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Findings: A brain MRI was performed. The patient's region of previous metastasis demonstrates an area of encephalomalacia on FLAIR images with surrounding increased signal, likely representing gliosis. In addition, a left frontal mass is seen with surrounding increased signal, likely representing vasogenic edema. T1 weighted post contrast axial images demonstrate ring enhancement of the left frontal lesion but no enhancement of the left parietal region. Finally, diffusion weighted images demonstrate abnormally increased signal centrally within the left frontal region but decreased signal centrally within the left parietal region.
Diagnosis: Cerebral abscess.
Discussion: The differential diagnosis for a ring enhancing brain lesion includes neoplasm which includes metastases as well as primary neoplasms such as glioblastoma multiforme. The differential often includes abscesses as well as a demyelinating process and contusions.
In this patient, it was unclear whether the new ring enhancing lesion represented a new metastasis or represented a cerebral abscess as the patient was also febrile. Therefore, diffusion weighted images were employed. A necrotic tumor should demonstrate decreased signal on diffusion weighted images while an abscess should demonstrate increased signal. Therefore, it was determined that the left frontal lesion which demonstrated increased diffusion weighted signal represented a cerebral abscess. This was confirmed with surgical drainage.
Cerebral abscesses are most often the result of hematogeneous dissemination from a primary infectious site. Direct extension from paranasal sinus infection and penetrating injury are also known causes. Abscesses classically occur at the gray-white matter junction.
References:
Grossman RI and Yousen DM. Neuroradiology: The Requisites. Mosby, 1994.
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