Another issue is the possibility of anaerobic glycolysis that could occur after TBI. In such a case the glucose levels measured by the PET are not tightly connected to the oxygen consumption of the patient's brain.[21] Third point regarding PET scans is the overall measurement per unit volume of brain tissue. The imaging can be affected by the inclusion of metabolically inactive spaces e.g. cerebrospinal fluidin the case of gross hydrocephalus, which artificially lowers the calculated metabolism.[22] Also the issue of radiation exposure must be considered in patients with already severely damaged brains and preclude longitudinal or follow-up studies.