"Innumerable" lesion burden on brain MRI ‒ a diagnostic approach.
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OBJECTIVES: The MR imaging lesion burden described as "innumerable" is rare, and can present a diagnostic challenge. Brain MR imaging with this descriptive term has not been systematically studied. We determine risk factors and MR imaging sequences helpful in a diagnostic algorithm for innumerable brain MR lesions. METHODS: Twelve thousand four hundred ninety-five brain MR imaging studies done at our institution from July 1, 2013 to June 30, 2016 were surveyed for the term "innumerable". Inclusion criteria included 50 or more parenchymal lesions. Patients were classified into active and chronic groups, based on MR characteristics and clinical features. RESULTS: One hundred and twenty three reports contained the term "innumerable". Thirty-one met inclusion criteria and 19 showed active, and 12 chronic brain process. The active group included 9 metastasis, 6 infarction, 2 microbleeds, and one each, foreign body granulomatous reaction and fungal abscesses. The MR feature accompanied or heralded onset of illness in eight patients. Malignancy was a risk factor in nine patients with metastasis and in 2 with infarct from cancer-associated hypercoagulation. Other risk factors included immunosuppression, endocarditis, long bone fracture and aortic dissection. CONCLUSIONS: MR defined active innumerable brain lesions occurred in 0.25 % of studies. Fifteen of 19 in the active group were due to metastasis or infarction, defined by T1-weighted gadolinium enhancement and restricted-diffusion respectively. A diagnostic algorithm based on MR imaging features and risk factors can guide critical decision for brain biopsy.