Abdominal X-Ray: Systematic Approach (ABDO X)

Overview

The Abdominal X-Ray (AXR) has largely been superseded by CT for complex acute abdominal pain. However, it remains a vital, low-radiation, first-line investigation for suspected bowel obstruction, toxic megacolon, and radio-opaque renal tract calculi (KUB).

The ABDO X Approach

To avoid missing critical peripheral pathology (like a subtle femoral hernia or lower rib fracture), adopt the systematic ABDO X reading method:

• Air (Extraluminal & Intraluminal)
• Bowel (Small vs. Large)
• Dense Structures (Bones & Calcifications)
• Organs & Soft Tissues
• Xternal Objects (Lines & Tubes)

A - Air (Gas Patterns)

The most critical finding on any AXR is free intraperitoneal air (Pneumoperitoneum). While an erect CXR is far more sensitive for detecting subdiaphragmatic free air, massive pneumoperitoneum can be spotted on a supine AXR.

• Rigler's Sign (Double Wall Sign): Normally, you only see the inside wall of the bowel (outlined by intraluminal gas). If you can clearly see BOTH the inside and the outside of the bowel wall, it means there is free gas residing in the peritoneal cavity outside the bowel.
• Falciform Ligament Sign: Free gas anterior to the liver outlines the falciform ligament, making it visible as a distinct vertical line in the right upper quadrant.
• Football Sign: Massive pneumoperitoneum outlining the entire peritoneal cavity in an oval shape, mimicking an American football.

B - Bowel (Small vs. Large)

Differentiating Small Bowel Obstruction (SBO) from Large Bowel Obstruction (LBO) is a core competency.

• Small Bowel: Central location. Contains Valvulae Conniventes (plicae circulares) which are closely spaced mucosal folds that completely cross the entire width of the bowel. Normal caliber is < 3cm.
• Large Bowel: Peripheral location (framing the abdomen). Contains Haustra, which are incomplete sacculations that do not cross the entire width. Normal caliber is < 6cm (and < 9cm for the Cecum).
• The "3-6-9 Rule" describes the upper limits of normal diameter for the Small Bowel (3cm), Large Bowel (6cm), and Cecum (9cm).

D - Dense Structures

Scan the osseous structures meticulously, as they often harbor incidental but highly significant pathology.

• Bones: Check the lower ribs, lumbar vertebrae, sacrum, pelvis, and proximal femurs for fractures, blastic/lytic metastases, or severe degenerative changes (e.g., Paget's disease).
• Calcifications: Look for classic right upper quadrant gallstones (only 10-20% are radio-opaque), renal/ureteric calculi along the kidney-ureter-bladder (KUB) tract, and most importantly, an expansile calcified abdominal aortic aneurysm (AAA) curving to the left of the lumbar spine.

High Yield Facts