Indeterminate marrow lesions: focal nodular marrow hyperplasia

Recognition of normal bone marrow pattern and corresponding changes with disease on MRI is an important diagnostic skill within musculoskeletal radiology. Occasionally, a marrow lesion does not conform to the predictable red-to-yellow marrow conversion pattern and therefore the MRI signal characteristics are not as expected. Such lesions are called ‘indeterminate,’ and are either followed up for a length of time, misinterpreted as malignancy and referred to a tertiary oncology centre for biopsy or even operated on. Following application of the Dixon MRI technique, the overwhelming majority of these lesions can be confidently diagnosed as islands of red marrow/focal nodular marrow hyperplasia (FNMH), an entity known to mimic bone metastases on MRI.

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