Document Type : Original Paper
Authors
1
M.Sc. Student, Department of Medical Physics and Biomedical Engineering and Research Centre for Science and Technology in Medicine, Tehran University of Medical Sciences, Tehran, Iran
2
Assistant Professor, Department of Medical Physics and Biomedical Engineering and Research Centre for Science and Technology in Medicine, Tehran University of Medical Sciences, Tehran, Iran
3
Associate Professor, Department of Medical Physics and Biomedical Engineering and Research Centre for Science and Technology in Medicine, Tehran University of Medical Sciences, Tehran, Iran
4
Assistant Professor, Department of Radiology, School of Medicine, Johns Hopkins University, Baltimore, MD, USA
Abstract
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 (6map). 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 511 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 2 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 K2HPO4 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-68 radionuclide source
acquired on the GE Discovery LS PET/CT scanner.
Results: An ROI analysis was performed on different positions of the resultant 6maps and the average
6value of each ROI was compared to the reference value. The results of the 6maps obtained for 511 keV
compared to the theoretical values showed that in the phantom for low concentrations of K 2 HPO 4 all
these methods produce 511 keV attenuation maps with small relative difference compared to gold
standard. The relative difference for scaling, segmentation, hybrid, bilinear and dual energy methods was
4.92, 3.21, 4.43, 2.24 and 2.29%, respectively. Although for high concentration of K 2 HPO 4 the three
methods; hybrid scaling/segmentation, bilinear and dual energy produced the lowest relative difference of
10.91, 10.88 and 5%, respectively. For patients it was found that for soft tissues all the mentioned energy
mapping methods produce acceptable attenuation map at 511 keV. The relative difference of scaling,
segmentation, hybrid, and bilinear methods compared to TX method was 6.95, 4.51, 7, and 6.45%
respectively. For bony tissues, the quantitative analysis showed that scaling and segmentation method
produce high relative difference of 26 and 23.2%, respectively and the relative difference of hybrid and
bilinear in comparison to TX method was 10.7 and 20%, 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.
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