Diagnostic imaging of acute abdomen

Paradigm shift in imaging

Historically, plain abdominal radiography (Plain Abdominal Radiograph, AXR) was the first and primary step in diagnosing emergency conditions. Today, its place has been firmly taken by computed tomography (CT). To minimize radiation exposure—which is especially critical for children, pregnant women, and patients with chronic pathologies (for example, Crohn's disease)—ultrasound and MRI are actively used. Low-dose CT protocols and iterative reconstruction are also being introduced into practice.

Plain abdominal radiography in non-emergent situations has extremely low diagnostic value. Exceptions include monitoring the passage of ingested radiopaque foreign bodies, searching for urinary tract calculi, and assessing transit time in constipation. For common acute inflammatory processes—cholecystitis, appendicitis, diverticulitis, or pancreatitis—radiography is uninformative. In these cases, the method should be avoided, prioritizing the clinical picture, ultrasound, or tomographic imaging.

Standard protocol and strict indications

Radiography for an "acute abdomen" is justified exclusively when its result is capable of changing the diagnosis or patient management strategy.

Justified indications:

• Diagnosis of hollow viscus perforation (searching for pneumoperitoneum).
• Assessment of loop dilatation in intestinal obstruction.
• Evaluation of the bowel wall status (in the setting of colitis or ischemia).
• Search for foreign bodies in the abdominal cavity.

The standard emergency protocol includes two examinations:

1. Supine abdominal radiography (supine AXR). Performed with an empty urinary bladder. The field of view must include anatomical structures from the diaphragmatic domes to the obturator foramina (herniolauricular orifices / hernia portals).
2. Erect chest radiography (erect CXR). This stage is critically important for detecting free gas. For maximum accuracy, the patient should spend about 10 minutes in a vertical position before the scan—this allows the gas to rise to the highest point.

Note: The previously popular erect abdominal radiography (erect AXR) for assessing fluid and gas levels is currently not recommended. It has been proven that the method is unreliable and frequently yields false results.

Pneumoperitoneum: searching for free gas

The presence of free gas almost always indicates hollow viscus perforation (the most frequent cause is a perforated peptic ulcer, whereas in perforated appendicitis this sign is rare). Gas can also appear due to bowel ischemia, severe obstruction, inflammatory bowel disease, or iatrogenic injuries following colonoscopy.

When analyzing an erect chest radiograph, gas is best seen beneath the right diaphragmatic dome—it clearly contrasts with the smooth surface of the liver. On the left, diagnosis is more difficult due to the overlay of the gastric bubble and the colon. Sometimes, collections beneath both domes merge in the midline, forming the cupola sign (Cupola sign).

Signs of gas on a supine radiograph (important for unconscious patients, trauma cases, or those on steroid therapy masking the clinical presentation):

• Gas in the right upper quadrant (in 50% of patients): accumulates around the liver, in the hepatorenal recess (Morison's pouch) or the falciform ligament fissure.
• Rigler's sign (Rigler's sign): the double wall phenomenon. Gas is present both in the lumen and in the abdominal cavity, clearly outlining the bowel wall.
• Contouring of ligaments: free gas makes the falciform ligament visible or outlines the medial umbilical ligament, forming the inverted V sign (Inverted V sign).
• Triangular-shaped gas collections between bowel loops.

Conditions mimicking pneumoperitoneum

Not every lucency beneath the diaphragm signifies perforation. There is a range of pathologies that create the picture of pseudopneumoperitoneum:

• Chilaiditi syndrome (Chilaiditi syndrome) — interposition of a bowel loop between the diaphragm and the liver.
• Subdiaphragmatic abscesses.
• Disc-like (subsegmental) atelectasis of the basal lung segments (Curvilinear atelectasis).
• Anatomic irregularities and bulges of the diaphragmatic contour.
• Accumulations of subdiaphragmatic adipose tissue.
• Cysts in the setting of pneumatosis intestinalis (Pneumatosis intestinalis).

In case of any diagnostic doubts, the most sensitive method is CT, which can detect minimal gas bubbles above the liver. As a compromise alternative, left lateral decubitus radiography can be used, where gas accumulates between the lateral edge of the liver and the abdominal wall.

Class and exam questions

What else to study on this topic

• Pathological distribution of gas in the retroperitoneal space and biliary tract.
• Assessment of bowel dilatation in various types of obstruction.
• Algorithm for imaging follow-up of ingested foreign bodies' progression.
• Reduction of cumulative radiation dose: principles of iterative reconstruction.

Other topics in the section "Diagnostic Radiology of the Abdominal Organs"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.