Abstract:
A multi-loop RF coil includes a plurality of channels and is formed of a plurality of coil elements. The coil includes a pair of coil elements that at least partially overlap with one another as part of a geometric decoupling scheme between the pair of coil elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. patent application Ser. No. 60/979,362, filed Oct. 11, 2007, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to imaging systems and more particularly, relates to coil decoupling schemes between various coil elements to offer improved performance and results. 
       BACKGROUND 
       [0003]    Specialized RF coils for application-specific imaging modalities are common in the field of magnetic resonance imaging (MRI). In particular, for the magnetic resonance imaging of the female breast in horizontal and vertical clinical MR instruments a number of single and multi-loop RF coil concepts for single channel, quadrature, and phased array configurations have been devised by a number of inventors. Examples of these systems are described in U.S. Pat. No. 7,084,631 (Qu et al., Aug. 1, 2006), U.S. Pat. No. 6,850,065 (Fujita et al., Feb. 1, 2005), U.S. Pat. No. 6,493,572 (Su et al., Dec. 10, 2002), U.S. Pat. No. 6,377,836 (Arakawa et al., Apr. 23, 2002), U.S. Pat. No. 6,163,717 (Su, Dec. 19, 2000), U.S. Pat. No. 6,023,166 (Eydelnan, Feb. 8, 2000), and U.S. Pat. No. 5,699,802 (Duerr, Dec. 23, 1997), each of which is hereby incorporated by reference in its entirety. 
         [0004]    Often the individual loops or coil elements are laid out in planar and orthogonal planes with respect to the main magnet in the clinical MR instrument. For example, in  FIG. 1 , the combination structure of a 7-channel RF coil configuration is illustrated and consists of four planar coils (or loops). Two coils are each placed on two planes that are positioned a certain distance apart. These coils can then be complemented by three additional coils vertically oriented with respect to the original four coils. 
         [0005]    In a multi-channel receiver system, the MR signal obtained by a multi-loop breast coil system can be configured such that each loop or coil element is selectively tuned to the resonance frequency of the MR instruments, while the remaining loops, or coil elements, are detuned. As a result, by tuning and detuning individual coil elements, parallel MR imaging is accomplished which enables high resolution imaging of selective regions of interest and concomitantly more rapid image formation. 
         [0006]    Unfortunately, when connecting each of the loops, or coil elements, to a receiving channel of the MR instrument, coupling between the tuned and detuned loops can occur. This is due to the fact that the signal received by one loop is also received by neighboring loops, despite detuning measures that involve preamplifier detuning. The coupling is most prominent for the orthogonal loop in the center of the configuration and its adjacently positioned planar loops. There is therefore a perceived need for a coil construction that offers provides improves coil decoupling schemes between various coil elements to offer improved performance and results. 
       SUMMARY 
       [0007]    A multi-loop RF coil according to one embodiment of the present invention includes a plurality of channels and is formed of a plurality of coil elements. The coil includes a pair of coil elements that overlap with one another as part of a geometric decoupling scheme between the pair of coil elements. 
         [0008]    In another embodiment, an RF coil that has a plurality of channels includes a plurality of coil elements and a hybrid decoupling scheme between the coil elements that is a combination of geometric and capacitive decoupling. 
         [0009]    In another embodiment, a multi-loop RF coil that has a plurality of channels that includes a plurality of coil elements. A first pair of coils that are disposed in one plane overlap with one another as part of a geometric decoupling scheme between the first pair of coils. In addition, one vertical coil is partially decoupled from the first pair of coils by a bridge over the overlapping portions of the first pair of coils. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0010]      FIG. 1  is a schematic of a structure of a conventional 7-channel RF breast coil consisting of two loops placed on two separate planes and complemented by three orthogonal loops, wherein the overall dimensions are given in meters; 
           [0011]      FIG. 2  is a schematic of a geometrical decoupling scheme between coil elements of a 7-channel coil system according to one exemplary embodiment; 
           [0012]      FIG. 3  is a schematic of the 7-channel coil system according to one embodiment showing the deployment of tuning, matching, and decoupling capacitors; 
           [0013]      FIGS. 4   a - g  are schematic illustrations of the 7-channel coil system showing current flow in each of the seven channels; and 
           [0014]      FIG. 5  is a schematic illustration of a magnetic field B 1 , produced in the left cross-sectional plane by the 7-channel coil configuration of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0015]      FIG. 2  is a schematic illustrating a 7-channel RF coil  100  according to one exemplary embodiment of the present invention. The 7-channel RF coil  100  is formed of seven coils, namely, a first coil (coil  1 )  110 ; a second coil (coil  2 )  120 , a third coil (coil  3 )  130 , a fourth coil (coil  4 )  140 , a fifth coil (coil  5 )  150 , a sixth coil (coil  6 )  160 , and a seventh coil (coil  7 )  170 . The RF coil  100  is formed of four planar or loop coils positioned on different planes and in particular, coils  110 ,  120 ,  130  and  140  are loop coils with coils  110 ,  120  disposed in one plane and coils  130 ,  140  disposed in another plane that is spaced from the one plane. 
         [0016]    The RF coil  100  has the following loop (or channel) assignments: coil  110  represents the upper right channel, coil  120  represents the upper left channel, coil  130  represents the lower right channel, coil  140  represents the lower left channel, coil  150  represents the vertical right channel, coil  160  represents the vertical middle channel, and coil  170  represents the vertical left channel. 
         [0017]    Moreover, each coil element shown in  FIG. 2  has a number of breaks which are populated by lumped components, including (a) tuning capacitors (seven), (b) matching capacitors (seven), and (c) decoupling capacitors (seven). The values of these components are chosen to ensure appropriate tuning, matching of each coil, as well as capacitive decoupling of certain coil pairs. 
         [0018]    Capacitive decoupling is used between the vertical middle loop  160  and the lower right loop  130  and lower left loop  140 . At the same time, the vertical middle loop  160  is only partially decoupled from the upper right loop  110  and upper left loop  120  by a bridge over the two overlapping segments of the lower right loop  130  and lower left loop  140 . Partial decoupling is sufficient in this case, because preamplifier decoupling (using low input impedance preamplifier) is additionally used for each loop as seen in  FIG. 2 . In the coil  100  of the present invention, capacitive decoupling between the middle vertical loop  160  and upper left and right loops  120 ,  130  is not used. This could create additional current paths, which would be very difficult to control. 
         [0019]      FIG. 2  thus illustrates a geometric decoupling strategy between coil elements  130 ,  140 ,  160  for the 7-channel RF coil arrangement displayed. 
         [0020]      FIG. 3  shows a suitable (exemplary) deployment and labeling of the tuning, matching, and decoupling capacitors for the coil element configuration of  FIG. 2 . 
         [0021]    More particularly, in  FIG. 3 , for example, “C dec  27 ” is a decoupling capacitor that decouples coils  2  and  7  (coil  120 , coil  170 ). “C dec  47 ” (as well as “C dec  35 ”) denotes four capacitors. The following coil pairs are decoupled by suitable choices of capacitors: 
         [0022]    1. Coil  1  (coil  110 )-Coil  2  (coil  120 ) 
         [0023]    2. Coil  1  (coil  110 )-Coil  6  (coil  160 ) 
         [0024]    3. Coil  2  (coil  120 )-Coil  6  (coil  160 ) 
         [0025]    4. Coil  1  (coil  110 )-Coil  5  (coil  150 ) 
         [0026]    5. Coil  2  (coil  120 )-Coil  7  (coil  170 ) 
         [0027]    6. Coil  3  (coil  130 )-Coil  5  (coil  150 ) 
         [0028]    7. Coil  4  (coil  140 )-Coil  7  (coil  170 ) 
         [0000]    Tuning capacitors for respective coils are indicated by “C tune x,” where x is the coil number. For example, “C tune  1 ” refers to a tuning capacitor for coil  1 . Similarly, matching capacitors are indicated by “C match x,” where x is the coil number. For example, “C match  1 ” refers to a matching capacitor for coil  1 . 
         [0029]    However, unlike previous capacitive decoupling attempts reported in the literature, an inductive decoupling is used for coils  3  and  4  (coils  130 ,  140 ) by creating an overlap of the conductive coil structures. As shown in  FIG. 2 , the coil  140  includes a portion  142  that overlaps a portion  132  of the coil  130  to create the inductive decoupling. As shown in  FIG. 2 , each of the coils  130 ,  140  includes an irregular portion that is different from the remaining portion of the coil. The irregular portions of the coils  130 ,  140  overlap one another and are positioned above the bridge  162 . The bridge  162  can be a continuous coil structure that has a number of bends formed therein to allow the overlapping portions of coils  3  and  4  to be disposed thereover. In other words, instead of being a more planar coil like coils  5  and  7 , the coil  6  is bent to allow the coil  6  to extend across the overlapping portions of the coil  3  and  4  without obstructing them since this bent portion lies in a different plane that is spaced from the plane(s) that contain the overlapped portions of the coils  3  and  4 . 
         [0030]    In the illustrated embodiment, the coil  4  lies over coil  3 ; however, the opposite can be true in that coil  3  can lie over coil  4 . 
         [0031]    As already mentioned above, Coil  6  (coil  160 ) features a bridge  162  that is positioned below the plane where Coils  3  and  4  (coils  130 ,  140 ) are located. Even though the pairs Coil  3 -Coil  6  (coils  130 ,  160 ) and Coil  4 -Coil  6  (coils  140 ,  160 ) are not capacitively decoupled, the bridge  162  helps to decrease the amount of coupling. 
         [0032]    The effectiveness of the disclosed geometric as well as the associated electrical decoupling is demonstrated in  FIGS. 4(   a )-( g ), where the current flow in the seven coil elements ( 110 ,  120 ,  130 ,  140 ,  150 ,  160 ,  170 ) is predicted through a Method of Moments numerical modeling program (details of the program are disclosed in the publication Time domain formulation of the method of moments for inhomogeneous conductive bodies at low frequencies, by Lemdiasov, R. A., Obi, A., and Ludwig, R,  IEEE Transactions on Antennas and Propagation,  v 54, n 2, pt. 2, February 2006, pp. 706-14), which is incorporated by reference in its entirety. In  FIG. 4 , cross hatching is used to denote the highest current flow, resulting in the respective MR signal acquisition of Coil elements  1  to  7 . Additional cross hatching is used to indicate small and very small current flow and a lack of cross hatching denotes no current flow.  FIG. 4   a  thus shows the excitation of coil element  1 ;  FIG. 4   b  thus shows the excitation of coil element  2 ;  FIG. 4   c  thus shows the excitation of coil element  3 ; FIG.  4   d  thus shows the excitation of coil element  4 ;  FIG. 4   e  thus shows the excitation of coil element  5 ;  FIG. 4   f  thus shows the excitation of coil element  6 ; and  FIG. 4   g  thus shows the excitation of coil element  7 . 
         [0033]    As can be seen in  FIGS. 4   a - 4   g , when individually tuned, each coil element carries its own current. Neighboring coil elements display very little current and therefore indicate good decoupling behavior. 
         [0034]    According to reciprocity principle, the sensitivity of a particular coil to the radiation from the biological load is proportional to the magnetic field of the coil, if the latter is driven by an external voltage source. The magnetic B 1  field of these coils can be calculated, provided that the coils take the same amount of input power. The signal-to-noise (SNR) of a coil is proportional to B 1 /√{square root over (P)}, where P is input power. Finally, the squares of magnetic fields of the seven coils can be added according to the formula: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    The resulting field is shown in  FIG. 5 .