Pelvic heterotopic ossification: when CT comes to the aid of MR imaging

OBJECTIVE: This article compares various imaging aspects of magnetic resonance (MR) and computed tomography (CT) of heterotopic ossification (HO) in the pelvic soft tissues in paraplegic patients. Our aim is to highlight the benefits of integrating MR and CT imaging in the diagnosis of immature HO, which may be challenging with MR images alone. METHODS: Paraplegic patients examined on the same day by contrast-enhanced 0.4-T pelvic MR and unenhanced CT for pressure-sore-related infections were selected. MR imaging was performed on a Hitachi-Aperto 0.4 T; the Open Magnet served as a more favourable configuration for the required limb positioning of these patients. CT images were attained on a six-slice Siemens-Somaton-Emotion. RESULTS: MR images of HO differ according to the degree of bone maturity. The more immature the HO process, the more heterogeneous is the signal, characterised mostly by focal iso-hypointensity on T1-weighted images and hyperintensity on T2-weighted/short TI inversion recovery (STIR). These characteristics correlate to different CT patterns. CONCLUSIONS: MR and CT features of pelvic HO in paralysed patients were reviewed with a focus on the different aspects associated with the degree of ossification. Based solely on the MR findings, immature heterotopic ossification may be difficult to differentiate from other soft tissue pelvic lesions. TEACHING POINTS: • The pelvis and hip are common locations of heterotopic ossifications (HO), often occurring in paraplegic patients. • With respect to HO, MR imaging allows for a confident diagnosis in mature ossified lesions only. The MR aspect of immature ossification may be confused with other pathologies. • Plain radiographs and CT may show various phases of ossification: amorphous calcification, immature and mature ossification. • Integrating MR with CT can help recognise HO foci and differentiate them from infections and other soft tissue lesions.