We report a case of a 61-year-old male who presented with a sudden change in mental status. From a psychiatric standpoint, his symptoms were consistent with a bipolar disorder. A neurology consult raised suspicion for vascular dementia, given the sudden onset of symptoms; however, the magnetic resonance angiography (MRA) was unremarkable. The magnetic resonance imaging (MRI) had findings that were suggestive of both vascular and frontotemporal lobe dementia based on parenchymal atrophy and a lacunar infarct near the thalamus. However, by co-registering the magnetic resonance images with a subsequent fluorine-18 Fluorodeoxyglucose positron emission tomography (F-18 FDG PET), and combining the functional data with the anatomic appearance, the diagnosis was narrowed to semantic dementia, which is one of the lesser known subtypes of frontotemporal lobe dementia (FTD).