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Concomitant findings and clinical significance of a fluid crescent between the iliacus muscle and iliac bone on MRI

Background Some routine magnetic resonance imaging (MRI) examinations show a thin line of fluid signal intensity along the iliac crest (“fluid crescent”) between the iliacus muscle and the iliac bone. This fluid crescent has not been described before. Purpose To evaluate the clinical context and significance of the fluid crescent. Material and Methods MRI examinations of the abdomen and pelvis performed over 1 year were retrospectively reviewed twice for the presence of a fluid crescent. The relationship between the presence of a fluid crescent and associated abnormal findings, including ascites, iliopsoas compartment, and bone and soft tissue pathologies, was evaluated. Results Forty-one out of 254 MRI studies (male:female ratio, 136:118; mean age, 42 years) demonstrated a fluid crescent (16%). Thirty-eight of them had associated MRI pathologies: edema of the hip muscles = 24, ascites = 11, iliac bone = 21, and iliopsoas compartment = 7. Correlations between the presence of a fluid crescent and pathological findings were highly significant (P < 0.0001), except for the presence of fluid in the hip joint. Conclusion A fluid crescent is an abnormal MRI finding strongly associated with iliopsoas compartment pathology, ascites, hip muscle edema, and pelvic bone abnormalities. The nature of this fluid crescent is yet to be determined.