From fluid loss to calcification: MRI and CT matrix features in cystic echinococcosis.

OBJECTIVE: To characterize MRI matrix signal and CT calcification patterns across WHO-IWGE stages, compare them with lesion mimicking cystic echinococcosis (CE), and derive an MRI-based subclassification for inactive cysts. MATERIALS AND METHODS: This retrospective multicenter study included 105 patients (174 CE cysts) imaged between 2012 and 2024, plus 30 mimickers. Inclusion required axial T1-weighted imaging with chemical-shift in-phase/opposed-phase acquisitions and at least two T2-weighted sequences. Two radiologists independently reviewed all examinations, and a consensus was reached. Cysts were classified as inactive if no free-fluid T2 hypersignal was present on index MRI and corroborated by follow-up imaging and/or adjunct ultrasound in equivocal cases. Diagnostic performance indices were calculated to identify inactivity. RESULTS: Fatty T1 hyperintensity was more frequent in inactive than active/transitional cysts (20/97 (21%) vs 2/77 (3%); p < 0.001). The "matrix flip-flop" sign was observed in 61/97 (63%) inactive cysts and in only two CE3b lesions (2/77 (3%); p < 0.001). Both signs showed high specificity for inactivity (75/77; 97.4%, 95% CI: 90.9-99.7). Matrix calcifications were associated with inactivity (46/97 (47%) vs 2/77 (3%); p < 0.001) and were rare among mimickers (2/30 (7%); p = 0.006). Homogeneous non-fatty T1 hyperintensity characterized mimickers (23/30 (77%) vs 10/174 (6%); p < 0.001). A three-tier inactive subclassification (early/intermediate/late) was derived from matrix T2 evolution. CONCLUSION: Matrix MRI/CT features-particularly fatty foci and the matrix flip-flop sign-provide ancillary evidence of CE inactivity and help distinguish CE from mimickers. T2 signal supported an MRI-based framework for stage-adapted "watch-and-wait" follow-up. KEY POINTS: Question MRI features of cystic echinococcosis remain insufficiently studied, particularly matrix signal abnormalities that may affect diagnosis and viability assessment on routine imaging. Findings T1- and T2-weighted matrix CE cysts analysis identified fatty foci and the newly described matrix flip-flop sign as specific features of inactive cysts. Critical Relevance Assessment of underrecognized MRI and CT cyst-matrix features can help stage cystic echinococcosis, assess viability, and distinguish cystic echinococcosis cysts from mimickers.