Quantitative characterization of the limiting resolution of a microscanning imager

A method to determine the limiting resolution of a microscanning imager is proposed. Specifically, both the sample-scene phase effects and aliasing effects due to microscanning are modeled in this method by combining the pixel transfer function and the squeeze modulation transfer function. Further, this model is used to calculate the amount of improvement from typical microscanning modes to the limiting resolution of the imager focusing on various blur factors. Analytical results show that the limiting resolution of the microscanning imager is closely related to microscanning modes. The amount of improvement from different microscanning modes to the limiting resolution is different and is closely associated with the fill factor and the blur factors. The conclusion obtained will be helpful in choosing the optimum microscanning mode according to the fill factor of the detector and system blur factors.     

Infrared image registration and high-resolution reconstructionusing multiple translationally shifted aliased video frames

Forward looking infrared (FLIR) detector arrays generally produce spatially undersampled images because the FLIR arrays cannot be made dense enough to yield a sufficiently high spatial sampling frequency. Multi-frame techniques, such as microscanning, are an effective means of reducing aliasing and increasing resolution in images produced by staring imaging systems. These techniques involve interlacing a set of image frames that have been shifted with respect to each other during acquisition. The FLIR system is mounted on a moving platform, such as an aircraft, and the vibrations associated with the platform are used to generate the shifts. Since a fixed number of image frames is required, and the shifts are random, the acquired frames will not fall on a uniformly spaced grid. Furthermore, some of the acquired frames may have almost similar shifts thus making them unusable for high-resolution image reconstruction. In this paper, we utilize a gradient-based registration algorithm to estimate the shifts between the acquired frames and then use a weighted nearest-neighbor approach for placing the frames onto a uniform grid to form a final high-resolution image. Blurring by the detector and optics of the imaging system limits the increase in image resolution when microscanning is attempted at sub-pixel movements of less than half the detector width. We resolve this difficulty by the application of the Wiener filter, designed using the modulation transfer function (MTF) of the imaging system, to the high-resolution image. Simulation and experimental results are presented to verify the effectiveness of the proposed technique. The techniques proposed herein are significantly faster than alternate techniques, and are found to be especially suitable for real-time applications     

A factorial experiment on image quality and radiation dose

To find if factorial experiments can be used in the optimisation of diagnostic imaging, a factorial experiment was performed to investigate some of the factors that influence image quality, kerma area product (KAP) and effective dose (E). In a factorial experiment the factors are varied together instead of one at a time, making it possible to discover interactions between the factors as well as major effects. The factors studied were tube potential, tube loading, focus size and filtration. Each factor was set to two levels (low and high). The influence of the factors on the response variables (image quality, KAP and E) was studied using a direct digital detector. The major effects of each factor on the response variables were estimated as well as the interaction effects between factors. The image quality, KAP and E were mainly influenced by tube loading, tube potential and filtration. There were some active interactions, for example, between tube potential and filtration and between tube loading and filtration. The study shows that factorial experiments can be used to predict the influence of various parameters on image quality and radiation dose.     

A framework for optimising the radiographic technique in digital X-ray imaging

The transition to digital radiology has provided new opportunities for improved image quality, made possible by the superior detective quantum efficiency and post-processing capabilities of new imaging systems, and advanced imaging applications, made possible by rapid digital image acquisition. However, this transition has taken place largely without optimising the radiographic technique used to acquire the images. This paper proposes a framework for optimising the acquisition of digital X-ray images. The proposed approach is based on the signal and noise characteristics of the digital images and the applied exposure. Signal is defined, based on the clinical task involved in an imaging application, as the difference between the detector signal with and without a target present against a representative background. Noise is determined from the noise properties of uniformly acquired images of the background, taking into consideration the absorption properties of the detector. Incident exposure is estimated or otherwise measured free in air, and converted to dose. The main figure of merit (FOM) for optimisation is defined as the signal-difference-to-noise ratio (SdNR) squared per unit exposure or (more preferably) dose. This paper highlights three specific technique optimisation studies that used this approach to optimise the radiographic technique for digital chest and breast applications. In the first study, which was focused on chest radiography with a CsI flat-panel detector, a range of kV(p) (50-150) and filtration (Z = 13-82) were examined in terms of their associated FOM as well as soft tissue to bone contrast, a factor of importance in digital chest radiography. The results indicated that additive Cu filtration can improve image quality. A second study in digital mammography using a selenium direct flat-panel detector indicated improved SdNR per unit exposure with the use of a tungsten target and a rhodium filter than conventional molybdenum target/molybdenum filter techniques. Finally, a third study focusing on cone-beam computed tomography of the breast using a CsI flat-panel detector indicated that high Z filtration of a tungsten target X-ray beam can notably improve the signal and noise characteristics of the image. The general findings highlight the fact that the techniques that are conventionally assumed to be optimum may need to be revisited for digital radiography.     

Balancing interpixel cross talk and detector noise to optimize areal density in holographic storage systems

We investigate the effects of interpixel cross talk and detector noise on the areal storage density of holographic data storage. A numerical simulation is used to obtain the bit-error rate (BER) as a function of hologram aperture, pixel fill factors, and additive Gaussian intensity noise. We consider the effect of interpixel cross talk at an output pixel from all possible configurations of its 12 closest-neighbor pixels. Experimental verification of this simulation procedure is shown for several fill-factor combinations. The simulation results show that areal density is maximized when the aperture coincides with the zero order of the spatial light modulator (SLM) (Nyquist sampling condition) and the CCD fill factor is large. Additional numerical analysis including finite SLM contrast and fixed-pattern noise show that, if the fixed-pattern noise reaches 6% of the mean signal level, the SLM contrast has to be larger than 6:1 to maintain high areal density. We also investigate the improvement of areal density when error-prone pixel combinations are forbidden by using coding schemes. A trade-off between an increase in areal density and the redundancy of a coding scheme that avoids isolated-on pixels occurs at a code rate of approximately 83%.     

Small-kernel superresolution methods for microscanning imaging systems

Two computationally efficient methods for superresolution reconstruction and restoration of microscanning imaging systems are presented. Microscanning creates multiple low-resolution images with slightly different sample-scene phase shifts. The digital processing methods developed here combine the low-resolution images to produce an image with higher pixel resolution (i.e., superresolution) and higher fidelity. The methods implement reconstruction to increase resolution and restoration to improve fidelity in one-pass convolution with a small kernel. One method uses a small-kernel Wiener filter and the other method uses a parametric cubic convolution filter. Both methods are based on an end-to-end, continuous-discrete-continuous microscanning imaging system model. Because the filters are constrained to small spatial kernels they can be efficiently applied by convolution and are amenable to adaptive processing and to parallel processing. Experimental results with simulated imaging and with real microscanned images indicate that the small-kernel methods efficiently and effectively increase resolution and fidelity.     

水化膜对原子力显微镜扫描图像的影响

原子力显微镜(AFM)有接触和轻敲两种工作方式,可以在气相和液相下工作.在液相工作状态下,浸没于水中的探针和样品表面会形成一层水化膜.在探针和样品表面形成的水化膜是否会对扫描图像产生影响以及产生何种影响是一个值得研究的问题.本文用原子力显微镜对1μm标准校正光栅在空气和水两种媒质中进行了图像扫描,扫描分别采用接触和轻敲两种工作方式.扫描图像显示,在液相工作环境中,水化膜在探针和样品表面的形成将严重影响轻敲工作方式所形成的图像,而接触方式的扫描图像基本不受影响.     

波分复用改善光纤传象束象质的导模分析

运用光纤导模（束缚模）理论和离散抽样理论分析了光纤象束象的质量，估算了决定波分复用象束系统象质的信息量Ｎ，Ｎ是到达象面的而且有与物场相位分布几乎相同的场模数，并估算了波分复用技术改善光纤象束系统象质的倍增率。     

Effects of multilevel phase masks on interpixel cross talk in digital holographic storage

We study the interpixel cross talk introduced to digital holographic data storage by use of a multilevel phase mask at the data-input plane. We evaluate numerically the intensity distribution at the output detector for Fourier plane hologram storage in a limited-aperture storage medium. Only the effect at an output pixel of interpixel cross talk from the four horizontal and vertical neighboring pixels is considered, permitting systematic evaluation of all possibilities. For random two-level and pseudorandom six-level phase masks, the influence of the pixel fill factor, as well as the aperture size of the storage medium, is studied. Our simulations show that, for a given aperture size, a random two-level mask is more susceptible to interpixel cross talk than is a pseudorandom six-level mask. Decreasing the pixel fill factor below 94% with a pseudorandom six-level phase mask makes it theoretically possible to have a system with no errors from interpixel cross talk if one particular 5-pixel pattern is forbidden through modulation coding. Reducing the input fill factor below 85% means that no patterns need to be excluded.     

Comparison of technical and anatomical noise in digital thorax X-ray images

Former studies by Hoeschen and Buhr indicated a higher total noise in a thorax image than expected from technical noise, i.e. quantum and detector noise. This difference results from the overlay of many small anatomical structures along the X-ray beam, which leads to a noise-like appearance without distinguishable structures in the projected image. A method is proposed to quantitatively determine this 'anatomical noise' component, which is not to be confused with the anatomical background (e.g. ribs). This specific anatomical noise pattern in a radiograph changes completely when the imaging geometry changes because different small anatomical structures contribute to the projected image. Therefore, two images are taken using slightly different exposure geometry, and a correlation analysis based on wavelet transforms allows to determining the uncorrelated noise components. Since the technical noise also differs from image to image, which makes it difficult to separate the anatomical noise, images of a lung phantom were produced on a low-sensitive industrial X-ray film using high-exposure levels. From these results, the anatomical noise level in real clinical thorax radiographs using realistic exposure levels is predicted using the general dose dependence described in the paper text and compared with the quantum and detector noise level of an indirect flat-panel detector system. For consistency testing, the same lung phantom was imaged with the same digital flat-panel detector and the total image noise including anatomical noise is determined. The results show that the relative portion of anatomical noise may exceed the technical noise level. Anatomical noise is an important contributor to the total image noise and, therefore, impedes the recognition of anatomical structures.     

Coloured computational imaging with single-pixel detectors based on a 2D discrete cosine transform

We propose and demonstrate a computational imaging technique that uses structured illumination based on a two-dimensional discrete cosine transform to perform imaging with a single-pixel detector. A scene is illuminated by a projector with two sets of orthogonal patterns, then by applying an inverse cosine transform to the spectra obtained from the single-pixel detector a full-colour image is retrieved. This technique can retrieve an image from sub-Nyquist measurements, and the background noise is easily cancelled to give excellent image quality. Moreover, the experimental set-up is very simple.     

Breast ultrasound image improvement by pixel compounding of compression sequence

Pixel compounding is a technique that synthesizes the information of an image sequence involving slow decorrelation of the speckle to form a detail-recovered and speckle reduced image. To avoid extra data acquisition time and patient exposure, reuse of the existing data is desirable. In the procedure of elasticity imaging, a set of B-mode images with slight changes due to deformation is produced, which provides an ideal input for the pixel compounding. The improvement in image quality is evaluated quantitatively using a figure-of-merit (FOM) that indicates the quality of boundary information recovery and the contrast-to-noise ratio (CNR) over the phantom images. The increase in average CNR is from 0.4 in the original images to 0.8 in the pixel compounded images. The improvement in average FOM is from 0.15 to more than 0.5 on a scale of 0 to 1. In vivo results with a breast cyst, a fibroadenoma, and a breast cancer are also presented and the image quality improvement is subjectively evaluated. The results suggest that B-mode breast images from compression procedures are suitable data for pixel compounding, and that a speckle-reduced and detail-recovered or detail-maintained image can be produced. The improved imaging may provide alternative or better information for detection and diagnosis. A similar approach could be extended to elasticity imaging with other modalities.     

Reduced depth of field in incoherent hybrid imaging systems

A hybrid imaging system combines a modified optical imaging module and a digital postprocessing step. We define what to our knowledge is a new metric to quantify the blurring of a defocused image that is more suitable than the defocus parameter for describing defocused hybrid imaging systems. We use this metric to design a pupil phase grating to reduce the depth of field, thereby increasing the axial resolution, of an incoherent hybrid imaging system using quasi-monochromatic illumination. By introducing this grating at the exit pupil and digitally processing the output of the detector, we reduce the depth of field by more than a factor of 2. Finally, we examine the effect of using a CCD optical detector, instead of an ideal optical detector, on the reduction of the depth of field.     

Characterization of the image resolution for the first-arriving-light method

A theoretical model is established for finding the resolution of imaging through random media with the first-arriving-light method. With this model analytical mathematical forms of the point-spread function are derived for transillumination and confocal scanning imaging modes combined with the first-arrivinglight method. Finally experiments were carried out with the holographic gating technique to demonstrate the validity of the theory. The experimental results show that the first-arriving-light method improves the image resolution by as much as a factor of 20 over the conventional transillumination mode.     

Current challenges of full field digital mammography

Full field digital mammography (FFDM) has advantages over screen-film mammography (SFM), but some important challenges remain. The first challenge is related to the specific characteristics of FFDM. It remains unclear, which shape and limiting values of the modulation transfer function have the most influence on the performance of a detector, such as the effect of the image display on the overall image quality and the effect of processing on cancer detection. In order to assess the image quality of FFDM, we have set up a scoring system. The second challenge is related to screening mammography: is the quality of an image the same when it is viewed on different monitors and with different processing algorithms? Is Computer Aided Diagnosis necessary in a screening environment? In FFDM, the effect of different detectors, processing and display possibilities on the image and on cancer detection are not clearly investigated.     

Optimal multi-scale geometric fusion based on non-subsampled contourlet transform and modified central force optimization

In the current era of technological development, medical imaging plays an important part in several applications of medical diagnosis and therapy. This requires more precise images with much more details and information for correct medical diagnosis and therapy. Medical image fusion is one of the solutions for obtaining much spatial and spectral information in a single image. This article presents an optimization-based contourlet image fusion approach in addition to a comparative study for the performance of both multi-resolution and multi-scale geometric effects on fusion quality. An optimized multi-scale fusion technique based on the Non-Subsampled Contourlet Transform (NSCT) using the Modified Central Force Optimization (MCFO) and local contrast enhancement techniques is presented. The first step in the proposed fusion approach is the histogram matching of one of the images to the other to allow the same dynamic range for both images. The NSCT is used after that to decompose the images to be fused into their coefficients. The MCFO technique is used to determine the optimum decomposition level and the optimum gain parameters for the best fusion of coefficients based on certain constraints. Finally, an additional contrast enhancement process is applied on the fused image to enhance its visual quality and reinforce details. The proposed fusion framework is subjectively and objectively evaluated with different fusion quality metrics including average gradient, local contrast, standard deviation (STD), edge intensity, entropy, peak signal-to-noise ratio, Q ^ab/f, and processing time. Experimental results demonstrate that the proposed optimized NSCT medical image fusion approach based on the MCFO and histogram matching achieves a superior performance with higher image quality, average gradient, edge intensity, STD, better local contrast and entropy, a good quality factor, and much more details in images. These characteristics help for more accurate medical diagnosis in different medical applications.     

Multiaperture imaging

We study the reconstruction of a high-resolution image from multiple low-resolution images by using a nonlinear iterative backprojection algorithm. We exploit diversities in the imaging channels, namely, the number of imagers, magnification, position, rotation, and fill factor, to undo the degradation caused by the optical blur, pixel blur, and additive noise. We quantify the improvements gained by these diversities in the reconstruction process and discuss the trade-off among system parameters. As an example, for a system in which the pixel size is matched to the diffraction-limited optical blur size at a moderate detector noise level, we can reduce the reconstruction root-mean-square error by 570% by using 16 cameras and a large amount of diversity. The algorithm was implemented on a 56 camera array specifically constructed to demonstrate the resolution-enhancement capabilities. Practical issues associated with building and operating this device are presented and analyzed.     

基于优先权和匹配度量的图像修复算法

目的 针对当前图像修复算法主要通过固定单一模板大小实现修复块与匹配块之间的匹配度量来完成图像复原,导致其存在一定的模糊效应以及振铃效应等不足,这里提出基于改进优先权和匹配优化度量的图像修复算法。方法 首先,利用数据项构造平滑因子,建立优先权模型,度量待修复像素点的优先权,选定优先修复块。然后,制定四级模板大小,利用误差平方和函数,结合模板大小特性,构造匹配度量模型,用于实现修复块和匹配块之间的动态匹配,选取最优匹配模板,对待修复块进行填充修复。最后,利用待修复像素点及其邻域像素点的灰度值构造邻域灰度差分模型,用于对修复区域的边缘进行缝合,优化修复效果。利用最优匹配度量结果,构造置信度更新模型,对置信度项进行更新,实现图像修复。结果 仿真实验结果显示,与当前图像修复算法相比,所提算法具有更高的修复质量,其输出图像无模糊效应与振铃效应。结论 所提算法能够较好地对损坏图像进行复原,在图像信息处理领域具有一定的参考价值。     

Influence of luminance and resolution on the perceived quality of black-and-white images on soft displays

Abstract

The perceived quality of an image displayed on a computer monitor depends on a number of different factors that can affect viewers' preferences. Two of these are the luminance of the monitor display and the resolution of the image. The effect of luminance is of interest for applications such as on-line access of images where the computer displays used for viewing these images could have different luminance settings. A relationship between cathode-ray tube (CRT) display luminance and resolution has been shown in previous studies. It was therefore interesting to investigate whether there is a relationship between image resolution and CRT display luminance on perceived image quality. Image resolution is related to the image file size, which is an important factor for applications such as on-line access of images. This work used a CRT display to study the effect of the above-mentioned factors on the perceived quality of the displayed image. Three sets of black-and-white images, each set with a different resolution, were presented to observers at three brightness settings of the computer monitor. Results are discussed regarding the effect of monitor display luminance and image resolution on perceived image quality and the interaction between them. Evaluation of results is further extended to the influence of the different backgrounds of the images. The scene content of the images was also shown to affect the viewers' judgement.     

Effect of a finite-size pinhole on noise performance in single-, two-, and three-photon confocal fluorescence microscopy

It is known that signal level in single-, two- and three-photon confocal fluorescence microscopy increases with the size of the detector. Here we evaluate the signal-to-noise and the signal-to-background criteria for these microscopes. We investigate the effect of pinhole size on their ability to detect a weakly fluorescent point object in the presence of a uniformly fluorescence background. Numerical results based on a paraxial approximation theory show that optimization of these criteria gives an optimal value for pinhole size, which results in an improved imaging performance. The resulting improvement in noise performance, compared with the use of a large detector, is greater for three-photon than for two-photon confocal fluorescence microscopes.