PET-CT is the fusion of functional and anatomic information acquired almost simultaneously that lets us see the body and disease in a way that is diagnostically very powerful. By combing the structural anatomic information with functional data, we are able to visualize form and function. An understanding of the normal and benign as well as the pitfalls and artifacts is essential to accurate interpretation.

The PET-CT scanners are essentially full ring coincidence detectors, the P.E.T. portion, physically mounted together with CT systems of various types. The PET tomographs are fitted with various crystals (BGO, LSO, GSO) that are used to detect the emission photons and convert them to light signals. This scintillation event is converted to an electric signal that can be displayed on a monitor.

Photons that originate from structures deeper in the body are more highly attenuated by the intervening soft tissue than those originating closer to the surface. This effect of attenuation is not accounted for in the non-attenuation corrected images, which appear to show high activity toward the surface and relatively low activity toward the center.

In addition to providing anatomic data, the CT transmission scan can be used to generate an attenuation map that can be used to correct this attenuation effect. Because of the lower photon energy of the CT x-rays (100-140kVp), the CT attenuation coefficients are scaled to reflect the attenuation of the high-energy 511keV emission photons first. Once scaled, they can be applied to the emission data to obtain the attenuation corrected image.

This correction process is essential for quantitative assessment (S.U.V. standardized uptake values) as well as improved image quality. Because artifacts can be introduced during this process, any suspected findings can be verified by seeing if the abnormality was present on the uncorrected emission image. If it was present on the uncorrected image as well, than it is likely a true focus of increased uptake.

Usually the CT transmission scan is acquired followed by the emission PET scan. Because the scanners are in the same gantry the patient remains on a single scanning table and in the same position for both parts of the scan. Therefore they are intrinsically registered as seen on the fusion image.