An image processing device includes: an image data acquisition unit for acquiring SPECT image data of a brain； a brain-region ROI definition unit for defining a brain-region ROI in the SPECT image； a striatum ROI definition unit for defining a striatum ROI in the SPECT image； and a threshold determination unit for, based on counts in the SPECT image's background which is the brain-region ROI except the striatum ROI, determining a threshold for distinguishing ventricles and sulci in the SPECT image； a region distinction unit for distinguishing between a region whose number of counts is smaller than or equal to the threshold and a region whose number of counts is larger than the threshold.
