Patients are instructed to fast for at least 4 hours before the PET appointment. Glucose containing drinks and intravenous (IV) glucose must be avoided at least 4 hours before FDG injection as well. The fasting state lowers the serum glucose level so that FDG has less competition for uptake by the tumor. Muscle uptake is also minimized by fasting through lowering the serum insulin level. Low FDG uptake in the muscles improves the tumor to background ratio and the image quality.
High glucose level in diabetic patients can also decrease the image quality. It is well known that high glucose level decreases the tumor intensity on FDG PET and that lesions might be missed. However, while a normal glucose level in diabetic patients before FDG injection is desirable, it often cannot be achieved. Therefore, from a practical stand point, it may be reasonable to perform the scan even with a high glucose level and interpret the findings. Should the images be “diagnostic enough,” and, for instance, the patient be upstaged, the low quality of images may not matter. A repeat scan should be considered, when a false-negative scan is suspected (for example, rising CA-125 in a patient with ovarian cancer, high glucose level, and negative FDG PET).