- Etiology: marrow starts out in the long bones as red (hematopoeitic) marrow and converts to yellow (fatty and hematopoeitically inactive) marrow over first two decades, the last red marrow seen is in the proximal metaphysis
- MRI:
— Best assessed on T1WI: red (hematopoietic) marrow is isointense to skeletal muscle in first few months of life and then hyperintense to skeletal muscle and intervertebral discs in children and adults, yellow (fatty) marrow is hyperintense and similar to subcutaneous fat
— On T2WI with fat saturation: red marrow has intermediate to mildly hyperintense signal intensity, yellow marrow is hypointense
— Marrow conversion occurs from peripheral (appendicular) skeleton to central (axial) skeleton, from distal to proximal (early epiphyseal conversion), from diaphyseal to metaphyseal
— Yellow marrow appears in the epiphysis within six months of ossification, in the diaphysis at 1-10 years, in the distal metaphysis at 10-15 years, in the proximal metaphysis in young adult - Complications: yellow to red marrow conversion occurs in anemia, post chemotherapy / GM-CSF, chronic inflammation, high altitudes, smokers
Pediatric Imaging 