Introduction 1 1 2 Crohn’s disease can be localized in any part of the gastrointestinal tract, although the location of predilection is the terminal ileum, involvement of the terminal ileum is observed in 90% of the patients with small-intestinal CD, who in turn constitute 30–40% of all CD patients. In 40–55% of the patients both ileum and colon are affected, while in a minority (15–25%) only a colonic localization is observed. The earliest change caused by CD occurs in the mucosa and submucosa and consists of hyperemia and edema. Tiny aphthoid or superficial ulcerations can be seen when disease progresses. In more severe disease, the disease extends transmurally with sometimes serosal involvement. In this stage, mucosal ulcerations merge to form deep longitudinal and transverse ulcerations while bowel wall thickening and narrowing of the bowel lumen can be observed due to significant mucosal edema and associated bowel spasms. In long-standing disease, chronic obstruction can develop due to scarring, luminal narrowing, and stricture formation. Extramural manifestations of CD are fistulas, abscesses, adhesions, creeping fat, and enlargement of lymph nodes. 3 4 Diagnostic modalities 5 6 1 Fig. 1. VCE image of a 14-year-old male patient with known CD. VCE was performed as small-bowel disease was suspected. Image shows severe inflammation of the small bowel with a stenosis.  For DBE, a high-resolution video-endoscope with a flexible overtube is used. By alternately inflating and deflating two balloons attached to the overtube and endoscope the small bowel is threaded on the overtube. Both an oral and an anal approach are possible; for the oral approach no specific preparation is required, although patients are asked to fast for at least 6 h before the procedure. If the anal approach is used, bowel cleansing such as is employed for traditional colonoscopy is used. By using both the anal and oral approach, DBE allows visualization of the entire small bowel, with the possibility of obtaining tissue for analysis and the added advantage of the possibility of endoscopic therapy (e.g. dilatation of a stricture, cauterization of a bleeding site). For DBE conscious sedation is a necessity. 5 7 8 9 Cross-sectional imaging modalities The trans- and extramural extent of IBD cannot be visualized with any of the abovementioned techniques. Much research has been directed toward the potential of cross-sectional imaging modalities for the diagnosis and evaluation of IBD as with these techniques the bowel lumen, the bowel wall and the extra-intestinal abdomen including the visceral fat, the lymph nodes and the vascular structures feeding and draining the bowel can be visualized. An added advantage of these techniques is the fact that they are limitedly to non-invasive. Ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI) are often used for the evaluation of the abdomen. While in the USA the technique of choice is CT, in Europe the focus is more on MRI and US. This inclination is reflected by the majority of CT studies on IBD patients originating from the USA, while the majority of published studies on MRI and US has been conducted in Europe. Ultrasonography Patient preparation and US technique Patients are usually asked to fast for several hours before the scan to diminish peristaltic movements and the amount of intraluminal air; in the available literature the minimum fasting time described is 4 h, whereas overnight fasting is also sometimes employed. Usually, no additional dietary or cleansing measures are taken. Due to the limited patient preparation necessary and the non-invasive nature of this examination, US can be considered to be a relatively patient-friendly and straightforward examination. 10 11 12 13 13 16 The use of Doppler-US might provide helpful additional information on IBD, particularly on the degree of disease activity. Using Doppler-US the vascularity of the bowel wall can be assessed according to the intensity of color signals and/or by the analysis of Doppler curves (measurement of resistive index) obtained from vessels detected within the bowel wall. Measurement of flow parameters of the superior and inferior mesenteric arteries can also be performed. Imaging criteria 2 17 Fig. 2. arrowheads  12 13 Diagnostic accuracy of US Most studies regarding diagnostic accuracy of US for diagnosis and follow-up of IBD have been conducted in the last decade. Although reported sensitivity and specificity values are high, with the state-of-the-art equipment diagnostic accuracy could possibly be higher than that previously reported. 15 18 19 10 20 21 22 23 24 3 Fig. 3. A arrowheads B arrowheads  13 18 19 25 26 27 28 29 30 Computed tomography Patient preparation 31 33 34 31 33 35 Imaging technique Technical developments have allowed the widespread use of multi-slice scanners. With these scanners volumes can be scanned in a very short breath hold, allowing the acquisition of isotropic voxels for multiplanar reformatting. Thin slices should be used to permit the detection of subtle pathology. 31 32 36 Imaging criteria 31 33 4 Fig. 4. arrowheads  37 39 38 Bowel wall enhancement can be transmural, but also layered. This layered enhancement pattern, which is represented by a thickened intestinal wall with a middle layer of low attenuation surrounded on each side by layers of higher attenuation, has been termed the target sign; this is due to the presence of edema or the deposition of fat in the submucosa. Diagnostic accuracy of CT 40 34 35 41 42 43 44 5 37 Fig. 5. 3 A arrowheads B arrowheads C arrowhead  45 46 Magnetic resonance imaging Patient preparation While in some studies on MRI a period of several hours of fasting was deemed sufficient, in others full bowel cleansing was performed, as the reference standard, (i.e., CS) was performed on the same day. There is no consensus yet as to what constitutes the optimal bowel preparation for MRI. However, as a limited bowel preparation does not seem to negatively affect accuracy, it might be sufficient to limit the bowel preparation to a fasting period taking into account the patient-friendliness of the respective preparations. 47 n 6 Fig. 6. A B  48 49 Imaging technique Mostly, both T2-weighted and T1-weighted sequences are used for the MRI evaluation of the bowel. On T2-weighted images the bowel wall can be appreciated and bowel wall stratification—if present—can be well apprehended. As feces can show bright signal intensity on T1-weighted sequences, it is important to perform a pre-contrast T1-weighted sequence in order to be able to determine whether high signal intensity was already present before intravenous contrast administration, indicating the presence of stool. 7 Fig. 7. A arrowheads B arrowheads  Imaging criteria 8 50 53 Fig. 8. 6 arrowheads  54 Extramural manifestations of disease that can be identified on MRI are fistulas, abscesses, fibrofatty proliferation, and enlarged lymph nodes. Diagnostic accuracy 52 55 56 57 47 52 58 9 Fig. 9. 3 5 A arrowheads B  59 60 61 51 62 63 64 65 Discussion Compared with conventional imaging methods, CT, US, and MRI are accurate methods for the detection of IBD of the small bowel, both in patients suspected of disease as in patients with known IBD. Although subtle lesions cannot be depicted with any of these modalities, clinically more relevant findings can be accurately depicted. Therefore, cross-sectional imaging should be incorporated in a comprehensive clinical evaluation of suspected IBD and for follow-up of CD. The exact role cross-sectional imaging techniques can play for follow-up in UC should be more extensively studied. As US is easily accessible, widely available, and inexpensive, it is recommended to use abdominal US as first-line modality in patients with suspected IBD of the small bowel. MR enterography would be a good alternative, especially as the assessment of the degree of disease activity can be better performed on MRI than on US. Although CT enterography is a very accurate technique and is used in many institutions, its role in IBD is limited by the ionizing radiation needed, especially due to the repetitive use for follow-up in often young individuals. If possible, it might be advisable to reserve this technique for patients in whom imaging is needed at very short notice as CT enterography can be performed very fast and is readily available. Although VCE has shown to be more accurate in depicting subtle lesions in the small bowel than MRI or CT, its role should be limited as of yet as the true benefit of VCE is not clear yet. As there are presently no standardized criteria for the diagnosis of CD with VCE, definitions with regard to what constitutes a positive finding might differ between studies. Moreover, the clinical significance of finding a single mucosal break or a few superficial aphthous lesions is not clear yet. Also, specificity and positive predictive values for VCE have not been established. At this time, it might be good to reserve VCE as a second-line modality if cross-sectional imaging has not shown abnormalities but the suspicion of disease remains standing despite these negative findings.