Abstract:
The tuning of an interventional receiver coil for magnetic resonance imaging signals is tuned by coupling a varactor tuned circuit with the coil and adjusting a DC voltage applied to the varactor to alter the tuning whereby the coil is tuned to the Larmor frequency of the MRI signals.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. patent application No. 60/365,396 filed Mar. 14, 2002, which is incorporated herein for all purposes. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to coils for use in interventional MRI.  
           [0003]    Magnetic resonance imaging of joints requires high resolution and contrast to visualize small tissues. These tissues, such as the glenoid labrum and knee meniscus, are critically important for joint function. MR arthrography has been developed to help improve contrast between joint structures and injected fluid. Miniature RF coils can be placed intra-articularly to take advantage of the minimally invasive nature of this procedure and the distinction of the joint space. These coils have the potential to improve signal to noise ratio (SNR) from tissue joint MRI signals.  
           [0004]    Intra-articular coils can utilize catheter coils, electroprobe designs, and flexible loop coils. Catheter coils are linear devices that can be placed through a single traducing sheath whose size is determined by the size of the coil itself. Electroprobe designs have the advantage of tailored sensitivity patterns and a larger field of view, however they are not a tuned coil but simply implanted conductors. The most promising design is a flexible coil that can be closed for introduction through a small diameter sheath and then opened after deployment. However, flexible coils require tuning after once deployed.  
           [0005]    The present invention is directed to a flexible interventional RF coil and particularly to the tuning thereof for MRI signal acquisition.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    In accordance with the invention, a coil is provided for placement interventionally for detecting MRI signals. In a preferred embodiment, the coil is flexible in design whereby the coil can be closed for fit inside a small sheath and placement interventionally, and then opened for signal reception after deployment.  
           [0007]    A tunable circuit interconnects the coil and an output port with the tunable circuit providing enhanced signal to noise in received MRI signals. More particularly, the tunable circuit includes a varactor and a voltage source for applying a bias voltage to the varactor for controlling capacitance thereof and thereby tuning the coil for enhanced signal reception.  
           [0008]    In accordance with a feature of the invention, the tunable circuit can be tuned either manually or automatically.  
           [0009]    The invention and objects and features thereof will be more readily apparent from the following description and appended claims when taken with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIGS. 1A, 1B illustrate a flexible coil closed configuration and in an open configuration, respectively.  
         [0011]    [0011]FIG. 2 is a schematic of a tunable circuit in accordance with one embodiment of the invention, interconnecting a flex coil and an output port.  
         [0012]    [0012]FIG. 3 is a block diagram of automatic tuning circuitry for the tunable circuit of FIG. 2.  
         [0013]    FIGS.  4 A- 4 C are phantom images of a flexible coil which are tuned, detuned, and retuned, respectively, along with graphs of the SNR for each image.  
         [0014]    [0014]FIG. 5 is a schematic diagram of an autotuning receiver circuit.  
         [0015]    [0015]FIG. 6 is a plot of impedance curves for tuned, detuned, and automatically retuned coils, respectively. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1A illustrates a flex coil in a closed configuration for insertion through a sheath for deployment, and FIG. 1B illustrates the flexible coil in an open state after deployment. It will be appreciated that the flexible coil can be opened to various configurations, each of which requires tuning for optimum reception of MRI signals.  
         [0017]    [0017]FIG. 2 is a schematic of a receiver for interventional MRI signal detection, including a flex coil  10  and a tunable circuit shown generally at  12 , which interconnects flex coil  10  to a receiver port  14 . The tunable circuit includes a varactor  16  connected in parallel with a capacitor  18 , which provides tuning for coil  10  in maximizing signal-to-noise ratio for optimum signal detection.  
         [0018]    A varactor is a variable capacitance diode having a pn junction with a diode depletion-layer capacitance. The depletion-layer capacitance is a function of voltage across the pn junction. Thus the varactor semiconductor diode has a strongly voltage-dependent shunt capacitance between terminals which can be used for tuning the flex coil.  
         [0019]    Tuning of the varactor is effected with a DC tuning voltage  20  which is applied through a manually-controlled potentiometer  22  to tune the varactor. Two 20 kohm resistors  23 ,  24  provide RF isolation, but do not degrade the quality Q of the coil. A large DC blocking capacitor  26  ( 10 nF) prevents the Q-spoiling PIN diode  28  at the output port from detuning the varactor.  
         [0020]    Tests were performed in a GE 0.5T Signa SP open scanner. The tuned Q varied between  55  and  65  depending on the coil shape. Higher Q&#39;s are possible if smaller non-magnetic varactors can be placed at the coil origin. To tune the coil, rapid gradient echo images with one to two second update rates were performed. The operator interactively tuned the tuning coil until the image achieved its maximum brightness, which corresponded to on resonance.  
         [0021]    [0021]FIG. 3 is a functional block diagram of automatic tuning apparatus including a microcontroller  30 , phase detector  32 , and frequency synthesizer  34 , which are connected through tune/receive switch  36  for use in autotuning of the RF coil  38 . Once the coil is tuned for a desired Larmor frequency, switch  36  connects the coil to a scanner  40  through pre-amplifier  42 . In testing the tuning, coil  38  was autotuned using the microcontroller circuitry and imaged. Then the coil shape was made narrower (by 50% into the plane direction) and an image was taken before retuning. Then, after reautotuning, a third image was acquired. The resulting images are shown in FIGS.  4 A- 4 C. Changing the coil width by 50% (narrower and wider, respectively) we tuned the coil by −4 and 2.5 MHz. Autotuning after the shape changes created approximately 70% improvement in SNR as noted in the plots under the images in FIGS.  4 A- 4 C. The plots show pixel-wise SNR for a central, horizontal line through each image.  
         [0022]    [0022]FIG. 5 is a more detailed schematic of the auto tuning receiver circuits in which an ATMEL  90 S 8515  microcontroller is used to control the PLL synthesizer  52  composed of an MC 145170 - 2  PLL, a mini-circuits POS- 100  VCO, and an LT 1227  tri-stateable current feedback amplifier. This block produces and drives a 63.9 MHz signal during tuning, and switches to 90 MHz and a high-Z state to avoid interfering with the coil  60  during signal receive mode. A PIN diode transmit/receive switch  54  provides 40 db of isolation, and along with a classic N 4  impedance transformation at  56 , enables appropriate impedance isolation for both tuning and receiving modes for varactor  62  and coil  60 . A capacitor in parallel with varactor  62  is not shown but can be used as described above to increase total capacitance.  
         [0023]    Phase comparator at  58  uses an AD 835  high-speed multiplier with an RC lowpass filter at the output, and AD 96685  ultra-fast voltage comparators to eliminate amplitude sensitivity. The varactor-diode bias voltage is generated by an LTC 1257  12-bit serial DAC shown at  64 , which the microcontroller steps through the tuning range while comparing the multiplier output to ground. When tuning, the impedance of a parallel resonant circuit is purely resistive on resonance, and is capacitive and inductive if resonant at frequencies below and above the target Larmor frequency, respectively. If a signal is applied at the Larmor frequency to a resonant circuit in series with a reference capacitor, their voltages will have a difference in phase of 90 degrees under tuned conditions.  
         [0024]    [0024]FIG. 6 shows impedance curves for the tunable receiver coil through a progression of conditions. The coil is designed to present 50 ohms when loaded, but loading also shifts the center frequency by as much as 2.8 MHz (curve a in FIG. 6). The automatic retuning circuit then retunes the impedance curve to center on 63.8 MHz, well within the 600 kHz 3 dB bandwidth of the MR signal. Curve a) is the coil loaded by a human fist and detuned to 66.6 MHz; curve b) is the same loading, but automatically tuned to 63.8 MHz; curve c) illustrates different human fist loading and detuned to 62.55 MHz; and curve d) illustrates the coil automatically tuned to 63.8 MHz.  
         [0025]    The use of varactor tuning of MRI coils has proved to be particularly advantageous with a flexible coil which is variable in configuration both during deployment and after deployment. The coil can be readily tuned either manually or automatically for optimum SNR value.  
         [0026]    Further, the computer control system allows the tuning to be synchronized to the dead time within an MRI pulse sequence. Also, the interventional coil can be rapidly detuned in the transmit period of an MRI sequence by varying the varactor voltage, thus further reducing the possibility of RF interaction artifacts in the final MRI image. The coil is retuned in time for the signal reception. This capability adds to the function of simple PIN diode Q spoiling to provide more robust artifact reduction and improved image quality.  
         [0027]    While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. For example, the phase comparison method is just one embodiment for automatic tuning. Another would use amplitude and phase comparison in the form of impedance measurement or complex ratios of received to transmitted signals when compared to a reference impedance (not just capacitor—it could be resistor). Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.