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

방사선방어학회지 2020년 45권 4호 p.171 ~ 177
Jung Seong-Moon, 김빛별, 김정인, 박종민, 최창헌,
소속 상세정보
 ( Jung Seong-Moon ) - Seoul National University Hospital Department of Radiation Oncology
김빛별 ( Kim Bit-Byeol ) - Seoul National University Medical Research Center Institute of Radiation Medicine
김정인 ( Kim Jung-In ) - Seoul National University Hospital Department of Radiation Oncology
박종민 ( Park Jong-Min ) - Seoul National University Hospital Department of Radiation Oncology
최창헌 ( Choi Chang-Heon ) - Seoul National University Hospital Department of Radiation Oncology

Abstract


Background: This study aims to determine the effective atomic number (Zeff) from dual-energy image sets obtained using a conventional computed tomography (CT) simulator. The estimated Zeff can be used for deriving the stopping power and material decomposition of CT images, 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 Zeff values 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 decomposition 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 calculated values with the reference Zeff values 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 applying the Rutherford, Schneider, and Joshi methods, respectively. Under dual-energy scanning (80 and 140 kVp), the patient exposure level was approximately twice that in general single-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 calculation based on single-energy exhibited appropriate feasibility.

키워드

Atomic Number; CT-SIM; DECT; Radiation Therapy

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