Abstract:
An arrangement to detune a reception antenna in a local coil of a magnetic resonance system, with at least one reception antenna that is fashioned as a loop antenna and that has at least one first capacitance. Radio-frequency signals of a magnetic resonance examination are received via the reception antenna. A switchable detuning circuit contains the first capacitance connected to an oscillating circuit and a first inductance. A reception device to receive a control signal is coupled with the oscillating circuit. The reception device switches the oscillating circuit into a high-resistance state given a received control signal so that a receipt of a radio-frequency signal via the reception antenna is prevented. The reception device is fashioned to receive a wirelessly transmitted radio-frequency control signal. The radio-frequency control signal has a frequency that lies outside of the bandwidth of the radio-frequency signal used for magnetic resonance examination.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns an arrangement to detune a reception antenna in a local coil of the type employing a switchable tuning circuit. 
     2. Description of the Prior Art 
     In modern magnetic resonance systems, magnetic resonance examination signals are acquired with the use of local coils placed close to the patient. 
     In addition to actual reception antennas (that are preferably fashioned as loop antennas), the local coil contains additional low-noise preamplifiers in order to be able to amplify the acquired magnetic resonance signals. The local coil additionally contains for each loop antenna, an associated active detuning circuit with which that reception antenna can be detuned during a transmission phase. 
     During the transmission phase the detuning is necessary in order to prevent an unwanted inductive injection of strong currents (which could be generated in the reception antenna by a received transmission signal) into the patient body. The unwanted reception of the transmission signal in the reception antenna is thus prevented with the use of the detuning circuit. 
       FIG. 5  shows an arrangement to detune a reception antenna LA of a local coil that is designed as a loop antenna. 
     The reception antenna LA shows four capacitances C 1  through C 4  that are fashioned to shorten the reception antenna LA. 
     A first capacitor C 1  together with a first inductance L 51  forms a block circuit that, together with a first diode D 51 , a second diode D 52  and a second inductance L 52  forms a detuning circuit VSS. 
     The first diode D 51 , the second diode D 52  and the second inductor L 52  are thereby connected in parallel with one another, and the two diodes D 51 , D 52  are connected with opposite polarity relative to one another. 
     This parallel circuit is connected at the input side with a first end of the first capacitor C 1  while the parallel circuit is connected at the output side via the first inductance L 51  with a second end of the first capacitor C 1 . 
     As described, during the transmission phase it is necessary to detune the reception antenna LA. For this purpose a controllable switch DCD is provided that is connected parallel to the two diodes D 51 , D 52 . 
     The switch DCD is preferably designed as a DC diode and is switched with the aid of a direct current signal DCSS as a control signal. The direct current signal DCSS as a control signal is directed via cables to the switch DCD. 
     Given a closed switch DCD, the first inductance L 51  together with the first capacitance C 1  forms a high-resistance block circuit, such that the detuning circuit VSS “detunes” and the loop-shaped reception antenna LA is therefore quasi-interrupted. In this case no reception of magnetic resonance signals is possible via the reception antenna LA. 
     If the first capacitance C 1  has a residual charge, or that the reception antenna LA nevertheless receives during the transmission phase (for example because the control signal DCSS is not present due to a line disconnection), either the first diode D 51  or the second diode D 52  (depending on polarity of the capacitance C 1 ) is switched through, such that the first inductance L 51  together with the first capacitance C 1  forms a high-resistance block circuit. The detuning circuit VSS is therefore also “detuned” for these cases. 
     The second inductance L 52  serves to discharge residual voltages of the two diodes D 51  and D 52 . This type of detuning is designated as “active detuning”. 
     Given such local coils the respective preamplifier and the respective detuning circuits are supplied via cables with operating voltage and operating power. 
     The received magnetic resonance signals are also forwarded on via cables for additional processing. 
     The DC control signals described above for the active detuning circuit are transferred via cables. 
     Due to this multiplicity of necessary cables, a cable bundle connected with the local coil is formed that is unwieldy and can be difficult to manage. 
     Due to the dimensions and the resulting weight, as well as the movement of the cable bundle, mechanical defects can occur at the respective plugs and at the individual cables. 
     Concepts are presently being developed that enable a wireless transmission of the received magnetic resonance signals from the local coil for additional processing. 
     The received magnetic resonance signals are thereby amplified only after a successful wireless transmission, such that the preamplifiers are no longer an integrated part of the local coil. 
     The mechanically susceptible cable connections that are required for the feed of the control signals for the active detuning circuit still remain. 
     SUMMARY OF THE INVENTION 
     An object of the present invention to provide, for a local coil whose received magnetic resonance signals are transmitted wirelessly for additional processing, an arrangement for detuning that is less error-prone with less expenditure. 
     This object is achieved by an arrangement according to the invention for detuning a reception antenna in a local coil having at least one reception antenna that is fashioned as a loop antenna and that ahs at least one first capacitance. Radio-frequency signals of a magnetic resonance examination are received by the reception antenna. A switchable detuning circuit contains the first capacitance a first inductance connected to an oscillating circuit. A reception device to receive a control signal is coupled with the oscillating circuit, and the reception device switches the oscillating circuit into a high-resistance state when a control signal is received. A receipt of a radio-frequency signal via the reception antenna is therefore prevented. 
     The reception device is fashioned to receive a wirelessly transmitted radio-frequency control signal. The radio-frequency control signal has a frequency that lies outside of the bandwidth of the radio-frequency signal used for magnetic resonance examination. 
     An active detuning that is activated or deactivated with the use of an additional wirelessly transmitted control signal as an auxiliary signal is achieved by the arrangement according to the invention. 
     The active detuning foregoes a wired feed of control signals so that the previously necessary connection cables for control signal transmission are spared. 
     The activation or deactivation of the detuning circuit no longer ensues via cables and is therefore less error-prone. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a first embodiment of the arrangement according to the invention. 
         FIG. 2  shows a second embodiment of the arrangement according to the invention. 
         FIG. 3  shows a third embodiment of the arrangement according to the invention. 
         FIG. 4  shows a fourth embodiment of the arrangement according to the invention. 
         FIG. 5  shows the arrangement described above for detuning a reception antenna of a local coil according to the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a first embodiment of the arrangement according to the invention with what is known as a loop antenna LA as a reception antenna. 
     The reception antenna LA shows four capacitances C 1  through C 4  that are fashioned to shorten the reception antenna LA. 
     A first capacitor C 1  together with a first inductance L 11  forms a block circuit that, together with a first diode D 11 , a second diode D 12  and a second inductance L 12 , forms a detuning circuit VSS. 
     The first diode D 11 , the second diode D 12  as well as the second inductance L 12  are connected in parallel to one another, wherein the two diodes D 11 , D 12  are additionally connected or, respectively, arranged inverse to one another. 
     This parallel circuit is connected on the input side with a first end of the first capacitor C 1  while the parallel circuit is connected at the output side via the first inductance L 11  with a second end of the first capacitor C 1 . 
     In a patient examination, magnetic resonance signals with a bandwidth BMR are received via the reception antenna LA. 
     These received magnetic resonance signals are induced by a transmission signal that is radiated towards the patient during a transmission phase via a transmission antenna (not shown here). 
     As described above, it is necessary to detune the reception antenna LA during the transmission phase. 
     For this purpose, not only the transmission signal but also an additional auxiliary signal HFS as a control signal are transferred during the transmission phase. The auxiliary signal HFS has a frequency f HFS  that is located just outside the bandwidth B MR  of the transmission signal used for examination. 
     In the embodiment shown here, the auxiliary signal HFS is received via the reception antenna LA. Either the first diode D 11  or the second diode D 12  is switched through by the auxiliary signal HFS, such that the first inductance L 11  together with the first capacitance C 1  forms a high-resistance block circuit. The detuning circuit VSS is therefore detuned. 
     The second inductance L 12  serves to discharge residual voltages of the two diodes D 11  and D 12 . 
       FIG. 2  shows a second embodiment of the arrangement according to the invention, with a loop antenna LA as a reception antenna. 
     The reception antenna LA shows four capacitances C 1  through C 4  that are fashioned to shorten the reception antenna LA. 
     A first capacitor C 1  together with a first inductance L 21  forms a block circuit that, together with a first diode D 21 , a second diode D 22  and a second inductance L 22 , forms a detuning circuit VSS. 
     The first diode D 21 , the second diode D 22  as well as the second inductance L 22  are thereby connected in parallel to one another, wherein the two diodes D 21 , D 22  are additionally connected or, respectively, arranged inverse to one another. 
     This parallel circuit is connected on the input side with a first end of the first capacitor C 1  while the parallel circuit is connected at the output side via the first inductance L 21  with a second end of the first capacitor C 1 . 
     In a patient examination, magnetic resonance signals with a bandwidth BMR are received via the reception antenna LA. 
     These received magnetic resonance signals are induced by a transmission signal that is radiated towards the patient during a transmission phase by a transmission antenna (not shown). 
     As described above, it is necessary to detune the reception antenna LA during the transmission phase. 
     For this purpose not only the transmission signal but also an additional auxiliary signal HFS as a control signal are transferred during the transmission phase. The auxiliary signal HFS has a frequency f HFS  that is located just outside the bandwidth BMR of the transmission signal used for examination. 
     In the embodiment shown here, the auxiliary signal HFS is received via an auxiliary antenna HA. 
     The auxiliary antenna HA here is exemplarily designed as a loop antenna and contains a first capacitance CS 21  and a second capacitance CS 22  for antenna shortening. 
     The auxiliary antenna HA is wired for coupling or connection with the detuning circuit VSS such that the first terminal of the first capacitance C 1  is connected with a second terminal of the capacitance CS 21  while the second terminal of the first capacitance C 1  is connected via the inductance L 21  with a first terminal of the capacitance CS 21 . 
     Either the first diode D 21  or the second diode D 22  is switched through by the auxiliary signal HFS, such that the first inductance L 21  together with the first capacitance C 1  forms a high-resistance block circuit. The detuning circuit VSS is therefore detuned. 
     The second inductance L 22  serves to discharge residual voltages of the two diodes D 21  and D 22 . 
     The advantage of this auxiliary antenna embodiment lies in that the actual reception antenna LA does not have to be modified with regard to its radio reception properties because the auxiliary signal reception is carried out independently via the auxiliary antenna HA. 
       FIG. 3  shows a variant (relative to  FIG. 2 ) of the arrangement according to the invention as a third embodiment of the invention. 
     Corresponding to  FIG. 2 , a first capacitor C 1  of the reception antenna LA forms a detuning circuit VSS together with a first inductance L 31 , a first diode D 31 , a second diode D 32  and a second inductance L 32 . 
     The first diode D 31 , the second diode D 32  as well as the second inductance L 32  are thereby connected in parallel to one another, wherein the two diodes D 31 , D 32  are additionally connected or, respectively, arranged inverse to one another. 
     This parallel circuit is connected on the input side via a λ/4 phase shifter with a first end of the first capacitor C 1 , while the parallel circuit is connected at the output side via the λ/4 phase shifter and via the first inductance L 31  with a second end of the first capacitor C 1 . 
     The λ/4 phase shifter contains two capacitances C 36  and C 37  in a shunt arm in the form of a T-circuit as well as an inductance L 33  in a series arm. 
     The first end of the capacitance C 1  is connected via the two capacitances C 36  and C 37  with the input of the parallel circuit. 
     The first end of the capacitance C 1  is connected via the capacitance C 36 , the inductance L 33  and the first inductance L 31  with the second end of the capacitance C 1 . 
     The second end of the capacitance C 1  is connected via the first inductance L 31  with the output of the parallel circuit. 
     In the embodiment shown here, the auxiliary signal HFS used as a control signal is received via an auxiliary antenna HA. 
     The auxiliary antenna HA here is exemplarily designed as a loop antenna and contains a first capacitance CS 31  and a second capacitance CS 32  for antenna shortening. 
     The auxiliary antenna HA is wired for coupling or, respectively, connection with the detuning circuit VSS such that the first terminal of the first capacitance C 1  is connected via the two capacitances C 36  and C 37  with a second terminal of the capacitance CS 31  while the second terminal of the first capacitance C 1  is connected via the inductance L 31  with a first terminal of the capacitance CS 31 . 
     The second inductance L 32  serves to discharge residual voltages of the two diodes D 31  and D 32 . 
     The mode of operation of the arrangement presented here differs with regard to the function described in  FIG. 2 . 
     If an auxiliary signal HFS is absent, the two diodes D 31  and D 32  are not switched through and thus form a radio-frequency open circuit at the λ/4 transformer. This open circuit is transformed via the λ/4 transformer into a radio-frequency short in the direction of the reception antenna LA so that the detuning circuit VSS is detuned via the capacitance C 1  and via the inductance L 31 . The capacitance C 1  and the inductance L 31  form a high-resistance block circuit. 
     If, in contrast to this, the auxiliary signal HFS is transferred and received via the auxiliary antenna HA, either the diode D 31  or the diode D 32  is switched through so that a radio-frequency short is formed at the λ/4 transformer. This short is transformed via the lamdat/4 transformer into a radio-frequency open circuit in the direction of the reception antenna LA so that the detuning circuit VSS is tuned. The capacitance C 1  is decoupled from the inductance L 31 ; the block circuit cannot act via the open circuit. 
     In summary, the detuning circuit VSS here is basically detuned. The detuning circuit VSS is essentially deactivated only given the transmission of the auxiliary signal HFS, such that the reception antenna LA is ready for operation. 
     The auxiliary signal HFS is correspondingly not transmitted during the transmission phase but rather during the duration of a desired acquisition phase. 
       FIG. 4  shows a fourth embodiment of the arrangement according to the invention, with a loop antenna LA as a reception antenna. 
     The reception antenna LA shows four capacitances C 1  through C 4  that are fashioned to shorten the reception antenna LA. 
     A first capacitor C 1  together with a first inductance L 41 , a first diode D 42 , a second diode D 43 , a second inductance L 42  and a third inductance L 43 , forms a detuning circuit VSS. 
     The first diode D 42  is a PIN diode while the second diode D 43  is designed as a rectifier diode. 
     The first diode D 42  and the second inductance L 42  are connected in series. The second diode D 43  and the third inductance L 43  are connected in series. 
     Both series circuits are in turn connected in parallel with one another, wherein the two diodes D 42 , D 43  are additionally connected or, respectively, arranged inverse to one another. 
     The parallel circuit is connected on the input side via the first inductance L 41  with a first end of the first capacitor C 1 , while the parallel circuit is connected at the output side with a second end of the first capacitor C 1 . 
     In a patient examination, magnetic resonance signals with a bandwidth BMR are received via the reception antenna LA. 
     These received magnetic resonance signals are induced by a transmission signal that is radiated towards the patient via a transmission antenna (not shown here) during a transmission phase. 
     As described above, during the transmission phase it is necessary to detune the reception antenna LA. 
     For this purpose not only the transmission signal but also an additional auxiliary signal HFS as a control signal are transferred during the transmission phase. The auxiliary signal HFS has a frequency f HFS  that is located just outside the bandwidth BMR of the transmission signal used for examination. 
     In the embodiment shown here, the auxiliary signal HFS is received via the reception antenna LA. Either the first diode D 42  or the second diode D 43  is switched through by the auxiliary signal HFS, such that the first inductance L 41  together with the first capacitance C 1  forms a high-resistance block circuit. The detuning circuit VSS is therefore detuned. 
     The parallel connection described above of the first series circuit with the elements L 42  and D 42  and the second series circuit with the elements L 43  and D 43  can seamlessly replace the parallel connection of the elements D 11 , D 12  and L 12  described in  FIG. 1 . 
     The same applies for the parallel connections of the elements D 21 , D 22  and L 22  from  FIG. 2  and the elements D 31 , D 32  and L 32  from  FIG. 3 . 
     The described embodiments in particular have the advantage that a control signal HFS received with a low amplitude causes a switching of the oscillating circuit. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.