Pediatric Marrow Conversion

  • 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
  • Imaging MRI: Best assessed on T1WI
    — 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
    — 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
    — T2WI with fat saturation: Red marrow has intermediate to mildly hyperintense signal intensity, yellow marrow is hypointense
  • DDX:
  • Complications: Yellow to red marrow conversion occurs in anemia, post chemotherapy or GM-CSF, chronic inflammation, high altitudes, smokers
  • Treatment:
  • Clinical:
  • Spine T1 marrow changes
    — Age less than 1 year – Vertebral body signal less than adjacent disc
    — 1 year – 5 years – Vertebral body signal = adjacent disc
    — Greater than 5 years – Vertebral body signal greater than adjacent disc
  • Extremity T1 marrow changes
    — There is fat in epiphysis within 6 months of radiographic appearance
    — Fatty marrow changes in diaphysis of humerus and femur begin by 1 year and complete by 10 years
  • Normal T1 marrow changes
    — During first and second decade of life red marrow gradually converts to yellow (fatty) marrow
    — In each extremity, conversion occurs from periphery (fingers and toes) to center of body (shoulders and hips)
    — In each bone, first epiphysis, second diaphysis, third metaphysis
  • Marrow reconversion with anemia
    — Order of transformation back to red marrow is reversed
    — Epiphyses are the last part to reconvert

Radiology Cases of Marrow Conversion