Before the PET-CT scan is performed, you will be asked to avoid stressful exercise for 24 hours. Starting about 12 hours before the scan, you should eat a high protein diet and limit the amount of carbohydrates you eat. Starting about 6 hours before the scan, you should not eat or drink anything other than water (fast) until after the scan is done. You may continue to take prescribed drugs with water during the fasting period.