An abdominal X-ray, also known as a plain film or a KUB (for kidneys, ureters, and bladder), is a quick, non-invasive imaging test that uses a small amount of radiation to produce pictures of structures within the abdominal cavity. It is a common initial diagnostic tool in emergency medicine for patients presenting with acute abdominal pain, nausea, vomiting, or suspected constipation. The X-ray provides a rapid assessment of gas patterns, soft tissue outlines, and calcifications, which can reveal signs of time-sensitive conditions like bowel obstructions, intestinal perforation, or radiopaque foreign objects. While advanced imaging like Computed Tomography (CT) offers greater detail, the speed and low cost of the X-ray make it valuable for initial triage.
Ensuring Image Quality and Proper View
Before interpretation, the radiograph’s quality must be assessed to ensure it is diagnostically adequate. This involves checking for proper exposure, which should allow for differentiation between the gray soft tissues and the whiter bone structures. The film must cover all necessary anatomical landmarks, spanning from the diaphragm down to the pubic symphysis, and including both flanks laterally. Failure to capture this entire area risks missing localized pathology.
While the most common view is the supine (lying on the back) anteroposterior (AP) projection, an erect (upright) view is often included in an acute abdominal series. The upright film is specifically useful because it allows free air and fluid to collect, making air-fluid levels visible within the bowel, which often signals obstruction. To allow small amounts of air to rise for detection, the patient must be positioned upright for 10 to 20 minutes before the image is taken.
Interpreting Bowel Gas Patterns
The appearance of gas within the intestines is the primary focus of an abdominal X-ray, providing immediate clues about bowel function. Normally, air is seen in the stomach, scattered minimally in the small bowel, and variably in the colon and rectum. The small and large bowels are distinguished by their location and the pattern of their internal folds.
Small bowel loops tend to be centrally located, and their internal folds, called valvulae conniventes, extend completely across the lumen. The small bowel is considered abnormally dilated if its diameter exceeds 3 centimeters. In contrast, the large bowel, or colon, is typically located peripherally and contains folds called haustra, which do not extend all the way across the width of the bowel lumen.
Abnormal gas patterns typically point toward either a mechanical obstruction or a functional ileus. In a mechanical small bowel obstruction, the bowel loops proximal (above) the blockage become dilated, while the colon distal (below) the obstruction often shows a lack of gas. A functional or adynamic ileus involves a lack of normal muscle movement, typically causing a more generalized, uniform dilation of both the small and large bowel. The presence of multiple, long air-fluid levels on an upright film strongly suggests an intestinal obstruction.
Assessing Soft Tissue and Skeletal Structures
Soft tissue organs and skeletal structures provide additional diagnostic information. The outlines of the liver, spleen, and kidneys are visible because of the surrounding fat, which has a different density on the X-ray. The psoas muscle shadows, which run obliquely alongside the lower spine, should also be examined; their clear definition suggests a lack of inflammation in the retroperitoneal space, while a loss of definition can indicate an inflammatory process.
Calcifications, which appear intensely white, should be noted for their location and appearance. This can include kidney stones, often seen along the expected course of the ureters, or vascular calcifications, such as those lining the walls of a potential abdominal aortic aneurysm. Any foreign bodies, such as swallowed objects or medical devices, will also be visible and their position should be confirmed. Finally, the bones visible on the film, including the lower ribs, spine, and pelvis, must be quickly scanned for any signs of fracture, degenerative changes like osteoarthritis, or other bone pathology.
Identifying Signs of Acute Abdominal Pathology
The ultimate goal of reading an abdominal X-ray in the acute setting is to identify high-alert findings that demand immediate intervention. The most significant finding is pneumoperitoneum, or free air within the abdominal cavity, which is a strong indicator of a perforated hollow organ, such as a burst ulcer or bowel. On an upright film, this free air collects beneath the diaphragm, appearing as a crescent-shaped lucency.
A different sign of free air, especially on a supine film, is the Rigler’s sign, where both the inner and outer walls of the bowel become visible due to air on both sides. In cases of severe small bowel obstruction, the “string of pearls” sign may be seen on upright or decubitus views, appearing as a row of small gas bubbles trapped between the internal bowel folds. Severe large bowel dilation, particularly in the colon, may be a sign of a volvulus, where a loop of bowel twists around itself, requiring emergency intervention.