Crohn's disease is a chronic, transmural, granulomatous inflammatory bowel disease that can involve any part of the gastrointestinal system. It most commonly affects the terminal ileum and cecum (40%). Characteristically shows skip lesions (diseased areas separated by healthy segments), transmural inflammation, fistulae, sinus tracts, and strictures. Mean age of onset is 15-35, with a second peak between 50-70 years. The disease is pathologically classified into active inflammation, fibrostenosis, and penetrating (fistulizing) phenotypes. CT enterography and MR enterography are fundamental modalities for evaluating disease activity, extent, and complications. MR enterography is preferred in young patients and follow-up due to absence of radiation. On imaging, mural stratification, mural enhancement increase, comb sign, creeping fat, and mesenteric lymphadenopathy are active disease findings.
Age Range
15-45
Peak Age
30
Gender
Equal
Prevalence
Uncommon
Crohn's disease pathogenesis is multifactorial: genetic predisposition (NOD2/CARD15, ATG16L1, IL23R mutations), environmental triggers (smoking, NSAIDs, antibiotic use), and intestinal microbiome dysbiosis together trigger the disease. Impaired intestinal barrier function → excessive immune response to luminal bacterial antigens → Th1/Th17-mediated chronic inflammation → transmural damage. Transmural inflammation affects all layers of the bowel wall — leading to fibrin deposition on the serosal surface, subserosal fat proliferation (creeping fat), and peritoneal adhesions. In the vascular component of chronic inflammation, neovascularization and vasodilation occur in mesenteric arterioles → 'comb sign' on imaging. Granuloma formation (in 30-50% of patients) is a Crohn-specific histological finding. Transmural inflammation predisposes to fistula and abscess formation — deep ulcers in the bowel wall reach the serosa creating abnormal connections with adjacent structures (other bowel loops, bladder, skin). The chronic inflammation-repair cycle leads to progressive fibrosis and stricture formation.
Areas of active disease separated by healthy segments — pathognomonic distribution pattern of Crohn's disease. Distinguishes from continuous involvement of ulcerative colitis.
Skip lesions in the terminal ileum and/or other small bowel segments on portal venous phase — areas of active disease separated by healthy segments. Mural stratification (target sign) in active segments: hypodense submucosal edema layer between enhancing mucosa and serosa. Wall thickness ranges 4-12 mm. Asymmetric involvement (more prominent on mesenteric border) is characteristic for Crohn's.
Report Sentence
Skip lesions and mural stratification in the terminal ileum are observed, consistent with active Crohn's disease.
Vasa recta dilation in the mesentery adjacent to the active disease segment on arterial phase — 'comb sign'. Dilated straight vessels extend parallel from mesentery to bowel wall resembling comb teeth. This finding reflects increased mesenteric blood flow and neovascularization. It is a reliable indicator of active inflammation.
Report Sentence
Comb sign in the mesentery adjacent to the active disease segment is observed, supporting active Crohn inflammation.
Mesenteric fat proliferation surrounding the bowel wall in the active disease segment on portal venous phase — 'creeping fat'. Increased density in mesenteric fat tissue and increased separation between bowel loops are observed. Creeping fat is most prominent in the terminal ileum and is a highly specific finding for Crohn's disease.
Report Sentence
Creeping fat in the terminal ileum is observed, consistent with chronic transmural inflammation and Crohn's disease.
Marked diffusion restriction in actively inflamed segments on DWI — bright signal on high b-value images and low ADC values. DWI is very valuable in differentiating active inflammation from fibrotic stricture — fibrotic stricture does not show diffusion restriction. DWI in MR enterography protocol is a reliable indicator of disease activity and does not require contrast agent.
Report Sentence
Diffusion restriction in active segments on DWI in MR enterography is observed, supporting active Crohn inflammation.
Abnormal tracts (fistulae) between bowel loops or bowel-skin/bowel-bladder and fluid collections with rim enhancement (abscesses) on portal venous phase. Enteroenteric fistulae are the most common type. Perianal fistulae are seen in 30-50% of Crohn patients. Abscesses are usually located in the ileocecal region or pelvis and show rim enhancement + central low density.
Report Sentence
Enteroenteric fistula and abscess with rim enhancement in the ileocecal region are observed, consistent with penetrating Crohn's disease.
High signal intensity wall thickening in active disease segments on T2-weighted MR enterography — edema. T2 hyperintensity is a reliable indicator of active inflammation. Fibrotic stricture shows low signal on T2 — very valuable in active-fibrotic differentiation. STIR sequence suppresses fat making wall edema more prominent.
Report Sentence
T2 hyperintense wall thickening in the terminal ileum on MR enterography is observed, consistent with active inflammatory Crohn's disease.
Criteria
Active mucosal and transmural inflammation — no stricture or penetration
Distinct Features
Mural stratification, comb sign, DWI positive. Good response to anti-TNF/anti-integrin therapy. Most common initial phenotype.
Criteria
Luminal narrowing with obstruction findings — fibrotic stricture
Distinct Features
T2 hypointense wall, DWI negative (pure fibrosis), prestenotic dilation. May require surgery or endoscopic dilation. Limited response to medical therapy.
Criteria
Fistula, sinus tract, and/or abscess formation — transmural complications
Distinct Features
Enteroenteric/enterocutaneous/enterovesical fistulae. Perianal fistulae common. Abscess drainage and surgery may be needed. Immunosuppressive therapy + surgery combination.
Distinguishing Feature
In TB, cecal/ileocecal involvement predominant, asymmetric wall thickening and contraction; in Crohn's, skip lesions and longitudinal ulcers. Omental caking and high-density ascites suggest TB
Distinguishing Feature
In lymphoma, aneurysmal dilation (obstruction rare), homogeneous enhancement; in Crohn's, stricture and obstruction common, mural stratification predominant
Distinguishing Feature
In ischemia, vascular pathology (thrombosis/embolism), segmental involvement and acute clinical; in Crohn's, skip lesions, chronic course and fistula/abscess
Distinguishing Feature
In adenocarcinoma, focal short-segment annular narrowing; in Crohn's, long-segment thickening, skip lesions and inflammatory findings
Urgency
urgentManagement
medicalBiopsy
NeededFollow-up
3-monthCrohn's disease treatment requires multidisciplinary approach. 5-ASA (mild), corticosteroids (moderate-severe), anti-TNF (infliximab, adalimumab), anti-integrin (vedolizumab), and anti-IL-12/23 (ustekinumab) are used in active inflammation. Fibrotic strictures require endoscopic balloon dilation or surgical resection. Abscess drainage (percutaneous or surgical) + antibiotic therapy. Role of imaging: evaluate disease activity, differentiate active vs fibrotic stricture, detect complications, monitor treatment response. MR enterography is preferred for follow-up due to absence of radiation.
Crohn's disease is a chronic condition with remission and flare periods. CT/MR enterography is critical for evaluating active inflammation, fistulae, abscesses, and strictures. Treatment approach depends on disease activity.