When you scan a subway car to see if your friend is on it, your eyes pass quickly across the riders. When you get a scan of your knee at the doctor's office, a much more detailed picture is taken, showing your bones, tendons, and muscles.
You might need a medical scan if you are in an accident or have a terrible headache. One of the most common kinds of scans is performed on a pregnant woman, to check out the size and health of her fetus. This kind of scan is called an "ultrasound" or a "sonogram." These use sound waves to take a picture of the inside of a person's body.