4^th largest organ, 5% of body weight

Components: trabeculae, hematopoietic cells, fat cells, stroma, and RES cells, sinusoids

1. Red marrow
   = hematopoietically active (by EGA of 20 weeks) with a rich + extensive vascular supply, composed of erythrocyte + leukocyte + platelet precursors
   • in adolescence: 40% fat, 40% water, 20% protein
   • at age 70: 60% fat, 30% water, 10% protein
2. Yellow / fatty marrow
   with a sparse vascular network
   • composed of 80% fat, 15% water, 5% protein

Distribution: conversion from red to fatty marrow throughout life

NUC:

1. Labeled leukocyte scintigraphy
2. Bone marrow scintigraphy

MR:

• Anatomic sites for MRI marrow screening:
  • spine (SAG images)
  • pelvis + femora (COR images)

• red marrow:
  • iso- / slightly hyperintense compared with muscle on T1WI + T2WI (longer T1 relaxation time)
  • hypointense compared with fatty marrow (shorter T2 relaxation time)
• yellow marrow:
  • isointense compared with subcutaneous fat on T1WI (relatively short T1 relaxation time compared with water)
  • T2-iso- / hypointense compared with subcutaneous fat
  • hyperintense compared with muscle on T2WI (long T2 relaxation time compared with water)

• Differences in SI are maximized on T1WI but diminished on T2WI
• Marrow signal iso- / hypointense to muscle + disk on T1WI in adults is abnormal!
• After 10 years of age red marrow is hyperintense to muscle on T1WI
• Fat-suppressed T2-FSE and STIR are very sensitive for pathology!