An orientation tracking device includes a memory for storing data therein including magnetic reference data relating to the direction of a static magnetic field of a magnetic imaging scanner in a selected principle co-ordinate frame and earth gravitational reference data relating to the direction of the gravitational field of the earth in the principle co-ordinate frame. The device also includes an accelerometer, a magnetometer and a communication module. A processor is connected to the accelerometer, the magnetometer and communication module and is adapted for receiving from the magnetometer a measurement of a magnetic field vector in a co-ordinate frame of the orientation tracking device, within the magnetic imaging scanner in which the orientation tracking device is placed in use. The processor further receives from the accelerometer a measurement of an acceleration vector in the orientation tracking device's. co-ordinate frame, within the magnetic imaging scanner in which the orientation tracking device is placed in use and the processor then determines the orientation of the device with respect to the principle co-ordinate frame by comparing the measured vectors from the accelerometer and magnetometer with the stored gravitational reference data and magnetic reference data respectively.
