旋转不变的非局域均值算法在磁共振图像去噪中的应用

磁共振成像（MRI）实验时常采用多次扫描累加平均提高图像信噪比（SNR）,但当扫描过程中运动引起图像变形时,简单地累加平均就无法奏效.为此,本研究组曾提出一种匹配加权平均方法（MWA）提高图像的信噪比.在此基础上,该文提出一种旋转不变的非局域均值算法（RINLM）,即选取圆形邻域区域并将其划分为一系列以中心像素为圆心的等面积圆环,再计算模式的相似性.RINLM算法可以更好地利用图像中旋转的冗余信息、找到更多的相似结构,提高算法的去噪性能.我们把该方法应用于低信噪比图像序列的平均和去噪中,可以更好地处理旋转的局部运动.与非局域均值算法（NLM）相比,RINLM算法可以进一步提高图像的信噪比;与MWA方法相比,其与RINLM算法的结合可以进一步提高磁共振图像序列信噪比,更好的保持图像边缘信息.     

基于局部位移校正的磁共振图像相干平均

多次扫描相干平均是提高磁共振图像信噪比的常用方法,但如果在多次扫描过程中病人发生自主或不自主的运动,使得图像中的组织发生位移,简单相干平均图像会导致图像模糊.本文受非局域均值算法的启发,提出了一种基于局部位移校正的相干平均方法.该算法通过比较多次采集的图像中组织结构的局部相似性,找出图像间的局部位移,利用该信息修正位移后进行加权平均,从而达到提高图像信噪比的目的.我们用模型及真实的肝脏弥散数据进行了实验.实验结果表明,对于不同次采样间存在运动的磁共振图像,该算法可有效地提高信噪比并保持结构边缘;其结果优于简单的相干平均,去噪效果也优于经典的非局域均值算法.     

辐射图像脉冲去噪方法

为去除辐射图像中的脉冲噪音,建立了一种先进行脉冲噪音检测再进行噪音数据修复的联合去噪音方法.首先采用测地膨胀方法从减灰度图像出发对原图像进行形态学灰度重构,再采用灰度阈值法从原图像与重构图像的差值图像中分割检测出脉冲噪音,然后采用改进非本地均值方法对噪音数据进行修复.改进非本地均值方法对相似像素的选择范围进行了限制,从原理上避免了传统非本地均值方法用于去除辐射图像脉冲噪音时易发生的以错纠错问题,本文对其进行了数学表述.实验证明,该方法对不同类型辐射图像的脉冲噪音均具有较好滤除效果,表现出良好的噪音数据修复和图像细节保护能力.     

辐射图像脉冲去噪方法

为去除辐射图像中的脉冲噪音,建立了一种先进行脉冲噪音检测再进行噪音数据修复的联合去噪音方法.首先采用测地膨胀方法从减灰度图像出发对原图像进行形态学灰度重构,再采用灰度阈值法从原图像与重构图像的差值图像中分割检测出脉冲噪音,然后采用改进非本地均值方法对噪音数据进行修复.改进非本地均值方法对相似像素的选择范围进行了限制,从原理上避免了传统非本地均值方法用于去除辐射图像脉冲噪音时易发生的以错纠错问题,本文对其进行了数学表述.实验证明,该方法对不同类型辐射图像的脉冲噪音均具有较好滤除效果,表现出良好的噪音数据修复和图像细节保护能力.     

基于改进非局部均值的CPMG信号去噪算法

采用低场核磁共振技术进行检测时,接收到的回波信号微弱且信噪比低,真实的信号容易淹没在背景噪声中,严重影响到后续的反演等操作的准确性．针对这一问题,提出利用非局部均值滤波算法对CPMG(Carr Purcell Meiboom Gill)回波信号进行降噪的方法．首先,对算法中至关重要的参数选择的方法进行分析,提出了利用Stein无偏风险估计的自适应参数选取方法;然后,根据回波信号的特性对算法进行改进,即利用信号点数据方差的不同,自适应地求取各点进行非局部均值滤波时的相似窗宽度;最后,求取利用最优参数进行降噪后的CPMG回波信号．对仿真数据和真实数据的反演结果对比分析表明,该改进的非局部均值滤波算法能够取得更好的滤波效果,能够获得较优的反演谱．     

三模高斯态光场非局域性的增强

提出了增强三模高斯态光场非局域性的方案. 结果表明：通过一个IPS（inconlusive photon subtraction）过程的非高斯操作,可以将三模高斯态退变为非高斯态,利用Bell不等式检测发现对于较弱的输入高斯态非局域性能够得到加强. 关键词： 非局域性 三模高斯态     

基于直方图均衡化的Robinson图像边缘检测算法

针对图像边缘检测,现有Robinson相关算法存在效率低、阈值设定随机性大、易出现伪边缘等问题,提出一种改进Robinson的图像边缘检测算法。该算法利用直方图均衡化对图像进行增强,然后将Robinson算子原有的八方向梯度依照两两垂直原则组合为八组,分别计算每组梯度的范数,并取其最大值作为该像素点的梯度。最后取整幅图像的灰度均值作为阈值来识别图像的边缘像素和背景像素。实验表明,相对于现有相关算法,该算法检测结果更加清晰完整,同时避免了传统算子人为设定阈值随机性大的问题。     

基于径向希尔伯特变换的图像边缘增强特性研究

研究了基于螺旋相位频谱滤波器滤波的径向希尔伯特变换对不同边缘分布图像的增强特性.给出了拓扑荷n=1的有限孔径螺旋相位滤波器点扩散函数的解析表达式,并利用圆孔、汉字和人像等三幅不同边缘分布密度的图像进行了边缘增强的模拟计算.结果表明,基于螺旋相位频谱滤波的径向希尔伯特变换会使边缘增强图像具有立体浮雕效果,并且随着输入图像...     

用于近红外宽带腔增强吸收光谱的小波去噪

为了有效抑制检测系统的噪声,提高气体浓度的反演精度,研究了近红外宽带腔增强气体传感系统的小波去噪方法。小波去噪方法的优化分析结果表明,选择db2小波函数作为小波基对含噪信号进行6级分层处理,并选择heursure阈值估计方法,采用局部阈值方式对噪声部分小波系数进行置零处理,可达到最优去噪效果。将近红外宽带腔增强吸收光谱技术与高分辨率傅里叶变换红外光谱仪相结合,建立了用于甲烷检测的气体传感系统,使用最小二乘拟合算法对去噪前后的甲烷吸收系数进行反演。结果表明,采用小波去噪后,反演浓度更接近真实值,反演精度提高7%,信噪比提高90%,系统检测下限降低45%,证明小波去噪算法可以有效提高系统的检测精度。     

脉冲形状对定域谱中水峰抑制作用的影响

报道了用MRI中STEAM方法获得定域谱(MRS)的实验结果,用相位正交、时间一长一短的二个特形脉冲对水峰进行预饱和照射,得到抑制水峰的MRS.这种方法抑制水峰的效果比用梯度散相压水峰的效果好。对于三种常见的特形脉冲,用GAUSS脉冲得到效果最好的实验结果。     

Generalized total variation-based MRI Rician denoising model with spatially adaptive regularization parameters

Magnetic resonance imaging (MRI) is an outstanding medical imaging modality but the quality often suffers from noise pollution during image acquisition and transmission. The purpose of this study is to enhance image quality using feature-preserving denoising method. In current literature, most existing MRI denoising methods did not simultaneously take the global image prior and local image features into account. The denoising method proposed in this paper is implemented based on an assumption of spatially varying Rician noise map. A two-step wavelet-domain estimation method is developed to extract the noise map. Following a Bayesian modeling approach, a generalized total variation-based MRI denoising model is proposed based on global hyper-Laplacian prior and Rician noise assumption. The proposed model has the properties of backward diffusion in local normal directions and forward diffusion in local tangent directions. To further improve the denoising performance, a local variance estimator-based method is introduced to calculate the spatially adaptive regularization parameters related to local image features and spatially varying noise map. The main benefit of the proposed method is that it takes full advantage of the global MR image prior and local image features. Numerous experiments have been conducted on both synthetic and real MR data sets to compare our proposed model with some state-of-the-art denoising methods. The experimental results have demonstrated the superior performance of our proposed model in terms of quantitative and qualitative image quality evaluations.     

Metabolite mapping of human filarial parasite, Brugia malayi with nuclear magnetic resonance

Metabolite mapping of human filarial parasite, Brugia malayi was carried out in vitro as well as in situ in host Mastomys coucha by ³¹P nuclear magnetic resonance (NMR) spectroscopy. Detection of parasites by visualizing contrast spots due to pathologic changes was observed by ¹H magnetic resonance imaging (MRI). Major metabolites of adult B. malayi observed by ³¹P-NMR spectroscopy were of sugar phosphates (SP), phosphomonoesters (PME), glycerophosphoryl-ethanolamine (GPE), -choline (GPC), phosphoenolpyruvate (PEP), inorganic phosphate (Pi), nucleoside diphosphosugar and nucleotides-mono, -di and -tri phosphates. PEP and GPC were present in high concentration; PEP being the major energy reservoir and GPC the major phospholipid in this species of filaria. The ³¹P NMR spectra of testis of mastomys, showed seven major peaks of SP, PME, phosphocreatine (PCr), phosphodiesters (PDE), Pi, and nucleotides di- and tri-phosphates. The ³¹P-NMR spectra of testis of B. malayi infected animal also consisted of seven major peaks with significant decrease in the SP and PME peak showing changes in the carbohydrate and lipid metabolism of filaria infected testis. Thus, in vivo ³¹P MRS provided a non-invasive assessment of tissue bioenergetics and phospholipid metabolism.     

A new approach to the use of EMMRS: Ionic Mobility MR Spectroscopy (IMMRS)

A new approach to the use of the previously proposed method called Electrical Mobility Magnetic Resonance Spectroscopy (EMMRS) has been worked out. It consists in obtaining mobility distribution, lifetimes, and activation energy of charge carriers by introducing into the medium a longitudinal elastic wave perpendicular to the electric field which causes displacement of charges (electrons, protons, ions, free radicals, and molecules related with charge carriers) present in the biological environment or inanimate matter. Such a method allows for determination of these quantities within small areas, thus offering possibilities for their imaging. We therefore suggest to call it Ionic Mobility MR Spectroscopy (IMMRS).     

Predictability of SNR and reader preference in clinical MR imaging

Fifty-four independent scans were performed in two volunteers covering one anatomic region in each (the brain and knee) with the purpose of ascertaining the agreement between predicted and measured signal-to-noise ratios (SNR). Systematically varied parameters were number of excitations (NEX), field of view (FOV), section thickness (d_z), and the number of phase-encoding steps (N_y). Correlation coefficients of measured versus predicted SNR were 0.82 and 0.86, respectively, in the anatomies studied. Significantly improved correlations were found for data subpopulations in which NEX was held constant. To assess the criteria guiding reader preference, a blinded study was performed in which radiologists were asked to rate images from least to most desirable. In order to quantitatively determine the criteria for reader preference, plots of mean rating versus SNR, voxel volume, and an image quality index [IQI = SNR/(voxel volume)] were performed. The latter was found to be a better predictor of reader preference than either SNR or spatial resolution alone. The data suggests T₁-weighted scan protocols yielding SNR of approximately 20 are preferable with any excess SNR being traded for smaller voxel size or shorter scan times.     

An automatic cerebellum extraction method in T1-weighted brain MR images using an active contour model with a shape prior

Purpose

The objective of this paper was to automatically segment the cerebellum from T1-weighted human brain magnetic resonance (MR) images.

Materials and Methods

The proposed method constructs a cerebellum template using five sets of 3-T MR imaging (MRI) data, which are used to determine the initial position and the shape prior of the cerebellum for the active contour model. Our formulation includes the active contour model with shape prior, which thereby maintains the shape of the template. The proposed active contour model is sequentially applied to sagittal-, coronal- and transverse-view images. To evaluate the proposed method, it is applied to BrainWeb data and a 3-T MRI data set and compared with FreeSurfer with respect to performance assessment metrics.

Results

The segmented cerebellum was compared with the results from FreeSurfer. Using the manually segmented cerebellum as reference, we measured the average Jaccard coefficients of the proposed method, which were 0.882 and 0.885 for the BrainWeb data and 3-T MRI data set, respectively.

Conclusion

We presented the active contour model with shape prior for extracting the cerebellum from T1-weighted brain MR images. The proposed method yielded a robust and accurate segmentation result.     

A method for the making and utility of gadolinium-labeled albumin microbubbles

Objective

Perfluorocarbon-exposed sonicated dextrose albumin microbubbles (PESDA) binds scavenger receptors and can be noninvasively imaged. To enhance imaging, gadolinium (Gd)-labeled PESDA was developed and tested in a model of vascular inflammation by magnetic resonance imaging (MRI).

Methods and Results

Purified human serum albumin (HSA) (5%) was labeled with Gd via the covalent binding of diethylenetriaminepentacetic acid. Abdominal aortic tissues in Sprague-Dawley rats (n=5 per group) were analyzed by 7-T MRI and scanning electron microscopy to evaluate PESDA binding. Labeling-purified 5% human albumin resulted in an average of 16.1 Gd atoms per albumin molecule as determined by atomic absorption. Forty-eight hours after balloon angioplasty, aortic tissue was enhanced with Gd-PESDA as compared to control tissue. 7-T MRI of explanted tissues was sensitive to the detection of retained PESDA. Enhancement of aortic tissue in vivo was present albeit to a lesser extent than explanted tissue from the same animals.

Conclusions

HSA was successfully labeled, and an albumin-based microbubble with Gd was synthesized. This contrast agent, Gd-PESDA, may serve as an additional agent for the MRI evaluation of innate inflammation and used to noninvasively image early vascular pathophysiologic processes.

Condensed Abstract

In this study, Gd-PESDA microbubbles and were synthesized and shown to detect the binding of these microbubbles using MRI in injured aortic tissue. The method for synthesizing Gd-PESDA is detailed, and the proposed utility of this new contrast agent is discussed.     

一种结合并行成像和压缩感知的快速磁共振成像新方法

压缩感知（CS）技术和并行成像技术（主要是SENSE技术、GRAPPA技术等）都能通过减少k空间数据的采集量来加快磁共振成像速度,目前已有一些将两种方法相结合进一步加速磁共振成像速度的方法（例如CS-GRAPPA）.本文针对数据采集和重建这两方面对现有CS-GRAPPA方法进行了改进,采集方式上采用了局部等间隔采集模板以满足GRAPPA重建的要求,并对采集模板进行随机放置以满足CS重建的要求;数据重建时,根据自动校正数据估算GRAPPA算法中欠采行的重建误差,并利用误差的大小确定在CS算法中保真的程度.不同磁共振图像重建实验的结果表明：与现有方法相比,本文方法能够更好地保留原有图像细节并有效减少伪影.