The invention provides for a medical instrument (100) comprising a magnetic resonance imaging system (104) with an imaging zone (132). The medical instrument further comprises an external beam radiotherapy system (102). A magnetic resonance antenna (129) surrounds the imaging zone. The magnetic resonance antenna comprises at least one detuning circuit (131) comprising at least one solid state switching element (308, 400, 900, 902) for switching the magnetic resonance antenna between a tuned mode and a detuned mode. The at least one solid state switching element conducts current in the detuned mode. The magnetic resonance antenna comprises at least one antenna element (300) comprising a tuning capacitor (302). The detuning circuit is connected in parallel with the tuning capacitor. The detuning circuit comprises a primary LC circuit (306) in series with the at least one solid state switching element.
