Introduction: Alzheimer's disease is the most prevalent form of dementia, which mostly affects older patients over the age of 65 (late-onset); nevertheless, around 4% of patients suffer from early-onset dementia (Baillon et al., 2019). The latter, accounts for around 30% of dementia cases in Southeast Asia (Vipin et al., 2021). Pathophysiology of Alzheimer disease often linked with extracellular senile plaques and intracellular neural fibrillary tangles (NFT). However, in young-onset Alzheimer disease it is mostly due autosomal dominant genetic mutation involving Amyloid precursor protein (APP), presenilin 1 (PSEN1) as well as presenilin 2 (PSEN2) genes (Van Giau et al., 2019). Case Presentation : This is a case of 40-year-old young lady presented cognitive decline and behavioural change. FDG PET-CT revealed hypometabolism at bilateral temporal- parietal, bilateral precuneus, bilateral posterior cingulate and bilateral occipital with sparing of the bilateral primary sensorimotor cortex and bilateral primary visual cortex. Her Z-scores at the aforementioned areas were positive (elevated) when compared with general normal data base. Conclusion : Neuroimaging using FDG PET/CT is a central part of diagnostic work up of Alzheimer disease as it can reveal pathological changes even before morphological changes. Signature pattern of FDG hypometabolism at the bilateral temporal-parietal, posterior cingulate and precuneus regions shown on brain PET/CT scan of our patients helps in further supporting the diagnosis young-onset Alzheimer as the cause of. Furthermore, the incorporation of quantitative measurement of Z score (analytical software) by Cortex ID Suite, GE healthcare improved the diagnostic accuracy of our neuroimaging.