Figure 1a. Normal hematopoietic (or red) marrow in a 21-year-old woman with knee pain and no additional medical problems. (a) Coronal T1-weighted MR image shows patchy regions of intermediate signal intensity (arrows) in the distal femur. These regions have signal intensity higher than that in nearby skeletal muscle (*), and this characteristic is suggestive of residual red marrow. (b) Coronal T2-weighted fat-suppressed MR image shows a patchy area (arrow) with signal intensity similar to that of skeletal muscle (*). MR images from 1%–2% of patients referred to the Orthopaedic Oncology Clinic showed similar features that are characteristic of residual red marrow but that were misinterpreted as potential malignancy.