Impacts of reduction of CT radiation dose on PET in PET/CT imaging

We studied the impacts of CT dose reduction on PET quantitation with combination of currently available techniques by phantom and human studies on a Toshiba PET/CT prototype scanner with TOF capability. For the phantom studies, an anthropomorphic torso phantom and an IEC phantom were scanned using both diagnostic quality CT protocols and dose-reduced CT protocols. To achieve reduced-dose CT scans, we used tube current reduction, spectral shaping, helical pitch optimization and AIDR 3DTM (Adaptive Iterative Dose Reduction). Then, both the high and low dose acquired CT images were resampled and used to generate attenuation maps for PET. The CT images, attenuation maps and reconstructed PET images were compared between high and low dose CT protocols. ROI-based analysis for each tissue type was also performed to see the impact of the dose reduction on PET quantitative accuracy among different dose protocols. Finally, the selected optimal low dose CT protocol was used for attenuation correction in whole-body PET/CT imaging on twelve patients. The PET data were reconstructed with fully 3D list-mode TOF ordered-subset expectation maximization algorithm with physical corrections of attenuation, scatter, random, detector normalization, decay, duration and dead time. The reconstructed PET images show that in PET/CT imaging, the PET diagnostic quality could be remained unchanged, while CT dose could be dramatically reduced compared to the diagnostic protocol, when CT is not used for diagnostic purpose. There is big room for CT dose reduction as long as the bias and noise in CT could be controlled in an acceptable range without introducing artifacts through CT-based PET attenuation correction. This study demonstrates that CT could be tailored for the intended purpose in PET/CT imaging to dramatically reduce patient radiation dose, while still maintaining high quality PET images.