The patient is injected with a radiopharmaceutical tracer called FDG (fluoro–deoxyglucose, a radioactive substance containing glucose) about one hour before the imaging is scheduled to begin. The radiotracer spreads in the body via the blood stream and gets absorbed by cells and starts emitting small amounts of energy (positrons). The positrons collide with electrons resulting in the production of photons that travel in opposite directions. The photon emission is captured by the PET scanner and is processed in a computer to give a three–dimensional image of the body tissues being studied. Cancerous cells display a high radiotracer uptake and are highly visible in the scan in comparison to healthy cells. But the PET scan has a limitation in terms of pinpointing the location of a tumor. This shortcoming is overcome by the combination of a PET and CT. The CT scan produces thin cross–sectional slices of internal organs. The images produced by the PET and the CT are combined together to obtain accurate details about the size and location of a tumor. Images can be obtained for the whole body or for any limited area of the body.
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