Abstract:
A local coil for magnetic resonance imaging employs a loop divided by a shunting conductor which reduces sensitivity and field strength at one end of the loop to provide improved homogeneity for different coil configurations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to magnetic resonance imaging (MRI) and in particular local coils for use in transmitting radio frequency excitation signals and/or receiving magnetic resonance signals in magnetic resonance imaging. 
     Magnetic resonance imaging is used to generate medical diagnostic images by measuring faint radio frequency signals (magnetic resonance) emitted by atomic nuclei in tissue (for example, protons) after radio frequency stimulation of the tissue in the presence of a strong magnetic field. 
     The radio frequency stimulation may be applied, and the resulting magnetic resonance signal detected, using a “local coil” having one or more single turn conductive “loops” serving as antennas. The loops of the local coil are tuned to a narrow band, for example, 64 megahertz for a 1.5 Tesla magnetic field, strength magnetic field, and adapted to be placed near or on the patient to decrease the effects of external electrical noise on the detected magnetic resonance signal. The detected magnetic resonance signal may be conducted through one or more signal cables to the MRI machine for processing. 
     A local coil may incorporate multiple loops whose signals may be combined prior to being processed by the MRI machine, for example, in a quadrature type coil where perpendicular loops are combined with a 90° phase shift. Alternatively, the signals of the multiple loops may be conducted independently to the MRI machine to provide for the so-called “phased array” detection. 
     An important characteristic of a local coil is the homogeneity of its field strength, the latter defined as the coil&#39;s sensitivity to magnetic resonance signals when operated in a receive mode, and the strength of the coil&#39;s transmission of radio frequency excitation signals when operated in the transmit mode. Homogeneity is particularly important for certain MRI procedures such as fat saturation where too much;or too little field strength may detrimentally affect the imaging process. 
     Field strength is a complex function of the design of the local coil and of the coil&#39;s interaction with the patient. Homogeneity is often a compromise with other desirable coil characteristics including signal-to-noise ratio and selection of a coil shape. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method of adjusting the field strength of different portions of loop by adding a shunting conductor that bypasses some current flow at one end of the loop. The shunt, in contrast to a resistive device, does not degrade the Q (quality factor) of the loop as would decrease the signal-to-noise ratio of the loop. By allowing control of the field strength independent of the coil geometry, the shunt permits field strength homogeneity to be increased or permits greater flexibility in other aspects of the coil design without significant loss of field strength homogeneity. 
     Specifically then, the present invention provides an MRI coil having a loop conductor split by a shunt conductor to divide a first loop portion from a second loop portion. The loop is tuned to a resonance frequency so that the current flow in the first and second loop portions are different. 
     It is thus one object of the invention to provide a means for controlling current flows within a loop to tune the field strength over different portions of the loop. 
     The MRI coil may include a matching network for conducting a signal related to the current flow in the first portion to an MRI machine. 
     Thus, it is another object of the invention to provide a field strength adjustment mechanism that permits a single tap point on the loop. 
     The area of the first loop portion may be different from the area of the second loop portion. 
     It is thus another object of the invention to allow altering the field strength profile of the loop by adjusting placement of the shunt conductor. 
     The MRI coil may include a patient support positioning the loop with respect to a patient so that the second loop portion is closer to the first loop portion of the loop and the proportion of current flow in the first and second loop portions may be adjusted to equalize the sensitivity of the first and second loop portions with respect to the patient. 
     Thus it is another object of the invention to provide a method of compensating for changes in field strength caused by variable separation distance between portions of the loop and the patient as may occur in certain desirable coil shapes. 
     The MRI coil may include multiple loops each split by a shunt conductor as described above. 
     It is thus another object of the invention to allow multi loop coils to have improved homogeneity using this technique. 
     The multiple loops may be placed adjacent to each other to form a tubular surface with the shunts parallel to the circumference of the tube, and in one embodiment, the tube may taper inward toward the center of the tube at one end of the tube. 
     Thus it is another object of the invention to allow construction of dome top tubular local coils with better homogeneity. 
     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 simple loop having a conductive shunt per the present invention, wherein the loop is tuned to provide co-cyclic current flow such as decreases current flow at one end of the loop for reduced field sensitivity at that end; 
     FIG. 2 is a cross sectional view of a head coil constructed of multiple simple loops similar to FIG. 1 showing increased proximity of a superior end of the loops to the patient as would normally produce an undesirable higher field strength which may be reduced by the shunt conductor of the present invention; 
     FIG. 3 is a perspective view of the head coil of FIG. 2 showing its domed top; and 
     FIG. 4 is a simplified, schematic of the coil of FIG.  1  and of individual coils of FIGS. 2 and 3 showing the use of series capacitors for tuning the coil to resonance 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a local coil  10  for use with an MRI system, provides a series resonant electrical loop  12  and having first and second opposed end conductors  14   a  and  14   b  joined by opposed side conductors  16   a  and  16   b . The form of the loop  12  as shown is rectangular, but the invention is not limited to this shape. 
     A shunt conductor  18  extending between the side conductors  16   a  and  16   b  generally parallel to the end conductors  14   a  and  14   b , cuts the loop  12  into two loop portions  20   a  and  20   b , loop portions  20   a  formed by end conductor  14   a  and shunt conductor  18  joined by portions of side conductors  16   a  and  16   b  and loop portions  20   b  formed by shunt conductor  18  and end conductor  14   b  joined by portions of side conductors  16   a  and  16   b . Thus, the shunt conductor  18  is shared between the loop portions  20   a  and  20   b.    
     A matching network  26  of a type well understood in the art may be connected to the local coil  10  at end conductor  14   b  to communicate through signal leads  28  to an MRI system (not shown) so that the local coil  10  may receive signals from the MRI system in a transmit mode and detect signals from the patient in a receive mode. 
     The local coil  10  is tuned into resonance through the use of capacitors  22  placed in series with the distributed inductances of the shunt conductor  18 , end conductor  14   a  and  14   b , and side conductors  16   a  and  16   b . The tuning is such as to ensure that the resonant mode of the local coil  10  provides currents in loop portions  20   a  and  20   b  are different by a desired amount. Generally, in the case of co-cyclic currents, current  24  passing through loop  20   b  in either direction splits at the junctures of the shunt conductor  18  and the side conductors  16   a  and  16   b  to pass partially through the shunt conductor  18  and partially through end conductor  14   a  so that the magnitude of the current  24  in loop  20   b  (being the measure of current in end conductor  14   b ) equals the magnitude of the current in the shunt conductor  18  summed with the magnitude of the current in the second loop portion  20   a  (being the measure of the current end conductor  14   a ). The currents need rot be co-cyclic how ever for different tuning methods. 
     This splitting of the current  24  means that a radio-frequency (RF) excitation signal introduced into the local coil  10  by matching network  26  attached at end conductor  14   b  (during an MRI transmit cycle) will provide less current flow (and hence less field strength) at loop  20   a  than would be the case if the shunt conductor  18  were absent. Likewise during an MRI receive cycle, the magnetic resonance signal received by loop  20   a  will make a smaller contribution to the signal conducted from matching network  26  than would be the case if the shunt conductor  18  were absent. 
     Generally, the shunt conductor  18  may be varied in position along the length of side conductors  16   a  and  16   b , with appropriate adjustment in the series capacitors  22 , to change the point at which field strength is reduced. Multiple shunt conductors  18  (not shown) may be used to create several loop portions of reduced field strength. 
     As mentioned above, the loop  12  may operate in either a transmit or receive mode and when operating as a receive only coil  10  may include passive or active de-coupling circuits of a type well known in the art. 
     Referring now to FIGS. 2 and 3, an example application of the present invention provides a domed-top head coil  30  having a cylindrical tubular section  33  capped by a hollow hemispherical domed section  34  at its superior end. The inferior end of the domed-top head coil  30  is open to receive the head of a patient  32 . The domed-top head coil  30  may include a patient support pillow  35  providing comfortable support of the patient&#39;s head and providing more uniformity in positioning of the patient within the volume of the domed-top head coil  30  so as to also enhance uniformity. 
     Loops  12 , as described above, may be arrayed about the surface of the domed-top head coil  30  so that their side conductors  16  extend generally along the axis of the cylinder and the shunt conductors  18  of each loop  12  are positioned to be circumferential with respect to the cylinder generally at the interface between the cylindrical tubular sections  33  and the hemispherical domed section  34 . Conductive ends  14   a , in this configuration are eliminated or reduced to extremely short segments so as to provide a tapering inward of the loop  12  as it approaches and covers the hemispherical domed section  34  accommodating the reduced circumference of that surface as one moves to its superior tip. 
     This tapering inward of the loop portions  20   a  of the loops  12  would normally be expected to cause increased field strength of loop portions  20   a  both because of their closer proximity to the patient  32  and because of their inward angulations. This increased field strength is offset, however, by the shunt conductor  18  which decreases the signal contributions to and by loop  20   a  as described above. 
     Each of the loops  12  in the domed-top head coil  30  may be separately connected by signal leads  28  and matching networks  26  to the MRI machine in a phased array mode of operation. Alternatively, each of the signal leads  28  may be joined to a combiner network properly phase shifting and adding these signals to produce one or more combination signals provided to the MRI machine. The signal leads  28  may be joined to follow along a grounding ring as taught in the U.S. patent application Ser. No. 10/227,072 filed Aug. 22, 2002, assigned to the assignee of the present invention and hereby incorporated by reference. 
     Referring now to FIG. 4 in the embodiment of the domed-top head coil  30 , the shunt conductor  18  may be placed so as to create a ratio of areas between loop portion  20   b  and  20   b  of 2:1. In this situation, a current splitting through shunt conductor  18  versus end conductor  14   a  of approximately 1 to 0.6 as found suitable. Other ratios may also be appropriate for different configurations of coils other than that of FIG. 2 as will be understood to those of ordinary skill in the art. For example, the location of the shunt may be such as to divide the area of the first and second loop in a ratio substantially in the range 1.5:1 to 3:1. Alternatively, the location of the shunt may be such as to divide the current flow between the first and second loop in a ratio of substantially in a range of 1:0.5 to 1:0.7. 
     It will be understood that the loops  12  may offer similar benefits in structures other than the domed-top head coil  30  but where portions of the patient anatomy may be closer or better received by portions of the loop or where the loop geometry would normally adversely affect field strength homogeneity in other ways. 
     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.