We will track the ultrasound probe with an Electro-Magnetic (EM) tracker. The tracker has small sensors that can be attached to the ultrasound probe under the sterile cover of the ultrasound probe, and therefore the sensor is not exposed. Also, the tracker has a 'transmitter' that is flat and is placed underneath the patient below a soft cushion. The sensor is attached to the ultrasound probe prior to the surgery and hencethe probe will remain sterile. The transmitter is flat and will be placed under a soft cushion so that it does not cause any pressure effects on patient.