An orientation tracking device, the device including: a 3-axis accelerometer; a 3-axis magnetometer; and a communication module; a memory with data stored 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; a processor connected to the accelerometer, the magnetometer and communication module, the processor 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; receiving from the accelerometer a measurement of a gravitational 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 determining 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. a 3-axis accelerometer; a 3-axis magnetometer; and a communication module; a memory with data stored 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; a processor connected to the accelerometer, the magnetometer and communication module, the processor 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; receiving from the accelerometer a measurement of a gravitational 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 determining 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. 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; receiving from the accelerometer a measurement of a gravitational 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 determining 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. <EOS>