisation in the target object is well above what would be expected in the low field. Thus it is possible, in principle, to enhance the sensitivity of a low field NMR experiment quite substantially using this technique as long as the time constants with which the magnetisation decays are long compared with the time taken to remove the pre-polarisation field.

Pre-polarisation has been proposed as a possibility for low field MRI but, as yet, has not been demonstrated in vivo. This is because at all except the lowest fields or with poorly coupled coils, detector noise in human MRI is dominated by noise from the patient's body itself, and most current machines operate at sufficiently high fields for this to be true. It has been pointed out that, even at best, the signal-to-noise ratio is proportional to B,, the magnitude of the main field. In practice, the gain is less than that because of timing limitations.

The Applicants have however appreciated that one major advantage of pre-polarisation as against attempting to increase the main magnetic field level is that the quality i.e.

degree of uniformity of pre-polarising field required is relatively low.

The rapid development of interventional MRI is the principal driver leading to the development of increasingly open magnets, allowing much better access to patients than that allowed by the traditional experimental configuration. Improved accessibility is, however, paid for by the difficulty of achieving high field levels in otherwise useful designs. Pre-polarisation offers the possibility of getting substantially enhanced images from local regions of the body of immediate concern to a clinician performing a procedure.

The invention provides apparatus for magnetic resonance imaging, comprising a magnet for generating a magnetic field through an imaging region, means for applying r.f. pulses to, and creating magnetic field gradients in, the imaging region, a high temperature superconducting magnet (HTSC) arranged adjacent to the imaging region to provide an increased main magnetic field, and means for rapidly withdrawing the HTSC magnet away from the imaging region.

The HTSC magnet provides a powerful pre-polarising field, but can be withdrawn to enable data acquisition to be conducted under the much higher quality field of the firstmentioned magnet. By high temperature superconductor, it is meant a superconductor which has a transition temperature of substantially more than 20"K.

Apparatus for magnetic resonance imaging constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic front view of a first embodiment of the invention employing one pre-polarising unit shown in a particular "C"-shaped iron-yoked magnet; Figure 2 is a diagrammatic front view of a second embodiment of the invention employing two pre-polarising units shown in the same form of magnet; and Figure 3 is a front view, partly diagrammatic and partly in section, showing a pre polarising unit of Figure 1 or Figure 2 in more detail.

Before describing the invention in more detail, the theory behind pre-polarisation will now be outlined.

If the region of the body to be enhanced is in a field By refs and a pre-polarisation field is applied which has a magnitude By pup at a point P, and is oriented to B0 re at an inclusive angle Op, then the magnetisation of the sample at P(Mpp) develops as: Mppα{B0ref+B0pp+2B0refB0 pp cos #} (1-exp(-t/T1 pp)) + B0 ref exp (-t/t1pp) where TlPp is the value of the T1 of the material at Pat the local (high) field (in vector notation (B0 + Bopp)), and t is the time after application of the enhancing field.

Assuming that Bores is constant (to within MRI limit of 5-lOppm) throughout the whole region of interest, the amplitude of its excited magnetisation will be modulated by the local values of B0 pp, and #p. However these are geometrically determined quantities (as well as being controlled by the applied field generating agency) and can be corrected for variations if necessary.

After a time t' after the removal of the pre-polarising field the magnetisation is then: Mppα{B0ref+B0pp+2B0 ref B0 pp cos #} (1-exp (-t/T1pp)) exp (-t/T1ref) + B0ref (1-exp (-t/T1pp)) exp (t'/T1ref) where Tires is the T1 at the point P in the low (Boref) field.

Typically, in tissue the value of T1 has been observed to have an empirical relation to the field strength given by:

where v is in the range of 0.3 - 0.4 in the range of fields normally used for whole body MRI. (B0A and BOB are the two field levels at which the comparison is being made).

By way of example, if we take the lower field of current interest (BOB) to be 0. iT (Tesla), and the maximum value of BOA which is likely to be really obtainable as 1.OT, then T1(A) = (2.0 - 2.5) T1(B).

The values of T1 for human tissue which are commonly encountered range from that of fat (of the order of 150 msec in the 0.1 - 0.2T range) to cerebo spinal fluid with a T1 of around 3.5-4 seconds, which does not vary significantly with the strength of the magnetic field. Typically T1 for the main tissues of interest lie in the range 250-800 msec in 0.1 to 0.2T fields.

A typical mid-range tissue might be grey matter in the brain with a T1 of 500-550 msec. Then application of a pre-polarisation field of 2.0T for two seconds, followed by data acquisition at 100 msec after nominal removal of the field would result in a magnetisation gain of 6.91 in a machine operating at 0.2T for tissue with a Tires of 500 msec. For simplicity it is assumed the pre-polarising field was applied orthogonal to the main field. Thus, locally, this would result in a signal-to-noise ratio gain of around 8.

The preferred method of producing a pre-polarising field of the intensity proposed is the use of a melt-quenched high temperature superconductor (HTSC). A block of this material (such as YBCO [yttrium barium copper oxide, typically having the composition Y Ba2 Cu3 O,J) l50mm in diameter, 40mm thick and magnetised to have a BREN5 of 4T can produce in excess of 2T at a depth of 30mm into the head of a patient (allowing for insulation, vacuum containment etc.).

One possible construction of a pre-polarising unit will now be described with reference to the drawings.

The first embodiment of magnetic resonance imaging apparatus comprises a C-magnet 1 comprising a yoke 2 joining pole pieces 3, 4. The faces of the pole pieces 3, 4 lie in a horizontal plane. The patient lies down horizontally with his/her axis extending perpendicular to the plane of the drawing. The head 5 of the patient rests upon a support 6 and the pre-polarising unit 7, shown in more detail in Figure 3, is adjacent to the patient's head.

In addition to the main field provided by the C-magnet 1, which is typically of the order of 0.2T, there is provided gradient coils (not shown) in order to enable the r.f.

signals detected to be spatially encoded. The gradient coils set up magnetic field gradients in three directions at right angles. An r.f. excitation coil (not shown) is provided in order to excite nuclear magnetic resonance in the nuclei of various tissues of the patient, and a receiving coil (not shown) is also provided to detect the r.f. signals emitted by the nuclei. One coil may be used to perform both functions.

In accordance with the invention, a HTSC magnet 8 is brought to a position adjacent to the patient's head to assist in the polarising process. It is then rapidly withdrawn to a position 8' to allow the r.f. signals from the patient to be detected under the influence of the field of the C-magnet 1.

The second embodiment of the invention described with reference to Figures 2 and 3 of the drawings, is the same as the first, except that an additional pre-polarising unit 7 is provided on the other side of the patient and except for size. The second unit 7 is the same as the first unit and therefore as shown in Figure 3. Typically, the magnets 8 of Figure 2 will be of larger diameter than those of Figure 1.

While the direction of the magnetic field between the pole pieces 3 and 4 is in a vertical direction, the direction of the magnetic field from the powerful magnet formed by the HTSC block is in a horizontal direction i.e. orthogonal to the field from the Cmagnet.

In general, the quality of the field (i.e. variation in strength across it) will vary with field strength. However, the field quality of the HTSC magnet 8 can be 1000 times less good than that of the C-magnet 1, but the amplitude will still only vary by around 1%. This would be totally unacceptable for data acquisition, but so long as the prepolariser 8 is withdrawn before data acquisition takes place, the use of the low quality high strength field for pre-polarisation is not a disadvantage.

The general form of the pre-polarising unit 7 is shown in detail in Figure 3. The HTSC block 8 is attached to a copper backplate (not shown) and insulating connecting rod 9 and runs inside an inner glass fibre tube 10 along guides (not shown). The block 8 and its copper support plate rest against cooling rings 11, 12 (split to stop eddy currents) at the front and rear extremities of its motion. These rings 11, 12, and the connecting guides are cooled by one cold finger attached to a two stage cryogenic refrigerator which has a cooling power of 5W at 20"K. The block 8 is therefore cooled at the extremities of its motion, but the motion is so fast that magnetisation is not lost in the process. The means for rapidly withdrawing the superconducting magnet from the imaging region is a linear motor consisting of an HTSC rotor 13 and three-phase windings 14. The motor is of the reluctance type. The rotor is cooled at the corresponding extremities of its motion by cooling rings 15 and 16 which are refrigerated via a separate cold fmger, and can be at a different temperature to the block 8.

The motor stator is wound from water-cooled hollow copper conductor, which can be transiently pulsed, with the windings correctly connected, to polarise the stator independently of the main HTSC block. The resulting field will extend radially from the rotor. The motor is designed to run the HTSC block rearwards very fast, to be stopped by a spring and clamped in the out position by a brake (not shown). The motor can be driven forward in a slower, controlled manner, (as this just means a slower build-up of magnetisation, and is not critical in terms of performance in the way removal is).

Typical sizes are a diameter of 35mm for the block 8 for Figure 1 and 150mm for Figure 2, and a run of 40cm between its inner and outer positions. The field is fixed in intensity at HTSC, but varies in space (and time as block is moved). This distance is enough to allow a magnetisation cycle comprising the following steps.

First, the rings 11, 12 are cooled to 20"K (thereby cooling the main block 8), while maintaining the cooling rings 15 and 16 above the critical temperature. The main block 8 is then polarised using an external high field magnet. This magnet is then removed, whereupon the rings 15, 16 are cooled to 20"K. The rotor 11 is now polarised by configuring the three-phase windings temporarily and pulsing the windings to generate a core field of 2 to 3T. The water cooling of the copper conductor is taken advantage of to pass a very high transient current (much more than normal) to excite the HTSC rotor. The block 8 is far enough away for its field to be dominated by that from the windings.

The inner glass fibre tube 10 is contained within an outer tube 17, and the space between them is evacuated, in order that the temperature on the outside of the prepolarising unit e.g. in the vicinity of the patient, is at room temperature.

MRI field qualities are such that even in the retracted position the field in the region of interest is significantly distorted by the permanent magnet pre-polarising unit 7.

With the small polariser of Figure 1, and small field of view, the r.f. detector system, which will initially be external to the cylinder, and at room temperature, will be sensitive to a volume small enough for components due to the residual field to be ignored, or, at worst, corrected by a simple shimming system (using the gradient coils, as linear corrections).

With the larger polariser of Figure 2, with which a refrigerated detector coil may be employed, the existence of transverse field gradients will become very apparent. With a 200mm diameter pre-polariser, even at the centre of an adult head, and assuming the parameters used previously, the boost in field is still a factor 2.47.

In order to make the system symmetrical and eliminate transverse field two pre polarisers are provided in the embodiment of Figure 2, one on each side of the head, and magnetised in the same direction (so that the south pole of one faces the north pole of the other). The purpose is to cancel the effect of the magnets 8 when data acquisition using the receiver coil takes place, both magnets being in the retracted position. Only one magnet 8 of course is adjacent to the patient in order to conduct the initial polarisation of the tissues to be examined. The enormous forces on the polarisers in close proximity means that only one side of the patient can be polarised at a time. In the sensitive regions, additional shim coils will be needed to correct for the residual field gradients. Even order axial shims will be included in the system as a whole (with odd orders being unnecessary as long as orthogonality between the prepolarisation axes and B0 is maintained). Shimming using iron powder discs or small coils, is needed to correct high order, and cross, terms.

The second pre-polarising magnet could be fixed in the retracted position, but it would be preferable for it to be retractable like the first, in order than either side of the patient may be imaged.

Though the complete image cycle is extended to perhaps 5 seconds, the use of techniques such as echo planar imaging allows acquisition of a complete image at each polarisation, so that a gain in sensitivity is achieved at each half cycle, and the effective gain in signal-to-noise ratio of the centre of the system is multiplied by2 over the 5 second cycle.

The noise can be assumed to be Johnson, so that an overall performance gain of 3.49 at the centre of the head rising to just over 7 at the edges can be predicted.

Even in the larger system of Figure 2, the weight of the moving part of the device is no more than about 6kg. The force required to overcome mechanical inertia and accelerate the pre-polariser fast enough is less than 900N with allowance for friction etc. The magnetic forces when the larger system is used as part of an alternated pair amount to a further 360N or so, mean the total starting drive force needed in the retraction unit is around 1.26kin. For the small, single sided pre-polariser, the power required is quite small - assuming a total weight of 1.5kg, and no magnetic attractions to overcome, the starting force needed is 260N (again with a small allowance for friction). A typical time for withdrawal of the magnets 8 to position 8' is lOOms.

Mechanical alignment of the pre-polarising system is important, as the force calculations above rely on the polariser travelling orthogonal to the main field.

Symmetry is important in maintaining performance, and will be needed in aligning the system. Some of the modern interventional magnets are being designed for rapid switching off and on, so in many instances it will be possible to position the prepolarising unit (if not always installed) in the absence of the main field. The prepolarisers therefore will be magnetised at the time of installation. Total unit weight of the smaller polariser (including refrigerator, but excluding its supports) is estimated to be 50kg, and of the large 120kg.

The pre-polarisers will require to be magnetised - and in a high field (of the order of 1.6 times the proposed Brem The method of magnetisation will be to pass the HTSC components in this assembly through a low quality, but very high field, cryogenic magnet. (Peak field is the important factor, and not the quality (uniformity) of the field). If located at a conveniently short distance from the actual machine, this unit can also be used to pre-polarise saline and other metabolites to be given by high speed injection to the patient. Once the pre-polariser is magnetised it should remain so until the temperature rises and the supercurrents quench.

Initially, the r.f. detector can be external to the cryostat for the pre-polariser, or it can be refrigerated and incorporated in it.

Typical overall dimensions are a cylinder approximately 90cm long in the case of the small unit of Figure 1 and 6cm in diameter, and 110cm long and 20cm in diameter in the case of the larger one of Figure 2.

Of course variations may be made from the above embodiments without departing from the scope of the invention. Thus, for example, the smaller unit of Figure 1 could employ two pre-polarisers as in Figure 2, or the larger unit of Figure 2 could employ one pre-polariser only. Also, alternatives to the linear motor 13, 14 are possible for rapidly withdrawing the superconducting magnet away from the imaging region. For example, the linear motor could be replaced by a rotary motor employing an HTSC rotating rotor with a multi-screw thread drive to the rod 9. Other forms of actuator could be used. Also, other materials could be used in place of YBCO for the high temperature superconductor for example the material known as BSCCO may be used.

(Bi2 Sr2 Cal Cu2 0s or Bi2 Sr2 Ca2 Cu3 0iso) Further, the invention can be used with magnets other than C-magnets, for example, H-magnets or other permanent magnets employing a yoke and pole pieces. The invention could also be used with samll solenoid-type magnets. Finally, the hollow conductor of the motor stator could be hollow alumimium conductor, and either the hollow copper or hollow aluminium could be cooled with oil or demineralised water.

Claims (13)

1. Apparatus for magnetic resonance imaging, comprising a magnet for generating a magnetic field through an imaging region, means for applying r.f. pulses to, and creating magnetic field gradients in, the imaging region, a high temperature superconducting magnet (HTSC) arranged adjacent to the imaging region to provide an increased main magnetic field, and means for rapidly withdrawing the HTSC magnet away from the imaging region.

2. Apparatus as claimed in Claim 1, including conduction cooling surfaces against which the HTSC magnet rests in its positions adjacent to the imaging region and withdrawn from the imaging region.

3. Apparatus as claimed in Claim 2, in which the cooling surfaces are contained within an evacuated enclosure.

4. Apparatus as claimed in Claim 3, in which the magnet is guided for movement within the enclosure.

5. Apparatus as claimed in any one of Claims 1 to 4, in which the means for rapidly withdrawing the superconducting magnet comprises a motor employing a HTSC magnet.

6. Apparatus as claimed in Claim 5, in which the magnet forms the rotor of a linear motor.

7. Apparatus as claimed in Claim 6, in which the motor has water-cooled windings.

8. Apparatus as claimed in Claim 7, in which the rotor is magnetisable by passing currents through the motor windings.

9. Apparatus as claimed in any one of Claims 1 to 8, in which the HTSC magnet providing an increased main magnetic field is polarisable by means of an external magnet.

10. Apparatus as claimed in any one of Claims 1 to 9, including a second superconducting magnet arranged adjacent to the imaging region on the opposite side thereof, in order to balance the field from the magnet when receiving r.f. signals.

11. Apparatus as claimed in any one of Claims 1 to 10, in which the first-mentioned magnet has a yoke coupling pole pieces.

12. Apparatus as claimed in Claim 11, in which the magnet is a C-magnet.

13. Apparatus for magnetic resonance imaging substantially as herein described with reference to the accompanying drawings.