Transmitting/receiving system for magnetic resonance imaging system
BACKGROUND OF THE INVENTION 
     The present invention relates to a transmitting/receiving system used for transmitting a high-frequency field to excite a magnetic resonance (MR) phenomenon and receiving and detecting an MR signal produced by the MR phenomenon in a magnetic resonance imaging (MRI) system for exciting the MR phenomenon in an object and for obtaining MR information in the object. 
     In an MRI system, a uniform static field is applied to a desired portion of an object, and a transmitting RF coil for forming a high-frequency field, i.e., an RF (radio frequency) field in a direction perpendicular to the static field is used. Thus, an MR phenomenon is caused in a specific slice from which a tomographic image is to be obtained. Then, MR signals generated from atomic nuclei after the RF field is removed are detected using a receiving RF coil. Upon excitation of the MR phenomenon and acquisition of the MR signals, in order to select a specific slice and/or to provide position information to MR signals, a gradient field is applied to the object, as needed. 
     FIG. 1 shows an arrangement of a conventional transmitting/receiving system in the MRI system. 
     A transmitting system comprises transmitting/receiving coil 1, variable capacitor 2, RF pulse oscillator 3, and cross diode switch 4. A receiving system comprises capacitors 8 and 15, preamplifiers 9 and 16, and first and second receivers 18 and 19. 
     Cross diode switch 4 has a basic arrangement in which two diodes are cross-coupled. Cross diode switch 4 serves as a high-frequency switch for passing a signal having an amplitude larger than a predetermined value and cutting off a signal having a smaller amplitude. RF pulses from oscillator 3 are applied to transmitting/receiving coil 1 through cross diode switch 4, and the RF pulses are transmitted from transmitting/receiving coil 1 to an object. 
     MR signals from the object are received through first or second receiver 18 or 19. 
     First receiver 18 has transmitting/receiving coil 1, variable capacitors 2 and 7, duplexer 5, and cross diode switch 6. Variable capacitor 7 is adopted for tuning, and is used for adjustment for tuning first receiver 18 with the MR signals from the object. Transmitting/receiving coil 1 is used for both transmission and reception, and normally comprises a saddle coil. Duplexer 5 is provided as needed, and is used for switching transmitting and receiving modes together with cross diode switches 4 and 6. Duplexer 5 is constituted by, e.g., a λ/4 cable or coil. First receiver 18 is used for receiving MR signals in a whole body (WB) imaging mode. 
     Second receiver 19 is constituted by receiving coil 12, cross diode switch 13, and variable capacitor 14 which are connected in parallel with each other. Variable capacitor 14 is used for adjustment for tuning second receiver 18 with the MR signals from the object. As receiving coil 18, a small coil having a good filling factor for obtaining a higher S/N ratio than that of transmitting/receiving coil 1 is adopted. Second receiver 19 is often used for receiving MR signals in a head portion imaging mode. A surface coil used for receiving MR signals in a local imaging mode for locally imaging a portion near a body surface belongs to receiving coil 12. 
     In the above arrangement, in the WB imaging mode, RF pulses are transmitted through transmitting/receiving coil 1, thus causing the MR phenomenon in an object. MR signals generated by the MR phenomenon are amplified by preamplifier 9, and are then processed by data processor 11. A method wherein excitation of the MR phenomenon and reception of MR signals are performed using only transmitting/receiving coil 1 is called a single coil method. 
     In the head portion (brain) imaging mode, RF pulses are transmitted through transmitting/receiving coil 1 to excite the MR phenomenon in the same manner as in the single coil method. In this case, MR signals are received through second receiver 19, and the reception signals are amplified by preamplifier 16. Thereafter, the signals are processed by data processor 11. In this manner, a method wherein transmission of the high-frequency field and reception of the MR signals are respectively performed by transmitting coil 1 and receiving coil 12 is called a cross coil method. 
     However, first and second receivers 18 and 19 are designed and adjusted to resonate, i.e., tuned with the MR signals in order to efficiently receive the MR signals from the object. When MR signals are received by first receiver 18, receiving coil 12 is unnecessary, and second receiver 19 including coil 12 is disabled. When MR signals are received through second receiver 19, transmitting/receiving coil 1 is necessary for transmitting a high-frequency field, and it cannot be disabled. In this case, since first receiver 18 is also adjusted to be tuned and resonate at a frequency of the MR signals, an inductive electromotive force to be induced in only receiving coil 12 is also partially induced in transmitting/receiving coil 1. As a result, a reception level of MR signals of second receiver 19 is decreased. A decrease in reception level of MR signals causes degradation in image quality of an MR image to be formed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a transmitting/receiving system capable of increasing a reception level of MR signals of a receiver including a receiving coil in an MRI system which can selectively use both a single coil method and a cross coil method. 
     A transmitting/receiving system according to the present invention comprises a first high-frequency coil section used for both transmission and reception, a transmitter for transmitting a high-frequency field for exciting an MR phenomenon to an object through the first high-frequency coil section, a first receiver which constitutes a first resonator, capable of resonating with an MR signal from the object, together with the first high-frequency coil section, for receiving the MR signal through the first high-frequency coil section, a second high-frequency coil section used for only reception, and a second receiver which constitutes a second resonator, capable of resonating with the MR signal from the object, together with the second high-frequency coil section, for receiving the MR signal through the second high-frequency coil section. The transmitting/receiving system further comprises a resonance controller for, when the second receiver is valid, changing a resonance condition of the first resonator to suppress resonance of the first resonator with respect to the MR signal from the object. 
     According to the transmitting/receiving system of the present invention, the first receiver is offset from a resonance point with respect to an MR signal frequency by the resonance controller so as not to resonate with the MR signal. Therefore, an inductive electromotive force is prevented from being partially induced in the transmitting/receiving coil by the MR signal. Thus, the MR signal is resonated with only the second receiver, and the reception level of the second receiver can be increased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a schematic arrangement of a conventional transmitting/receiving system in an MRI system; 
     FIG. 2 is a circuit diagram showing a schematic arrangement of a transmitting/receiving system according to a first embodiment of the present invention; 
     FIGS. 3 and 4 are graphs respectively showing characteristics of a variable capacitance diode and resonance characteristics of a first receiver, for explaining an operation of the embodiment shown in FIG. 2; 
     FIG. 5 is a circuit diagram showing a schematic arrangement of a transmitting/receiving system according to a second embodiment of the present invention; 
     FIG. 6 is a circuit diagram showing a schematic arrangement of a transmitting/receiving system according to a third embodiment of the present invention; and 
     FIG. 7 is a circuit diagram showing a schematic arrangement of a transmitting/receiving system according to a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described hereinafter in detail. 
     FIG. 2 shows a schematic arrangement of a transmitting/receiving system according to the first embodiment of the present invention. The system shown in FIG. 2 employs variable capacitance diode 17 as a tuning capacitor in first receiver 18A. The capacitance of variable capacitance diode 17 is varied and adjusted by controller 10. Variable capacitance diode 17 and controller 10 constitute resonance controller 21. 
     The capacitance of variable capacitance diode 17 changes in accordance with a DC control voltage applied to diode 17, as shown in FIG. 3. Upon a change in capacitance of diode 17, a resonance frequency of first receiver 18A is changed as shown in FIG. 4. Therefore, when a control voltage output from controller 10 is changed, the resonance frequency of first receiver 18A can be changed. 
     When MR signals are received by first receiver 18A like in a WB imaging mode of an object, the capacitance of diode 17 is set to be a predetermined value under the control of controller 10, and first receiver 18 can be set in a resonant state with respect to the MR signals. 
     When MR signals are received by second receiver 19 like in a brain imaging mode of an object, the capacitance of diode 17 is appropriately shifted from the predetermined value under the control of controller 10, and first receiver 18 is set in a non-resonant state with respect to the MR signals. Thus, the MR signals can no longer be received by first receiver 18A. More specifically, no electromotive force is generated in transmitting/receiving coil 1 by MR signals. Therefore, an electromotive force induced in receiving coil 12 is increased accordingly, and hence, an MR signal reception level of second receiver 19 can be increased. 
     Note that transmission of RF pulses and processing after reception of MR signals are the same as those in the conventional system. 
     The embodiment of the present invention has been described. However, the present invention is not limited to the above embodiment. 
     For example, in a second embodiment of the present invention, as shown in FIG. 5, a series circuit of capacitor 20 and switch 24 is connected in parallel with a tuning capacitor to constitute first receiver 18B. Switch 24 is ON/OFF controlled by controller 10A. When MR signals are received by first receiver 18B, switch 24 is turned off by controller 10A. When MR signals are received by second receiver 19, switch 24 is turned on. Upon turning on of switch 24, capacitor 20 is parallel-connected to tuning capacitor 7, and the resonance frequency of first receiver 18B is greatly offset from the MR signal frequency. With this arrangement, the same effect as in the first embodiment can be obtained. In this case, resonance controller 21A is constituted by capacitor 20, switch 24, and controller 10A. Switch 24 need not be automatically controlled by controller 10A but may be manually operated. In this case, a resonance controller is constituted by manual switch 24 and capacitor 20. 
     In a third embodiment of the present invention, as shown in FIG. 6, switch 22 is connected to the output terminal of duplexer 5, and can be turned off when second receiver 19 receives MR signals. When switch 22 is turned off, no resonator is formed in first receiver 18C, and first receiver 18C is set in a non-resonant state with respect to MR signals. Therefore, with this arrangement, the same effect as in the above embodiments can be obtained. In this case, switch 22 constitutes a resonance controller. 
     In a fourth embodiment of the present invention, as shown in FIG. 7, switch 23 is provided in series with tuning capacitor 7, and can be turned off only when MR signals are received by second receiver 19. When switch 23 is turned off, no resonator is formed in first receiver 18D, and first receiver 18D is set in a non-resonant state with respect to MR signals. In this case, switch 23 constitutes a resonance controller. 
     Note that as switches 22 and 23, ones having a very small ON resistance are adopted in order to prevent a decrease in Q (quality) factor of a resonator. The switches can comprise mechanical switches or semiconductor switches.