本文選題：超聲成像 + 自適應(yīng)波束形成��； 參考：《華中科技大學(xué)》2014年博士論文

【摘要】：作為四大醫(yī)學(xué)影像技術(shù)之一,醫(yī)學(xué)超聲具有安全、便攜、實(shí)時(shí)、低成本等優(yōu)點(diǎn),在臨床診斷中已得到了廣泛應(yīng)用。在超聲成像系統(tǒng)中,波束形成處于核心位置,對(duì)成像質(zhì)量起著決定性作用。其中,延時(shí)疊加(delay-and-sum, DAS)波束形成技術(shù)能簡(jiǎn)單而有效地實(shí)現(xiàn)超聲圖像的重建,已在商業(yè)產(chǎn)品中得到廣泛的應(yīng)用。但是,其所形成的波束主瓣寬度較寬、旁瓣水平較高,降低了圖像空間分辨率和對(duì)比度。傳統(tǒng)的變跡技術(shù)使用一組預(yù)先確定的權(quán)重(如漢明窗)來降低旁瓣干擾,但會(huì)增加主瓣寬度,犧牲了圖像分辨率。自適應(yīng)波束形成技術(shù)通過從接收信號(hào)中提取特征信息來確定孔徑權(quán)重,在成像區(qū)域中形成更窄主瓣寬度和更低旁瓣級(jí)的高質(zhì)量波束,能夠同時(shí)增強(qiáng)分辨率和對(duì)比度。本論文以相干因子(coherence-based factor)自適應(yīng)加權(quán)和最小方差(minimum variance, MV)波束形成為主要研究對(duì)象,分析它們?cè)诔暢上裰械膬?yōu)點(diǎn)和不足,并提出了相應(yīng)的改進(jìn)方法,具體包括： 首先,幅度相干因子法(the coherence factor, CF)和相位/符號(hào)相干因子(thephase/sign coherence factor, PCF/SCF)法能以較低的計(jì)算復(fù)雜度顯著抑制旁/柵瓣,減少雜波,但是會(huì)降低圖像平均亮度、增大斑點(diǎn)方差甚至損失部分紋理信息。本論文使用復(fù)解析孔徑數(shù)據(jù),設(shè)計(jì)了一種基于相量離散度的相干因子,通過對(duì)相干波束和的加權(quán),抑制旁瓣和雜波,減少離軸干擾信號(hào)和隨機(jī)噪聲。仿真和真實(shí)體模數(shù)據(jù)的實(shí)驗(yàn)結(jié)果表明：相比于DAS,該方法能同時(shí)提高分辨率和對(duì)比度。但是,它也會(huì)產(chǎn)生斑點(diǎn)紋理上的負(fù)面效果。為了解決該問題,我們通過一個(gè)用戶定義的參數(shù)α來調(diào)節(jié)離軸干擾抑制和圖像紋理信息之間的平衡。結(jié)果表明,一個(gè)0.9至1之間、稍小于1的α值能獲得最佳的成像結(jié)果。 其次,MV波束形成方法能顯著提高空間分辨率,但是對(duì)旁瓣和對(duì)比度的改善尚顯不足。為此,本文提出將MV波束形成與相位相干成像相結(jié)合的改進(jìn)方法。實(shí)驗(yàn)表明,該方法在保持高分辨率的同時(shí),減少了旁瓣和雜波,但會(huì)造成圖像亮度降低、斑點(diǎn)方差增大等問題。 然后,詳細(xì)指出了使用相干因子波束形成方法(如CF、PCF/SCF)重建圖像所存在的缺陷,包括：整體圖像亮度降低,斑點(diǎn)方差增大,高回聲反射體周圍出現(xiàn)黑洞偽影以及遠(yuǎn)場(chǎng)點(diǎn)目標(biāo)亮度被過低估計(jì)等。為解決這些問題,本論文從理論上分析了它們產(chǎn)生的原因,進(jìn)而提出一種新的空時(shí)平滑處理技術(shù)來修正相干因子類方法。由此所設(shè)計(jì)的空時(shí)平滑相干因子在一個(gè)發(fā)射脈沖的持續(xù)時(shí)間上測(cè)量所劃分重疊子陣列的波束和之間的信號(hào)相干性。仿真和真實(shí)體模數(shù)據(jù)的實(shí)驗(yàn)結(jié)果表明,該方法能明顯提高相干因子成像的魯棒性,減少斑點(diǎn)方差,消除黑洞偽影,具有較強(qiáng)的雜波抑制能力,從而提高圖像對(duì)比度,增強(qiáng)了圖像中囊腫的可檢測(cè)性。 最后,為了進(jìn)一步改善空時(shí)平滑相干因子方法的成像質(zhì)量,提出了三種改進(jìn)方案,即對(duì)該相干因子進(jìn)行空間濾波、將其與空間復(fù)合技術(shù)相結(jié)合以及與最小方差波束形成相結(jié)合。仿真實(shí)驗(yàn)結(jié)果表明：空時(shí)平滑相干因子經(jīng)過空間濾波后能減少由于因子值的波動(dòng)引起的斑點(diǎn)亮度變化,消除黑洞偽影,進(jìn)一步增強(qiáng)圖像對(duì)比度；與空間復(fù)合的結(jié)合能更好地減少斑點(diǎn)噪聲和雜波,使背景更加均勻,顯著提高對(duì)比度；與MV波束形成相結(jié)合,可同時(shí)增強(qiáng)空間分辨率和對(duì)比度,并且較好地保持了背景的紋理信息,獲得整體圖像質(zhì)量的提升。 綜上所述,本論文通過對(duì)相干因子和MV波束形成方法的改進(jìn),彌補(bǔ)了它們?cè)诔暢上裰械牟蛔?提高了其成像性能,使之更適合臨床應(yīng)用。尤其是,空時(shí)平滑相干因子自適應(yīng)加權(quán)方法的提出,為提高超聲成像質(zhì)量、增強(qiáng)超聲系統(tǒng)的檢測(cè)診斷能力提供了有益的嘗試。
[Abstract]:As one of the four major medical imaging techniques, medical ultrasound has the advantages of safety, portability, real time, low cost and so on. It has been widely used in clinical diagnosis. In the ultrasonic imaging system, the beam forming is at the core position and plays a decisive role in the imaging quality. Among them, the delay-and-sum, DAS beam forming technology can be simple. The effective realization of the reconstruction of ultrasonic images has been widely used in commercial products. However, the width of the main lobe of the beam is wide, the side lobe level is high, and the spatial resolution and contrast of the image are reduced. The traditional technique of tracking uses a set of predetermined weights (such as Han Mingchuang) to reduce the sidelobe interference, but it will increase the main point. The adaptive beamforming technology determines the aperture weight by extracting feature information from the received signal to form a higher quality beam with narrower main lobe width and lower sidelobe level in the imaging region, which can enhance the resolution and contrast at the same time. This paper uses the coherent factor (coherence-based factor) self adaptation in this paper. Minimum variance (MV) beamforming is the main research object, and the advantages and disadvantages of them in ultrasonic imaging are analyzed, and the corresponding improvement methods are put forward, which include:
First, the the coherence factor (CF) and the phase / symbol coherent factor (thephase/sign coherence factor, PCF/SCF) can significantly reduce the side / gate flap with a lower computational complexity, reduce the clutter, but reduce the average image brightness, increase the spot variance and even lose some texture information. This paper uses complex analysis. In aperture data, a coherent factor based on phasor dispersion is designed. By weighting the coherent beam and reducing the sidelobe and clutter, the off axis interference signal and random noise are reduced. The experimental results of the simulation and real model data show that the method can increase the resolution and contrast at the same time compared to the DAS. However, it also produces spots. In order to solve this problem, we use a user defined parameter alpha to adjust the balance between off axis interference suppression and image texture information. The results show that an alpha value of less than 1 between 0.9 and 1 can obtain the best imaging results.
Secondly, the MV beamforming method can significantly improve the spatial resolution, but the improvement of the sidelobe and contrast is still inadequate. Therefore, this paper proposes an improved method combining the MV beam forming with phase coherent imaging. The experiment shows that the method reduces the sidelobe and clutter while maintaining high resolution, but reduces the brightness of the image, The enlargement of the speckle variance.
Then, the defects in the image reconstruction using the coherent factor beamforming (such as CF, PCF/SCF) are described in detail, including the reduction of the brightness of the whole image, the increase of the speckle variance, the black hole artifact around the high echo reflector and the low estimation of the brightness of the far point target. In order to solve these problems, the paper has theoretically analyzed the problem A new space time smoothing technique is proposed to modify the coherent factor class method. The designed space-time smooth coherence factor is used to measure the beam and signal coherence between the overlapped subarrays on the duration of a emission pulse. This method can obviously improve the robustness of coherent factor imaging, reduce the speckle variance, eliminate the black hole artifact, and have strong clutter suppression ability, thus improving the contrast of images and enhancing the detectability of the cysts in the image.
Finally, in order to further improve the imaging quality of the space-time smoothing coherent factor method, three improvements are proposed, namely, the spatial filtering of the coherent factor, combining it with the spatial composite technology and combining with the minimum variance beamforming. The simulation results show that the space time smoothing coherent factor can be reduced after spatial filtering. Less speckle brightness changes caused by the fluctuation of the factor value, eliminate the black hole artifact and further enhance the contrast degree of the image. The combination with the space compound can better reduce the speckle noise and clutter, make the background more uniform and improve the contrast significantly; combined with the MV beam formation, the spatial resolution and contrast can be enhanced at the same time, and better, and better The background texture information is maintained and the overall image quality is improved.
To sum up, by improving the coherent factors and MV beamforming methods, this paper makes up their shortcomings in ultrasonic imaging, improves their imaging performance and makes it more suitable for clinical applications. Especially, the adaptive weighting method of space-time smooth coherence factor is proposed to improve the quality of ultrasound imaging and enhance the detection and diagnosis of ultrasonic systems. Ability provides a useful attempt.
【學(xué)位授予單位】：華中科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2014
【分類號(hào)】：R445.1

【參考文獻(xiàn)】

中國(guó)博士學(xué)位論文全文數(shù)據(jù)庫(kù) 前1條

1 李雅琴;非衍射超聲聲源的仿真與實(shí)現(xiàn)研究[D];華中科技大學(xué);2012年

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本文編號(hào)：1946781