Introduction: The advent of dual-modality PET/CT scanners has revolutionized clinical oncology by improving lesion localization and facilitating treatment planning for radiotherapy. In addition, the use of CT images for CT-based attenuation correction (CTAC) decreases the overall scanning time and creates a noise-free attenuation map (۶map). CTAC methods include scaling, segmentation, hybrid scaling/segmentation, bilinear and dual energy methods. All CTAC methods require the transformation of CT Hounsfield units (HU) to linear attenuation coefficients (LAC) at ۵۱۱ keV. The aim of this study is to compare the results of implementing different methods of energy mapping in
PET/CT scanners. Materials and Methods: This study was conducted in ۲ phases, the first phase in a phantom and the second one on patient data. To perform the first phase, a cylindrical phantom with different concentrations of K۲HPO۴ inserts was CT scanned and energy mapping methods were implemented on it. For performing the second phase, different energy mapping methods were implemented on several clinical studies and compared to the transmission (TX) image derived using Ga-۶۸ radionuclide source acquired on the GE Discovery LS
PET/CT scanner. Results: An ROI analysis was performed on different positions of the resultant ۶maps and the average ۶value of each ROI was compared to the reference value. The results of the ۶maps obtained for ۵۱۱ keV compared to the theoretical values showed that in the phantom for low concentrations of K ۲ HPO ۴ all these methods produce ۵۱۱ keV attenuation maps with small relative difference compared to gold standard. The relative difference for scaling, segmentation, hybrid, bilinear and dual energy methods was ۴.۹۲, ۳.۲۱, ۴.۴۳, ۲.۲۴ and ۲.۲۹%, respectively. Although for high concentration of K ۲ HPO ۴ the three methods; hybrid scaling/segmentation, bilinear and dual energy produced the lowest relative difference of ۱۰.۹۱, ۱۰.۸۸ and ۵%, respectively. For patients it was found that for soft tissues all the mentioned energy mapping methods produce acceptable attenuation map at ۵۱۱ keV. The relative difference of scaling, segmentation, hybrid, and bilinear methods compared to TX method was ۶.۹۵, ۴.۵۱, ۷, and ۶.۴۵% respectively. For bony tissues, the quantitative analysis showed that scaling and segmentation method produce high relative difference of ۲۶ and ۲۳.۲%, respectively and the relative difference of hybrid and bilinear in comparison to
TX method was ۱۰.۷ and ۲۰%, respectively. Discussion and Conclusion: Based on the result obtained from these two studies it can be concluded that for soft tissues all energy mapping methods yield acceptable results while for bony tissues all the mentioned methods except the scaling and segmentation yield acceptable results.