PET-CT is a new technology that combines two diagnostic imaging techniques, PET and CT, into one. The full name of PET is: Positron Emission Tomography (PET); the full name of CT is: Computed Tomography (CT). PET is a functional imaging technique that can diagnose early and subtle lesions in organs and tissues, but not the physiological anatomical location and shape of the lesion area, while CT is an anatomical imaging technique that can diagnose the precise anatomical location and shape of the lesion area, but not the early and subtle lesions. PET-CT combines the functional images of PET with the high-resolution anatomical images of CT. It has become the most advanced medical imaging equipment today, which is important in early diagnosis, localization, staging, recurrence monitoring of tumors, determination of biological target areas for radiotherapy, determination of tumor hypoxic areas, determination of precise radiotherapy protocols and monitoring of tumor efficacy; PET-CT is a non-invasive examination, and patients only need to inject a minute amount of nuclide tracer for examination, and the radiation dose to patients is much less than that of X-ray. At the same time, PET-CT is a high-grade health care screening tool. The unique features of PET-CT are a kind of functional plus structural imaging; it can quantitatively reflect the biological characteristics of the lesion; it can understand the overall condition of the whole body in one examination; it is sensitive and accurate; PET-CT technology is the best way to achieve the medical imaging diagnosis. PET-CT technology is a reliable way to achieve the “four definitions” (“localization”, “characterization”, “quantification”, “periodicity”) of medical imaging diagnosis. PET-CT technology has laid a reliable foundation for the realization of “four determinations” (“localization”, “qualitative”, “quantitative” and “periodic”) of medical imaging diagnosis; PET-CT examination is safe, comfortable and non-invasive.