Red marrow is present in the fetal skeleton and converts to yellow marrow after birth. In sickle cell anemia (SCA) destruction of RBCs causes anemia which prevents the conversion of red marrow to yellow marrow. The persistent red marrow is seen in all bones of patients with SCA and has pathologic and radiologic consequences. Red marrow stimulation leads to widening of the medullary spaces and cortical thinning which can cause pathologic fractures. Coarse trabeculation and osteopenia may also be seen. Persistent red marrow presents as low signal intensity in osseous structures on T1WI. On skull radiographs the widening of the diploic spaces is a manifestation of bone marrow expansion. The hair on end appearance of the skull is secondary to coarse trabeculation from new bone formation. In the spine, cortical thinning results in biconcave deformities of the vertebral bodies sometimes referred to as the fish-mouth appearance. REFERENCES Ejindu VC, Hine AL, Mashayekhi M, et al. Musculoskeletal manifestations of sickle cell disease. Radiographics 2007;27:1005-21.