Comparing 511 keV Attenuation Maps Obtained from Different Energy Mapping Methods for CT Based Attenuation Correction of PET Data

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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