Thickening of the bowel wall (Fig. 5.1) occurs due to edema and infiltration with inflammatory cells in active inflammation or due to deposition of collagen in fibrostenosing disease, and is one of the most important imaging abnormalities in CD [3, 9, 18, 26]. Normal bowel wall thickness (in optimal distended bowel) ranges from 1 to 3 mm and in case of CD inflammation has values ranging from 5 to 10 mm. The bowel wall thickening may be asymmetric due to the preferential inflammatory involvement of the mesenteric side of the bowel wall. The post-contrast T1-images and the HASTE sequences allow better assessment of bowel wall thickness compared to the true FISP images. The black boundary artefact seen on the true FISP images can confound evaluation of wall thickness whereas the HASTE sequences are relatively insensitive to this artifact.

Increased signal intensity [3, 9, 18] of the thickened bowel wall on the HASTE sequences can suggest bowel wall edema and indicate active inflammation, whereas low to moderate T2 signal intensity may suggest underlying fibrosis. However, the absence of high signal intensity in the thickened wall does not exclude active disease. High T2 bowel wall signal intensity can also be present in case of intramural fat deposition, found in chronic inflammation, and can be differentiated from edema by using fat-saturated T2 sequences (Fig. 5.2).

Good bowel distension and absence of flow artefacts are crucial to assess fold abnormalities, to avoid false-positive and false-negative results. In case of suspected fold abnormalities and insufficient bowel distension more MR sequences can be acquired after additional ingestion of oral contrast material. True FISP images are superior for evaluating some fold abnormalities given the minor flow artifacts. The latter are also reduced by anti-spasmodic agents, given that they reduce fluid flow in the bowel lumen.

Alterations in fold patterns are more evident along the mesenteric border, characteristic of the disease and can manifest in different ways. The three main patterns of fold abnormalities are diffuse fold thickening, ulceration of folds, or in more severe disease cobblestoning [11, 20].

Early areas of mucosal ulceration (Fig. 5.3) present as small foci of hyper-intensity surrounded by a rim of edema, deeper ulcerations present as lines of high signal intensity in the thickened bowel wall on the T2 images, paralleling the lumen or protruding transversely into the wall. Cobblestone appearance is the result of a combination of longitudinal and circumferential ulcers and fissures separating islands of mucosa.

Better assessment of mucosal ulcerations can be obtained by using high resolution MR imaging compared to standard MRE [27], although it remains inferior compared to endoscopy.

These are segments of persistent narrowing of the bowel lumen (Figs. 5.4 and 5.5) with or without bowel wall thickening. They are considered to be functionally significant if there is an upstream bowel dilatation of more than 3 cm and defined as nonfunctional if the bowel lumen is narrowed more than 10 % compared to neighboring bowel loops in the absence of bowel dilatation [20]. Differentiating active from chronic strictures has important therapeutic implications and remains a diagnostic challenge on MRE.

Non-fibrotic strictures tend to have wall thickening with high signal intensity on the T2 sequences, suggesting the presence of bowel wall edema, whereas fibrotic strictures have lower signal intensity in the bowel wall on T1- and T2-weighted images.

Bowel wall enhancement is increased compared to the surrounding normal bowel loops and correlates with bowel inflammation and disease activity [11, 20]. The evaluation of the different patterns of bowel wall enhancement can be useful in determining the level of disease activity.

Three-layered pattern of contrast enhancement (Fig. 5.6), also called mural stratification, consists of strong enhancement of the mucosa, relatively poor enhancing submucosa, and strong enhancement of the serosa; this type of enhancement, in combination with bowel wall edema, is found in active inflammation. Strong mucosal enhancement is one of the most sensitive indicators of active CD.

Diffuse and intense homogenous contrast enhancement suggests transmural inflammation, but remains nonspecific given the fact that it can be found in active and chronic disease.

Minimal and heterogeneous bowel wall enhancement is rather seen in segments where fibrosis predominates.

Pseudodiverticulum and pseudosacculation are the result of the asymmetric and preferential inflammation of the mesenteric side of the bowel wall and relative sparing of the opposite bowel wall. Fibrosis in the diseased mesenteric wall results in shorting of this side of the bowel wall and dilatation of the opposite wall, leading apparent sacculation or formation of a diverticulum [18]. In contrast to diverticular disease in the colon, CD involves all layers of the bowel wall, and focal dilations are called pseudodiverticula or pseudosacculations (Fig. 5.7). As this is seen in the chronic setting of CD, other concomitant signs of chronicity are usually present.

Engorgement of the vasa recta is the result of increased blood flow through the vasa recta to the inflamed bowel segments. If these vessels have a perpendicular course to the long axis of the diseased segments it is called the comb sign (Fig. 5.8). Fine lines of low signal intensity on the true FISP images and high signal lines on the post-contrast T1 images are seen.

The comb sign is seen in the setting of active inflammation [3, 18].

Inflammation of the mesenteric fat or mesenteric edema surrounding the inflamed bowel segments, together with the comb sign, bowel wall edema and strong enhancement of the bowel wall, strongly suggest active disease [3].

Fibrofatty proliferation is usually seen in patients having a history of longstanding CD and can be a useful diagnostic discriminator as it is rarely seen with other differential diagnosis.

Hypertrophy of fat surrounding diseased bowel loops, can be symmetric or asymmetric [18], in the latter preferentially involving the mesenteric border of the bowel, and can produce mass effect on the surrounding bowel loops or organs.

Enlarged mesenteric lymph nodes (Fig. 5.9) are often seen in patients with active and inactive CD and they are frequently, but not always located in the area of the diseased bowel loops, often around the ileocolic vessels, given the preferred side of the disease. A study [28] showed that the mesenteric lymph nodes in CD present with a different degree of homogenous contrast enhancement, depending on the disease subtypes and therefore quantification of enhancement ratios could be useful for disease subtype classification.

If multiple enlarged mesenteric lymph nodes are present, lymphoproliferative disease should be excluded, given the additional risk as a consequence of CD treatment.

When a transmural ulcer communicates with an adjacent epithelial surface, it becomes a fistula [9, 18]. Most fistulas (Figs. 5.10 and 5.11) arise between small bowel loops (entero-enteric fistula) or between a small bowel loop and a colon segment (entero-colic fistula), but can also communicate with other organs, such as bladder, skin or even local muscles. The fistulas are usually seen as high signal tracts on the T2 images, with variable degrees of enhancement after administration of intravenous contrast material. These fistula tracts may vary from single to complex, sometimes having stellate appearance with multiple tracts radiating from a central point to adjacent bowel, other organs or even local muscles, such as the psoas muscle. Fistulas can easily be depicted on MR, with the exception of enterocutaneous fistulas. The latter are harder to visualize because of their superficial location and the compression in the often prone positioning of the patient makes it more difficult to assess these enterocutaneous tracts. If there is clinical suspicion of cutaneous fistula, supine positioning of the patient is preferred and additional sequences in the sagittal plane can be taken to better assess the location and extent of the fistula. In some cases MR images provide insufficient assessment and additional conventional radiology techniques are necessary, such as fistulography.

If a transmural ulceration does not communicate with another epithelial surface, it is a blind-ending tract and called a sinus. These sinuses have the same MR characteristics as fistulas.

An abscess (Fig. 5.11) is an encapsulated fluid collection with peripheral contrast enhancement [18]. They can contain air and their content is often heterogeneous due to the presence of solid material and gas. Abcesses are easily depicted on MR, having high signal on T2 and low signal on T1. On the other hand, small amounts of air in a collection might be difficult to visualize. The detection of an abscess is important, because it is a relative contra-indication to the use of anti-TNF-alpha drugs. In some cases these abscesses can first be treated with imaging-guided drainage before starting medical therapy.