利用蒙地卡羅方法量化數位X光影像之骨質密度

Quantification of bone mineral density using Monte Carlo techniques and digital radiography

林祐瑄(Yo-Shan Lin)

指導教授 ： 鄭凱元 ； 吳杰

中臺科技大學/健康科學院/醫學影像暨放射科學系暨研究所/碩士(2012年)

摘要

準確的測量病人之骨質密度將可以有效的治療或預防骨質疏鬆症與骨折發生。臨床上 BMD 之量測主要使用雙能量X光吸收儀（dual-energy x-ray absorptiometry, DEXA）為主，DEXA 具有較昂貴的儀器成本且只能做骨質密度的量測無其他診斷應用，本實驗將提出一套以 X 光放射吸收測量法搭配蒙地卡羅技術以絕對定量 BMD 方法。實驗以 K2HPO4 做為骨骼等效假體，並配置成0.4 - 1.6 g/cm2的 BMD，調配之骨假體使用 DEXA 儀器進行驗證，首先使用70 kVp、5 mAs 條件進行照射並搭配蒙地卡羅模擬得到假體的灰階值與線性衰減係數，在假體中加入 0-4 cm 的水做為軟組織厚度，並以蒙地卡羅來進行軟組織衰減與散射校正，最後以迴歸分析法定量骨質密度。假體密度 0.40、0.80、1.20 （g/cm2）由 DEXA 驗證結果為 0.42、0.83、1.16（g/cm2），相對誤差各為 5.7%、4.4%、-3.6% ，R2 值為 0.996，假體密度 0.60、1.03、1.40（g/cm2），利用回歸分析法量測軟組織 1 cm 下假體密度各為 0.55、1.04、1.36，相對誤差各為 -8.56 %、1.19 %、-2.76 % ，在軟組軟組織2、3、4 cm 下誤差皆小於 ±10 % ，DEXA 測量豬骨結果各為 1.27、1.39 （g/cm2） 以迴歸分析在豬骨於軟組織 1 cm 下結果各為 1.21、1.47 （g/cm2），在軟組軟組織 2、3、4 cm 下誤差平均小於 ±10 %。本實驗提出使用一般 X 光機定量骨假體之 BMD，相較於 DEXA，具有低成本與低劑量的優勢，或許可以成為一個具有評估骨質密度潛力的量測方法。

關鍵字

放射吸收測量法 ； 散射校正 ； 蒙地卡羅 ； 骨質密度

並列摘要

World Health Organization （WHO） indicates that low bone mineral density increases the risk of bone fractures. Dual-energy X-ray absorptiometry （DEXA） has been widely applied in the clinical for the measurement of bone mineral density （BMD） and the estimation of the risk of bone fractures. In this study, we proposed a radiographic absorptiometry-based BMD measurement method, which converts the raw pixel values of digital radiography to BMD distribution map with the scatter correction by Monte Carlo simulation. To evaluate and validate the measurement results of proposed method, the bone equivalent phantoms were made by the K2HPO4 solution with different concentrations to mimic the bone with various BMDs from 0.4 to 1.6 g/cm2, and the gray-level values of these phantoms were obtained by X-ray digital radiographic imaging. Then, the actual imaging condition for bone equivalent phantoms was also simulated by using Monte Carlo software to obtain the attenuation coefficients of phantoms with various BMD and to estimate the scatter components under the different thicknesses of water covering. At last, the digital radiography can be converted to BMD map by using the linear relationships between gray-level values and the attenuation coefficients, and between the BMDs and the attenuation coefficients. The BMDs of bone equivalent phantoms were validated by DEXA, and the percent errors were lower than ±6%. By using the relationships determined in this study to estimate the BMDs of phantoms under different thicknesses of water covering, the percent errors were lower than ±10%. We concluded that the proposed method is conveniently applied in the clinical for patient BMD assessment.

並列關鍵字

Radiographic absorptiometry ； BMD ； Monte Carlo ； Scatter correction

參考文獻

Adams J E 2008 Dual-Energy X-Ray Absorptiometry 105-24

Adams J E 2009 Quantitative computed tomography European journal of radiology 71 415-24

Adams J E 2010 Radiogrammetry and radiographic absorptiometry Radiologic clinics of North America 48 531-40

Augat P, Fuerst T and Genant H K 1998 Quantitative bone mineral assessment at the forearm: A review Osteoporosis International 8 299-310

Bonnick S L 2010 Densitometry Techniques