Abstract:
A dual-tuned volume coil for performing MR imaging according to one embodiment includes an inner cylinder having a first coil structure disposed on an inner surface thereof and a second coil structure disposed on an outer surface thereof. The first coil structure operates at a first resonance frequency and the second coil structure operates at a second resonance frequency. The volume coil includes an outer shield disposed about the inner cylinder, with the first and second coil structure being connected to the outer shield by means of a plurality of capacitors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. patent application Ser. No. 60/670,605, filed Apr. 11, 2005, which is hereby incorporated by reference in its entirety. The present application is also related to and incorporates by reference in its entirety, International Patent Application No. PCT/US2004/027532, entitled MICROSTRIP COIL DESIGN FOR MRI APPARATUS, filed on Aug. 23, 2004 and published as WO 2005/020793 A2 on Mar. 10, 2005. 
     
    
     BACKGROUND  
       [0002]     Microstrip radio frequency (RF) transmission line coil technology for magnetic resonance (MR) imaging developed by Insight Neuroimaging Systems, LLC (the assignee of the present application) has been successfully applied to a diverse set of magnet systems, ranging from 3 T human to 11.7 T animal scanners. The coils have been developed for linear and quadrature mode of operation and can function in combination with separate receiver coils.  
         [0003]     In conventional MR imaging applications, the RF coils are tuned to the hydrogen ( 1 H) resonance frequency determined by the main magnet field strength. Depending on the biomedical application, however, it may prove desirable to extend the RF coil capabilities beyond proton resonance imaging. For instance, by tuning to other atom resonance frequencies such as phosphorous, fluoride, or carbon, the range of applications for the RF coils can be significantly extended.  
       SUMMARY  
       [0004]     A dual-tuned volume coil for performing MR imaging according to one embodiment includes an inner cylinder having a first coil structure disposed on an inner surface thereof and a second coil structure disposed on an outer surface thereof. The first coil structure operates at a first resonance frequency and the second coil structure operates at a second resonance frequency. The volume coil includes an outer shield disposed about the inner cylinder, with the first and second coil structure being connected to the outer shield by means of a plurality of capacitors.  
         [0005]     A dual-tuned volume coil includes a single inner coil structure having a plurality of microstrips that operate at two different resonance frequencies to provide two imaging modes and an outer shield that is electrically coupled to the inner coil structure. The widths of the microstrips vary as a function of their location about the coil. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0006]     The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of the illustrative embodiments of the invention where like reference numbers refer to similar elements and in which:  
         [0007]      FIG. 1A  is a cross-sectional view of a dual-tuned resonator volume coil according to one exemplary embodiment of the present invention;  
         [0008]      FIG. 1B  is a perspective view of the dual-tuned resonator volume coil according to  FIG. 1A ;  
         [0009]      FIG. 2A  shows a reflection coefficient (S 11 ) as a function of frequency for a dual-tuned resonator volume coil in accordance with a first embodiment of the present invention and at a lower frequency of operation;  
         [0010]      FIG. 2B  shows the B 1  field behavior for the dual-tuned resonator volume coil in accordance with the first embodiment, at the lower frequency of operation;  
         [0011]      FIG. 3A  shows a reflection coefficient (S 11 ) as a function of frequency for the dual-tuned resonator volume coil in accordance with the first embodiment of the present invention and at a higher frequency of operation;  
         [0012]      FIG. 3B  shows the B 1  field behavior for the dual-tuned resonator volume coil in accordance with the first embodiment, at the higher frequency of operation;  
         [0013]      FIG. 4A  shows a reflection coefficient (S 11 ) as a function of frequency for a dual-tuned resonator volume coil in accordance with a second embodiment of the present invention and at a lower frequency of operation;  
         [0014]      FIG. 4B  shows the B 1  field behavior for the dual-tuned resonator volume coil in accordance with the second embodiment, at the lower frequency of operation;  
         [0015]      FIG. 5A  shows a reflection coefficient (S 11 ) as a function of frequency for the dual-tuned resonator volume coil in accordance with the second embodiment of the present invention and at a higher frequency of operation;  
         [0016]      FIG. 5B  shows the B 1  field behavior for the dual-tuned resonator volume coil in accordance with the second embodiment, at the higher frequency of operation;  
         [0017]      FIG. 6A  shows a reflection coefficient (S 11 ) as a function of frequency for a dual-tuned resonator volume coil in accordance with a third embodiment of the present invention and at a lower frequency of operation;  
         [0018]      FIG. 6B  shows the B 1  field behavior for the dual-tuned resonator volume coil in accordance with the third embodiment, at the lower frequency of operation;  
         [0019]      FIG. 7A  shows a reflection coefficient (S 11 ) as a function of frequency for the dual-tuned resonator volume coil in accordance with the third embodiment of the present invention and at a higher frequency of operation;  
         [0020]      FIG. 7B  shows the B 1  field behavior for the dual-tuned resonator volume coil in accordance with the third embodiment, at the higher frequency of operation;  
         [0021]      FIG. 8A  shows a reflection coefficient (S 11 ) as a function of frequency for a reference volume coil operating at a lower frequency of operation;  
         [0022]      FIG. 8B  shows the B 1  field behavior for the reference volume coil at the lower frequency of operation;  
         [0023]      FIG. 9A  shows a reflection coefficient (S 11 ) as a function of frequency for reference volume coil at a higher frequency of operation;  
         [0024]      FIG. 9B  shows the B 1  field behavior for the reference volume coil at the higher frequency of operation;  
         [0025]      FIG. 10A  shows a reflection coefficient (S 11 ) as a function of frequency for a dual-tuned, single layer resonator volume coil in accordance with a fourth embodiment of the present invention and at a lower frequency of operation;  
         [0026]      FIG. 10B  shows the B 1  field behavior for the dual-tuned, single layer resonator volume coil in accordance with the fourth embodiment, at the lower frequency of operation;  
         [0027]      FIG. 11A  shows a reflection coefficient (S 11 ) as a function of frequency for a dual-tuned, single layer resonator volume coil n accordance with the fourth embodiment of the present invention and at a higher frequency of operation;  
         [0028]      FIG. 11B  shows the B 1  field behavior for the dual-tuned, single layer resonator volume coil in accordance with the fourth embodiment, at the higher frequency of operation;  
         [0029]      FIG. 12A  shows a reflection coefficient (S 11 ) as a function of frequency for a reference volume coil operating at a lower frequency of operation;  
         [0030]      FIG. 12B  shows the B 1  field behavior for the reference volume coil at the lower frequency of operation;  
         [0031]      FIG. 13A  shows a reflection coefficient (S 11 ) as a function of frequency for reference volume coil at a higher frequency of operation; and  
         [0032]      FIG. 13B  shows the B 1  field behavior for the reference volume coil at the higher frequency of operation. 
     
    
     DETAILED DESCRIPTION  
       [0033]     In an exemplary embodiment, the present invention provides a dual-tuned RF transmit/receive coil  100  for MR imaging that has the ability to tune selectively to two separate resonance frequencies.  
         [0034]     The present invention relates to resonator volume coils  110 ,  120  that enable imaging at two different resonance frequencies. In a first exemplary embodiment, two widely spaced resonance frequencies, such as 170 MHz ( 1 H imaging at 4 Tesla) and 40 MHz ( 13 C imaging at 4 Tesla) are achieved using two separate coil structures  110 ,  120  placed on the inner and outer surfaces, respectively, of an inner cylinder and connected to an outer cylindrical shield  130  via fixed and/or variable tuning capacitors  140 .  FIGS. 1A and 1B  show cross-sectional and perspective views, respectively, of such an embodiment. The aforementioned capacitors  140  can be seen in  FIG. 1B .  
         [0035]     Several variants of the exemplary two-layer coil  100  of  FIGS. 1A and 1B  are possible in accordance with the present invention and are referred to herein as variants I, II and III. In a first variant, variant I, the inner coil  110  of the two-layer coil is the lower frequency coil. In variant II, the inner coil  110  is the higher frequency coil and in variant III, the inner coil  110  is the lower frequency coil and extends beyond the outer coil  120 .  
         [0036]     Simulations were conducted to compare the field linearity performance of the exemplary, two-layer coil  100  of the present invention to that of a conventional, single-layer microstrip coil, referred to as the “reference coil” and shown schematically in cross-section in  FIGS. 8B and 9B  and again in  FIGS. 12B and 13B .  
         [0037]     An exemplary embodiment of coil  100  according to the present invention has the following parameters:  
         [0000]     Shield Support Cylinder  
         [0000]    
       
         
           
              OD=8.313 in (211 mm)  
              ID=8.188 in (208 mm)  
              Wall thickness=0.063 in (1.6 mm)  
              Material=acrylic 
 
 Coil Support Cylinder 
 
              OD=7 in (178 mm) (location of outer coil)  
              ID=6.5 in (165 mm) (location of inner coil)  
              Wall thickness=0.25 in (6.4 mm)  
              Material=acrylic 
 
 Coil length=5 in (127 mm) 
 
 Number of strips=8 per layer (2 layers) 
 
 Using “optimal” strip width for single-layer coils (a function of ID/OD): 
 
              Relative strip width=66.7% of center-to-center spacing  
              Inner coil strip width=1.701 in (43 mm)  
              Outer coil strips width=1.833 in (47 mm) 
 
 Outer coil is rotated 22.5° with respect to the inner coil. 
 
 Operating frequencies for the coil  100  in accordance with one embodiment of the present invention: 
 
 f   low =40.8 MHz 
 
 f   high =170.2 MHz
 
           
         
       
     
         [0049]     Table I summarizes the results of the simulations for the three variants of coil  100  and the reference coil implementations.  
                                                                                                       TABLE I                                       Lower frequency   Higher frequency                    B 1     Normalized           B 1     Normalized                   (linear),   B 1 ,   Mode       (linear),   B 1 ,   Mode       Coil Type   Q   μT/{square root over (W)}   μT/{square root over (Q · W)}   separation   Q   μT/{square root over (W)}   μT/{square root over (Q · W)}   separation                    Variant I:   396   6.61   0.332   14.5%   320   0.517   0.0289   1.3%       Dual coil, inner       coil is the low       frequency coil       Variant II:   326   4.53   0.251   11.5%   328   2.08   0.115   4.5%       Dual coil, inner       coil is the high       frequency coil       Variant III:   432   6.35   0.305   13.2%   464   0.842   0.0391   27.1%        Dual coil, inner       coil is the low       frequency coil       and extends       beyond outer       coil       Reference   433   7.30   0.351    9.5%   699   4.61   0.174   9.4%       design (single       coil)                  
 
         [0050]     Exemplary embodiments of the aforementioned variants will now be described in greater detail.  
         [0051]     An exemplary embodiment of Variant I, in which the lower frequency coil is on the inside  110 , has the following capacitor values: 
        C low =651 pF     C high =65.1 pF        
 
         [0054]     For the lower frequency (e.g., 40.8 MHz) operation of this embodiment: 
        Q=396     B 1 =6.61 uT/sqrt(W)     B 1 (Q-normalized)=0.332 uT/sqrt(Q*W)     Mode separation=14.5%        
 
         [0059]      FIG. 2A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the lower frequency of operation.  
         [0060]      FIG. 2B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the lower frequency of operation.  
         [0061]     For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment: 
        Q=320     B 1 =0.517 uT/sqrt(W)     B 1 (Q-normalized)=˜0.0289 uT/sqrt(Q*W)     Mode separation=1.3%        
 
         [0066]      FIG. 3A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the higher frequency of operation  
         [0067]      FIG. 3B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the higher frequency of operation.  
         [0068]     An exemplary embodiment of Variant II, in which the higher frequency coil is on the inside  110 , has the following capacitor values: 
        C low =798.7 pF     C high =52.2 pF        
 
         [0071]     For the lower frequency (e.g., 40.8 MHz) of operation of this embodiment: 
        Q=326     B 1 =4.53 uT/sqrt(W)     B 1 (Q-normalized)=0.251 uT/sqrt(Q*W)     Mode separation=11.5%        
 
         [0076]      FIG. 4A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the lower frequency of operation  
         [0077]      FIG. 4B  shows the B 1  field behavior (black contour: ±1dB boundary) for the same embodiment, at the lower frequency of operation.  
         [0078]     For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment: 
        Q=328     B 1 =2.08 uT/sqrt(W)     B 1 (Q-normalized)=0.115 uT/sqrt(Q*W)     Mode separation=4.5%        
 
         [0083]      FIG. 5A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the higher frequency of operation  
         [0084]      FIG. 5B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the higher frequency of operation.  
         [0085]     An exemplary embodiment of Variant III, in which the lower frequency coil is on the inside  110  and the inner coil  110  extends beyond the outer coil  120 , has the following capacitor values: 
        C low =554 pF     C high =67.2 pF        
 
         [0088]     In this exemplary embodiment, the microstrips of the inner coil  110  extend by 0.5″ beyond both ends of the outer coil  120  for a total length of 6″.  
         [0089]     For the lower frequency (e.g., 40.8 MHz) of operation of this embodiment: 
        Q=432     B 1 =6.35 uT/sqrt(W)     B 1 (Q-normalized)=0.305 uT/sqrt(Q*W)     Mode separation: 13.2%        
 
         [0094]      FIG. 6A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the lower frequency of operation  
         [0095]      FIG. 6B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the lower frequency of operation.  
         [0096]     For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment: 
        Q=464     B 1 =0.842 uT/sqrt(W)     B 1 (Q-normalized)=0.0391 uT/sqrt(Q*W)     Mode separation=27.1%        
 
         [0101]      FIG. 7A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the higher frequency of operation.  
         [0102]      FIG. 7B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the higher frequency of operation.  
         [0103]     For comparison, the performance of the reference coil will now be described.  
         [0104]     For the lower frequency (e.g., 40.8 MHz) of operation, the value of the capacitors used is 598 pF, and the resultant parameters are: 
        Q=433     B 1 =7.30 uT/sqrt(W)     B 1 (Q-normalized)=0.351 uT/sqrt(Q*W)     Mode separation=9.5%        
 
         [0109]      FIG. 8A  shows the reflection coefficient (S 11 ) as a function of frequency for the reference coil operating at the lower frequency of operation, whereas  FIG. 8B  shows the B 1  field behavior.  
         [0110]     For the higher frequency (e.g., 170.2 MHz) of operation, the value of the capacitors used is 33.3 pF, and the resultant parameters are: 
        Q=699     B 1 =4.61 uT/sqrt(W)     B 1 (Q-normalized)=0.174 uT/sqrt(Q*W)     Mode separation=9.4%        
 
         [0115]      FIG. 9A  shows the reflection coefficient (S 11 ) as a function of frequency for the reference coil operating at the higher frequency of operation, whereas  FIG. 9B  shows the B 1  field behavior.  
         [0116]     In yet a further exemplary embodiment in accordance with the present invention, a single coil is provided having relatively closely spaced resonances frequencies, such as, for example 170 MHz ( 1 H at 4 T) and 160 MHz ( 19 F at 4 T). In this embodiment, the two imaging modes (typically degenerate, used in quadrature) are moved to two different frequencies. An advantage is that the modes are orthogonal, and thus decoupled, while homogeneity and efficiency are preserved. It is noted that this coil configuration can be used at linear-only modes of operation for both frequencies.  
         [0117]      FIGS. 10B and 11B  show cross-sections of such an embodiment. As can be seen, the widths of the coil strips vary as a function of their location. In an exemplary embodiment, the strip widths vary sinusoidally from their nominal width, with the width of the strips at the 0° and 180° positions being −30% of the nominal width, and the width of the strips at the 90° and 270° positions being +30% of the nominal width.  
         [0118]     As in the above described embodiments, capacitors (not shown) are coupled between each strip of the coil and an outer cylindrical shield. In an exemplary embodiment, the values of the capacitors vary sinusoidally as a function of the position of their corresponding strip. In an exemplary embodiment, the capacitors of the strips at the 0° and 180° positions are −17.6% of the nominal capacitance, and the capacitors of the strips at the 90° and 270° positions are +17.6% of the nominal capacitance.  
         [0119]     An exemplary embodiment of the coil has the following parameters:  
         [0000]     Shield Support Cylinder  
         [0000]    
       
         
           
              OD=7.25 in (184 mm)  
              ID=7 in (178 mm)  
              Wall thickness=0.125 in (3.2 mm)  
              Material=acrylic 
 
 Coil Support Cylinder 
 
              OD=6 in (152 mm)  
              ID=5.75 in (146 mm)  
              Wall thickness=0.125 in (3.2 mm)  
              Material=acrylic  
              Coil length=5 in (127 mm)  
              Number of strips=12  
              Nominal strip width=1.018 in (25.8 mm or 67.6% of center-to-center spacing)  
           
         
       
     
         [0131]     The operating frequencies are as follows: 
        f low =160.8 MHz     f high =170.2 MHz        
 
         [0134]     Table II summarizes the results of simulations for the aforementioned embodiment and the reference coil.  
                                                                 TABLE II                                       Lower frequency   Higher frequency                        Normalized   B 1  ± 1 dB           Normalized   B 1  ± 1 dB               B 1 ,   B 1 ,   diameter,       B 1 ,   B 1 ,   diameter,       Coil Type   Q   μT/{square root over (W)}   μT/{square root over (Q · W)}   mm   Q   μT/{square root over (W)}   μT/{square root over (Q · W)}   mm               Dual   679   5.52   0.212   108   699   5.19   0.196   104       tuned,       single coil       Reference   693   5.48   0.208   111   697   5.35   0.203   111       single-       tuned       linear                  
 
         [0135]     The performance of an exemplary embodiment of the dual-tuned, single layer coil of the present invention will now be described.  
         [0136]     In this exemplary embodiment, the capacitor values and strip widths are as follows: 
        C(nominal)=26.4 pF     C variation=sinusoidal: −17.6% at 0° and 180°, +17.6% at 90° and 270°    Strip width=1.004 in (25.5 mm or 66.7% of center-to-center spacing)     Strip width variation=sinusoidal: −30% at 0° and 180°, +30% at 90° and 270°       
 
         [0141]     For the lower frequency (e.g., 160.8 MHz) of operation of this embodiment: 
        Q=679     B 1 =5.52 uT/sqrt(W)     B 1 (Q-normalized)=0.212 uT/sqrt(Q*W)     B 1 ±1 dB diameter=108 mm        
 
         [0146]      FIG. 10A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the lower frequency of operation  
         [0147]      FIG. 10B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the lower frequency of operation.  
         [0148]     For the higher frequency (e.g., 170.2 MHz) of operation of this embodiment: 
        Q=699     B 1 =5.19 uT/sqrt(W)     B 1 (Q-normalized)=0.196 uT/sqrt(Q*W) p 1  B 1 ±1 dB diameter=104 mm        
 
         [0152]      FIG. 11A  shows the reflection coefficient (S 11 ) as a function of frequency for this embodiment, at the higher frequency of operation  
         [0153]      FIG. 11B  shows the B 1  field behavior (black contour: ±1 dB boundary) for the same embodiment, at the higher frequency of operation.  
         [0154]     For comparison, the performance of the single-tuned reference coil will now be described.  
         [0155]     For the lower frequency (e.g., 160.8 MHz) of operation, the value of the capacitors used is 30.0 pF, and the resultant parameters are: 
        Q=693     B 1 =5.48 uT/sqrt(W)     B 1 (Q-normalized)=0.208 uT/sqrt(Q*W)     B 1 ±1 dB diameter=111 mm (linear)        
 
         [0160]      FIG. 12A  shows the reflection coefficient (S 11 ) as a function of frequency for the reference coil operating at the lower frequency of operation, whereas  FIG. 12B  shows the B 1  field behavior.  
         [0161]     For the higher frequency (e.g., 170.2 MHz) of operation, the value of the capacitors used is 24.9 pF, and the resultant parameters are: 
        Q=697     B 1 =5.35 uT/sqrt(W)     B 1 (Q-normalized)=0.203 uT/sqrt(Q*W)     B 1 ±1 dB diameter=111 mm (linear)        
 
         [0166]      FIG. 13A  shows the reflection coefficient (S 11 ) as a function of frequency for the reference coil operating at the higher frequency of operation, whereas  FIG. 13B  shows the B 1  field behavior.  
         [0167]     Having described embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.