During an ultrasound, a trained technician called a sonographer applies gel to your skin and presses a handheld device against your body, which sends sound waves through your tissue and converts the echoes into real-time images on a monitor. The whole process typically takes 30 minutes to an hour, requires no radiation, and is usually painless. But the exact steps vary depending on which type of ultrasound you’re having and what part of your body is being examined.
Why the Gel Matters
Before the scan begins, the sonographer spreads a water-based gel over the area being examined. This step looks minor but is actually essential. Air blocks ultrasound waves almost completely: the difference in density between air and body tissue is so extreme that even a tiny air gap between the device and your skin causes the sound waves to bounce back before they reach anything useful. The gel fills in those microscopic pockets and creates a smooth path for the waves to travel into your body. The gel is water-based, safe, and wipes off easily from skin and clothing.
What Happens During an External Scan
Most ultrasounds are done externally, meaning the device (called a transducer) stays on the surface of your skin. After applying the gel, the sonographer holds the transducer like a pen and presses it against the area of interest. They stabilize it with their fingers while watching the monitor in real time.
To build a complete picture, the sonographer moves the transducer in several ways. They slide it side to side or up and down to scan across the area. They tilt or fan it to get views from different angles, giving a more three-dimensional perspective. They rotate it to switch between lengthwise and crosswise views. And they sometimes press down firmly to get a clearer image of deeper structures. When they spot something worth capturing, they freeze the image on screen. Most machines also have a roll-back feature that retrieves images from a few seconds earlier, so nothing important gets missed.
You’ll feel pressure as the transducer is moved and pressed against your body. For most people this is comfortable, though it can feel mildly uncomfortable over tender areas or when the sonographer needs to push down harder to see deeper organs. The gel often feels cool at first.
How Internal Ultrasounds Differ
Some scans require a transducer to be inserted into the body for a closer view. The two most common types are transvaginal ultrasounds (used to examine the uterus, ovaries, and early pregnancies) and transrectal ultrasounds (used primarily to examine the prostate).
The internal transducer is shaped like a thin tube, roughly the width of a finger. It’s covered with a protective sheath and coated with gel before insertion. For a transrectal ultrasound, you typically lie on your side with your knees bent. Your care team may ask you to use an enema beforehand to clear the rectum, and you may have a brief manual rectal exam before the transducer is placed. The sonographer moves the device gently to capture images from different angles, and the entire exam takes about 20 minutes.
Internal scans can feel uncomfortable due to the pressure and the sensation of the device inside the body, but they shouldn’t be painful. They produce much sharper images of nearby organs than an external scan can, which is why they’re used when precision matters.
Doppler Scans: Listening to Blood Flow
A Doppler ultrasound follows the same basic process as a standard scan, but it’s designed to measure blood flow rather than just create pictures of organs. The transducer sends sound waves into your body that bounce off moving blood cells. As those cells move toward or away from the device, the pitch of the returning sound waves shifts slightly, and a computer translates that shift into information about how fast your blood is moving and in which direction.
The main difference you’ll notice is sound. During a Doppler exam, you may hear a rhythmic whooshing from the machine as it picks up blood flow. The results often appear as color-coded images or graphs rather than the grayscale pictures you see in a standard ultrasound. Doppler scans are commonly used to check for blood clots, narrowed arteries, or circulation problems in the legs, arms, or neck.
How to Prepare
Preparation depends entirely on what’s being scanned. For an abdominal ultrasound, you may be asked to fast for up to eight hours beforehand. An empty stomach reduces gas in the intestines and gives the sonographer a clearer view of organs like the liver, gallbladder, and pancreas. Drinking small amounts of water to take daily medications is usually fine during a fasting period.
For pelvic ultrasounds, you may need to drink a specific amount of water before your appointment and arrive with a full bladder. A full bladder pushes the intestines aside and acts as a window for the sound waves to pass through, improving image quality. This part can be uncomfortable, especially toward the end of the exam, but it makes a real difference in what the sonographer can see. Your provider’s office will give you specific instructions when you schedule the scan.
Who Performs the Scan and Reads the Results
The person in the room with you is the sonographer, a trained technician who specializes in operating ultrasound equipment and capturing diagnostic images. They position you, perform the scan, and analyze the images as they go. However, the sonographer typically doesn’t deliver a diagnosis during your appointment. The images are sent to a radiologist or your ordering physician, who interprets them and sends a report to your care team. This means you usually won’t get results the same day, though timelines vary by facility.
Safety and What Ultrasound Cannot Do
Ultrasound uses sound waves, not ionizing radiation, which makes it fundamentally different from X-rays or CT scans. There’s no radiation exposure involved, which is why it’s considered safe during pregnancy and for repeated use. The FDA does note that ultrasound introduces energy into the body, and at diagnostic levels it can produce minor physical effects in tissue, including slight pressure oscillations and a small rise in temperature. In practice, these effects are well within safe limits for standard diagnostic exams.
Ultrasound does have limitations. It struggles to image through bone and air-filled structures like the lungs, and image quality depends heavily on body type. In people with more body fat, deeper organs can be harder to visualize, and the sonographer may need to press harder or recommend a different imaging method. It’s also less detailed than MRI or CT for certain conditions, which is why your provider sometimes orders additional imaging after an ultrasound.