Introduction
Mild cognitive impairment (MCI) was described as a transitional phase from normal to Alzheimer's disease. Cerebral blood flow (CBF) measurement using single photon emission computed tomography (SPECT) in MCI patients is less sensitive than in Alzheimer's disease patients. and changes in CBF are small and patterns of CBF abnormality are not uniform in MCI patients. In this study, using fractal analysis we evaluated the distribution of CBF as measured by SPECT. The purpose of this study is to evaluate the clinical usefulness of fractal analysis in patients with MCI.
Subjects
The subjects were 14 clinically diagnosed amnestic MCI patients (MCI group: mean age 70.6 years, mean MMSE score 25.8 +/− 1.6) and age matched 18 cognitively normal subjects (control group: mean age 66.9 years, mean MMSE score 28.5 +/− 1.3). Methods: The device we used was a high performance SPECT system (SPECT2000 H, Hitachi Medico Co. Ltd., Tokyo, Japan). Subjects underwent CBF SPECT with Tc-99 m HMPAO at resting condition. Reconstructed transaxial data (64 × 64 matrix, 44 slices, 4 × 4 × 4 mm voxel) were analyzed by a 3-dimensional fractal analysis. The analysis calculates the fractal dimension (FD) as the next equation. M(a) = ka-FD, where a = percent threshold against peak voxel value in the whole brain, k = constant, M(a) = number of voxel which have values above corresponding threshold. High FD value indicates complexity of the voxel value distribution among data set (total voxi in the whole brain), in another word heterogeneity.
Results
The mean FD value in MCI group (1.071 + 0.194 (mean +/− SD)) was significantly (p<0.001) higher than in control group (0.853 +/− 0.062). In MCI group, FD value correlated with neither MMSE score nor age. When cut off value was set at FD = 0.92, sensitivity and specificity to detect MCI by FD value were 78% and 83%, respectively. Out of 14 patients in MCI group, 4 had progressed dementia in the following 2 years. There was no difference in FD at initial SPECT between converters (1.05 +/− 0.11) and non-converter (1.08 +/−0.22) in MCI group.
Conclusion
Distribution of CBF in MCI patients was more heterogeneous than in normal subjects. Fractal analysis with CBF SPECT was a helpful tool to separate MCI patients from normal subjects. However, FD value alone seemed not predict the converter from MCI to dementia.