Abstract:
As the static magnetic field used in Magnetic Resonance Imaging (“MRI”) instruments increases the resonance frequency also increases. Consequently, the signal lost due to the coil becomes an issue. To compensate for this loss, it is possible to use an active device, such as a diode, a transistor, etc., with the radio frequency coil, MRI arrangement and method according to exemplary embodiments of the present invention to generate negative resistance to cancel the coil loss resistance. In this manner, the efficiency of the RF coil at high frequency can be improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. Patent Application No. 60/551,145, filed Mar. 8, 2004, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Using a magnetic resonance imaging (“MRI”) system, a substantially uniform main magnetic field is usually generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject (or a portion thereof) being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the main magnetic field by transmitting radio frequency excitation signals into the examination region. Conventionally, radio frequency pulses that are transmitted via a radio frequency (“RF”) coil assembly move the dipoles out of alignment with the main magnetic field, and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. Then, a corresponding radio frequency magnetic signal is generated. The RF magnetic resonance signal is received by the RF coil assembly, and from the received RF signals, an image representation can be reconstructed for display, and/or data obtained for other purposes. 
     Intrinsic signal-to-noise ratio (“SNR”) of the MRI system can be proportional to the static magnetic field. The MRI technology development is moving towards higher static magnetic fields (e.g., 7 T, 8 T, or 9.4 T). However, an increase in the static magnetic field can increase the resonance frequency. Such higher resonance frequency can result in a more significant coil loss that otherwise could be neglected in a low field (e.g., 1.5 T or below). This coil loss may includes a radiation loss and a conductor loss. Thus, the efficiency of conventional passive radio frequency (“RF”) coils is reduced, likely significantly, when the resonance frequency increases. Such reduction in efficiency of the conventional passive RF coils may compromise the higher SNR that is obtained by increasing the static magnetic field. One of the objects of the present invention is to address such efficiency reduction, e.g., using an active RF coil. 
     Previously, active devices have been used in active antenna and active filter designs. However, the use of the active devices not been significantly utilized in the conventional MRI coil designs. One of the reasons for such lack of utilization is because the sample loss (as opposed to coil loss) is dominant in common low field (1.5 T) RF detection. As the static magnetic field increases, the resonance frequency increases, and signal lost due to the coil becomes an issue, and therefore minimizing loss due to the coil is desirable. 
     SUMMARY OF THE INVENTION 
     In order to compensate for the coil loss in the high field, and in accordance with at least one of the objects and exemplary embodiments of the present invention, it is possible to use an active device (e.g., a diode, a transistor, etc.) to generate a negative resistance to cancel the coil loss resistance. In this manner, the efficiency of the RF coil at a high frequency can be improved. 
     According to one exemplary embodiment of the present invention, a radio frequency coil (“RF”), MRI system and method can be provided adapted for high field magnetic resonance imaging. In particular, an arrangement can be provided therefor exhibiting negative resistance characteristics. The arrangement can include a negative resistance device which has a field effect transistor. The field effect transistor can include a Metal Shottky field effect transistor (“MESFET”), a Metal Oxide Semiconductor Field Effect Transistor (“MOSFET”), a high electron mobility transistor (“HEMT”) and/or a Pseudomorphic High Election Mobility Transistor (“pHEMT”). Further, the field effect transistor can include a common-gate which includes a inductive feedback device, a common-source which includes a capacitive feedback and drain output device, and a common-source which includes a capacitive feedback with gate output device. 
     According to another exemplary embodiment of the present invention, the arrangement can be a bipolar-junction transistor. The bipolar-junction transistor may include a NPN transistor and/or a PNP transistor. Further, the arrangement can include a diode. The diode may include a Gunn diode, an Impact Ionization Avalanche Transit-Time diode (IMPATT diode) and/or a tunnel effect diode. The arrangement can also be an operational amplifier. 
     According to still another exemplary embodiment of the present invention, at least one capacitor can be provided which is capable of controlling the arrangement. For example, the arrangement can be controlled using the at least one capacitor by controlling a transconductance and/or a feedback capacitance. The arrangement may be capable of exhibiting capacitive properties. The arrangement is capable of exhibiting inductive properties. In addition, the arrangement can be a negative resistance device which is shunted to a ground with a copper strip. 
     In a further exemplary embodiment of the present invention, a varactor may be provided which is capable of controlling the negative resistance of the arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an exemplary simplified circuit diagram of an Inductance-Capacitance-Resistance (“LCR”) equivalent circuit for a conventional low field RF coil; 
         FIG. 2  is an exemplary circuit diagram of a field-effect transistor (“FET”) type negative resistance device used with an active RF coil, in accordance with one exemplary embodiment of the present invention; 
         FIG. 3A  is an exemplary display illustration of a real part of a negative resistance measurement of the FET negative resistance device; 
         FIG. 3B  is another exemplary display illustration of a negative resistance measurement of the FET negative resistance device expressed in a Smith Chart where all the measured points are provided outside of a zero resistance circle (e.g., a negative resistance); 
         FIG. 4  is an exemplary circuit diagram of a resonator circuit which includes a negative resistance device according to an exemplary embodiment of the present invention to compensate for loss due to the characteristics of the coil; 
         FIG. 5A  is an exemplary active array element with the negative resistance device shunted to the ground shield in a front end with a copper strip according to one exemplary embodiment of the present invention; and 
         FIG. 5   b  is a further exemplary active array element with the negative resistance device shunted to the ground shield in series with a copper strip according to another exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components, or portions of the illustrated embodiments. Moreover, while the present invention will now be described in detail with reference to the Figures, it is done so in connection with the illustrative embodiments. 
     DETAILED DESCRIPTION 
     Conventional MRI RF coils have utilized passive resonators (“PR”) which can be modeled with a variety of LCR circuits, as described in Chen C-N et al., Biomedical Magnetic Resonance Technology, Adam Hilger (1989). Although there are some active parts on various coils, such as diodes, these active parts are likely only provided for a transmit/receive (“TR”) switch, and generally do not contribute to coil tuning and matching. To demonstrate the limitation of passive resonators in lossy conditions which is likely the reality in high field, it is possible to analyze a simplified circuit model for a conventional surface coil  10  (shown in  FIG. 1 ) In this conventional coil  10 , L is the inductance of copper strip, C is the tuning capacitor, and a resistor R is the sum of sample loss and coil loss. As shown in  FIG. 1 , the resistor R and the inductor L are provided in series, and such arrangement of the resistor R and the inductor L is provided in parallel with the capacitor C. The resonance frequency (ω) and Q-factor of this circuit  10  are as follows: 
     
       
         
           
             
               
                 
                   
                     ω 
                     = 
                     
                       
                         
                           1 
                           LC 
                         
                         - 
                         
                           
                             R 
                             2 
                           
                           
                             L 
                             2 
                           
                         
                       
                     
                   
                   , 
                   
                       
                   
                   ⁢ 
                   
                     Q 
                     = 
                     
                       
                         
                           ω 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           L 
                         
                         R 
                       
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     As indicated herein above, the higher the magnetic field, the higher the coil loss. Therefore, Equation [1] indicates the following. First, when the frequency-dependent coil loss is increased to the extent that the resistance of the resistor R becomes larger than the square root of L/C, then ω becomes an imaginary number, which means that this passive LCR circuit likely cannot resonate. Second, a higher resistance of the resistor R may lead to a lower Q-factor. To avoid the diminishing consequence of high coil loss in high field, e.g., in a 7 T body coil, it is preferable to provide a negative resistance from a transistor circuit, e.g., to cancel at least a portion of the resistance, keep the resonance frequency ω from becoming an imaginary number, and maintain the coil&#39;s Q high. 
     The resistance of the resistor R is defined by Ohm&#39;s law R=V/I, in which V is voltage and I is current. However, various active devices, such as diodes and transistors, do not always follow Ohm&#39;s law. A more broad definition of resistance can be provided by a ratio of a differential voltage to a differential current, i.e., R=dV/dI. In general, the current tends to rise with an increasing voltage. However, for transistors and diodes, there are certain conditions where current falls as voltage rises. This can result in a negative resistance, which can be realized by, for example, (i) a Gunn Diode, (ii) an FET, or (iii) a bipolar-junction-transistor (“BJT”), as described in Mantena N.R. et al., “Circuit Model Simulation of Gunn Effect Devices,” IEEE Trans. on Microwave Theory and Techniques (1969) MTT-17, p. 363, Karacaoglu U. et al., “Dual-Mode Microstrip Ring Resonator Filter with Active Devices for Loss Compensation,” IEEE MTT-S International Microwave Symposium Digest (1993), pp. 189-192 (“Karacaoglu et al.”), and Adams D.K. et al., “Active Filters for UHF and Microwave Frequencies,” IEEE trans. on Microwave Theory and Techniques (1969) MTT-17, pp. 662-670, respectively. 
     One example of the negative resistance that can be obtained by utilizing the FET is shown in  FIG. 2 , which illustrates an exemplary circuit diagram of a field-effect transistor (“FET”) type negative resistance device  20  used with an active RF coil, in accordance with one exemplary embodiment of the present invention. This device can be included as a part of the MRI system  25  and/or provided in communications with (either wired or wireless) therewith. In particular, the device includes a transistor  30 , and a resistor  35  which connected to the drain terminal thereof. In addition, a capacitor C fb  can be attached to the transistor  30  as well. For example, the impedance of the device  20  can be analyzed in Karacaoglu et al. according to formula 2. 
                       Z     i   ⁢           ⁢   n       =       -       g   m         ω   2     ⁢     C   gs     ⁢     C   fb           -     j   ⁡     (       1     ω   ⁢           ⁢     C   gs         +     1     ω   ⁢           ⁢     C   fb           )           ,           [   2   ]               
In particular, C gs  is the gate-source capacitance, C jb  is the feed back capacitance, g m  is the mutual conductance. The exemplary circuit  20  shown in  FIG. 2  provides negative resistance and capacitance.
 
     The device  20  of  FIG. 2  can be arranged to observe the negative resistance on a network analyzer, e.g., Agilent E5070B (CA). The selected FET  30  can be, e.g., ATF-34143-BLK manufactured by Agilent. The trans-conductance of this exemplary FET  30  can be g m =230, and the noise value may be 0.5 dB. When the feedback capacitor C fb  is 200 pF, the drain of the FET  30  may be DC biased and/or not biased, the gate port thereof may provide a negative resistance of −11 Ω at 300 MHz.  FIG. 3A  shows is an exemplary display illustration of a real part of a negative resistance (or impedance) measurement of the FET negative resistance device, it can be confirmed that the resistance of the circuit is negative, e.g., where (a) is the real part of the impedance measurement.  FIG. 3B  shows another exemplary display illustration of a smith chart measurement in which the measurement points may all be provided outside of zero resistance circle. Therefore, using this exemplary device  20  shown in  FIG. 2  and described herein above, a desired negative resistance can be obtained. In further exemplary embodiments of the present invention, the desired negative resistance can be obtained utilizing other active devices, such as diodes and bipolar junction transistors (“BJT”). In still a further exemplary embodiments of the present invention, the active device may be selected to obtain enhanced power performance. 
     One having ordinary skill in the art with the benefit of this disclosure would clearly realize that other active devices may be utilized to obtain the objects, features and advantages of the present invention. For example a variety of configurations of negative resistance devices according to the present invention may be utilized at, e.g., 300 MHz at 7 T. As known by those of ordinary skill in the art, there may be three common types of solid state devices that can be used for generating negative resistance. For example, these devices may include: (a) FET which can be a Metal Shottky Field Effect Transistor (“MESFET”), a Metal Oxide Semiconductor Field Effect Transistor (“MOSFET”), a High Electron Mobility Transistor (“HEMT”) and a Pseudomorphic High Electron Mobility Transistor (“pHEMT”); (b) BJT which can be either a NPN transistor or a PNP transistor; or (c) a diode, which can be Gunn diode, an Impact Ionization Avalanche Transit-Time diode (“IMPATT” diode), or a tunnel effect diode. In a further exemplary embodiment of the present invention, the negative resistance can also be realized by operational-amplifiers or certain composite materials. Each type of negative resistance device may also have a different circuit topology. For example, in the case of FET, the negative resistance device can be: (a) common-gate with inductive feedback; (b) common-source with capacitive feedback and drain output; or (c) common-source with capacitive feedback with gate output. 
     In a further exemplary embodiment of the present invention, the different types and topologies of the negative resistance devices may be controlled for use as part of the RF coil. For example, it may be possible to adjust the negative resistance values by varying certain portions of the negative resistance device. In particular, with reference to  FIG. 2 , the negative resistance of the device can be calculated using Equation [2], which indicates that the negative resistance can be adjusted by selecting different transconductance g m , and/or feedback capacitance C gs . Such controllability may provide adjustability to increase or maximize the compensation of the signal loss due to the RF coil, without becoming an oscillator. 
     According to another consideration for providing an optimal negative resistance device according to yet another exemplary embodiment of the present invention, an absolute value of the negative resistance may be provided to be in the vicinity of the coil loss resistance, e.g., R CL =R r +R c , where R r  is the radiation loss resistance and R c  is the conductor loss resistance. The R r  and R c  can be determined analytically, based on the conductor geometry of the coil, and/or estimated experimentally. Other considerations for the exemplary design according to the present invention may include the noise considerations and power handling of the device of the present invention. In order to transmit signals, the negative resistance device&#39;s noise figure may not be crucial. However, the power tolerance and power saturation threshold should preferably be optimized. For example, it may be preferable for the power tolerance to be about 200 W for 16 channels and 100 W for 32 channels, given a maximum power output from power amplifier of 8 kW. For receiving signals, the negative resistance device&#39;s noise figure should preferably be as low as possible (e.g., less than 0.5 dB), while power handling should preferably be on the order of milliwatts (mW). 
     In a still further exemplary embodiment of the present invention, the Q of the resonators can be enhanced by integrating the negative resistance device to compensate the loss. Various different negative resistance circuits can be used for such purpose. As one exempla, the common-source FET negative resistance circuit  210  provided in an MRI arrangement  200  shown in  FIG. 4  as a simplified equivalent circuit of a single FET active resonator may preferably be used because such circuit can utilize (or include) a varactor  215  as the source feedback element to control the negative resistance more precisely, as described, e.g., in Karacaoglu U. et al., “MMIC Active Bandpass Filters Using Varactor-Tuned Negative Resistance Elements,” IEEE trans. on Microwave Theory and Techniques (1995) 43, pp. 2926-2932. 
     In other exemplary embodiments of the present invention, an active array element may be used as part of a high field MRI radio frequency coil. One such exemplary embodiment includes the active array element which has a negative resistance device, either (i) shunted to the ground shield in the front end, or (ii) provided in series with a copper strip, for example, as shown in  FIGS. 5A and 5B , respectively. In still a further exemplary embodiment, the negative resistance device can be either inductive or capacitive depending on its topology. 
     For example, the exemplary embodiment shown in  FIG. 5A  is provided in the MRI arrangement  250 , and includes a serially connected capacitors  255  (C 1 ) and  260  (C 2 ). This capacitor combination is connected in parallel to the negative resistance device (“NRD”)  265 . The combination of the capacitors  255 ,  260  and the NRD  265  are connected at one end to a copper strip  270 , and on the other end to a ground shield  275 . On an opposite end, the copper strip  270  and the ground shield  275  are connected in series to another capacitor  280  and an enable device  285 . In this manner, an array element configuration can be formed which may also be used in the active filter arrangement according to the present invention, as shown in  FIG. 4 . In the exemplary embodiment of  FIG. 5B  which can also be provided in the MRI arrangement  300 , the capacitors  255 ,  260  are provided in series with the NRD  265  (instead of in parallel as shown in  FIG. 5A ). Although the negative resistance of this exemplary embodiment can directly cancel the conductor loss of the copper strip  270 , if the NRD  265  is inductive, it may increase the total inductance C total , and possibly force capacitors  255  (C 1 ) and  280  (C 3 ) to become too small to be realistic. If the NRD  265  is capacitive, it may cause the match to become more complicated, and the exemplary device shown in  FIG. 5A  can be used. 
     The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present invention. All publications and references cited above are incorporated herein by reference in their entireties.