Clinical presentation and epidemiology
The disease develops in genetically predisposed individuals as a result of an inappropriate immune response to antigens in the intestinal lumen, including normal microflora. An interesting trend: in developed countries, against the background of a decrease in common intestinal infections, there is a noticeable increase in inflammatory bowel diseases.
Patients most often present with non-specific symptoms: diarrhea, weight loss, and cramping abdominal pain. However, a third of patients reveal characteristic specific signs—perianal fistulas (fistulae), fissures (fissures), skin tags (tags), and aphthous ulcers. In the acute stage, the pathology can mimic an "acute abdomen," clinically indistinguishable from acute appendicitis, or manifest as acute intestinal obstruction due to stricture formation. In children, the disease is often accompanied by growth retardation.
Radiological signs and CT semiotics
Diagnostic imaging is crucial for assessing the severity and extent of inflammation. Traditional barium studies can reveal a variety of characteristic patterns:
- Aphthous ulcers: small contrast collections surrounded by a radiolucent halo of edema.
- Slit-like (fissure ulcers) and longitudinal ulcers occurring with deep involvement of the bowel wall.
- Cobblestone sign (cobblestoning): a specific mucosal pattern formed by the intersection of longitudinal and transverse ulcers separating areas of intact mucosa.
- Skip lesions (skip lesions): discontinuous bowel involvement, strictly specific to Crohn's disease.
- Strictures: multiple lumen narrowings with prestenotic dilation proximally. The presence of multiple strictures virtually confirms the diagnosis.
Computed tomography (CT) provides detailed visualization of changes not only in the bowel wall, but also in the surrounding mesenteric structures. Active inflammation manifests as the target sign (layered enhancement of the thickened wall) and the comb sign (comb sign — mesenteric hypervascularization due to engorged vasa recta). A characteristic chronic feature is fibro-fatty proliferation—expansion of mesenteric fat, which causes a mass effect and visually separates the bowel loops.
MR enterography and ultrasound
Because Crohn's disease has a relapsing course, patients are exposed to a high cumulative radiation dose from repeated CT scans. To address this issue, low-dose CT protocols with iterative reconstruction are being introduced, and MR enterography (MRE) is also actively used.
MRE excellently visualizes wall thickening, fistulas, and the cobblestone sign. The fat-suppression sequence (T2-FS) allows the detection of submucosal edema, and post-contrast T1-weighted images show active mucosal enhancement. The inclusion of diffusion-weighted imaging (DWI / DWI) with apparent diffusion coefficient (ADC) calculation helps differentiate healthy bowel from inflamed bowel, which is especially important for young patients with a small amount of intra-abdominal fat.
Ultrasound (US) is highly operator-dependent, but is an excellent non-radiation first-line modality. Segmental wall thickening greater than 3 mm, wall rigidity, and hypervascularization on Doppler imaging are reliable markers of an active process. In addition, ultrasound allows the rapid exclusion of hepatobiliary pathology and the detection of abscesses.
Extraintestinal manifestations and complications
When analyzing images, the radiologist is obligated to perform a targeted search for extraintestinal manifestations. Such patients frequently have renal calculi (renal calculi), erosive sacroiliitis, and signs of primary sclerosing cholangitis. A vital task of imaging is the search for extramural complications (phlegmons, sinuses, and abscesses). Identifying an interloop or pelvic abscess is critically important, as their presence strictly contraindicates the use of modern immunosuppressive drugs (e.g., infliximab).
One must also maintain oncologic vigilance. Patients with Crohn's disease are at high risk for developing GI adenocarcinoma and carcinoid tumors. Furthermore, long-term immunosuppressive therapy (anti-TNF agents or azathioprine) significantly increases the risk of lymphoma.
Briefly: answers to topical questions
How does the imaging appearance of Crohn's disease differ from ulcerative colitis (UC)?
Radiological signs of colitis are often non-specific and of little help in differential diagnosis. The sources list the following characteristic features for Crohn's disease, and features of severe colitis/toxic megacolon for ulcerative colitis.
| Sign | Crohn's disease | Ulcerative colitis / severe colitis according to sources |
|---|---|---|
| Nature of the lesion | Skip lesions (skip lesions), asymmetric bowel wall involvement | In toxic megacolon, haustration is always absent; if preserved, the diagnosis of toxic megacolon is excluded |
| Mucosal changes | Cobblestone sign (cobblestoning) | "Mucosal islands" (mucosal islands) in extremely extensive ulceration |
| Bowel wall | Transmural inflammation, wall thickening, target sign | In severe colitis/toxic megacolon, the bowel wall acquires the consistency of "wet blotting paper" |
What radiological stages of Crohn's disease exist?
The sources do not provide a strict classification of the radiological stages of Crohn's disease. It is indicated that radiological signs vary depending on the severity, stage, and extent of the disease.
- Signs of active inflammation: cobblestone sign (cobblestoning), bowel wall thickening with the target sign (target sign), perienteric fat stranding (fat stranding), comb sign (comb sign) as mesenteric hypervascularization, high DWI signal in the terminal ileum due to restricted diffusion against the background of active inflammation.
- Clinically inactive phase / remission: mesenteric fibro-fatty proliferation may persist and cause separation of bowel loops; in the remission stage, overgrown fibro-fatty tissue separates the small bowel loops.
What types of fistulas can form in Crohn's disease?
The sources mention the following types of fistulas in Crohn's disease:
- Perianal fistulas (fistulae) — indicated as specific signs of IBD.
- Enteroenteric fistulas — indicated among complications of Crohn's disease and indications for surgical/colorectal consultation.
- External enterocutaneous fistulas — indicated among indications for surgical/colorectal consultation.
- Fistulas involving hollow organs — indicated among indications for surgical/colorectal consultation.
Cross-sectional imaging modalities allow the diagnosis of extramural complications, including fistulas.
A mnemonic for remembering the active-phase imaging findings is the acronym SMRT: Skip lesions, Target (layered wall), Comb (engorged mesenteric vessels), Tears (deep ulcers).
Class and exam questions
Why is it so crucial to detect abdominal abscesses in Crohn's disease?
The presence of interloop, pelvic, or abdominal abscesses is a strict contraindication for prescribing modern immunosuppressive therapy, specifically TNF inhibitors.
What is the "cobblestone" sign?
This is a pathognomonic radiological pattern that occurs at the intersection of deep longitudinal and transverse ulcers separating protruding unaffected areas of the mucosa.
Is it possible to completely eliminate patient radiation exposure during disease monitoring?
Yes, MR enterography or ultrasound is used for this purpose. MRE is comparable to CT in lesion assessment accuracy and excellently demonstrates extramural complications without the use of ionizing radiation.
What else to study on this topic
- Radiological signs of celiac disease and differential diagnosis with Crohn's disease
- Causes of small bowel strictures: infectious, vascular, and neoplastic factors
- CT enteroclysis in the diagnosis of primary small bowel neoplasms
- Cavitary mesenteric lymph node syndrome in enteropathies
- Low-dose CT protocols and iterative image reconstruction algorithms
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