本文選題：低劑量CT + 先驗圖像��； 參考：《南方醫(yī)科大學(xué)》2014年博士論文

【摘要】：X射線計算機(jī)斷層成像(CT)以其高的時間分辨率、空間分辨率及對比度分辨率成為現(xiàn)代影像學(xué)的杰出代表,在臨床診斷和治療中廣泛使用。但過量照射X光可誘發(fā)癌癥、白血病或其他遺傳性疾病,因此CT的劑量安全問題成為業(yè)界關(guān)注的焦點(diǎn)。如何以最小的代價和最低的X射線劑量獲得最佳的CT圖像質(zhì)量,已經(jīng)成為臨床最為迫切的需求。 降低X射線劑量的方法有多種,如通過降低管電流和縮短曝光時間來降低X射線球管的毫安秒(mAs)或者減少掃描角向采樣數(shù)目等直接性的措施來降低X謝線劑量。但與此同時,上述掃描模式下解析重建的圖像質(zhì)量將嚴(yán)重退化,圖像中的噪聲和偽影影響了臨床診斷和應(yīng)用。近年來,針對低劑量CT圖像優(yōu)質(zhì)重建的研究方興未艾,主要集中于對解析重建的低劑量CT圖像進(jìn)行后處理濾波、對低劑量CT投影數(shù)據(jù)進(jìn)行恢復(fù),以及包括代數(shù)迭代和統(tǒng)計迭代的迭代重建算法。其中,針對迭代重建算法的研究尤其熱烈。相對于解析重建算法,迭代重建算法在CT固有的物理系統(tǒng)建模方面具有較大的優(yōu)勢,如可精確模擬系統(tǒng)成像幾何,X線多能光譜、線束硬化、散射、噪聲等。因此,迭代重建算法能保持或者提高圖像空間分辨力的同時抑制偽影和噪聲。進(jìn)一步的,統(tǒng)計迭代重建算法能基于光子統(tǒng)計學(xué)建立多種更精確的噪聲模型,在降低圖像噪聲方面有更佳的表現(xiàn)。常規(guī)的,在迭代重建過程中,準(zhǔn)確建模投影數(shù)據(jù)的測量方程是得到優(yōu)質(zhì)重建的前提和基礎(chǔ),而先驗知識的合理利用對于求解的穩(wěn)定性及獲得高精度重建圖像具有非常重要的意義。 目前,在疾病的臨床診斷和治療過程中,常用到反復(fù)的CT掃描,如在灌注CT成像、4D-CT成像、CT圖像引導(dǎo)的活體組織穿刺檢查以及圖像引導(dǎo)放療等過程中。以圖像引導(dǎo)放療為例,除計劃階段CT掃描外,在整個放射治療階段,在分次治療前需對患者進(jìn)行CBCT掃描來進(jìn)行定位,在此情況下,患者所接受的累加輻射劑量較常規(guī)CT檢查掃描將會很高,反復(fù)CT掃描增加了罹患腫瘤的風(fēng)險。在上述反復(fù)CT掃描過程中,對于同一個病人,先后兩次CT掃描的圖像之間除了幾何位置和某些不自主運(yùn)動引起的不同外,大部分解剖結(jié)構(gòu)是相同的。換句話說,兩次掃描獲取的CT圖像之間存在大量的結(jié)構(gòu)冗余信息。傳統(tǒng)的針對低劑量CT的研究方法僅僅引入圖像自身的局部鄰域的約束作為先驗信息,未能考慮同一病人先前掃描獲取的圖像所能提供的先驗信息來導(dǎo)引當(dāng)前CT的圖像的重建。鑒于此,本論文針對基于先前圖像的先驗信息的合理引入、先驗信息引入后算法的優(yōu)化等問題進(jìn)行了深入的研究,同時本文對基于平板探測器的CBCT(Cone BeamCT)成像系統(tǒng)的噪聲特性進(jìn)行了進(jìn)一步實(shí)驗分析和驗證,為后續(xù)迭代重建建立了理論和系統(tǒng)模型基礎(chǔ)。歸納起來,本文的主要工作有： 1)為合理的引入先驗圖像信息,同時突破傳統(tǒng)先驗單純依賴于目標(biāo)圖像局部鄰域內(nèi)的像素灰度信息的約束,本文利用非局部的思想通過在固定大小的搜索窗內(nèi)檢測基于圖像塊匹配的像素相似性而形成正則化項來實(shí)現(xiàn)基于先驗圖像的先驗信息的有效引入。新方法在能夠引入先驗圖像信息的同時弱化了對當(dāng)前待重建目標(biāo)圖像和先驗圖像間配準(zhǔn)精度的要求。數(shù)值仿真,物理體模以及臨床數(shù)據(jù)實(shí)驗表明,新方法能夠提高當(dāng)前低劑量圖像的重建質(zhì)量,同時不引入偽結(jié)構(gòu)信息。 2)針對腦灌注CT成像的兩大特點(diǎn)：(1)灌注掃描前的平掃描標(biāo)準(zhǔn)劑量圖像的分辨率高且噪聲低,可以為低劑量灌注序列增強(qiáng)圖像的重建提供極為豐富的形態(tài)結(jié)構(gòu)冗余信息；(2)對同一感興趣層面反復(fù)掃描所得的增強(qiáng)序列圖像除增強(qiáng)信息外,各圖像之間的相對形變量較��；本文提出一種標(biāo)準(zhǔn)劑量平掃描圖像導(dǎo)引的低劑量CT腦灌注序列圖像重建方法。新方法首先利用利用標(biāo)準(zhǔn)劑量平掃描圖像豐富的冗余信息實(shí)現(xiàn)基于改進(jìn)的非局部均值算法的低劑量增強(qiáng)掃描圖像的優(yōu)質(zhì)恢復(fù),然后對恢復(fù)后的圖像做去卷積迭代重建。數(shù)值仿真體模和病人數(shù)據(jù)實(shí)驗表明,該算法能有效抑制圖像中噪聲,提高圖像信噪比,從而使得相應(yīng)血流動力學(xué)參數(shù)的計算更加準(zhǔn)確。 3)為解決先驗圖像和當(dāng)前待重建圖像之間的結(jié)構(gòu)不匹配問題,本文提出了一種利用當(dāng)前測量所得投影數(shù)據(jù)與先驗圖像配準(zhǔn)的方法獲取與當(dāng)前待重建圖像相近或類似的先驗圖像用于基于先驗圖像的CBCT少角度重建。具體來說,本文算法是通過當(dāng)前測量投影數(shù)據(jù)與先驗圖像在當(dāng)前成像幾何下的前向投影數(shù)據(jù)的匹配來實(shí)現(xiàn)圖像域變形場的估計,然后將該變形場作用于先驗圖像來獲取配準(zhǔn)的先驗圖像,用于基于先驗圖像的重建策略中。特別的,本文采用的重建算法為PICCS(Prior Image Compress Constrained Sensing)算法。本文提出的算法能夠有效避免圖像空間配準(zhǔn)時FDK算法重建的圖像中噪聲和條形偽影對配準(zhǔn)精度的影響。XCAT仿真實(shí)驗以及臨床數(shù)據(jù)實(shí)驗結(jié)果表明,本文提出的方案能夠獲取優(yōu)質(zhì)先驗圖像,重建結(jié)果優(yōu)于傳統(tǒng)PICCS算法。 4)鑒于CBCT成像系統(tǒng)中平板探測器與常規(guī)扇形束CT探測器物理設(shè)計上的不同,本文實(shí)驗性的針對瓦里安TrueBeam系統(tǒng)中的平板探測器采集信號進(jìn)行噪聲相關(guān)性研究。通過對固定角度下反復(fù)測量的的探測數(shù)據(jù)的分析得出投影數(shù)據(jù)在探測器單元之間存在噪聲相關(guān)性。實(shí)驗結(jié)果表明,二維探測器八鄰域內(nèi)的噪聲相關(guān)性是明顯可見的,且一階鄰域內(nèi)的相關(guān)系數(shù)明顯高于二階鄰域內(nèi)相關(guān)系數(shù)。同時,該噪聲相關(guān)性與掃描所用X線劑量無關(guān)。本文將該噪聲相關(guān)性應(yīng)用于基于投影數(shù)據(jù)噪聲模型的懲罰加權(quán)最小二乘恢復(fù)算法中,重建圖像結(jié)果表明,該噪聲相關(guān)性的引入能夠提高重建圖像質(zhì)量。該實(shí)驗研究為CBCT圖像重建提供了更精準(zhǔn)的系統(tǒng)建模。
[Abstract]:X - ray computed tomography (CT) with its high temporal resolution, spatial resolution and contrast resolution has become an outstanding representative of modern imaging, widely used in clinical diagnosis and treatment. But excessive irradiation of X light can cause cancer, leukemia or other genetic diseases, so dose CT security issues become the focus of attention of the industry. How to minimize the cost and the lowest CT X ray dose obtained the best image quality, has become the most urgent demand for clinical.
Method of reducing the X radiation dose has a variety of, such as by reducing the tube current and shorten the exposure time to reduce the X ray tube milliamperemeter (mAs) or reduce the scan angle to the number of sampling direct measures to reduce X Xie line dose. But at the same time, the image quality of the scanning mode of analytical reconstruction will be severely degraded that image noise and artifacts affect clinical diagnosis and application. In recent years, the study of low dose CT image quality reconstruction is just unfolding focuses on image analysis, the reconstruction of low dose CT postprocessing filter for low-dose CT projection data recovery, and iterative reconstruction algorithms including algebraic and statistical iteration. Among them, research on the iterative reconstruction algorithm is particularly warm. Compared with the analytical reconstruction algorithm, iterative reconstruction algorithm has more advantage in the modeling of physical systems inherent in the CT, such as fine Indeed the imaging geometry simulation system, X-ray energy spectrum, beam hardening, scattering, noise and so on. Therefore, while suppressing noise artifacts and iterative reconstruction algorithm can maintain or improve spatial resolution of the image. Further, statistical iterative reconstruction algorithm can establish a more accurate noise model based on photon statistics, have better performance to reduce the image noise. The conventional iterative reconstruction, in the process, the measurement equation of projection data is obtained accurately modeling the premise and foundation of high quality reconstruction, and the rational use of prior knowledge is very important for solving the stability and achieve high precision of image reconstruction.
At present, the clinical diagnosis and treatment of disease, commonly used to repeated CT scan, such as perfusion CT imaging, 4D-CT imaging, biopsy examination and image guided radiotherapy CT image guidance. In image guided radiotherapy as an example, in addition to planning CT scan, treatment stage in the whole radiation. In the time before treatment for patients with a CBCT scan to locate, in this case, the cumulative radiation dose received by the patients compared with the conventional CT scans will be high, repeated CT scanning and raises the risk of cancer. In the repeated CT scanning process, for the same patient, has between two CT scan images in addition to different geometry and some involuntary movements caused by the most anatomical structure is the same. In other words, there is a large amount of redundant information structure between CT image two scans obtained for traditional. Study on the method of low dose CT only into the local neighborhood of the image itself as constraint of prior information, failed to consider the image of the same patient previously scanned can provide information to guide the current CT image reconstruction. In view of this, this paper introduced the previous image based on prior information, in-depth study on the optimization of a priori information after the introduction of this paper on the flat panel detector based on CBCT (Cone BeamCT) the noise characteristics of the imaging system for further analysis and experimental verification, based on model theory and system for the subsequent iterative reconstruction. To sum up, the main works of this paper are:
1) for the a priori information of the image, pixel information and break through the traditional a priori simply rely on the local neighborhood within the constraints of the target image, using non local thought through the detection of pixel image block matching based on the similarity and the formation of a regularization term to achieve effective introduction of the prior image based on prior information in the search window fixed size. The new method weakens on the reconstructed object image and prior image registration accuracy requirements can be introduced in the prior image information at the same time. The numerical simulation shows that, the physical phantom and clinical data of the experiment, the new method can improve the image quality of low dose, while not introducing pseudo structural information.
2) according to the two characteristics of CT perfusion imaging: (1) high dose image scan standard perfusion scanning before the resolution and low noise, can provide abundant information for the redundant structure of low dose perfusion sequence to enhance image reconstruction; (2) to enhance the sense of the same image sequence in addition to enhance information the outer layer of interest face repeatedly scanned, relative deformation is smaller between each image; this paper presents a scanning image guided flat standard dose of low dose CT cerebral perfusion image sequence reconstruction method. The new method first use standard dose scan image redundant information rich can enhance the quality of low dose improved scanning image restoration the non local means algorithm based on image restoration, and then after the deconvolution iterative reconstruction. Numerical simulation shows that the phantom and patient data experiments, this algorithm can effectively suppress the noise,. The high image signal to noise ratio (SNR) makes the calculation of the corresponding hemodynamic parameters more accurate.
3) to solve the prior image and the current image to be reconstructed structure mismatch between the problem, this paper proposes a method of using the measured projection data and prior image acquisition and image registration prior the image to be reconstructed or similar to the prior image CBCT based on small angle reconstruction. Specifically, the algorithm is through the estimation of the current measurement of projection data and prior image in the image geometry prior to projection data, to realize the image deformation, and the deformation field to the prior image to obtain the prior registration for image based on prior image reconstruction strategy. In particular, the reconstruction algorithm used in this paper is PICCS (Prior Image Compress Constrained Sensing) algorithm. This algorithm can effectively avoid image space registration FDK reconstruction algorithm in noise and strip The influence of artifact on registration accuracy is demonstrated by.XCAT simulation experiment and clinical data experiment. The results show that the proposed scheme can get high-quality prior images, and the reconstruction result is better than the traditional PICCS algorithm.
4) in view of the flat panel detector CBCT imaging system and the conventional fan beam CT detector on different physical design, this paper studies the noise correlation experimental system for Varian TrueBeam flat panel detector signal acquisition. Through repeated measurements of the fixed angle of the detection data analysis shows the presence of noise in the projection data correlation between detector unit. The experimental results show that the noise correlation in the neighborhood of the two-dimensional detector eight is visible, and the correlation coefficient of first order in the neighborhood was higher than the two order neighborhood correlation coefficient. At the same time, the noise correlation with scanning X-ray dose related. The noise correlation applied to the penalized weighted least-squares projection data based on the noise model. The reconstruction of image restoration algorithms, the results show that the introduction of the noise correlation can improve the quality of image reconstruction. The experimental research The study provides a more accurate system modeling for CBCT image reconstruction.

【學(xué)位授予單位】：南方醫(yī)科大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2014
【分類號】：R814.2

【參考文獻(xiàn)】

相關(guān)期刊論文 前1條

1 M.Fujita;K.Kitagawa;T.Ito;Y.Shiraishi;Y.Kurobe;M.Nagata;唐光健;;降低動態(tài)CT負(fù)荷心肌灌注成像的輻射劑量:80kV/370mAs與100kV/300mAs掃描的對照研究[J];國際醫(yī)學(xué)放射學(xué)雜志;2014年03期

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本文編號：1762868