T2 restoration and noise suppression of hybrid MR images using Wiener and linear prediction techniques

The authors address the problem of enhancing hybrid magnetic resonance (MR) images degraded by T2 effects and additive measurement noise. To reduce imaging time, MR signals are acquired using hybrid imaging (HI) sequences such as rapid acquisition relaxation-enhanced (RARE) and fast spin-echo (FSE). With these techniques, T2 effects act as a distortion filter. This T2 filter affects the signal and results in image spatial resolution and/or contrast loss. Furthermore, the amplitude and phase discontinuities in the T2 filter frequency response function may generate serious ringing artifacts. These distortions will damage image quality and affect object detectability. The authors use the Wiener filter and linear prediction (LP) technique to process HI MR signals in the spatial frequency domain (K-space) and the hybrid domain, respectively. Based on the average amplitude symmetry constraint of the spin echo signal, the amplitude frequency response function of the T2 distortion filter can be estimated and used in the Wiener filter for a global T2 amplitude restoration. Then, the linear prediction technique is utilized to obtain the local signal amplitude and phase estimates around the discontinuities of the frequency response function of the T2 filter. These estimates are used to make local amplitude and phase corrections. The effectiveness of this combined technique in correcting T2 distortion and reducing the measurement noise is analyzed and demonstrated using experiments on both phantoms and human studies.     

Motion-induced phase error estimation and correction in 3D diffusion tensor imaging

A multishot data acquisition strategy is one way to mitigate B0 distortion and T2? blurring for high-resolution diffusion-weighted magnetic resonance imaging experiments. However, different object motions that take place during different shots cause phase inconsistencies in the data, leading to significant image artifacts. This work proposes a maximum likelihood estimation and k-space correction of motion-induced phase errors in 3D multishot diffusion tensor imaging. The proposed error estimation is robust, unbiased, and approaches the Cramer-Rao lower bound. For rigid body motion, the proposed correction effectively removes motion-induced phase errors regardless of the k-space trajectory used and gives comparable performance to the more computationally expensive 3D iterative nonlinear phase error correction method. The method has been extended to handle multichannel data collected using phased-array coils. Simulation and in vivo data are shown to demonstrate the performance of the method.     

Advanced Data Acquisition Methods for Quantitative Single Photon Emission CT

This paper proposes some advanced data acquisition methods for quantitative single photon emission CT. One example of such methods is the odd number angular sampling method over 360 degrees for a widely distributed object, and the other is fine angular sampling of 180 degrees for a nonsymmetrically localized small object in an attenuation cross section. The image quality, such as the spatial resolution and the signal-to-noise ratio, becomes better than that reconstructed from conventional conjugate projection data. The reconstruction technique used in computer simulation is the iterative correction matrix method, but other techniques are also applicable. Comparison between the weighted back projection method and the correction matrix method is also presented.     

Combined MR data acquisition of multicontrast images using variable acquisition parameters and K-space data sharing

A new technique to reduce clinical magnetic resonance imaging (MRI) scan time by varying acquisition parameters and sharing k-space data between images, is proposed. To improve data utilization, acquisition of multiple images of different contrast is combined into a single scan, with variable acquisition parameters including repetition time (TR), echo time (TE), and echo train length (ETL). This approach is thus referred to as a "combo acquisition." As a proof of concept, simulations of MRI experiments using spin echo (SE) and fast SE (FSE) sequences were performed based on Bloch equations. Predicted scan time reductions of 25%-50% were achieved for 2-contrast and 3-contrast combo acquisitions. Artifacts caused by nonuniform k-space data weighting were suppressed through semi-empirical optimization of parameter variation schemes and the phase encoding order. Optimization was assessed by minimizing three quantitative criteria: energy of the "residue point spread function (PSF)," energy of "residue profiles" across sharp tissue boundaries, and energy of "residue images." In addition, results were further evaluated by quantitatively analyzing the preservation of contrast, the PSF, and the signal-to-noise ratio. Finally, conspicuity of lesions was investigated for combo acquisitions in comparison with standard scans. Implications and challenges for the practical use of combo acquisitions are discussed.     

幅相误差下基于CMCAP-JDL的低空风切变风速估计

针对机载气象雷达存在幅相误差的情况下，导致杂波抑制性能下降，低空风切变风速估计结果不准确的问题，本文提出了一种基于组合空时主通道-局域联合空时自适应处理（Combined Space-time Main Channel Adaptive Processing, CMCAP-JDL）低空风切变风速估计方法。该方法首先对雷达回波数据进行距离依赖性矫正及杂波协方差矩阵估计；然后对空域和时域加权，形成空域主波束和时域主通道，同时增加多个局域处理区域参与联合处理的方式来构造自适应降维处理器；最后使用权矢量对降维后的回波数据实现自适应滤波，完成幅相误差情况下风场速度的准确估计。仿真结果表明，该方法能够在幅相误差的情况下，实现低空风切变风速的准确估计。      

An Approach to Optimal Threshold Selection on a Sequence of Two-Dimensional Echocardiographic Images

In our laboratory, thresholding has been found to have a remarkable effect on the detection of moving boundaries in two-dimensional echocardiographic images (2D echo). In this note, a technique for selection of an optimum threshold for a sequence of 2D echo frames by means of counting the maximum number of moving points in two consecutive frames is explored. This threshold is applied to the sequence of images to convert them to binary form prior to dynamic scene analysis. Experimental results of threshold images are shown with 2D echos obtained from two patients.     

Improved iterative image reconstruction with automatic noise artifact suppression

A method for stabilizing iterative image reconstruction techniques has been developed for improving the image quality of position emission tomography. A damping matrix is introduced, which suppresses noisy correction on a pixel-by-pixel basis, depending on the statistical precision of the iterative correction. The precision is evaluated by comparing a certain number of correction submatrices, each of which is formed from a subset of the projection data. Simulation studies showed that statistical noise is effectively suppressed, while the image of the source object is reconstructed with high resolution, as long as the signal level is higher than the local noise level. In the application to the MLE (maximum likelihood estimator), the minimum RMS error of the image was reduced to 84% for 500 k total counts, and the RMS error increased more slowly with further iterations as compared with the simple MLE. The method was also applied to the FIR (filtered iterative reconstruction) algorithm, and the images were found to be better than those obtained by the convolution backprojection method.     

存在站址误差下的无源雷达稀疏成像

无源雷达稀疏成像要求精确已知系统观测矩阵,然而在实际应用中通常存在发射机和接收机站址误差,会使得雷达回波模型中的观测矩阵部分未知,导致回波测量值与观测矩阵失配,将大大减弱传统稀疏成像算法的性能。首先对存在收发阵元位置误差下的无源成像进行了建模分析,接着提出一种基于优化迭代技术的自适应相位误差校正成像方法,可以在重构目标图像的同时消除相位误差对无源成像的影响。仿真结果验证了所提成像方法的有效性。     

Optimal CT scanning plan for long-bone 3-D reconstruction

Digital computed tomographic (CT) data are widely used in three-dimensional (3-D) construction of bone geometry and density features for 3-D modelling purposes. During in vivo CT data acquisition the number of scans must be limited in order to protect patients from the risks related to X-ray absorption. The aim of this work is to automatically define, given a finite number of CT slices, the scanning plan which returns the optimal 3-D reconstruction of a bone segment from in vivo acquired CT images. An optimization algorithm based on a Discard-Insert-Exchange technique has been developed. In the proposed method the optimal scanning sequence is searched by minimizing the overall reconstruction error of a two-dimensional (2-D) prescanning image: an anterior-posterior (AP) X-ray projection of the bone segment. This approach has been validated in vitro on 3 different femurs. The 3-D reconstruction errors obtained through the optimization of the scanning plan on the 3-D prescanning images and on the corresponding 3-D data sets have been compared. 2-D and 3-D data sets have been reconstructed by linear interpolation along the longitudinal axis. Results show that direct 3-D optimization yields root mean square reconstruction errors which are only 4%-7% lower than the 2-D-optimized plan, thus proving that 2-D-optimization provides a good suboptimal scanning plan for 3-D reconstruction. Further on, 3-D reconstruction errors given by the optimized scanning plan and a standard radiological protocol for long bones have been compared. Results show that the optimized plan yields 20%-50% lower 3-D reconstruction errors     

基于CRC纠错与迭代干扰消除的星载AIS接收机

为了提升星载船舶自动识别系统( AIS)的卫星接收性能,提出了一种采用4 b循环冗余校验( CRC)纠错与迭代干扰消除算法的星载AIS接收方案。针对星载AIS系统中多路信号碰撞明显的情况,首先,通过分析星载AIS系统中CRC错误类型,采用比特反转的方式进行纠错;其次,对纠错后的序列进行AIS数据重构,采用迭代干扰消除方法消除重构信号,并将余下信号作为输入信号再次进入系统进行解调。仿真结果表明,和传统的CRC纠错系统相比,针对典型的两路信号碰撞情况,基于4 b CRC校验与迭代干扰消除的星载AIS系统能够带来1~2 dB的误码率增益,对星载AIS系统的性能提升有重要指导意义。     

Performance analysis of a hybrid DS/SFH CDMA system usinganalytical and measured pico cellular channels

This paper presents the throughput and delay analysis of a packet-switched code division multiple access (CDMA) network based on the hybrid direct sequence (DS)/slow frequency hopping (SFH) spread-spectrum multiple access (SS MA) technique with Q-, B-, and D-PSK modulation using analytical and measured pico cellular channels. The performance of the hybrid DS/SFH, DS, and SFH multiple access techniques have been compared in a pico cellular personal communications network (PCN) environment. Multipath and multiple access interference are considered. The performance is evaluated for a given delay spread and a fixed bandwidth. The effects of forward error correction (FEC) coding and diversity techniques, such as selection diversity and maximal ratio combining on the performance, are also investigated     

Diagnosis and correction of multiple logic design errors in digitalcircuits

This paper presents a technique to correct multiple logic design errors in a gate-level netlist. A number of methods have been proposed for correcting single logic design errors. However, the extension of these methods to more than one error is still very limited. We direct our attention to circuits with a low multiplicity of errors. By assuming different error dependency scenarios, multiple errors are corrected by repeatedly applying a single error search and correction algorithm. Experimental results on correcting double-design errors and triple-design errors on ISCAS and MCNC benchmark circuits are included     

Registration of 3-D images using weighted geometrical features

The authors present a weighted geometrical feature (WGF) registration algorithm. Its efficacy is demonstrated by combining points and a surface. The technique is an extension of Besl and McKay's (1992) iterative closest point (ICP) algorithm. The authors use the WGF algorithm to register X-ray computed tomography (CT) and T2-weighted magnetic resonance (MR) volume head images acquired from eleven patients that underwent craniotomies in a neurosurgical clinical trial. Each patient had five external markers attached to transcutaneous posts screwed into the outer table of the skull. The authors define registration error as the distance between positions of corresponding markers that are not used for registration. The CT and MR images are registered using fiducial paints (marker positions) only, a surface only, and various weighted combinations of points and a surface. The CT surface is derived from contours corresponding to the inner surface of the skull. The MR surface is derived from contours corresponding to the cerebrospinal fluid (CSF)-dura interface. Registration using points and a surface is found to be significantly more accurate then registration using only points or a surface     

Navigator echo motion artifact suppression in synthetic aperture ultrasound imaging

We develop a simple yet effective technique for motion artifact suppression in ultrasound images reconstructed from multiple acquisitions. Assuming a rigid-body motion model, a navigator echo is computed for each acquisition and then registered to estimate the motion in between acquisitions. By detecting this motion, it is possible to compensate for it in the reconstruction step to obtain images that are free of lateral motion artifacts. The theory and practical implementation details are described and the performance is analyzed using computer simulations as well as real data. The results indicate the potential of the new method for real-time implementation in lower cost ultrasound imaging systems.     

Propagation of measurement noise through backprojection reconstruction in electrical impedance tomography

A framework to analyze the propagation of measurement noise through backprojection reconstruction algorithms in electrical impedance tomography (EIT) is presented. Two measurement noise sources were considered: noise in the current drivers and in the voltage detectors. The influence of the acquisition system architecture (serial/semi-parallel) is also discussed. Three variants of backprojection reconstruction are studied: basic (unweighted), weighted and exponential backprojection. The results of error propagation theory have been compared with those obtained from simulated and experimental data. This comparison shows that the approach provides a good estimate of the reconstruction error variance. It is argued that the reconstruction error in EIT images obtained via backprojection can be approximately modeled as a spatially nonstationary Gaussian distribution. This methodology allows us to develop a spatial characterization of the reconstruction error in EIT images.     

LROC analysis of detector-response compensation in SPECT

Localization ROC (LROC) observer studies examined whether detector response compensation (DRC) in ordered-subset, expectation-maximization (OSEM) reconstructions helps in the detection and localization of hot tumors. Simulated gallium (Ga-67) images of the thoracic region were used in the study. The projection data modeled the acquisition of attenuated 93- and 185-keV photons with a medium-energy parallel-hole collimator, but scatter was not modeled. Images were reconstructed with five strategies: 1) OSEM with no DRC; 2) OSEM preceded by restoration filtering; 3) OSEM with iterative DRC; 4) OSEM with an ideal DRC; and 5) filtered backprojection (FBP) with no DRC. All strategies included attenuation correction. There were four LROC studies conducted. In a study using a single tumor activity, the ideal DRC offered the best performance, followed by iterative DRC, restoration filtering, OSEM with no DRC, and FBP. Statistical significance at the 5% level was found between all pairs of strategies except for restoration filtering and OSEM with no DRC. A similar ranking was found for a more realistic study using multiple tumor activities. Additional studies considered the effects of OSEM iteration number and tumor activity on the detection improvement that iterative DRC offered with respect to OSEM with no DRC.     

Hybrid acquisition of direct sequence CDMA signals

In direct sequence code division multiple access (DS-CDMA) systems, signal acquisition is necessary before communication can commence. Recent work has shown that the problem of acquisition may be even more restrictive than the problem of error control in limiting the capacity and performance of these systems. Passive matched filters and parallel search schemes have been shown to be able to acquire signals rapidly, but they do so at the cost of high or prohibitive complexity. In contrast, straight serial search schemes have lower complexity but acquire the signal much more slowly. In this paper, we studyhybrid active correlation schemes that provide flexibility in the trade-off between acquisition speed and complexity. These techniques test several phases concurrently and either decide that a particular phase is correct, in which case the decision is verified by a binary hypothesis test, or that none of the phases are correct, in which case another group of phases is tested. Two new hybrid acquisition schemes are presented; the first is based on a fixed-sample-size weighted MAP test, and the second is based on anM-ary sequential hypothesis test called MSPRT. It is shown that the weighted MAP-based scheme hypothesis test called MSPRT. It is shown that the weighted MAP-based scheme outperforms a standard MAP-based scheme. It is also shown that considerable performance gain can be obtained using sequential testing. In particular, it is shown through numerical examples that MSPRT-based schemes are several times faster than corresponding fixed-sample-size schemes.This paper has been presented in part at the 1994 IEEE International Conference on Communications, New Orleans, Louisana, May 1–5, 1994.     

Rapid MRI of short t2 species with multiple small angle excitations and time-varying gradients

Images weighted by transverse relaxation time T2 have established efficacy in tissue characterization and enhanced the sensitivity and specificity of the diagnosis of pathology in many organ systems. While relatively long echo times (TE) on the order of 50-100 ms appear to improve image contrast, they do so at the expense of a loss of information, namely short T2 species. This precludes the correlation of complex determinants of tissue T2 decay such as the modulation of intrinsic dipolar interactions by molecular processes like proton exchange, proton transfer, and diffusion, with biological factors such as the degree of tissue hydration (compartmentalization), oxygen content, and temperature. We present a rapid MRI technique that employs multiple small angle excitations synchronized to periodic gradient fields to image both a short T2 species that is not visualized and several long T2 species that are seen with a current spin echo and gradient recalled echo technique (TE = 12.5 ms). The unique phase modulation to which each point in space is subjected is decoded through long term integration of the product of the observed periodic steady-state (dynamic equilibrium) signal and a phase demodulation kernel. Whereas gradient recalled echo techniques with static readout gradients are extremely sensitive to main magnet inhomogeneities, susceptibility changes, and chemical shifts, the decoding operation above can be modified to provide a relatively high degree of immunity to these variations. Furthermore, continuous cosinusoidal gradients enable the use of resonant gradient circuitry to eliminate switching transients and to dissipate less power for comparable amplitudes.     

Motion correction for coronary stent reconstruction from rotational X-ray projection sequences

This paper presents a new method for 3-D tomographic reconstruction of stent in X-ray cardiac rotational angiography. The method relies on 2-D motion correction from two radiopaque markerballs located on each side of the stent. The two markerballs are on a guidewire and linked to the balloon, which is introduced into the artery. Once the balloon has been inflated, deflated, and the stent deployed, a rotational sequence around the patient is acquired. Under the assumption that the guidewire and the stent have the same 3-D motion during rotational acquisition, we developed an algorithm to correct cardiac stent motion on the 2-D X-ray projection images. The 3-D image of the deployed stent is then reconstructed with the Feldkamp algorithm using all the available projections. Although the correction is an approximation, we show that the intrinsic geometrical error of our method has no visual impact on the reconstruction when the 2-D markerball centers are exactly detected and the markerballs have the same 3-D motion as the stent. Qualitative and quantitative results on simulated sequences under different realistic conditions demonstrate the robustness of the method. Finally, results from animal data acquired on a rotational angiography device are presented.     

Electromagnetic imaging of underground targets using constrainedoptimization

Several high-frequency electromagnetic techniques have been used in recent years to detect and identify buried objects. Post-processing of the collected data is performed in many of these techniques to obtain high-quality images of buried targets. Accurate reconstructions of the target's constitutive parameters can be obtained by casting the imaging problem in terms of an inverse electromagnetic scattering problem. A number of techniques have been put forth recently to invert the electromagnetic data to obtain such images. The authors use a frequency-domain Born iterative method to reconstruct images of shallow targets. The Born iterative technique requires successive solutions to a forward scattering problem followed by an inverse scattering problem at each iteration step. They use a finite-difference time-domain (FDTD) algorithm to solve the forward scattering problem and constrained optimization for the inverse problem. Two-dimensional simulated data for several canonical objects buried in the ground are obtained using the FDTD technique. The same FDTD code is also used in calculating the Green's function required for solving the constrained optimization problem. Lossy, inhomogeneous ground models are used in several simulations to illustrate the use of this technique for practical situations. The inversion process can be used to reconstruct images for many realistic dielectric contrasts for which a linear Born approximation fails. Moreover, it is also shown that a small number of measurements results in accurate reconstructions with this technique. Use of multiple frequencies is also investigated