Heterogeneous bone marrow signal

Heterogeneous bone marrow signal is a common but challenging MRI finding. It is usually an incidental finding but in a small minority can represent a serious underlying condition such as malignancy. It is most commonly described in the lumbar spine ^1-3.

Terminology

No uniform term is used in the literature to describe this imaging finding, additional terms include vertebral marrow signal abnormality and abnormal bone marrow signal ^1,2.

Epidemiology

The incidence of heterogeneous bone marrow signal on MRI is reported at ~2% (range 0.25-4%) ^1,2. It was most commonly reported in patients aged in their mid-to-late 50s. ^1,2. Approximately 5% (range 1.5-10%) of patients with heterogeneous bone marrow signal were reported to have a malignant cause ^1,2.

Pathology

The cause of heterogeneous bone marrow signal is often not determined, however, causes include ^1-4:

• benign

  • normal aging

  • bone marrow reconversion (diffuse hematopoietic marrow hyperplasia)

  • focal nodular marrow hyperplasia (FNMH)

  • lipid-poor (atypical) intraosseous hemangioma

  • degenerative changes

  • chronic anemia

• malignant

  • multiple myeloma

  • skeletal metastases

  • lymphoma

Radiographic features

MRI

Heterogeneous bone marrow signal imaging features are not well defined and rely on the subjective impression of the radiologist ^1,2. Various patterns have been described including, focal, diffuse, combined diffuse and focal, multifocal, variegated, and normal ^1,3.

Signal loss >20% (1.5 T) or >25% (3 T) between in-phase and out-of-phase imaging (e.g. chemical shift imaging, DIXON method) is generally considered the threshold that distinguishes normal marrow as well as benign conditions (e.g. red marrow conversion, FNMH, osseous hemangioma, and bone marrow edema) from marrow infiltration ⁴. Complete fat suppression on pulse sequences also indicates a benign lesion ⁵. Absence of signal loss is not specific for a malignant cause ⁵.

Worrisome features of heterogeneous bone marrow signal include ³:

• marked decrease in signal intensity (particularly T1 signal lower than skeletal muscle)

• diffuse homogeneous abnormal signal

• focal lesions with sharp margins

• high signal after contrast administration

• high signal intensity after fat saturation

• increase in size or number of focal lesions / heterogeneous marrow on follow-up MRI

Treatment and prognosis

There is no consensus recommendations for the work-up of heterogeneous bone marrow signal with variable recommendations ¹. Patients are often referred to oncology or other specialties, where work-ups performed include further imaging, laboratory investigations, and bone marrow or other biopsies ^1,2.