Comparative assessment of different energy mapping methods for generation of 511-keV attenuation map from CT images in PET/CT systems: A phantom study

The use of X-ray CT images for CT-based attenuation correction (CTAC) of PET data results in the decrease of overall scanning time and creates a noise-free attenuation map (mumap). The linear attenuation coefficient (LAC) measured with CT is calculated at the x-ray energy rather than at the 511 keV. It is therefore necessary to convert the linear attenuation coefficients obtained from the CT scan to those corresponding to the 511 keV. Several conversion strategies have been developed including scaling, segmentation, hybrid, bilinear and dual-energy decomposition methods. The aim of this study is to compare the accuracy of different energy mapping methods for generation of attenuation map form CT images. An in-house made polyethylene phantom with different concentrations of K₂HPO₄ was used in order to quantitatively measure the accuracy of the nominated methods, using quantitative analysis of created mumaps. The generated mumaps using different methods compared with theoretical values calculated using XCOM cross section library. Accurate quantitative analysis showed that for low concentrations of K₂HPO₄ all these methods produce acceptable attenuation maps at 511 keV, but for high concentration of K₂HPO₄ the last three methods produced the lowest errors (10.1% in hybrid, 9.8% in bilinear, and 4.7% in dual energy method). The results also showed that in dual energy method, combination of 80 and 140 kVps produces the least error (4.2%) compared to other combinations of kVps.