The purpose of functional imaging in clinical practice is to increase diagnostic accuracy when differentiating between dementia disorders. The techniques most often used are imaging of regional cerebral blood flow (rCBF), using single photon emission computed tomography (SPECT), and imaging of glucose metabolism using positron emission tomography (PET). rCBF assessed with SPECT has been used to study vascular reactivity and to evaluate the effect of dementia treatment. It has also been studied in CADASIL, a hereditary form of vascular dementia (VaD). PET shows the same disease pattern as SPECT when ligands are used for investigating rCBF. It is also used to study regional glucose metabolism using Fluoro-Deoxy-D-Glucose (FDG), oxygen metabolic extraction rate and cerebral oxygen metabolic rate. According to the meta review by Dougall and his group, it is clear that the clinical usefulness of SPECT in differentiating VaD from Alzheimer's disease is limited.