An ultrasound can show the size, shape, and condition of most soft tissues and organs in your body, from a developing baby to gallstones to blood clots in your legs. It works by sending high-frequency sound waves into the body and capturing the echoes that bounce back from different tissues. Because each type of tissue reflects sound differently, the machine can build a real-time picture of what’s happening inside you without radiation or incisions.
How Ultrasound Creates an Image
A handheld device called a transducer sends pulses of sound into your body. When those pulses hit a boundary between two different tissues (say, the edge of your liver or the wall of your gallbladder), some of the sound bounces back. The machine measures how long each echo takes to return and how strong it is, then uses that data to build a grayscale image on screen.
Dense tissues like bone reflect almost all the sound, creating a bright line on the image but blocking everything behind them. Air does the same thing. This is why ultrasound works well for soft, fluid-filled, and solid organs but struggles with the lungs (full of air) and the brain in adults (encased in bone). Fluid-filled structures like cysts or a full bladder appear dark because sound passes straight through them with almost no reflection.
What an Abdominal Ultrasound Reveals
An abdominal scan gives your doctor a direct look at your liver, gallbladder, kidneys, spleen, and pancreas. It can detect gallstones, kidney stones, cysts, tumors, fatty liver disease, gallbladder inflammation, and an enlarged spleen. If you’ve been having unexplained stomach pain, this is often one of the first imaging tests ordered because it’s quick, painless, and shows the most common culprits.
For an abdominal scan, you’ll typically need to fast for eight hours beforehand. Water and medications are fine. If a pelvic ultrasound is being done at the same time, you’ll also be asked to drink about 32 ounces of water an hour before the exam. A full bladder pushes the intestines out of the way and creates a clear “window” for the sound waves to reach the uterus or bladder. Kidney and testicular ultrasounds usually require no preparation at all.
Pregnancy Scans
Prenatal ultrasound is probably the most familiar use of this technology, and it shows far more than just whether you’re having a boy or a girl. In the first trimester, sonographers measure the length from the top of the baby’s head to the bottom (crown-rump length) to confirm how far along the pregnancy is. This measurement is most accurate before 12 weeks.
By the 20-week anatomy scan, the focus shifts to four key measurements: head circumference, the diameter across the skull, abdominal circumference, and the length of the thigh bone. At 20 weeks, average values are roughly 17.5 cm for head circumference, 14.9 cm around the belly, 4.9 cm across the skull, and 3.2 cm for the thigh bone. These numbers help track growth and flag potential problems early. The anatomy scan also checks the brain, spine, heart chambers, kidneys, and placenta.
3D and 4D ultrasounds (which add real-time motion to the 3D image) are sometimes used when doctors need to evaluate abnormalities that are hard to see on a standard flat image. Cleft lip, for example, is much easier to assess in 3D. These modes also help parents visualize a problem that a sonographer can spot on a 2D scan but that can be difficult for a non-expert to interpret.
Heart Imaging (Echocardiogram)
When ultrasound is aimed at the heart, it’s called an echocardiogram. It shows the size of all four chambers, the thickness of the heart walls, and how well the muscle contracts with each beat. One of the most important numbers it produces is the ejection fraction, which tells your doctor what percentage of blood the left ventricle pumps out per beat.
An echo also evaluates the four heart valves. It can detect problems like mitral valve regurgitation, where the valve doesn’t close completely and blood leaks backward, or aortic stenosis, where the valve doesn’t open wide enough and the heart has to work harder to push blood through. Thickening of the left ventricle wall, a condition that often develops from long-standing high blood pressure, is another common finding.
Blood Flow and Blood Clots
Doppler ultrasound adds a layer of information that standard imaging can’t provide: it measures the speed and direction of blood flow. This is especially useful for detecting deep vein thrombosis (DVT), a blood clot in the leg veins. The sonographer presses the transducer against the vein. A healthy vein compresses flat under pressure. A vein with a clot inside it won’t compress, and the Doppler signal shows disrupted flow.
Color Doppler can also distinguish between a complete and partial blockage and detect clots in smaller or deeper veins that are harder to compress directly. If the waveform in the femoral vein appears flat and continuous rather than pulsing with your breathing, that’s a sign of obstruction higher up in the pelvis or abdomen, even if the clot itself isn’t visible at that location. This technique is also used to check blood flow through the carotid arteries in the neck, the blood vessels supplying the kidneys, and the umbilical cord during pregnancy.
Muscles, Tendons, and Joints
Musculoskeletal ultrasound has become a go-to tool for evaluating soft tissue injuries, particularly in the shoulder, elbow, knee, and ankle. It can show tendon damage at every stage: early tendinopathy appears as thickening of the tendon with subtle changes in texture, while more advanced disease shows a loss of the tendon’s normal fibrous pattern and new blood vessel growth within the damaged area. Those findings often correlate with worsening pain.
For rotator cuff tears, ultrasound can distinguish between partial and complete tears. A partial tear shows up as a small dark area within the tendon, sometimes with a sliver of fluid in the gap. A full tear appears as a clear defect where the tendon fibers have separated, and the surrounding tissue may herniate into the space. One advantage over MRI is that the examiner can move your arm during the scan, watching the tendon in real time to confirm whether torn edges are still in contact or have pulled apart.
What Ultrasound Cannot Show
Ultrasound has real blind spots. Bone and air reflect sound so completely that structures behind them become invisible. This means it can’t image the inside of bones (fractures are diagnosed with X-rays or CT), and it can’t reliably see the adult brain through the skull. Lung tissue, filled with air, is largely off-limits, though ultrasound can detect fluid collecting around the lungs.
It also has depth limitations. In larger patients, deeper structures may be harder to visualize because the sound waves lose energy as they travel through tissue, a process called attenuation. When too much energy is absorbed, structures farther from the transducer simply don’t appear on the image. For these situations, CT or MRI may be needed to complete the picture.
Safety Profile
Diagnostic ultrasound does not use ionizing radiation, which is the primary reason it’s the preferred imaging method during pregnancy. Professional guidelines recommend keeping energy output as low as reasonably achievable. For scans on pregnant patients, the thermal index (a measure of tissue heating) is kept at or below 0.7, and the mechanical index (related to pressure effects on tissue) stays below 1.0. At these levels, decades of clinical use have not demonstrated harmful effects on patients or developing babies.