PET, or positron emission tomography, monitors the biochemical functioning of cells by detecting how they process certain compounds, such as glucose (sugar). Cancer cells metabolize glucose at a much higher level than normal tissues. By detecting increased glucose use with a high degree of sensitivity, PET identifies cancerous cells—even at an early stage when other modalities may miss them. However, PET cannot pinpoint the exact size and location of tumors to a precision necessary for optimal diagnosis and treatment planning. Learn more about PET scans.