Clinical utility of magnetic resonance enterography for pediatric Crohn's disease: a review with illustrative cases and a focus on Japan.

Magnetic resonance enterography (MRE) is increasingly used in pediatric Crohn's disease (CD) because it enables repeated assessment of disease extent, transmural inflammation, strictures, fistulas, abscesses, and treatment response without ionizing radiation. This is particularly important in children, in whom CD may be extensive and active, symptoms can be nonspecific, and long-term follow-up is required. Pediatric MRE requires both adequate luminal distention and patient cooperation during scanning; therefore, preparation should be adapted to age, body size, and tolerance. Oral MRE is the mainstay technique for assessing small-bowel CD, whereas MR enterocolonography (MREC) generally refers to an oral MRE-based approach with additional bowel preparation to facilitate colonic assessment rather than true MR colonography requiring transanal catheterization. The core protocol elements and inflammatory findings used for activity assessment are broadly similar to those in adults, although pediatric protocols should be adapted to patient cooperation and institutional practice. T2-weighted imaging provides the morphological basis for evaluating bowel wall thickening, luminal narrowing, prestenotic dilatation, ulcer-related mural deformity, mural edema, and perienteric inflammatory changes. When gadolinium is used, fat-suppressed T1-weighted imaging provides complementary information on mural hyperenhancement, stratified enhancement, and penetrating complications. Diffusion-weighted imaging can complement morphological assessment and is incorporated into pediatric activity scoring. Cine balanced steady-state free-precession imaging may provide adjunctive information on motility and functionally significant strictures but should be used selectively. For activity monitoring, MRE-based indices translate imaging findings into reproducible scores. This review summarizes MRE preparation, imaging technique, interpretation, and quantitative activity assessment in pediatric CD, with emphasis on the Pediatric Inflammatory Crohn's MRE Index (PICMI), the Japanese 5-point MREC classification, and the derived MREC score, using representative pediatric cases.