Crohn’s disease also has a variety of appearances at CT or MR depending on whether the activity is acute inflammatory or chronic fibrostenosing and whether there are complications such as fistula or abscess. The optimal distension of the small bowel loops is important for the accurate evaluation of the bowel wall because collapsed bowel can hide or mimic disease. CT or MR which is performed after oral contrast ingestion to achieve small bowel distension is called CT or MR enterography. The negative oral contrast agents, such as polyethylene glycol solution (PEG), suspension of 0.1 % barium sulfate (Volumen), and water-methylcellulose solution, are preferred because they allow better depiction of bowel wall enhancement [1].

On CT or MR, enteric findings such as mural hyperenhancement, bowel wall thickening, mural stratification, and extraenteric findings such as engorged vasa recta (“comb sign”) [4] and increased attenuation of the mesenteric fat are features of active inflammatory small bowel Crohn’s disease (Figs. 8.4 and 8.5) [3, 5]. Among these findings, combinations of mural hyperenhancement and bowel wall thickening are the most sensitive findings suggesting the presence of active inflammation [5]. It is important to differentiate active inflammatory small bowel strictures from fibrotic strictures in patients with Crohn’s disease because the former are mostly managed medically, whereas the latter may require endoscopic or surgical interventions (e.g., balloon dilation, strictureplasty, or bowel resection) [6]. In fibrostenosing Crohn’s disease (Figs. 8.6 and 8.7), mural stratification may be absent because of the transmural fibrosis and/or muscular hypertrophy and collagen deposition leading to a homogeneous and less-intense enhancement [7] (Table 8.1). Low-signal intensity of the stricture site on T2-weighted MR imaging may be helpful for diagnosing fibrostenotic Crohn’s disease [8]. However, active inflammation and fibrosis often coexist in the same patient or even in the same affected bowel segments in Crohn’s disease.

CT or MR has an important role in evaluating extraenteric complications of Crohn’s disease. The most common extraenteric complications include fistula, sinus tract, and abscess [9–11]. On CT or MR, sinuses or fistulas are demonstrated as tethering of bowel loops and visualization of linear enhancing tracts with or without communication with adjacent structures such as peritoneal or retroperitoneal spaces, skin or adjacent organs, or bowel, respectively (Figs. 8.8, 8.9, and 8.10) [12]. Abscesses are usually contiguous to the diseased bowel segment and are seen in the mesentery or retroperitoneal space (Fig. 8.11) [3]. The accurate detection of abscesses and fistulas has high importance because it affects the decision to treat medically or surgically. Particularly, in the identification of perianal fistula tracts, MR imaging is useful because of its better multiplanar imaging capability and soft tissue contrast than those of CT (Fig. 8.10). Bowel perforation can be developed in Crohn’s disease. It is associated with bowel distension with increased intraluminal pressure proximal to an obstruction or ischemic hypothesis (Fig. 8.12) [13]. Other extraenteric manifestations of Crohn’s disease, such as mesenteric lymphadenopathy, cholelithiasis, nephrolithiasis, sacroiliitis, and primary sclerosing cholangitis, can also be evaluated [3].

Radiation concern is an important issue in CT because patients with Crohn’s disease are relatively younger and are expected to undergo multiple follow-up CT studies [14]. In terms of radiation issue, MR enterography is an emerging diagnostic tool for evaluating patients with known or suspected Crohn’s disease by virtue of its ability to help physician confirm the diagnosis, assess its extent and inflammatory activity, and detect extraintestinal complications (Figs. 8.9 and 8.10). Major MR enterographic findings of Crohn’s disease are not different from those of CT. The two diagnostic modalities appear to be similar in terms of detecting active inflammation, fibrosis, and extraenteric complications [15]. However, CT is preferred in elderly patients because MRI is more time consuming and sometimes requires breath-holding technique [16]. Moreover, CT should be preferred in emergency settings such as suspicious bowel perforation or obstruction.