FIGURE 9. ¹⁸F-FDG PET scans of a patient with false-positive findings and an AD patient. Shown are representative axial (left) and sagittal (middle) slices through an ¹⁸F-FDG PET brain scan of a 79-y-old woman undergoing clinical evaluation for cognitive impairment. The patient had a history of depression and thyroid disease and was receiving thyroid hormone replacement therapy at time of PET. The interpreting nuclear medicine physician had read the scan as consistent with early neurodegenerative changes in an Alzheimer-like pattern, because of the apparent relative decreased activity in the parietal cortex (bold white arrows), relative to the adjacent frontal and temporal cortex (thin white arrows). Longitudinal clinical follow-up for 2.5 y after the scan showed no progressive dementia, so the scan interpretation was classified as false positive. If this scan were being read now, the false-positive interpretation might have been avoided by comparing the thalamic activity (red arrows) to the parietal activity, with which it is approximately isometabolic, and to the (higher) frontal activity—in contrast to the pattern found in AD (right), in which parietal cortex becomes hypometabolic relative to the (preserved and normally isometabolic) thalamus.