Magnetic Resonance Signal Detection Using Remotely Positioned Receive Coils

Abstract

The receive coil arrangement includes an inner local volume coil adjacent the part to be imaged so as to maximize the received MR signal and an outer coil, which may be the built in body coil of the magnet, connected by cable to the signal processing system. Both the coils are individually tuned to the common resonant frequency and the local volume coil include an arrangement to halt current flow therein during the transmit stage. The local volume coil has no cable and is arranged to communicate the MR signal therein to the signal processing system through the outer coil by inductive coupling to the outer coil. Despite inherent losses by interfering with the tuning of the loops and in the inductive coupling this magnifies the MR signal and makes the local volume coil wireless.

1 . Apparatus for MR imaging of a subject comprising
 a MR magnet with gradient coil operable to generate a variable magnetic field to be applied to the subject;   an RF transmit arrangement for generating an RF pulse in a transmit stage to be applied to the subject to be imaged such that the subject generates an MR signal in response to the magnetic field and the RF pulse applied;   a receive coil arrangement for acquiring the MR signal in a receive stage;   and a signal processing system for receiving the MR signal for carrying out signal processing by which an image is generated;   the receive coil arrangement comprising:
 a birdcage coil configured to surround the subject so as to receive the MR signal; 
 at least one receive coil; 
 said at least one receive coil having at least one signal communication cable connected to the signal processing system for transferring the MR signal therein to the signal processing system; 
 said at least one receive coil and said birdcage coil being individually tuned to a common resonant frequency for receiving said MR signal; 
 all coil loops of said birdcage coil and said at least one receive coil which act only in the receive stage and do not transmit the applied RF pulse in the transmit stage having therein an arrangement to halt current flow therein at the resonant frequency during the transmit stage so as to prevent the presence of said all coil loops from interfering with the RF pulse during the transmit stage; 
 said birdcage coil being arranged to communicate the MR signal therein to the signal processing system through said at least one receive coil by inducing the MR signal onto said at least one receive coil. 
   
     
     
         2 . The apparatus according to  claim 1  wherein said at least one receive coil is located at a spacing from said birdcage coil such that:
 the signal from said birdcage coil is induced onto said at least one receive coil at an efficiency of induction sufficient that the MR signal on said at least one receive coil is greater than the MR signal which would be generated in the absence of said birdcage coil; and 
 mutual inductance between said birdcage coil and said at least one receive coil is insufficient to change the tuned common resonant frequency of the volume coil and the receive coil sufficiently to reduce the MR signal at said at least one receive coil to a value which is less than the MR signal which would be generated in the absence of said birdcage coil. 
 
     
     
         3 . The apparatus according to  claim 1  wherein said birdcage coil is free from a wired cable carrying the MR signal to the signal processing system. 
     
     
         4 . The apparatus according to  claim 1  wherein said birdcage coil is arranged to be located as close as physically possible to the subject. 
     
     
         5 . The apparatus according to  claim 1  wherein said birdcage coil includes a plurality of loops and each loop includes an addressable switch operable remotely to halt flow of current in the loop so that each loop can be activated in turn. 
     
     
         6 . The apparatus according to  claim 1  wherein the signal processing system includes a plurality of channels for individual processing of separate MR signals and wherein there is provided an arrangement for generating the separate MR signals for the separate channels from the signal induced onto said at least one receive coil. 
     
     
         7 . The apparatus according to  claim 6  wherein said birdcage coil includes a plurality of separate first loops and wherein each first loop includes an addressable switch operable remotely to halt flow of current in the first loop so that each first loop can be activated in turn, wherein said at least one receive coil comprises a single second loop and wherein there is provided a signal dividing system arranged to receive the signal from the single second loop and to calculate the separate MR signals for the separate channels from the signal induced onto said single second loop. 
     
     
         8 . The apparatus according to  claim 6  wherein said at least one receive coil comprises a plurality of separate loops each providing a signal to a respective one of the channels. 
     
     
         9 . The apparatus according to  claim 1  wherein the arrangement to halt current flow in the loops comprises an arrangement to temporarily de-tune the loop from the resonant frequency. 
     
     
         10 . The apparatus according to  claim 1  wherein said at least one receive coil comprises a body coil carried on to the magnet.