About 12 hours before a positron emission tomography, or PET, scan, the doctor puts the patient on a diet low in carbohydrates, explains North Shore University Health System. Foods such as meat, butter, eggs and starchless vegetables are acceptable, but cereals, milk, breads, pasta and sugars are not. A patient can eat and drink if he doesn't receive contrast media during a CT scan, according to Johns Hopkins Medicine.
However, the patient must stop eating at least three hours before a CT scan involving contrast media, notes Johns Hopkins Medicine. The patient can still drink water and other clear fluids that do not interfere with the contrast media. A diabetic patient can eat a light meal, drink and take his usual medications up to three hours before the scan. After the scan, the diabetic should not take his medication for 48 hours.
A patient who has a PET scan in the late afternoon can eat a low-carbohydrate breakfast, according to North Shore University Health System. The patient must stop eating altogether six hours before the scan. Water, however, is acceptable.
Hospitals can tailor their instructions to the specific needs of their patients, report North Shore University Health System and Johns Hopkins Medicine.