Ultrasound Scan: How It Works, Types & What to Expect

What Is Ultrasound and How Does It Work?

Ultrasound (also called sonography or ultrasonography) is a medical imaging technique that uses high-frequency sound waves to create real-time images of structures inside your body. A device called a transducer sends sound waves into your body, which bounce off organs and tissues and return as echoes that are converted into images on a screen.

Ultrasound technology represents one of the most significant advances in medical imaging. Unlike X-rays and CT scans, which use ionizing radiation to create images, ultrasound relies entirely on sound waves. These sound waves are at frequencies far above what the human ear can detect—typically between 2 and 18 megahertz—and they travel through soft tissues in the body, reflecting back when they encounter boundaries between different tissue types.

The physics behind ultrasound imaging is elegantly simple yet remarkably effective. When the transducer emits sound waves, these waves travel through your body at different speeds depending on the density and composition of the tissues they encounter. When sound waves hit a boundary between tissues of different densities—such as between fluid and solid tissue—some of the energy reflects back to the transducer. The transducer then converts these returning echoes into electrical signals, which a computer processes into visual images displayed on a monitor.

One of the most remarkable features of modern ultrasound is its ability to create real-time moving images. This means doctors can observe organs in motion, watch blood flowing through vessels, and see a developing baby moving in the womb. This real-time capability makes ultrasound invaluable for guiding procedures such as biopsies, where doctors need to see exactly where their needle is positioned.

The technology behind ultrasound imaging

Modern ultrasound machines contain sophisticated computer technology that processes millions of data points every second. The transducer contains piezoelectric crystals that vibrate when electrical current passes through them, generating sound waves. These same crystals convert returning sound waves back into electrical signals. Advanced signal processing algorithms then create detailed grayscale images, with brighter areas indicating tissues that reflect more sound and darker areas indicating tissues that reflect less.

Different types of transducers are designed for different purposes. Linear transducers produce high-resolution images of superficial structures like blood vessels and the thyroid gland. Curved (convex) transducers provide a wider field of view for examining abdominal organs. Phased array transducers use electronic beam steering and are particularly useful for cardiac imaging, where they can fit between the ribs to visualize the heart.

Types of ultrasound imaging modes

Healthcare providers use several different ultrasound modes depending on what they need to visualize. B-mode (brightness mode) is the most common, producing the familiar two-dimensional grayscale images. M-mode (motion mode) displays movement over time and is particularly useful for measuring heart valve motion. Doppler ultrasound detects blood flow by measuring the frequency shift of sound waves bouncing off moving blood cells, often displayed in color to show flow direction and speed.

What Are the Different Types of Ultrasound Examinations?

Ultrasound examinations are categorized into external (on the skin) and internal (inside body cavities) types. External ultrasound examines organs like the heart, liver, and kidneys through the skin. Internal ultrasound uses specialized probes inserted through the mouth, vagina, or rectum for closer views of specific organs.

The type of ultrasound examination you receive depends on which part of your body needs to be evaluated and what information your doctor needs to obtain. Each type has specific advantages and is chosen based on the clinical question being asked. Understanding the different types can help you know what to expect and how to prepare appropriately.

External (transabdominal) ultrasound

External ultrasound is the most common type and involves placing the transducer on the skin surface. A water-based gel is applied to eliminate air between the transducer and your skin, as air can interfere with sound wave transmission. The sonographer then moves the transducer across your skin, applying gentle pressure to obtain images from different angles.

External ultrasound can examine virtually any organ accessible through the skin surface. Common applications include abdominal ultrasound to evaluate the liver, gallbladder, pancreas, spleen, and kidneys; pelvic ultrasound to examine the bladder, uterus, and ovaries in women or the prostate in men; and musculoskeletal ultrasound to assess muscles, tendons, and joints. Echocardiography, or cardiac ultrasound, uses specialized techniques to evaluate heart structure and function.

Internal (endocavitary) ultrasound

Internal ultrasound provides closer, more detailed views of certain organs by placing a specialized transducer inside a body cavity. Because the transducer is positioned closer to the organs being examined, internal ultrasound often produces higher-resolution images than external scanning.

Transesophageal echocardiography (TEE) involves inserting a transducer through the mouth into the esophagus to obtain detailed images of the heart. Because the esophagus lies directly behind the heart, TEE provides exceptional visualization of heart valves and chambers. This technique is often used during heart surgery or to detect blood clots before certain procedures.

Transvaginal ultrasound uses a slim probe inserted into the vagina to examine the uterus, ovaries, and fallopian tubes. This technique provides much clearer images of reproductive organs than transabdominal scanning and is commonly used in early pregnancy, fertility evaluations, and to investigate abnormal bleeding or pelvic pain.

Transrectal ultrasound (TRUS) involves inserting a probe into the rectum to examine the prostate gland and surrounding structures. This technique is commonly used to guide prostate biopsies and to evaluate prostate size and abnormalities.

How Should I Prepare for an Ultrasound Scan?

Preparation for ultrasound varies depending on the type of scan. Abdominal ultrasounds often require fasting for 8-12 hours. Pelvic ultrasounds may require a full bladder. Always follow the specific instructions from your healthcare provider, and inform them of any medications you take.

Proper preparation is essential for obtaining high-quality ultrasound images and accurate results. The preparation requirements vary significantly depending on which part of your body is being examined. Your healthcare facility will provide specific instructions, but understanding the general principles can help you prepare effectively.

The most common preparation requirement is fasting before abdominal ultrasound. When you eat, your gallbladder contracts to release bile, which helps digest fats. An empty gallbladder is smaller and more difficult to evaluate. Additionally, eating causes gas to accumulate in the intestines, and gas blocks ultrasound waves, creating dark shadows that obscure underlying structures. Fasting for 8-12 hours before an abdominal scan minimizes these issues.

Preparation by scan type

For abdominal ultrasound, you will typically need to fast for 8-12 hours before your appointment. You may drink water, but avoid other beverages and foods. If your scan is scheduled for the morning, you can usually eat a light dinner the night before and then fast overnight.

For pelvic ultrasound, especially transabdominal pelvic scanning, you may need a full bladder. A full bladder pushes the intestines out of the way and provides an acoustic window through which sound waves can travel to reach the uterus and ovaries. You may be asked to drink 32 ounces (about 1 liter) of water an hour before your appointment and avoid urinating until after the scan.

For transvaginal ultrasound, an empty bladder is usually preferred for patient comfort. If you are having both transabdominal and transvaginal scans, the transabdominal portion with a full bladder will be performed first, after which you can empty your bladder for the transvaginal portion.

Medications and special considerations

Always inform your healthcare provider about any medications you take. Most medications do not affect ultrasound imaging, but some situations may require adjustment. For example, if you take medication for diabetes and need to fast, your provider may adjust your medication timing to prevent low blood sugar.

If you require contrast-enhanced ultrasound, inform your provider of any allergies, particularly to blood products or previous contrast agents. Ultrasound contrast agents are different from CT or MRI contrast and are generally well-tolerated, but allergic reactions, though rare, can occur.

What Happens During an Ultrasound Examination?

During external ultrasound, you lie on an examination table while the sonographer applies gel to your skin and moves the transducer over the area being examined. You may be asked to change positions or hold your breath briefly. The procedure is painless and typically takes 15-45 minutes depending on the examination type.

Understanding what happens during an ultrasound can help reduce anxiety and ensure you get the most accurate results possible. The examination is performed by a trained sonographer (also called an ultrasound technologist) or sometimes by a doctor, depending on the type of scan and the facility.

When you arrive for your appointment, you will be asked to change into a hospital gown if the area being examined requires removal of clothing. You will then be positioned on an examination table, usually lying on your back, though some examinations require side-lying or other positions. The room will be dimly lit so the sonographer can see the images on the monitor clearly.

The external ultrasound process

The sonographer begins by applying a water-based gel to your skin over the area to be examined. This gel serves a crucial function: it eliminates air pockets between the transducer and your skin. Air reflects sound waves almost completely, so without the gel, very little ultrasound energy would enter your body, and image quality would be severely compromised. The gel may feel cool at first but warms quickly to body temperature.

Once the gel is applied, the sonographer places the transducer on your skin and begins moving it across the area of interest. They may press firmly at times to push aside overlying structures or to get closer to deeper organs. If this pressure causes discomfort, let the sonographer know, and they can usually adjust their technique. You will hear the machine making various sounds as it processes the returning echoes.

Throughout the examination, you may be asked to change positions—for example, rolling onto your left or right side—to allow visualization of different organs from different angles. You may also be asked to take a deep breath and hold it briefly, as this can help move certain organs into better position and momentarily stops respiratory motion that can blur images.

Internal ultrasound procedures

For transesophageal echocardiography (TEE), you will receive local anesthetic spray in your throat to reduce gagging. You may also receive a mild sedative. You will lie on your left side with a mouth guard to protect your teeth. The lubricated probe is gently advanced through your mouth and down your esophagus while you swallow. The procedure may feel uncomfortable but should not be painful. The actual scanning usually takes 10-15 minutes.

For transvaginal ultrasound, you will be positioned as for a pelvic examination, either in stirrups or with your knees bent. The slim, lubricated transducer is gently inserted into the vagina. You may feel pressure but not pain. The sonographer slowly moves and angles the transducer to obtain views of the uterus and ovaries. The examination can be performed during menstruation if necessary.

For transrectal ultrasound, you will lie on your left side with your knees bent toward your chest. A lubricated, finger-sized transducer is gently inserted into the rectum. You may feel pressure and a sensation of needing to have a bowel movement, but the procedure should not be painful. If a biopsy is being performed simultaneously, you may receive local anesthesia.

What Happens After an Ultrasound?

After most ultrasound examinations, you can resume normal activities immediately. The gel is wiped off your skin, and you can dress and leave. If you received sedation for internal ultrasound, you may need to rest and should not drive until the next day. Results are typically available within 1-3 days.

One of the significant advantages of ultrasound compared to other imaging techniques is the minimal recovery time required. For external ultrasounds, there is essentially no recovery period at all. Once the examination is complete, the sonographer will wipe the gel off your skin, and you can immediately get dressed and leave. You can eat, drink, and return to all normal activities right away.

For internal ultrasound procedures, the recovery depends on whether sedation was used. After transvaginal or transrectal ultrasound without sedation, you can leave immediately and resume normal activities. You may experience mild spotting or discomfort afterward, which typically resolves quickly.

If you received sedation for a procedure like transesophageal echocardiography, you will need to stay at the facility for observation until the sedation begins to wear off, typically 1-2 hours. You should not drive, operate machinery, or make important decisions for the remainder of the day. Arrange for someone to take you home. Your throat may feel sore from the probe, and you should avoid hot foods or drinks until the local anesthetic fully wears off, as you may not feel if something is too hot.

When you get your results

The timing of results depends on your facility and the urgency of your examination. In some cases, particularly if the examination is being performed by a doctor or if immediate findings affect your care, you may receive preliminary results right away. The doctor may come in during or after the scan to discuss what they see.

More commonly, the images are saved and sent to a radiologist (a doctor specializing in medical imaging) for detailed review and interpretation. The radiologist examines all the images, measures relevant structures, and prepares a written report. This process typically takes 1-3 days. Your referring doctor will then contact you to discuss the results, either at a follow-up appointment or by phone.

If you have not heard about your results within the expected timeframe, do not hesitate to contact your doctor's office. Important findings that require immediate attention are typically communicated quickly, but it is always appropriate to follow up on your own health care.

Is Ultrasound Safe? Understanding Risks and Side Effects

Diagnostic ultrasound is considered extremely safe with no confirmed biological effects on patients when used appropriately. Unlike X-rays, ultrasound does not use ionizing radiation, making it safe for pregnant women and fetuses. Rare complications can occur with internal ultrasound but are uncommon.

The safety profile of diagnostic ultrasound is exceptional, which is why it has become one of the most widely used imaging techniques in medicine. Since the introduction of medical ultrasound in the 1950s, extensive research has been conducted on its biological effects, and no confirmed harmful effects have been documented when used according to established guidelines.

The sound waves used in diagnostic ultrasound interact with body tissues in two main ways: they can cause slight heating and they can cause tiny gas bubbles to form (cavitation). However, the energy levels used in diagnostic imaging are carefully regulated to keep these effects well below any threshold that could cause biological harm. Medical ultrasound equipment includes output displays that help sonographers ensure they are using the lowest power necessary to obtain good images.

Ultrasound during pregnancy

Ultrasound has been used extensively in pregnancy for over 50 years, and no link has been established between diagnostic ultrasound and any harmful effects on mothers or babies. Major medical organizations including the World Health Organization, American College of Obstetricians and Gynecologists, and American Institute of Ultrasound in Medicine all confirm that ultrasound is safe when used for medical indications by trained operators.

However, experts recommend avoiding unnecessary ultrasound exposure, particularly during pregnancy. Non-medical "keepsake" ultrasound sessions at commercial facilities are discouraged because they may use longer exposure times than medically necessary and are often performed by individuals without medical training who might miss important findings. Ultrasound should be performed when there is a medical reason and used judiciously with the minimum exposure needed to answer the clinical question.

Complications of internal ultrasound

While external ultrasound has virtually no risks, internal procedures carry small risks related to probe insertion. Transesophageal echocardiography rarely can cause minor bleeding or, very rarely, esophageal perforation. The risk is extremely low—estimated at less than 1 in 10,000 examinations—and TEE is only performed when the clinical benefit justifies this minimal risk.

Transvaginal and transrectal ultrasound may occasionally cause minor bleeding or discomfort, but serious complications are rare. If you experience significant bleeding, fever, or severe pain after any internal ultrasound, contact your healthcare provider immediately.

How Do I Understand My Ultrasound Results?

Ultrasound results are provided in a written report from a radiologist describing what was seen and its clinical significance. Normal findings indicate structures appear typical for your age and condition. Abnormal findings may require further investigation, follow-up imaging, or treatment depending on what is discovered.

Understanding your ultrasound results can help you participate more actively in your healthcare decisions. Ultrasound reports typically include several components: a description of the technical quality of the examination, measurements of relevant structures, descriptions of any abnormalities observed, and an impression or conclusion summarizing the key findings.

Ultrasound images display different tissues in shades of gray. Fluid-filled structures like the bladder or gallbladder appear black (anechoic) because fluid transmits sound waves without reflecting them back. Dense structures like bones appear bright white (hyperechoic) because they reflect most of the sound energy. Most soft tissues appear in various shades of gray depending on their density and composition.

Common ultrasound findings

Some common findings on ultrasound are entirely normal or represent benign variations. Simple cysts—round, fluid-filled structures with thin walls—are common in the kidneys, liver, and ovaries and are almost always harmless. Hepatic hemangiomas are benign blood vessel tumors in the liver found in up to 7% of the population. Gallstones are a frequent finding and may or may not require treatment depending on whether they cause symptoms.

Other findings may require further evaluation. Complex cysts with thick walls, internal debris, or solid components may need additional imaging or monitoring. Masses or nodules in organs like the thyroid, liver, or kidneys may require further characterization with other imaging techniques or biopsy. Dilated ducts or blood vessels may indicate obstruction or other conditions requiring investigation.

Your doctor will explain what your specific results mean in the context of your symptoms, medical history, and other test results. A finding that would be concerning in one clinical situation may be completely insignificant in another, so the interpretation always depends on the complete clinical picture.

How Can I Participate in My Healthcare?

Active participation in your healthcare leads to better outcomes. Ask questions about why ultrasound is recommended, what the results mean, and what the next steps are. Request copies of your reports and images for your records. If you do not understand something, ask for clarification.

Being an active participant in your healthcare is increasingly recognized as essential for achieving the best possible outcomes. This means not just passively receiving care but engaging with your healthcare providers, understanding your conditions and treatments, and making informed decisions about your health.

Before your ultrasound examination, consider asking your doctor why this test is being recommended and what they hope to learn from it. Understanding the purpose of the examination can help you appreciate its importance and ensure you prepare appropriately. Ask about any specific preparation requirements and what will happen during and after the procedure.

Questions to ask about your results

When you receive your results, make sure you understand what they mean. Ask your doctor to explain any medical terminology you do not understand. Key questions include: What did the ultrasound show? Are the results normal or abnormal? If abnormal, what might this indicate? Do I need any additional tests or follow-up? What are the next steps in my care?

You have the right to receive copies of your medical records, including ultrasound reports and images. Having these records can be valuable if you see other healthcare providers or move to a new location. Many facilities now offer patient portals where you can access your results online.

If English is not your first language or if you have hearing difficulties, you have the right to interpreter services or other communication assistance. Do not hesitate to request these services to ensure you fully understand your care.