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

Abstract

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.

Fig. 1

A 58-year-old paraplegic man, post trauma. a On axial STIR a diffuse increased signal intensity involving the right obturator externus-adductor muscles suggests an inflammatory reaction as observed in the early phase of ossification. b On CT axial scan a circumscribed amorphous calcification showing ill-defined margins and no trabecular structure is evident within the periphery of the adductor brevis muscle belly

Fig. 2

A 55-year-old tetraplegic male patient following vertebral trauma. a Axial CT scan shows a bilateral ossifying process about the femural metaphyseal region with some amorphous calcifications in the ileopsoas muscle area. A left throcanteric calcific bursitis is also observed. b–d MR imaging: (b) axial STIR and (c) T1-weighted axial images show bilateral ileopsoas bursitis (small arrows) on the right merging to a mature ossification. d T1-weighted fat-suppressed images after contrast administration show, on the right side, peripheral enhancement of the bursal walls (large arrow)

Fig. 3

A 62-year-old male patient with traumatic paraplegia. CT imaging shows soft-tissue thickening adjacent to the great trochanter due to bursitis. An immature ossification with an initial trabecular structure and faint margins is evident within the paratrochanteric bursa

Fig. 4

Different patterns of HO maturity shown on CT. a Amorphous calcification located within the right quadratus femoris muscle, characterised by ill-defined margins and no recognisable trabecular structure. b Immature ossification located within the right medius gluteus muscle. Lesion shows poorly-defined margins and an initial trabecular formation is recognisable. c Mature ossification located within the left ileopsoas muscle, outlined by cortex with well-defined cancellous bone inside

Fig. 5

A 72-year-old tetraplegic male patient with spinal injury. Different grades of maturity may coexist in the same clinical setting. a Axial CT scan shows an HO formation located within the gluteus minimus muscle with the typical features of maturity: well-structured cancellous bone outlined by cortex and adherent to cortical bone. Immature ossification and amorphous calcifications coexist in various regional muscles. b–d Axial corresponding MR imaging shows the mature ossification with a high-fat bone marrow signal outlined by low signal cortical bone (c) (small arrow). The amorphous calcifications contiguous to the posterior iliac bone (large arrow) are characterised by elevated signal intensity on STIR images (b) and intense contrast-enhancement on T1-weighted fat-saturated (d) compared with unenhanced T1-weighted images (c). The small immature ossification in the gluteus medius muscle (asterisks) is clearly demonstrated by CT (a), while it is not easily recognisable on MR imaging

Fig. 6

A 35-year-old male paraplegic patient after spinal injury. a First CT axial scan shows swelling and hypodensity of the right ileopsoas muscle. b One-year follow-up examination shows the appearance of an immature ossification within the ileopsoas muscle

Fig. 7

The same patient as in Fig. 6: the corresponding MR examination (a, b) performed on the same day as CT examination shown in Fig. 6b. Muscle swelling and heterogeneity (large arrow) is clearly noticeable on STIR images (a) and T2-weighted images (b) associated with inflammatory changes of contiguous retroperitoneum tissue (asterisks). A marked intra-articular fluid collection is also evident to the right of the coxofemural joint (small arrows). T1-enhanced (d) compared with T1-weighted unenhanced images (c): after i.v. gadolinium administration, a heterogeneous and prominent contrast-enhancement is evident inside the right ileopsoas muscle encircling a “dark spot” corresponding to the ossification

Fig. 8

A 41-year-old male paraplegic following spinal trauma. a CT axial scan defines a bulky calcific mass located within the anterior compartment of the right thigh and the gluteus minimus muscle. On MR images (b–d), the mass is characterised by heterogeneity on STIR (b) and T1-weighted (c) images. d Focal contrast-enhancement appears on T1-weighted imaging following i.v. gadolinium administration (arrows)

Fig. 9

A 61-year-old male paraplegic patient following spinal injury. a CT axial scan reveals an elongated ossifying lesion within the right pectineus muscle. b On STIR images the lesion (small arrow) is essentially isointense to muscle, while on (d) T1-weighted images it shows a considerable low signal intensity). c No contrast-enhancement is noticeable at this site. On the left ischiatic region, a deep infected soft tissue ulcer with bone involvement is shown (large arrow)

Fig. 10

A 35-year-old male patient with leg contractures, affected by posterior element incomplete fusion (spina bifida occulta). a MR T1-weighted axial images evidence a soft tissue abscess (small arrow) of the perineal region related to pressure sores. Lesion is determining a certain retraction over the superficial layers of the dermis. b STIR images show the focal inflammatory lesion partially filled with fluid and a large bursitis of the right gluteal region (large arrow). c On coronal T1-weighted fat-sat images after gadolinium administration, the abscess walls appear highly vascularised. The gluteal bursitis seems to be communicating with the infectious process, suggesting a septic bursitis. A prominent contrast enhancement is also seen to the sacrum, suggesting a bony involvement by the septic process. d Axial CT imaging performed with patient in a different leg positioning . The exam validates the connection between the perineal abscess and the gluteal bursitis. Amorphous calcifications (asterisks) are enclosed within the septic bursitis at the right gluteal region

Fig. 11

A 34-year-old paraplegic male following spinal cord injury. Examinations were performed to evaluate abscess and osteomyelitis of the left ischial tuberosity (large arrow). The MR study reveals a lesion in the area of the right psoas muscle (small arrow): (a) central hypo-intense area on axial T1-weighted non-enhanced image (small arrow), (b) enhanced tissue and some fluid after contrast administration (small arrow). Note that the sterile HO is similar to the abscess of the left ischial region. c The corresponding CT shows the mass as non-homogeneous with some subtle calcifications on the medial side. d The CT study repeated 10 months later shows transformation of the immature bone into a mature, heterotopic ossification that adheres to the femur

Fig. 12

A 30-year-old male patient affected by posterior element incomplete fusion (spina bifida occulta). a On first CT axial scan a bulky immature ossification is revealed adjacent to the left iliac wing. This HO was not comprised into the MR acquisition volume that was lower centred because of the clinical relevancy of a posterior ischiofemural abscess. b A 10-month follow-up CT scan shows the lesion appears to be fragmented and partially resorbed. A marked soft tissue swelling is now visible caused by an underlying infectious process