Process Algorithms for Resolution Improvement and Contrast Enhancement in Optical Coherence Tomography

We proposed two process algorithms for resolution improvement and contrast enhancement in the images of optical coherence tomography (OCT). An OCT system with a non-Gaussian light source spectrum or dispersion mismatch usually results in sidelobes in the interference fringe envelope that may produce artifacts and reduce image contrast. Based on the concept of deconvolution, we proposed two different process algorithms and demonstrated their effectiveness in retrieving sample structures. The effects of the process algorithms were examined by numerical simulations and real OCT scanning images. After processing with the proposed procedures, the effects of sidelobes were tremendously suppressed and the image qualities were improved.     

Spectral Filtering in OCT System Implemented by Mechanically Induced Long-Period Fiber Grating

A spectral filtering method that can enhance the axial resolution and suppress sidelobes of the point spread function in Optical Coherence Tomography (OCT) system is demonstrated. A mechanically induced long-period fiber grating (MLPG) is used to tailor the spectral shape of a homemade Er-doped fiber amplified spontaneous emission (ASE) source. By using a MLPG, the high and narrow characteristic peak, centered at 1533 nm, of the ASE source was fairly well removed, which enabled us to use a broader spectral bandwidth of the source. The experimental results show that the proposed spectral filtering method can suppress the sidelobes in the axial point spread function up to 5.4 dB and thus enhance the axial resolution of an OCT system by four times. The OCT images of an extracted human premolar taken with and without the spectral filtering are presented and compared with each other.     

Enhanced resolution for EPR imaging by two-step deblurring

The broad spectrum of spin probes used for electron paramagnetic resonance imaging (EPRI) result in poor spatial resolution of the reconstructed images. Conventional deconvolution procedures can enhance the resolution to some extent but obtaining high resolution EPR images is still a challenge. In this work, we have implemented and analyzed the performance of a postacquisition deblurring technique to enhance the spatial resolution of the EPR images. The technique consists of two steps; noniterative deconvolution followed by iterative deconvolution of the acquired projections which are then projected back using filtered backprojection (FBP) to reconstruct a high resolution image. Further, we have proposed an analogous technique for iterative reconstruction algorithms such as multiplicative simultaneous iterative reconstruction technique (MSIRT) which can be a method of choice for many applications. The performance of the suggested deblurring approach is evaluated using computer simulations and EPRI experiments. Results suggest that the proposed procedure is superior to the standard FBP and standard iterative reconstruction algorithms in terms of mean-square-error (MSE), spatial resolution, and visual judgment. Although the procedure is described for 2D imaging, it can be readily extended to 3D imaging.     

基于子谱分析的白光频域OCT

为了提高频域OCT系统轴向分辨率和信噪比,提出了一种采用白光光源的频域OCT系统,研究了所用的CCD相机的横向像素数、白光光源的光谱与系统轴向分辨率之间的关系,建立了一种基于子谱分析的白光频域OCT系统。通过选择所需的白光光源及其接收频段,既能保证最大探测深度和高轴向分辨率,又能提高信噪比。实验结果验证了所提出的光源的相干长度和系统及图像轴向分辨率测量方法,证明了白光频域OCT的可行性。     

Real-time implementation of coherent speckle suppression in B-scan images

Early experiences of new forms of adaptive filtering for ultrasound speckle reduction and parametric imaging, using off-line conventional digital processing, have been sufficiently encouraging to warrant examining the feasibility of implementing specific algorithms in real-time. A hardware two-dimensional real-time filter is described which consists of a hybrid digital/analogues system in which the video signal from any scanner is sampled to 256 points per line and passed sequentially through a series of shift registers, in order to derive a 5 x 5 window of values which surrounds the image point currently being processed. These 25 video signals are then used as inputs to an analogue processor, which provides the filtered output. The real-time processed images show clear evidence of speckle smoothing without blurring of tissue structural information but possess limited pixel resolution.     

A combined multiple-SLED broadband light source at 1300 nm for high resolution optical coherence tomography

We demonstrate a compact, inexpensive, and reliable fiber-coupled light source with broad bandwidth and sufficient power at 1300 nm for high resolution optical coherence tomography (OCT) imaging in real-time applications. By combining four superluminescent diodes (SLEDs) with different central wavelengths, the light source has a bandwidth of 145 nm centered at 1325 nm with over 10 mW of power. OCT images of an excised stage 30 embryonic chick heart acquired with our combined SLED light source (<5 μm axial resolution in tissue) are compared with images obtained with a single SLED source (∼10 μm axial resolution in tissue). The high resolution OCT system with the combined SLED light source provides better image quality (smaller speckle noise) and a greater ability to observe fine structures in the embryonic heart.     

Imaging of subcutaneous microcirculation vascular network by double correlation Optical Coherence Tomography

Spatial distribution of superficial blood vessels in human skin in vivo has been observed by using the double correlation Optical Coherence Tomography (OCT). To remove background noise, reduce the artifacts associated with patient motions and to increase the overall quality of the experimental OCT images an adaptive Wiener filtering technique has been employed. Fourier domain correlation has been subsequently applied to enhance spatial resolution of images of vascular network in human skin in vivo. Image processing has been performed on Graphics Processing Units (GPUs) utilizing Compute Unified Device Architecture (CUDA) framework in the frequency‐domain. This approach allows carrying out image processing in parallel significantly speeding up the computations. The presented results show that the double correlation method permits obtaining 2D/3D OCT images of subcutaneous microcirculation vascular network and its spatial distribution within the human skin with higher spatial resolution compare to the other OCT correlation‐based techniques developed earlier.     

基于偏振滤波图像增强和动态散斑照明的宽场荧光层析显微镜

提出了一种基于偏振滤波图像增强和动态散斑照明的新型宽场荧光层析显微镜. 该显微镜采用了一种新型的偏振滤波图像增强技术, 基于激发光与荧光偏振态的差异, 利用偏振器件滤除激发光; 并利用动态散斑照明实现宽场层析. 该荧光层析显微镜具有结构简单、低成本、响应速度快、容易操作等特点. 实验研究结果表明, 本文提出的滤波方案能够显著地提高图像质量, 利用动态散斑照明实现宽场层析具有较高的纵向分辨能力. 研究丰富了在荧光显微镜中, 从强激发光中提取弱荧光信号的技术手段, 为今后发展具有快速响应, 波长可调谐的多光谱荧光层析等高端的显微镜具有重要参考意义.     

Iterative sparse reconstruction of spectral domain OCT signal

We propose and study an iterative sparse reconstruction for Fourier domain optical coherence tomography （FD OCT） image by solving a constrained optimization problem that minimizes L-1 norm. Our method takes the spectral shape of the OCT light source into consideration in the iterative image reconstruction procedure that allows deconvolution of the axial point spread function from the blurred image during reconstruction rather than after reconstruction. By minimizing the L-1 norm, the axial resolution and the signal to noise ratio of image can both be enhanced. The effectiveness of our method is validated using numerical simulation and experiment.     

利用变迹术和相干门相结合实现光学相干层析成像术轴向超分辨

光学相十层析成像（光学相干层析成像术）的轴向分辨力由光源带宽和探测光束的聚焦条件共同决定。提高光学相干层析成像术轴向分辨力的方法主要基于带宽光源技术。提出了一种将变迹术与光学相十层析成像术相干门有机结合的方法来提高其轴向分辨力。通过适当形式的光瞳滤波器．使光学相干层析成像术系统轴向响应的主瓣宽度缩小到相干门之内,而其旁瓣则处于相干门之外．不对相干成像产生有效贡献。这样．就能在光源带宽不变的条件下,有效提高光学相十层析成像术的轴向分辨力,避免了采用宽带光源所带来的费用昂贵和系统复杂等缺陷。     

基于受激辐射信号的谱域光学相干层析分子成像方法

鉴于不同生理病理状态下组织复折射率实部的变化不大,传统光学相干层析(OCT)成像技术在分子特异性识别上存在先天不足.为此,本文提出了基于受激辐射信号的OCT成像方法,可在实现传统散射成像的同时,实现基于受激辐射信号的分子成像.在超高分辨率谱域OCT系统的基础上,通过增设光谱分光与调制抽运光支路,建立了基于单宽谱光源的抽运探测谱域OCT系统,详细推导了调制抽运下受激辐射信号的获取与成像公式.利用搭建的抽运探测谱域OCT系统,实现了瞬态受激辐射信号的相干探测.基于同时获取的受激辐射OCT信号和传统OCT信号,成功重构了氮化物粉末构建样品的基于受激辐射信号的分子对比OCT图像.     

基于GPU的多帧盲解卷积图像复原技术并行化实现

多帧盲解卷积图像复原技术能够进一步提高自适应光学图像的分辨力,但其算法比较复杂,处理耗时过长,对序列图像复原经常需要几分钟甚至几十分钟的计算时间,对实际应用造成了极大不便。为了提升算法的运行速度,改善其耗时过长的问题,通过研究和分析盲解卷积算法原理和算法结构,采用目前高速发展的中央处理器（CPU）和图形处理器（GPU）异构加速技术,主要对耗时最长的矩阵卷积运算进行优化,通过使用库函数与算法结构微调相结合的方法并行加速,实现多帧盲解卷积的图像复原算法的并行化。使用并行算法对图像进行复原处理,针对16帧以上分辨率为256256像素的空间目标图像,可以实现17的加速比,为图像复原的实时/准实时提供一种可行的方案。     

基于微通道板的二维位置灵敏像探测器X射线成像研究

研制了一套基于微通道板的二维位置灵敏像探测器,开展了X射线成像实验研究。对游标卡尺构建的狭缝成像显示,狭缝图像清晰可见,图像的边缘分辨率能达到约500 μm。对组合样品进行了成像测试,在X射线机靶流为1 mA的条件下,像探测器系统在1 ms内即可实现成像,且成像结果清晰。采用中值滤波与锐化、盲反卷积、维纳滤波等方法,对游标卡尺的成像结果进行了初步的图像处理。结果显示,三种方法都能改善图像的质量,其中维纳滤波法效果最优,相比原图边缘分辨率提高了约7%。     

Comparison of SAFT and two-dimensional deconvolution methods for the improvement of resolution in ultrasonic B-scan images

This Paper compares the properties of the well known synthetic aperture focusing technique (SAFT) with those of two deconvolution methods, which have previously been less widely applied to digitized ultrasonic data. The linear deconvolution method of Wiener filtering and the non-linear maximum entropy method are applied to two-dimensional unrectified (RF) B-scan data, to improve the resolution simultaneously in the lateral and range directions. With SAFT, only an improvement in lateral resolution is possible. Data from linear scans of several unfocussed transducers over machined reflectors at different depths in steel were collected and used to quantify the resolution obtainable by the three processing methods. Wiener filtering is shown to give higher lateral resolution than SAFT processing, by factors of between 1.3 and 2, depending on the depth of the reflector. Maximum entropy gives even higher apparent lateral resolution, but has a tendency to underestimate reflector size. The improvement in range resolution obtainable with Wiener filtering was approximately a factor of 1.5, whereas maximum entropy gave corresponding factors of between 1.7 and 5. For far-field reflectors, deconvolution of rectified (envelope-detected) data is shown to give inherently lower lateral resolution than the deconvolution of the equivalent RF data.     

激光相干场成像散斑噪声复合去噪方法

噪声是影响激光相干场高分辨成像系统像质的重要因素,激光相干场成像系统既受背景光加性噪声影响,又受激光乘性散斑噪声影响.为解决激光相干场成像系统受激光乘性散斑噪声和背景光加性噪声叠加引起的成像像质退化效应问题,从噪声抑制角度提高激光相干场系统高分辨成像像质,研究建立了激光散斑乘性噪声和背景光加性噪声对大气下行链路激光回波场信号影响干扰模型,并基于该模型提出了一种基于同态滤波和稀疏基追踪级联复合去噪算法.首先基于同态滤波理论将激光乘性散斑噪声转化为加性噪声,再由高通滤波器滤除散斑噪声,最后采用基追踪稀疏理论方法抑制背景光等加性噪声对像质的影响.研究表明,较现有单一去噪方法,该级联复合去噪方法可一次性消除激光乘性散斑噪声和背景加性噪声两种不同性质的噪声,有效改善了激光相干场成像质量.     

改正重谱中噪声偏差的正性约束方法和天文高分辨率像复原实验

虽然斑纹掩模法能有效地消除大气流噪声引起的像质衰变,实现望远衍射极限分辨率像复原,但实际的斑纹探测器都不同程度地存在着多种噪声,它们在斑纹图平均功率谱和重谱中产生的偏差将使模和位相的复原受到严重干扰而最终导致像复原的失败。本文讨论了斑纹掩模法像复原中的噪声问题。提出正重谱噪声偏差的正性约束方法,报道了对天文目标：双星ＡＤＳ１６８００和三星Ｈｌｄ１７１ＡＤＳ１６６４８）的高分辨率像复原观测。     

Enhancement of low-quality reconstructed digital hologram images based on frequency extrapolation of large objects under the diffraction limit

During the reconstruction of a digital hologram, the reconstructed image is usually degraded by speckle noise, which makes it hard to observe the original object pattern. In this paper, a new reconstructed image enhancement method is proposed, which first reduces the speckle noise using an adaptive Gaussian filter, then calculates the high frequencies that belong to the object pattern based on a frequency extrapolation strategy. The proposed frequency extrapolation first calculates the frequency spectrum of the Fourier-filtered image, which is originally reconstructed from the +1 order of the hologram, and then gives the initial parameters for an iterative solution. The analytic iteration is implemented by continuous gradient threshold convergence to estimate the image level and vertical gradient information. The predicted spectrum is acquired through the analytical iteration of the original spectrum and gradient spectrum analysis. Finally, the reconstructed spectrum of the restoration image is acquired from the synthetic correction of the original spectrum using the predicted gradient spectrum. We conducted our experiment very close to the diffraction limit and used low-quality equipment to prove the feasibility of our method. Detailed analysis and figure demonstrations are presented in the paper.     

互相关自适应加权的医学超声成像算法研究

根据超声成像系统的超声回波信号互相关性,提出互相关自适应加权超声成像算法.该算法根据散射点回波信号之间的空间相关性设置加权系数,对不同位置的散射点进行自适应加权成像,从而降低了成像系统的旁瓣,抑制了相关性较差的噪声.通过Field II仿真的点目标和吸声斑目标处理结果表明该方法成像的横向和纵向分辨率高,成像速度快.相对于延时叠加(DAS)算法,该算法对散射点成像可提高对比度16 dB,对于吸声斑成像可提高对比度0.85 dB.最后采用完备数据集进行实验,结果表明该算法成像分辨率优于DAS算法,对比度提高了17 dB.     

散斑干涉相位条纹图的频域滤波处理

为了解决在数字散斑干涉技术测量时,散斑干涉相位条纹图像中大量噪声对相位解包裹结果和精度产生严重影响的问题,介绍了一种条纹正余弦分解和频域低通滤波结合的方法,实现了散斑干涉相位条纹图的高精度滤波。该方法的基本思路是在对相位图像进行滤波处理前,先将相位图通过正余弦函数进行映射转换成两幅图,分别经过频域滤波,然后再合成为相位图。这种分解频域滤波方法可以在滤波的同时,有效保留相位跳变信息。实验结果表明：与传统的图像降噪方法相比,该方法能够在保留图像“尖峰”信息的基础上,较好地滤除图像中的散斑噪声,方法简单有效,有效解决了传统滤波方法应用在相位条纹图中,相图灰度信息丢失10%~40%的问题。     

Adaptive filtering for reduction of speckle in ultrasonic pulse-echo images

Current medical ultrasonic scanning instrumentation permits the display of fine image detail (speckle) which does not transfer useful information but degrades the apparent low contrast resolution in the image. An adaptive two-dimensional filter has been developed which uses local features of image texture to recognize and maximally low-pass filter those parts of the image which correspond to fully developed speckle, while substantially preserving information associated with resolved-object structure. A first implementation of the filter is described which uses the ratio of the local variance and the local mean as the speckle recognition feature. Preliminary results of applying this form of display processing to medical ultrasound images are very encouraging; it appears that the visual perception of features such as small discrete structures, subtle fluctuations in mean echo level and changes in image texture may be enhanced relative to that for unprocessed images.