Functional brain imaging represents cerebrovascular status, neuronal status, and neuronetwork status, and plays an increasingly important role in the evaluation of vascular cognitive impairment or vascular dementia. In this article, the roles of functional brain imaging are discussed in evaluation of subcortical lesions disrupting cortico-subcortical circuits and in the study of white-matter changes in dementia. Also mentioned is an ongoing clinical trial of extracranial-intracranial artery bypass surgery for hemispheric hemodynamic misery perfusion where single-photon emission tomography is used as a surrogate measure.