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

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.