In most institutions, the majority of MR studies are performed as an enterography rather than enteroclysis, as discussed.

Luminal distension with oral contrast is vital for detailed assessment of the bowel wall. Additional benefits include the displacement of air from the lumen by oral contrast. This is necessary as air produces susceptibility artefact and degrades image quality.

Oral contrast utilization will be determined by several factors including local availability, patient tolerance, cost and the effectiveness of distension. In addition to the choice of agent there is departmental variation in the volumes administered and the timing of image acquisition following ingestion of the contrast material. In some instances there may be rapid transit of contrast to the right colon but in the majority of individuals a delay of 40–60 min from the start of contrast ingestion is adequate to obtain small bowel distension.

Significant variance exists between the agents available and it is important to be cognizant of the differences. Agents can be stratified into several categories:

In our department we utilize a technique that is well reported in the literature and consists in a total volume of between 1–2 L of contrast being ingested in divided aliquots over a 45–60 min time period. Water is an undesirable biphasic contrast agent due to its rapid absorption and erratic distension of the distal small bowel. This can be problematic as the distal small bowel is most commonly involved in Crohn’s disease. There are many commercially available biphasic contrast agents. In our practice we have utilized both dilute barium 0.1 % wt/vol solution with sorbitol as well as a bulk fibre laxative, which helps luminal expansion by retaining water. High osmotic agents such as mannitol and sorbitol can have undesirable side effects depending on the sugar alcohol concentration. They can cause diarrhoea, excessive gas and cramping. This can be improved by keeping the concentration below 2.5 %.

An optional method for simultaneous large bowel assessment is administration of a rectal water enema. This allows the colonic wall to be examined for disease involvement (vide infra).

Gadolinium derivatives are typically administered intravenously at a dose of 0.2 mmol/kg body weight. Peak bowel enhancement is thought to occur between 70 and 75 s following contrast material administration but can vary between individuals [38]. It is therefore standard practice to perform multi-phasic MRI commencing in the arterial phase at 25 s and obtaining at least two further acquisitions. If patient is know to have strictures, the addition of a delayed (7 min) phase is a consideration, as it better allows the characterization of the presence/degree of wall fibrosis [39]. In circumstances where gadolinium usage is relatively or absolutely contra-indicated, unenhanced sequences usually suffice in demonstrated areas of mural abnormality as well as mesenteric hyperaemia in diseased segments.

MRE allows not only the static display of morphology but can be combined with ultrafast imaging techniques (“cine MRI” or “MR fluoroscopy”) for analysis of bowel motility. Traditional X-ray fluoroscopic imaging confirms that involvement with Crohn’s may result in motility disorders. Pathological changes in the bowel wall result in areas of aperistalsis or diminished motility. When used in conjunction with routine MRE it can improve lesion detection. In a study performed to assess if cine MRI improves additional lesion detection in patients with Crohn’s, 40 patients with CD underwent cine MR enterography in addition to a standard MR protocol. Blinded reads were performed with and without cine MRI. Overall cine MRI detected more Crohn’s specific findings than static MR enterography alone (p = 0.007) and more patients with CD relevant MR findings (p = 0.03) [40].

There is growing literature supporting the use of cine MR enterography. Froehlich et al. [40] showed that the addition of cine MR enterography identified more patients with MR findings of Crohn’s disease compared to static MR enterography alone because altered motility on cine imaging highlighted abnormal bowel segments and may lead to increased diagnostic confidence and because Cine MR enterography can facilitate diagnosis in otherwise equivocal cases, especially if other sequences are motion-degraded. Abnormal motility on cine MR enterography in Crohn’s disease has also been shown to correlate with inflammatory markers, biopsy results, and clinical disease activity scores [41, 42].

Multiphasic imaging can also be used to evaluate for adhesions or stenoses and helps distinguish true strictures from temporarily contracting or underdistended segments [43] (Fig. 18.2). Cine MR enterography has also been shown to increase visualization of the proximal small bowel compared to static MR enterography, simulating the benefits of enteroclysis [44].

Although still not widely used in routine clinical practice in most institutions, there is growing interest in cine MR enterography as a valuable technique for motility evaluation. The joint European Crohn’s and Colitis Organisation (ECCO) /European Society of Gastrointestinal and Abdominal Radiology (ESGAR) guidelines in 2013 acknowledge that small bowel motility evaluation may increase lesion detection in Crohn’s disease compared to static MR enterography alone [45]. The consensus statement from the Society of Abdominal Radiology (SAR) Crohn’s disease-focused panel published in 2015 includes multiphasic imaging as an optional additional sequence in MR enterography evaluation [46].

Cine MRI can be performed by single slice techniques, usually SSFP sequences, which are repeated at intervals of 0.5–1 s in the same plane but in a stepwise location from anterior to posterior. After the images are acquired, the cine loop function of the workstations allows visualization of bowel motility. Anti-spasmodics should not be administered until the cine MRI sequence is obtained, otherwise they will artificially reduce peristaltism. Consequently, the SSFP sequences that allow cine MRI are the first sequence in our protocol. Since with MR radiation is not utilized, potential sites of disease can be repeatedly evaluated without incurring any risk. It must be emphasized, however, that cine MRI does not have sufficient spatial resolution to be used in isolation when performing diagnostic MRI.

Diffusion weighted imaging (DWI) relies on differences in the motion of water molecules between tissues to provide image contrast. It is more and more becoming a standard application in routine imaging and it has already been incorporated in two MRI scores (the Clermont score, which is very similar to the MaRIA score but replaces contrast enhancement by restricted diffusion), and the Nancy score [47], but DWI’s low spatial resolution is a drawback compared with the anatomical details provided by the high-resolution post- gadolinium sequences. It remains to be determined whether DWI’s low specificity for detecting activity [47], can be overcome by including it in an overall index. However, in the presence of other imaging findings of inflammation, DWI is associated with more severe disease [48] (Fig. 18.3).

In the future, if DWI is proven to be of benefit in larger patient groups, it may circumvent the need for gadolinium and potentially avoid issues related to cost and patient safety with this intravenous contrast media.

Conventional colonoscopy remains the gold standard for mucosal evaluation of the colon and allows biopsy performance for histological confirmation of a suspected diagnosis of inflammatory bowel disease. Entire colonic visualization and stepwise biopsies are considered mandatory and IBD is typically correctly classified in 80–90 % of cases at the time of initial examination. In specific circumstances, however, imaging is a suitable alternative for the establishment of disease extent and severity. These include the presence of a fulminant colitis when the patient is at higher risk of colonic perforation. Similarly, for both patient and technical reasons, incomplete colonoscopy may preclude colonic assessment in between 5–20 % of cases. Additionally, submucosal and mesenteric involvement, which is common in Crohn’s disease, cannot be assessed with colonoscopy; consequently, the evaluation of the mucosa with colonoscopy alone may result in underestimation of the extent and activity of the disease, as when assessing small bowel involvement. Imaging is also invaluable for assessment of colonic anastomoses [49]. The assessment of an anastamosis can be challenging on endoscopy and is the most common site of disease recurrence.

The earliest changes of Crohn’s disease such as erythema, superficial apthous ulcers and nodularity of the mucosa are difficult to appreciate on MR. An attempt to address this deficiency has been made by authors who have obtained high resolution thin section images (2–3 mm thick) aligned parallel to the bowel segment [53]. This allows a detailed depiction of the mucosa and provides in plane resolution of 1–2 mm.

However, there are several imaging findings that are characteristic of inflammatory bowel disease and they are discussed in some detail below.