At present, most PET studies apply 2-(18F)-FDG (Fluoro-2-Deoxy-D-Glucose). F-18 is preferred for clinical studies because of its favorable half-life (see Table 12.6). Since many kinds of tumor cells have increased glucose metabolism, the most com­mon indication is searching for tumors, especially to explore whether lymph node and distant metastases are present and to identify remnants or recurrent tumors after therapy. After surgery or radiation therapy, the environment of a former tumor will be distorted anyway, so it is hard to distinguish harmless scars from tumor lesions using structural imaging methods like CT and MRI.

Figure 12.12 The detectors of PET, packed in rings around the patient, detect annihilation radiation in coincidence mode.

Most recently produced PET devices are combined with a CT. In this way, stud­ies with both modalities can be completed sequentially, keeping the same position of the patient on the table. Attenuation correction of the PET images is much faster and less noisy when using CT rather than a separate set of transmission images obtained with an external radioisotope source. Moreover, the more detailed CT image helps determine a better localization of the abnormalities, thus reducing the fraction of false positive reports compared to PET alone. On the other hand, hybrid PET/CT has a much higher sensitivity for the detection of tumors than CT alone (Figure 12.13).