Advanced MRI Sequences for Structural Lesion Assessment in Sacroiliitis

<b>Background/Objectives:</b> Assessing structural damage in pediatric sacroiliitis is challenging, necessitating radiation-free alternatives to computed tomography (CT). This study evaluated the diagnostic performance of advanced MRI sequences-3D-MENSA (Multi-Echo in Steady-State Acquisition), 3D-MERGE (Multiple-Echo Recombined Gradient Echo), and Zero Echo Time (ZTE)-against conventional T1-weighted sequences for detecting structural lesions. Low-dose computed tomography (LDCT) served as the reference standard. A secondary objective was to qualitatively assess the visibility of active inflammatory lesions and fat metaplasia. <b>Methods:</b> In this cross-sectional study, 23 pediatric patients with enthesitis-related arthritis (ERA) were included. To adhere strictly to radiation safety principles, the study used pre-existing ldCT datasets from a clinical cohort as the reference standard. No new CT scans were performed for this study. Structural lesions (erosions, sclerosis, and joint-space changes) were independently scored by two blinded radiologists. Interobserver agreement was assessed using intraclass correlation coefficients (ICC). <b>Results:</b> Advanced sequences (ZTE, 3D-MENSA, 3D-MERGE) demonstrated high agreement with ldCT for erosion detection (ICC range: 0.924-0.998) and significantly outperformed conventional T1-weighted MRI (ICC: 0.707). 3D-MENSA provided distinct contrast, effectively differentiating the ligamentous component of the sacroiliac joint from both the synovial component and the adjacent bone cortex. Qualitatively, 3D-MENSA also identified bone marrow edema and fat metaplasia, which cannot be visualized by ZTE or ldCT. <b>Conclusions:</b> 3D-MENSA and 3D-MERGE enable comprehensive evaluation of structural sacroiliitis lesions in pediatric patients with diagnostic accuracy comparable to ldCT. Specifically, 3D-MENSA demonstrates the potential to detect both active and chronic lesions in a single, rapid, radiation-free acquisition. These findings suggest that it should be considered for routine pediatric imaging protocols.