Cause:

1. Anemia:
   1. Sickle cell disease
   2. Thalassemia
   3. Hereditary spherocytosis
   4. Paroxysmal nocturnal hemoglobulinuria
2. Chronic / severe blood loss
3. Marrow replacement by neoplastic cells
4. Treatment with GCSF (= granulocyte-macrophage colony-stimulating factor)

Reconversion = recruitment of yellow marrow for hematopoiesis once hematopoietic capacity of existing red marrow stores is exceeded

Order of reconversion: spine >flat bones >skull >long bones (proximal >distal metaphysis >diaphysis >epi- / apophyses)

MR:

• SI of hyperplastic marrow similar to muscle on T1WI + T2WI + STIR
• red marrow hypointense relative to fat on T2WI
• low SI of renal cortex on T1WI + T2WI ← hemosiderin deposition (after intravascular hemolysis)

Cx:

1. Hemosiderosis (histologic term of iron deposition in tissue) = excess iron in cells of RES ← repeated blood transfusions
   • magnetic susceptibility effects of hemosiderin produce hypointense marrow on T2WI (+ T1WI if hemosiderosis severe)
2. Medullary infarction (common in sickle cell disease)

DDx: leukemia (hyperintense on STIR)