Magnetic resonance imaging (MRI) of the knee: a pattern approach for evaluating bone marrow edema.

The applications of magnetic resonance imaging (MRI) have become numerous for the assessment of disorders involving the knee. Once a technique used nearly exclusively in the evaluation of internal derangement of this joint, it now plays an important role in the diagnosis of processes that affect the bone marrow, including those that occur as a result of trauma, infection, tumor, and rheumatologic disorders. Signal alterations in the bone marrow frequently are present in association with these pathological processes. When the signal is of low intensity on T1-weighted images and becomes heterogeneously increased in intensity on T2-weighted images, it indicates the presence of edema in the bone marrow. Two types of marrow edema are presented in this review: posttraumatic and reactive. In many situations, the area of altered signal intensity is radiographically occult. The distribution of marrow edema often reflects the mechanism of injury in trauma and may correlate with additional injuries to the surrounding soft tissues. Reactive marrow edema occurs either in response to an inflammatory focus in the bone and/or joint or a neoplastic process in or adjacent to the bone.