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 positron emission tomography (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 four-dimensional (4-D) extended cardiac-torso phantom (XCAT) to simulate maximum of 2, 3, and 4 cm respiratory motions. The respiratory cycle was divided into 13 phases, with average activity and attenuation maps to represent 18F-fluorodeoxyglucose (18F-FDG) distributions with average respiratory motions and CACT, respectively. The end-inspiration, end-expiration, and midrespiratory phases of the XCAT attenuation maps represented three different helical CTs (i.e., HCT-1, HCT-5, and HCT-8). The IACTs were generated using: 1) 2 extreme+11 interpolated phases (IACT 2o ); 2) 2 phases right after the extreme phases+11 interpolated phases (IACT 2s); 3) 4 original+9 interpolated phases (IACT 4o). A spherical lesion with a target-to-background ratio (TBR) of 4:1 and a 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 Software for Tomographic Image Reconstruction, respectively, using ordered subset expectation maximization(OS-EM) with up to 300 updates. Normalized mean-square error, mutual information (MI), TBR, image profile, and noise-contrast tradeoff were analyzed. The PET reconstructed images with AC using CACT showed least difference as compared to the original phantom, followed by IACT 4o, IACT 2o , IACT 2s, HCT-5, HCT-8, and HCT-1. Significant artifacts were observed in the reconstructed images using HCTs for AC. The MI differences between IACT 2o and IACT 4o /CACT were <0.41% and <2.17%, respectively. With a slight misplacement of the two extreme phases, IACT 2s was still comparable to IACT 2o with MI difference of <2.23%. The IACT is a robust and accurate low-dose alternate to CACT.