Small bowel imaging with fluoroscopic barium small bowel follow-through (SBFT) was at one time considered the gold standard in evaluating pediatric IBD and is still used in the initial diagnosis of IBD despite the increasing availability of magnetic resonance enterography (MRE) and computed tomography enterography (CTE). This fluoroscopic study involves drinking barium contrast with serial X-ray images as the contrast progresses through the small intestine to the cecum. SBFT can provide an assessment of the small intestinal luminal anatomy by evaluating for strictures or wall thickening but generally does not provide information on colonic inflammation (Figure 1). SBFT has many benefits including relatively low cost, wide availability, and the ability to complete the study without sedation in the pediatric population. The downsides of SBFT include radiation exposure, length of study, operator dependent quality of images, and lack of extraintestinalevaluation (Table 1). The effective dose of radiation for SBFT in a pediatric patient is estimated to be 1.8-2.2 millisieverts (mSv)[6] with an average effective dose of 5 mSv in the adult population[7], though the actual radiation exposure can increase based on number of films obtained and radiologist technique.

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Figure 1 Small bowel follow-through examination in two patients with Crohn’s disease. A and B demonstrate mucosal irregularity and luminal narrowing of the terminal ileum (arrows).

Based on several retrospective studies among pediatric patients with IBD, the sensitivity of SBFT in detecting terminal ileum inflammation using histology as a gold standard is 45%-76% with specificity of 67%-96%[8-10]. A prospective study among adults with newly diagnosed IBD similarly showed the sensitivity and specificity of SBFT detecting terminal ileum inflammation was 67%-72% and 100% respectively[11]. This study found that there was not a statistically significant difference in the sensitivity in detecting terminal ileitis between SBFT, CTE and MRE, but CTE and MRE had significantly greater sensitivity for detection of extraintestinal complications[11]. Overall, SBFT is an imaging technique that has a role in identifying small bowel inflammation given its low cost and ease of performing the study. However, it has been falling out of favor given risks of radiation and improved sensitivity in detecting extraintestinal complications with newer cross sectional imaging techniques.

CTE was first described in 1997 as a modification of standard abdominal computed tomography (CT) to better evaluate the small bowel in CD[12]. Patients typically drink 1-2 Lofa neutral or low-density oral contrast mixture and receive intravenous (IV) contrast during the study to optimize luminal distention and assessment of the bowel wall[13]. Diagnostic criteria for IBD disease activity using CTE include bowel wall thickening, bowel hyperemia, submucosal fat deposition, and lymphadenopathy[14,15] (Figure 2). This cross-sectional imaging technique can evaluate for complications of IBD including bowel obstruction, fistula, perforation, or abscess[14,15]. The advantages of CTE include rapid scan time, cross-sectional imaging for evaluation of extraintestinal complications, relatively lower cost compared to MRE, and ability to perform the study without sedation in children (Table 1).

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Figure 2 Axial computed tomography image of the pelvis in an adolescent boy with Crohn’s disease. It demonstrates bowel wall thickening of the distal small bowel and enhancement of the mucosa (arrow). There is surrounding free fluid (arrowhead).

The main disadvantage to CTE is exposure to ionizing radiation, though the need to ingest a large volume of contrast and cost may also be prohibitive. The estimated effective dose of radiation is approximately 10 mSv for standard abdominal CT and 10-20 mSv for CTE in the adult population[7,16]. In the pediatric population, estimated effective doses as low as 2.9-4 mSv have been reported for abdominal CT using multiple detector computed tomography[6] and varied 64-320 detector CT scanners[17]. Newer adaptive iterative dose reduction techniques have been described that greatly reduce the radiation exposure among pediatric patients undergoing CTE from 16.7 milligray (mGy) to 6.1 mGy, with minimal reduction in diagnostic sensitivity and specificity[18]. Similarly, newly proposed CTE imaging techniques using low-dose radiation and noise reduction techniques among adults can reduce effective dose radiation exposure by 53%-60% from 15-20 mSv to 5-7 mSv[19]. Ultimately, effective doses of radiation from abdominal CT and CTE are dependent on protocols at individual institutions and can vary greatly, but promising new techniques may be able to reduce the radiation exposure to levels equivalent to SBFT.

CTE was previously recommended as the imaging study of choice in initial diagnosis and suspected complications of CD among adults and children[20], but it has fallen out of favor as MRE has become more widely available with faster scanning protocols for pediatrics[20-22]. According to a recent meta-analysis,sensitivity and specificity of CTE in diagnosing IBD is 84% and 95% respectively[23], with growing evidence that CTE is more sensitive than SBFT in diagnosing IBD among adults and children[11,24-26]. CTE is useful both in the initial diagnosis of IBD as well as monitoring disease activity and screening for complications over the course of a patient’s lifetime. CTE findings including unsuspected penetrating disease, fistula, abscess, or stricture have been shown to alter medical management plans in 61% of patients and lead to interventional procedures in 18% of patients with known or suspected CD[27]. CTE remains an instrumental study in diagnosing IBD, monitoring disease activity, and identifying complications; though the risk of ionizing radiation often limits its use to emergency situations when MRE is not feasible.

MRE has become an increasingly important cross sectional imaging modality in the initial diagnosis of IBD as well as disease activity monitoring. Patients typically drink 1-2 L of a hyper osmolar oral contrast material to distend the bowel lumen, which can be difficult for younger patients and is often not well tolerated. Intravenous gadolinium contrast and spasmolytic medications such as glucagon are often administered during the study[28,29]. The imaging procedure generally takes 1-2 h to complete, and patients must comply with instructions to hold their breath intermittently. Historically, young children undergo anesthesia for this procedure, but newer protocols to reduce scan time, limit oral contrast, and enlist child life team support have made MRE without anesthesia more feasible[30-32]. Signs of active IBD using MRE include bowel wall thickening, increased T2 bowel wall signal, bowel wall hyperemia, and creeping fat[29,33] (Figure 3). The major benefit of MRE is the absence of ionizing radiation.Other advantages include the ability to evaluate extraintestinal manifestations of disease activity and to obtain a dynamic assessment of the bowel with real time imaging sequences. The potential disadvantages of MRE are lack of availability at certain centers, longer scan time with possible need for sedation in younger children, and higher cost than other imaging techniques (Table 1).

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Figure 3 Magnetic resonance enterography study of the pelvis in an adolescent patient with active Crohn’s disease. T2 weighted image (A) demonstrating free fluid (arrowhead) and bowel wall thickening (arrow); T1 weighted image (B) with contrast demonstrating enhancement of the bowel and increased mesenteric vascularity (arrow).

MRE is now recommended as the imaging modality of choice for the diagnosis of IBD among children, monitoring disease activity among children and adults, and evaluation of perianal disease among children and adults[20-22]. A meta-analysis of prospective studies shows MRE has a sensitivity of 93% and specificity of 93% in diagnosing IBD[23]. MRE is the preferred study for evaluation of perianal disease and possible fistulas[34,35]. There is not a statistically significant difference between CTE and MRE in diagnostic accuracy for detecting active inflammation in IBD[36]. However, MRE is superior to CTE for differentiating bowel fibrosis from active inflammation (sensitivity 57% and 42%, specificity 82% and 68% respectively)[36]. The addition of diffusion weighted imaging on MRE has been shown to aid in identifying colonic inflammation and improve diagnostic confidence among children with IBD without the need for IV contrast[37,38]. Newer techniques such as automated motility mapping analysis can improve the identification of inflammatory lesions among patients with IBD[39]. MRE has also been shown to detect endoscopic remission with 83% accuracy and aphthous ulcer healing with 90% accuracy in a prospective multicenter study[40]. The ability of MRE to confirm the absence of disease rather than to identify inflammation or complications of IBD is novel. Future studies are needed to determine if this imaging modality will play a larger role in noninvasive routine CD monitoring.

Transabdominal ultrasound is a well-established imaging technique for the evaluation of IBD among children and adolescents; though primarily used in Europe, it is gaining popularity in North America. More recently, intraluminal contrast enhanced ultrasound (SICUS) has been used with improved bowel visualization. It involves drinking a relatively small volume (200-500 mL) of non-absorbable contrast solution approximately 30 min prior to abdominal ultrasound, and the scan time generally takes less than an hour. Conventional ultrasound is typically performed first using a 3.5-5 MHz probe to evaluate for extraintestinal abnormalities; then a high frequency 7.5-17 Hz probe is used to evaluate bowel wall thickness as well as Doppler assessment of blood flow to the intestine[41]. Sonographic evidence of active IBD include bowel wall thickening greater than 3 mm and bowel wall hyperemia[41] (Figure 4). Abdominal ultrasound can also assess for extraintestinal complications such as abscess, lymphadenopathy, or other complications of active IBD such as stricture or fistula. Bowel ultrasound has many advantages including low cost, lack of ionizing radiation, dynamic real-time bowel assessment, and no need for sedation in the pediatric population (Table 1). The potential disadvantage of bowel ultrasound is the need for a skilled operator to provide optimal sensitivity and imaging, which may not be available in all centers.

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Figure 4 Transabdominal ultrasound in a 12-year-old boy with Crohn’s disease. It demonstrates bowel wall thickening of the terminal ileum.

Ultrasound has been shown to have similar sensitivity and specificity in identifying IBD compared to MRE and CTE[23]. Though most studies use bowel wall thickening greater than 3 mm as a marker of active inflammation, studies have also demonstrated that bowel hyperemia as measured by Doppler blood flow is associated with active bowel inflammation[41,42]. A meta-analysis showed abdominal ultrasound had a sensitivity of 89.7% and a specificity of 95.6% for diagnosing IBD[23]. One group demonstrated 57% sensitivity in detecting undiagnosed CD among adults[43] and 76% sensitivity among undiagnosed children[44] with transabdominal ultrasound. Addition of enteral contrastdemonstrated improved sensitivity to as high as 94% among adults and 100% among children. Further studies are needed to confirm these findings. SICUS has excellent accuracy in diagnosing complications of CD such as stricture, abscess, and fistula compared to surgical findings[45]. Abdominal ultrasound findings have also been shown to correlate with endoscopic severity in moderate to severe UC[46]. Though still an experimental technique, some studies have shown that IV microbubble contrast can also be used to better detect vascular density and predict IBD disease activity[47].

Ultrasound is cost-effective, highly accurate in detecting bowel inflammation, and does not involve ionizing radiation or sedation. It is particularly beneficial in the pediatric population and in monitoring disease activity over time given these attributes. The routine use of bowel ultrasound has not been adopted in North America though it is widely used in Europe. There is concern that the diagnostic accuracy is operator dependent, and this may impact the utility in more widespread use.