Abstract:
A coil for magnetic resonance imaging operates in a transmit mode with multiple loops locked together in a phase relationship like a birdcage coil to provide a uniform transmission field, but in a receive mode like a phased array coil, each coil operating independently to produce a separate signal for reception by the MRI machine. Phasing of the coil during transmit mode is provided by a ring resonator controllably coupled to the loops. Controllable coupling is provided by a series of current limiting elements interposed between the resonant ring and the loops of the coil.

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 present invention relates generally to magnetic resonance imaging (MRI) and in particular to local coils for use in MRI. 
   In magnetic resonance imaging, a uniform magnetic field is applied to an imaged object, for example, a patient and a radio frequency (RF) excitation signal pulse is applied to excite nuclei within the patient into resonance. This RF excitation pulse will also referred to herein as a “transmit” signal. 
   After application of the transmit signal, one or more magnetic gradient fields are superimposed on the uniform magnetic field to spatially encode the precessing nuclei, and nuclear magnetic resonance (NMR) signal from the nuclei are received and processed mathematically to produce an image. The NMR signal will also be referred to herein as a “receive” signal. 
   The transmit and receive signals may be transmitted and received by loop antennas termed “coils”, such as a whole body coil built into the MRI machine to encompass the entire patient. 
   Improvement in the signal-to-noise ratio of the receive signal can be obtained by placing “local coils” on the patient. The local coil has a smaller reception pattern than a whole body coil and therefore can focus on a smaller region of interest to obtain a stronger signal and to receive less noise. Locals coils may also provide improved application of the transmit signal. 
   A common local coil design is the “birdcage” coil which provides two conductive rings separated along a common axis to define the two bases of a cylindrical volume. A number of conductive axial struts are spaced regularly about the circumference of rings to join the rings. The rings may be excited into resonance at the transmit signal frequency so that a traveling wave progressively promotes current flow in each of the struts to produce a highly uniform rotating magnetic vector within the cylindrical volume. The same coil can be used to collect the receive signal, the rings serving to combine the signals from each of the struts inducted by the rotating magnetic vector of the nuclei. 
   Phased array local coils are multiple loop local coils where the outputs from each loop are independent and may be processed independently to improve signal-to-noise ratio or to obtain additional spatial information. The loops of a phased array coil may be arranged about a cylindrical volume, and thus may resemble and provide similar coverage to a birdcage coil, but unlike the birdcage coil, the phased array coil provides multiple independent signals to the MRI machine in contrast to the birdcage coil which provides a signal as a combination of the current flows in each strut. 
   In order to obtain independent electrical signals from each loop, a phased array coil must normally provide decoupling between the loops. This decoupling may employ a controlled overlap between adjacent loops or capacitive decoupling networks or the like. 
   While phased array coils provide advantages for the reception of the receive signals (receive mode), a standard birdcage structure can provide a more uniform field for transmitting the transmit signal (transmit mode). Accordingly, there is considerable interest in creating a hybrid local coil that may operate as a birdcage coil while transmitting and, as a phased array coil when receiving. 
   Another approach is to combine the physical structures of the phased array coil and birdcage coil in two concentric arrays. One array is activated during transmission and the other during reception. This approach increases the weight and size of the coil and interaction between the two arrays can interfere with the uniformity of the field and/or reduce the signal-to-noise ratio of the received signals. 
   An alternative approach connects each of the loops of the phased array coil directly with the output to a different phase shifter providing phase shifted transmit signals mimicking the phase shifting provided by the birdcage coil end rings. Unfortunately, variations in the loading of each loop often result in widely differing loop currents creating transmit field distortions that result in poor performance for phase sensitive MRI techniques such as fat saturation sequences. 
   Variations in the current flows from each phase shifter can be controlled by the addition of a radio frequency amplifier between the phase shifter and each loop. This approach is expensive and significantly increases the bulk and weight of the coil and for this reason may be practical only for whole body coils. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a ring resonator, analogous to the end rings of a birdcage coil, that may be controllably coupled to phased array loops during transmission to produce an extremely uniform excitation field and, then disconnected to allow the loops to operate independently for a reception. Coupling between the ring resonator and the loops is controlled through a set of passive, power-limiting circuits overcoming the problems of phase distortion caused by variation in the loading of the loops by nearby body tissue and the dielectric resonance effects at ultra-high MRI frequencies corresponding to magnetic field strengths of 3 Tesla and higher. 
   Specifically, the present invention provides a local coil for use with magnetic resonance imaging systems having a set of MRI antenna loops supported about a patient and a set of transmit receive switches alternatively connecting the loops to a transmit signal from the MRI system and to multiple receive inputs to the MRI system. A ring resonator provides a series of phase splitting taps that are connected through passive power-limiting circuits to each of the loops. 
   Thus it is one object of at least one embodiment of the invention to provide a uniform transmit field, while reducing of phase distortion caused by large variations in coupling to the ring resonator by the different loops. 
   The power-limiting circuits may provide a non-reactive current limiter. 
   Thus it is an object of at least one embodiment of the invention to provide a phase splitting circuit that acts like a current source and which does not de-tune the ring resonator. 
   The power-limiting circuits may be 90-degree, phase shifting circuits. 
   Thus it is another object of at least one embodiment of the invention to make use of a well-understood circuit that can transform a low impedance at the ring resonator to a high impedance at the loops to provide current limiting. 
   The ring resonator may include N phase shifting elements connected in series in a ring. The ring need not be circular. 
   Thus it is another object of at least one embodiment of the invention to provide a simple, discrete element implementation of the ring resonator that is more compact than the ring used in a conventional birdcage coil. 
   The phase shifting elements may be links of coaxial cable. 
   Thus it is another object of at least one embodiment of the invention to provide robust, low loss phase shifting element that may be readily fabricated. 
   The inductance required by the 90-degree phase shift circuit used as a power limiter may be lumped into the inductance of the coaxial cable to eliminate a separate inductor. 
   Thus it is another object of at least one embodiment of the invention to decrease the resistive loss associated with the use of a discrete inductor in the power-limiting circuit. 
   The phase shifting elements may provide for equal phase shifts or different phase shifts summing to 360 degrees. 
   Thus it is an object of at least one embodiment of the invention to provide for a variety of coil designs where the loops may or may not be spaced at equal angular intervals about the patient. 
   Each power-limiting circuit may have the same electrical characteristics to provide equal coupling between the ring resonator and the loops or may provide for different electrical characteristics to provide for different couplings to at least one loop. 
   Thus it is another object of at least one embodiment of the invention to allow the adjustment of current through different loops of the coil. 
   The coil may include an interface circuit between the MRI machine and the ring resonator coupling the transmit signal to the ring resonator and also coupling combined NMR signals from the loops to the ring resonator. 
   Thus it is an object of at least one embodiment of the invention to provide a coil that may operate both in phased array reception mode and also in a birdcage coil-like reception mode in which the signals from the loops are combined by the ring resonator. This latter mode may be useful for calibration purposes of the coil. 
   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 schematic diagram of the present invention showing a ring resonator coupled to loops of a head coil to provide for different modes of transmission and reception; 
       FIG. 2  is a detailed schematic representation of the ring resonator of  FIG. 1  showing the use of discrete elements of coaxial cable and the circuitry of a 90-degree phase shifter used as the power-limiting element; 
       FIG. 3  is a detail of  FIG. 2  showing a replacement of the coaxial cable with a standard pi-network phase shift element; 
       FIG. 4  is a schematic similar to that of  FIG. 1  showing application of the invention to a knee/foot coil in which different amounts of phase shifting and coupling may be desired for different coils; and 
       FIG. 5  is an alternative implementation of the power-limiting circuit of  FIG. 2  in which the inductance of  FIG. 2  has been absorbed into the inductance of the coaxial elements in order to reduce electrical losses. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , a head coil  10  suitable for use with the present invention may provide for a series of rectangular conductive loops  12   a – 12   h  arrayed about a cylindrical volume  14  to follow the curved circumference of the cylindrical volume. 
   Each loop  12  includes axial struts  16  which may be shared by adjacent loops  12  and circumferential struts  18  at one end and circumferential struts  20  at a second end of the volume  14 . 
   The circumferential struts  20  include capacitive de-coupling circuitry reducing the coupling between adjacent loops  12  as described in U.S. patent application Ser. No. 10/122,476 filed Apr. 12, 2002, and assigned to the same assignee as the present invention and hereby incorporated by reference. 
   Circumferential struts  18  include loop interface circuits  22  which each provide an electrical coupling to a loop  12   a – 12   f  so that transmit signals may be input to the loops  12   a – 12   f  and receive signals may be collected from the loops  12   a – 12   f.    
   Each of the loop interface circuits  22  connects to an associated lead  24  which may connect to a pole of a single pole, double throw transmit/receive switches  26 , the number of transmit/receive switches  26  being equal in number to the number of loops  12 . While the transmit/receive switches  26  are shown as mechanical switches, it will be understood to one of ordinary skill in the art that these switches normally will be implemented through diode-type switching circuits known in the art in which a switching signal from the MRI system biases diodes into conduction to effect the switching between throws. 
   The pole of each transmit/receive switch  26  may alternately connect to a first throw (as shown in  FIG. 1 ) communicating to a ring resonator  28  via coaxial cable  40  as will be described below, or to a second throw associated with a terminal labeled Rx.sub. 1 –Rx.sub. 8  in  FIG. 1 , the latter communicating with separate input lines of the MRI system (not shown). During a transmit mode, each transmit/receive switch  26  will connect to the first throw as shown in  FIG. 1  so that signals may pass from the ring resonator  28  to the individual loops  12   a – 12   h . Normally during a receive mode, each transmit/receive switch  26  will connect to the second throw and to terminals R.sub.X 1  through R.sub.X 8  so that the independent signals from each of the loops  12   a – 12   h  may pass to the inputs for phased array reception of the MRI machine. 
   Referring still to  FIG. 1 , the ring resonator  28  provides a set of phase shift elements  30   a  through  30   h  connected in series and in a ring so that the sum of the phase shifts as one passes around the ring through each phase shift element  30   a  through  30   h  totals substantially 360 degrees. In an 8-loop design shown in  FIG. 1 , there will be eight phase shift elements  30 , each having a phase shift of approximately 45 degrees. 
   In between each phase shift element  30 , at the junctions of adjacent phase shift elements  30 , are phase splitting taps  32 . The ring resonator  28  may be excited into resonance by a transmit signal Tx received from the MRI machine and passed through an input of a 90-degree quadrature splitter/combiner  36  of a type well known in the art producing two outputs  38   a  and  38   b  having a 90-degree phase difference. These outputs will be attached appropriate taps  32  having a corresponding 90-degree phase separation along the ring. In this case, the outputs  38   a  and  38   b  are attached to the taps  32  separated by the two phase shift elements  30   c  and  30   d.    
   The transmit signal Tx may thus be received by the 90-degree quadrature splitter/combiner  36  and coupled to the ring resonator  28  to create a traveling wave about the ring resonator  28  providing for a series of phase shifted outputs at each phase splitting tap  32 . 
   Each phase splitting tap  32  is coupled through a power-limiting element  34  and cable  50  to the first throws of the transmit/receive switches  26  and thus, during the transmit mode, to particular loops  12 . 
   The 90-degree quadrature splitter/combiner  36  also provides an output optionally providing, under certain conditions, a receive signal Rx. This receive signal Rx is obtained by holding the transmit/receive switches  26  in a state of connection to the first throws during the receive phase of the MRI acquisition. In this case, the ring resonator  28  combines signals received through each of the power-limiting elements  34  in the manner analogous to a standard head coil. This mode may be used for calibration of the MRI system, for example, by obtaining images of a water phantom placed within the volume  14  as the mode may provide improved homogeneity possibly at the expense of signal-to-noise ratio desirable for such calibration procedures. 
   Referring now to  FIG. 2 , each of the phase shift elements  30   a – 30   h  may be a short segment of coaxial cable  40  providing by virtue of its distributed characteristics and length, the necessary phase shift from one end to the other. These segments of coaxial cable  40  may be connected in series in a compact configuration, for example, in a single linear configuration eliminating the need for a bulky circular ring. As is understood in the art, each segment of coaxial cable  40  provides a distributed inductance and capacitance that serves to produce the desired delay through a center conductor when an outer shield is grounded. The distributed inductance of the segments of coaxial cable  40  may be augmented by capacitors  41  connecting taps  32  to ground as will be understood to those of ordinary skill in the art. 
   Referring now to  FIG. 3 , alternatively, the phase shift elements  30  may be provided by single pi-networks in which an inductor  42  is connected between the taps  32 . The inductor  42  is flanked by capacitors  44  leading to ground. Other phase shift networks may also be possible. 
   Referring still to  FIG. 2 , the power-limiting elements  34  may be 90-degree phase shift circuits formed of a capacitor  44  connecting between each tap  32  at an input terminal of the power-limiting element  34 , and the first throw of each transmit/receive switch  26  at an output terminal of the power-limiting element  34 . An inductor  46  connects the input terminal of the power-limiting elements  34  to ground and an inductor  48  connects the output terminal of the power-limiting elements  34  to ground. 90-degree phase shift circuits of this type are well known in the art. 
   An advantage to the 90-degree phase circuit is that it presents a neutral or non-reactive impedance to the loop formed of the phase shift elements  30   a  through  30   h  and that it converts the impedance at this loop to a high impedance as seen from each of the loops  12 . This impedance seen from each loop  12  is substantially higher than the impedance of the loops  12  thus providing for an effective current limiting caused by the fact that changes in the impedance of loops  12  caused by variations in patient loading and inter-loop coupling are slight with respect to the impedance of the power-limiting elements  34 , and thus do not significantly affect current flow through the power-limiting elements  34 . The effect is to control the coupling between the loops  12  and the ring resonator  28  by the creation of an effective current source therefrom. 
   Referring to  FIG. 5 , an alternative implementation of the power-limiting circuit shown in  FIG. 2  in which the inductance of  FIG. 2  has been absorbed into the inductance of the coaxial elements in order to reduce electrical losses is shown. Referring to  FIGS. 2 and 5 , the inductor  46  of the power-limiting elements  34  may be absorbed into the inductance naturally present in the segments of coaxial cable  40  thus eliminating the losses of the independent inductor. Equivalently, the inductor  48  of the power-limiting elements  34  may be absorbed into the inductance naturally present in the segments of coaxial cable  40 . 
   Referring now to  FIG. 4 , the coil  10  need not be a head coil but, for example, may be a knee/foot coil such as that described in U.S. Pat. No. 5,277,183 entitled: “NMR Local Coil For Foot Imaging” and U.S. Patent Publication 20040220469 entitled “Knee-Foot Coil With Improved Homogeneity” assigned to the assignee of the present invention and hereby incorporated by reference. In these designs, there may be some variation in the angular spacing of the struts  16  as they are placed on a form that provides for intersecting hollow tubes communicating at their intersection, in this instance, to provide for a chimney portion  50  extending upward from a tubular foot holding portion  52 , the former allowing extension of the patient&#39;s toes upward while the ankle and remainder of the foot are enclosed in the tubular foot holding portion  52 . Variation in angular spacing of the struts  16  may be readily accommodated by changing the phase shifts of certain of the phase shift elements  30 ′ in contrast to phase shift elements  30  connected to the transmit/receive switch  26  shown in  FIG. 4  while the sum of all phase shifts of all the phase shift elements  30  and  30 ′ remains substantially 360 degrees. 
   Likewise, selected of the power-limiting elements  34 ′ may provide for greater, in this example, or lesser coupling between the ring resonator  28  and the loops  12  of the coil  10  to accommodate differences in those loops  12  both in terms of loop area and proximity to the patient. 
   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.