Bone marrow is ubiquitous throughout the skeleton, primarily composed of haematopoietic cells and fat cells between bony trabeculae and fibrous retinacula. It performs numerous physiological functions and dynamically changes during normal ageing and in response to stressors and pathology. Although bone marrow can be seen or inferred from many imaging modalities, MRI is the best way to visualise its constituents directly.
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Gross anatomy
Bone marrow contains:
haematopoietic cells
supporting stroma
reticulum (phagocytes and undifferentiated progenitor cells)
scattered fat cells
rich vascular supply
Although all normal bone marrow contains the same constituents, the proportion of haematopoietic cells and fat varies dramatically depending on age and region. This allows normal bone marrow divided into red and yellow marrow based on its relative fat content.
Red marrow
Red marrow is most abundant in infancy, occupying the entire skeleton (see below), and is primarily composed of haemopoietic cells. At birth, red marrow comprises almost 100% haematopoietic cells. Over time, this component decreases, with increased contribution of fat cells, such that by young adulthood the haematopoietic component has decreased to 60% 10.
By weight, the red marrow of adults is composed of 40% water, 40% fat and 20% protein 10. Thus, even red marrow has a significant component of fat that influences bone marrow signal and results in signal loss in chemical shift imaging (see below) 11,12.
Yellow marrow
Yellow marrow has the same constituents as red marrow, but is dominated by fat with little vascularity. On average it consists of 80% fat, 15% water and 5% protein 10.
Normal marrow conversion
During infancy, red marrow occupies the entire ossified skeleton except for the epiphyses and apophyses. Gradually, red marrow "retreats" centrally and by 25 years of age it is essentially confined to the axial skeleton (pelvis, spine, shoulder girdle, skull). The conversion of red to yellow marrow progresses from distal to proximal of the extremities, so first hands and feet, then forearms/lower legs, then humeri/femora, then pelvis/spine.
Within the long bones, the epiphysis is the first to undergo conversion followed by the diaphysis, before extending to the metadiaphysis 5,6.
Although the distribution varies with age and from one individual to another, it should be symmetric.
Heterogeneous bone marrow signal on MRI is common and can be challenging to distinguish from pathology. Islands of red marrow may also be seen anywhere in the skeleton, typically in a subcortical distribution, often with central yellow marrow giving it a bull's-eye appearance on axial MRI.
Additionally, red marrow is found in subchondral crescents. Typical locations include the proximal humerus and femur 2. Similarly, focal fatty deposits in bone marrow may be seen essentially anywhere within the skeleton.
Yellow marrow can also be seen focally within vertebral bodies. The pattern of normal red and yellow bone marrow distribution in the spine is variable 9:
yellow marrow visible around the basivertebral veins (particularly common in younger patients, predictably seen following normal regression of paediatric widespread red marrow) 9,11
band-like and triangular-like areas of yellow marrow in the vertebral body corners and abutting the endplates (adjacent to degenerative disc disease and Schmörl nodes)
speckled pattern (punctate foci of red and yellow bone marrow)
larger areas of yellow marrow and poorly circumscribed areas of red marrow
Radiographic features
Bone marrow is best evaluated with MRI although FDG PET and CT, particularly with dual energy, can offer insights.
MRI
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T1
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T2
red marrow: slightly hyperintense to muscle, usually its signal intensity is slightly lower than that of yellow marrow, but sometimes it can be difficult to distinguish the two
yellow marrow: hyperintense to muscle and iso- to slightly hypointense to subcutaneous fat
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fat-suppressed T2
red marrow: remains hyperintense
yellow marrow: is saturated out (hypointense)
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T1 C+ (Gd)
normal bone marrow in adults does not enhance visibly, whereas there may be a significant contrast enhancement in normal marrow of a neonate or a small child
enhancement in adults occurs only in pathological marrow, which can be accentuated on post-contrast images with fat saturation
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chemical shift imaging
in/out of phase should show significant decrease (>50%) in signal intensity in normal marrow 12
>20% signal loss indicates a benign cause of bone marrow abnormality 12
Related pathology
Bone marrow can demonstrate changes as a result of a very wide variety of pathologies, either due to direct involvement or as a response to disease elsewhere. Examples include:
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