Parallel processor implementation in computerized tomography using transputers : a thesis presented in partial fulfilment of the requirement for the degree of Master of Technology in Production Technology at Massey University

Image reconstruction by computerized tomography provides a nonintrusive method of imaging the internal structure of objects. From measurements of radiation (e.g. X-rays or gamma rays) passed through an object, it is possible to observe the internal structure. The reconstruction process is computationally intensive and requires imaginative parallel processing algorithms to attain 'real­ time' performance. The Inmos transputer makes parallel processing algorithms both feasible and relatively straight forward. In this thesis, a modification to the backprojection algorithm is introduced in order to improve the speed of the implementation. Work carried out has involved evaluating how these algorithms ( convolution, backprojection and interpolation ) can be used in multiprocessor concurrent architecture to obtain rapid image reconstruction. Several suitable transputer network structures have been advanced to simulate the image reconstruction. The reconstruction time is decreased very greatly and the image reconstruction result is good

Parallel algorithms for three dimensional electrical impedance tomography

This thesis is concerned with Electrical Impedance Tomography (EIT), an imaging technique in which pictures of the electrical impedance within a volume are formed from current and voltage measurements made on the surface of the volume. The focus of the thesis is the mathematical and numerical aspects of reconstructing the impedance image from the measured data (the reconstruction problem). The reconstruction problem is mathematically difficult and most reconstruction algorithms are computationally intensive. Many of the potential applications of EIT in medical diagnosis and industrial process control depend upon rapid reconstruction of images. The aim of this investigation is to find algorithms and numerical techniques that lead to fast reconstruction while respecting the real mathematical difficulties involved. A general framework for Newton based reconstruction algorithms is developed which describes a large number of the reconstruction algorithms used by other investigators. Optimal experiments are defined in terms of current drive and voltage measurement patterns and it is shown that adaptive current reconstruction algorithms are a special case of their use. This leads to a new reconstruction algorithm using optimal experiments which is considerably faster than other methods of the Newton type. A tomograph is tested to measure the magnitude of the major sources of error in the data used for image reconstruction. An investigation into the numerical stability of reconstruction algorithms identifies the resulting uncertainty in the impedance image. A new data collection strategy and a numerical forward model are developed which minimise the effects of, previously, major sources of error. A reconstruction program is written for a range of Multiple Instruction Multiple Data, (MIMD), distributed memory, parallel computers. These machines promise high computational power for low cost and so look promising as components in medical tomographs. The performance of several reconstruction algorithms on these computers is analysed in detail

Image reconstruction technique for ultrasonic transmission tomography

Precise flow control has always been a necessity for developing easier approaches or instrumentation for two-phase flow regime. An important method for monitoring this process is called process tomography such as electrical tomography, optical tomography and ultrasonic tomography (UT). In the case of high-acoustic impedance mixtures e.g. bubbly flow, UT has the advantages in monitoring real time data. Although various researches were conducted using UT systems in bubbly flow regimes, there are still weaknesses especially in real time image reconstruction techniques for monitoring the process. Some efforts such as linear back projection (LBP), filter back projection (FBP), convolution back projection (CBP) and iterative techniques are utilized for reconstructing the image with few views data for UT system. Regardless of the utilized method there still exist two main issues in UT image reconstruction both in forward and inverse problems. In the case of forward problem, the gaps between sensitivity maps cause artifacts in a reconstructed image. Moreover, for inverse problem, limited number of sensors causes artifacts in reconstructed image. In the case of high noisy environment, the LBP, FBP and CBP methods are not capable of totally removing the noise and artifacts level. Dynamic motion of flow regime is considered as another issue in UT system which causes inaccuracy in image reconstruction. Therefore, these issues were considered in developing a modified image reconstruction algorithm which was based on improving the CBP algorithm both in forward and inverse problems. A modified sensitivity map based on Gaussian distribution was utilized to combat the gaps in forward problem, and for the case of inverse problem, the wavelet fusion technique was applied to reduce the noise level, artifacts and the effects of dynamic motions. The simulation and the experimental works had been conducted based on different static profiles. Various types of image reconstruction algorithms were implemented and compared with the proposed technique. The quality of the final reconstructed images was evaluated using structural similarity (SSIM) and peak signal to noise ratio (PSNR). Results show that the WCBP outperforms LBP and CBP in case of SSIM and PSNR. Comparing to LBP, the SSIM and PSNR were improved at least by 30% and 5% respectively while for CBP the improvement were about 5% and 1% respectively

Image reconstruction technique via ultrasonic tomography system for metal pipe

Detection of concentration of the gas/liquid mixture inside the pipe with Ultrasonic Tomography (UT) has been investigated seriously with various researchers in the recent decade. To date, most of the researches of ut focus on Acrylic or PVC as pipe material. This research investigates the usage of metal pipe for ut application. The attenuation problem of metal pipe is due to high acoustic impedance mismatch between liquid and metal pipe. Based on the problems of metal pipe for application in the UT system, various alternatives are presented in this research. Modelling of the UT using COMSOL software is studied to visualize the real UT system. Various frequencies are tested to determine the optimum frequency of the UT system. The hardware of UT system is developed after selection of the suitable transceiver. The structures of the transmitter and receiver circuits are developed in order to improve the Signal to Noise Ratio (SNR) and functionality of circuits. The sampled signals are preceded to the computer via Data Acquisition (DAQ) system. Various algorithms are investigated to produce the best image reconstruction of the UT system. As the basic and convenient algorithm, Linear Back Projection (LBP) is used for reconstructing the primary image. Median Filter Back Projection (MFBP) and Disk Filter Back Projection (DFBP) are applied to improve the image quality of LBP algorithm. Additionally, the Circular Thresholding Segmentation (CTS) algorithm is applied to produce the segmented thresholding images. Based on the simulation results, 40 kHz is determined as the optimum frequency of UT system. The designed UT system for the metal pipe is experimentally tested and cross-sectional images are extracted from metal pipe. Additionally, this thesis presents the static and dynamic results of UT system from metal pipe. Based on the comparison between the performances of applied algorithms, the CTS algorithm has the best results due the minimum errors between original images and reconstructed images. The obtained results corroborate the

Application of constrained optimisation techniques in electrical impedance tomography

A Constrained Optimisation technique is described for the reconstruction of temporal resistivity images. The approach solves the Inverse problem by optimising a cost function under constraints, in the form of normalised boundary potentials. Mathematical models have been developed for two different data collection methods for the chosen criterion. Both of these models express the reconstructed image in terms of one dimensional (I-D) Lagrange multiplier functions. The reconstruction problem becomes one of estimating these 1-D functions from the normalised boundary potentials. These models are based on a cost criterion of the minimisation of the variance between the reconstructed resistivity distribution and the true resistivity distribution. The methods presented In this research extend the algorithms previously developed for X-ray systems. Computational efficiency is enhanced by exploiting the structure of the associated system matrices. The structure of the system matrices was preserved in the Electrical Impedance Tomography (EIT) implementations by applying a weighting due to non-linear current distribution during the backprojection of the Lagrange multiplier functions. In order to obtain the best possible reconstruction it is important to consider the effects of noise in the boundary data. This is achieved by using a fast algorithm which matches the statistics of the error in the approximate inverse of the associated system matrix with the statistics of the noise error in the boundary data. This yields the optimum solution with the available boundary data. Novel approaches have been developed to produce the Lagrange multiplier functions. Two alternative methods are given for the design of VLSI implementations of hardware accelerators to improve computational efficiencies. These accelerators are designed to implement parallel geometries and are modelled using a verification description language to assess their performance capabilities

Tomography applied to Lamb wave contact scanning nondestructive evaluation

The aging world-wide aviation fleet requires methods for accurately predicting the presence of structural flaws that compromise airworthiness in aircraft structures. Nondestructive Evaluation (NDE) provides the means to assess these structures quickly, quantitatively, and noninvasively. Ultrasonic guided waves, Lamb waves, are useful for evaluating the plate and shell structures common in aerospace applications. The amplitude and time-of-flight of Lamb waves depend on the material properties and thickness of a medium, and so they can be used to detect any areas of differing thickness or material properties which indicate flaws. By scanning sending and receiving transducers over an aircraft, large sections can be evaluated after a single pass. However, while this technique enables the detection of areas of structural deterioration, it does not allow for the quantification of the extent of that deterioration. Tomographic reconstruction with Lamb waves allows for the accurate reconstruction of the variation of quantities of interest, such as thickness, throughout the investigated region, and it presents the data as a quantitative map. The location, shape, and extent of any flaw region can then be easily extracted from this Tomographic image. Two Lamb wave tomography techniques using Parallel Projection tomography (PPT) and Cross Borehole tomography (CBT), are shown to accurately reconstruct flaws of interest to the aircraft industry. A comparison of the quality of reconstruction and practicality is then made between these two methods, and their limitations are discussed and shown experimentally. Higher order plate theory is used to derive analytical solutions for the scattering of the lowest order symmetric Lamb wave from a circular inclusion, and these solutions are used to explain the scattering effects seen in the Tomographic reconstructions. Finally, the means by which this scattering theory can be used to develop Lamb wave Tomographic algorithms that are more generally applicable in-the-field, is presented

A tomographic imaging system for pneumatic conveyors using optical fibres.

This thesis presents an investigation into the application of optical fibre sensors to a tomographic imaging system.Several sensing mechanisms for measurement using non-intrusive techniques are discussed and there relevance to pneumatic conveying discussed. Optical systems are shown to be worthy of investigation. The optical sensor is modelled to predict the expected sensor output voltage profiles arising from different, artificially produced flow regimes. These artificial flow regimes are created by placing a shaped obstruction inside a gravity drop conveyor in the path of the flowing solids. It is shown that for two arrays, each consisting of sixteen transducers, approximately 30% of the measurement volume is sampled.An image reconstruction method for optical tomography is described, based on the back projection between view lines algorithm.The design of the optical tomography system is described, with emphasis on preparation of the ends of the optical fibre, beam collimation and design of the transmitter and receiver circuits.The optical sensors are evaluated singly and as a tomographic array. Results relating to concentration measurement are presented for solids flow using sand with a mean of 300 micron and plastic beads of 2 mm nominal diameter. Measurements were made with a single optical sensor using the gravity flow rig. The results demonstrate the suitability of the optical sensor for concentration measurement for lightly loaded flows (up to approximately 2% solids by volume in the test). The test is extended to all thirty-two sensors using a range of solids mass flow rates from 40 to 320 gm/s with both dry sand and plastic beads over a range of artificially created flow regimes. The results obtained by comparing the measured and predicted flowrates show good general agreement. The statistical parameters for the error of the sand flow measurement have been calculated as having a mean of 6.76% and standard deviation of 3.94% and for plastic beads is 5.43% and standard deviation of 0.21%. The results also demonstrates that the system is reasonably independent of flow regime and so the optical fibre system is suitable as a concentration meter.Back projection is used to generate tomographic images as an alternative representation of the data on concentration measurement. This provides a visual representation of optical density (concentration) information which is not obvious from the concentration measurements.Results from experiments on particles with different sizes are presented. The results are analysed using frequency spectrum techniques and shown to be dependent upon the particle size for approximately spherical particles with diameters between 600 |im and 5 mm.Suggestions for further work on optical fibre sensors and optical fibre tomographic measurements are made

Institute of Safety Research, Annual Report 1995

The report gives an overview on the scientific work of the Institute of Safety Research in 1995

Progress Report No. 23

Progress report of the Biomedical Computer Laboratory, covering period 1 July 1986 to 30 June 1987