14.1 Abdomen

The KUB (AP Supine Abdomen)

The abdominal radiograph is one of the most frequently performed procedures and a heavily tested item in the ARRT Procedures category. The routine survey image is the KUB - short for Kidneys, Ureters, and Bladder - which is an AP supine abdomen. The patient lies supine with the midsagittal plane centered to the midline of the grid, the pelvis and shoulders equidistant from the tabletop so there is no rotation, and the arms placed away from the body. Use a lengthwise 35 x 43 cm (14 x 17 inch) image receptor (IR). A broad hypersthenic habitus may require the IR crosswise or two exposures to cover the width, whereas a slender asthenic patient fits easily on a single lengthwise IR.

The central ray (CR) is directed perpendicular to the level of the iliac crests, which corresponds roughly to the L4-L5 interspace. Centering at the crests places the exposure field so the symphysis pubis is included at the bottom of the image, ensuring the urinary bladder is demonstrated. When the patient is very tall and the crest-to-symphysis distance is long, a second lower KUB or bladder image is added rather than shifting the primary KUB inferiorly.

Respiration and Evaluation Criteria

The abdomen is exposed on suspended expiration. On expiration the diaphragm moves superiorly, the abdominal organs shift up, and more of the abdominal contents are projected onto the IR; expiration also reduces motion. A well-positioned KUB shows the psoas muscles, the inferior margin of the liver, the lower ribs, the lumbar spine with symmetric transverse processes, and the pelvis. Signs of no rotation include symmetric iliac wings, spinous processes aligned in the midline of the vertebral bodies, and symmetric obturator foramina and ischial spines.

The Acute Abdominal Series

When bowel obstruction, ileus, or a perforated viscus is suspected, the physician orders an acute abdominal series (also called a three-way or obstruction series). Its purpose is to detect free intraperitoneal air (pneumoperitoneum) and air-fluid levels. The classic series has three parts:

The erect PA chest is included because the thin, high-contrast interface at the domes of the diaphragm makes even a few milliliters of subdiaphragmatic free air conspicuous. A common exam trap is forgetting that a chest image belongs in an abdominal series.

The Erect (Upright) AP Abdomen

For the upright AP abdomen the CR is horizontal and centered about 2 inches (5 cm) above the iliac crests so both hemidiaphragms are included at the top of the image. This higher centering is essential because free air collects immediately beneath the diaphragm and air-fluid levels form in dilated bowel loops. If the diaphragm is cut off, the study fails its purpose. The patient should be truly upright, not reclined.

The Left Lateral Decubitus Abdomen

Many acutely ill patients cannot stand. The substitute is the left lateral decubitus abdomen: the patient lies on the left side with a horizontal (cross-table) CR centered about 2 inches above the level of the iliac crests (which is now near the midline). The left side is chosen so free air rises to the nondependent region over the right lobe of the liver, where it is not confused with the normal gastric air bubble in the stomach on the left. A right lateral decubitus would let free air collect near the stomach gas and be missed. The left lateral decubitus can reveal as little as a few milliliters of free air.

Abdominal Landmarks and Body Habitus

Accurate centering depends on surface landmarks. The iliac crest sits at approximately L4-L5, the xiphoid tip at about T9-T10, the inferior costal margin near L2-L3, the ASIS at the level of the S1-S2, and the greater trochanters roughly at the level of the symphysis pubis. Soft-tissue structures a well-exposed KUB should reveal include the liver, spleen, kidney shadows, the lateral margins of the psoas muscles, the properitoneal fat stripes, and gas within the stomach and colon. The most common exposure error on the abdomen is quantum mottle from too little mAs in a large patient, or motion blur from failing to give clear breathing instructions.

Body habitus dictates IR placement. A sthenic or asthenic patient fits a lengthwise 14 x 17 inch IR. A hypersthenic patient, whose abdomen is broad and short, often needs the IR crosswise and may require two exposures - one high for the diaphragm on an upright and one low for the bladder. A dorsal decubitus (patient supine, horizontal beam, lateral IR) is an alternative cross-table view for demonstrating air-fluid levels or aneurysm calcification when the patient cannot be moved into a decubitus or upright position.

Timing and Common Traps

Before an erect or decubitus abdomen, the patient should be in position for at least 5 minutes (ideally 10 to 20) so free air can rise and fluid can settle into distinct levels; exposing too early hides small pneumoperitoneum. Remember the direction rules: abdomen is expiration, but the chest is inspiration. Center the KUB at the crests, but raise centering 2 inches for the upright to catch the diaphragm. When in doubt about free air and the patient cannot stand, choose the left lateral decubitus - never the right. Finally, always place an anatomic side marker in the collimated field at the moment of exposure; markers added afterward electronically are not acceptable for a legal record.