CT-based attenuation correction in PET image reconstruction for the Gemini system

The Gemini system is a combined CT/PET imaging system newly developed by Philips Medical Systems. It has a unique open gantry design that allows for variable separation between the CT and PET gantries. The Gemini system incorporates CT-based attenuation correction (CT-AC) into a three-dimensional row-action maximum likelihood algorithm (RAMLA) for PET image reconstruction. It uses several unique techniques to achieve high accuracy while reducing patient X-ray dose. These new techniques include (1) using low-dose CT protocols to obtain CT images with adequate quality and quantitation for CT-AC while keeping patient X-ray dose low; (2) using a CT truncation compensation technique to improve the accuracy of CT-AC; and (3) using a generalized model for the conversion of CT images to attenuation maps at 511 keV. In this paper, we report the workflow and performance of Gemini CT-AC using phantom and patient studies. For comparison, attenuation maps obtained from PET transmission scans are also used for attenuation correction (TX-AC). Both phantom and patient studies show that PET images with CT-AC have image quality equivalent to or better than those with TX- AC.