基于水平集的三維放射治療CT圖像彈性配準(zhǔn)

石翔翔唐 濤龐皓文*

基于水平集的三維放射治療CT圖像彈性配準(zhǔn)

石翔翔①唐 濤①龐皓文①*

目的:將基于水平集的圖像彈性配準(zhǔn)應(yīng)用于放射治療CT圖像中,為精確評(píng)估患者腫瘤及危及器官變化過程及其累加劑量提供技術(shù)支持.方法:根據(jù)水平集進(jìn)化理論,通過matlab軟件編程,分別對(duì)宮頸癌患者與鼻咽癌患者放射治療分次間的兩組CT圖像實(shí)施三維全自動(dòng)的彈性配準(zhǔn).結(jié)果:宮頸癌患者配準(zhǔn)前后對(duì)比顯示,最小均方誤差(MSE)減少55.1%,相關(guān)系數(shù)(CC)提高了5.3%;鼻咽癌患者配準(zhǔn)前后對(duì)比顯示,MSE減少32.1%,CC提高4.6%.結(jié)論:從配準(zhǔn)前后差分圖以及評(píng)價(jià)參數(shù)可見,該理論取得了較好的試驗(yàn)效果,但要真正應(yīng)用到臨床放射治療中,需進(jìn)一步探尋更精確的數(shù)學(xué)算法以模擬人體器官的運(yùn)動(dòng).

水平集;彈性配準(zhǔn);放射治療

石翔翔,男,(1985- ),碩士研究生,技師.西南醫(yī)科大學(xué)附屬醫(yī)院腫瘤科,從事腫瘤放射治療工作.

[First-author's address] Department of Oncology, Affiliated Hospital of Southwest Medical University, Luzhou 646000, China.

彈性配準(zhǔn)研究是放射治療中的熱點(diǎn)和前沿課題,放射治療的整個(gè)實(shí)施過程中腫瘤的退縮以及周圍正常組織的變化是影響整個(gè)放射治療劑量精確的關(guān)鍵,精確評(píng)估每例患者腫瘤及危及器官變化過程及其累加劑量是放射治療熱點(diǎn)與難點(diǎn)[1-5].彈性配準(zhǔn)算法為統(tǒng)計(jì)這些變化過程提供了技術(shù)支持,也是放射治療四維模型建立的基礎(chǔ)[6-7].目前,許多彈性配準(zhǔn)方法已經(jīng)被用于放射治療圖像中,如基于樣條函數(shù)的方法、光流法以及危及器官中植入標(biāo)記點(diǎn)的方法等[8-14].本研究嘗試應(yīng)用Vemuri等[15]提出的水平集理論,通過matlab軟件編程對(duì)放射治療分次間CT圖像實(shí)施三維全自動(dòng)彈性配準(zhǔn).

1 資料與方法

1.1 一般資料

隨機(jī)選取西南醫(yī)科大學(xué)附屬醫(yī)院腫瘤科10例宮頸癌放射治療患者不同分次時(shí)的兩組CT圖像,并與10例鼻咽癌放射治療患者不同分次時(shí)的兩組CT圖像進(jìn)行仿真試驗(yàn)驗(yàn)證.

1.2 方法

根據(jù)Vemuri等[15]在水平集進(jìn)化理論的基礎(chǔ)上提出的水平集移動(dòng)理論,假定有兩組圖像為M(ν)和S(ν),設(shè)ν=(x1,x2,x3)為空間3個(gè)方向,M(ν)是待配準(zhǔn)圖像灰度值,S(ν)是參考圖像灰度值,給定圖像彈性形變的位移場(chǎng)為u(ν),M(ν)與S(ν)之間就存在一個(gè)相應(yīng)的灰度映射關(guān)系,該關(guān)系可以通過兩組灰度圖像之間的水平集的映射來實(shí)現(xiàn),可以用一非線性的雙曲偏微分方程(partial differential equations PDE)表達(dá).因此,兩組圖像的彈性配準(zhǔn)就可以直觀的認(rèn)為是要將待配準(zhǔn)圖像的灰度函數(shù)的水平集變換成目標(biāo)參考圖像的灰度函數(shù)的水平集,即可以認(rèn)為將M(ν)的水平集沿著法線方向移動(dòng),直到其與參考圖像S(ν)保持一致.則兩組圖像的彈性形變的位移場(chǎng)為u(ν)可表達(dá)為公式1～3:

則為公式5:

式中σ=2.

1.3 彈性配準(zhǔn)步驟

⑴首先初始化位移場(chǎng),迭代次數(shù)n=0,u(ν)0=0.

⑵設(shè)n=n+1,根據(jù)公式5更新位移場(chǎng)u(ν)n+1,更新浮動(dòng)圖像為Mn(ν+u(ν)n+1).

⑶判斷迭代次數(shù)大于設(shè)定值,則轉(zhuǎn)步驟(4),否則執(zhí)行步驟(2).

⑷迭代終止,Mn(ν+u(ν)n+1)為最終的配準(zhǔn)圖像,配準(zhǔn)過程中采用多分辨策略.

2 結(jié)果

通過Matlab7.6軟件,根據(jù)上述水平集算法原理,編寫三維全自動(dòng)彈性配準(zhǔn)程序,并用此程序?qū)?0例宮頸癌放射治療患者不同分次時(shí)的兩組CT圖像與10例鼻咽癌放射治療患者不同分次時(shí)的兩組CT圖像進(jìn)行仿真試驗(yàn)驗(yàn)證.試驗(yàn)平臺(tái)為Intel Core i5-3210M CPU,2.5 GHz,4 G內(nèi)存.宮頸癌患者CT圖像斷層大小均為512X512,層距為2.5 mm,共采集64層;鼻咽癌患者CT圖像斷層大小均為512X512,層距為2.5 mm,共采集145層.試驗(yàn)參數(shù)見表1.

表1 試驗(yàn)評(píng)價(jià)參數(shù)

試驗(yàn)結(jié)果如圖1、圖2所示;宮頸癌患者配準(zhǔn)前后對(duì)比,均方誤差(mean squared error,MSE)減少55.1%,平均相關(guān)系數(shù)(correlation coefficient,CC)提高了5.3%;鼻咽癌患者配準(zhǔn)前后對(duì)比,平均MSE減少57.0%,平均CC提高4.6%.

3 討論

圖1 1例基于水平集的宮頸癌放射治療患者CT圖像彈性配準(zhǔn)冠狀面差分圖

圖2 1例基于水平集的鼻咽癌放射治療患者CT圖像彈性配準(zhǔn)冠狀面差分圖

在本研究中,基于根據(jù)Vemuri等在水平集進(jìn)化理論的基礎(chǔ)上提出的水平集移動(dòng)理論,該理論是一種迭代近似估計(jì)法,通過水平集移動(dòng)理論計(jì)算待配準(zhǔn)圖形的位移場(chǎng),通過matlab軟件對(duì)放射治療CT圖像進(jìn)行三維全自動(dòng)彈性配準(zhǔn)仿真實(shí)驗(yàn),從配準(zhǔn)前后差分圖的比較及表1兩種算法的評(píng)價(jià)參數(shù)可見,該理論取得了較好的試驗(yàn)效果,但要真正應(yīng)用到臨床放射治療中,仍顯精度不夠.

4 結(jié)語

為了更精確評(píng)估每例患者腫瘤及危及器官變化過程及其累加劑量,本研究期望彈性配準(zhǔn)的整個(gè)形變過程能更加準(zhǔn)確地模擬腫瘤器官的退縮與周圍正常組織的運(yùn)動(dòng).通過不同的數(shù)學(xué)方法模擬人體組織的運(yùn)動(dòng)是非常困難的事情,而如何獲得整個(gè)放射治療期間器官真實(shí)的累加劑量,可能需要更進(jìn)一步地改進(jìn)算法,并充分考慮效率問題,為個(gè)體化的自適應(yīng)放射治療提供更加精確的支持.

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Level set motion-based elastic registration of 3D CT images in radiotherapy

SHI Xiangxiang, TANG Tao, PANG Hao-wen

Objective: To apply level set motion-based elastic registration method to radiotherapy CT image, then to provide technical support for the accurate evaluation of tumor and changing process of patients with endanger organ and its accumulative dose. Methods: Based on Vemuri's level set motion method, we wrote the algorithm program using Matlab software and applied on two sets of 3D CT images from patients with cervical cancer and NPC for fully automatic elastic registration. Results: Comparing CT images before and after registration, for the cervical cancer patient, the minimum mean square error (MSE) decreased by 55.1% and correlation coefficient (CC) increased by 5.3%. For the NPC patient, MSE decreased by 32.1% and CC increased by 4.6%. Conclusion: From the image difference and evaluation parameters, the efficacy of level set motion-based elastic registration method was preliminarily demonstrated. In order to apply this method to clinical radiotherapy, dit needs to find a more accurate mathematical algorithm further in order to compute human anatomy deformation through image motion.

Level set motion; Elastic registration; Radiotherapy

1672-8270(2016)067-0020-03 [中圖分類號(hào)] R814.42

A

10.3969/J.ISSN.1672-8270.2016.07.007

①西南醫(yī)科大學(xué)附屬醫(yī)院腫瘤科 四川 瀘州 64600

279165416@qq.com

2016-01-08