Performing an ultrasound involves applying gel to the skin, placing a handheld probe against the body, and manipulating that probe to generate real-time images of internal structures on a monitor. The core technique is straightforward, but getting useful images depends on proper preparation, probe handling, and body mechanics. Here’s how the process works from start to finish.
Why Gel Is Essential
Before the probe ever touches skin, you need ultrasound coupling gel, and understanding why makes the entire process make more sense. Sound waves travel well through liquid and soft tissue but reflect almost completely off air. The acoustic impedance of air is about 0.0004 MRayl, while biological tissue sits around 1.50 MRayl. That mismatch means even a paper-thin air gap between the probe and skin can bounce nearly all the ultrasound energy back, producing no usable image.
Gel fills that gap. Its impedance closely matches tissue (typically 1.5 to 1.7 MRayl), so sound waves pass from the probe into the body with minimal reflection. Apply it generously. A thin, patchy layer creates air pockets that degrade the image. If you’re using a probe cover (common for sterile or internal exams), apply gel directly to the probe face first, then pull the cover tightly over the tip to squeeze out all air bubbles before adding more gel on the outside.
Patient Preparation by Scan Type
Different exams require different preparation, and getting this wrong can make organs harder to visualize.
- Abdominal ultrasound: No food or drink for eight hours before the exam. Water and medications are fine. Fasting reduces gas and bile activity in the gallbladder, making organs easier to see.
- Pelvic ultrasound: Drink 32 ounces (four glasses) of water one hour before the scan. A full bladder pushes the intestines out of the way and creates an acoustic window to the uterus and ovaries. You can use the bathroom if needed, as long as you keep drinking water.
- Combined abdominal and pelvic: Fast for eight hours and drink the 32 ounces of water one hour before.
- Kidney or testicular ultrasound: No special preparation needed.
Position the patient before scanning in whatever way gives the best access to the target area while keeping both the patient and the person scanning comfortable. For abdominal exams, that usually means lying flat on the back. For some kidney views, you may need the patient on their side.
How to Hold and Move the Probe
Hold the probe the way you’d hold a pen, with your fingers wrapped around it near the tip. Rest your other fingers against the patient’s body to stabilize your hand. This anchoring is important: without it, small hand tremors get magnified on screen, and you’ll struggle to hold a steady image.
There are six basic probe movements, and each one changes the image differently:
- Slide: Move the entire probe across the skin surface to scan a new area.
- Sweep/Fan: Keep the probe footprint in place and tilt it side to side, creating a panoramic effect that gives a more three-dimensional sense of the anatomy beneath.
- Rock: Tilt the probe along its long axis, angling the beam to catch structures that sit at an oblique angle.
- Compression: Push the probe downward into the tissue. This is useful for checking whether a vein collapses (which helps distinguish it from an artery) or for displacing bowel gas.
- Rotation: Spin the probe clockwise or counterclockwise to switch between longitudinal and transverse views of the same structure.
In practice, you’ll combine several of these movements fluidly. Start by sliding to find the general area of interest, then fan and rock to optimize the image before freezing it.
Probe Orientation and Screen Layout
Every ultrasound probe has an orientation marker, a small bump or ridge on one side. This marker corresponds to a dot on the monitor, usually in the upper left corner. The standard convention is to point the probe marker toward the patient’s right side (for transverse views) or toward the head (for longitudinal views). When oriented correctly, the patient’s right side appears on the left side of the screen, matching how anatomy is displayed in radiology.
Cardiac exams are the exception. When you select a cardiac preset on the machine, the marker dot flips to the right side of the monitor. This is a long-standing cardiology convention, so if you’re switching between cardiac and abdominal exams, double-check your preset to avoid left-right confusion.
Freezing and Measuring Images
Once you’ve optimized a view, press the freeze button to capture a still frame. Most machines let you scroll back through the last several seconds of imaging to find the best frame. To take measurements, place on-screen calipers at the start and end points of whatever you’re measuring (organ diameter, fluid collection depth, fetal length) and press select to lock them in. The machine calculates the distance automatically.
Saving images with proper labels and measurements matters for documentation. Most systems let you annotate directly on the frozen image before storing it.
Ergonomics During Scanning
Sonographers have high rates of repetitive strain injuries in the shoulder, wrist, and neck. If you’re scanning regularly, body mechanics matter as much as image quality. The CDC recommends positioning the equipment monitor directly in front of you so you’re not twisting your neck to one side. Your keyboard arm should hang relaxed at your side with the elbow bent at roughly 90 degrees.
The exam table should be as narrow as possible, ideally 24 to 27 inches wide, so you don’t have to reach across a wide surface to access the far side of the patient. Use a motorized height-adjustable table to bring the patient to a level where your scanning arm stays close to your body rather than elevated or extended. An adjustable chair with proper footrests and seat height controls also helps. These details sound minor, but over hundreds of exams they’re the difference between a sustainable practice and chronic shoulder pain.
Bedside Versus Formal Scans
There are two broad categories of ultrasound exams, and the technique differs between them. A formal diagnostic ultrasound is a comprehensive study performed in an imaging department, typically by a trained sonographer, with dozens of standardized images reviewed by a radiologist. These exams follow strict protocols and produce detailed reports.
Point-of-care ultrasound (POCUS) is a focused, rapid exam performed by a physician at the bedside to answer a specific clinical question: Is there fluid around the heart? Is this vein clotted? Is the bladder full? POCUS has become standard practice in emergency medicine, obstetrics, and musculoskeletal care. It reduces diagnostic time significantly. For lung conditions, POCUS cuts emergency department diagnostic time by about two hours compared to standard imaging like X-rays or CT scans.
For certain conditions, bedside scans perform just as well as formal studies. POCUS is equivalent to formal sonography for screening abdominal aortic aneurysms and equally accurate for diagnosing deep vein thrombosis. It can identify full-thickness rotator cuff tears with the same accuracy as MRI, based on a review of 20 studies covering over 1,100 shoulders. For partial-thickness tears and some other shoulder conditions, it’s less reliable than MRI. The key distinction is that POCUS answers a binary yes-or-no question quickly, while formal ultrasound provides a comprehensive anatomical survey.