A system is disclosed for determining a position and a change in the position of an anatomical structure. The system utilizes a surgical navigation system and a substrate that is capable of being removably mounted to an outer surface of a patient's body. The substrate includes a sensor that is tracked by the surgical navigation system and an ultrasonic imaging device that determines the position of an anatomical structure relative to the sensor. The concatenation of the position of the sensor and the relative position of the anatomical structure allows a global position of the anatomical structure to be determined by a computer system and displayed to the user.