- 學位論文
不同體型之孩童與成人電腦斷層劑量長度積轉換有效劑量因數評估
Size-specific Conversion Factors Used to Evaluate Effective Dose from Dose-Length Product for Children and Adult at CT Scans: Phantoms Study
摘要
臨床電腦斷層(computed tomography, CT)檢查前後，在其操作的螢幕端能顯示該次掃描的劑量資訊-劑量長度乘積(dose-length product, DLP)。惟DLP僅是直徑限制在16公分或32公分壓克力假體的劑量參數，尚不能直接代表不同體型或胖瘦的有效劑量。藉由DLP轉換有效劑量的因數，或稱k值，可將DLP換算成有效劑量，但仍受限於不同的體型大小。因此本研究的目的是提供一種簡易可對應不同體型的轉換因數，方便將電腦斷層掃描後的DLP直接換算成有效劑量。我們將分析孩童與成人對應之頭、頸、胸、腹部、骨盆和全身六個電腦斷層掃描部位的劑量長度乘積轉換有效劑量因數。本研究使用擬人假體實驗和蒙地卡羅模擬兩種方式，進行六個掃描部位k值的評估。我們將校正後的熱發光劑量計填放至1、5、10歲和成人擬人假體內，進行掃描取得掃描之DLP和有效劑量後，進而分析實驗的k值(k-factorE)。另以蒙地卡羅劑量模擬軟體，取得1、5、10歲和成人數學假體掃描之劑量結果，再針對模擬的k值(k-factorM)進行分析，其中模擬的掃描參數與掃描範圍均參考擬人假體實驗。由兩種方式分析的k-factorE與k-factorM結果顯示，頭部掃描的k值皆為六個掃描部位中最小的，k值最大的部位皆為胸部或腹部。另外隨著假體體型越小其k-factor會越大，上述擬人實驗與模擬之k值有相同現象。綜合k-factorE與k-factorM的結果，我們導入AAPM第204號報告的有效直徑概念，推估得到不同有效直徑的假體，進行電腦斷層掃描時六個部位的轉換因數函數。期待藉由臨床直接量測和估算病人體型的有效直徑，簡便地對應於掃描部位的k值函數，能提供DLP對應不同體型病患之有效劑量，以利醫師或放射技術師對施做該檢查的參考。
並列摘要
The computed tomography (CT) terminal can show dose-length product (DLP) on the monitor before and after clinical scan. However, DLP only presents the dose information for the diameter of 16 cm or 32 cm acrylic phantoms. It cannot be directly represented the effective dose (ED) for different size body. Using conversion factor, or k- factor, DLP can be simply converted into effective dose, but it is still limited to some assigned body sizes or weights. The purpose of this study is to provide k-factors which directly convert the DLP into ED for specific-size of the body. This study would analyze the k-factor, which may correspond to different size of children and adult, for six clinical CT scan ranges, that are head, neck, chest, abdomen, pelvis and whole body (WB). The k-factor was studied by two methods. One is experimental method with the anthropomorphic phantom equipped with thermoluminescent dosimeters (TLD), and the other is simulation method with mathematical phantom and Monte Carlo calculation. In the phantom experiment, the phantoms aged of 1, 5, 10 years and adult underwent the CT scans individually. After acquiring the DLP and ED, the experimental k-factor, k-factorE, can be obtained. In the simulation method, the simulated k-factor, k-factorM, can be obtained after analyzing the DLP and ED that derived from the CT Monte Carlo simulation software. The head scan shows a lowest value and the chest or abdomen shows a highest value of k-factors. The trend of experimental and simulation shows the lower of body size and higher of k-factor value. Finally, this study introduces the concept of effective diameter that mentioned in the AAPM Report no.204. We combine k-factor and the effective diameter estimated by the anthropomorphic and mathematical phantoms. The k-factor functions of effective diameter for six scanned regions were presented. They can be easily and practically applied to evaluate the effective dose of the patients after measuring the physical size of the patient on the scanned region. The k-factor function for specific-size of body could be a convenient dose reference for physician, radiologist or radiology technician in CT clinical scan procedure.