Ultrasound technology uses sound waves to create images of the body’s internal structures. When these waves encounter air, they are reflected, creating an image disruption known as an artifact. This interference prevents the sound waves from reaching deeper tissues, obscuring the view of underlying organs and requiring careful interpretation.
The Appearance of Air on an Ultrasound
Air in the body creates a distinct image on an ultrasound screen. Due to its different density compared to body tissues, air reflects most sound waves. This reflection appears as a bright, white, or “hyperechoic,” area on the display, indicating a pocket of gas.
Beneath this bright line, a characteristic shadow appears. Unlike the clean shadow from a solid object like bone, this one is a hazy and chaotic “dirty shadow.” This effect is caused by sound waves scattering off the gas, which degrades the image quality and makes it difficult to see structures located behind the air pocket.
Common and Benign Causes of Air Artifacts
Often, air on an ultrasound is a normal finding that does not indicate a problem. The most frequent source of this benign air is gas within the gastrointestinal tract. During an abdominal or pelvic ultrasound, it is common to see air in the stomach, small intestine, or colon as a natural part of digestion.
This bowel gas can pose a challenge by blocking the view of nearby organs. For instance, a large amount of intestinal gas might obscure the pancreas, aorta, or ovaries, making a complete evaluation difficult. While an inconvenience for the scan, this is considered a normal anatomical feature.
Air can also be introduced during certain medical procedures. For instance, small amounts of air can enter the bladder after a urinary catheter is placed, creating a temporary and harmless artifact. Similarly, air can be introduced into the uterus following some gynecological procedures, which is an expected consequence of the intervention.
Medically Significant Causes of Air
Air appearing in locations outside of the lungs or digestive tract can signal a significant medical condition. One example is pneumoperitoneum, the presence of air within the abdominal cavity but outside the organs. This is often caused by a perforation, or hole, in the stomach or intestine that allows air to leak out.
Gas-producing bacteria are another source of concerning air. In some infections, bacteria create gas within tissues, leading to an abscess. An ultrasound may show this as a fluid collection containing bright gas bubbles with dirty shadowing, seen in conditions like emphysematous cholecystitis or a splenic abscess.
In rare cases, air in the uterine wall, known as uterine emphysema, may indicate a severe infection. Air can also appear in the soft tissues under the skin, a condition called subcutaneous emphysema, resulting from trauma or infections like necrotizing fasciitis. Both findings point to an underlying problem requiring prompt medical evaluation.
Diagnostic Interpretation and Next Steps
The significance of air on an ultrasound depends on its location and the patient’s clinical picture, including symptoms and medical history. A radiologist will interpret the finding based on where the air is located. For example, air within the bowel is considered normal, whereas air found freely in the abdominal cavity suggests a problem.
When an ultrasound reveals air in a concerning location, further steps are taken to confirm the diagnosis. A common next step is ordering a different imaging study, such as a computed tomography (CT) scan. CT scans are more sensitive than ultrasound for detecting and pinpointing the exact location of small amounts of gas.
If a patient has severe symptoms like intense abdominal pain or fever, the discovery of abnormal air may prompt immediate medical or surgical intervention. The ultrasound finding, combined with the clinical context, helps guide the medical team. This determines the appropriate action, such as further imaging, antibiotic treatment, or surgery.