Computed tomography sound velocity reconstruction using incomplete data

An approach based on limited-angle transmission tomography for reconstruction of the sound velocity distribution in the breast is proposed. The imaging setup is similar to that of x-ray mammography. With this setup, the time-of-flight data are acquired by a linear array positioned at the top of the compressed breast that both transmits and receives, and a metal plate is placed at the bottom as a reflector. The setup allows acoustic data acquisition for simultaneous B-mode image formation and the tomographic sound velocity reconstruction. In order to improve the sound velocity estimation accuracy, a new reconstruction algorithm based on a convex programming formulation has been developed. Extensive simulations for both imaging and time-of-flight data based on a 5-MHz linear array were performed on tissues with different geometries and acoustic parameters. Results show that the sound velocity error was generally 1-3 m/s, with a maximum of 5.8 m/s. The radii of the objects under investigation varied from 2 to 6 mm, and all of them were detected successfully. Thus, the proposed approach has been shown to be both feasible and accurate. The approach can be used to complement conventional B-mode imaging to further enhance the detection of breast cancer.     

Limited-angle computed tomography for sandwich structures using data fusion

Both conventional and limited-angle computed-tomography (CT) are ill-posed problems, but where conventional CT has a small null space with few dimensions, limited-angle CT has a much larger null space and is severely ill-posed. The nature of the ill-posedness of limited-angle CT dictates that any limited-angle reconstruction method must interpolate from partial data in the Fourier domain. Large aluminum-core sandwich structures, such as those of aircraft control surfaces, simultaneously prevent full range data acquisition for CT and have face sheets that lie primarily in the limited-angle Radon transform null space. Consequently, interpolation of missing data is not possible and limited-angle CT for large sandwich structures is not feasible. This paper presents a novel method for limited-angle CT that addresses this problem. The method uses constraint-based data fusion to reduce the null space of the problem in a manner consistent with the specimen. Measurements of spatial support and face sheet thickness provide data for the additional constraints. By using the method, accurate reconstruction of sandwich structures becomes possible, where it would otherwise be impossible. The method of convex projections solves the inverse problem posed by the fusion system well because it is fast (for this problem) and easily incorporates the fusion constraints. Experimental results based on synthetic data show the improvement in reconstruction accuracy obtained by the fusion method. Real data results show the successful application of the method to a plexiglass phantom.     

超声衍射CT及其在不完全投影条件下的实验研究

超声衍射层析成像术是一种利用声波散射数据所携带的信息来反演物体内部结构的技术，具有广泛的应用前景。本文对这一领域的研究现状进行综述，慨述一些典型的超声衍射层析成像重构方法，同时探讨在残缺投影数据下的重构问题，对于不完全投影数据下的图像重构进行了实验研究。最后对实验结果进行讨论。     

Ultrasonic computed tomography reconstruction of the attenuation coefficient using a linear array

The attenuation coefficient distribution and sound velocity distribution in the breast can be used to complement B-mode ultrasound imaging in the detection of breast cancer. This study investigated an approach for reconstructing the attenuation coefficient distribution in the breast using a linear array. The imaging setup was identical to that for conventional B-mode breast imaging, and the same setup has been used for reconstruction of sound velocity distributions in previous studies. In this study, we further developed a reconstruction method for the attenuation coefficient distribution. In particular, the proposed method incorporates the segmentation information from B-mode images and uses the sound velocity distribution to compensate for refraction effects. Experiments were conducted with a setup consisting of a 5-MHz, 128-channel linear array, a programmable digital array system, a phantom, and a computer. The constructed phantom contained materials mimicking the following breast tissues: glandular tissue, fat, cysts, high-attenuation tumors, and irregular tumors. Application of the proposed technique resulted in all the cysts and tumors (including high-attenuation and irregular tumors) being distinguished by thresholding the reconstructed attenuation coefficients. We have demonstrated that it is possible to use the same imaging setup to acquire data for B-mode image, sound velocity distribution, and attenuation coefficient distribution simultaneously. Moreover, the experimental data indicate its potential in improving the detection of breast cancer.     

扩展经验正交函数沿海声层析的声速反演

为了实现沿岸海域声速的实时连续监测,针对目前单收发换能器沿海声层析系统的剖面反演声线不足和接收信号不稳定的问题,提出了双收发换能器沿海声层析（Double-transcever Coast Acoustical Tomography,DCAT）系统。通过计算机进行仿真模拟,结果表明DCAT系统声速反演精度提高了一个数量级,验证了该系统对存在问题的改善及其有效性。在声速反演过程中,通过使用扩展经验正交函数（Extension Empirical Orthogonal Function,EEOF）表示声速剖面,实现了剖面全水深的声速反演。DCAT系统的提出和EEOF方法的应用,为海上实时连续观测系统的建立及数据处理,提供了理论基础和有效的实施方法。     

利用残缺样本声速重构声速剖面

用经验正交函数(experiential orthogonal functions,EOF)表示声速剖面受限于样本声速的测量深度,应用该方法重构声速剖面只能计算到样本中最浅剖面的深度。要想进行全海深声速剖面的重构,必须对残缺的样本声速进行合理地外延。为此,首先对样本中温度和盐度进行了外延,然后根据声速经验公式计算得到了全海深的样本声速。在此基础上,通过解多元方程组的办法求解经验正交函数系数达到了声速剖面重构的目的。结果表明,提出的声速剖面外延方法是有效的。另外,只要知道声速剖面变化较剧烈深度上的3个点的声速值就能重构声速剖面,对于文中的数据来说,重构的均方根误差可达到0.872 m/s;增加经验正交函数的阶数能提高重构精度,但5阶以上,阶数的继续增加对精度的提高将不会有显著的影响。     

多波束测深中声速剖面的分层EOF自适应重构

在海水性质变化剧烈地区利用重构声速剖面进行多波束测深时,传统的经验正交函数(Empirical Orthogonal Functions,EOF)方法在阶次选取时未顾及声速浅水的复杂性和深水的平稳性的特点。针对传统EOF方法存在计算量大、精度低等问题,给出了一种声速剖面EOF重构中分层阶次的确定方法。根据常梯度声线跟踪计算深度,在满足0.25%倍水深限差的要求下,统计有效波束比,采用自适应方法确定出合理的阶次。实验结果表明,该方法相较于传统EOF重构声速剖面测深的阶次选取方法,降低了运算量,提高了精度。     

Performance evaluation of iterative tomography algorithms for incomplete projection data

Projection data obtained through optical techniques for tomographic measurements, such as interferometry for refractive-index-based measurements, are often incomplete. This is due to limitations in the optical system, data storage, and alignment and vignette issues. Algebraic iterative reconstruction techniques are usually favored for such incomplete projections. A number of iterative algorithms, based on additive and multiplicative corrections, are used with a known simulated phantom and noise source to assess the reconstruction performance of incomplete data sets. In addition, we present reconstructions using experimental data obtained from a coherent gradient sensing interferometer for a steady temperature field in a fluid medium. We tested the algorithms using the simulated data set for incompleteness conditions similar to those found in the experimental data, and the best-performing algorithm is identified.     

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.     

基于COMSOL的超声层析成像仿真及图像重建

为了验证三角形和四边形准则的二值逻辑反投影(Binary Logic Back Projection,BLBP)算法在超声层析成像重建过程中的可行性,以超声层析成像的重建区域中同时含有圆柱形和椭圆柱形障碍物为例,利用COMSOL对成像区域内存在障碍物的声场进行仿真,得到声场声压分布。然后,通过提取和处理所获取的声压分布,获得信号矩阵。最后,将信号矩阵代入二值逻辑反投影算法进行图像重建。仿真结果表明:当入射波的波长λ远小于障碍物的尺寸时,声衍射的现象并不明显。重建结果表明:两种准则在几何近似理论下可行,且四边形准则的效果优于三角形准则。     

A study of errors due to reconstruction filters in computed tomography

In the summation convolution backprojection method of image reconstruction in computed tomography, the final image accuracy depends on the convolution filter used. Filters are designed to attenuate high spatial frequencies when noisy projection data are used. This paper explores the differences between the images reconstructed using a range of filters, and compares the results with the case of the ramp filter that provides the “best” image for ideal, noise-free, projection data. It is shown that systematic errors between these images and the best image exist, and that these errors are related to the second differential of the reconstruction filter with respect to spatial frequency. This error determination may be used to correct computed tomography images that have been reconstructed using inappropriate filters, and this theory is tested using noise-free projection data from two computer simulated images. It is shown that the corrected images are far closer to the original images.     

Directional velocity estimation using focusing along the flow direction. II: Experimental investigation

For pt. I see ibid., vol. 50, no. 7, p. 857 (2003). A new method for directional velocity estimation is investigated through a number of flow rig measurements. The method uses beam-formation along the flow direction to generate data, where the correct velocity magnitude can directly be estimated from the shift in position of the received consecutive signals. The shift is found by cross-correlating the beamformed lines. The approach can find the velocity in any direction, including transverse to the traditionally emitted ultrasound beam. The method is investigated using a flow rig with a peak velocity of 0.15 m/s. A 7-MHz linear array transducer is used together with a dedicated sampling system to acquire signals from 64 transducer elements simultaneously. A technique for obtaining 128-element data using multiplexing is also presented. The data is beamformed off-line on a PC. A relative standard deviation of 1.4% can be obtained for a beam-to-flow angle of 45/spl deg/ and 4.3% at 90/spl deg/. Color flow images are displayed showing that the correct velocity magnitude can be obtained with the method for beam-to-flow angles of 60 and 90/spl deg/ with an accuracy of 3 to 4%.     

A new projection-based iterative image reconstruction algorithm for dual-energy computed tomography

Dual-energy CT can be represented as the dual-energy equations by decomposing the linear attenuation coefficient of the X-ray scanned object into two material basis functions of photoelectric absorption and Compton scatter. To solve the dual-energy equations, in this paper, we apply the mean-value theorem for integrals and propose a new projection-based iterative algorithm. We discuss the convergence of the proposed algorithm and carry out various numerical simulations for demonstrating its feasibility.     

Time‐varying reconstruction in single photon emission computed tomography

We propose a new method for reconstruction of images in single photon emission computed tomography (SPECT), when the activity distribution of the object is time‐varying. The activity evolution is modeled with the first‐order Markov model, and linear observation model is used to characterize the measurement system. The state‐space representation of the measurement sequence reduces to an ill‐conditioned state estimation problem, which is solved recursively using the Kalman filter and smoother algorithms. Two special models, the compartmental model and the diffusion model, for the time variation are discussed. The method is evaluated using simulations. © 2005 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 14, 186–197, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ima.20023     

Column-relaxed algebraic reconstruction algorithm for tomography with noisy data

A tomographic reconstruction algorithm similar to the well-known algebraic reconstruction technique (ART) is presented. Similar to ART, the approximate algebraic reconstruction technique (AART) technique consists of a sequence of displacements of the image vector based on the projection error. AART is a column-relaxation technique that is a series of vector displacements of the image vector parallel to its coordinate axes. AART is compared with ART, a standard conjugate-gradient technique, and a conjugate-gradient technique augmented by nonnegativity. The use of relaxation parameters to improve the performance of both ART and AART in the presence of noise is discussed, and the use of an iteration termination criterion based on random generalized cross validation is illustrated.     

Three-dimensional CAD model reconstruction from image data of computer tomography

Three-dimensional (3D) shape reconstruction from 2D image data has been receiving extensive attention in medical engineering. It can enhance accurate diagnosis of the disease from medical images of computer tomography (CT) and magnetic resonance imaging (MRI). An algorithm based on reverse engineering technique was proposed in this work for 3D surface reconstruction of CT images. Several image processing techniques were applied first to detect the 2D contour of the object for each of the CT images. A surface lofting approach was then employed to fit the 2D contours into a 3D surface model. A brain example was presented to demonstrate the feasibility of the proposed method. © 1999 John Wiley & Sons, Inc. Int J Imaging Syst Technol 10, 328–338, 1999     

基于不完备实测模态数据的结构损伤识别方法研究

在传统的基于模型修正的损伤识别中,由于实测模态信息有限而待识别参数过多,往往导致损伤识别方程出现较大误差,从而限制了该方法在复杂结构中的应用。为解决这一问题,对结构的自由度进行了分解,将损伤结构中模态振型的未测量部分表达为已测量到的模态振型、模态频率以及结构其它参数的函数。将损伤视为结构单元刚度的减小,利用完好结构的计算模态数据以及损伤结构扩充后的实测模态数据,建立了结构的损伤识别方程。运用信赖域优化算法对具有双重约束条件的目标函数进行最小化,识别出了结构各单元的刚度损伤参数。通过两个损伤识别数值仿真算例及实验验证,结果表明,在测点数量有限及测试噪声等不利因素影响下,所提方法只需运用少量的实测模态信息,即可实现结构损伤位置及程度的准确识别,同时算法具有较好的鲁棒性。     

Patient doses using multidetector computed tomography scanners in Kenya

Assessment of patient dose attributed to multislice computed tomography (CT) examination. A questionnaire method was developed and used in recording the patient dose and scanning parameters for the head, chest, abdomen and lumbar spine examinations. The patient doses due to brain, chest and abdomen examination were above the international diagnostic reference levels (DRLs) by factors of between one and four. The study demonstrated that the use of multislice CT elevates patient radiation dose, justifying the need for local optimised scanning protocols and the use of institutional DRL for dose management without affecting diagnostic image quality.