| Summary: | 碩士 === 慈濟科技大學 === 放射醫學科學研究所 === 105 === Abstract
Purpose: This study aimed to (1) evaluate CT doses to reduce patient radiation exposure as low as reasonably achievable and (2) evaluate the efficacy of automatic exposure control (AEC) in order to generate low-dose CT protocols for patients of different ages undergoing cardiac PET/CT and SPECT/CT.
Methods: One PET/CT and one SPECT/CT without AEC were used to acquire CT images for four anthropomorphic phantoms representative of adult, 10-, 5-, and 1-year-old children. For each CT scan, a region of interest was placed at the position of heart to quantify image noise. CT dose measurements were made by using anthropomorphic phantoms with thermoluminescent dosimeters. One PET/CT and one SPECT/CT with AEC were used to acquire CT images for four anthropomorphic phantoms representative of 1-, 5-, 10-year-old children and adult. For the AEC system on the hybrid systems investigated in this study, the tradeoff between radiation dose and image quality depends on the selection of predefined image quality index and mA range. In addition, the volume
CT dose index (CTDIvol) and dose-length product (DLP) displayed on scanners' console after each scan were recorded. Multiple linear regression methods were used to (1) analyze image data from anthropomorphic phantom studies to investigate the effects of chest size and CTDIvol on image noise. (2) analyze image data from anthropomorphic phantom studies for investigating the effects of body size and predefined image quality index on CT radiation dose in cardiac PET/CT and SPECT/CT scans.
Results: In CT dose measurements, the organ dose values of heart and breast were higher in PET/CT, while higher thyroid and gonad doses were observed in SPECT/CT. The effective dose of CT scan with default protocol in PET/CT without AEC was 2.64, 2.81, 2.88 and 3.16 mSv for anthropomorphic phantoms simulating adult, 10-, 5- and 1-year-old children, respectively. With SPECT/CT without AEC, the corresponding results were 2.37, 2.40, 2.69 and 4.18 mSv. Regression models relating the effects of chest size and CTDIvol on CT image noise were built for protocol optimization. The regression relationships have coefficient of determination larger than 0.8, indicating a good fit to the data. According to the regression models, low-dose protocols with the use of AEC technique were optimized for patients of different ages. When using the optimized protocols in cardiac PET/CT, 32.2%, 63.7% and 79.2% dose reduction can be achieved for anthropomorphic phantoms simulating 10-, 5- and 1-year-old children, respectively. The corresponding results for cardiac SPECT/CT are 8.4%, 51.5% and 72.7%.
Conclusion: AEC is a practical way to reduce CT radiation dose in cardiac PET/CT and SPECT/CT, but the AEC settings should be determined properly for optimal effect. Our results show that AEC does not eliminate the need for pediatric protocols, and CT examinations with the use of AEC technique should be optimized for pediatric and adolescents patients to reduce radiation dose as low as reasonable achievable. The regression models determined in this study can be used to tailor CT protocols for pediatric patients undergoing cardiac PET/CT or SPECT/CT imaging to reduce radiation exposure while providing sufficient diagnostic image quality.
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