Focal nodular marrow hyperplasia

Focal nodular marrow hyperplasia is a rare, benign condition where there is a localized conversion of red/hematopoietic from yellow/fatty bone marrow. Its main relevance is of having a pseudotumor appearance mimicking skeletal metastases on MRI ¹.

Pathology

Location

Most commonly located in the spine (~60%) but can also commonly occur in the femora, sacrum and ilium ².

Radiographic features

Plain radiograph

Focal nodular marrow hyperplasia is occult ³.

CT

Focal nodular marrow hyperplasia can have mild medullary sclerosis but can often appear normal ^2,3.

MRI

Focal nodular marrow hyperplasia appears as an ill-defined, solitary or multifocal round-to-oval lesion without marrow edema. The average size is ~20 mm (range 8-55 mm) ^1,2. 

Signal characteristics

• T1: iso to mild high signal to skeletal muscle; low signal to marrow
• T2: low signal compared to skeletal muscle and marrow
• T2FS/STIR: variable including isointense to yellow marrow
• T1C+: no enhancement ^1-3

Signal intensity drop of >20% between in-phase and out-of-phase quantitative chemical shift imaging (e.g. Dixon method) is shown in most (~90%) cases ².

Nuclear medicine

Bone scintigraphy

Normal uptake is demonstrated ². 

PET-CT

Focal nodular marrow hyperplasia demonstrates mild increased uptake ². 

Differential diagnosis

• skeletal metastases: T2 signal tends to be higher, <20% signal drop on chemical shift imaging ²