CD has a bimodal age distribution [1], with two age peaks at 20-40 years old (but some patients are diagnosed even before the age of 10) and 60-70 years old. Therefore, age cannot eliminate CD.

The diagnosis is based on a comprehensive analysis of several aspects, such as the patient’s symptoms, faecal calprotectin level, endoscopic studies (limited to the endoluminal involvement), histopathological analysis, and the presence of specific antibodies (positive ASCA antibodies) [2].

Fig 1: Crohn’s disease diagnostic. Faecal calprotectin helps differentiate inflammatory bowel disease (values higher than >150 mcg/g suggest active inflammation) from irritable bowel syndrome. A value between 50 and 150 mcg/g is equivocal and should be repeated after eliminating causes such as infections or non-steroidal anti-inflammatory drugs. ANCA – anti-neutrophil cytoplasmic antibodies, ASCA - anti-Saccharomyces cerevisiae antibodies.

The imaging studies have several roles: to assess the extension of the disease (mural extension and multifocality) and complications in a recently diagnosed patient, to discern between the predominance of active inflammation or fibrosis, to monitor the disease activity in patients with high suspicion of CD but normal endoscopic studies, and as a complement for patients with perianal fistula [3].

Fig 2: Imaging role. It is fundamental to differentiate between the predominance of active inflammation (medical treatment first) and fibrosis (surgical treatment). Most patients have a mixture of active inflammation and fibrosis features — the role of the radiologist is to depict which one is predominant.

The appropriate imaging study and protocol should be chosen. Entero-MRI is the preferred imaging modality for the baseline extension study (better contrast resolution), while CT is reserved for more urgent settings [4] to spare these patients the irradiation (they will need multiple check-ups throughout their lives).

Fig 3: Imaging. Some centers prefer imaging in the prone position since it yields higher-quality images (flattened bowel loops). However, this causes the patient significant discomfort and a real risk of emesis (especially after the antiperistaltic agent administration). The role of metoclopramide is to evacuate the stomach since the water is needed to distend the bowel loops. The inflammation should be evaluated primarily on the post-contrast sequences. The diffusion restriction indicates active inflammation but might be falsely elevated if the bowel loops are not adequately distended. The SSFP sequence evaluates the digestive motility. SS-FSE – single-shot fast spin echo, DWI – diffusion-weighted imaging, SSFP – steady-state free precession.

Ultrasound can evaluate certain changes in a patient (e.g. the evolution of a previously diagnosed collection or terminal ileum inflammation). Since it’s easier to tolerate, it can be used even as a first-line imaging modality in children with right lower quadrant pain (primarily to assess the terminal ileum).

The systematic approach to an intestinal study (MRI, CT, or ultrasound) should focus on the following aspects: parietal analysis, mesentery changes, and complications [5].

Fig 4: Systematic analysis.

Parietal analysis

The wall thickness is classified as mild, moderate, or severe, and when it’s greater than 15 mm, neoplasia should be considered until proven otherwise. The report should state the length of the thickening and whether more than one bowel segment is affected [5].

Fig 5: Parietal analysis – wall thickening. (a), (b) Thickened terminal ileum, as seen on ultrasound and MRI. (c), (d) To assess the multifocality, do not hesitate to use the diffusion sequence to see the areas with increased signal (arrows). The DWI sequence should be interpreted cautiously – if the loops are insufficiently distended, they will falsely have diffusion restriction (higher cellularity compared to the distended bowel), mimicking active disease (high cellularity because of inflammation). © Radiology Department, Centre Hospitalier d’Arras/ France.

The enhancement pattern can orient toward active inflammation (arterial) or fibrosis (late).

Fig 6: Parietal analysis – contrast enhancement. (a) Arterial enhancement (white arrow) of the terminal ileum in keeping with active inflammation. (b), (c) Enhancement during the late phases (black arrows), indicating the presence of fibrosis. © Radiology Department, Centre Hospitalier d’Arras/ France.

Fig 7: Parietal analysis – contrast enhancement. Doppler mode can be used to evaluate the presence of active inflammation vs. fibrosis. The machine needs to have the parameters correctly set – the pulse repetition frequency (PRF) should be calibrated for low velocities (as in the two circles). (a) High colour Doppler signal indicating active inflammation. (b) Scarce power Doppler signal, in keeping with fibrosis. © Radiology Department, Centre Hospitalier d’Arras/ France.

Submucosal oedema indicates active inflammation and should not be mistaken for intramural fat. Careful evaluation of all the sequences is necessary.

Fig 8: Parietal analysis – submucosal edema. (a) Thickening and submucosal edema (intermediate T2 signal) of the terminal ileum. (b), (c), (d) High T2 submucosal signal (asterisks), which also has a high T1 signal and saturates on the fat-sat sequence, in keeping with fat (from chronic inflammation). Not to be mistaken for edema because of the high T2 submucosal appearance. © Radiology Department, Centre Hospitalier d’Arras/ France.

A stricture is defined as the luminal narrowing associated with upstream dilatation over 3 cm. It has a higher risk of penetrating disease [5, 7].

Fig 9: Parietal analysis – stricture. (a) Stricture of the terminal ileum associated with upstream dilatation of 37 mm. (b) Same patient, MRI follow-up. Persistence of the stricture (arrow) with stable upstream dilatation. © Radiology Department, Centre Hospitalier d’Arras/ France.

Ulcers represent discontinuity of the mucosal lining. They can be superficial (only visible on endoscopic studies) or deep.

Fig 10: Parietal analysis – ulceration. (a), (b) Irregularities of the mucosal surface (arrows) representing deep ulcers. Superficial ulcers are only visible on endoscopic studies. © Radiology Department, Centre Hospitalier d’Arras/ France.

Sacculations are outpouchings along the antimesenteric border of the bowel loop. They are due to acute or long-standing bowel wall inflammation and/or fibrosis.

Fig 11: Parietal analysis – sacculation. (a), (b) Note two sacculations (arrows) along the antimesenteric border in two patients. © Radiology Department, Centre Hospitalier d’Arras/ France.

Mesentery changes

The engorged vasa recta, represented on imaging as the comb sign, is another sign of active inflammation [4, 5].

Fig 12: Mesentery changes – engorged vasa recta. (a) Engorged vasa recta (between the two arrows) in the left flank, giving the typical comb sign appearance associated with reactive local adenopathies (dashed circle). (b) Six-month follow-up of the same patient – there are no more engorged vasa recta, and the lymph-nodes have diminished in size. © Radiology Department, Centre Hospitalier d’Arras/ France.

Fibrofatty proliferation (also called creeping fat or mesenteric sclerolipomatosis) might be slightly hyperattenuating compared to normal fat due to the influx of inflammatory cells and fluid. This is a specific finding for CD [4-6].

Fig 13: Mesentery changes – fibrofatty proliferation. (a), (b) At the level of the right iliac fossa, the dashed circle delineates the area of fibrofatty proliferation around the terminal ileum (which appears thickened), as opposed to the left iliac fossa where there is only a little amount of fat between the small bowel loops. © Radiology Department, Centre Hospitalier d’Arras/ France.

The loco-regional adenopathies are reactive to active inflammation and measure up to 1–1.5 cm in short-axis diameter [5].

Fig 14: Mesentery changes – adenopathy. (a), (b) Ileocolic reactive adenopathies (dashed circle). Both DWI and SSFP sequences have good lymph node conspicuity. © Radiology Department, Centre Hospitalier d’Arras/ France.

Complications

Fistulas can be simple or complex, blind-ended, or communicating with a nearby organ (large or small intestine, abdominal wall). They usually arise from the mid or proximal aspect of a stricture in the setting of active inflammation [5, 8].

Fig 15: Complications - fistula. (a) Fistula (white arrow) originating from a moderately thickened terminal ileum and feeding an abscess (abscess). (b) Different patient, with a stricture at the level of the terminal ileum (asterisk – dilated bowel loop). A fistula (black arrow) originating from the stricture is feeding an abscess in the belly of the iliac muscle (not shown). © Radiology Department, Centre Hospitalier d’Arras/ France.

Fig 16: Complications - fistula. (a)-(f) Cranial to caudal CT slices from the same patient. White arrows following a complex fistula (note the star shape in (c)), with some tracts ending blindly - for example, the most cranial one in (a) or a medial one in (d) and another one feeding a large abdominal wall abscess (asterisks). The abscess has an air-fluid level (in keeping with its digestive tract origin). It has an intra-abdominal component (the two most posterior asterisks in (f)) and a fistulous tract opening to the skin (black arrow). © Radiology Department, Centre Hospitalier d’Arras/ France.

The abscesses are well-defined collections fed by fistulas. They have the following characteristics: rim enhancement, restricted diffusion (pus filling), and might contain gas inside.

Fig 17: Complications – inflammatory mass. (a) Inflammatory mass at the level of the right lower quadrant fed by two fistulas, one from the caecum (arrow) and a more superior one from the terminal ileum (not shown). © Radiology Department, Centre Hospitalier d’Arras/ France.

An inflammatory mass is a dense mesenteric inflammation without a well-defined fluid component not confined by a wall. It might develop into an abscess.

Fig 18: Complications - abscess. (a) Same patient as on the previous slide. The inflammatory mass progressed toward abscess formation (white arrow) – a well-defined collection with hypoattenuating central areas (pus), with a gas focus inside, fed by a fistula (black arrow). (b) Different patient. Abcess of the right iliac muscle (dashed circle). Its evolution was followed-up by ultrasound (circle). Notice the echogenic surrounding fat with colour Doppler signal indicating inflammatory changes. © Radiology Department, Centre Hospitalier d’Arras/ France.

The venous thrombosis occurs in the drainage territory of the inflamed bowel loops. It can be acute (distended vessel) or chronic (narrowed or interrupted vessel and collateral circulation) [5].

Perforations are rare and usually occur due to occlusion. They might be complicated by peritonitis. Their management is surgical [4, 5].

Fig 19: Complications – venous thrombosis and perforation. (a) 2 foci (arrow) of intra-peritoneal air associated with a dilated small bowel loop (double arrow) in a patient with CD, in keeping with small bowel occlusion and perforation. © Radiology Department, Centre Hospitalier d’Arras/ France.