本文選題：錐形束CT + 幾何校正　； 參考：《南方醫(yī)科大學(xué)》2017年碩士論文

【摘要】：X射線計(jì)算機(jī)斷層成像技術(shù)(Computed Tomography,CT)是一種利用物體不同角度的射線投影來(lái)重建物體內(nèi)部組織結(jié)構(gòu)的成像技術(shù)。其中,錐形束CT(Cone-beamCT,CBCT)相比傳統(tǒng)的二維CT(螺旋CT),具有掃描速度快、射線利用率高、空間分辨率高、重建三維圖像等優(yōu)點(diǎn)。CBCT利用平板探測(cè)器采集被檢測(cè)物體在不同角度的X射線投影數(shù)據(jù),采用重建算法重構(gòu)出被檢測(cè)物體的三維斷層圖像。在重建算法中,機(jī)械幾何結(jié)構(gòu)需要滿足:第一,射線源中心射線應(yīng)經(jīng)過(guò)旋轉(zhuǎn)軸并垂直于平板探測(cè)器的中心;第二,旋轉(zhuǎn)中心軸與探測(cè)器的列方向平行。但是,實(shí)際情況中,由于機(jī)械設(shè)計(jì)和安裝水平等原因,CBCT的幾何結(jié)構(gòu)與重建算法要求的理想幾何結(jié)構(gòu)之間存在一定程度的偏移,導(dǎo)致重建圖像中產(chǎn)生幾何偽影。幾何偽影嚴(yán)重影響圖像的細(xì)節(jié)和醫(yī)生對(duì)病人病情的診斷。針對(duì)CBCT圖像中的幾何偽影問(wèn)題,本文以優(yōu)化重建圖像質(zhì)量為目標(biāo),進(jìn)行以下三方面的研究:第一,提出一種幾何校正效果的評(píng)價(jià)模體和方法。評(píng)價(jià)模體由低密度正方體和高密度球體組成。首先通過(guò)CT掃描模體獲取三維圖像并通過(guò)Hough變換計(jì)算出中心層圖像高低密度的邊界,然后將高低密度區(qū)域CT平均值的差值的絕對(duì)值作為評(píng)價(jià)幾何校正效果的指數(shù)。通過(guò)設(shè)置不同幾何參數(shù)的數(shù)字模體和真實(shí)模體實(shí)驗(yàn),結(jié)果表明提出的評(píng)價(jià)指數(shù)能夠客觀評(píng)價(jià)CT圖像的幾何校正效果,可為評(píng)價(jià)幾何校正效果提供統(tǒng)一的標(biāo)準(zhǔn)。第二,提出一種圖像質(zhì)量驅(qū)動(dòng)的單投影幾何校正方法。通過(guò)引入上文中的評(píng)價(jià)指數(shù)作為衡量幾何偽影嚴(yán)重程度的目標(biāo)函數(shù),通過(guò)迭代修正校正模體上標(biāo)記點(diǎn)的坐標(biāo),最終達(dá)到校正幾何參數(shù)和消除CBCT圖像幾何偽影的目的。為提高運(yùn)行效率,采用GPU對(duì)CBCT圖像重建部分進(jìn)行加速。通過(guò)仿真實(shí)驗(yàn)、bench-top CBCT、口腔CT數(shù)據(jù)實(shí)驗(yàn)結(jié)果表明,系統(tǒng)幾何參數(shù)的誤差由19.07%降到0.115%,消除了 CBCT圖像的幾何偽影。該方法還降低了校正模體加工誤差對(duì)校正精度的影響,大幅度減少校正模體制作成本。第三,提出一種基于簡(jiǎn)易模體的整體投影幾何校正方法。在圓軌道CBCT下,由機(jī)械運(yùn)動(dòng)和安裝調(diào)試引起的幾何誤差,雖與真實(shí)值有一定的偏差但每個(gè)角度下投影的幾何參數(shù)比較相近。該方法通過(guò)采集簡(jiǎn)易模體的整體投影,解析計(jì)算系統(tǒng)的幾何參數(shù),并以評(píng)價(jià)指數(shù)為目標(biāo)函數(shù),對(duì)幾何參數(shù)進(jìn)一步迭代修正。實(shí)驗(yàn)結(jié)果表明,系統(tǒng)幾何參數(shù)的誤差由2.2%降到0.105%,該方法進(jìn)一步提高了現(xiàn)有解析幾何校正方法的校正精度,改善重建圖像的質(zhì)量。本文針對(duì)CBCT幾何偽影問(wèn)題,提出了一種評(píng)價(jià)幾何校正效果的標(biāo)準(zhǔn),并以優(yōu)化重建圖像質(zhì)量為目標(biāo)提出了基于單張投影和整體投影的CBCT幾何校正方法,效果明顯,實(shí)用價(jià)值突出。
[Abstract]:Computed Tomography (CTT) is an imaging technique for reconstruction of the internal structure of an object by means of ray projection at different angles. The Cone-Beam CTT (CBCTT) of cone-beam has higher scanning speed, higher ray-utilization ratio and higher spatial resolution than the conventional 2D CTT (helical CTT). CBCT uses flat panel detector to collect X-ray projection data of the detected object at different angles, and uses the reconstruction algorithm to reconstruct the 3D tomography image of the detected object. In the reconstruction algorithm, the mechanical geometry should be satisfied: first, the central ray of the ray source should pass through the axis of rotation and perpendicular to the center of the plate detector; second, the axis of the center of rotation should be parallel to the column direction of the detector. However, in practice, due to mechanical design and installation level, there is a certain degree of deviation between the geometric structure of CBCT and the ideal geometric structure required by the reconstruction algorithm, resulting in geometric artifacts in the reconstructed image. Geometric artifacts seriously affect the details of the image and the doctor's diagnosis of the patient's condition. Aiming at the problem of geometric artifacts in CBCT images, this paper aims at optimizing the quality of reconstructed images, and studies the following three aspects: first, a geometric correction evaluation motif and method are proposed. The evaluation motif consists of a low density cube and a high density sphere. Firstly, 3D images are obtained by CT scanning motifs and the boundary of high and low density of center layer images is calculated by Hough transform. Then, the absolute value of the difference of CT mean values in high and low density regions is taken as the index to evaluate the geometric correction effect. Through digital and real motifs experiments with different geometric parameters, the results show that the proposed evaluation index can objectively evaluate the geometric correction effect of CT images and provide a unified standard for evaluating geometric correction effects. Secondly, a single projection geometric correction method driven by image quality is proposed. By introducing the evaluation index mentioned above as the objective function to measure the severity of geometric artifacts, and by iteratively modifying the coordinates of the labeled points on the correction motifs, the purpose of correcting geometric parameters and eliminating geometric artifacts of CBCT images is finally achieved. In order to improve the running efficiency, GPU is used to accelerate the reconstruction of CBCT image. The experimental results of oral CT data show that the error of geometric parameters of the system is reduced from 19.07% to 0.115, and the geometric artifacts of CBCT images are eliminated. The method also reduces the effect of machining error on the correction accuracy and greatly reduces the cost of the calibration mode system. Thirdly, a global projection geometric correction method based on simple motifs is proposed. The geometric error caused by mechanical motion and installation and debugging in circular track CBCT is different from the real value, but the geometric parameters projected at each angle are close to each other. By collecting the global projection of the simple motifs, the geometric parameters of the system are analyzed, and the evaluation index is taken as the objective function to further iterate and modify the geometric parameters. The experimental results show that the error of geometric parameters of the system is reduced from 2.2% to 0.105. This method can further improve the correction accuracy of the existing analytical geometry correction methods and improve the quality of reconstructed images. In this paper, aiming at the problem of CBCT geometric artifact, a standard for evaluating geometric correction effect is put forward. Aiming at optimizing the quality of reconstructed image, a CBCT geometric correction method based on single projection and global projection is proposed. The effect is obvious and the practical value is outstanding.
【學(xué)位授予單位】：南方醫(yī)科大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TP391.41;O434.1

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