Procedures · Chest, abdomen and GI3 min read5-question check
Abdomen: KUB and acute series
KUB: iliac crests, expiration. Erect and decubitus images catch free air under the diaphragm.
Supine KUB
The KUB is a supine AP abdomen at 40 inches, or 100 cm. The patient lies flat with the midsagittal plane centered to the table, arms at the sides. Center the central ray perpendicular to the midsagittal plane at the level of the iliac crests. Expose on expiration, because the diaphragm rises and the abdomen is not compressed. The symphysis pubis must be included at the bottom of the image.
What a good image shows
A good KUB shows the lower liver margin, kidney outlines, psoas muscles, and transverse processes of the lumbar spine, from the upper abdomen down to the symphysis pubis. No rotation means the iliac wings are symmetric, the obturator foramina match, and the spinous processes are centered in the vertebral bodies.
Acute abdomen series
The acute abdomen series looks for free air, obstruction, and air-fluid levels. It usually includes a supine AP, an erect AP, and an erect PA chest, which is the best single image for free air under the diaphragm. For the erect AP, center about 2 inches above the iliac crests so the diaphragm is included. Keep the patient upright at least 5 minutes before exposure, and 10 to 20 minutes if possible, so a small amount of free air can rise. If the patient cannot stand, use a left lateral decubitus with the right side up and a horizontal beam, centered about 2 inches above the crests. Free air rises over the liver, where it is not confused with the gas bubble in the stomach. Use the same wait, and position for air: make sure the upper side of the abdomen is fully on the image.
Fixing it
On the KUB, centering too high cuts off the symphysis; recheck the crests and center there. On the erect and decubitus images, centering at the crests can cut off the diaphragm, where free air collects; move up about 2 inches. Exposing too soon on either view may miss a small amount of free air.
Clinical Example
An ER patient with severe abdominal pain is too weak to stand. You turn him onto his left side on a pad, wait 10 minutes, and use a horizontal beam centered 2 inches above the crests. A thin crescent of free air shows over the liver.
Common Mistakes
- Exposing a KUB on inspiration, which pushes the organs down.
- Doing a right lateral decubitus, which puts free air near the stomach bubble.
- Centering the erect abdomen at the crests and cutting off the diaphragm.
- Skipping the waiting time after moving the patient upright or onto the side.
Why It Matters
Free air under the diaphragm means a perforated bowel, which needs immediate surgery. The registry tests centering, breathing, which side goes up, and wait times.
Key Relationships
- KUB: supine AP at the iliac crests, on expiration, symphysis included
- Erect AP abdomen: center about 2 inches above the crests
- Erect PA chest is the best image for free air
- Left lateral decubitus: right side up, free air over the liver
- Wait at least 5 minutes before erect or decubitus exposures; 10 to 20 minutes is preferred for free air