CT enterography / colonography with contrast

General principles of CT imaging of the bowel

To obtain a complete diagnostic picture, intravenous (IV) contrast enhancement is a practically mandatory condition. The pattern of contrast accumulation by the altered bowel wall (mural enhancement pattern) provides the diagnostician with the primary clue for establishing the main diagnosis.

In addition to IV enhancement, oral contrasts are used. Traditional routine abdominal CT uses positive agents (iodine-containing preparations or 2% barium suspension), which the patient begins taking an hour or even a day before the scan. When perforation or surgical anastomosis leakage is suspected, rectal contrast administration is specifically used.

Normal CT criteria include:

• A wall thickness of no more than 3 mm (provided there is adequate distension).
• Homogeneous perienteric adipose tissue with a small number of vascular channels.
• Active contrast enhancement of the normal mucosa following IV injection.

Large bowel imaging: CT colonography

CT colonography (CT colonography or CTC) is an examination of the large bowel under conditions of gas distension. Insufflation significantly increases the accuracy of pathology detection, although it limits the method's use in acute emergency situations due to its invasiveness.

A mandatory preparation step is the "tagging" (tagging) of residual bowel contents. The patient takes high-osmolarity contrast agents (e.g., Gastrografin) to give fecal masses a high X-ray attenuation (attenuation). The procedure itself can be performed either after classical full or partial cleansing with laxatives or without them entirely (prep-less CTC).

Note: Double-contrast barium enema (DCBE) is currently considered obsolete for detecting polyps or colorectal cancer and has been completely replaced by CT colonography.

Small bowel imaging: CT enterography

CT enterography (CT enterography or CTE) targets the small bowel. Here, it is critically important to achieve adequate loop distension. To do this, the patient rapidly drinks large volumes of neutral-density oral contrast (0–30 HU).

Water, milk, polyethylene glycol (PEG)-based solutions, or a 0.1% low-density barium sulfate solution (VoLumen) are used as neutral agents.

• PEG preparations provide the best distension because they are less absorbed and remain in the lumen longer, but they more frequently cause spasms and patient discomfort.
• Neutral/negative contrast is ideal for evaluating mucosal diseases and the enhancement pattern of the wall.

The peak of mucosal contrast accumulation (enteric phase) is recorded approximately 50 seconds after intravenous administration (or 14 seconds after the aortic enhancement peak is reached).

The role of CT in vascular pathologies

The diagnosis of vascular catastrophes, such as acute mesenteric ischemia, requires a specific technique: rapid IV bolus injection, dual-phase scanning, and the use of ordinary water as an oral contrast (to better visualize the bowel wall).

CT semiotics depends on the type of circulatory disorder:

1. Acute transmural infarction: the bowel wall thins (mural thinning), the lumen dilates, and IV wall enhancement is reduced or absent.
2. Non-occlusive ischemia: the wall thickens, and mucosal hyperenhancement is observed (mucosal hyperenhancement).
3. Venous thrombosis: pronounced wall thickening, mesenteric stranding (mesenteric stranding), vascular congestion, and frequently ascites.

Formidable radiological signs of bowel infarction include pneumatosis intestinalis (pneumatosis intestinalis) and the appearance of gas in the mesenteric or portal vein.

CT also assists in diagnosing vasculitides (polyarteritis nodosa, SLE) by revealing aneurysms of the mesenteric vessels and plays an indisputable role in searching for sources of occult gastrointestinal bleeding using multiphase protocols.

Specific contrast examinations (enemas)

Although tomographic methods dominate, classical techniques retain significance in narrow clinical niches. A water-soluble contrast enema (e.g., Gastrografin diluted with water 3:1) allows for real-time anatomical evaluation.

The method is indicated for:

• Checking the integrity of surgical anastomoses.
• Evaluating bowel caliber (in megacolon).
• Searching for colorectal fistulas (e.g., rectovaginal).
• Ruling out mechanical obstruction.

It is important to remember complications: hypertonic solutions cause diarrhea, and in the presence of strictures, the movement of fluid proximal to the narrowing can provoke overdistension and perforation. Therefore, isotonic preparations are increasingly being used.

Class and exam questions

What else to study on this topic

• Radiopaque small bowel examinations: barium follow-through and enteroclysis
• Multiphase MDCT protocols for occult gastrointestinal bleeding
• CT semiotics of systemic vasculitides with gastrointestinal involvement (SLE, polyarteritis nodosa)
• Portal venous gas: differential diagnosis and prognostic significance
• Radiological diagnosis of surgical anastomosis leakage and pelvic fistulas

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

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