WO2000070356A1

WO2000070356A1 - Magnetically equivalent rf coil arrays

Info

Publication number: WO2000070356A1
Application number: PCT/US2000/013696
Authority: WO
Inventors: Michael J. Hennessy
Original assignee: Intermagnetics General Corporation
Priority date: 1999-05-19
Filing date: 2000-05-18
Publication date: 2000-11-23
Also published as: AU5441100A

Abstract

A group of matched, self-resonant high Q, high temperature superconducting coils (10) are positioned in a symmetric array (18) configuration and tuned using inductive coupling without need of complex circuitry. In the array, each coil is positioned to be magnetically equivalent to other coils in the array, for example, having the same number of neighbors, nearest neighbors, spacing to neighbors. The array acts as a single resonant system. In a coherent mode, all currents are in phase at resonance.

Description

MAGNETICALLY EQUIVALENT RF COIL ARRAYS This application claims the benefit of the prior filed copending United States of America provisional application 60/134 843 (filed May 19, 1999) .

This development was supported in part by the National Institute of Standards and Technology Award #70NANB5H1068.

BACKGROUND OF THE INVENTION

Methods for designing phased arrays for MRI (Magnetic Resonance Imaging) have historically taken advantage of the ability to overlap coils in order to cancel the mutual inductance between coils as reported by Roemer J.P.B. et al. (Magnetic Resonance Medicine 16, 192 (1990) . In this method each coil can be connected to a separate preamplifier and recombined in software to obtain a noise minimized image. Because of the relatively low Q of copper coils, coupling with the next nearest neighbor magnetic can be ignored and each coil behaves independently with a single resonance.

With the advent of ultra high Q superconducting MRI coils, this configuration is somewhat cumbersome to arrange from a practical viewpoint because of cryostat restrictions. The coils have to be physically aligned to minimize the mutual inductance. This is very difficult to do in a cryogenic container. HTS (high temperature superconducting) coils have extremely high Q's which means that the positioning relative to each other is very critical and magnetic coupling between the individual coils, even next nearest neighbors, results in multiple resonances .

What is needed is an array of RF HTS coils for MRI that performs independently as a single unit with a single resonance as did the low Q copper coils, but including the advantages inherent in high Q HTS coils. SUMMARY OF THE INVENTION

Recent progress has been made in the fabrication of high temperature superconducting (HTS) RF coil arrays for MRI applications. Several HTS based RF coils have been fabricated over the past decade. To maintain the high Q properties of HTS coils, the coils are usually designed as self-resonant resonators to avoid resistive contacts. The highest performance coils are fabricated with epitaxial HTS films deposited on a lanthanum aluminate substrate or sapphire substrate. These coils have Q's in the range of 10,000 to 100,000 depending on the frequency. One limitation of this technology is the quality of the epitaxial growth over large area depositions. The highest quality coils are commercially made with diameters of three inches or less. This construction is currently changing as improvements in materials and deposition equipment are being made. A well-known advantage for using small RF coils in MRI, especially when used as surface coils, is that the effective RF coil noise resistance due to the patient is lower, leading to a higher signal-to-noise ratio (SNR) . A well known disadvantage is that imaging depth of field is reduced. This disadvantage can be alleviated to some extent by building arrays of coils. When enough coils are assembled, a larger field of view can be achieved. The signal-to-noise ratio of an array of small coils is better than for a single large coil when the signals are properly summed.

To test the viability of HTS coils in a coil array an integrated cryogenic system was constructed that housed two HTS RF coils. The cryogenic system was cooled with liquid nitrogen and integrated with an Intermagnetics

General Corp. 0.15 T permanent magnet whole body imaging system. The cryogenic system was sealed with external ventilation to protect personnel from liquid nitrogen boiloff. The HTS coils were typically cooled to 77K. The HTS thin film material used for the RF coils was Tl₂Ba₂CaCu₂0₂ (TBCCO) deposited on both sides of a two inch diameter lanthanum aluminate substrate and patterned to form a resonant structure, resonating at the nuclear magnetic resonant frequency for protons in the nominal 0.15 tesla magnetic field (i.e., approximately 6.2 MHz. The unloaded Q's were measured to be above 10,000 for each coil .

The coils were mounted in the cryostat approximately 32 mm apart, and tuned and matched in the presence of the magnetic field. The coils, configured as receiver coils, were interfaced to the imaging console which was a narrow band 6.2 MHz system. The RF transmitter field was generated by a larger whole body coil.

The Q and resonant frequency of the HTS RF coils were substantially unaffected by the 0.15T field. The magnetic field was transverse (horizontal) to the bore of the magnet and the RF coil array was positioned on a plane perpendicular to the field, a direction favorable to HTS performance. Images were taken of a phantom filled with a solution of distilled water/doped copper sulfate solution (5 grams per liter) and NaCl (17 grams per liter) . The phantom, supplied by the Phantom Laboratory in Salem, NY was representative of a full size human head. Included in the phantom was another small plastic phantom partially filled with water which was used for calibration purposes.

The resultant images showed an enlarged field of view from two coils. Although the coils were only two inches in diameter, most of the head region was imaged due to the high sensitivity of the coils. The HTS coils were able to detect signals received far from the plane of the array. Symmetric array configurations which have been conceived, enable one to tune a group of high Q HTS coils using inductive coupling without the need of complex circuitry or direct electrical connections to the low temperature spaces. Though the configuration may not result in the absolute minimum noise figure image, it is a practical solution that can generate exceedingly good images, especially if several coils are used in a group. These configurations are herein called "magnetically equivalent RF coil arrays". What is meant by this term is that an array, comprising several matched self resonant coils, each coil having the same Q and resonant frequency, is positioned and oriented so that each coil in the array is magnetically equivalent to the other coils in the array. Each coil in the array has the same number of neighbors, nearest neighbors, etc., is spaced at the same distance, and has the same relative mutual inductance as every other coil in the array, respectively. If one coil is interchanged with another in the array it "sees" the same effective magnetic interactions with its new neighbors as before the interchange. In this magnetically equivalent configuration, it can be demonstrated empirically and mathematically that all coils have the same frequency response. The frequency response is complex, in general, but the modes can be sorted out in terms of in-phase and out-of-phase coherent modes. The in-phase component allows one to obtain an image with the least complexity.

In a receiving mode, the array or sub-arrays can be coupled to a single pick up loop or a series of pickup loops, having respective outputs which can be summed to obtain a single resonance signal. In this manner the coils in an array can be made to act collectively as a single resonant system. The most useful mode is the coherent mode where all currents are in phase at resonance.

Tests have verified that arrays with two and three planar ring coils are realizable. A four-coil set with two coils in each of two parallel planes, mirroring one another, has also been favorably tested. A six-coil set with three coils in each plane was being developed.

Accordingly, it is an object of the present invention to provide an array of high Q radio-frequency, high temperature superconducting coils for MRI that performs independently as a single unit with a single resonance. The invention accordingly comprises the features of construction, combinations of elements, an arrangements of parts that will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: Figs. 1 a-d schematically illustrate basic planar RF arrays in accordance with the invention;

Figs. 1 e-h schematically illustrate basic orthogonal RF arrays in accordance with the invention;

Figs. 2 a-c schematically illustrates paired RF arrays in accordance with the invention;

Figs. 3 a-b schematically illustrate paired oblique RF arrays in accordance with the invention;

Fig. 4 is a schematic representation of a self- resonant high Q coil; Fig. 5a is a top view of two coil arrays in two parallel planes, each array including two coils; and

Fig. 5b is an elevational view of the two coil arrays of Fig. 5a, and showing the parallel planes.

DESCRIPTION OF PREFERRED EMBODIMENTS

With regard to self-resonant circular coils, it will be understood by those skilled in the art that such a circular coil especially a superconducting coil, will not in practice necessarily be a coil of wires wrapped on a core. More likely the coil will be a substrate disk on whose surface or surfaces has been deposited a patterned layer of superconducting material. For purposes of discussion in this application, the coil 10 will be considered to be a circular disk 12, (Fig.4 ) having a geometric center 14. Capacitance is inherent in any winding of wires or pattern deposited on a surface. Thus, inductive and capacitive elements are present to make the pattern or coil self-resonant at a preselected resonant frequency. The resonators in accordance with the present invention are all very high Q (1000 to 100,000) as would be expected in using superconducting conductors without contact resistances.

The simplest or basic type of a magnetically equivalent array includes matched self-resonant coplanar coils uniformly spaced around a center point in a ring. Several other examples are also shown in the Figures.

There are also placements on a sphere (not shown) of coils which satisfy magnetic equivalence. Some arrays of coils are not easily configured around an object that is being imaged and sense RF fields parallel to the magnetic field. Such configurations are not of much value in conventional NMR although there may be some physical measurements which could benefit from the detection of RF fields in all directions .

The issue of magnetic equivalence can be treated rigorously mathematically with group theory employing all the symmetry groups of magnetic coupling, including angular symmetry, mirror symmetry and permutation symmetry. The complete classification of all groups which are magnetically equivalent requires additional study. Mathematical support for array constructions described herein is found in provisional application No. 60/134,843, filed by the present applicant on May 19, 1999, which application is incorporated herein in its entirety by reference. Accordingly, this mathematical material is not repeated here. Because each RF coil 10 is equivalent magnetically with respect to its neighbors, i.e., the magnetic coupling from one coil to its neighbors is the same in each position of the coil array, it can be shown mathematically and experimentally that in these magnetically equivalent coil arrays, a single resonant mode exists corresponding to a coherent, in-phase mode for the entire structure. Other modes also exist at other frequencies.

Coupling or pick-up loops 16, known in the MR arts, are used to detect the various resonant modes of the coil structure. Loops for pickup can be placed at each coil 10 of the array and received signals from the respective coils can be combined by using external circuitry, or a single loop 16 may be placed in a symmetric location where each coil contributes the same amount of flux to a received signal. Such locations for a pickup loop are shown in Figure la-d, in-plane ring structures, described hereinafter. The advantage of using a single pickup loop

16 to detect the performance of several coils 10 is the reduction in wiring complexity and reduced need for adjustment. The disadvantage is that it is difficult to compensate for small mismatches in coils 10 or coil positions .

Fig. la illustrates two equivalent self-resonant circular coils 10 lying in a common plane 18. A single pickup loop 16 lies symmetrically positioned between the coils 10 on the imaginary line connecting the coil centers. Fig. lb illustrates an array of three equivalent self resonant coils 10 lying in a common plane 18

(represented by the paper of the drawing) with the pickup loop 16 located symmetrically between the three coils 10. Fig. lc illustrates a coil array having four equivalent resonant circular coils 10 symmetrically positioned about a central pickup loop 16. Fig. Id illustrates an array of

N equivalent circular self resonant coils 10 equally spaced around a circle 20 having the pickup loop 16 located at the center 22 of the circle. As illustrated in

Figs, la-d, every coil 10 in each array lies in the same plane 18, represented in the Figures la-d as the plane of the paper.

Figs, le-h illustrate further coil arrays that have the same number of coils 10 respectively with magnetic equivalence as in the arrays of Figs. la-d. However, the coils of Figs, la-d have been pivoted by 90 degrees about a diameter 24 to produce Figs. le-h. The centers 14 of each coil 10 of an array and a center line (diameter) of each coil 10 remain in a common plane 18. The center 14 of each coil 10 in an array connects to the pickup loop 16 by an imaginary ray that is perpendicular to the plane of the coil itself as illustrated in Figs. le-h. There may be 2, 3, 4, ...N coils 10 in a basic array.

As illustrated in Fig. 2a-c, each of the basic arrays of Fig. la-h can be constructed with a coplanar pair of coils 10 in place of the single coil 10 at each position in Fig. la-h. Paired arrangements that would have been Fig. 2d and 2h corresponding to Fig. Id and lh are omitted from the drawings, for convenience in illustration. In each instance, the individual coils 10 are matched for the resonant frequency, and each pair of coil 10 is the magnetic equivalent of any other pair of coils 10 in the paired array. In each paired array of Fig. 2a-c, each coil 10 is coplanar (18) ith the other coils in the respective array.

It should be understood that the arrangements of Fig. 2a-c (and omitted Fig. 2d) can be rotated 90 degrees (not shown) so as to correspond to the constructions illustrated in Figs. le-h. Every pair in an array is the magnetic equivalent of any other pair in that same array and the coil centers are coplanar in any given array. Further, with reference to Figs. 3 a,b, the individual coils 10 of the paired arrays in Figs. 2a, b are illustrated rotated about their centers. However, the rotation is not 90 degrees as described above, but instead the individual coils 10 are rotated to a position oblique with the common plane 18 that passes through the coil centers in each array. The rotations are such that magnetic equivalence is maintained. Coplanar, perpendicular, and oblique orientations are possible for every magnetically equivalent array that has coplanar coil centers 14.

In summary, in each array whether a basic array or a paired array, each coil in that array has its geometric center 14 on the same plane 18. Each coil 10 is positioned for magnetic equivalence with each other coil

10 in that array. Each individual coil 10 is magnetically matched and self-resonant to the same RF frequency. The pickup loop 16 or loops are positioned so that each portion of an array, whether that portion includes, 2, 3, 4, ...N coils 10 contributes equally to the RF signals that are picked up from the overall array.

It should further be understood that whereas the number of individual coils 10 in the arrays of Figs, la-c were doubled to make the paired arrays of Fig. 2a-c (and those paired arrays omitted from the Figures) each coil 10 in a basic array of Figs, la-h can be increased in number by adding 1 (paired), 2, 3, 4, ... N additional coils 10 to provide a very complex array. In each construction, magnetic equivalence is maintained for each of the sub- arrays that together make up the complex array. As previously stated, every coil in every array discussed above has its center 14 located on the common plane of that particular array.

It should further be understood (Figs. 5a, b) that a pair of similar arrays of coils 10 may lay with the coil centers 14 of a first array in a first plane 18 and the coil centers 14 of a second array in a second plane 26. There are four coils 10 in two opposed pairs in the illustrated example. The arrays in the respective planes are mirror images of each other and are aligned in the respective first and second parallel planes in a mirror relationship of coils 10. Thus a test subject may be positioned between the two planes 18, 26. Each array in its respective pattern provides magnetic equivalence such that all of the coils 10 in one plane act together as a single system having a single frequency of operation for purposes of receiving RF signals from a test subject.

It should also be understood that magnetic equivalence and behavior of a plurality of magnetically matched coils as a single system may be achieved with a spherical, symmetrical placement of coils 10. The array that is in a first plane 18 and is used with a similar array in a parallel second plane 26 may have a configuration of coils 10 that is either a basic configuration, a paired configuration, or a more complex configuration. In every instance magnetic equivalence is achieved in the array of the respective plane.

High Q coils are used in the arrays discussed above. It is proposed that these high Q values be achieved by use of high temperature superconductive coils, which have Q values in the range to 1000 to 100,000. However, the applications of the present inventions are not to be considered as limited to high Q coils produced only with superconductive materials, nor is the invention limited to self-resonant coils. Coils may be tunable. Placement of individual coils relative to each other is difficult and requires high precision when working with high Q superconductive coils because one coil can even affect non-neighbor coils, resulting in the possibility of many frequencies from each coil. However, in accordance with the invention the symmetrical arrangement of individual coils gives the same frequency performance for each coil (although there may be more than one frequency) . Each coil acts the same. The signals from each frequency are the same and in-phase when using resonant frequency plus a differential. (The differential in coil frequency response results from coupling between the coils as discussed in the provisional application and hereinafter) . The symmetrical arrangement of individual coils forms a single collective in phase mode resonating at a common frequency. When a self resonant RF coil is placed in the vicinity of equivalent others, the flux generated by a first coil is picked up or coupled into the others. The amount of flux coupled between two coils is related to the magnetic coupling coefficient which is a function of the mutual inductance between the coils and self inductance of the individual coils. When two coils at a resonant frequency approach one another, the coupling causes the single resonance to split into two resonances. The degree of splitting is related to the magnetic coupling coefficient. The more there is coupling, the more there is splitting. The two resonances occur at fo+df/2 and fo-df/2, where df is the frequency split, and is approximately proportional to the magnetic coupling. (This concept is valid if the two RF coils are very high Q coils, having a bandwidth smaller than df) . Magnetic coupling not only causes the appearance of two resonances but also forces the coils to act in unison on resonance rather than independently.

As an example, consider the case of two RF coils in a plane. Assume the coils are not overlapping. In this case, currents in the coils at the upper resonance are flowing in phase with one another. At the lower resonance, the currents flow opposite to one another. These currents exhibit a collective phenomena referred to as current modes. There are two modes, the in phase mode and out of phase mode for the two coil system.

When three magnetically equivalent coils are placed together, the picture is more complex. For simplicity, assume all coils are in the same plane and are not overlapping. Assume that two coils Cl and C2 are near each other and another coil C3 is far away. In this case, the two coils closest together act in unison generating two resonances as described above. The other coil C3 does not "feel" the Cl and C2 because C3 is far away and resonates independently at its own self resonance fo. The collection of coils exhibits three resonances, corresponding to two collective modes (the in phase and out of phase mode) and one independent mode C3. Two resonances occur at fo+df/2 and fo-df/2 and one at fo . The two coils Cl and C2 are in phase at the highest frequency fo+df/2, but the third coil is far away from this resonance and does not contribute a signal or current. At the intermediate frequency fo, only the third coil C3 is resonant and behaves independently from Cl and C2. At the lowest frequency fo-df/2, Cl and C2 partake in this resonance with their currents out of phase from each other. The third coil C3 does not contribute a signal or current at the frequency fo-df/2 because its resonance is far away.

When the third coil is moved into a magnetic equivalent position, equidistant from the other coils, its behavior changes from an individual coil response to part of a collective response. There are three collective modes, but the resonant frequencies shift. For this case there are only two resonant frequencies. One mode corresponds to the highest frequency. Two modes resonate at the same lower frequency (degeneracy) . The frequency shifts due to the fact that more flux is interchanged. The highest frequency fo+df corresponds to all three coils being in phase. Other modes are mixed modes and contain currents which are unequally weighted in phase and out of phase currents. At other geometric positions which are not magnetically equivalent, there are three separate resonances, all of which are mixed modes.

For larger numbers of coils in a plane, the same phenomena occur. In the case of four RF coils magnetically equivalent positioned, there are four independent modes, one of which corresponds to the in phase mode which resonates at the highest frequency. The other modes are mixed modes. At non-magnetically equivalent positions, all the modes are mixed modes. The same is true for a larger number of coils.

Claims

What is claimed is:

1. A first Rf coil array comprising: a plurality of first matched, resonant coils, each said first coil having high Q, said first coils being coupled and geometrically positioned relative to each other in said first array, each first coil being magnetically equivalent to each other first coil of said array and having a same frequency response, said array of first coils acting as a collective resonant system.

2. A first RF coil array, as in claim 1, wherein said selected resonant frequency mode is in-phase for each said first coil.

3. A first RF coil array, as in claim 1, wherein said first coils have geometric centers in a first common plane .

4. A first RF coil array as in claim 3, wherein said first coils are substantially planar, and one of coplanar with said first common plane, or orthogonal to said first common plane, and oblique to said first common plane.

5. A first RF coil array as in claim 4, wherein the number of first coils is 2, 3, 4... N.

6. A first RF coil array as in claim 5, wherein each said first coil is paired with a similar second coil, each said first coil and said second coil in a pair being in combination a magnetic equivalent of every other said pair in a paired array.

7. A first RF coil array, as in claim 5, wherein each said first coil is positioned with a respective plurality of similar second coils, said respective second coils associated with each said first coil numbering 2, 3, 4....N, each said first coil and each said plurality of said second coils being a combination that is a magnetic equivalent of every other combination of said first coils and second coils to form a complex array.

8. A first RF coil array as in claim 3, further comprising a second coil array similar to said first coil array, said second coil array including resonant coils having geometric centers in a second common plane, said first common plane and said second common plane being parallel to each other, said first coil array and said second coil array mirroring each other and being magnetically equivalent to each other.

9. A first RF coil array as in claim wherein said coils are self resonant.

10. A first RF coil array as in claim 9, wherein said coils are superconductive.

11. A first RF coil array as in claim 10, wherein said coils are high temperature superconductive material.

12. A first RF coil array as in claim 4 wherein said coils are self resonant.

13. A first RF coil array as in claim 12, wherein said coils are superconductive.

14. A first RF coil array as in claim 13, wherein said coils are high temperature superconductive material.

15. A first RF coil array as in claim 6 wherein said coils are self resonant.

16. A first RF coil array as in claim 15, wherein said coils are superconductive.

17. A first RF coil array as in claim 16, wherein said coils are high temperature superconductive material.

18. A first RF coil array as in claim 8 wherein said coils are self resonant.

19. A first RF coil array as in claim 18, wherein said coils are superconductive.

20. A first RF coil array as in claim 19, wherein said coils are high temperature superconductive material.

21. A first RF coil array as in claim 1, further comprising at least one RF signal pickup located relative to each said first coil, said at least one RF signal pickup outputting a signal that represents a signal from each said first coil in said array.

22. A first RF coil array as in claim 21, wherein said signal output by said at least one pickup represents each said first coil equally.

23. A first RF coil array as in claim 2, further comprising at least one RF signal pickup located relative to each said first coil, said at least one RF signal pickup outputting a signal that represents a signal from each said first coil in said array, said at least one pickup for sensing said in phase resonant frequency mode.

24. A first RF coil array as in claim 23, wherein said signal output by said at least one pickup represents each said first coil equally.

25. A first RF coil array as in claim 4, further comprising at least one RF signal pickup located relative to each said first coil, said at least one RF signal pickup outputting a signal that represents a signal from each said first coil in said array, said pickup being tuned to an in-phase resonant frequency mode.

26. A first RF coil array as in claim 25, wherein said signal output by said at least one pickup represents each said first coil equally.

27. A first RF coil array as in claim 6, further comprising at least one RF signal pickup located relative to each said coil, said at least one RF signal pickup outputting a signal that represents a signal from each said coil in said array, said pickup for sensing said in- phase resonant frequency mode.

28. A first RF coil array as in claim 27, wherein said signal output by said at least one pickup represents each said coil equally.