What causes the Double Line Sign in the femoral head on T2-weighted MRI?
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The Double Line Sign represents the pathological interface between viable and necrotic bone in avascular necrosis (AVN). It is composed of two distinct layers: an inner bright line representing granulation tissue with edema, and an outer dark line representing adjacent fibrotic or sclerotic bone. This sign reflects the reactive process that develops at the margin between infarcted bone marrow and surrounding normal marrow. The Double Line Sign is observed in approximately 80-85% of AVN cases and is considered highly specific for the diagnosis. Associated conditions leading to AVN include femoral neck fractures, idiopathic osteonecrosis, steroid-induced necrosis, sickle cell disease, and other conditions causing bone infarction. The sign may be accompanied by joint effusion, cartilaginous thinning, subchondral collapse, and fragmentation in advanced stages.
Why is it called so?
The sign is named for its distinctive appearance on T2-weighted MRI, where two lines of contrasting signal intensity are visualized at the demarcation zone between necrotic and viable bone. The term “double line” directly describes the morphological appearance of this two-component interface seen on imaging.
Pathophysiology
The Double Line Sign develops through a sequential process following bone infarction. Initially, the infarcted bone becomes demarcated from surrounding viable tissue through inflammatory response and revascularization. The inner bright line represents granulation tissue and bone marrow edema at this interface. The outer dark line represents the sclerotic response of living bone adjacent to the necrotic region, as viable bone develops fibrosis and sclerosis as a reaction to the presence of dead bone. This sclerotic zone acts as a demarcation line and reflects new bone formation on dead trabeculae. The varying signal intensities on T2-weighted MRI create the characteristic two-line appearance that is pathognomonic for AVN.
Other associated named signs:
Crescent Sign – represents a subarticular fracture line in necrotic bone seen on radiographs, indicating collapse of the articular surface
Doughnut Sign – seen on bone scintigraphy, representing central photopenia surrounded by increased uptake indicating revascularization
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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