WO1992017799A1

WO1992017799A1 - Rf saddle coil for use with pulsed magnetic fields

Info

Publication number: WO1992017799A1
Application number: PCT/US1992/002738
Authority: WO
Inventors: Xuan Z. Ni; Howard Hill
Original assignee: Varian Associates, Inc.
Priority date: 1991-04-05
Filing date: 1992-04-03
Publication date: 1992-10-15

Abstract

A simple RF saddle coil structure includes at least two members (42, 40) parallel with a z axis and terminating on two ring shaped members (46, 48). The introduction of at least one slot (70) through one parallel member (40) and the associated ring members (46, 48) removes circulating eddy currents from the ring members when a pulsed, z-directed magnetic field is applied.

Description

DLE COIL FOR USE WITH PULSED MAGNETIC FIELD

Field of the Invention The present invention relates generally to instrumentation based upon magnetic resonance phenomena and particularly relates to reduction of transient magnetic field perturbations occasioned by eddy currents.

Background of the Invention

In the practice of modern Fourier transform nuclear magnetic resonance, time dependent magnetic fields can introduce undesirable transient effects. One such effect is the inducement of eddy currents in local conductive structures with the resulting occurrence of transient magnetic effects due to these eddy currents. Pulsed magnetic gradients are commonly associated with NMR instruments employed for the study of diffusion effects or for the spatial resolution of NMR phenomena.

The sample volume is ordinarily limited by the bore of a superconducting magnet housed in a cryostat of complex construction.

The pulsed gradient coils occupy a peripheral region of the bore and the RF coil occupies another (inner) region within the bounds of the gradient coil(s). As a result of the pulsing of a gradient coil, eddy currents may be induced on the inner structural members of the cryostat and also on the RF coil conductor disposed within the gradient coil(s).

The eddy currents decay exponentially and there results an exponentially time dependent contribution to the magnetic field experienced by the sample. There is substantial interest in the reduction of the effects of eddy currents. The present work is directed to reduction of eddy currents induced on the RF coil. In the present invention the reduction is achieved by distinct structural features. A known single turn RF saddle coil structure (shown in FIG.

2(a)) comprises two parallel conductors 40 and 42 both of which terminate on an upper ring-shaped conductor 46. A lower ring shaped conductor 48 is disposed in contact with one of the par-axial conductors 40 and that one, or the ring 48 itself, provides one terminal or feed to the coil. The second par-axial conductor 42 is disposed so as not to contact the lower ring 48, and it forms the return for the RF current. The structure was intended to provide for a constant axial distribution of coil conductor material in the sensitive region of an NMR instrument. This structure is described in U.S. 4,563,648, commonly assigned herewith.

Another RF coil structure of prior art is described in U.S. 4,641,098 and resembles the aforedescribed structure in providing an upper and lower ring structure with two par-axial conductors linking the lower and upper rings. The feed and return terminals are provided by introducing isolation in the form of a pair of gaps in the lower ring with capacitors inserted therebetween, following the tracking of Alderman and Grant, J. Mag. Res., v.36, pp. 447-451 (1979). Patent 4,641,098 directs that for each one of an even number of RF current loops, there be inserted at least one slot collinear with the resonant current path and these slots provide an interruption in eddy current path for one conditioning ring.

Both of the above-discussed references were directed to the RF energy applied to the coil from an RF power source and/or intercepted by the coil from resonating nuclei of a sample within the coil volume. Structure following the above references was anticipated to be operated within a steady magnetic field. The use of pulsed magnetic gradient coils in proximity to RF coil structure introduces eddy currents on such coil structure which are effectively reduced by use of the present invention.

In one embodiment of the present invention, a single turn saddle coil is disposed about a cylindrical locus forming two current loops on opposite (azimuthal) sides of a cylindrical locus. The loops are formed by conductive portions parallel to the axis and by cylindrical (ring) portions joining the par-axial portions. In the present invention, one of the par-axial members includes a slot along the entire axial length thereof which also interrupts the end ring portions. The slot effectively interrupts an eddy current which would appear around (continuous) ring portions and separates one of the par-axial portions into two sub-portions. These sub-portions support parallel current flow originating from one terminal of the coil. Another embodiment combines the above structure with a slotted coaxial cylinder disposed within one of the ring portions. The outer ring comprises additional slots situated on either side of the full length slot to provide for RF terminals geometrically antisymmetrical with respect to the full length slot.

A Brief Description of the Figures

FIG. 1 shows the general context of the system embodying this invention. FIG. 2 shows a prior art coil.

FIG. 2(b) shows the coil of FIG. 2(a) unrolled onto a plane. FIG 3(a) shows another known RF coil. FIG. 3(b) shows the RF coil of FIG. 3(a) unrolled onto a plane. FIG. 4(a) shows an embodiment of the present invention. FIG. 4(b) shows the RF coil of FIG. 4(a) unrolled onto a plane.

FIG. 5 shows a pulse diagram for examining eddy current effects. FIG. 6 is another embodiment of the invention.

FIG. 7 compares the Q of a prior art coil of FIG. 2b with the coil of FIG. 6.

FIG. 8 shows another embodiment of the invention.

Detailed Description of the Present Invention

Portions of a typical NMR data acquisition instrument are schematically illustrated on FIG. 1. An acquisition/control processor 10 communicates with an RF transmitter 12, modulator 14 and receiver 16, including analog-to-digital convertor 18 and a further digital processor 20. The modulated RF power irradiates an object

(not shown) in a magnetic field 21 through a probe assembly 22 and response of the object is intercepted by probe 22 communicating with receiver 16. The response typically takes the form of a transient time domain waveform or free induction decay. This transient waveform is sampled at regular intervals and samples are digitized in adc 18. The digitized time domain wave form is then subject to further processing in processor 20. The nature of such processing may include averaging the time domain waveform over a number of similar of such waveforms and transformation of the average time domain wave form to the frequency domain yields a spectral distribution function directed to output device 24. Alternatively this procedure may thus be repeated with variation of some other parameter and the transformation(s) from the data set may take on any of a number of identities for display or further analysis. The magnetic field 21 which polarizes the sample and defines the Larmor frequency thereof, is established by an appropriate means, not shown. Coil(s) 19 are employed for imposing a desired spacial and time dependence of magnetic field.

FIG. 2(a) is one prior art single loop saddle coil which includes in its construction a gap 50 and ring portion 48. This gap 50 would (fortuitously) serve to reduce circulating eddy current induced from a pulsed magnetic field in proximity thereto and having an axial component. The upper ring 46 would support such induced eddy currents, in contrast to the lower ring. The RF current supported in the lower ring depends upon the capacitance between ring 48 and conductor 42 across gap 50.

FIG. 2(b) is a variant of FIG. 2(a) and does not include the gap 50. This form of coil serves conveniently as a standard by which to reference performance for the present invention.

FIG. 3(a) is another prior RF coil, discussed above, which includes slots 56 and 58 and chip capacitors 60 and 62 respectively bridging gaps 64 and 66 in a lower ring portion 68. This structure is mapped onto a plane in FIG. 3(b) with the eddy current distribution qualitatively shown. (The edges BB and B'B' coincide to form the structure of FIG. 3(a). One observes that the circulations of an eddy current in one ring, e.g., upper ring, is interrupted by the slots 56 and 58 whereas the lower ring 68 supports a transient current through chip capacitor 60 and 62 depending upon the values of the capacitances. This prior art is similar to that of

FIG. 2a where the circulating current is interrupted in one ring only to the extent of the capacitance between corresponding members.

Turning now to FIG. 4(a) there is shown one embodiment of the present invention. The structure resembles that of FIG. 2(b), with the addition of slot 70 which removes the possibility of circulating eddy currents around both ring members 46 and 48. In FIG. 4(b), the coil structure of FIG. 4(a) is mapped onto a plane and a qualitative current distribution is indicated. (The edges AA and A Α' coincide to form the structure of FIG. 4(a).) The single slot 70 is placed in such a position as to leave unaffected the RF current paths compared to prior art but to introduce substantially different conditions for circulating eddy currents introduced by pulsed magnetic fields. The physical comparison of the embodiment of FIG.

4(a) with the prior art coil of FIG. 2(b) was effected by measuring, for the identical system and sample, the NMR responses as a function of time as described in FIG. 5 where a magnetic transition (typically in a gradient coil) is followed after an interval τ with an RF pulse. This excitation is then observed and transformed to yield a spectral distribution. This spectral distribution will, for sufficiently short values r, exhibit an effective magnetic field which includes a decaying component indicated by the dotted line due to eddy currents. Preliminary results were obtained in the form of a set of spectral responses for a variety of intervals r in the range .001 to 1 second for the system including a coil of the type indicated in FIG. 2(a) and separately for the identical system employing the coil of FIG. 4(a). Over the range of r surveyed the spectral characteristic of the FIG. 4a coil exhibited independence of the value of r over a greater range than was observed for the FIG. 2a coil.

FIG. 6 is a variation of the prior art structure of FIG. 2b where the terminal extension of conductor 42 of that prior art is extended azimuthally to form lower ring 49 (FIG. 6) in known manner. The variation employed here adds two slots 82 and 84 to the outer ring 49. This structure is driven from a balanced feed derived from an RF source, not shown, instead of employing inner ring 48 (of FIG. 2b) for one such terminal.

This structure does not inhibit circulating eddy currents in either the upper ring or the lower, inner ring. The balanced RF feed provides that the floating inner ring 84 and the common ring 85 behave as virtual grounds. The performance of a structure following FIG. 6 has been compared with a prior art coil (FIG. 2b including outer ring 49). The Q of each of these coils was compared by measurement as a function of distance along the axis for insertion of a capillary tube (from the top direction in the drawings) containing a sample substance. The Q value is derived from measurement of the RF pulse duration (for constant amplitude) required to obtain a 180° nutation of sample spins. The Q of the coil is proportional to the inverse square of the 180° pulse duration. This comparison is shown in FIG. 7. Q as a function of displacement along the axis of the coil was found to decrease as expected with the capillary insertion and also found to continue to decrease as the insertion of the capillary tip continued beyond the operative edge 69 into the lower ring structure 49 of the prior art coil. For the coil structure of FIG. 6, the measured Q became independent of insertion length beyond the corresponding aperture edge 69' as shown by the dotted line as the capillary tip entered the region surrounded by the ring structure 49. In FIG. 8 there is shown another embodiment. The construction here follows that of FIG. 4a with the addition of slots 64 and 66 in the lower ring 68 to provide for a balanced feed from an RF source not shown. An electrically floating inner coaxial lower ring 72 is provided to furnish desired capacitance coupling between portions of the (segmented) lower ring. This inner coaxial ring is also slotted to substantially eliminate support for circulating eddy currents. The slot in the inner coaxial ring 72 is properly aligned with slot 70 or aligned in 180° relationship with slot 70. That is, the slot coincides with a locus of vanishing RF current distribution. This structure provides the benefits of shielding provided by the FIG. 6 embodiment with the substantial elimination of eddy currents in the ring members as shown in the embodiment of FIG. 4a. In operation with the balanced RF feed applied to terminals 90 and 92 the conductor 60a and that portion of the lower ring between gaps 70 and 66 are inductively coupled to the symmetrically corresponding portions of the coil with the result that the interior value of the coil is uniformly irradiated by an RF field of TM-like symmetry. In alternative arrangements for excitation of the coil structure of FIG. 8., the balanced RF feed may be applied to terminals 92 and 94 or terminals 90 and 94. Both of these alternatives are rich in RF resonant response requiring values for the several capacitances and inductances even for quantitative analysis. In general, the simplest excitation via terminals 92 and 94 yields an RF field distribution of

TE-like symmetry (which is not useful for NMR purposes if the polarizing magnetic field coincides with the axis of the coil structure). Excitation via terminals 90 and 94 produces a radially directed RF magnetic field gradient which is useful for certain classes of NMR measurement.

It is recognized that slot 70 need not be continued through lower ring 68 in the embodiment of FIG. 8 because slots 66 and 68 serve the purpose of impeding a circulating eddy current.

It is to be understood that many changes can be made in the specifically described embodiments without departing from the scope of the invention and that the invention is to be determined from the scope of the following claims, without limitation to the specifically described embodiments.

Claims

What is Claimed is:

1. An RF saddle coil comprising:

(a) at least two par-axial conductor segments disposed parallel to an axis located equi-distant therebetween to define a cylindrical locus of radius of r,

(b) a first ring conductor of radius r disposed at a first end of said par-axial conductor segments in contacting both said par-axial conductor segments, (c) a second ring conductor disposed in a plane orthogonal to said axis and contacting one said par-axial conductor segment, the other par-axial conductor segment disposed electrically isolate from said second ring conductor, said first and second ring conductors and a first par-axial conductor segment including a continuous slot to interrupt eddy currents induced in said RF saddle coil, said second ring conductor comprising one terminal of said RF saddle coil and second par-axial conductor segment proximate said second ring conductor comprising a second terminal of said coil.

2. An RF saddle coil comprising:

(b) a first ring conductor of radius r disposed at a first end of said par-axial conductor segments in contacting both said par-axial conductor segments,

(c) a second ring conductor disposed in a plane orthogonal to said axis and contacting one said par-axial conductor segment, the other par-axial conductor segment disposed electrically isolate from said second ring conductor, said first and second ring conductors and a first par-axial conductor segment including a continuous first slot dividing said RF saddle coil into two coil portions whereby eddy currents induced in said first and second rings of said RF saddle coil, are integrated, said second ring conductor comprising two further slots disposed on either side said first slot whereby said second ring comprises three portions, and said second ring conductor comprises two terminals connected to ring portions adjacent one said further slot.

(d) a third ring conductor disposed coaxially within said second ring conductor, said third ring conductor comprising a slot to interrupt eddy currents circulating azimuthally therein.

3. The RF saddle coil of claim 2 wherein said two further slots of said second ring are disposed symmetrically about said first slot.

4. The RF saddle coil of claim 3 further comprising a balanced feed RF source adapted for communication with said two terminals.

5. A system for excitation and observation of magnetic resonance phenomena, comprising polarizing magnetic field means oriented to providing a steady magnetic field along a first axis, pulsed magnetic field means for generating a time dependent magnetic field along a selected axis, RF source means for providing RF energy to irradiate a sample at the Larmor frequency thereof to excite magnetic resonance of said sample, said RF source means further comprising:

(c) a second ring conductor disposed in a plane orthogonal to said axis and contacting one said par-axial conductor segment, the other par-axial conductor segment disposed electrically isolate from said second ring conductor, said first and second ring conductors and a first par-axial conductor segment including a continuous first slot dividing said RF saddle coil into two coil portions whereby eddy currents induced in said first par-axial conductor segment includes a continuous slot to interrupt eddy currents induced in said rf saddle coil, due to said pulsed magnetic field means.