A tracking device (20, 20') for tracking a tip (14) of an interventional instrument such as a catheter (10) during an interventional procedure performed on an associated subject (12) and monitored by magnetic resonance imaging includes a resonant circuit (22) disposed at the tip (14) of the catheter (10). The resonant circuit (22) includes a coil (32, 32') having a coil inductance and a light-sensitive metal-insulator-semiconductor capacitor (30) optically coupled with an optical fiber (36) and having a selected capacitance determined by an intensity of light delivered by the optical fiber (36). A selected resonance frequency of the resonant circuit (22) is determined by the coil inductance and the selected capacitance. The resonance frequency is adjusted by modulating the intensity of light delivered to the light-sensitive metal-insulator-semiconductor capacitor (30).