Abstract:
A phase array coil uses asymmetric loops and selective overlapping to provide a more uniform field sensitivity to magnetic flux passing through the surface defined by multiple such phased array loops.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   BACKGROUND OF THE INVENTION 
   The field of the invention is magnetic resonance imaging and in particular a local coil for use in magnetic resonance imaging. 
   Magnetic resonance imaging (“MRI”) provides images, for example, of a human patient, by detecting faint signals from precessing hydrogen protons under the influence of a strong magnetic field (termed the B 0  field) after a radio frequency excitation. 
   The quality of the image produced by MRI is strongly dependent upon the strength of the received signal. For this reason, it is known to use radio frequency receiving coils placed in close proximity to the volume being imaged. Such coils are called local coils. 
   A particular type of local coil, termed a “phased array coil”, provides a series of small loops that together define a surface covering a region of interest. Magnetic flux from NMR signals within the region of interest passes through the surface defined by the loops and is detected separately at each loop. The relatively smaller area of the loops provide improved signal-to-noise ratio, better spatial localization for certain MRI techniques, and extended coverage of the patient made possible by switching different sets of loops into communication with limited MRI machine inputs. 
   In order to ensure continuous coverage of a region of interest, the constituent loops of a phased array coil are normally placed in close proximity. This proximity promotes a coupling between the loops that reduces signal-to-noise ratio in the received NMR signal. For this reason, the coupling is normally reduced by one of several isolation techniques. 
   In a first decoupling method, a slight but precise overlap of the loops is created to promote a “flux sharing” of countervailing flux between the loops sufficient to decouple the coils from each other. This technique is described in U.S. Pat. No. 5,256,971 assigned to the assignee of the present invention and hereby incorporated by reference. 
   Decoupling coils by flux sharing can be difficult in practice because the amount of overlap must be precisely controlled and can change depending on the imaging environment. Accordingly, second methods of decoupling may be used instead or in addition to flux sharing, such methods including the use of networks of decoupling capacitors and/or the use of preamplifiers with input impedances selected to reduce current flow in the loops. 
   Despite many advantages of phased array coils, uniform sensitivity over the surface covered by the loops is hard to obtain because of a drop-off in sensitivity of the loops at their edges. In coils employing symmetric patterns of loops, the drop off may be most pronounced at the center of body structure where sharp imaging and high signal-to-noise ratio signals are required. 
   BRIEF SUMMARY OF THE INVENTION 
   The present inventors have realized that the patterns of sensitivity of individual loops of phased array coils may be shifted to eliminate low signal strength regions in combinations of these loops, particularly at the centers of interfaces of such loops. This shifting is accomplished by transforming the loops from their typical symmetric outline to an asymmetric shape. The asymmetry can be used to move the peak sensitivity pattern of the coil toward its interface with other loops. 
   For example, for a phased array coil of four loops, asymmetry in the loops may be used to shift the sensitivity of each loop toward the common center of the array counteracting a normal falloff of signals in this area. This technique may be used with or without selective overlap of the coils. 
   Specifically then, the present invention provides a phased array local coil for magnetic resonance imaging comprising a set of loops fitting together to define a surface to couple with magnetic flux passing through the surface. Each loop includes a conductor for independently communicating a signal between the loop and an MRI machine, as is typical with phased array coils, and each loop is radially asymmetric about a normal to the surface of the loop to provide improved uniformity of coupling between the loops and the magnetic flux passing through the surface. 
   Thus, it is one object of the invention to provide a method of reducing variations in sensitivity over a surface covered by phased array loops. 
   The loops may be arranged about a center of the surface and the asymmetry of the loops may cause the center of gravity of an area enclosed by each loop to be displaced toward the center. 
   It is thus another object of the invention to overcome a difficulty particularly occurring in symmetric phased array coils where there is considerable falloff toward the center of the coils where each of the coils abuts and such as may cause a loss of signal in clinically important areas. 
   The loops may be N sided polygons where N is an odd number greater than or equal to five. 
   Thus, it is another object of the invention to provide a simple method of introducing asymmetry through the use of irregular polygon coils. 
   The loops may be arranged in columns and rows with the rows extending along an axis of a polarizing magnetic field of the MRI machine and the loops may overlap with loops in adjacent columns and not with loops in adjacent rows. 
   Thus it is another object of the invention to augment the loop asymmetry of the present invention with an overlapping of the loops, the latter overcoming loss of sensitivity caused by insensitivity of the loop conductors, near the loop conductors, to surface normal flux. This is not a problem for loops positioned transversely with respect to the B 0  field where surface parallel flux is present and may be detected. 
   The loops may be flexible conductors and the phased array coil may conform to an arched space admitting a patient. 
   It is thus another object of the invention to provide a method of adjusting the sensitivity of loops in a phased array coil that may be particularly useful for flexible coils that conform to a non-planar shape. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a torso coil constructed according to the present invention with two phased array sets, each having four loops, where each loop has a center-shifting asymmetry; 
       FIG. 2  is a top plan view of one of the phased array sets of  FIG. 1  showing the conductor pattern, such as produces the asymmetric loop, together with the routing of independent conductors from each loop along a ground ring to preamplifier units providing decoupling; and 
       FIG. 3  is a simplified representation of axial and transverse conductors of the loops of  FIG. 2  showing a loss of sensitivity of a transverse conductor to surface normal magnetic field as may be overcome by an overlap of the transverse conductors of coils. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a phased array torso coil  10  includes an upper phased array unit  12  and a lower phased array unit  14  being mirror images of one another about a horizontal plane separating them. 
   Generally, the lower phased array unit  14  may lay flat or nearly flat along a patient table (not shown) while the upper phased array unit  14  may curve about an axis  16  parallel to and aligned with the surface of patient table and the polarizing magnetic field of the MRI machine to drape about the sides of the patient&#39;s torso for better coverage. 
   Referring now to  FIGS. 1 and 2 , each of the upper phased array unit  12  and lower phased array unit  14  may include four phased array loops  20   a ,  20   b ,  20   c , and  20   d.  Loops  20   a  and  20   c,  and loops  20   a  and  20   d,  are arranged each in adjacent rows extending along the axis  16 . As such, loops  20   a  and  20   b  form a perpendicular first column and coil  20   d  and  20   c  form a perpendicular second column to fit within rectangular area  21 . 
   Each of the loops  20   a - 20   c  comprises a conductor, such as copper foil on a flexible printed circuit board substrate, such as may be encased in a soft flexible protective material, such as fabric or foam to fit comfortably against a patient. As is generally understood in the art, the conductors are tuned to a resonance of the NMR signal by the use of series capacitances (not shown) and are decoupled from one another by preamplifier decoupling reducing current flow in the loops or by a capacitive decoupling network  22  of a type as is described in U.S. co-pending application Ser. No. 10/303,586 entitled: Decoupling Circuit for Magnetic Resonance Imaging Local Coils, filed Nov. 22, 2002, assigned to the assignee of the present invention and hereby incorporated by reference. 
   Each of the loops  20   a - 20   d  is provided with a separate conductor  24   a - 24   d  that communicates an independent NMR signal to a preamplifier circuit  26  attached to the upper phased array unit  12  or lower phased array unit  14 . The preamplifier circuit  26  amplifies the signals from the conductor  24   a - 24   d  before transmitting them along MRI cables  28  to the MRI machine. 
   In the preferred embodiment, a conductive grounded ring  30  may circumscribe the four coils  20   a - 20   d , and the cables  24   a - 24   b  may be attached to the grounded ring  30  so as to pass to a convenient exit point near the preamplifiers  26  without being subjected to coupling magnetic fields, such as may introduce interference into the MRI signal. 
   Referring still to  FIG. 2 , each of the loops  20   a - 20   d  is asymmetric about an axis normal to the surface over which they lie (the normal being perpendicular to the sheet on which  FIG. 2  is drawn). What is meant by radially asymmetric is that when the loop is flattened to lie within a plane, there is no point in the loop where each line, for all angles within the plane through the point, intersects the loop at two locations equal distance from the point. In particular outer conductors of each loop  20   a - 20   d  near corners of the rectangular area  21  pass diagonally inward away from the corners of the rectangular area  21  turning an otherwise square outline of the loops  20   a - 20   d  into a generally 5-sided asymmetric figure. 
   The result of this asymmetry is to move the centers  34   a - 34   d  of each loop  20   a - 20   d  (center of gravity) inward toward a center region  40  of the upper phased array unit  12  and a lower phased array unit  14 , increasing the sensitivity of the loops  20   a - 20   d  to this center region  40  above that which would have been obtained had loops  20   a - 20   d  retained their nearly square outlines. The amount of asymmetry may be readily controlled to tailor the signal boost desired in this center region  40 . 
   Referring still to  FIG. 2 , the loop in adjacent rows (i.e.,  20   a  and  20   b , and  20   d  and  20   c ) do not overlap, however, the loops in adjacent columns (i.e.,  20   a  and  20   d , and  20   b  and  20   c ) do overlap at area  51  to overcome a falloff in sensitivity that occurs at the column-to-column interface. 
   Referring to  FIG. 3 , this falloff occurs because the lines of flux of the NMR signal are largely confined to planes perpendicular to axis  16 . Thus, transverse conductor  50  of loops  20   a - 20   d  (running perpendicular to axis  16 ) are blind to the NMR signals directly below the transverse conductor  50 , being sensitive principally to axial magnetic fields  52  (by the right hand rule). Overlap of transverse conductors  50  moves these dead areas into a region of sensitivity of the adjacent loop  20   a - 20   d , ensuring that there is no dead area. 
   Conversely, axial conductors  54  of loops  20   a - 20   d  (running parallel to axis  16 ) are sensitive to the principally transverse magnetic fields  56  produced by the NMR signals. Overlap here is not necessary and would only increase undesirable coupling. 
   Thus through a combination of asymmetry shifting inward the center of gravity of the coils to regions of low signal strength, and selective overlap, a more uniform phased array coil may be obtained. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.