What is an ultrasound scan

What it is

Ultrasound imaging - also called sonography - uses high-frequency sound waves, far above what the ear can hear, to make pictures of the inside of the body. A small handheld probe placed on the skin sends sound into the body and picks up the echoes that bounce back; a computer turns the returning echoes into a moving image.

Because it uses sound rather than radiation, ultrasound involves no ionizing radiation exposure, which is a large part of why it is used so widely, including during pregnancy.

What it is used for

The best-known use is monitoring pregnancy - checking the baby's development, position, and due date - but that is only one of many. Ultrasound examines abdominal organs (liver, gallbladder, kidneys), the thyroid, blood vessels and blood flow (Doppler ultrasound), the heart (echocardiography), muscles and tendons, and breast lumps.

It also guides procedures in real time: needle biopsies, fluid drainage, and line placement are all safer when the operator can see the needle on screen.

Some specialized forms place the probe inside the body - for example transvaginal or transesophageal ultrasound - to get closer to the organ of interest for a sharper image.

What the experience is like

A standard ultrasound is painless and usually takes 15 to 45 minutes. The person lies on a table while clear gel is spread on the skin - the gel conducts the sound - and the probe is moved over the area.

Preparation depends on the target: a full bladder is needed for some pelvic scans, and fasting for some abdominal ones, because gas and food obscure the view. Most scans need no preparation at all.

Results are interpreted by a specialist and usually reach the referring clinician within days.

Strengths and limits

Ultrasound is safe, widely available, relatively inexpensive, and shows movement in real time - blood flowing, a heart beating, a baby moving. It has no known harmful effects at diagnostic levels, which is why it can be repeated freely.

Its limits are physical: sound does not pass well through air or bone, so ultrasound is poor for the lungs and the adult brain, and image quality drops in larger bodies where the sound must travel farther. When a question needs to see through bone or air, CT or MRI answers it better. Each imaging method has questions it handles best, and clinicians choose accordingly.