Experimental: FET PET

Upon recruitment, a single study visit will be scheduled where subjects undergo a limited 18F-FET PET/CT of the brain in SGH.~After the study visit, from time of recruitment, they will continue their regular clinic visits as clinically indicated, where they will be monitored for at least a year for stability or deterioration. If clinically indicated, they may undergo conventional MRI, alternative MRI imaging and/or histopathological correlation in their respective primary institutions.~During these follow-up visits, any adverse effects possibly attributed to the 18F-FET PET/CT can also be flagged up.~Any alternative MRI imaging performed (as part of clinical practice in the respective primary institutions) within 4 weeks of the 18F-FET PET/CT study will also be included in the comparative analysis.