GB2159958A - RF field generating and detecting arrangements - Google Patents

Abstract

An arrangement for generating or detecting RF magnetic fields, more especially for use in nuclear magnetic resonance imaging apparatus, comprises arcuate sheets 1, 3, 5, 7 of non-magnetic electrically conducting material disposed at spaced positions around the curved surface of a cylindrical volume and having interconnections 35, (37) at their ends to provide closed loop paths for current flow embracing the axially extending gaps between the sheets. In each gap between the sheets there is a further member (27, 29, 31, 33) of non-magnetic electrically conductive material so that current flow in such a closed loop path induces, or is induced by a current flowing in the further member in a loop substantially parallel to the closed loop path current, but in the reverse direction. The further members 27, 29, 31, 33 thus act as parasitic elements and reduce concentration of current in the regions of the gaps, and render the field more uniform. In a modification the members 27-33 have a central slot and the input and output feeders 17-23 are connected thereto in a lieu of sheets 1-7. In a second arrangement the four arcuate sheets each have a complete or partial longitudinal slot across which a respective feeder is connected (Figures 5-7, not shown), whereby an opposed pair of sheets acts as further members for the other opposed pair. <IMAGE>

Description

SPECIFICATION Magnetic field generating and detecting arrangements This invention relates to arrangements for use in generating or detecting radio frequency (RF) magnetic fields.

More especially, although not exclusively, the invention relates to such arrangements suitable for use in nuclear magnetic resonance (NMR) imaging apparatus.

In NMR imaging apparatus nuclear magnetic spins are excited in a body to be imaged by applying to the body an RF magnetic field in a direction transverse to the direction of a relatively strong static magnetic field applied to the body. The excited nuclear magnetic spins have a rotating vector in a plane perpendicular to the static magnetic field and so may be detected using the same arrangement as was used to apply the RF magnetic field, or another similar arrangement.

A proposed arrangement for use in generating or detecting RF magnetic fields in an NMR imaging apparatus essentially comprises two substantially semi-circular arcuate sheets of non-magnetic electrically conductive material, e.g. copper, disposed with a small gap between them so as to embrace a cylindrical volume into which the body to be imaged is inserted during examination, and through which volume the static magnetic field extends axially. The sheets are electrically connected to one another at each axial end, and to produce an RF magnetic field, currents are caused to flow in the sheets in directions generally parallel to the axis of the volume with the currents on opposite sides of each gap between the two sheets in opposite directions.The excited currents thus effectively flow in loops embracing the gaps and an RF field is produced which is directed perpendicular to the planes of the loops, and hence to the axis of the volume and the directions of the static magnetic field.

Such an arrangement is normally designed to be resonant at the frequency of the RF field, and to this end the interconnections between the sheets normally include capacitance which forms a resonant circuit with the inductance presented by the sheets per se.

The arrangement is suitably excited by generators connected across the gaps between the sheets, and it will be appreciated that the arrangement can be used to detect an RF magnetic field directed perpendicular to the planes of the loops by connecting a receiver across the gaps.

In a typical arrangement for use in generating and detecting RF fields, four 90 arcuate sheets disposed at spaced positions around the curved surface of a cylindrical volume are used. The arrangement is excited by balanced emfs from a transmitter applied respectively across each of one diametrically opposite pair of the gaps between adjacent sheets, and balanced inputs for a receiver for detecting an RF field are derived from across the other pair of diametrically opposite gaps. This arrangement has the advantage that during excitation no inputs are applied to the receiver. It will be appreciated that although the receiver only responds to RF fields orthogonal to the excited RF field this is acceptable in NMR imaging apparatus since the NMR spin vectors rotate.

Such proposed arrangements have the disadvantage that the currents do not distribute themselves uniformly around the sheets but tend to concentrate in the regions of the gaps between the sheets across which the excitation emfs are applied. The resultant magnetic field is consequently stronger near these gaps than elsewhere.

It is an object of the present invention to provide an arrangement for use in generating or detecting RF magnetic fields wherein this difficulty is alle viateel.

Aceording to the present invention an arrangement for use in generating or detecting RF magnetic field comprises: at least two arcuate sheets of non-magnetic electronically conducting material disposed at spaced positions around the curved surface of a cylindrical volume so as to provide substantially axially extending gaps between the sheets at substantially diametrically opposite posi tions.around said surface, the sheets being electrically ton:reected to one another at each of their axial ends to provide closed loop paths for current flow in' s;åid sheets and interconnections embracing said gaps; and in each said gap, a further member of non-magnetic electrically conductive material ar ranged so that current flowing in a said loop constituted by said sheets and interconnections inducçs, or is induced by current flowing in a said further member in a loop substantially parallel to said loop constituted by said sheets and intercon nectiong, but in the reverse direction.

It wlll he appreciated that in an arrangement according to the invention the field produced by currents flowing in the further members oppose the field produced by currents flowing in the sheets and interconnections so reducing the strength of the field;n the regions of the gaps between the sheets.

In an arrangement according to the invention coupling means for applying an input for exciting the artan-gement, andXor for deriving an output from th8larrangement, e.g. for application to detection means, will normally also be provided.

Each sJa'td coupling means may comprise a feeder connected between a pair of said sheets defining a said gap.

Alternatively, each said coupling means may comprise a coupling loop positioned adjacent a said further member in a said gap.

In a further arrangement each said further member is in the form of a continuous member provided with a central axially extending slot and each said coupling means comprises a feeder connected between the edges of a said slot.

The invention will now be further explained and two arrangements in accordance with the invention will be described, by way of example, with reference to the accompanying drawings in which Figure 1 is an end view of a previously proposed arrangement; Figure 2 is a side view of the arrangement of Figure 1; Figure 3 is an end view of a first arrangement in accordance with the invention; Figure 4 is a side view of the arrangement of Figure 3; Figure 5 is an end view of a second arrangement in accordance with the invention; Figure 6 is a side view of the arrangement of Figure 5, and Figure 7 illustrates a modification of the arrangement of Figures 5 and 6.

In our United Kingdom Patent Application No.

8504371 there is described a novel arrangement suitable for use in an NMR imaging apparatus for generating an RF magnetic field directed perpendicular to the direction of a static magnetic field in which a body to be imaged is placed in use of the apparatus, thereby to excite nuclear magnetic spins in the body, and for use in detecting the resultant nuclear magnetic spins in the body.

Referring to Figures 1 and 2, the arrangement comprises four arcuate sheets 1, 3, 5 and 7 of nonmagnetic electrically conductive material, e.g. copper. The four sheets lie at spaced positions around the curved surface of a cylindrical volume, each sheet subtending an angle of approximately 90 at the axis of the volume.

Over the major part of their lengths there is a small gap 9 between the adjacent edges of each adjacent pair of the sheets, but at one end of one longer edge of each sheet there is a tab 11 by means of which that sheet is electrically connected to the adjacent sheet, thereby to form an electrically conductive ring interconnecting the sheets at one axial end.

At the other end of the sheets there is a ring 13 of non- magnetic electrically conductive material, separate from the sheets, but capacitively coupled to the sheets. The capacitive coupling may be by way of discrete capacitors, for example, one at each side of each sheet as illustrated in the drawing by capacitors 15. Alternatively, the ring may be positioned sufficiently closely to the ends of the sheets with or without interposed dielectric material to achieve the desired capacitive coupling. In this case the ring 13, instead of being as shown in Figures 1 and 2, may conveniently be a short tubular member (not shown) fitting around the outside of the ends of the sheets 1 to 7.

The capacitive coupling serves to tune the arrangement approximately to the frequency at which it is intended to operate.

It will be appreciated that if desired the tabs 11 may be omitted and a ring capacitively coupled to the sheets provided at both ends of the sheets.

Connection of the arrangement to an RF source for NMR excitation purposes, and to a receiver for NMR detection purposes, is made by way of four balanced feeders 17, 19, 21 and 23. The conductors of each feeder are connected to corresponding points on the adjacent edges of a respective pair of the sheets, at an intermediate position along the length of the gap 9 between that pair of the sheets.

In operation, for NMR excitation, balanced RF emfs from a transmitter (not shown) are applied to each of one diametrically opposite pair of the feeders, say feeders 17 and 21.

Similarly, balanced outputs for application to a receiver (not shown) for NMR detection purposes are derived by way of the other diametrically opposite pair of feeders i.e. feeders 19 and 23 in the present example.

The emf applied by way of feeder 17 causes currents to flow in loops embracing the gap 9 between the sheets 1 and 7 to which the feeder 17 is connected, which loops are constituted by those sheets 1 and 7 and the conductive rings at the ends of the sheets. The emf applied by way of feeder 21 similarly causes currents to flow in loops embracing the gap 9 betwen the sheets 3 and 5 to which it is connected and constituted by those sheets 3 and 5 to and the conductive rings at the ends of the sheets.As a result, with the two emfs appropriately phased the currents in the sheets and rings at any instant have directions indicated by the plus and minus signs and arrows in Figures 1 and 2, and an RF magnetic field directed parallel to the plane 25 indicated in Figure 1, i.e. perpendicular to the planes of the loops, is produced in the volume embraced by the coil arrangement.

However, it will be appreciated that during excitation no emfs are applied via feeder 19 or 23 to the receiver since the loops constituted by the conductive rings and the sheets 1 and 3 or 5 and 7 with which each of the feeders 19 and 23 is connected are parallel to the direction of the field produced during excitation, i.e. there is no linkage of these loops with the magnetic flux produced during excitation.

During detection the rotating magnetic spin vectors will clearly induce emfs in the loops associated with feeders 19 and 23, thus causing balanced emfs to be applied to the receiver.

The arrangement of Figures 1 and 2 suffers from the disadvantage that there is a tendency during excitation for the currents in the sheets to concentrate in the regions of the gaps 9 across which the exciting emfs are applied, with a consequent nonuniformity of the RF magnetic field.

Referring now to Figures 3 and 4, this difficulty is overcome in accordance with the invention by en larking the gaps 9 between the sheets 1, 3, 5 and 7 and providing in each such enlarged gap a plate 27, 29 31 or 33 or other continuous member of electrically conductive non-magnetic material. The plates.27 to 33 are each spaced from the adjacent edges of'the adjacent ones of the sheets, 1, 3, 5 and 7 and the rings by which the axial ends of the sheets are interconnected, and are thus electrically insulated from (although inductively coupled to) the rest of the arrangement. Rings 35 and 37 of the alternative short tubular form mentioned above are shown in Figures 3 and 4 by way of example.

In operation of the arrangement for NMR excitation electric currents flow in the sheets 1, 3, 5 and 7 as described above with reference to Figures 1 and 2. The plates 27 and 31 in the gaps 9 across which the excitation emfs are applied by way of feeders 17 and 21, being in regions of strong magnetic field, have induced currents flowing in them in loops, as illustrated by the plus and minus signs and arrows in Figures 3 and 4, the induced loop currents in the plates 27 and 31 being in the opposite sense to the the loop currents flowing in the sheets 1, 3, 5 and 7 and the rings 35 and 37, in accordance with Lenz's law. Hence the strong fields in the regions of th gaps 9 across which the excitation emfs are applied are reduced, and the field is made more uniform in the cylindrical volume embraced by the arrangement. Thus the plates 27 and 31 act as parasitic elements.

It will be understood that the week field in the gap 9 between sheets 1 and 3 and the gap 9 between sheets 5 and 7 are little affected because the field is parallel to the plane 25 and does not induce appreciable currents in the plates 29 and 33.

It will be understood that by reducing the strongest fields, the efficiency of the arrangement is reduced, but this is normally acceptable in the interest of improved field uniformity.

It will be appreciated that whilst the strongest fields could be reduced to some extent simply by increasing the widths of the gaps 9, this tends to produce an unacceptably large increase in the inductance of the arangement. In the arrangement of Figures 3 and 4, however, the parasitic currents flowing in the plates 29 and 31 reduce the inductance of the arrangement.

It will be appreciated that in operation of the arrangement of Figures 3 and 4 for NMR detection purposes the plates 29 and 33 in enlarged gaps 9 renders the arrangement more uniformly responsive throughout the volume embraced by the arrangement.

In a modification of the arrangement of Figures 3 and 4 the excitation voltages and outputs are applied and derived via small coupling loops (not shown) appropriately positioned adjacent the plates 27, 29, 31 and 33, for example in the spaces 39 shown in Figure 4, instead of by means of feeders having their conductors connected to the edtes of the sheets 1,3,5 and 7.

Such a modified arrangement has the advantage that the positions of the loops relative to the spaces 39 may easily be made adjustable thereby to vary the impedance presented by the arrangement. By this means the impedance of the arrangement may be more accurately matched to a source of excitation voltage, or the impedance of the arrangement may be adjusted to compensate for the effect of a body being placed in the volume embraced by the arrangement.

In a further modification the excitation voltages and outputs are applied and derived across central slots in the plates 27 and 33 extending parallel to the cylindrical volume axis. It will be appreciated in this connection that instead of using plates or other continuous members in the enlarged gaps 9, closed loops of conducting material may be used, since negligible currents flow in the central parts of the plates.

A second arrangement in accordance with the invention wherein the excitation voltages and outputs are applied across central slots in the parasitic elements will now be described with reference to Figures 5 and 6.

The arrangement includes four arcuate sheets 41, 43, 45 and 47 and two rings 49 and 51 interconnecting the sheets at their axial ends arranged with gaps 65 between the sheets substantially as described above with reference to Figures 1 and 2, except that each sheet has formed in it a central axially extending slot 53.

Balanced RF emfs for excitation purposes are respectively applied via feeders 55 and 59 across the slots 53 in each of one pair of opposite sheets 41 and 45. For detection purposes balanced outputs are derived via feeders 57 and 61 from across the slots 53 in the other pair of diametrically opposite sheets 43 and 47.

The emf applied by way of feeder 55 during excitation causes current to flow in the sheet 41 and the rings 49, 51 in loops which embrace the slot 53 in the sheet 41 and are perpendicular to the plane 63 shown in Figure 5. Similarly the emf applied by way of the feeder 59 causes current to flow in the sheet 45 and rings 49, 51 in loops which embrace the slot 53 in the sheet 45 and are perpendicular to the plane 63, the two applied emfs being phased so that the loop currents in the two sheets 41 and 45 and rings 49, 51 are in the same sense. By induction further currents flow in loops perpendicular to plane 63 provided by the sheets 43 and 47 and the rings 49, 51, but in this case each loop includes-parts of both sheets 43 and 47, so that the currents in each sheet 43 or 47 at any instant are all in the same directions, as indicated by the plus and minus signs in Figure 5.

As a result, an RF magnetic field directed parallel to the plane 63 is produced.

Thus, in the arrangement of Figures 5 and 6 during excitation the loop currents flowing in the sheets 43 and 47 and rings 49, 51 together produce a field equivalent to that produced by the currents flowing in the sheets 1, 3, 5 and 7 of the arrangement of Figures 3 and 4, and the loop currents in each of the sheets 41 and 45 weaken the strong field in the gaps between the edges of plates 43 and 47, thus effectively performing the same role as the plates 27 and 31 in the arrangement of Figures 3 and 4 during excitation.

It will be appreciated that during detection similar current flows take place but with the role of sheets 41 and 45 reversed with that of sheets 43 and 47.

In a modification of the arrangement of Figures 5 and 6 the central slot 53 in each of the sheets 41, 43, 45 and 47 may be divided into two parts 53a, 53b, as illustrated in Figure 7, thereby modifying the loop currents which flow in the sheets.

It will be appreciated that in a further modification of the arrangement of Figures 5 and 6 excitation voltages may be applied between sheets 43 and 47 instead of across the slots 53 in sheets 41 and 45. The loop currents in sheets 41 and 45 are then produced during excitation by induction rather than directly by the excitation voltages, and the loop currents in sheets 43 and 47 are produced directly rather than by induction. It will be appreciated that the slots 53 in sheets 41 and 45 are then no longer necessary. Similarly outputs for detection purposes may be derived from between sheets 41 and 45 instead of across the slots 53 in sheets 43 and 47, which slots are then no longer necessary.

In general, the arrangement of Figures 5 and 6 may be expected to exhibit rather better uniformity and efficiency than the arrangement of Figures 3 and 4, but a higher inductance.

It is pointed out that an arrangement in accordance with the invention may be adapted for use in an NMR imaging apparatus for only excitation or detection purposes, and not both purposes. Thus the arrangement of Figures 3 and 4 may be adapted for excitation purposes only by omitting the feeders 19 and 23 and replacing the sheets 1 and 3 and the plate 29 between then by a first substantially semi-circular arcuate sheet (not shown), and replacing the sheets 5 and 7 and the plate 33 between them by a second such single sheet (not shown). The arrangement of Figures 5 and 6 may be similarly adapted by omitting the feeders 57 and 61 and the slots 53 in the sheets 43 and 47.

Claims (13)

1. An arrangement for use in generating or detecting RF magnetic fields comprising: at least two arcuate sheets of non-magnetic electrically conducting material disposed at spaced positions around the curved surface of a cylindrical volume so as to provide substantially axially extending gaps between the sheets at substantially diametrically opposite positions around said surface, the sheets being electrically connected to one another at each of their axial ends to provide closed loop paths for current flow in said sheets and interconnections embracing said gaps; and in each said gap, a further member of non-magnetic electrically conductive material arranged so that current flowing in a said loop constituted by said sheets and interconnections induces, or is induced by, current flowing in a said further member in a loop substantially parallel to said loop constituted by said sheets and interconnections but in the reverse direction.

2. An arrangement according to Claim 1 including at least one coupling means for applying an input for exciting the arrangement and/or deriving an output from the arrangement.

3. An arrangement according to Claim 2 wherein each said coupling means comprises a feeder connected between a pair of said sheets defining a said gap.

4. An arrangement according to Claim 2 wherein each said coupling means comprises a coupling loop positioned adjacent a said further member in a said gap.

5. An arrangement according to Claim 4 wherein the position of said coupling loop relative to the associated said further member is adjustable.

6. An arrangement according to Claim 4 wherein each said further member is in the form of a continuous member provided with a central axially extending slot and each said coupling means comprises a feeder connected betwen the edges of a said slot.

7. An arrangement according to any one of the preceding Claims comprising four said arcuate sheets.

8. An arrangement according to any one of Claims 1 to 6 comprising two said arcuate sheets.

9. An arrangement according to Claim 8 wherein said sheets and said further members are substantially identical so that the role of said sheets may be reversed with that of said further members.

10. An arrangement according to any one of the preceding claims wherein said closed loop paths incorporate capacitance.

11. An arrangement according to any preceding claims wherein said sheets are interconnected via two rings,, one at each end of said sheets.

12. An arrangement according to Claim 11 when dependent on Claim 10 wherein at least one of said rings is capacitively coupled to the adjacent ends of said sheets.

13. An'arrangement for use in generating or detecting- RF magnetic fields substantially as hereinbefore described with reference to figures 3 and 4 or Figures 5 and 6 of the accompanying drawings.