Why is fasting before a cat scan necessary?

Its not safe because something might go wrong in the scan and the antibiotic/sleeping injection might react with food and it won't show a clear and good picture of your cat's insides. -------------------------------------------------Improvement By the Wording of the question, 'cat' scan may actually refer to CT scan or Computed (Axial) Tomography scan. Fasting before the scan for 4 to 6 hours or more may be required for certain scans requiring contrast media (iodine-based dyes to visualize certain organs better) or scans of the abdomen to avoid compounding results since certain food items may mimic human tissue in its response to X rays or reduce the effectiveness of contrast provided by contrast media.