Embodiments described herein relate to detectors and their method of use for sensing electromagnetic fields, electromagnetic signals, biochemical analytes, and/or other conditions in subjects. The device may include an inductively-coupled implantable coil-based transducer that converts electrical, photonic, biochemical signals, and/or other appropriate signals and/or conditions originating in tissues and/or transplanted tissue grafts into changes in a property of the transducer, such as a resonance frequency, that may be detected using an alternating magnetic field that may be provided by a magnetic resonance imaging (MRI) signal and/or other appropriate source. In some embodiments, the detector comprises a FET that changes state upon detection of a subject condition of interest. The change in the FET may change the resonance frequency of an associated LC or RLC circuit. The change in resonance frequency may change the brightness and/or intensity of the detector when detected by an MRI scanner or other appropriate scanner.