Hippocampal perfusion in mild Alzheimer's disease

Abstract

Perfusion and metabolic studies in patients with Alzheimer's disease (AD) have so far yielded conflicting results on the functional status of the hippocampal region, whose deep location in the brain makes it critical to optimize the image-reconstruction technique employed in emission tomography. We used a brain-dedicated device (CERASPECT) to perform single photon emission computed tomography (SPECT) studies with 99mTc-hexamethylpropylene-amine-oxime in 22 consecutive patients (mean age: 74±6.5 years) with mild [mini-mental status examination (MMSE) score ≥15, mean 20.8±3.2], probable AD. The control subjects were 11 healthy elderly people (mean age: 70.5±6.5 years). In patients, the total score on the selective reminding test (SRT) was used as an index of memory function. Counts from a hippocampal and a temporoparietal region of interest in each hemisphere were referred to the average thalamic counts. To optimize SPECT images, we used conventional filtered back-projection (FBP) reconstruction and a new iterative method of conjugate gradients (CG), which takes into account the geometrical and physical characteristics of the gamma-camera. Hippocampal perfusion in the two hemispheres was significantly lower in patients than in control subjects, regardless of which reconstruction method was used, and correlated with the MMSE score. The correlation between hippocampal perfusion and the SRT score was significantly (bootstrap procedure) higher with the CG method than with the FBP method (CG: r=0.52 and 0.54; FBP: r=0.39 and 0.47, for the right and left hemisphere, respectively). These results show hippocampal hypoperfusion in patients with mild AD, a correlation between hippocampal perfusion and the severity of cognitive impairment, and enhanced identification of these subtle perfusional changes with the use of an alternative image-reconstruction method that improves the spatial resolution of SPECT images.