The characteristic motor symptoms present in Parkinson's disease are often accompanied by a range of non-motor symptoms; in particular, cognitive impairment leading to dementia has an 80% cumulative prevalence in PD but with incredibly variable time for dementia onset. This study presents and compares early-phase [18F] Florbetaben (FBB) positron emission tomography (PET) and arterial spin labeling magnetic resonance imaging (ASL MRI) of perfusion in the context of cognitive decline and imminent conversion to dementia in Parkinson's disease. Patients underwent a comprehensive neuropsychological battery prior to inclusion in the study, which was used to classify patients as PD with normal cognition (PD-N, n=4), mild cognitive impairment (PD-MCI, n=41) or dementia (PDD, n=5), and to assign each patient a summary global cognitive score and an individualized Parkinsons disease dementia risk score (PDDRS). Early-phase FBB PET images, and structural and arterial spin labeled MR images were acquired for each patient, which were then coregistered, normalized and smoothed in preparation for analysis. The relative measures of brain function given by each imaging modality were analysed using the general linear model, in order to identify any association with cognitive status and dementia risk in the subject group. Cognitive decline and increased dementia conversion risk were found to be significantly associated with distinct regions of cortical hypoperfusion as quantified by the ASL data. FBB-derived images did not exhibit any significant association with cognition or dementia risk, and did not correlate significantly with ASL perfusion measures, suggesting that the two techniques are measuring di erent physiological phenomenon. A network based approach using principal component analysis identified networks of cortical hypoperfusion in the ASL data that related significantly to cognition and risk of conversion to dementia. This thesis raises questions regarding the physiological information presented by earlyphase PET, which remains a worthwhile area of further investigation. The PDDRS-related perfusion network developed here presents a potential biomarker of imminent conversion to dementia in Parkinsons disease.