How are you? The length of fasting varies and depends on the radiological imaging requested whether there is presence of contrast media or not. Fasting usually refers to no food, drink, for a certain period of time before the diagnostic test. There are other instances wherein a fast has been prescribed and ingestion of clear liquids up to 2 hours before (water, smooth fruit juices, tea, weak dark coffee, sodas) is allowed, some requires 6 hours fasting and some others for longer periods. It is recommended that such requirement prior to any procedure be discussed with the attending physician to clear any doubt. Take care and regards.