RF Receiver: Receive Coil Safety

RF Receiver · Safety

A receive coil is passive while it listens. The safety question is what happens during the transmit pulse, when it is sitting inside a strong field it was never meant to absorb.

Receive loop with its detuning elements, and the timing of transmit pulses and detuning — Every receive element is detuned while the transmit coil fires: active and passive detuning, preamplifier decoupling and cable traps.

Close-up of a receive loop array with detuning components — A receive loop array with its detuning circuits, set up on phantoms.

The question

Every receive element must be detuned while the transmit coil is firing. If that detuning fails, the element becomes a resonant loop in a strong radiofrequency field, and the current it carries can heat the conductor and anything in contact with it. The failure is local, it is fast, and it is on the patient side of the system, so it is designed against rather than detected after the fact.

What this means in the design

Detuning that fails safe. Both the active detuning driven by the system and the passive detuning that must work when the system does not.
Cable and common-mode currents, which carry energy along the outside of a cable to places the coil design never considered.
Preamplifier decoupling, which is a sensitivity technique and a safety technique at once: an element that is well decoupled carries less induced current.