To ensure optimal image quality patients will be required to adhere to a high fat, low carbohydrate diet for 48 hours prior to the scan. The subject will then be cannulated to enable injection of the 18F-NaF tracer. They will then rest in a quiet environment for 60 minutes to enable the tracer to reach the valve before transfer to the imaging suite. Those with a heart rate of >65 /min will be given beta-blockade if it is deemed suitable and safe. This is routine practice in cardiac CT. A scout CT will be performed to allow optimal alignment of the PET and CT scanners (so we can be sure the radiation we are detecting is coming from the aortic valve). The patient will then be asked to lie still for 30 minutes so that the PET data can be acquired.