The most recent PET scanners are made up of a large number of detectors arranged on numerous rings and are able to identify pairs of 511 KeV photons emitted at 180° to each other which are formed following the annihilation of a positron when it meets an electron. These data are converted in tomographic images, with coronal, transaxial and sagittal sections 3-4 mm thick capable of revealing lesions with a diameter of about 4-5 mm 2.
As the two 511-keV gamma-rays pass through the body, there is likelihood that one or both of the gamma-rays will be absorbed or will be scattered away from the two detectors they would have hit (attenuation). Guaranteeing optimal quality requires the attenuation correction with measuring attenuation correction factors. For this purpose, acquisition of transmission scan and emission images were done.
-Transmission scan: Positioning the patient in the scanner( the patient has not been given an injection of positron emitter at this stage) and acquire scan with activity from the external rod or point source using a radionuclide source of germanium-68 emitting positrons or cesium-137 emitting gamma rays.
-Emission scan: acquire scan from the patient after injection of positron emitter 8 .
The acquisition of transmission images enables correction for the attenuation of emission images to be performed. This not only leads to a significant improvement in image quality, but also enables a quantitative analysis of the extent of radiotracer uptake within the tissues to be performed 2, 8.
In a PET/CT scanner, the PET and CT tomographs are housed in a single gantry with a single patient bed and workstation. The CT tomography is usually in the front of gantry, and the PET tomography in the back. PET/CT scanners can be used either as a dedicated PET scanner or as a dedicated CT scanner. The CT scanner can be dual or multislice, with axial or helical acquisition modes and different rotation speeds 9. One main advantage of a PET/CT scanner is that it uses the CT images(as transmission images) for attenuation correction of the PET data rather than relying on a rotating transmission rod source. PET/CT scanners are able to perform the registration of the transmission images in extremely short times -less than a minute - with the PET study acquisitions performed immediately after. Upon reconstruction, both the PET images and the CT images are displayed side by side and overlaid (fused) 10. Use of the CT scan reduces the total PET acquisition time which translates into increased patient comfort and therefore increased patient cooperation 2. In addition, the availability of high quality transmission images enables improved accuracy and precision attenuation correction of the emission images and a more precise localization and interpretation of the hypermetabolic lesions, thanks to the availability of anatomical landmarks 2, 10.
