A sonogram is done by pressing a handheld device called a transducer against your skin, which sends sound waves into your body and captures the echoes that bounce back to create a real-time image on a screen. The whole process typically takes 20 to 30 minutes, involves no needles or radiation, and is painless for most people. Depending on what part of your body is being examined, the transducer may be used externally on your skin or internally through the vaginal or rectal canal.
How Sound Waves Become Images
Inside the transducer is a crystal made of a material that vibrates when electricity hits it, converting electrical pulses into high-frequency sound waves. Those waves travel through your tissue, and every time they hit a boundary between different types of tissue (fluid and organ wall, for example), some of the sound bounces back. The returning echoes hit the same crystal, which vibrates again and generates a tiny electrical signal. A computer measures the strength and timing of each returning signal to calculate how deep the structure is and how dense it is, then assembles all of that data into a grayscale image on the monitor.
Because sound waves travel poorly through air, a water-based gel is applied to your skin before the exam. The gel fills the microscopic air gap between the transducer and your body so the sound waves pass through cleanly. It feels cool and slippery but wipes off easily with a towel.
What Happens During an External Sonogram
You’ll lie on an exam table, usually on your back, though the technician (called a sonographer) may ask you to shift to your side for certain views. The sonographer spreads gel over the area being examined, then presses the transducer firmly against your skin and glides it in slow, deliberate paths. You may feel moderate pressure, especially if they need to angle the transducer to see around a rib or through a full bladder, but it shouldn’t be painful.
As the transducer moves, images appear on a screen in real time. The sonographer will pause periodically to freeze a frame, take measurements, and save specific images. They may ask you to hold your breath briefly or change position so they can capture a clearer view. Once they have all the images they need, they wipe the gel off your skin, and you’re done. Most external sonograms wrap up in under 30 minutes.
Internal Sonograms
When organs are too deep or too small to image well from outside the body, a slim, specially shaped transducer is inserted internally. The two most common types are transvaginal and transrectal.
For a transvaginal sonogram, you lie on your back with your feet in stirrups, similar to a pelvic exam. The sonographer covers the transducer with a disposable sheath, applies lubricating gel, and gently inserts it into the vaginal canal. The probe is roughly finger-width and gives much sharper images of the uterus and ovaries than an external approach, which is why it’s often used in early pregnancy or when evaluating pelvic symptoms. Most people describe the sensation as pressure rather than pain.
A transrectal ultrasound follows a similar concept. You lie on your side with your knees bent, and a thin, lubricated transducer is inserted a short distance into the rectum. This approach is commonly used to examine the prostate. The exam takes about 20 minutes and can cause mild discomfort, though numbing gel is available if needed. Your care team may ask you to use an enema at home beforehand to clear the rectum.
Preparation Varies by Type
How you prepare depends on what’s being scanned. For a pelvic ultrasound done externally, you’ll typically be asked to drink at least 24 ounces of clear fluid about an hour before your appointment and avoid using the bathroom until after the exam. A full bladder pushes the intestines out of the way and creates an acoustic window that lets sound waves reach the uterus and ovaries more easily. For a transvaginal ultrasound, the opposite applies: you empty your bladder right before the procedure.
Abdominal sonograms that focus on the gallbladder, liver, or pancreas often require fasting for 8 to 12 hours beforehand, because eating triggers the gallbladder to contract and produces gas in the intestines, both of which can obscure the view. Most other ultrasound exams, including those of the thyroid, breast, or extremities, need no special preparation at all.
Doppler Mode: Watching Blood Flow
Sometimes the sonographer switches the machine into Doppler mode during your exam. Standard ultrasound shows the shape and structure of organs, but Doppler measures movement, specifically the flow of blood through your vessels. It works by detecting tiny shifts in the frequency of echoes bouncing off moving red blood cells. Cells flowing toward the transducer reflect sound at a slightly higher frequency, while cells flowing away reflect it at a lower frequency.
The machine can display this information in several ways. Color Doppler overlays red and blue onto the grayscale image to show the direction and speed of blood flow. Spectral Doppler plots the flow as a graph, which helps identify blockages or narrowing in a vessel. You won’t feel anything different when Doppler is activated; the sonographer simply adjusts settings on the machine. You may hear a rhythmic whooshing sound, which is the audio representation of your blood flow.
Is It Safe?
Ultrasound uses no ionizing radiation, which is the type associated with X-rays and CT scans. The FDA classifies it as non-ionizing, meaning it does not carry the DNA-damage risks of radiation-based imaging. The sound waves do introduce a small amount of energy into tissue, which can cause slight warming and pressure oscillations at the cellular level. At diagnostic levels, these effects have not been shown to cause harm in humans, which is why ultrasound remains the default imaging tool during pregnancy.
Getting Your Results
The sonographer captures and saves the images during your appointment, but they typically do not interpret the findings for you on the spot. A radiologist or specialist reviews the images afterward and sends a written report to the provider who ordered your exam. Turnaround times vary by facility. Urgent or inpatient scans are often read within an hour, while routine outpatient results may take one to three business days. Your ordering provider then contacts you to discuss the findings, or you may be able to see the report through an online patient portal before that call comes.