In one embodiment a radio frequency receiver for a magnetic resonance imaging system is provided. The radio frequency receiver comprises a RF coil for receiving one or more radio frequency signals transmitted through an object to be imaged so as to enable a reconstruction processor to generate an image representation of the object based on the received radio frequency signals, a local oscillator configured for generating a stimulus, the stimulus comprising a range of radio frequency signals having different frequencies and a flux probe coupled to the local oscillator, the flux probe configured for applying the stimulus to the RF coil. Further, the RF coil is configured for returning a reflected signal in response to the stimulus applied and comprises at least one digitally tunable capacitor.