Focal fatty deposits in bone marrow

Last revised by Daniel J Bell on 22 Dec 2023

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.

Common in older individuals, related to age but not to sex.

Focal fatty deposits might be associated with degenerative changes or chronic inflammatory conditions such as 1-3:

This process is a normal variant and does not account for pathology 2. The bone marrow is composed of hematopoietic and adipocytic cells, with the red bone marrow having a higher proportion of hematopoietic cells and the yellow bone marrow having a significantly higher proportion of adipocytes. The chemical composition of focal fatty marrow deposits is characterized by a higher proportion of lipids in relation to water and protein than the surrounding bone marrow 4,5.

Histologically, focal fatty deposits are characterized by a replacement of normal hematopoietic bone marrow by a higher percentage of adipocytic cells 4.

Usually appear normal, with preserved trabecular pattern 4.

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.

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%

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.

Imaging differential diagnoses of focal fatty deposits include 4,5:

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