Deriving the Effective Atomic Number with a Dual-Energy Image Set Acquired by the Big Bore CT Simulator

Background: This study aims to determine the effective atomic number (Zeff) from dual -ener-gy image sets obtained using a conventional computed tomography (CT) simulator. The esti-mated Zeff can be used for deriving the stopping power and material decomposition of CT imag-es, thereby improving dose calculations in radiation therapy.Materials and Methods: An electron-density phantom was scanned using Philips Brilliance CT Big Bore at 80 and 140 kVp. The estimated Zeffvalues were compared with those obtained using the calibration phantom by applying the Rutherford, Schneider, and Joshi methods. The fitting parameters were optimized using the nonlinear least squares regression algorithm. The fitting curve and mass attenuation data were obtained from the National Institute of Standards and Technology. The fitting parameters obtained from stopping power and material decompo-sition of CT images, were validated by estimating the residual errors between the reference and calculated Zeff values. Next, the calculation accuracy of Zeff was evaluated by comparing the cal-culated values with the reference Zeffvalues of insert plugs. The exposure levels of patients under additional CT scanning at 80, 120, and 140 kVp were evaluated by measuring the weighted CT dose index (CTDIw).Results and Discussion: The residual errors of the fitting parameters were lower than 2%. The best and worst Zeff values were obtained using the Schneider and Joshi methods, respectively. The maximum differences between the reference and calculated values were 11.3% (for lung during inhalation), 4.7% (for adipose tissue), and 9.8% (for lung during inhalation) when ap-plying the Rutherford, Schneider, and Joshi methods, respectively. Under dual-energy scan-ning (80 and 140 kVp), the patient exposure level was approximately twice that in general sin-gle-energy scanning (120 kVp).Conclusion: Zeff was calculated from two image sets scanned by conventional single-energy CT simulator. The results obtained using three different methods were compared. The Zeff calcula-tion based on single-energy exhibited appropriate feasibility.