Pelvic heterotopic ossification: MR imaging characteristics.

PURPOSE: To evaluate the magnetic resonance (MR) signal intensity characteristics of pelvic heterotopic ossification (HO) in various stages of maturation. MATERIALS AND METHODS: Thirty-six patients with HO proved at computed tomography (CT) (n = 17) or radiography (n = 19) who underwent 1.5-T pelvic MR imaging within 3 months were included. HO was defined at CT or radiography as grade 1, fluid attenuation without calcification at CT; grade 2, calcification; grade 3, immature ossification; or grade 4, mature ossification. The location and MR signal intensity of all HO sites were noted. RESULTS: HO was determined to be grade 1 at 20 of 141 sites, grade 2 at 39, grade 3 at 30, and grade 4 at 52. With increasing HO grade, the following findings were observed: (a) decreasing T2 signal intensity (grade 1, 70%; grade 2, 58%; grade 3, 44%; grade 4, 4%), (b) increasing fat and cortical bone signal intensity at T1-weighted imaging (grade 1, 0%; grade 2, 3%; grade 3, 13%; grade 4, 86%), and (c) decreasing contrast enhancement (from 100% for grade 1 to 20% for grade 4). Fifteen (88%) patients with CT correlation had HO in the anatomic area of the trochanteric or iliopsoas bursa (55 [60%] of 91 sites). CONCLUSION: With progressive maturity of HO, T2 signal intensity and contrast enhancement decrease, but fat and cortical bone-equivalent signal intensity increases.