MRI Bone Marrow Signal Patterns

This is most likely physiological red marrow reconversion. In endurance athletes, increased haematopoietic demand leads to reconversion of yellow marrow back to red marrow. Key differentiating features: (1) The signal is intermediate on T1 but still BRIGHTER than muscle - pathological infiltration would be darker than muscle. (2) Distribution is symmetric and follows anatomic pattern (proximal metaphyses). (3) No cortical destruction or soft tissue mass. (4) May see bull's eye sign (central fat dot). (5) Clinical context supportive (athlete). I would be reassured by these benign features.

The focal T1 hypointense lesions darker than muscle are highly suspicious for metastatic disease, particularly given the prostate cancer history. Prostate metastases are classically sclerotic on X-ray but still show T1 hypointensity on MRI. I would assess: (1) Distribution - multiple vertebral bodies suggests haematogenous spread. (2) Pedicle involvement - metastases commonly involve pedicles (cf infection spares pedicles). (3) Disc space - typically preserved in metastases (destroyed in infection). (4) Soft tissue mass - epidural extension and cord compression. (5) STIR signal - will be variable but often high. (6) Enhancement pattern with contrast if available.

This is avascular necrosis (AVN) of the femoral head. The classic MRI features are: geographic area of abnormal signal demarcated by a dark band on T1 (interface between viable and necrotic bone) and the double-line sign on T2 (inner bright line representing granulation tissue, outer dark line representing sclerotic bone). Corticosteroid use is a major risk factor. Staging: I would use the Steinberg or ARCO classification based on extent of femoral head involvement and presence of collapse. Early stages show MRI changes without X-ray abnormality. Advanced stages show subchondral collapse (crescent sign). Bilateral involvement occurs in approximately 50%, so the contralateral hip must be imaged.