Ultrasound is the scan most people have heard of through pregnancy, but it is used far more widely. It makes pictures from sound that is too high for people to hear, so there are no X-rays involved and, for most scans, no needles. This page explains how a scan works, what it feels like, what it is used to look at and what it cannot do. For how it fits beside the other methods, see the overview of types of medical imaging.
How a sound wave becomes a picture
The key part is a handheld device called a transducer, which both sends sound waves into the body and listens for the echoes that come back. The U.S. National Institute of Biomedical Imaging and Bioengineering (NIBIB) explains that the waves are reflected back by boundaries between tissues, for example between fluid and soft tissue or between tissue and bone. Using the speed of sound and the time each echo takes to return, the scanner calculates the distance to each boundary and builds a two-dimensional image.
RadiologyInfo.org compares the principle to the sonar used by bats and ships. When a sound wave strikes an object it bounces back, and measuring the echoes shows how far away the object is, as well as its size, shape and consistency, including whether it is solid or filled with fluid. The picture appears on a monitor straight away, in real time, which is why ultrasound is good at showing movement such as a beating heart.
What the scan is like
For most scans you lie on an examination table, often on your back, and may be asked to turn to either side. The person doing the scan, a sonographer or a radiologist, spreads a water-based gel on your skin. The gel matters: it removes the air pockets between the transducer and skin that would block the sound. The transducer is then moved back and forth over the area of interest.
RadiologyInfo.org says there is usually no discomfort from the pressure, though if an area is tender you may feel pressure or minor pain. The gel is wiped off afterwards, and it does not usually stain clothing. Most exams take about 30 minutes, and more extensive ones can take up to an hour. Afterwards you can usually resume normal activities immediately. A scan with a Doppler setting, which measures blood flow, may add pulse-like sounds that change in pitch.
Some ultrasound exams use a probe placed inside the body. The same source lists transesophageal echocardiography (through the food pipe, for the heart), transrectal ultrasound (for the prostate) and transvaginal ultrasound (for the uterus and ovaries), and notes that these may cause minimal discomfort. Your own team will explain which applies to you and why.
Preparing for an ultrasound
Preparation depends on the type of exam. According to RadiologyInfo.org, you should wear comfortable, loose clothing, and you may need to remove clothing and jewellery in the area examined or change into a gown. For some scans you may be told not to eat or drink for up to 12 hours, a shorter time for babies and young children. For others you may be asked to drink up to six glasses of water about two hours beforehand and avoid urinating, so that the bladder is full when the scan begins. These instructions are scan specific, so follow the ones your imaging team gives you rather than a general rule. The longer guide to preparing for a scan and your results covers the other methods too.
What ultrasound is used to look at
RadiologyInfo.org says doctors use ultrasound to evaluate pain, swelling and infection, and lists organs such as the heart and blood vessels, liver, gallbladder, spleen, pancreas, kidneys, bladder, uterus and ovaries, eyes, and the thyroid. In infants it is commonly used to look at the brain, hips and spine. NIBIB adds that one of the most common uses is during pregnancy to monitor the growth and development of the fetus, and that images may be displayed in 2D, 3D or 4D (3D in motion).
Ultrasound also guides procedures. Because it works in real time, doctors can watch a needle as it is steered toward a target, as in a needle biopsy, and the same real-time view helps with other minimally invasive procedures. Ultrasound of the heart is commonly called an echocardiogram, or "echo" for short.
The Doppler technique deserves a line of its own. Doppler ultrasound evaluates movement, mainly blood flow. RadiologyInfo.org says it helps a doctor see blockages to blood flow such as clots, narrowing of vessels, and reduced or absent blood flow to organs. NIBIB describes using it to determine whether plaque build-up in the carotid arteries is blocking blood flow to the brain, and describes elastography, which measures how stiff tissue is and can be used to test for liver fibrosis.
What it cannot do well
The limits come from physics. RadiologyInfo.org states that ultrasound waves are disrupted by air or gas, so the technique is not ideal for the air-filled bowel or for air-filled lungs, though it can detect fluid around or within the lungs. Ultrasound also has difficulty penetrating bone, so it can see only the outer surface of bony structures. The exception it names is infants, whose skeletons contain more cartilage, and it adds that doctors typically use other methods such as MRI to see the internal structure of bones or certain joints. NIBIB makes the same point, noting that diagnostic ultrasound is not good for imaging bones or tissues that contain air.
Size can matter too. RadiologyInfo.org says larger patients are more difficult to image because greater amounts of tissue weaken the sound waves as they pass deeper and return to the transducer. If a clear picture is not possible, your doctor may suggest another method, and the page on X-ray, CT and MRI compared describes what those can show instead.
Safety, in short
RadiologyInfo.org describes standard diagnostic ultrasound as having no known harmful effects on humans and not using radiation. NIBIB is a little more cautious in its wording: diagnostic ultrasound is generally regarded as safe and produces no ionizing radiation, but it can produce some biological effects under specific settings, so the U.S. Food and Drug Administration requires devices to operate within acceptable limits. NIBIB adds that the FDA and many professional bodies discourage casual use, such as keepsake videos, and recommend it be used only when there is a true medical need. Questions about scanning in pregnancy are covered further on the page about imaging, children and pregnancy.
Frequently asked questions
Does an ultrasound hurt?
Most ultrasound exams are painless, fast and easily tolerated, according to RadiologyInfo.org. If the area being scanned is tender you may feel pressure from the transducer, and scans that use an internal probe may cause minimal discomfort.
Why is gel used in an ultrasound?
The gel lets sound travel between the transducer and the skin. Without it, air pockets would block the sound waves from passing into the body.
Does ultrasound use radiation?
Diagnostic ultrasound uses sound waves, not X-rays, and NIBIB says it does not produce ionizing radiation. Therapeutic ultrasound, a separate use that is not about making pictures, can treat or destroy tissue.
Why might I need to drink water before an ultrasound?
For some scans a full bladder helps the sound waves pass through and gives a clearer view of nearby organs. RadiologyInfo.org lists drinking up to six glasses of water about two hours before and not urinating as one example of a preparation, but only your imaging team can say whether it applies to your scan.
The short version
Ultrasound turns echoes of high-frequency sound into live pictures, using a gel and a handheld transducer, with no X-rays involved. It is widely used for organs, blood flow, pregnancy and guiding needles, and it is weak where air or bone is in the way. How to prepare depends entirely on the scan, so your imaging team's instructions are the ones to follow.