Robert Alvarez - Academia.edu (original) (raw)
Address: Mountain View, California, United States
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Universidad Nacional de Colombia (National University of Colombia)
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Papers by Robert Alvarez
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Physics in medicine and biology, 1976
All X-ray computerized tomography systems that are available or proposed base their reconstructio... more All X-ray computerized tomography systems that are available or proposed base their reconstructions on measurements that integrate over energy. X-ray tubes produce a broad spectrum of photon energies and a great deal of information can be derived by measuring changes in the transmitted spectrum. We show that for any material, complete energy spectral information may be summarized by a few constants which are independent of energy. A technique is presented which uses simple, low-resolution, energy spectrum measurements and conventional computerized tomography techniques to calculate these constants at every point within a cross-section of an object. For comparable accuracy, patient dose is shown to be approximately the same as that produced by conventional systems. Possible uses of energy spectral information for diagnosis are presented.
Radiology, 1981
Prereconstruction dual-energy computed tomography (PREDECT) produces rigorously exact reconstruct... more Prereconstruction dual-energy computed tomography (PREDECT) produces rigorously exact reconstructions that accurately separate the total attenuation coefficient into two values, representing the Compton and photoelectric contributions. The images are obtained without polychromatic distortion and with an acceptable dose. PREDECT was used to scan objects and solutions of varying atomic number and electron density, specimens of normal and pathologic brain and other body tissues, and nine patients. The values for Compton and photoelectric attenuation of the different specimens were distinctive enough to provide "tissue signatures" of potential clinical usefulness. Eight of the nine patients studied provided acceptable images, which produced some tissue characterization. PREDECT appears to represent an advance over the previously used postreconstruction methods; areas of greatest potential are differential diagnosis, improved detection of abnormalities, and elimination of the p...
Neuroradiology, 1978
Some of the prior efforts at using postreconstruction dual kilovoltage CT scans to obtain an effe... more Some of the prior efforts at using postreconstruction dual kilovoltage CT scans to obtain an effective atomic number and electron density are reviewed. A prereconstruction method which avoids beam hardening is applied to a phantom and a beginning clinical trial is described.
Journal of Computer Assisted Tomography, 1979
The spatial resolution and noise properties of a computed tomography (CT) image may be altered by... more The spatial resolution and noise properties of a computed tomography (CT) image may be altered by two-dimensional linear filtering of the initial image. In this paper, we derive filters that minimize the noise variance subject to a constraint on the spatial resolution. The resulting filter functions can reduce the noise variance by 17% in comparison with conventional filters. The method for obtaining these filters requires knowledge of the noise and imaging properties of the system. We derive theoretical expressions for these properties and introduce experimental techniques for their measurement. The statistical characteristics are shown to be anisotropic, spatially variant, and object dependent. We discuss the implications of this result both for optimal filtering and for the general problem of CT image noise property measurement.
Journal of Computer Assisted Tomography, 1984
ABSTRACT
IEEE Transactions on Nuclear Science, 2000
A system is described for measuring the amount of bone and soft tissue in a path through any poin... more A system is described for measuring the amount of bone and soft tissue in a path through any point in a radiograph. The basic information required is the transmitted intensity with two different source spectra. It is shown experimentally that this data may be used to accurately calculate the bone and soft tissue thicknesses.
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Proceedings of SPIE - The International Society for Optical Engineering
ABSTRACT
Physics in medicine and biology, 1976
All X-ray computerized tomography systems that are available or proposed base their reconstructio... more All X-ray computerized tomography systems that are available or proposed base their reconstructions on measurements that integrate over energy. X-ray tubes produce a broad spectrum of photon energies and a great deal of information can be derived by measuring changes in the transmitted spectrum. We show that for any material, complete energy spectral information may be summarized by a few constants which are independent of energy. A technique is presented which uses simple, low-resolution, energy spectrum measurements and conventional computerized tomography techniques to calculate these constants at every point within a cross-section of an object. For comparable accuracy, patient dose is shown to be approximately the same as that produced by conventional systems. Possible uses of energy spectral information for diagnosis are presented.
Radiology, 1981
Prereconstruction dual-energy computed tomography (PREDECT) produces rigorously exact reconstruct... more Prereconstruction dual-energy computed tomography (PREDECT) produces rigorously exact reconstructions that accurately separate the total attenuation coefficient into two values, representing the Compton and photoelectric contributions. The images are obtained without polychromatic distortion and with an acceptable dose. PREDECT was used to scan objects and solutions of varying atomic number and electron density, specimens of normal and pathologic brain and other body tissues, and nine patients. The values for Compton and photoelectric attenuation of the different specimens were distinctive enough to provide "tissue signatures" of potential clinical usefulness. Eight of the nine patients studied provided acceptable images, which produced some tissue characterization. PREDECT appears to represent an advance over the previously used postreconstruction methods; areas of greatest potential are differential diagnosis, improved detection of abnormalities, and elimination of the p...
Neuroradiology, 1978
Some of the prior efforts at using postreconstruction dual kilovoltage CT scans to obtain an effe... more Some of the prior efforts at using postreconstruction dual kilovoltage CT scans to obtain an effective atomic number and electron density are reviewed. A prereconstruction method which avoids beam hardening is applied to a phantom and a beginning clinical trial is described.
Journal of Computer Assisted Tomography, 1979
The spatial resolution and noise properties of a computed tomography (CT) image may be altered by... more The spatial resolution and noise properties of a computed tomography (CT) image may be altered by two-dimensional linear filtering of the initial image. In this paper, we derive filters that minimize the noise variance subject to a constraint on the spatial resolution. The resulting filter functions can reduce the noise variance by 17% in comparison with conventional filters. The method for obtaining these filters requires knowledge of the noise and imaging properties of the system. We derive theoretical expressions for these properties and introduce experimental techniques for their measurement. The statistical characteristics are shown to be anisotropic, spatially variant, and object dependent. We discuss the implications of this result both for optimal filtering and for the general problem of CT image noise property measurement.
Journal of Computer Assisted Tomography, 1984
ABSTRACT
IEEE Transactions on Nuclear Science, 2000
A system is described for measuring the amount of bone and soft tissue in a path through any poin... more A system is described for measuring the amount of bone and soft tissue in a path through any point in a radiograph. The basic information required is the transmitted intensity with two different source spectra. It is shown experimentally that this data may be used to accurately calculate the bone and soft tissue thicknesses.