False perfusion defects simulating Alzheimer's disease can be attributed to skull thickness and head holders of a SPECT scan table. Brain phantom SPECT was performed using a different type of head holder in order to evaluate the effect of attenuation due to head holders and skull thickness on brain SPECT. Skull thickness was measured in 10 normal or non-Alzheimer's patients and 10 patients with suspected Alzheimer's disease. Visual and quantitative analysis of regional perfusion showed a 19% count loss due to attenuation by commercially available head holders and average skull thickness, less than 18% count difference between frontal-temporal and parietal-occipital skull in non-Alzheimer's disease patients, and more than 22% count difference observed in suspected Alzheimer's patients.