根據聯合國原子輻射效應科學委員會2000年公布的數據，醫用輻射劑量約佔全部人造輻射劑量的98%以上，其中以電腦斷層檢查的貢獻最大。由於電腦斷層影像對軟組織具有高解析度與高對比度的解剖影像，因此在醫學診斷上的應用非常普遍，然而電腦斷層給予受檢者的輻射劑量是一般X光照影的十數倍甚至百倍，因此，電腦斷層檢查病患的劑量與風險評估十分重要。近年來，國際上有許多研究報告探討電腦斷層的輻射劑量，特別是小兒電腦斷層檢查的劑量評估方面，資料顯示在相同有效劑量的條件下，嬰兒的致癌風險亦比成年人高出許多。而我國在此部份的數據，卻極為缺乏。為提升醫療品質、與重視輻射防護，研究台灣小兒電腦斷層的輻射劑量，有其重要性和必要性。
本研究首先建立量測上的分齡小兒圓柱形PMMA假體，使用衛生署公布的國人身高、體重資料，製作直徑分別為10、16、20、24及32 cm之假體，代表年齡層為新生兒、1至5歲、5至10歲、10至15歲及成年人，另製直徑為5 cm假體，以探求與空氣中量測之關係。採用長棒狀電腦斷層專用游離腔，度量五台電腦斷層掃描機的各種電腦斷層指標(CTDI)，掃描機射束品質，與探討單次掃描劑量分布情況。並針對Siemens Somatom Sensation 64此台電腦斷層掃描機建立蒙地卡羅模擬程式，以期將來能應用於計算各廠牌電腦斷層掃描機所造成之劑量。研究結果發現，GE系列的nCTDI最高，於頭部、軀幹掃描約為18.81、9.39 mGy/100mAs；Siemens Somatom Sensation 64的nCTDI最小，於頭部、軀幹掃描掃描為10.67、5.41 mGy/100mAs。分析相關結果，建議臨床上進行小兒電腦斷層掃描時，使用之技術條件可降為80kVp，如此一來，可在不犧牲影像品質下，降低約69%的劑量。另發現隨著假體直徑增加(5、10、16、20、24及32 cm)，於中軸造成的散射輻射比例為39%、63%、77%、81%、85%及89%。模擬計算方面，初步建立的電腦斷層模型所造成劑量與量測值誤差在16%之內。以這些資料數據為基礎，作為台灣小兒電腦斷層輻射劑量的初探，對瞭解小兒電腦斷層劑量之分析是極具價值。

According to the UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) Report 2000, the largest man made source of ionizing radiation exposure to the world population is from diagnostic medical x-rays. The increasing use of computed tomography (CT) in clinical practice and the relatively high dose delivered to the patient make CT a significant contributor to the population dose from diagnostic medical x-rays. CT images with excellent resolution and high contrast of soft tissue have led to significant advances in medical radiology. Recently, much attention has been shifted to pediatric CT dosimetry due to the increased patient dose and the extra health detriment. Therefore, there is a need for the assessments of radiation dose and health detriment to pediatric CT patients.
To study pediatric CT dosimetry, age-dependent cylindrical phantoms are required for the measurements of CTDI (Computed tomography dose index). In this project, we propose to fabricate several age-dependent cylindrical PMMA phantoms based on the weight- and height-data published by the Department of Health and other references. Age-dependent CT phantoms with diameters in 10, 16, 20, 24 and 32 cm are represented patients with ages in newborn, 1-5y, 5-10y, 10-15y and adult. CTDI in various locations and beam quality are measured by a CT pencil-type ionization chamber and make using traditional and modern CT scanners, i.e. the GE Prospeed Plus, the Simens Somatom Sensation 64, the Philips BrillianceTM 40, and the GE Lightspeed VCT. GE series have the highest nCTDI (18.81(head), 9.39(body) mGy/100mAs) and Siemens Somatom Sensation 64 has the lowest (10.67(head), 5.41(body) mGy/100mAs). More importantly, parameters should be adjusted based on children size. For scanning the pediatric body, 80kVp usually reduced radiation dose for ~69% without sacrificing diagnostic images quality instead of using the same protocol of adults. Furthermore, the contributions from scattered beam to CTDIC are 39%, 63%, 77%, 81%, 85% and 89% with phantom diameters in 5, 10, 16, 20, 24 and 32cm. Also, CT source model and object models with Siemens Somatom Sensation 64 to calculate CT dose have been established for MCNP and the difference between simulated and measured CTDI is within 16%. With these dose data available, a single-scan dose analysis will be applied to derive the pediatric dose guidance levels. The results of this study are very valuable to understand the pediatric CT radiation dose in Taiwan.

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