Bone marrow is primarily red (composed of hematopoietic elements) at birth and converts to yellow with skeletal maturation. The two types of marrow differ in their composition: red marrow is approximately 40% water and 40% microscopic fat, while yellow marrow is approximately 80% fat. This lends to the differing MR imaging appearances of red and yellow marrows.
Reversal of a normal marrow conversion pattern (marrow reconversion) can occur with various stressors. Specifically, reconversion occurs in healthy individuals with changing metabolic demands, for example those enlisting in an athletic training program or altitude climbers. Slightly different is the idea of marrow repopulation which happens in patients with a chronic illness such as sickle cell disease or thalassemia. A third cause of increased red marrow in adults is marrow stimulation which occurs in patients who are receiving red cell and/or granulocyte stimulating factors.
The differential diagnosis for increased red marrow in adults includes marrow deposition disease (Gaucher's), myelofibrosis, leukemia and lymphoma.
Poulton TB, Murphy WD, Durek JL, et al. Bone marrow reconversion in adults who are smokers: MR imaging findings. AJR Am J Roentgenol 1993;161(6):1271-21.
Zajick DC Jr, Morrison WB, Schweitzer ME, et al. Benign and malignant processes: normal values and differentiation with chemical shift MR imaging in vertebral bodies. Radiology 2005;237(2):590-6.