Interpolated average CT for attenuation correction in PET - a simulation study

Previously we proposed using an interpolated average CT (IACT) method for attenuation correction (AC) in PET, which is a good low-dose approximation of cine average CT (CACT) to reduce misalignments and improve quantification in PET/CT. This study aims to evaluate the performance of IACT for different motion amplitudes. We used the digital 4D Extended Cardiac Torso phantom (XCAT) to simulate maximum of 2 cm, 3 cm and 4 cm respiratory motions. The respiratory cycle was divided into 13 phases, with average activity and attenuation maps to represent ¹⁸F-FDG distribution with average respiratory motions and CACT respectively. The end-inspiration, end-expiration and the mid-respiratory phases represented 3 different helical CTs (HCT-1, HCT-5 and HCT-8). The IACTs were generated using: (a) 2 extreme + 11 interpolated phases (IACT_2o11i); (b) 2 phases right after the extreme phases + 11 interpolated phases (IACT_2s11i); (c) 4 original + 9 interpolated phases (IACT_4o9i). A spherical lesion with target-to-background ratio (TBR) of 4:1 and diameter of 25 mm was placed in the base of right lung. The noise-free and noisy sinograms with attenuation modeling were generated and reconstructed with different noise-free and noisy AC maps (CACT, HCTs and IACTs) by STIR (Software for Tomographic Image Reconstruction) respectively, using OS-EM with up to 300 updates. Normalized-mean square error (NMSE), mutual information (MI), TBR and image profiles were analyzed. The PET reconstructed images with AC using CACT showed least difference as compared to the original phantom, followed by IACT4_o9i, IACT_2o11i, IACT_2s11i, HCT-5 and HCT-1/HCT-8. Significant artifacts were observed in the reconstructed images using HCTs for AC. The MI differences between IACT_2o11i and IACT_4o9i/CACT were <;0.41% and <;2.17% respectively. With a slight misplacement of the two extreme phases, IACT_{2s11i was still comparable to IACT2o11i with difference of <;2.23%. The IACT is a robust, accurate low dose alternate to CACT and works well for over 90% of the clinical patients.}