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