GB2277160A - Electric field screening arrangement for MRI RF coil - Google Patents

Abstract

An electric field screening arrangement for a single turn surface coil, especially those designed for use in NMR apparatus, comprises a conducting loop of substantially the same dimensions as the surface coil, arranged to lie between the coil and a body to be examined, and split in two positions 11, 12a quarter of the distance around the coil from each of its two output ends. The screen renders the coil relatively insensitive to frequency detuning. The capacitance between coil 1 and half-loop 14 forms part of the coil tuning capacitance. <IMAGE>

Description

ELECTRIC FIELD SCREENING ARRANGEMENT This invention relates to screening arrangements for surface coils, especially those designed for use-in nuclear magnetic resonance (NMR) apparatus. In such apparatus a static magnetic field is applied to a body under investigation to define an equilibrium axis of magnetic alignment in the region of the body being examined.

A radio frequency (r.f.) magnetic field is then applied to said region in a direction orthogonal to the static magnetic field direction, to excite magnetic resonance in the region, and resulting r.f. signals generated by resonating nuclei within the region are detected and processed.

The exciting r.f. field is applied, and resulting signals detected, by r.f. coils placed adjacent the body. Separate coils are normally used for excitation and detection, although the same coil or coils may sometimes be used for both purposes.

One form of r.f. coil employed in NMR apparatus is the so-called surface coil, which can be located closely adjacent the surface of a region of a body being examined with its plane lying approximately parallel to that part of the surface, and such a coil may be in the form of a single conducting loop extending between two ends which provide the output therefrom.

The presence of the body under examination, can however lead to frequency detuning, and it is therefore desirable to screen the r.f. coil in order to reduce such frequency changes.

It is also desirable to screen the r.f. coil against the emission of any appreciable electric r.f. field which could lead to increased dielectric heating of the subject under examination and also interfere with the operation of adjacent electrical equipment.

Various ways of achieving such screening are in use or have been proposed, and an object of the present invention is to provide a form of screening for a single loop surface coil which has advantages over the earlier arrangements as will subsequently be described.

According to one aspect of the invention an electric field screening arrangement for a single turn surface coil comprises a conducting loop of substantially the same dimensions as the surface coil, and disposed so as to lie adjacent the surface coil between the latter and a body to be examined, the conducting loop being split in two positions a quarter of the distance around the coil from each of the two ends of the coil.

Such an arrangement has an important advantage in that it is relatively insensitive to frequency detuning compared with existing forms of screening.

Usually both the surface coil and the screening loop will be approximately circular, the screening loop being split at positions 90 and 270 around the coil between one output end and the other.

However in some instances the surface coil and screening loop may be in shapes other than circular.

The invention will be further explained by way of example with reference to Figures 1 to 7 of the accompanying schematic drawings, in which Figures 1 to 3 illustrate in diagrammatic form three known forms of screening arrangements, Figure 4 illustrates one form of screening arrangement in accordance with the invention, Figure 5 illustrates a modification of the arrangement described with reference to Figure 4, and Figures 6 and 7 illustrate two further modifications.

Referring first to Figure 1, this illustrates a single turn coil 1 in the form of a conducting loop extending between two output ends 2, 3, the latter being interconnected by a tuning capacitor C.

A conducting body 4, which may be provided by a body under examination in the case where the coil comprises a surface coil for use in NMR apparatus, is disposed closely adjacent the coil as shown, the body being at ground potential.

If the coil 1 carries an a.c. current I sin t, there will be a distributed capacitance to ground; the displacement current associated with this capacitance can be minimised by grounding the centre point of the coil to the conducting body as at 5.

The current flowing in the coil produces a field B, the voltaqe across the ends 2, 3 of the coil being given by the surface integral of B over the whole area of the coil, i.e.

Because of the balanced nature of the arrangement one end of the coil will be at +V/2 with respect to the ground body, and the other at -V/2 with respect to the ground body as indicated.

It is common practice to utilise a coaxial line to provide a surface coil, the outer conductor forming a faraday screen, with the coil arranged to feed an unbalanced amplifier. Such an arrangement is illustrated in Figure 2, the outer conductor 6 being broken 180 from the output ends of the coil 1. Now an unbalanced loop on its own will be worse than the arrangement of Figure 1 because the whole of the induced voltage will be across the stray capacitance SC at one end, the voltage at the opposite end being zero. This gives rise to a net displacement current to ground, as well as a displacement current from one end through ground to the opposite end.

The effect of the screen provided by the outer conductor 6 will give an improvement, because a voltage V will be induced between the ends 2, 3, and the centre point between the output ends 2, 3 of the loop 1 will be notionally at ground potential. This effectively makes the net displacement current to ground equal, although the current passing through the capacitance to ground and then to the other end of the loop will be unaffected.

However, one way of reducing the effect of the capacitance to ground is to reduce the voltage along the loop, and this may be achieved by distributing the tuning capacitance in the loop, as indicated in Figure 3. In this case tuning capacitors 7, 8 are provided between the output ends 2, 3 of the loop 1, and at a position 180 displaced therefrom respectively as shown.

Although the capacitance to ground will not itself be affected, because the voltage will be reduced, the displacement currents will also be reduced.

Thus, for the arrangement of Figure 1 the displacement current through one end of the loop to the other via ground will be in the form

where L is half the total length of the loop, V1 is the voltage at some point 1 along the loop and #c is the capacitance per unit length.

The voltage V1 varies uniformly along the loop, that is to say V1 = V . 1 where V is the.voltage across the ends of 2 L the loop.

Accordingly

Now for the arrangement of Figure 3

Any loss in the ground appears as a series impedance to the current path, so that as I is reduced by a factor of 2 the power loss is reduced by a factor of 4. In the practical case of surface coils near bodies, the loss is so dominated by eddy currents that any reduction of losses due to electric fields is not a major factor.

However an advantage of the Figure 3 arrangement is that tuning of the coil will be less affected by the presence of the body being examined, and this improvement is also a factor or 4.

Nevertheless the need for two tuning capacitors 7,8 results in increased expense, and makes the arrangement more difficult to control.

This is avoided by the present invention as illustrated in Figure 4.

In this arrangement the loop 1 is the same as that of Figure 1. However a second, screening, loop 9, of the same diameter as the first loop 1 (the tuning loop), is disposed so as, to lie between the tuning loop and the lossy conducting body 4. The screening loop is cut at the +90 and -90 points 11, 12 with respect to the ends 2, 3 of the tuning loop 1 as shown. The centre point of the tuning loop 1 will be at zero volts as will the centre point of the semicircle 13 of the screening loop further from the output ends 2, 3 of the tuning loop 1. Consequently there will be no electric field between that semicircle of the screening loop 9 and the tuning loop 1.

The semicircle 14 of the screening loop 9 adjacent the output ends 2, 3 of the tuning loop 1 will form part of the tuning capacitance. As the circuit of this tuning capacitance will be from end 2 at +V to semicircle 14, along the semicircle and back to the 2 loop at end 3 at -V, the mean voltage of the semicircle 14 of the 2 semicircle 14 of the screening loop 9 will also be zero.

The voltages at the ends of the semicircle 14 will be +V and V and those at the ends of semicircle 13 will be 4 4 -V and +V as indicated.

4 4 Accordingly the voltage between the tuning loop 1 and the screen loop 9 will be +V at one end of semicircle 14 and -V at the other end. 2 2 Such an arrangement gives an improvement in insensitivity to the proximity of the body 4 similar to that of the Figure 3 arrangement, but has the advantage of dispensing with the need for two tuning capacitors.

Some further improvement may be obtained by the use of a second screen in the form of a further broken loop 15 as indicated in Figure 5. In this Figure, although the tuning loop 1, the screening loop 9 and the second screening loop 15 are of the same diameter, they have been shown with different diameters for the sake of clarity.

This second screening loop 15 is broken at 45 , 135 , -45 and -135 , and further improves the electric field effects.

Nevertheless in practice the second screening loop decreases valuable filling factor, and as the additional improvement it provides is relatively small, such an arrangement may not be convenient for some purposes.

As there is little or no electric field between the coil 1 and the section 13 of the split loop 9 in the arrangement illustrated in Figure 4, that section of the loop can in some cases be omitted, as illustrated in Figure 6, with only a small difference in performance, the voltage at the ends of the section being almost identical to the nearest points on the coil 1. Similarly in the arrangement of Figure 5, the sections of the screening loop 15 which lie adjacent and opposite to the output ends 2, 3 can also be omitted, as shown in Figure 7, without any essential loss of performance.

It will be understood that the invention includes within its scope NMR apparatus utilising surface coils incorporating screening arrangements as above described with reference to Figures 4 to 7. Moreover the invention may also have other applications where it is desired to screen a single turn r.f. coil.

Claims (6)

1. An electric field screening arrangement for a single turn surface coil comprising a conducting loop of substantially the same dimensions as the surface coil, and disposed so as to lie adjacent the surface coil between the latter and a body to be examined, the conducting loop being split at two positions a quarter of the distance round the coil from each of the two ends.

2. A screening arrangement according to Claim 1 modified in that the section of the loop between said two positions further from said two ends of the coil is omitted.

3. A screening arrangement according to Claim 1 or Claim 2 further including a second conducting loop of substantially the same dimensions as the surface coil and disposed so as to lie adjacent the surface coil between the latter and the body to be examined, the second conducting loop being split at positions an eighth and three-eighths of the distance round the coil from each of the two ends.

4. A screening arrangement according to Claim 3 modified in that the two sections of the second loop which lie adjacent and opposite said two ends of the coil are omitted.

5. A screening arrangement according to any preceding claim wherein said coil is circular.

6. An electric field screening arrangement substantially as hereinbefore described with reference to Figure 4, or Figure 5 or Figure 6 of the accompanying drawings.