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At least 703 records · Page 39Linked to original sources
PubMed1977

Noise due to photon counting statistics in computed X-ray tomography.

A general expression is derived for the noise due to photon counting statistics in computed X-ray tomography. The variance is inversely proportional to the cube of the resolution distance. For scanners using a water box, the noise in the reconstructed image depends inversely on the number of detected primary photons, summed over all angles, that have passed through a resolution element. Predictions of this formula agree well with the results of computer simulations. It is shown how this formula can be used to determine such parameters as required X-ray flux, detector counting rate, and dose, with special emphasis on tradeoffs between these parameters and resolution. It is also shown that to determine the X-ray attenuation coefficient of a resolution element to a given precision, the number of photons required by computed X-ray tomography is close to a theoretical limit.

Elementary Particles↗
PubMed1977

Emission computer assisted tomography with single-photon and positron annihilation photon emitters.

Computed transverse section emission tomography using 99mTc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and 68Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144 views. The positron ring has a LSF of 6 mm at the center with a transverse section reconstruction LSF of 7.5 mm FWHM. Correction for uniformity of detector response and accurate center of rotation determination is essential in both techniques. The signal-to-noise ratio in a reconstruction is diminished by a factor of 1.2 x (number of resolution elements)1/4 over that expected from the average number of events per resolution element. Attenuation compensation causes more noise to appear in the center than the edge for both modes and an average increase in uncertainty of 30%. The effects of attenuation result in more loss of data for positron coincidence imaging than for single-photon imaging even at energies of 80 keV. For a 20-cm cylinder imaged in transverse section, only 20% of the positron annihilation events are not scattered; however, at 140 keV, 40% of the photons are not scattered. The relative crystal efficiency gives single-photon imaging an advantage of 5. On the other hand, the solid angle advantage of positron photon coincidence imaging is about 100 for the comparisons of this paper. Taking these factors into account, we find positron-computed section imaging has a tenfold increase in sensitivity over multiple-view imaging with the scintillation camera, which gives multiple sections but requires camera or patient rotation.

Elementary Particles↗
PubMed1978

Noise limitations in X-ray computed tomography.

A lower bound for the statistical accuracy in X-ray computed tomography, which, under certain conditions, is independent of the reconstruction algorithm, is derived. An evaluation of this bound indicates that little improvement is possible over the performance of the convolutional algorithm.

Elementary Particles↗
PubMed1978

Nuclear scattering radiography of the spine and sphenoid bone.

Nuclear scattering radiographs of a portion of a spine and a sphenoid bone have been obtained using a 1 GeV proton beam. The ability of the method to yield three-dimensional representations is illustrated by three series of adjacent pictures corresponding to the three orthogonal planes (elementary volume: 5.2 mm3 and 0.9 mm3). The sensitivity of the method is discussed. Nuclear scattering radiographs are compared with ones obtained by conventional X-ray tomography and computed tomography. Nuclear scattering radiography also may be used to analyze the partition of hydrogen within the tissures. Hydrogen radiographs obtained in this way are shown.

Elementary Particles↗
PubMed1979

Data collection for cross-sectional image reconstruction by a moving ring of positron annihilation detectors.

A ring of positron annihilation detectors capable of two independent motions in the plane of the ring is considered. It can rotate around its center, and the whole ring may move so that its center traces a circular path. Provided certain parameters are chosen according to given formulas, data collected by such a detector ring can be reorganized (rebinned), in a computationally very inexpensive fashion, so that the data items in each bin are measurements along parallel lines spaced at small equal intervals. Efficient high resolution reconstruction algorithms that assume such data organization can therefore be applied to data collected collected by a moving positron ring detector. The method is demonstrated on a realistic example.

Computers↗
PubMed1979

Quantitation in positron emission computed tomography: 1. Effect of object size.

The effect of object size on the capability of positron emission computed tomography to measure isotope concentrations in a cross section was studied. The relationship between the apparent isotope concentration in an image and the true concentration was measured as a function of object size for three instrument resolutions and four convolution filters. The relationship between image size and object size was also measured under the same conditions. Depression of apparent isotope concentration in an image for objects equal in size to the instrument resolution (FWHM) was significant (50% for a cylinder and 25% for a bar). For objects larger than 1.0 FWHM, accurate object sizes can be estimated from the images. Thus, reasonably accurate and practical schemes of compensation for object size effects can be implemented for objects larger than 1.0 FWHM. Accuracy in quantitating isotope concentrations in smaller objects is seriously compromised by the loss of sensitivity to the object size and the large correction factors required to compensate for instrument response. The results of the measurements were found to be in good agreement with theoretical predictions for ideal systems of comparable resolution.

Copper↗
PubMed1979

Assessment of regional myocardial perfusion by positron emission tomography after intracoronary administration of Gallium-68 labeled albumin microspheres.

Positron emission computed tomography (ECT) of the normal and ischemic canine heart was undertaken in eight anesthetized dogs after the intracoronary administration of 68gallium labeled human serum albumin microspheres (SAM). Bolus doses of 200 muCi of 68Ga-SAM were injected into the left anterior descending (LAD) and left circumflex coronary arteries of four normal dogs and four dogs undergoing LAD ligation. Positron imaging of 68Ga-SAM distribution was performed with a multicrystal positron camera and computer reconstruction. In normal dogs, five to six axial transverse section images of the heart demonstrated the transmural distribution of 68Ga-SAM in an annular pattern around the left ventricular cavity. In dogs with LAD ligation, perfusion defects in the anterolateral wall and septum were observed in tomographic cuts through ischemic areas. In conclusion, these studies demonstrate the feasibility of positron ECT imaging of the heart after intracoronary administration of 68Ga-SAM.

Animals↗
PubMed1980

Design of a high resolution positron emission tomograph: the Neuro-PET.

The design of a high resolution positron emission tomograph is described. The scanner has four rings of detectors with an inside diameter of 38 cm and produces seven simultaneous slices, including three cross-slices. Each ring contains 128 bismuth germanate scintillation detectors with dimensions of 8.25 x 20 x 35 mm; adjacent crystals are separated by tapered tungsten septa that extend to within 7.5 mm of the front faces. The anticipated geometrical spatial resolution of the scanner is 4.5 mm full width at half maximum (FWHM) at the center of the image, and the sensitivity is 44,000 true counts/sec/ring (390,000 counts/sec total) for a uniform phantom 20 cm in diameter containing 1 muCi/cc activity. There are interchangeable collimators for use in high count rate studies, for narrowing the slice width from 1 cm FWHM to 5 mm FWHM, and for ultra-high resolution studies with a 2.5 mm FWHM geometrical point spread function. The electronic circuitry has separate timing and energy verification channels and can detect 95% of the coincidences with a timing window (twice the maximum time between two coincident pulses) of 14 nsec. The count rate capability of the electronics is 150,000 counts/sec/ring and 1.5 million counts/sec total.

Bismuth↗
PubMed1989

Inhibition of growth of Chlamydia trachomatis by the calcium antagonist verapamil.

Treatment of BGM (African Green Monkey kidney) cells with the calcium antagonist Verapamil resulted in a reduced yield of chlamydial infectious particles. The inhibitory effect was concentration-dependent, the maximal effect being achieved at 200 microM-Verapamil, which produced a 99.99% reduction of infectious particle yield. Electron microscopy showed that control Chlamydia trachomatis-infected BGM cells contained typical large inclusions in which most of the particles were elementary bodies, whereas Verapamil-treated infected cells contained small inclusions consisting predominantly of reticulate bodies. The findings indicate a possible therapeutic use of this calcium antagonist as an anti-chlamydial drug.

Animals↗
PubMed1986

Theoretical antideuteron-nucleus absorptive cross sections.

Antideuteron-nucleus absorptive cross sections for intermediate to high energies are calculated using an ion-ion optical model. Good agreement with experiment (within 15 percent) is obtained in this same model for mean p-nucleus cross sections at laboratory energies up to 15 GeV. We describe a technique for estimating antinucleus-nucleus cross sections from N mean N data and suggest that further cosmic ray studies to search for antideuterons and other antinuclei be undertaken.

Cosmic Radiation↗
PubMed1986

Theoretical estimates of photoproduction cross sections for neutral subthreshold pions in carbon-carbon collisions.

Using the Weizsacher-Williams method of virtual quanta, total cross section estimates for the photoproduction of neutral subthreshold pions in carbon-carbon collisions at incident energies below 300 MeV/nucleon are made. Comparisons with recent experimental data indicate that the photoproduction mechanism makes an insignificant contribution to these measured cross sections.

Carbon↗
PubMed1986

Comparison of abrasion model differences in heavy ion fragmentation: optical versus geometric models.

Using an abrasion-ablation collision model, which includes contributions from frictional-spectator interactions and electromagnetic dissociation, analyses of the sensitivities of predicted fragmentation cross sections to the choice of a particular abrasion formalism are made using both geometric and optical potential abrasion models. Most cross section differences obtained using the two abrasion models are less than the present experimental uncertainties, suggesting that either abrasion model is suitable for estimating isotopic and elemental fragment distributions.

Elementary Particles↗
PubMed1987

Trends of total reaction cross sections for heavy ion collisions in the intermediate energy range

Direct measurements of total reaction cross sections (sigma R) have been performed in the energy range of 10-300 MeV/nucleon for heavy ion collisions. A decrease of sigma R with increasing energy was observed for a wide range of masses of the colliding systems. The data suggest that sigma R reaches a minimum located around 300 MeV/nucleon independently of the projectile target combination. A dependence of sigma R on mass asymmetry of the svstem is also demonstrated. Trends of sigma R in this energy range are well reproduced by the predictions of a simple microscopic model based on individual nucleon-nucleon collisions. Our data have been employed in this framework to derive a new semi-empirical parametrization of sigma R. Most of the experimental results in the intermediate and high energy range have been reproduced by this parametrization using a single energy-dependent parameter.

Carbon↗
PubMed1990

Charge dependence and electric quadrupole effects on single-nucleon removal in relativistic and intermediate energy nuclear collisions.

Single-nucleon removal in relativistic and intermediate energy nucleus-nucleus collisions is studied using a generalization of Weizsäcker-Williams theory that treats each electromagnetic multipole separately. Calculations are presented for electric dipole and quadrupole excitations and incorporate a realistic minimum impact parameter, Coulomb recoil corrections, and the uncertainties in the input photonuclear data. Discrepancies are discussed. The maximum quadrupole effect to be observed in future experiments is estimated and also an analysis of the charge dependence of the electromagnetic cross sections down to energies as low as 100 MeV/nucleon is made.

Electromagnetic Fields↗
PubMed1990

Electric quadrupole excitations in relativistic nucleus-nucleus collisions.

Calculations are presented for electric quadrupole excitations in relativistic nucleus-nucleus collisions. The theoretical results are compared to an extensive data set and it is found that electric quadrupole effects provide substantial corrections to cross sections, especially for heavier nuclei.

Electromagnetic Phenomena↗
PubMed1991

Calculations of hadronic dissociation of 28Si projectiles at 14.6A GeV by nucleon emission.

An optical potential abrasion-ablation collision model is used to calculate hadronic dissociation cross sections for one, two, and three nucleon removal for the first time for a 14.6A GeV 28Si beam fragmenting in aluminum, tin, and lead targets. These estimates are compared with recent semi-inclusive measurements. Significant differences between some calculated and measured semi-inclusive cross sections exist which cannot be resolved without measurements of the exclusive channel hadronic cross sections. Calculations for each exclusive reaction channel contributing to the semi-inclusive cross sections are presented and discussed.

Aluminum↗
PubMed1993

Geometric model for nuclear absorption from microscopic theory.

A parameter-free geometric model for nuclear absorption is derived from microscopic theory. The expression for the absorption cross section in the eikonal approximation taken in integral form is separated into a geometric contribution, described by an energy-dependent effective radius, and two surface terms which are shown to cancel in an asymptotic series expansion. For collisions of light nuclei, an expression for the effective radius is derived using harmonic-oscillator nuclear density functions. A direct extension to heavy nuclei with Woods-Saxon densities is made by identifying the equivalent half density radius for the harmonic-oscillator functions. Coulomb corrections are incorporated and a simplified geometric form of the Bradt-Peters type obtained. Results spanning the energy range of 1 MeV/nucleon to 1 GeV/nucleon are presented. Good agreement with experimental results are obtained.

Elementary Particles↗