Showing papers on "Attenuation published in 1978"

A Method for Attenuation Correction in Radionuclide Computed Tomography

Abstract: The development of algorithms for Radionuclide Computed Tomography (RCT) is complicated by the presence of attenuation of gamma-rays inside the body. Some of the existing RCT reconstruction algorithms apply approximation formulas to the projection data for attenuation correction, while others take attenuation into account through some iterative procedures. The drawbacks of these algorithms are that the approximation formulas commonly used are generally inadequate and the iterative procedures are usually very time-consuming. The method for attenuation correction in RCT, which we propose, applied a simple, effective two-step procedure to the uncorrected image. In this procedure the filtered back-projection algorithm is used for its fast speed. A simple mathematical basis and description of the procedure together with some illustrative computer results are given in this paper.

Physical constraints on acoustic communication in the atmosphere: Implications for the evolution of animal vocalizations

Abstract: 1. Acoustic communication requires not only detection of the signal but also discrimination of differences among signals by the receiver. Attenuation and degradation of acoustic signals during transmission through the atmosphere will impose limits on acoustic communication. Attenuation of sound during atmospheric transmission results primarily from atmospheric absorption, ground attenuation, scattering of a sound beam, and deflection of sound by stratified media. For maximum range of detection, therefore, animals should favor optimal positions in their habitat and optimal weather conditions. Frequency-dependent attenuation seems not to differ consistently among major classes of terrestrial habitats, such as forests and fields. Increased scattering of higher frequencies from vegetation in forests is in part matched by scattering from micrometerological heterogeneities in the open. 2. In addition to frequency-dependent attenuation, two kinds of degradation during atmospheric transmission will limit a receiver's ability to resolve differences among acoustic signals: the accumulation of irregular amplitude fluctuations from nonstationary heterogeneities, often atmospheric turbulence, and reverberation. Both types of degradation affect temporal patterns of amplitude or intensity modulation more than patterns of frequency modulation. Both effects should increase with carrier frequency, as they depend on the relationship between wavelength and the dimensions of scattering heterogeneities. Irregular amplitude fluctuations are more severe in open habitats and primarily mask low frequencies of amplitude modulation; reverberations are more severe in forested habitats and primarily mask high frequencies of amplitude modulation and rapid, repetitive frequency modulation. This difference between forested and open habitats could explain previous reports that birds in the undergrowth of tropical forests avoid rapid frequency modulation in their long-range vocalizations. 3. Maximum range of detection is probably not the primary selection pressure on many animal vocalizations, even for territorial advertisement, except perhaps in tropical forests. Instead, acoustic signals might incorporate features that degrade predictably with range to permit a receiver to estimate the signaler's distance. Future investigations might explore the propagation of animal vocalizations in relation to the usual spacing of animals in their habitat. Features that encode different kinds of information, such as individual and species identity, might propagate to different distances. 4. Measurements of the transmission of sound in natural environments have often not controlled several important parameters. First, the effects of gound attenuation and scattering are not linear with range; consequently measurements of excess attenuation over different ranges in the same environment might differ. Second, the directionality of speakers and microphones will affect measurements of attenuation and reverberations in scattering environments. Third, as stationary waves shift with frequency, any single microphone placement will lie in a null for some frequencies and in a maximum for others.

Tissue mimicking materials for ultrasound phantoms

Abstract: Up until now, no material has been found whose attenuation and speed of sound properties not only mimic those of human soft tissue, but are controllable in magnitude. We have discovered such a material in the form of water-based pharmaceutical gels containing uniform distributions of graphite powder and known concentrations of alcohol. The magnitude of the attenuation coefficient can be controlled easily between 0.2 and 1.5 dB/cm at 1 MHz, by varying the concentration of graphite. These attenuation coefficients are nearly proportional to the frequency. The speed of sound varies between 1520 and 1650 m/s at room temperature, depending primarily upon the concentration of alcohol. Bacterial invasion has been prevented by sterilization procedures and the introduction of appropriate preservatives. The ultrasonic properties exhibit temporal stability and change little over the range of room temperatures.

Signal Processing of Broadband Pulsed Ultrasound: Measurement of Attenuation of Soft Biological Tissues

Abstract: In this paper we will discuss the measurement of attenuation of soft tissues using broadband pulsed ultrasound. While all the methods presented here may be used for measuring the attenuation coefficient of single layers, some of the methods can also be used for measuring integrated attenuation of composite layers of soft tissue. These latter methods do not require knowledge of reflection coefficients at tissue interfaces and are well suited to generating projection data for computerized attenuation tomography of soft tissues.

High-Speed Computation of Single and Coupled Microstrip Parameters Including Dispersion, High-Order Modes, Loss and Finite Strip Thickness

Abstract: Based on an optimized rigorous hybrid mode solution for covered/open zero thickness microstrip patterns the following frequency dependent single and coupled line data are evaluated with very short CP-times: The characteristic impedances of the even and odd quasi-TEM modes, the propagation/attenuation constants and associated strip current density components of these and the higher order modes, the loss of the dominant modes under consideration of nonuniform strip current, substrate surface roughness and dielectric loss tangent. Finite strip thickness is introduced by a correction of the strip width input values.

Regional variation of the scaling law of earthquake source spectra

Abstract: The dependence of the seismic source spectrum on earthquake magnitude was determined with improved accuracy in the frequency range from 1 to 25 Hz using coda waves from local earthquakes. Separation of the source effects from the effects of scattering and attenuation was achieved by the method of Aki and Chouet (1975) assuming that the earth is a randomly heterogeneous medium with a uniform statistical property. Scaling laws constructed for seismic regions in Japan, California, and Hawaii show marked variations which may be attributed to the differences in the scale length of inhomogeneity and strength of the earth's crust among these regions. In the magnitude range from 1 to about 5, the stress drop increases in most areas from roughly 1 bar to about 1 kbar. The exception is Hawaii where the stress drop is very low and almost constant. The dependence of the stress drop on earthquake magnitude reflects the heterogeneous material properties in the seismic zone and is explained by a fault plane with barriers which may be skipped, unbroken, when the tectonic stress is low.

General relationships between ultrasonic attenuation and dispersion

Abstract: General relationships between the ultrasonic attenuation and dispersion are presented. The validity of these nonlocal relationships hinges only on the properties of causality and linearity, and does not depend upon details of the mechanism responsible for the attenuation and dispersion. Approximate, nearly local relationships are presented and are demonstrated to predict accurately the ultrasonic dispersion in solutions of hemoglobin from the resuts of attenuation measurements.

The acoustic characteristics of the skull

Abstract: Many investigators have stated that the difficulties of imaging with acoustical energy through the skull result from the marked attenuation of the energy by the skull. In the literature measurements of total attenuation have been confused with those for absorption. Measurements made by us show that absorption by compact bone varies between 2–3 dB cm −1 MHz −1 and, in the low megaHertz region appears to be directly proportional to frequency. It has also between shown that the convoluted inner surface of the ivory bone of the inner table of the skull may degrade the collimation and directionality of the beam by refraction. Cancellous bone, such as is present in the dipole of the skull, greatly attenuates the energy. It is postulated that this largely results from scattering. It is also postulated that the energy propagates through cancellous bone as two components, one in the soft tissues and the other partly in the bony spicules. Observations suggest that attenuation due to scattering much more markedly affects the latter of these components and scatters more greatly the higher frequencies in a pulse of broad bandwidth. The energy in each component has varying propagation paths so that the later cycles in the pulse of each component are subject to increasing interference as a result of the variations in propagation times. The two components moreover may have different propagation times so that interference may occur between the pulses of each component as well. All of these phenomena degrade the collimation, coherence, directionality, beam width, pulse length, frequency and other properties of the ultrasonic energy upon which imaging through the skull depends. The interference effects described above are least for the first cycle in the pulse which usually is not the cycle of highest amplitude. Since, in the free field, most of the energy is concentrated around the beam axis, most of the energy in the field which is deflected from its normal propagation path is deflected away from the beam axis. Thus the directionality of the beam is least degraded in the beam axis. The effects of the skull in degrading the properties of the ultrasonic pulse would therefore be lessened if the amplitude of the first cycle of the pulse and the directionality of its energy could be used for imaging.

A perturbation analysis of the attenuation and dispersion of surface waves

Abstract: A perturbation formulation of the equations of linear piezoelectricity is obtained using a Green’s function approach. Although the resulting equation for the first perturbation of the eigenvalue strictly holds for real perturbations of real eigenvalues only, it is formally extended to the case of purely imaginary perturbations of real eigenvalues. The extended equation is applied in the calculation of the attenuation of surface waves due to the finite electrical conductivity of thin metal films plated on the surface and air loading. The influence of the viscosity of the air is included in the air‐loading analysis, and the calculated attenuation increases accordingly. Since the metal films are thin compared with a wavelength, an approximate thin‐plate conductivity equation is employed in the determination of the attenuation due to the electrical conductivity of the films. The resulting attenuation is obtained over a very large range of values of sheet conductivity. This is accomplished by using the equatio...

Simultaneous dual-energy computer assisted tomography

Abstract: A dual-energy detector system for use in computer assisted tomography. This system produces two independent sets of information from one scan, namely, high-energy and low-energy data. The system employs two cooperating detectors. The first one responds primarily to low-energy photons, allowing most high-energy photons to pass through. The second detector lies behind the first and detects the remaining photons. Thus, two electrical signals are generated which contain information in two different energy ranges, which signals can be computer-processed. The attenuation coefficients at these two energies are sufficiently different so that differential diagnosis and chemical identification may be aided. The computer-processed signals may be employed to provide any of (a) beam-hardening correction, (b) chemical identification and composition of tissues, such as lesions, bone, etc., (c) localization of injected contrast material, or (d) attenuation coefficients for radiation therapy planning.

Lateral variation of surface-wave anelastic attenuation across the pacific

Abstract: Seismograms from island stations within or near the edge of the Pacific plate were used to obtain surface-wave attenuation coefficients in the period range 18 to 110 sec for Rayleigh waves and 20 to 110 sec for Love waves. The average Rayleigh-wave attenuation coefficient values range from a maximum of 1.64 × 10 −4 km −1 at short periods to a minimum of 0.72 × 10 −4 km −1 at longer periods. Corresponding extreme values for Love waves are 3.30 × 10 −4 km −1 and 0.60 × 10 −4 km −1 . The data are characterized by relatively large standard deviations which reflect departures from an ideal medium having laterally homogeneous elastic and anelastic properties. The possibility of regional variations in anelastic properties was examined by dividing the Pacific into three regions according to age (0 to 50 m.y., 50 to 100 m.y., >100 m.y.). A systematic decrease in attenuation coefficient values over most of the period range is readily apparent from the data. Q β −1 models obtained by stochastic inversion of the attenuation data suggest that the observed differences are produced by increasing values of Q β in the low Q zone (at depths of 40 to 200 km), and possibly also by increasing values of Q β in the lithosphere, as the age of the Pacific sea floor increases.

Iterative Correction for Attenuation of 5 cm Radar in Rain

Abstract: The attenuation of 5 cm radar in rain is investigated theoretically for stratiform and thunderstorm drop size distributions. An iterative attenuation estimation scheme is presented. The effects of attenuation on radar precipitation measurements and the capabilities of the attenuation estimation technique are considered for a variety of hypothetical storm sizes and errors in radar calibration, assumed temperature and assumed drop size distribution. This study indicates that 5 cm radar is an adequate precipitation measuring radar for storms under about 50 dBZ, and that if calibrated correctly and used with the iterative attenuation correction scheme, the 5 cm radar can function moderately well up to about 60 dBZ. Radar calibration accuracy is seen to be a limiting criterion for attenuation correction. The results of this study point out the need for raingages in most situations requiring accurate rainfall measurement.

Loss in cleaved Ti‐diffused LiNbO3 waveguides

Abstract: The attenuation coefficient and coupling loss were measured in single‐mode Ti‐diffused LiNbO3 waveguides with cleaved end faces. Coplanar electrodes increase the attenuation substantially but a dielectric buffer layer can eliminate most of the electrode absorption.

Correction for beam hardening in computed tomography.

Abstract: Corrections for beam-hardening artifacts in computed tomography can be made by using a model which assumes that water and bone mineral are the only constituents of tissue. With this model, a correction factor for the measured transmission values can be calculated such that the reconstructed attenuation coefficients have values corresponding to a monoenergetic source of known energy. Systematic errors in the uncorrected attenuation coefficients, which may be 5%, can be reduced to less than 1% if corrected transmission values are used.

Differential mode attenuation measurements in graded-index fibers

Abstract: An automated apparatus is described for measuring mode-dependent scattering and total attenuation in graded-index waveguides. Measurements are reported for two low-loss waveguides and several high-loss waveguides. The results illustrate the ability of this technique to identify different types of loss mechanisms.

Radio Communication in Tunnels

Abstract: The attenuation constant of radio waves in tunnels was obtained experimentally and theoretically. According to this study, the tunnel is a transmission channel of high-pass type. It is found that the higher the frequency, the smaller the attenuation constant. The experimental values of attenuation constants are similar to the theoretical values of the the TE/sub 01/ and EH/sub 11/ and modes when the tunnel is regarded as a circular waveguide with the same cross-sectional area as the tunnel. Radio communication using the tunnel was proven to be fully possible in spite of the standing wave effects due to the interference of the propagation modes.

Ultrasonic attenuation in ceramics

Abstract: Ultrasonic attenuation has been measured for a range of ceramic polycrystals. An extreme‐value approach for attenuation analysis has been developed, based on numerical scattering cross sections and an extreme‐value characterization of the microstructure. A good correlation between predicted and measured attenuation has been obtained in each case. The implications of the analysis for ultrasonic failure prediction and microstructural characterization have been explored.

Experimental studies of attenuation in sediments

Abstract: Recent theoretical work suggests that attenuation of acoustic waves in ocean sediments depends on two distinct types of energy loss; these losses result from inelasticity of the skeletal frame and motion of the pore water relative to the frame In this paper, experimental evidence is presented which verifies the type of response predicted by the theoretical model Torsional resonance and logarithmic decrement are measured over a wide frequency range in both water‐saturated and dry sediments A changeover from dominance by one type of loss to the other is clearly demonstrated by a change of response that is observed as the frequency is varied Specially designed equipment is used which allows the intergranular stress to be controlled to simulate various depths of embedment in the sea floor Both the theory and the experiments suggest that for some sediments the attenuation will vary in a manner quite different from the usual dependency on the first power of frequency that is often assumed

Transient and impulse responses of a one-dimensional linearly attenuating medium — II. A parametric study

Abstract: We investigate one-dimensional waves in a standard linear solid for geophysically relevant ranges of the parameters. The critical parameters are shown to be T*= t_u/Q_m where t_u is the travel time and Q_m the quality factor in the absorption band, and τ^(-1)_m, the high-frequency cut-off of the relaxation spectrum. The visual onset time, rise time, peak time, and peak amplitude are studied as functions of T* and τ_m. For very small τ_m, this model is shown to be very similar to previously proposed attenuation models. As τ_m grows past a critical value which depends on T*, the character of the attenuated pulse changes. Seismological implications of this model may be inferred by comparing body wave travel times with a ‘one second’ earth model derived from long-period observations and corrected for attenuation effects assuming a frequency independent Q over the seismic band. From such a comparison we speculate that there may be a gap in the relaxation spectrum of the Earth's mantle for relaxation times shorter than about one second. However, observational constraints from the attenuation of body waves suggest that such a gap might in fact occur at higher frequencies. Such a hypothesis would imply a frequency dependence of Q in the Earth's mantle for short-period body waves.

Split-pulse laser method for measuring attenuation coefficients of transparent liquids: application to deionized filtered water in the visible region

Abstract: Attenuation coefficients α(λ) for collimated quasi-monochromatic radiation passing through deionized filtered water were measured throughout the 418.6–640.3-nm wavelength region by use of a split-pulse laser method, which employs reference and sample cells arranged in a geometry similar to that of a Michelson interferometer. The radiant source was a pulsed wavelength-tunable dye laser possessing a relatively short coherence time. This paper includes descriptions of theoretical and experimental techniques applicable to the split-pulse laser method and includes a tabulation of α(λ) measured for deionized filtered water at 26.4 ± 1.7°C.

Radio propagation in urban areas

Abstract: This paper reviews earlier work on radio propagation in urban areas, including data, studies of multi-path fading, and empirical propagation models A model is described, for use with a digital computer, which provides a rapid means of calculating both the median attenuation and the location, or path-to-path, variaability

The propagation of attenuated sh channel waves

Abstract: The problem of detecting discontinuities which interrupt coal seams is of great importance to the coal mining industry. One possible method of detecting such discontinuities is that of in-seam seismology where both source and detectors are placed under- ground on the coal face. In this paper the propagation of SH waves from a line source in the seam is investigated. There exists a particular set of waves–channel waves–, confined to the coal seam. These waves are dispersive and have an associated Airy phase whose geometrical attenuation is least. However, if absorption of energy within the coal is included then the Airy phase is the dominant wavegroup only for distances less than a certain maximum. If a detection criterion proposed by Dresen and Freystatter applies, then there is a lower limit to the frequency that should be used for exploration. This requirement may be in conflict with attenuation considerations.

Acoustic impedance variations at burn–nonburn interfaces in porcine skin

Abstract: The success of the ultrasonic pulse‐echo technique for measurement of the depth of burn necrosis in porcine skin [Med. Phys. 4, 259–263 (1977)] has led to the present investigation of the specific acoustic impedance difference between the necrotic (burn) tissue and the underlying viable (nonburn) tissue. Experimental results show that the ultrasonic group velocities and mass densities are approximately the same in these tissues yielding average values of (1.72±0.12) ×105 cm/s for group velocity and (1.093×0.009) g/cm3 for the density. The characteristic impedance (density times velocity) differs by at most 3% between necrotic and viable tissues. Measurements of acoustic attenuation show a difference of approximately 70% between these tissues. porcine skin is due primarily to the difference in acoustic attenuation.

A Simple Procedure for Estimating Global Daily Radiation on Any Surface

Abstract: Monthly means of measured daily radiation data from a defined horizontal site are used to estimate the prevailing atmospheric attenuation parameter. The estimation method uses the properties of the Fourier structure of attenuation, as a function of the hour angle, at that site, for a range of attenuation parameters. The Fourier structure of the parameter estimate is then used to calculate the global daily radiation of a “nearby” site, of any orientation, by direct formula substitution.

Effect of waveguide optical scattering on the integrated optical spectrum analyzer dynamic range

Abstract: The dynamic range limited by optical scattering in the integrated optical spectrum analyzer is considered. Calculations of dynamic range as a function of waveguide attenuation and surface roughness characteristics are presented. Dynamic range is found to decrease with increasing attenuation, to be nearly independent of the surface roughness correlation length for a fixed attenuation, and to be independent of the Fourier transform lens focal length. Adjacent channel rejection is found to fall off slowly. The effect of lens scattering is shown only to be important for cases in which waveguide scattering is very small and lens scattering is very high.

Effect of ionizing radiation on the optical attenuation in doped silica and plastic fiber‐optic waveguides

Abstract: The radiation‐induced optical attenuation has been measured over a wide range of dose and time in state‐of‐the‐art step‐ and graded‐state 60Co and pulsed electron irradiation. Results are reported for previously uncharacterized silica fibers containing dopants such as Ge,P,B,F,Tl, and Cs. At short times following the irradiation, fibers which contain Ge exhibit an extremely high attenuation that is not observed when these fibers contain P. Recovery is typically more complete in the silica core fibers than in the doped silica core fibers, which can have losses 25–1000 times the intrinsic loss 3 months after an irradiation of 105–106 rad.

Light waves guided by a single curved metallic surface.

Abstract: Propagation of TE-waves along a single curved metallic surface with radius of curvature much larger than wavelength is investigated both theoretically and experimentally. Approximate analytic expressions for the field configuration yield that power concentrates in a small region near the metal. The attenuation constant per unit angle of bend (radian) is given by the real part of the inverse of the refractive index, independent of the radius of curvature and of the mode number. In agreement with theory experiments with 10-μm radiation showed that low loss guiding can be realized.