A switchable birdcage coil functioning as a transmitter coil and a
separate RF surface coil functioning as the receiver coil are utilize in
NMR and MRI apparatus. To prevent the switchable birdcage coil from
absorbing energy by coupling to the receiver coil or by absorbing power
from the RF field produced by the precessing nuclear spins, one or more
of the reactive elements of the birdcage coil are switched from a normal
reactive impedance state to a high impedance state. The high impedance
state is formed by switchably combining the reactive element with a
complementary reactive element thereby forming parallel resonant high
impedance circuit that is tuned to resonate at the NMR frequency.