Final answer:
The Trendelenburg position is used during a myelogram when the patient's head is lower than the feet, which involves the patient being supine.
Step-by-step explanation:
During a myelogram, when a patient's head is put lower than his feet, the position they are in is called the Trendelenburg position. This position involves the patient being supine, which means lying on their back, with the body tilted so that the feet are higher than the head. It's different from the other positions listed: Fowler's position involves a semi-upright seating position, prone means lying face down, and Sim's position is when the patient lies on their side with the lower arm behind the body and the upper thigh flexed.