The present invention relates to a magnetic resonance imaging system and a
corresponding method having a transmit phase and a receive phase.
Further, the present invention relates to a detuning circuit and a
corresponding detuning method for detuning an RF receive coil during the
transmit phase in such a magnetic resonance imaging system. In high-field
MRI systems the transmit mode operating frequency is higher than normal
high breakdown voltage rectifiers can handle when they are used to
forward bias a passive detuning circuit PIN diode switch. The proposed
circuit uses a current-limiting capacitor (C5) in series with a fast
(e.g. schottky) rectifier diode (V2) with a reverse breakdown voltage of
e.g. 20 volts and a fast reverse recovery time to generate a DC current.
The rectifying circuit is isolated from the PIN diode (V1) with a
relatively high-value inductor (L2), which ensures that no harmful
transient current spikes can flow from the PIN diode anode to the
rectifying circuit. The inductor (L2) still passes and maintains the DC
current generated by the rectifying circuit through the PIN diode, thus
enabling the robust forward-biasing of the PIN-diode during transmit
mode. The use of a fast (and thus low-power) rectifier results in less
dissipation on the detuning circuit, and helps in fulfilling the surface
temperature limits posed on receiver coils.