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Background and Purpose: The diagnosis of Dementia with Lewy Bodies (DLB) is challenging due to various clinical presentations and clinical and neuropathological features that overlap with Alzheimer's disease (AD). The use of 18F‐Fluorodeoxyglucose‐PET (18F‐FDG‐PET) can be limited due to similar patterns in DLB and AD. However, metabolism in the posterior cingulate cortex is known to be relatively preserved in DLB and visual assessment of the "cingulate island sign" became a helpful tool in the analysis of 18F‐FDG‐PET. The aim of this study was the evaluation of visual and semiquantitative 18F‐FDG‐PET analyses in the diagnosis of DLB and the differentiation to AD as well as its relation to other dementia biomarkers. Methods: This retrospective study comprises 81 patients with a clinical diagnosis of DLB or AD that underwent 18F‐FDG‐PET/CT. PET scans were analyzed visually and semiquantitatively and results were compared to clinical data, cerebrospinal fluid results, dopamine transporter scintigraphy, and 18F‐Florbetaben‐PET. Furthermore, different cingulate island ratios were calculated to analyze their diagnostic accuracy. Results: Visual assessment of 18F‐FDG‐PET showed an accuracy of 62%‐77% in differentiating between DLB and AD. Standard uptake values were significantly lower in the primary visual cortex and the lateral occipital cortex of DLB patients compared to AD patients. The cingulate island ratio was significantly higher in the DLB group compared to the AD group and the ratio posterior cingulate cortex to visual cortex plus lateral occipital cortex showed the highest diagnostic accuracy to discriminate between DLB and AD at 81%. Conclusions: Semiquantitative 18F‐FDG‐PET imaging and especially the use of an optimized cingulate island ratio are valuable tools to differentiate between DLB and AD. [ABSTRACT FROM AUTHOR]