The immature sacroiliac joints on cross-sectional imaging: developmental anatomy and diagnostic pitfalls

The immature sacroiliac joint (SIJ) has developmental imaging appearances that frequently simulate sacroiliitis in children and adolescents evaluated for juvenile spondyloarthritis, and false-positive interpretation increases when adult MRI and structural criteria are applied. This review integrates developmental anatomy with MRI and CT findings and discusses CT-like MRI and MRI-based synthetic CT as structural adjuncts for cortical assessment in the immature SIJ. We emphasize the most relevant false-positive pathways in daily practice, including peri-physeal fluid-sensitive marrow hyperintensity, developmental cortical contour variants, transitional intersegmental fusion interfaces, well-corticated intra-articular developmental ossification centers, physiologic enhancement patterns, and intra-articular gas (vacuum phenomenon). The review also outlines modality-specific strengths and limitations and provides practical guidance on how to integrate fluid-sensitive and structural information across modalities. The figure set is organized as a teaching atlas focused on pattern recognition, multimodality correlation, and explicit interpretive safeguards to help radiologists distinguish physiologic maturation from true inflammatory disease in skeletally immature patients.