Ultrasound examination, also called diagnostic medical sonography or sonography, is an imaging method that uses high-frequency sound waves to produce precise images of structures within your body. These images often provide information that's valuable in diagnosing and treating disease.

Ultrasound may be used, among other things, to:

• Evaluate a fetus
• Diagnose gallbladder disease
• Evaluate flow in blood vessels
• Guide a needle biopsy
• Guide the biopsy and treatment of a tumor
• Check your thyroid gland
• Study the heart
• Diagnose some forms of infection
• Diagnose some forms of cancer
• Reveal abnormalities in the scrotum and prostate

How you prepare for an ultrasound depends on which area of your body needs evaluation. Some ultrasound exams require no preparation. Others require that you forego food and liquids for up to 12 hours before the exam. Still others require that you not urinate before the exam so that your bladder will be full. When your ultrasound is scheduled, ask your doctor for specific instructions for your particular situation.

Ultrasound is based on the same principles as sonar — a technology used to detect underwater objects. During an ultrasound, a technician trained in ultrasound imaging (sonographer) presses a small hand-held device (transducer), about the size of a bar of soap, against your skin. The transducer generates and receives high frequency sound waves that can't be heard by the human ear.

As the sonographer places the transducer on your skin, crystals inside the transducer emit pulses of sound waves that travel into your body. Your tissues, bones and body fluids reflect the sound waves and bounce them back to the transducer. The transducer then sends this information to a computer, which composes detailed images based on the patterns created by the sound waves.

Though the majority of ultrasound exams are performed with a transducer on your skin, some ultrasounds are done inside your body (invasive ultrasounds). For these exams, the transducer is attached to a probe that's inserted into a natural opening in your body. Examples of these exams include:

• Transesophageal echocardiogram. The transducer is inserted into your esophagus to obtain images of the heart.
• Transrectal ultrasound. The transducer is inserted into a man's rectum to view the prostate.
• Transvaginal ultrasound. The transducer is inserted into a woman's vagina to view the uterus and ovaries.

During an ultrasound exam, you lie on an examination table and a small amount of warm gel is applied to your skin. The gel helps eliminate the formation of air pockets between the transducer and your body. The sonographer presses the transducer against your skin over the area of your body being examined, moving from one area to another as necessary.

Ultrasound usually is a painless procedure. However, you may experience some discomfort as the sonographer guides the transducer over your body, especially if you're required to have a full bladder. A typical ultrasound exam takes from 20 minutes to an hour.

When your exam is complete, the sonographer and a radiologist generally view the ultrasound images on film or on a computer monitor. The radiologist analyzes the images and sends a report of the findings to your doctor.

Diagnostic ultrasound is a safe procedure that uses low-power sound waves. Higher-power sound waves are used for treatment purposes to heat and even destroy some types of tissue or tumors.

Although ultrasound is a valuable tool, it does have its limitations. Sound doesn't travel well through air or bone, so ultrasound isn't effective at imaging parts of your body that have gas in them or that are obscured by bone. Rather than using ultrasound to view these areas, your doctor may instead order other imaging tests, such as CT scans, MRI or X-rays.

Be sure the sonographer conducting your ultrasound exam is credentialed by the American Registry for Diagnostic Medical Sonographers or is supervised by a sonographer who is credentialed by this organization. Some schools offer certificates in sonography without requiring the proper training to conduct these exams.