Performance evaluation of interpolated average CT for PET attenuation correction in different lesion characteristics

Previously we demonstrated the effectiveness of the interpolated average CT (IACT) for attenuation correction (AC) in PET in simulations and clinical patients. This study aims to evaluate the performance of IACT for thoracic lesions with different sizes, uptake ratios and locations. The XCAT phantom was used to simulate noisy ¹⁸F-FDG distribution based on the clinical count level with respiratory motion amplitude of 2 cm and 3 cm. The average activity and attenuation maps represented static PET and cine average (CACT) respectively. IACT was generated by the end-inspiration and end-expiration phases of the attenuation maps (HCT-in and HCT-ex) using deformable registration method. Spherical 10 mm and 20 mm lesions were simulated at 4 locations individually, including the lower left lung (LLL), lower right lung (LRL), middle right lung (MRL) and upper right lung (URL). Four target-to-background ratios (TBR), including 4:1 and 8:1 for respiratory motion of 2 cm, 6:1 and 12:1 for respiratory motion of 3 cm, were modeled. The noisy sinograms with attenuation modeling were generated and reconstructed with different AC maps by STIR (Software for Tomographic Image Reconstruction), using OS-EM with up to 300 updates. Normalized mean square error (NMSE), mutual information (MI) and TBR were analyzed. The NMSE and MI results showed that PET_CACT and PET_IACT were more similar to the original phantom as compared to PET_HCTs. For TBRs, the differences between CACT/IACT and HCTs AC were more significant for lesions in the lower lung with PET_HCT-ex showed higher TBR and PET_HCT-in showed lower TBR as compared to PET_CACT/PET_IACT for all lesion sizes, uptake ratios and respiratory motion amplitudes. The TBRs for 10 mm lesion were more difficult to be recovered in all AC schemes. Better lesion localization and more stable quantitation for different lesion characteristics make IACT a good alternate for AC as compared to conventional HCT/CACT.