Catheter tracking and guidance in the body is essential for interventional procedures. However, traditional catheters are invisible under magnetic resonance imaging (MRI). We present an acousto-optic sensor for tracking catheter position during interventional MRI. A coil antenna collects local MRI signal. In order to eliminate the RF induced heating, optical fiber is used to carry the MRI signal. An acousto-optic modulator based on fiber Bragg grating (FBG) is developed to convert the electrical signal from the antenna to optical signal to be carried by the fiber. Sensor was successfully tested for position detection in phantom under MRI.