Ultrasound scans use sound waves to build up a picture of body tissues. An endorectal ultrasound scan can show the size and location of a cancer in the anus or rectum.
For the test, you lie on your left side with your knees bent up. A nurse or doctor gently passes a small, lubricated probe into the back passage. This produces an image of the rectum on a screen.
The scan takes about 10 minutes and you can usually go home as soon as it is over.