Investigation of scan time for solitary pulmonary nodule discrimination

Current recommendation for using PET imaging in the assessment of solitary pulmonary nodules is limited to sizes greater than 8mm. Our objective was to assess the scan time required in order to improve discrimination of small lesions with realistic FDG uptake using XCAT phantom based simulations. Dynamic time-activity curves were generated to simulate uptake in different organs and a subset of these data were used to simulate static PET activity. An XCAT phantom of the thorax (with respiratory motion) was created with six spherical lesions (6mm and 8mm diameter with SUV ∼ 2 SUV ∼ 4) placed in different axial planes to create 4 datasets. For each dataset, 6 scan times ranging from 2.85–42 minutes were created to represent the ungated PET data. The corresponding optimal gated PET data were created by using 35% of total counts that of ungated data. Time-of-flight (TOF) sinograms modelling image acquisition physics, including randoms, scatter and PSF were simulated and Poisson noise added to obtain 25 noise realizations for each case. Each dataset was reconstructed with nonTOF PSF (PSF) and TOF PSF (PSFTOF) reconstruction using a matched and free-breathing attenuation map. In addition, joint estimation TOF-MLAA method was also investigated with free-breathing attenuation map. Discrimination of lesions was performed by assigning data with SUV = 2 as “benign” and SUV = 4 as “malignant”. Signal-to-noise ratio as well as area under the ROC curve were computed and used as a figure of merit. The maximum joint sensitivity and specificity was also computed from the ROC curve in order to estimate the scan time equivalence for different methods. It was observed that optimal gating and TOF based reconstruction provided improved performance for all cases. Ungated PET data provided poor performance even when scan time was increased. Optimal gating with TOF reconstruction approached AROC=1 earlier compared to nonTOF based reconstruction. Maximum time was required by nonTOF PSF based reconstruction with optimal gating (42 minutes) and the shortest was with TOF PSF based reconstruction with optimal gating and free-breathing CT (10 minutes). The use of longer scan durations, optimal gating and TOF based reconstructions help in improved discrimination of solitary pulmonary nodules.