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We present the case of a 53-year-old woman with a known history of hereditary multiple exostosis disease referred for further imaging work-up after ultrasound and computed tomography leading to the suspicion of malignant transformation of an osteochondroma (exostosis) located on the posteromedial aspect of the right proximal femur. Imaging examinations suggested an ischiofemoral impingement resulting in a secondary bursitis associated with the development of an internal lipoma arborescens. This rare association explained the complexity of the diagnosis. Magnetic resonance imaging (MRI) played a key role in correctly identifying this benign complication of the osteochondroma and in distinguishing those observations from a secondary exostotic chondrosarcoma. MRI findings were subsequently confirmed at surgery and pathological examination., (© The Foundation Acta Radiologica 2019.)

The magnetic resonance imaging (MRI) appearance of an "established" bone marrow infarct is well-known, consisting of an area of preserved bone marrow signal surrounded by a serpiginous line. We report the uncommon observation of the very early phases of appearance of a bone marrow infarct, showing its progressive de novo appearance on MR images paralleling clinical symptoms and high-dose systemic steroid administration in a young female patient, presenting with acute knee pain. The initial knee MR examination performed one week after pain onset showed no abnormality. One week later, a second examination showed subtle ill-defined dotted signal abnormalities of the bone marrow of uncertain significance, of high signal on PDFS sequences. A third MR study obtained again one week later showed more evident findings with confluence of the high signal "dots" into a serpiginous line with a geographical appearance of the lesion, corresponding to the typical MRI presentation of bone marrow infarcts. Follow-up MRI at seven weeks showed definitive stability of this bone marrow infarct. A whole-body MRI performed for whole skeleton screening revealed multiple bone marrow infarcts typical for systemic avascular necrosis. This case represents a novel observation of the "birth" of a bone marrow infarct, from early intriguing changes to its typical ring-shaped appearance on MR images. It also reminds of the key role of MRI for early diagnosis of bone marrow infarcts and illustrates the emerging role of whole-body MRI for the detection of multifocal, asymptomatic skeletal involvement by ischemic lesions in systemic osteonecrosis.

Multifocal eosinophilic granuloma (EG) is a rare observation within the spectrum of histiocytosis X, generally described in children. We report the case of a 33-year-old man with multifocal EG showing an asynchronous evolution of bone lesions during a follow-up of 11 years. We also present the therapeutic approach chosen for this patient and the repeated magnetic resonance imaging (MRI) examinations used to monitor the disease with a final favorable outcome.

In this case report, we describe an "uncommon" case of axial gouty arthropathy in a 69-year-old woman with bilateral sciatica that was thoroughly evaluated with conventional radiography, CT scan, magnetic resonance imaging, bone scintigraphy, and PET-CT. Axial gouty arthropathy should be included in the differential diagnosis of chronic low back pain, mainly when several risk factors for gout are present.