An ultrasound is a study performed using high-frequency sound waves to study local anatomy.  This test does not involve any radiation. Because the sound waves cannot penetrate that deeply into the body, this test can only evaluate structures that are near an accessible surface, such as the abdominal wall.  It is also used for local evaluation of the rectal wall and the anal canal.  These tests are called a rectal ultrasound and an anal ultrasound.  The rectal ultrasound is generally used for pre-treatment staging of rectal cancers, while the anal ultrasound is used to evaluate anal sphincter muscle anatomy for fecal incontinence or complex peri-rectal fistulae.