Neural network approach to image reconstruction from projections

In this article, we formulate the image reconstruction problem in terms of a multicriteria optimization-based neural network model, and study its performance. The value of neural network as a computational device is to be shown. We also demonstrate the efficiency of our implementation of neural net processing and compare its performance with respect to the convolution-backprojection reconstruction techniques based on computer-generated noisy projections. The method presented here has the particular ability to solve ill-posed reconstruction problems. The algorithm described was implemented on a serial PC/Intel 586 microcomputer, but was cast in a form which is ideally suited for parallel processing. © 1998 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 9, 381–387, 1998     

Quantitative optical image reconstruction of turbid media by use of direct-current measurements

We present a detailed experimental study concerning quantitative optical property reconstruction of heterogeneous turbid media by use of absolute dc data only. We performed experiments by using tissuelike phantoms in both single-target and multitarget configurations in which variations in target size and optical contrast with the background were explored. Our results show that both scattering and absorption images can be reconstructed quantitatively by use of dc data only, whereas it was impossible to obtain such quantitative information in previously reported studies. We believe that this improvement is primarily a result of the realization of a novel data preprocessing/optimization scheme for accurately determining several critical parameters needed for reconstruction. The use of this data preprocessing/optimization scheme also eliminates the calibration reference measurement previously required for reconstruction. Experimental confirmation of this scheme is given in detail.     

Frequency-domain optical image reconstruction in turbid media: an experimental study of single-target detectability

An experimental study of the detectability of an object embedded in optically tissue-equivalent media by frequency-domain image reconstruction is presented. The experiments were performed in an 86-mm-diameter cylindrical phantom containing an optically homogeneous cylindrical target whose absorption and scattering properties presented a 2:1 contrast with the background medium. The parameter space explored during experimentation involved object size (15-, 8-, and 4-mm targets) and location (centered, 20-mm off-centered, and 35-mm off-centered) variations. Image reconstruction was achieved with a previously reported regularized least-squares approach that incorporates finite-element solutions of the diffusion equation and Newton's method solutions of the nonlinear minimization problem. Also included during image formation were image enhancement schemes-(1) total variation minimization, (2) dual meshing, and (3) spatial low-pass filtering-which have recently been added. Quantitative measures of image quality including the size, location, and shape of the heterogeneity along with errors in its recovered optical property values are used to quantify the image reconstructions. The results show that a near 22:1 ratio of tissue thickness relative to detectable object size has been achieved with this approach in the laboratory conditions and parameter space that have been investigated.     

Image reconstruction by backprojection from frequency-domain optical measurements in highly scattering media

The reconstruction of the location and optical properties of objects in turbid media requires the solution of the inverse problem. Iterative solutions to this problem can require large amounts of computing time and may not converge to a unique solution. Instead, we propose a fast, simple method for approximately solving this problem in which calculated effective absorption and reduced scattering coefficients are backprojected to create an image of the objects. We reconstructed images of objects with centimeter dimensions embedded in a diffusive medium with optical characteristics similar to those of human tissue. Data were collected by a frequency-domain spectrometer operating at 120 MHz with a laser diode light source emitting at 793 nm. Intensity and phase of the incident photon density wave were collected from linear scans at different projection angles. Although the positions of the objects are correctly identified by the reconstructed images, the optical parameters of the objects are recovered only qualitatively.     

一种基于模型的光学层析图像重建方法

光学层析图像重建是个病态问题,测量误差会在重建过程中被放大,对此,提出一种以广义高斯马尔可夫随机场模型为先验信息的光学层析图像重建方法.重建过程是对目标函数的优化过程,目标函数关于光学参数的梯度计算是算法中的难点,因此,提出一种基于梯度树的梯度计算方法.文中分别给出了吸收系数和散射系数的重建结果,并引入三个指标因子衡量重建图像的质量,进而列出不同重建算法下,重建图像的指标值.最后通过对重建结果和指标因子取值的比较,分析基于模型的重建算法的有效性.     

Volume image reconstruction for diffuse optical tomography

Optical tomography is a potential diagnostic method for visualizing optical properties of tissues in vivo. We present an optical tomography method that has been designed for imaging of the human testes, particularly for spectroscopic tumor differentiation. In this application we need to compute three-dimensional distributions of the optical contrast (absorption coefficient) in the tissue in real time. Thus we have given special care to elaborate an efficient inverse algorithm that takes the limitations of spatial resolution and data space point density into account. Our inverse solution is based on a linearization approach and a dedicated object space discretization. Furthermore, we introduce the concept of fuzzy voxels, which enables a reconstruction-inherent image smoothing.     

Enhanced frequency-domain optical image reconstruction in tissues through total-variation minimization

Optical image reconstruction in heterogeneous turbid media is sensitive to noise, especially when the signal-to-noise ratio of a measurement system is low. A total-variation-minimization-based iterative algorithm is described in this paper that enhances the quality of reconstructed images with frequency-domain data over that obtained previously with a regularized least-squares approach. Simulation experiments in an 8.6-cm-diameter circular heterogeneous region with low- and high-contrast levels between the target and the background show that the quality of the reconstructed images can be improved considerably when total-variation minimization is included. These simulated results are further verified and confirmed by images reconstructed from experimental data by the use of the same geometry and optically tissue-equivalent phantoms. Measures of imaging performance, including the location, size, and shape of the reconstructed heterogeneity, along with absolute errors in the predicted optical-property values are used to quantify the enhancements afforded by this new approach to optical image reconstruction with diffuse light. The results show improvements of up to 5 mm in terms of geometric information and an order of magnitude or more decrease in the absolute errors in the reconstructed optical-property values for the test cases examined.     

Benefits of optical system diversity for multiplexed image reconstruction

Algorithms that use optical system diversity to improve multiplexed image reconstruction from multiple low-resolution images are analyzed and demonstrated. Compared with systems using identical imagers, systems using additional lower-resolution imagers can have improved accuracy and computation. The diverse system is not sensitive to boundary conditions and can take full advantage of improvements that decrease noise and allow an increased number of bits per pixel to represent spatial information in a scene.     

Tomographic reconstruction of stratified fluid flow

A method for imaging a moving fluid is proposed and evaluated by numerical simulation. A cross-section of a three-dimensional fluid is probed by high-frequency acoustic waves from several different directions. Assuming straight-ray geometric acoustics, the time of flight depends on both the scaler sound speed and the vector fluid velocity. By appropriately combining travel times, projections of both the sound speed and the velocity are isolated. The sound speed is reconstructed using the standard filtered backprojection algorithm. Though complete inversion of velocity is not possible, sufficient information is available to recover the component of fluid vorticity transverse to the plane of insonification. A new filtered backprojection algorithm for vorticity is developed and implemented. To demonstrate the inversion procedure, a 3-D stratified fluid is simulated and travel time data are calculated by path integration. These data are then inverted to recover both the scaler sound speed and the vorticity of the evolving flow.     

Tomographic reconstruction technique for pulsed-echo imaging

This article proposes a tomographic reconstruction technique using divergent ultrasonic waves of a circular arc wavefront which are formed by one-dimensional thin-phased array. The projection data which are obtained by phasing of transmitting and receiving signals are applied to the filtered back projection algorithm so that the image at the spherical surface is reconstructed. The experiments using a small steel sphere and a layer phantom of agar gel are performed using a commercially available phased-array system. The divergent waves are formed by the slit in the front of the array. As a result, images with a space-invariant, high-resolution unlike conventional B-scan image are obtained. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 432–437, 1997     

Dynamically adaptive mesh refinement technique for image reconstruction in optical tomography

A novel adaptive mesh technique is introduced for problems of image reconstruction in luminescence optical tomography. A dynamical adaptation of the three-dimensional scheme based on the finite-volume formulation reduces computational time and balances the ill-posed nature of the inverse problem. The arbitrary shape of the bounding surface is handled by an additional refinement of computational cells on the boundary. Dynamical shrinking of the search volume is introduced to improve computational performance and accuracy while locating the luminescence target. Light propagation in the medium is modeled by the telegraph equation, and the image-reconstruction algorithm is derived from the Fredholm integral equation of the first kind. Stability and computational efficiency of the introduced method are demonstrated for image reconstruction of one and two spherical luminescent objects embedded within a breastlike tissue phantom. Experimental measurements are simulated by the solution of the forward problem on a grid of 5x5 light guides attached to the surface of the phantom.     

高分辨率SAR与光学图像融合的建筑物三维重建研究

分析了用于建筑物测量的高分辨率SAR叠掩及角反射器效应等成像要素,并将其与高分辨率快鸟卫星图像融合,实现了建筑物屋顶信息的提取。进一步利用SAR叠掩得到的建筑物高度信息和快鸟图像得到的建筑物屋顶信息实现了建筑物的三维重建。与实地测量数据相比较,计算结果精度较高,从而验证了这种三维重建方法的可行性。     

Phosphorescence lifetime imaging in turbid media: the inverse problem and experimental image reconstruction

Three-dimensional phosphorescence lifetime imaging is a novel method for the mapping of oxygen concentration in biological tissues. We present reconstruction techniques for recovering phosphorescent objects in highly scattering media based on the telegraph equation and two regularization methods, i.e., the Tikhonov-Phillips regularization and the maximum entropy method. Theoretical results are experimentally validated, and the reconstructed images of phosphorescent objects rendering oxygen maps in a layer are presented.     

Tomographic reconstruction of sound fields using TV holography

Combining TV holography recording with acoustic phase stepping and image processing, we measure the integrated density distribution in sound fields that propagate in air. We record a given number of two-dimensional cross sections that are tomographically backprojected to give the amplitude and phase distributions of the emitted sound field. The validity of the procedure is demonstrated.     

Region-of-interest reconstruction from noisy projections using fractal models and Wiener filtering

In this paper, we present a method for region-of-interest (ROI) tomography using noisy projections. A wavelet decomposition down to the coarsest level is done on the noisy signal. The signal at various levels is estimated using a Wiener filter. By assuming that the projections are 1/f processes, the Wiener filtering reduces to a scalar multiplication. Using the Wiener filter and regularity property of the wavelets, we combine the estimation and localisation of the noisy projections for ROI imaging. Experimental results are shown on Shepp-Logan phantom and actual CT images. The validity of the 1/f model for projections of real life images is also shown. The names of the authors are in alphabetical order     

Tomographic optical breast imaging guided by three-dimensional mammography

We introduce a modified Tikhonov regularization method to include three-dimensional x-ray mammography as a prior in the diffuse optical tomography reconstruction. With simulations we show that the optical image reconstruction resolution and contrast are improved by implementing this x-ray-guided spatial constraint. We suggest an approach to find the optimal regularization parameters. The presented preliminary clinical result indicates the utility of the method.     

基于数字希尔伯特变换的OCT信号处理与系统实现

信号处理方法的选择是光学相干层析(OpticalCoherenceTomography,OCT)系统研制的重要环节,直接决定着OCT系统的硬件构建和实现。应用硬件实现OCT信号处理的方法虽然速度快,但存在硬件开销大,系统配置可调节性差等缺点。进而采用了基于数字希尔伯特变换的软件方法来处理OCT信号。该数字处理方法对探测得到的离散干涉实信号进行解析拓展,得到同时具有振幅信息和相位信息的复信号,其中的振幅信息被用于OCT的图像重建。介绍了研制的光纤OCT系统和各驱动单元的同步时序控制,最后给出了人体皮肤和洋葱的OCT高清晰层析图。