Investigation of Sub-Centimeter Lung Nodule Quantification for Low-Dose PET

With the recent development of high resolution time-of-flight (TOF) positron emission tomography (PET) and resolution recovery incorporated in the reconstruction algorithm, accurate detection, and quantification of sub-centimeter nodules might become feasible. In this paper, we performed a comprehensive simulation and mini-Derenzo phantom study to explore the quantitative accuracy of sub-centimeter nodules using the Siemens Biograph mCT scanner. We simulated nodules ranging from 4 to 10 mm in diameter, with 2:1 to 8:1 contrast level, at 1% to 100% (70 million) count-level, and with realistic respiratory motion amplitudes (superior-inferior/anterior-posterior directions) of 5/3, 10/6, and 20/12 mm. Images were reconstructed using motion-compensation ordered subset expectation maximization list-mode algorithm for resolution-recovery reconstruction. We also investigated different reconstruction voxel sizes of 0.5, 1.0, and 2.0 mm for both simulation and phantom studies. Reconstructions with 1.0- and 2.0-mm voxel size were upsampled to those with 0.5 mm prior to evaluation and different upsampling methods were compared. The results from simulation and phantom studies were consistent. We found that nodules sized 6 mm or greater resulted in a bias of mean standardized uptake value (SUVmean) smaller than 20%, even when the count levels dropped to 4%. SUVmean was reduced with 5/3 mm motion compared to static scans by 18 ± 4% with the 100% count-level, with smaller additional reductions found for larger motion amplitudes. Images reconstructed with voxel sizes of 1.0 and 0.5 mm resulted in more accurate quantification and reduced distortion compared to those with 2-mm voxels. The results indicated that it is feasible to achieve accurate quantification for nodules ≥ 6 mm using low-dose PET, with respiratory motion correction and fine reconstruction voxel size.