Bone marrow composition

	Red marrow	Yellow marrow	Osseous component
Function	Haematopoietic	Energy storage, space-filler, haematopoietic backup (by Red marrow reconversion)	Structural support
Chemical composition	W: 40%, F 40%, P 20%	W 15%, F 80%, P 5%	Hydroxyapatite
Vasculature	Rich sinusoidal network with sluggish flow → favours metastasis, infection (vertebral osteomyelitis), and sickling events	Relatively fewer vessels	-
Normal distribution in adult	Axial skeleton + proximal humeri and femora	Remaining appendicular skeleton	-

MRI appearances

	Red marrow	Yellow marrow
T1WI	Intermediate - ↓ than yellow marrow/subcutaneous fat, but ↑ than muscle/disc	Very high, similar to subcutaneous fat
STIR/T2WI FS	Intermediate - iso-mildly ↑ compared with muscle	Dark, similar to other fat (fat signal is suppressed)
Post Gd T1WI with fat suppression	Mild-moderate enhancement	Mild enhancement