CT of the rectum

Role of CT in the diagnosis of inflammatory bowel disease (IBD)

IBD (Inflammatory Bowel Disease) includes ulcerative colitis, Crohn's disease (Crohn disease), and unclassified forms. The pathogenesis is based on dysregulated immune responses to microflora in genetically predisposed patients.

Although ileocolonoscopy with biopsy remains the gold standard for diagnosis, cross-sectional imaging (including CT) is critically important. It helps determine the extent of the disease, assess complications, and select candidates for surgical treatment.

Severe cross-sectional imaging findings include:

• An inflamed large bowel surrounded by marked inflammatory fat stranding.
• Enlarged regional lymph nodes.
• Presence of microabscesses.
• No rectal sparing (no rectal sparing).

It is especially noteworthy that in young children, the colon and rectum are frequently involved even in Crohn's disease, and the disease often presents in a severe fistulizing form.

Pseudomembranous colitis and rectal involvement

Pseudomembranous colitis (Pseudomembranous Colitis) may develop after antibiotic therapy (specifically clindamycin, lincomycin, or similar agents). The causative agent is Clostridium difficile, which is frequently isolated in stool cultures.

Pathological findings on imaging are detected in about one-third of cases. These include:

1. Colonic dilatation (in 32% of cases).
2. Haustral thickening and mucosal abnormalities.
3. The «thumbprinting» sign (thumbprinting).

Inflammation may affect the entire colon, with the transverse colon being the most commonly affected. Meanwhile, the rectum is involved in the pathological process in the vast majority of cases.

Large bowel ischemia and wall thickening

Computed tomography is a much more sensitive method than plain radiography for detecting bowel wall thickening. On CT, the wall is clearly outlined by gas and adjacent adipose tissue.

In large-bowel ischemia (Large-Bowel Ischaemia), the wall—particularly its submucosal layer—becomes significantly thickened as a result of hemorrhage and edema. Radiologically, this is also described by the term thumbprinting sign (thumbprinting).

Complications of ischemia:

• The affected segment acts as a functional obstruction, causing distension (bloating) of the right colon.
• In the long term, the affected segment may undergo fibrosis resulting in stricture formation.

CT is also indispensable for detecting small pockets of intramural gas (pneumatosis), which is a ominous sign of necrosis.

Dilatation, pseudo-obstruction, and special techniques

There are numerous causes of colonic distension without mechanical obstruction, including paralytic ileus and pseudo-obstruction.

Pseudo-obstruction (Pseudo-Obstruction) typically occurs in elderly patients and clinically mimics true obstruction. Imaging reveals a markedly dilated colon. Performing a CT scan is a mandatory step to rule out mechanical obstruction.

To detect intraluminal lesions (polyps, neuroendocrine tumors, mucosal metastases), special techniques are used: CT enterography (enterography) or enteroclysis. Neutral oral contrast agents (water or diluted polyethylene glycol) combined with intravenous contrast administration are used for better visualization of endoluminal lesions.

Class and exam questions

What else to study on this topic

• Imaging findings of large bowel ischemia and watershed zones.
• Toxic megacolon: high-risk criteria.
• CT findings of strangulation and closed-loop obstruction in bowel obstruction.
• MR enterography as a radiation-free alternative to CT.
• Ultrasound assessment of bowel wall thickness in children.

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

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