以修正乳房厚度評估乳房攝影之劑量：可壓迫假體之研究

Mammographic dose assessment using corrected breast thickness：A compressible phantom study

洪思穎(Szu-Ying Hung)

指導教授 ： 董尚倫

中山醫學大學/醫學科技學院/生物醫學科學學系碩士班/碩士(2015年)

https://doi.org/10.6834/CSMU.2015.00164

摘要

前言
X光乳房攝影是早期偵測乳房攝影的第一線工具，為了得到高品質的影像與病人接受到低的平均乳腺劑量，乳房是必須自動控制壓迫板進行機械式的壓迫。由於在測試品質控制的程序中，乳房系統之系統顯示厚度是有容許±5 mm的誤差，因此準確的評估乳房壓迫厚度與平均乳腺劑量在乳房攝影上是很重要的議題。本研究的目的是使用可壓迫乳房假體與厚度量測裝置評估平均乳腺劑量。
材料與方法
本研究使用西門子數位乳房攝影系統(Mammomat NovationDR)，搭配24×30彈性式、24×30固定式、18×24固定式壓迫板。為了決定乳房壓迫厚度，本研究設計厚度量測裝置，再以長度8~15 cm之方形Bolus假體來模擬女性乳房，並使用8~20 kg進行壓迫。在量測厚度時是測量Bolus假體不同位置之厚度，並應用量測厚度與攝影系統顯示厚度來發展厚度修正模式。在驗證方面，是以直徑13 cm之圓形Bolus假體進行壓迫來驗證厚度修正模式。在評估平均乳腺劑量方面，是將上述Bolus假體以靶/濾片組合W/Rh搭配管電壓28 kV進行攝影，以不同壓迫厚度之攝影條件評估乳腺含量比與平均乳腺劑量。
結果
本研究之結果顯示，4 cm Bolus假體之最大壓迫厚度介於31~39 mm，對相同的壓力而言，量測厚度會隨著垂直胸壁方向的量測位置而改變；對相同的量測位置而言，量測厚度會隨著壓力的上升而下降，同時，模式修正厚度與量測厚度之差異為-0.8~1.1 mm。在評估乳腺含量比方面，使用模式修正厚度與量測厚度所求得之乳腺含量比非常接近，而以系統顯示厚度所計算之乳腺含量比則高於100%。對4 cm的Bolus假體而言，以系統顯示厚度、模式修正厚度與量測厚度所計算之平均乳腺劑量分別為0.80~1.30、0.77~0.86與0.77~0.85 mGy。
討論
應用Bolus假體搭配厚度量測裝置，可量測不同大小假體之厚度。由假體實驗結果顯示，Bolus假體的形狀對壓迫厚度僅有些微的影響，而攝影過程中的施加壓力會對壓迫厚度的影響較大。在壓迫厚度評估方面，厚度修正模式可準確地評估乳房攝影中之壓迫厚度，但系統顯示厚度則明顯小於量測厚度。以系統顯示厚度來計算乳腺含量比時，上述厚度低估的效應會導致乳腺含量比的高估，同時也會造成平均乳腺劑量的高估。
結論
在乳房攝影過程中，Bolus適合用來設計成可壓迫假體以評估壓迫厚度，同時本研究發展新的厚度量測裝置來評估壓迫厚度與平均乳腺劑量，也建立了厚度修正模式來快速的評估壓迫厚度。本研究結果顯示，使用系統顯示厚度會高估平均乳腺劑量與輻射風險，使用厚度修正模式後可有效的修正此一缺失。

關鍵字

乳房攝影 ； Bolus ； 壓迫厚度 ； 平均乳腺劑量

並列摘要

Introduction
X-ray mammography is the initial imaging tool for the early detection of breast cancer. In order to obtain a mammogram with good image quality and result in low average glandular dose (AGD) to patient, the breast is compressed mechanically by a motor-controlled compression paddle. Because there is ±0.5 cm tolerance on actual and displayed CBT form the mammography system during the quality control procedure, the determination of accurate compressed breast thickness (CBT) and AGD has been an important issue in mammography. The purpose of this study is the estimation of average glandular dose using compressible breast phantom and thickness measurement device in mammography.
Materials and Method
A digital mammography unit (Mammomat NovationDR, Siemens) with flexible compression paddle (FP) , 24 cm × 30 cm compression paddle (RP), 18 cm × 24 cm RP were used. To determine the CBT, a homemade thickness measurement device (TMD) was developed and rectangular Bolus phantoms with length of 8-15 cm are used to simulate breasts of female in mammography. Compression forces applied were ranged from 8 to 20 kg. The thicknesses of compressed Bolus phantom were measured at different positions using the TMD, and the measurement thicknesses mentioned above and the displayed thicknesses of the mammography system were used to develop a thickness correction (TC) model. A circular Bolus phantom with diameter of 13 cm was used to verify the TC model. For AGD assessment, Bolus phantoms were imaged using the anode/filter combination of W/Rh with tube voltage 28 kV. The percentage glandular content (PGC) and AGD values of each exposure for different compressed thicknesses of Bolus phantom were calculated.
Results
Result from this study show that the maximum compressed thickness of 4 cm bolus phantom is ranged from 31 to 39 mm. For the same compression force, the measured thickness changed along the measurement position of perpendicular direction to chest wall. For the same measurement position, the thickness decreased with increasing compression force. The error of corrected thickness using the TC model is ranged from -0.8 to 1.1 mm compare to the maximum measured thickness using the TMD. For PGC assessments, all the PGC values estimated from the displayed thickness were higher than 100%. The PGC values estimated from corrected thicknesses were comparable to those from measured thicknesses. For 4 cm bolus phantom, the AGD values calculated using displayed thicknesses, corrected thicknesses and maximum measured thicknesses are 0.80~1.30, 0.77~0.86 and 0.77~0.85 mGy, respectively.
Discussion
By applying Bolus phantom combined with TMD, the thickness distributions of compressible phantom with different sizes can be measured. The shape of bolus phantom seems play a little effect on determination of compressed thickness. The compressed thickness was strongly dependent on compression force applied in mammography. Results from the phantom study show that the TC model can accurately correct compressed thickness in mammography. The displayed thicknesses acquired from the mammography system are significantly less than the measured thicknesses. This effect leads to an overestimation of PGC value and an overestimation of AGD when the displayed thicknesses were applied in mammographic dosimetry assessment.
Conclusion
The Bolus phantom is suitable for construction of a compressible mammographic phantom. A new TMD was developed for assessments of compressed thickness and AGD. The TC model was proposed for correction of compressed thickness in mammography. Results from this study show that the calculated AGD values and radiation risks using displayed thicknesses were overestimated. By applying the corrected thicknesses from the TC model, these uncertainties mentioned above can be minimized.

並列關鍵字

mammography ； bolus ； compressed breast thickness ； AGD

參考文獻

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