Seventy percent of patients with CD have small-bowel lesions [6]; therefore, small intestinal disease detection is important. Historically, barium studies such as small barium follow-through (SBFT) and conventional enteroclysis have been standard CD evaluation techniques. However, these barium studies are limited with regard to observing the bowel wall, extraluminal extension, and overlapping bowel loops.

Recently, non-invasive imaging has played an increasingly important role in the assessment of CD [7]. Video capsule endoscopy (VCE) performs well, compared to other small bowel imaging modalities, in patients with CD. In a meta-analysis, the yields of VCE and MR enterography did not significantly differ in either suspected or established CD patients [8]. However, VCE has important limitations, including a high risk of retention due to stenosis, poor localization of bowel abnormalities, and a lack of tissue diagnosis [9]. Ultrasonography (US) is an inexpensive modality with widespread availability, although operator dependence and difficulty with gastrointestinal tract viewing are significant disadvantages [10]. Computed tomography (CT) enterography has been the cross-sectional imaging modality of choice when evaluating CD patients. However, recent recognition of the potential long-term effects of repeated ionizing radiation exposure from CT scans has led to increased interest in the application of non-ionizing-radiation-based cross-sectional imaging modalities for patients with chronic diseases [1, 11].

CD is typically diagnosed at a young age. These patients often experience chronic relapses during their lifetimes, necessitating multiple imaging examinations involving radiation. The most recent Australian population-based cohort study assessed the risks in children and adolescents following exposure to low-dose ionizing radiation from a diagnostic CT. All exposures to the funded CT in people aged 0–19 years were identified from a cohort of 10.9 million people from Australian Medicare records. The overall cancer incidence was 24% greater for those exposed to radiation than for those unexposed. A dose–response relationship was also observed, with a 0.16 increase in the IRR for each additional CT scan [12]. In children, limiting radiation exposure is particularly important. CD, a long-term disease, often affects young patients and usually requires the collection of periodic control images; thus, radiation limitation is a serious consideration [13]. CT should thus, be used with caution in young patients with CD.

As mentioned above, patients with CD must undergo repeated multiple imaging examinations to monitor disease activity and guide appropriate treatment. For those reasons, the desirable imaging modality would be reproducible, well tolerated, and free of ionizing radiation. Recent studies and reviews have focused on the roles of new MR techniques optimized for bowel imaging in the evaluation of bowel disorders [14–16]. Relative to MR, CT has advantages such as better spatial resolution, superior image quality, and lower acquisition time, whereas the advantages of MR over CT include a lack of ionizing radiation, high-contrast soft-tissue resolution, and a superior intravenous contrast safety profile. The disadvantages of MRI include higher costs and reduced availability. Recent advances in MRI have allowed the rapid acquisition of high-resolution images, leading to ultrafast sequences and the assessment of bowel disorders. MR can simultaneously assess the bowel surface, bowel wall, and perianal lesions such as perianal fistulae and perianal abscesses without the issue of overlapping bowel loops. A previous study demonstrated that MR and CT provided equally accurate assessments of CD activity and bowel damage [17]. Furthermore, a meta-analysis of 44 studies found no significant differences in sensitivity and specificity among MRI, US, and CT [18]. Therefore, in the second European evidence-based consensus of the European Crohn’s and Colitis Organization (ECCO) it was stated that MR had the highest diagnostic accuracy and was the current standard for assessing the small intestine in CD [19].

Four methods are available for MRI-based intestinal evaluation: MR enteroclysis, MR enterography, MR colonography, and MR enterocolonography (Table 6.2).

MR enteroclysis requires nasojejunal intubation, sometimes with conscious sedation, and the administration of 1500–2000 ml of contrast agent solution via manual injection. This technique provides superior distension of both the jejunum and ileum. A previous study showed that MR enteroclysis better described mucosal changes relative to MR enterography, and had a high level of accuracy equivalent to that of conventional enteroclysis. On the other hand, some studies reported equivalent sensitivities of MR enterography and MR enteroclysis for moderate to severe CD lesions.

However, MR enteroclysis not only requires a more intensive time commitment and is less tolerable, but also requires exposure to additional ionizing radiation with nasojejunal intubation. For these reasons, MR enteroclysis is recommended only as an initial examination in patients with suspected CD [20].

MR enterography (MRE) requires the oral administration of a large amount of solution. Although several different ingestion algorithms have been used, a volume of 1350–2000 ml is adequate in the majority of cases. Typically, the total volume of PEG is administered via division into multiple smaller volumes within 60 min prior to scanning. The sensitivity and specificity for the detection of active inflammation range was from 88% to 98% and from 78% to 100%, respectively [14–16]. MRE can be used to follow up and monitor disease activity and the effects of medical therapies such as immune-modulating agents, because it is free from the radiation exposure and discomfort associated with nasojejunal intubation.

For MR colonography, patients are required to ingest 1000–3000 ml of PEG orally 4 h before MR for bowel cleansing [21, 22], and 1000–2000 ml of solution is retrogradely instilled into the colon through a rectal balloon catheter. MR and endoscopy results were found to exhibit an acceptable concordance, with a sensitivity of 87–89%, specificity of 85–100%, and significant correlation between CDAI and the MR index [21]. In a recent study, diffusion-weighted imaging (DWI-MRI) colonography without an oral and rectal preparation detected endoscopic inflammation with a sensitivity and specificity of 58% and 85% respectively [23].

MR enterocolonography (MREC) was developed for the simultaneous evaluation of both small- and large-bowel lesions [24]. Magnesium citrate (34 g/200 ml) is ingested the day before the procedure, and 1000 ml of PEG is taken orally 60 min before MR scanning without a nasojejunal intubation or rectal preparation. Compared to the gold-standard balloon-assisted enteroscopy, the respective sensitivities and specificities of MREC for active lesions were 82.4% and 87.6% in the small intestine and 82.8% and 93.2% in the colon [25].

To assess bowel lesions, most centers use a torso coil and a 1.5-T imager to enhance access and image quality reproducibility. Imaging at 3 T yields a higher signal-to-noise ratio and higher spatial resolution but is limited by dielectric effects, banding, and other pulse sequence-related artifacts. MRI scanning protocols comprise various combinations of sequences that highlight different aspects of tissues. The specific sequences, which are defined in Table 6.2, are as follows [16, 26] (Table 6.3): (1) FASE or SSFSE, HASTE, (2) True SSFP or FIESTA, true FISP, bFFE, (3) FSE or TSE with fat-saturation, (4) Diffusion-weighted EPI, (5) SPGR-LAVA or VIBE, eTHRIVE with fat-saturation, and (6) Delayed post contrast 2D T1-weighted SPGR with fat-saturation.