Your prostate sits directly above the rectal wall between the bladder and the base of the penis. A TRUS biopsy consists of the insertion of an ultrasound probe approximately the size of an adult man's thumb into your rectum. This probe sends out high-energy sound waves which are bounced off internal tissues or organs to create echoes. These echoes form a picture of body tissue, called a sonogram. Further, the ultrasound image helps the physician to navigate to certain areas in the prostate and to obtain needle core samples from the gland. These samples are used to exclude or determine the possibility of cancer being present.