Focal fatty deposits, also known as marrow islands or replacements in the bone marrow, are well-defined focal fat islands within the bone marrow of the spine or other parts of the axial skeleton 1,2.
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Epidemiology
Common in older individuals, related to age but not to sex.
Associations
Focal fatty deposits might be associated with degenerative changes or chronic inflammatory conditions such as 1-3:
inflammatory arthritis (e.g. axial spondyloarthritis)
Anatomy
This process is a normal variant and does not account for pathology 2. The bone marrow is composed of haematopoietic and adipocytic cells, with the red bone marrow having a higher proportion of haematopoietic cells and the yellow bone marrow having a significantly higher proportion of adipocytes. The chemical composition of focal fatty marrow deposits is characterised by a higher proportion of lipids in relation to water and protein than the surrounding bone marrow 4,5.
Histology
Histologically, focal fatty deposits are characterised by a replacement of normal haematopoietic bone marrow by a higher percentage of adipocytic cells 4.
Radiographic features
Plain radiograph
Usually appear normal, with preserved trabecular pattern 4.
CT
On CT focal fatty deposits are characteristically more hypodense than the normal marrow, they are however more difficult to discern from normal marrow than on MRI unless dual energy applications such as e.g. calcium suppression are used ref.
MRI
Focal fatty deposits in the bone marrow are usually seen as well-defined, rounded, sometimes coalescing areas of high T1 and T2 signal intensity and low signal intensity on fat suppression sequences 5.
T1: high signal
T2: high signal
STIR/T2FS/T1FS: low signal
IP/OP: signal drop out usually ≥20% 6
-
Dixon
water: low signal
fat: high signal
IP/OP: high signal with drop out ≥20%
Clinical importance
Focal fatty deposits are seen and can be conveniently diagnosed on MR imaging by a prudent selection of suitable sequences. On routine protocols, however, they can lead to confusion and can be mixed up with other variants or pathological conditions 4. However, they might also help in the diagnosis of certain pathologic conditions such as axial spondyloarthropathies.
Differential diagnosis
Imaging differential diagnoses of focal fatty deposits include 4,5:
-
other variants
basivertebral vein fat: around the basivertebral veins in the central posterior of the vertebrae
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pathologies
vertebral haemangiomas: they usually have coarsened trabeculae, high signal on T1WI and mostly high on T2WI +/- high on STIR depending on the degree of fat and vascular components
bone marrow haemorrhage
lipoma: very rare
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