(RSNA 2018, Mon Nov 26 2018 3:30PM - 3:40PM ROOM Z44) PURPOSE Posterior cortical atrophy (PCA) is a form of dementia considered to be an atypical variant of Alzheimer's disease (AD) and Dementia with Lewy bodies (DLB) is a type of posterior dementia characterized by fluctuating levels of cognition, changes in behavior, visual hallucinations with accompanying extrapyamidal motor symptoms. We attempt to identify specific core areas on FDG PET imaging which are common to both and also establish the differences which may be helpful to differentiating the two. METHOD AND MATERIALS We retrospectively analysed of 30 patients with clinically suspected posterior dementia. All the subjects underwent F-18 FDG PET CT scan of the brain and the studies were analyzed both qualitatively (visually) and semi-quantitatively. The subjects had undergone dopamine transporter imaging with Tc 99 m TRODAT 1 on a prior date. The subjects were divided into possible PCA with TRODAT scan normal (n=10) and possible DLB with abnormal (n=20). The FDG uptake patterns were recorded and areas of cortical hypometabolism in the cerebral cortex that were two standard deviations from the mean were considered as abnormal. RESULTS All the subjects had an abnormal pattern of F-18 FDG uptake on PET scan, both on visual inspection and semiquantitative analysis. Bilateral parieto-occipital hypometabolism was consistently found in all the subjects. Hypometabolism in precuneus, posterior cingulate and the cortex around the angular gyrus was present in all the subjects of PCA with relative sparing of the medial occipital cortices. DLB subjects showed variable degrees of involvement of the medial occipital cortices with relative sparing of posterior cingulate and precuneus. CONCLUSION FDG PET scan can act as a non-invasive diagnostic modality in differentiating the two posterior cortical dementias despite significant clinical and imaging overlap. CLINICAL RELEVANCE/APPLICATION We present features based on which FDG-PET can be used to diagnose and differentiate Posterior Cortical Atrophy (PCA) and Dementia with Lewy Bodies (DLB) in the clinical practice.