Showing papers on "Iterative reconstruction published in 1975"

Theory of image reconstruction in computed tomography.

Abstract: Mathematical methods are of central importance in the new technologies of radiographic and radioisotopic image reconstruction. The most important procedures are classified as Back-projection, iterative, and analytical (Two-dimensional Fourier, Filtered Back-projection). Back-projection played an important historical role but is no longer used because of sizable artifacts. Analytical methods excel in speed and accuracy when a large number of projections are available and are extensively used in x-ray imaging. Iterative reconstruction is more attractive when the number of views is limited, if noise is significant, and if additional factors, e.g., gamma-ray attenuation, are present. For these reasons, iterative methods are widely used in radioisotope imaging.

Ripple suppression during reconstruction in transverse tomography

Abstract: In many transverse tomographic reconstruction techniques using filtered projections, ringing or overshoot is found in regions where the attenuation suddenly changes. Such artifacts are obtained with iterative techniques as well. A modification of the filter often used in corrected tomography which produces very small ringing in such regions is described. A transmission device such as the EMI Scanner was computer simulated. Projections were calculated through a phantom and the phantom then reconstructed using different filters. The phantom consisted of a disc with diameter 32 arbitrary units centred at the origin of the plane.

Picture Reconstruction from Projections

Abstract: In this paper we will present two methods for the reronstruction of a two-dimensional picture from its one-dimensional projections at various angles. The first method is recursive, and the second is nonrecursive. Both of these methods have potential applications in picture transmission, and offer valuable insight into the more general problem of reconstruction of three-dimensional objects from their two-dimensional projections. We will illustrate the numerical details of each method by considering the reconstruction of a picture discretized on a 32 X 32 grid, and compare their relative merits. These two methods will also be compared with other methods suggested in the literature.

Axial tomography and three dimensional image reconstruction

Abstract: A number of existing $gamma$ cameras for imaging of radioisotope distributions give depth information about the distribution. These devices have in common that they provide tomographic images of the object, that is, that images of a given object plane have that plane in focus and all other object planes contribute an out-of-focus background superimposed on the in-focus image. A method is described for three dimensional reconstruction of these axial tomographic images which removes the blurred off-plane activity from a number of transverse planes simultaneously. The method is applicable to a number of tomographic cameras, such as the multiple single-pinhole camera, the rotating slanted-hole collimator, the Anger focussing tomographic scanner, and the positron camera. The method can be implemented on a small computer having a disc system. (auth)

Spectral effects on three-dimensional reconstruction from x-rays

Abstract: Continuous bremsstrahlung spectra were calculated for 120 kVp for constant and sinusoidal potentials. Fluorescent radiation for the tungsten target was added to the bremsstrahlung, and the spectra were attenuated through various filter materials. A drawing of an object to be scanned was divided into an array of small squares in which the composition was assumed to be constant. Transmission data for 120 rays at each of 120 angles spanning a range of 180$sup 0$ were calculated. Two algorithms for the reconstruction of attenuation coefficients from projection data, an algebraic reconstruction technique (ART) and the convolution method, were utilized to reconstruct effective coefficients. The effect of spectral filtration on the quality of the reconstruction was evaluated. Lightly filtered x-ray beams give rise to severe distortions in image quality, with values of the reconstructed coefficients rising toward the periphery of the object. Highly filtered beams give rise to images with less pronounced distortion. (AIP)

A Comparative Study of 3-D Image Reconstruction Algorithms with Reference to Number of Projections and Noise Filtering

Abstract: A comparative study of four popular 3-D image reconstruction algorithms has been made. Particular attention was given to artifacts generated and noise sensitivity. The methods considered include two spatial domain convolution algorithms, the Linear Superposition with Compensation (LSC) and a Fourier Convolution Method (FCM), a direct Fast Fourier Transform method (FFT), and an algebraic technique, the Simultaneous Iterative Reconstruction Technique (SIRT). The methods were compared by computing reconstructed images for an identical input phantom image. The phantom image contains several edges and a 2% contrast object. Variations, artifacts and noise sensitivity are easily visualized by perspective plots of the reconstructed images. Considerations as to the optimum algorithm for a particular application are discussed.

Iterative relaxation methods for image reconstruction

Abstract: The problem of recovering an image (a function of two variables) from experimentally available integrals of its grayness over thin strips is of great importance in a large number of scientific areas. An important version of the problem in medicine is that of obtaining the exact density distribution within the human body from X-ray projections.One approach that has been taken to solve this problem consists of translating the available information into a system of linear inequalities. The size and the sparsity of the resulting system of inequalities (typically, 25,000 inequalities with less than 1% of the coefficients nonzero) makes methods using successive relaxations computationally attractive. A variety of such methods have been proposed with differing relaxation parameters.

Image Reconstruction (I): Computerized X-Ray Scanners

Abstract: Medical science tends to advancc incrementally, and full-fledged breakthroughs are rare. The discovery of the xray by Wilhelm Conrad Roentgen in 1895 and the subsequent development of the science of radiography is one notable example. In the last 3 years, a new x-ray device known as the CAT-scanner (for computerized axial tomography) has been appearing in an increasing number of hospitals and clinics. On the basis of their experience so far, many radiologists are saying that these computerized x-ray scanners are the greatest advance in diagnostic medicine since Roentgen's discovery, while others are only somewhat less effusive in their praise. CAT-scanners have had an indisputably marked effect on the way radiologists and surgeons diagnose their patients, but it is still too soon to evaluate what the overall contribution of the scanners to the quality of health care will be. The enthusiasm for CAT-scanners derives from their superior ability to detect abnormalities (lesions) in the brain as compared with such conventional neuroradiological techniques as standard skull xradiography (roentgenography), angiography, pneumoencephalography, and radionuclide scanning. Radiologists also cite the relatively noninvasive character of the scanners and their potential for reducing the cost of health care for patients who otherwise would be hospitalized. In the diagnosis of numerous abnormalities of the brain, radiologists at the Mayo Clinic have reported an overall error rate with CAT-scanning of 4 percent on 12,000 scans over a little more than 2 years, for example (1). Disorders visualized included brain atrophy, degeneration of the brain, hydrocephalus, cysts, tumors of the brain and the eye, infarcts (dead areas of the brain due to loss of blood supply), and hemorrhage (Fig. I). In addition, they find that CAT-scanning is applicable to all of the above-mentioned categories of abnormalities, whereas the other methods are each limited to certain ones only. In conventional x-radiography, the image obtained on a film after a diverging xray beam passes through the subject is a projection or shadow of everything standing between the x-ray source and the film. Thus, the image may contain many overlapping organs and tissues which are difficult to separate. In addition, whereas an observer can easily distinguish between air, soft tissue, and bone in an x-ray photograph, the same viewer cannot easily see the few percent difference in the attenuation of x-rays by normal and diseased tissue, even when overlapping images are not a complicating factor. The method embodied in computeriied x-ray scanners to overcome these difficulties is a specific example of a general mathematical technique called reconst ruction of images from projections. In principle, if x-ray photographs are made of' a person's head at an infinite number of angles, it is mathematically possible to reconstruct a full three-dimensional image of the skull and its contents from these projections. Such reconstructions can be made from a finite number of projections, but the reconstructed image is no longer exact. A number of researchers have made reconstructions of two-dimensional cross sections normal to an axis of rotation of (an object (transverse axial tomography) from x-ray photographs taken at equal angular intervals around the axis. This procedure overcomes the problem of overlapping, but the cumulative x-ray dose to a patient would be excessive. In addition, scattering of x-rays by parts of the patient's body would cause a loss of contrast, as it does in conventional x-rav radiography. The use of an electronic detector in place of the x-ray film together with a collimated, narrow x-ray beam and computer processing solves these problems. Since the detector records only a small region at a time, in order to duplicate the

Best Linear Decoding of Random Mask Images

Abstract: In 1968 Dicke proposed coded imaging of x and ? rays via random pinholes. Since then, many authors have agreed with him that this technique can offer significant image improvement. We present a best linear decoding of the coded image and show its superiority over the conventional matched filter decoding. Experimental results in the visible light region are presented.

Image Reconstruction (II): Computerized Scanner Explosion

Abstract: Computerized x-ray scanners have had a dramatic impact on the practice of neuroradiology since their introduction 3 years ago. Efforts are now under way to extend the range of applicability of image reconstruction from projections (the principle on which x-ray scanners are based) in medicine still further. In particular, researchers are working on fast x-ray scanners, on the combination of image reconstruction with nuclear medicine, and on low dose methods of imaging. As now performed, the scanning procedure is necessarily slow. The basic scanning action consists of the motion of a narrow x-ray beam and a single detector across the head or body of a patient; this linear scan is followed by a rotation of the frame holding the x-ray source and detector about the patient and another linear scanning motion. A two-dimensional image of the cross section of the head or body in the plane that is traced out by the x-ray beam during the scanning is typically reconstructed from 180 or more one-dimensional projections, one of which is produced by each linear scan (Science, 7 November, p. 542). The time to read the detector 40,000 or more times and the time to move the x-ray source and detector repeatedly through the linear and rotational motions together add up to about 4.5 minutes. Ways to reduce this time include the use of many detectors in the plane of the cross section, so that many data points are taken simultaneously, and the reduction or elimination of all mechanical motion. One reason for needing faster scanners is that any motion of the patient during the scanning introduces artifacts into the reconstructed image, and it is often difficult for sick patients to remain sufficiently immobile for 5 minutes. Even chest motion during respiration could be overcome with 5to 20-second scanning times because the breath could be held that. long. And more patients could be accommodated, if the examination time were reduced. Manufacturers and academic investigators who are now in the process of building computerized x-ray scanners that are equipped with multiple detector arrays are also using an x-ray source geometry called a fan beam. In conventional x-radiography, the x-ray tube emits a diverging cone of radiation that may be 40 centimeters in diameter by the time it reaches the film. In the first generation of computerized x-ray scanners, the x-rays are collimated into a thin beam which, in conjunction with a

Real-time optical reconstruction of microwave holograms

Abstract: Real-time optical reconstruction from microwave hologram data is accomplished by means of a noncoherent-to-coherent image converter based on an erasable liquid-crystal photoconductor cell operating in a dynamic scattering mode. Reconstructed image quality is shown to be comparable to that which is obtained directly from photographic transparency records of the microwave hologram.

Three-dimensional image reconstruction by means of two-dimensional Radon inversion

Abstract: In this report we consider the problem of three-dimensional reconstruction of structures from their projections. formulated as a mUltiple two-dimensional problem. Part I contains a discussion of some reconstruction techniques. that are known in literature, including the analytic backgrounds of some of them. In Part II a method is discussed that is based on orthogonal polynomial expansion. Notably. attention is payed to the calculation of the resulting finite series of orthogonal polynomials. Finally some numerical experiments concerning the reconstruction of testpatterns are presented. In part III a variant of the convolution method is derived and some numerical experiments are discussed. Part 1 and II were prepared in order to obtain the master's degree (ir.) at the Technological University Eindhoven. I am greatly indebted to prof. G.W. Veltkamp, for his stimulating critism during the preparation of this report. AMS Subject Classifications 65R05

Physical And Mathematical Aspects Of Transmission 3-Dimensional Image Reconstruction

Abstract: 3-Dimensional Structure Analysis or image reconstruction is reviewed in view points of physics. Optimum energy of probe radiation in particular reference to photons and related aspects such as image enhancement using reference materials (-water like) are discussed. Existing computer algorithms are briefly reviewed in particular reference to the linear superposition with compensation (LSC) technique.© (1975) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Numerical image reconstruction from an off-axis sound-wave hologram

Abstract: A numerical image reconstruction technique is proposed, where the images are calculated by performing a Fourier transform once instead of transforming twice as in the conventional method. This technique enables us to reconstruct images from off-axis holograms. An image reconstruction experiment was done with a sound-wave hologram of 18 kHz.

An Inexpensive Laboratory X-Ray Tomographic Scanner

Abstract: A computerized transverse-axial x-ray scanning tomography system has been constructed and operated using inexpensive and readily available components. The system is used only for the scanning of phantoms and the image reconstruction is carried out on an IBM 360/75. Images reconstructed from data taken by the device are able to resolve radiographic density variations of about 1/2% over distances of 0.5 cm.