Bone marrow conversion

At birth, all bone marrow are predominantly red marrow → conversion from red marrow to yellow marrow occurs shortly after birth and progress throughout life. Adult pattern of bone marrow is typically reached around the age of 25 years.

Appendicular skeleton

General patterns

• Peripheral → central (fingers/toes → forearms/legs → arms/thighs)
• Epiphyses → diaphysis → distal metaphysis → proximal metaphysis (this pattern is reversed in Red marrow reconversion).

Pattern by ages (tubular bones focused)

1. Infancy
   • Primary ossification center (metadiaphysis) is filled with red marrow.
   • Secondary ossification center (epiphyses and apophyses):
     • Un-ossified: cartilage → intermediate T1, high T2.
     • Ossified: yellow marrow within the ossification centers.
2. Childhood
   • Complete marrow conversion of fingers and toes.
   • Yellow marrow replaces in diaphysis.
   • Red marrow remains within the metaphyses.
3. Adolescent
   • Progressive conversion to yellow marrow (distal > proximal)
   • Can see patchy islands of red marrows in distal femora
4. Adult
   • Yellow marrow predominate.
   • Except proximal metaphyses of the femora and humeri.

Axial skeleton

Spine

First decade

• Predominantly red marrow → lower SI than adjacent disc on T1WI
• Yellow marrow infiltration can be seen around the basivertebral vein

Older than 10 years

• Progressively decrease amount of red marrow → higher SI than adjacent disc on T1WI
• 4 patterns of yellow fat marrow conversion
  • Basivertebral vein
  • Peripheral
  • Multiple small regions
  • Multiple large regions

Pelvis