Magnetic resonance imaging (MRI) is very sensitive for the detection of marrow abnormalities. Bone marrow edema on MRI has been defined as an area of low signal intensity on T1-weighted images, associated with intermediate or high signal intensity findings on T2-weighted images. The bone marrow edema pattern is a nonspecific finding with multiple etiologies. The knee is a common place for bone marrow signal abnormalities to appear on MRI. Besides contusions and fractures from acute trauma, there are a variety of other causes of the bone marrow edema pattern. It is important for the interpreter of the study to be aware of the different etiologies responsible for producing these changes and to be able to narrow the differential diagnosis without mistaking such a pattern for acute trauma or infiltrative tumor. This article concentrates on those entities that produce a bone marrow edema pattern not related to acute trauma including red marrow proliferation, stress, osteochondral lesions, osteonecrosis, bone marrow edema syndrome, arthropathy, infection, Paget's disease, and marrow replacement disorders.
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