Suppression of slice selection axis motion artifacts in MRI

Patient motion during data acquisition in magnetic resonance imaging causes artifacts in the reconstructed image, which for two-dimensional Fourier transform imaging techniques appear as blurring and ghost repetitions of the moving structures. T. Mitsa et al. (1990) proposed a technique for suppressing artifacts from periodic motion along the slice selection axis. A different approach to the same problem is presented which is not restricted to periodic motion. The algorithm is verified using a simulated phantom and motion. It is also shown to perform well in the presence of noise and motion within the imaging plane.     

Correction of periodic motion artifacts along the slice selection axis in MRI

The effect of periodic motion of a single magnetic resonance imaging (MRI) slice in the direction of the slice selection axis is modeled as amplitude modulation of the raw data with a motion kernel along the phase encoding direction in the Fourier domain. It is shown that this motion can be detected in 1-D projections of the raw data along the frequency encoding direction which in combination with appropriate filtering leads to the recovery of the motion kernel. It is demonstrated by means of simulation examples that significant reduction in the amplitude of ghost artifacts is obtained when the image is filtered by the inverse of the motion kernel. Some issues to be investigated before the technique can be used in a clinical environment are mentioned.     

Correction of computed tomography motion artifacts using pixel-specific back-projection

Cardiac and respiratory motion can cause artifacts in computed tomography scans of the chest. The authors describe a new method for reducing these artifacts called pixel-specific back-projection (PSBP). PSBP reduces artifacts caused by in-plane motion by reconstructing each pixel in a frame of reference that moves with the in-plane motion in the volume being scanned. The motion of the frame of reference is specified by constructing maps that describe the motion of each pixel in the image at the time each projection was measured; these maps are based on measurements of the in-plane motion. PSBP has been tested in computer simulations and with volunteer data. In computer simulations, PSBP removed the structured artifacts caused by motion. In scans of two volunteers, PSBP reduced doubling and streaking in chest scans to a level that made the images clinically useful. PSBP corrections of liver scans were less satisfactory because the motion of the liver is predominantly superior-inferior (S-I). PSBP uses a unique set of motion parameters to describe the motion at each point in the chest as opposed to requiring that the motion be described by a single set of parameters. Therefore, PSBP may be more useful in correcting clinical scans than are other correction techniques previously described.     

Image restoration in computed tomography: the spatially invariant point spread function

Image restoration to deblur smoothing caused by the finite-size X-ray beam profile for a simulated computed tomography (CT) system is presented. Three simple image restoration methods are compared when the point-spread-function (PSF) is spatially invariant. In the first restoration method, an iterative least squares solution, regularized with the image norm and constrained by the boundary of the object, is obtained from the projection data. In the second method, a Wiener filter, designed using the power spectrum of CT noise, is applied to the reconstructed CT image. The third method obtains a weighted least-squares solution, by iteration, from the reconstructed CT image; the solution is regularized with the weighted image norm. Restored images were compared with the image obtained using filtered backprojection method. Differences between these images were evaluated qualitatively.     

点扩散函数的颜色影响性研究

点扩散函数是图像处理的核心元素,是研究图像复原,增强的基础参数,也是构建退化函数的主要部分。在此应用Matlab程序,对拥有不同影响因素的实验图像进行点扩散函数的计算、归纳、比对和总结。探究图像颜色对点扩散函数的影响因素,图像自身参数影响因素,从而为建立应用更为广泛的退化模型奠定基础,也为图像复原工作的准确性提供更准确的依据。     

长曝光大气湍流退化图像点扩散函数估计

大气湍流能明显降低光学系统的成像质量,距离目标越远,曝光时间越长,受大气扰动越严重,图像越模糊。利用大气湍流退化点扩散函数可以对模糊图像进行复原,但实际自然条件下的点扩散函数往往难以准确获得。结合课题研究背景,针对长曝光大气湍流退化图像复原提出了近似等腰三角形模型,通过该模型能得到准确的大气湍流点扩散函数,并采用维纳滤波获得清晰复原图像。实验表明该方法能够对大视场、远距离条件下获得的长曝光大气湍流退化自然图像估计出准确的点扩散函数,复原图像拥有较好的视觉效果,通过计算灰度平均梯度值和拉普拉斯梯度模两个客观评价标准,进一步证实了该算法的有效性。     

Reduction of motion artifacts using a two-frequency impedanceplethysmograph and adaptive filtering

The authors measured transthoracic impedance in nine presumed healthy adult subjects with a two-frequency plethysmograph at 57 kHz and 185 kHz. The measurement protocol included periods of normal breathing without motion and periods of motion without breathing. The authors analyzed the cross-correlation and the ratio between the signals at both frequencies for all the different maneuvers. The correlation coefficient was between 0.97 and 1 for breathing, the minimal cross-correlation (0.81) was for simulated obstructive apnea. The authors found that the amplitude ratio between the two-frequency signals was different for normal breathing and for motion. Based on these results, the authors designed and tested an adaptive filter to increase the signal-to-artifact ratio (SAR). The increase in SAR (mean±standard deviation) compared with the signal at 57 kHz was: 183%±117% for arm movement, 133%±93% for leg movement, and 34%±62% for simulated obstructive apnea     

Imaging heart motion using harmonic phase MRI

This paper describes a new image processing technique for rapid analysis and visualization of tagged cardiac magnetic resonance (MR) images. The method is based on the use of isolated spectral peaks in spatial modulation of magnetization (SPAMM)-tagged magnetic resonance images. We call the calculated angle of the complex image corresponding to one of these peaks a harmonic phase (HARP) image and show that HARP images can be used to synthesize conventional tag lines, reconstruct displacement fields for small motions, and calculate two-dimensional (2-D) strain. The performance of this new approach is demonstrated using both real and simulated tagged MR images. Potential for use of HARP images in fast imaging techniques and three-dimensional (3-D) analyses are discussed.     

Experiment-based estimation of point spread function in electron-beam lithography: Forward-scattering part

The point spread function (PSF) plays an important role in electron beam (e-beam) lithography, e.g., estimation of resist profile, proximity effect correction, etc. One of the essential tasks is how to estimate the PSF with accuracy and efficiency. Conventional approaches include estimation of PSF’s based on certain functions or through a Monte Carlo simulation. A new approach to estimating PSF’s based on experimental data is proposed to provide an alternative to the conventional approaches. It utilizes the relationship between a PSF and a line spread function (LSF), and that between a LSF and the remaining resist profile. Since effects of all phenomena and processes involved in the exposure step are reflected in experimental results, the proposed approach has a good potential to generate realistic PSF’s for any substrates and e-beam tools as long as experiments can be carried out. In this paper, the implementation of the new approach for estimation of the forward scattering part of PSF is described along with the simulation and experiment results.     

Electrode motion artifacts in electrical impedance pneumography

For electrical impedance pneumography, we measured electrode motion artifacts for four different electrode circuits (bipolar, tetrapolar, guarded bipolar, and guarded tetrapolar). Experiments used 10 different electrode configurations each with 4 different subject activities on 10 subjects. These show that 1) the common small-area bipolar electrode configuration is the worst in several measures of artifact sensitivity; 2) increasing the electrode area from 0.8 to 33 cm2 reduces the mean compliant, arm-motion and body-motion artifacts to 57, 45, and 32 percent, respectively, of normal; 3) changing the circuit from bipolar to tetrapolar without changing area changes these artifacts to 106, 65, and 65 percent of normal; 4) changing the circuit from bipolar to guarded bipolar without changing area reduces these artifacts to 87, 57, and 55 percent of normal; and 5) changing the circuit from guarded bipolar to guarded tetrapolar without changing area changes these artifacts to 106, 91, and 110 percent of normal. These results suggest that developers of apnea monitors use larger area electrodes and consider use of a tetrapolar or guarded bipolar circuit.     

Automatic correction of motion artifacts in magnetic resonanceimages using an entropy focus criterion

Presents the use of an entropy focus criterion to enable automatic focusing of motion corrupted magnetic resonance images. The authors demonstrate the principle using illustrative examples from cooperative volunteers. Their technique can determine unknown patient motion or use knowledge of motion from other measures as a starting estimate. The motion estimate is used to compensate the acquired data and is iteratively refined using the image entropy. Entropy focuses the whole image principally by favoring the removal of motion induced ghosts and blurring from otherwise dark regions of the image. Using only the image data, and no special hardware or pulse sequences, the authors demonstrate correction for arbitrary rigid-body translational motion in the imaging plane and for a single rotation. Extension to three-dimensional (3-D) and more general motion should be possible. The algorithm is able to determine volunteer motion well. The mean absolute deviation between algorithm and navigator-echo-determined motion is comparable to the displacement step size used in the algorithm. Local deviations from the recorded motion or navigator-determined motion are explained and the authors indicate how enhanced focus criteria may be derived. In all cases they were able to compensate images for patient motion, reducing blurring and ghosting     

Image restoration in computed tomography: estimation of the spatially variant point spread function

An indirect method for determining the point-spread-function (PSF) in computed tomography (CT) is described. Unlike experimental techniques in which a resolution phantom is scanned to obtain the system PSF, the approach estimates the parameters of a model which describes the two-dimensional X-ray beam profile at each point as a convolution of the appropriately scaled focal spot intensity and detector sensitivity distributions. The model was validated by experimental measurement of the focal spot intensity distribution. Using known X-ray beam profiles, the PSF of a CT scanner can be derived by simulating the data collection process and applying conventional image reconstruction techniques. Visual comparison of directly measured and computed PSFs reveals an asymmetry resulting from misregistration of the phantom wires and the image grid.     

点扩散函数的一维数值计算及其MATLAB实现

通过应用复指数函数与Bessel函数的展开关系和将径向对称的光瞳函数展开成方位角的Fourier级数的方法,可以将衍射受限光学成像系统的点扩散函数的二维Fourier变换的计算转换为一维Fourier变换和一维Hankel变换的计算。并借助MATLAB软件在计算机上实现了编程计算。与二维Fourier变换计算方法相比,一维计算可以方便而清晰地获得像平面上任一确定方向的点扩散强度分布。与Fortran、Basic和C等编程计算语言相比,应用MATLAB语言编程计算,程序语言简洁,且大大降低了使用者对数学基础和计算机语言知识的要求,是进行科学与工程研究的高效工具。     

基于点扩散函数的机载光电吊舱图像增强融合

为更好地融合机载光电吊舱中多源图像的特征信息,提出一种利用点扩散函数(PSF)对多源图像进行增强融合的方法。采用刃边法和小波变换法分别对红外和可见光图像进行点扩散函数求取,利用维纳滤波恢复得到增强图像,然后采用高斯和双边联合滤波对图像进行多尺度混合分解,针对不同尺度下的分解图像,在确定融合权重时引入红外和可见光图像点扩散函数的半幅全宽(FWHM)比值作为分辨率补偿因子,以获得更高质量的融合图像。实验结果表明通过引入点扩散函数信息,光电吊舱系统的获得的融合图像能够具有更好的对比度和分辨率信息,主观和客观评价结果都有所改善。     

与任意点扩散函数卷积的模糊图像的恢复

本文给出了实现全息逆转滤波器的条件,制作了三种具有较大空间带宽积的点扩散函数的全息逆转滤波器,给出了二维模糊图像的恢复结果.     

Impact of point spread function on infrared radiances from geostationary Satellites

The blurring from diffraction for the infrared (IR) radiances on a geostationary satellite (GEO) e.g., the next generation of Geostationary Operational Environmental Satellite (GOES-R) was simulated by using Moderate Resolution Imaging Spectroradiometer Airborne Simulator data and the point spread function (PSF) model for an unobscured telescope. The portion of the total radiance contributed from each nearby geometrical field of view (GFOV) was calculated. For 90% ensquared energy (EE) (equivalent to 10% of the energy coming from outside the footprint), the closest GFOVs contribute 7%; the contribution from the closest GFOVs increases to 22% for 70% EE. The increased portion from the nearby GFOVs causes larger blurring and degrades the pixel-based retrieval product accuracy. Radiance contamination from the nearby field for the GEO IR radiances with 90%, 80%, and 70% EE causes 0.2-, 0.3-, and 0.4-K blurring errors, respectively, in the 12-/spl mu/m IR longwave window band in clear 300-K scenes. The blurring error is doubled in cloudy 230-K scenes. For the 13.8-/spl mu/m absorption band, the blurring error will be smaller than that of the 12-/spl mu/m band because the atmospheric layer where the temperature sensitivity peaks for the 13.8 /spl mu/m is more uniform than the surface where the 12 /spl mu/m is most sensitive. This indicates that the PSF has a greater impact on a heterogeneous surface. Similar blurring errors occur at both 4- and 10-km spatial resolution IR sensors. The blurring error is not random, and it varies spectrally. These conclusions are very relevant to the design of a cost-effective GEO IR sounder that meets the science requirements.     

基于改进点扩散函数的遥感图像超分辨率重建

为了提高遥感图像空间域重建质量, 采用改进凸集投影(POCS)算法的点扩散函数, 提出了一种改进的POCS超分辨率重建算法。首先给出POCS算法基本原理以及具体实现步骤, 在此基础上对算法做出改进, 即对待重建的高分辨初始帧进行边缘检测, 对检测到的边缘像素点应用改进的点扩散函数(PSF), 使边缘处像素点对应的PSF水平方向与垂直方向系数依据边缘斜率变化而设置不同的权重; 最后分别采用两组数据集对改进POCS算法的有效性进行验证。结果表明, 该改进的POCS算法有效地提高了图像重建的效果, 两组实验平均绝对误差效果分别提升了0.79%和0.26%, 达到了提高图像重建质量的目的。该算法具有较好的实际应用价值。     

Fast point spread function computation from aperture functions in high-resolution positron emission tomography

The problem of extracting point spread functions from detector aperture functions in high-resolution PET is addressed. In the limit of very small size detectors relative to the ring dimensions, assumptions are made that lead to a fast and simple computation model yielding point spread functions with negligible errors due to the reconstruction algorithm. The methods allows one to assess accurately the intrinsic performance of a PET tomograph, and it appears to be adequate to relate the imaging capabilities in every point of the camera reconstruction field to the geometric and physical characteristics of the detection system. The method was developed as an investigation tool to help design the next generation of very-high-resolution PET tomographs.     

基于高斯型点扩展函数的红外图像热源复原

为了复原红外图像的热源,采用高斯点扩展函数的方法来增强热源的清晰度和对比度。首先,确定热源图像的高斯点扩展函数,建立其退化模型;然后,采用维纳滤波的方法复原红外图像中的热源,对复原图像通过YIQ变换来复原其温度场彩色信息;最后,通过边缘锐度和标准差评价分析热源复原的质量,并与盲复原算法对比。结果表明,边缘锐度边缘锐度和标准差分别提高了0.502%和0.124%。基于高斯型点扩展函数的方法对红外图像的热源复原具有明显的效果。     

数字共焦显微镜实验3维点扩散函数的构建

为了获取作者所在实验室数字共焦显微镜光学系统实验3维点扩散函数,采用荧光微珠模拟点光源,利用该显微镜对荧光微珠不同散焦量的切片图像进行采集,采用多图像平均法对切片图像进行降噪处理,以此构建显微镜光学系统的实验3维点扩散函数。以该实验点扩散函数进行3维显微图像复原,并与理论点扩散函数的复原效果进行了比较。结果表明,两种点扩散函数对图像复原时均获得良好的复原效果,但是以正确实验方法获取的实验3维点扩散函数,更符合该光学系统的光学传递特性,复原效果更准确。所构建的实验3维点扩散函数,为下一步的生物显微图像复原处理提供了一种较好的选择,为数字共焦显微镜实验3维点扩散函数的构建提供了一种有效的方法。