Computer tomography (CT) still is the most widely used cross-sectional imaging modality in patients with CD given its wide availability. Its major role in children with CD is in the evaluation of disease extent and in assessing for complications, particularly in the acute situation. CT enteroclysis has been shown to be more accurate than SBFT in the diagnosis of CD, but neither is able to detect the early mucosal changes of CD [2]. Additionally, as with any enteroclysis study, this technique requires the introduction of a nasojejunal tube, generally not well accepted in the pediatric population. Changes readily detected by CT include bowel wall thickening, luminal narrowing, and mesenteric involvement. Mesenteric findings include thickening due to fibrofatty infiltration, lymphadenopathy, and fatty encroachment of the affected loop of bowel.

Patients with known CD, who present with new acute symptoms suspicious for complications or a deteriorating clinical course, are best imaged with CT to assess for progressive disease or the onset of complications such as obstruction, fistulae, abscesses, or malignant change (Fig. 19.2). Extraintestinal manifestations of CD in the hepatobiliary, pancreatic, urinary, and musculoskeletal systems are also readily assessed by CT. Specific CT findings of complications and extraintestinal manifestations of CD are discussed below.

The sensitivity of CT scan in patients with CD is increased by optimal opacification and distension of the bowel by administering oral contrast at an age- and weight-appropriate dose, or by the enteroclysis technique. Bowel wall thickening greater than 3 mm in pediatric patients is generally considered abnormal [3]. Given that this young patient population frequently undergoes multiple studies, the current trend is moving toward MRI evaluation in the non-acute setting, thus minimizing exposure to ionizing radiation.

MRI offers unique advantages to the pediatric patient because, in addition to being noninvasive, it avoids exposure to ionizing radiation. In many cases, MRI can replace or complement CT because its excellent soft tissue contrast and three-dimensional capabilities are ideal properties for imaging the bowel [4]. In the past, motion artifacts often limited MRI, but this problem has been largely overcome by the recent introduction of respiration-suspended sequences. Other technological advances, including improved coils, fat suppression, use of oral agents and intravenous gadolinium, powerful gradient systems, and ultrafast pulse sequences have led to overall improvement in gastrointestinal imaging. Optimal image quality depends greatly on adequate luminal distension with contrast medium. Without enteric contrast, MRI has produced inconsistent results in children with CD [5, 6].

The method of enteric contrast administration has proved to be a critical factor because oral ingestion of contrast agents that do not provide adequate bowel distention such as routine positive contrast agents or water, while patient-friendly, produces inadequate luminal distension, downgrades the image quality, and may limit the ability to detect early or minimal disease. The two techniques found to have the greatest success to evaluate for CD of the small bowel include MR enteroclysis and MR enterography utilizing a negative contrast agent that provides adequate bowel distention. The choice of oral contrast agent for MR enterography varies with institution. However, at our institution, we utilize VoLumen, a low-concentration barium (0.1% weight/volume) that contains sorbitol to aid in bowel distention.

Magnetic resonance enteroclysis requires duodenal intubation to permit volume challenge, which causes reflex bowel atony and produces superb contrast for evaluating luminal, transmural, and extramural changes. It has been postulated that by combining the advantages of enteroclysis with three-dimensional cross-sectional imaging, MR enteroclysis has been touted as the only imaging modality that can provide comprehensive diagnostic information on small bowel CD [4]. However, routine use of MR enteroclysis in children has not been widely adopted because of the need to insert a duodenal tube fluoroscopically, entailing exposure to ionizing radiation and, the potential need for intravenous sedation.

Prospective comparison of MR enterography and CT enterography in the evaluation of small bowel CD has been performed [7] with the sensitivities for detecting active small bowel disease found to be similar (90.5% vs. 95.2%, respectively). Although MR enterography had a slightly lower sensitivity and specificity, this difference was not statistically significant for the 30 patients who underwent both imaging studies. However, image quality across the study cohort was better with CT enterography. In another study [8], MR enterography demonstrated good sensitivity in the detection of active CD and found good correlation between MR and CT enterography in the evaluation of wall thickening with mucosal hyperenhancement and the presence of the comb and halo signs. While CT enterography was found to be superior in the detection of fibrofatty proliferation and mesenteric lymph nodes, MR enterography was superior in the evaluation of fistulas. Currently, however, because MR enterography has a diagnostic effectiveness comparable to that of CT enterography [9], the trend is increasing toward MR enterography as a radiation-free alternative for the evaluation of patients with CD. Indeed, one study [10] concluded that MR enterography can be substituted for CT as the first-line imaging modality in pediatric patients with CD. This viewpoint is based on the ability of MR enterography to detect intestinal pathologic abnormalities in both small and large bowel as well as extraintestinal disease manifestations. Furthermore, MR enterography provides an accurate noninvasive assessment of CD activity and mural fibrosis and can aid in formulating treatment strategies for symptomatic patients and assessing therapy response [10].

The technique for MR enterography begins with the oral ingestion of contrast, but the type of oral contrast used again is controversial and is usually institution-specific. Again, at our institution the utilization of VoLumen, low-concentration barium that contains sorbitol to aid in bowel distention, has been well tolerated and shown to produce good quality images. The patient is asked to ingest three 450 mL bottles over approximately 1–1½  h as tolerated, with each bottle being ingested over approximately 20 min. The field of view includes the abdomen and majority of the pelvis to evaluate the entirety of the small bowel. Imaging of the bowel begins with coronal T2 single-shot fast-spin echo imaging, which is reviewed by the radiologist to ensure adequate oral preparation with contrast reaching the colon. If adequate, the remainder of the MR enterography protocol is performed including axial T2-weighted axial diffusion-weighted sequence to evaluate for restricted diffusion in areas of pathologic edema, coronal pre-gadolinium T1-weighted, and dynamic steady-state free precession imaging in the coronal plane to evaluate for bowel peristalsis. The evaluation is enhanced utilizing intravenous glucagon to inhibit bowel motion in preparation for the longer post-gadolinium sequences following the administration of IV contrast. These include axial, coronal, and sagittal T1-weighted sequences to evaluate enhancement pattern. MR findings of active CD affecting the small bowel include mucosal hyper enhancement, wall thickening (Fig. 19.3a, b), restricted diffusion (Fig. 19.4), ulcers, mesenteric hypervascularity (Comb sign, Fig. 19.5), mesenteric inflammation, and reactive mesenteric nodes. Fibrostenotic lesions (Fig. 19.6a, b) may show homogenous T2 hyperintensity, uniform contrast enhancement, and minimal adjacent inflammatory changes. Complications of CD include penetrating disease and bowel obstruction, sinus tracts, fistulas, and abscess formation [11].

In addition to MR enterography in the assessment of IBD, we currently also use MRI of the pelvis for the evaluation of complex perianal disease, as discussed below.

The lack of ionizing radiation and noninvasive nature of US make it an ideal method of evaluation in children. In addition, for routine US imaging, bowel cleansing is not required, nor is enteric or intravenous contrast. However, because it is operator dependent, its role in patients with CD is currently generally limited to the evaluation of complications, particularly abscesses, and extraintestinal disease manifestations. It is rarely used for primary diagnosis. Affected bowel segments demonstrate wall thickening, lack of peristalsis, and poor stratification of the different layers (Fig. 19.7a) [12]. Similar to adults, US findings in children with CD show good correlation with endoscopy [13]. The most promising use of US may be in the ongoing evaluation of disease activity as well as response to treatment. In children, the sonographic value of bowel wall thickening as an index of increased disease activity has been demonstrated [14, 15]. With moderate–severe disease, the predictive value of increased bowel wall thickening greater than 2.5 mm in the ileum as an index of active disease was 88% (82% for colon >3 mm) [14]. Assessment of disease severity can also be enhanced by measuring the vessel density in the affected bowel segment using color Doppler US (Fig. 19.7b) [16]. When incorporated into a clinical protocol, US may reduce the need for contrast studies [14, 17]. In expert hands, US has been used to assess fistulae and strictures, and also monitor postoperative disease recurrence [18].

There are a number of limitations to the use of US in CD. Although the assessment of terminal ileal disease with US is quite good, the proximal small bowel and distal portions of the colon are poorly imaged. In addition, superficial lesions as seen in early disease can be missed in both children and adults [13].

Ulcerative colitis is a chronic, idiopathic, inflammatory disease of the rectal and colonic mucosa that is characterized by mucosal inflammation, edema, and ulceration. Several distinguishing features permit clinical and radiological distinction from CD. As a rule, UC nearly always affects the rectum and extends proximally to involve a variable length of colon in a contiguous fashion. Other than the occasional “backwash ileitis” of the terminal ileum, the small bowel is not affected. On rare occasions, variants with transmural involvement or without rectal inflammation also occur. Radiologic features of UC are quite distinct, although in the majority of cases, diagnosis is dependent on clinical presentation, laboratory tests, and findings on colonoscopy and biopsy.

Patients with IBD whose clinical, endoscopic, pathologic, and radiologic presentation cannot easily be differentiated into CD or UC are assigned the diagnosis of indeterminate colitis (IC). Indeterminate colitis appear to be more common in children compared to adults, with a prevalence rate of nearly 30% recently reported in a cohort of 250 children with IBD [26].

One study has shown that the probability of making a definitive diagnosis of either CD or UC increases with age [11]. Careful radiologic evaluation may play a significant role in subsequent reclassification of patients with IC. The distinction between CD and UC may be important in selecting appropriate treatment and in determining prognosis.

Demonstration of small bowel inflammation, skip lesions, and mesenteric extension usually indicates CD, particularly in the absence of colonic disease. Patients classified as IC usually have normal small bowel imaging on the SBFT and CT. As noted previously, newer techniques such as US, radionuclide scans, and MRI may demonstrate small bowel disease, particularly in the distal ileum. Again, MR enterography is becoming a promising technique and with recent advances in imaging, the detection of IBD has improved for both small bowel and colonic disease. However, in isolated colonic disease, the ability to distinguish UC and CD with high specificity remains a challenge. When extensive colonic disease is present, the finding of terminal ileal inflammation may be misleading because some patients with UC may have “backwash ileitis.”

Although the colon is usually accessible for endoscopic evaluation, cross-sectional imaging with CT or MRI may demonstrate transmural involvement, extension into mesenteric fat, fistulae, or abscesses, which may prompt a more definitive reclassification as CD. Using US to differentiate between CD and UC have so far proved unreliable in children, although it may be useful for monitoring disease activity [13, 15, 27].