Showing papers on "Attenuation published in 2004"

Implications of cosmological gamma-ray absorption - II. Modification of gamma-ray spectra

Abstract: Bearing on the model for the time-dependent metagalactic radiation field developed in the first paper of this series, we compute the gamma-ray attenuation due to pair production in photon-photon scattering. Emphasis is on the effects of varying the star formation rate and the fraction of UV radiation assumed to escape from the star forming regions, the latter being important mainly for high-redshift sources. Conversely, we investigate how the metagalactic radiation field can be measured from the gamma-ray pair creation cutoff as a function of redshift, the Fazio-Stecker relation. For three observed TeV-blazars (Mkn501, Mkn421, H1426+428) we study the effects of gamma-ray attenuation on their spectra in detail.

Poly(vinyl alcohol) cryogel phantoms for use in ultrasound and MR imaging

Abstract: Poly(vinyl alcohol) cryogel, PVA-C, is presented as a tissue-mimicking material, suitable for application in magnetic resonance (MR) imaging and ultrasound imaging. A 10% by weight poly(vinyl alcohol) in water solution was used to form PVA-C, which is solidified through a freeze-thaw process. The number of freeze-thaw cycles affects the properties of the material. The ultrasound and MR imaging characteristics were investigated using cylindrical samples of PVA-C. The speed of sound was found to range from 1520 to 1540 m s(-1), and the attenuation coefficients were in the range of 0.075-0.28 dB (cm MHz)(-1). T1 and T2 relaxation values were found to be 718-1034 ms and 108-175 ms, respectively. We also present applications of this material in an anthropomorphic brain phantom, a multi-volume stenosed vessel phantom and breast biopsy phantoms. Some suggestions are made for how best to handle this material in the phantom design and development process.

Fog attenuation prediction for optical and infrared waves

Abstract: The principal disadvantage of using free space optics (FSO) telecommunication systems is the disturbing role played by the atmosphere on light propagation and thus on the channel capacity, availability, and link reliability. The wavelength choice is currently a subject of disagreement among designers and users of FSO equipments. Generally this equipment operates in the visible and the near IR at 690, 780, 850, and 1550 nm. Several authors affirm that equipment working at 1550 nm presents less atmospheric attenuation in the presence of fog and thus better link availability. Others consider that for dense fogs (visibility<500 m), all wavelengths are attenuated in the same way (wavelength independence). Fog attenuation in the visible and IR regions is reviewed from an empirical and theoretical point of view. Laser system performance in the presence of fog (advection and convection) in the 0.4- to 15-µm spectral zone is investigated using FASCOD computation. A transmission gain of 42% for a lasercom system working at 780 nm is observed compared to the same system working at 1550 nm. This gain reaches 48% if the same system works at 690 nm. Finally, we propose a fast transmission relationship based on an exact Mie theory calculation valid in the 0.69- to 1.55-µm spectral bands. It enables us to predict fog attenuation according to visibility without using heavy computer codes.

Quantitative measurement of attenuation coefficients of weakly scattering media using optical coherence tomography

Abstract: From calibrated, weakly scattering tissue phantoms (2-6 mm-1), we extract the attenuation coefficient with an accuracy of 0.8 mm-1 from OCT data in the clinically relevant 'fixed focus' geometry. The data are analyzed using a single scattering model and a recently developed description of the confocal point spread function (PSF). We verify the validity of the single scattering model by a quantitative comparison with a multiple scattering model, and validate the use of the PSF on the calibrated samples. Implementation of this model for existing OCT systems will be straightforward. Localized quantitative measurement of the attenuation coefficient of different tissues can significantly improve the clinical value of OCT.

Comparative study of lead borate and bismuth lead borate glass systems as gamma-radiation shielding materials

Abstract: Gamma-ray mass attenuation coefficients have been measured experimentally and calculated theoretically for PbO–B2O3 and Bi2O3–PbO–B2O3 glass systems using narrow beam transmission method. These values have been used to calculate half value layer (HVL) parameter. These parameters have also been calculated theoretically for some standard radiation shielding concretes at same energies. Effect of replacing lead by bismuth has been analyzed in terms of density, molar volume and mass attenuation coefficient.

Empirical Attenuation of Ground-Motion Spectral Amplitudes in Southeastern Canada and the Northeastern United States

Abstract: A database of 1700 digital seismograms from 186 earthquakes of mag- nitude mN 2.5-5.6 that occurred in southeastern Canada and the northeastern United States from 1990 to 2003 was compiled. Maximum-likelihood regression analysis of the database was performed to determine a model for the attenuation of Fourier spectral amplitudes for the shear window, for the vertical and horizontal component of motion, for frequencies from 0.2 to 20 Hz. Fourier amplitudes follow a hinged trilinear attenuation model. Fourier spectral amplitudes decay as R 1.3 (where R is hypocentral distance) within 70 km of the source. There is a transition zone from 70 to 140 km as the direct waves are joined by strong postcritical reflections, where the attenuation is described as R 0.2 ; spectral amplitudes actually increase with distance in this range for low frequencies. Beyond 140 km, the attenuation is well described by R 0.5 , corresponding to geometric spreading in two dimensions. The associated model for the regional quality factor for frequencies greater than 1 Hz can be ex- pressed as Q 893f 032 . Q can be better modeled over a wider frequency range (0.2- 20 Hz) by a polynomial expression: log Q 3.052 0.393 log f 0.945 (log f ) 2 0.327 (log f ) 3 . The polynomial expression accommodates the observation that Q values are at a minimum (about 1000) near 1 Hz and rise at both lower and higher frequencies. Correction factors for the spectral amplitude model that describe the effects of focal depth on the amplitudes and their attenuation are developed using the subset of events with known focal depth. The attenuation model is similar to that determined from an earlier study with more limited data (Atkinson and Mereu, 1992), but the enlarged database indicates more rapid near-source amplitude decay and higher Q. The attenuation model is used to play back attenuation effects to determine the apparent source spectrum for each earthquake in the database and hence determine moment magnitude (M) and Brune stress drop. The events have moment magnitude in the range from 2.5 to 5. Stress drop increases with moment magnitude for events of M 4.3, then appears to attain a relatively constant level in the range from 100 to 200 bars for the larger events, as previously noted in Atkinson (1993b). The results of this study provide a useful framework for improving regional ground-motion relations in eastern North America. They further our understanding of attenuation in the region through analysis of an enlarged ground-motion database. In particular, the inclusion of the three-component broadband data gathered over the last decade allows extension of attenuation models to both horizontal and vertical components over a broad frequency range (0.2-20 Hz).

Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies

Abstract: We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the UV to the FIR/submm and can also account for the surface brightness distribution in both the optical/NIR and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 AA and 2.2 micron, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Halpha/Hbeta line ratio.

Modelling the spectral energy distribution of galaxies. III. Attenuation of stellar light in spiral galaxies

Abstract: We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the ultraviolet (UV) to the Far-infrared (FIR)/submillimeter (submm) and can also account for the surface brightness distribution in both the optical/Near-infrared (NIR) and FIR/submm. The calculations are based on the model of Popescu et al. (2000), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912A and 2.2 μm, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the Hα/Hβ line ratio.

Connectivity vs capacity in dense ad hoc networks

Abstract: We study the connectivity and capacity of finite area ad hoc wireless networks, with an increasing number of nodes (dense networks). We find that the properties of the network strongly depend on the shape of the attenuation function. For power law attenuation functions, connectivity scales, and the available rate per node is known to decrease like 1//spl radic/n. On the contrary, if the attenuation function does not have a singularity at the origin and is uniformly bounded, we obtain bounds on the percolation domain for large node densities, which show that either the network becomes disconnected, or the available rate per node decreases like 1/n.

Shear wave attenuation and dispersion in melt-bearing olivine polycrystals: 2. Microstructural interpretation and seismological implications

Abstract: [1] The torsional forced oscillation tests of melt-bearing olivine aggregates reported by Jackson et al. [2004] consistently show a peak in attenuation that is absent from melt-free aggregates tested under similar conditions and grain sizes. Characterization by SEM shows that the melt resides in triple junction tubules and larger pockets as previously described. TEM imaging and EDS analysis reveals that olivine-olivine grain boundaries are characterized by a region ≤1 nm wide which is structurally and chemically distinct from olivine grain interiors. From the possible mechanisms that can produce an anelastic attenuation peak, melt squirt can be eliminated for our samples and experimental conditions. We attribute the observed attenuation peak to elastically accommodated grain boundary sliding, requiring that the grain boundaries are weak relative to olivine grain interiors but have a significantly higher viscosity than bulk melt. While the nanometer scale grain boundary structure in the melt-bearing aggregates is essentially the same as for melt-free aggregates studied previously, elastically accommodated sliding in the latter is apparently inhibited by tight three-grain edge intersections. The exponentially increasing high temperature background attenuation in both types of aggregate is attributed to diffusionally accommodated grain boundary sliding. Extrapolation to mantle grain sizes shows that the broad peak may be responsible for nearly frequency independent attenuation in partially molten regions of the upper mantle.

Seismic attenuation and mantle wedge temperatures in the Alaska subduction zone

Abstract: [1] Anelastic loss of seismic wave energy, or seismic attenuation (1/Q), provides a proxy for temperature under certain conditions. The Q structure of the upper mantle beneath central Alaska is imaged here at high resolution, an active subduction zone where arc volcanism is absent, to investigate mantle thermal structure. The recent Broadband Experiment Across the Alaska Range (BEAAR) provides the first dense broadband seismic coverage of this region. The spectra of P and SH waves for regional earthquakes are inverted for path averaged attenuation operators between 0.5 and 20 Hz, along with earthquake source parameters. These measurements fit waveforms significantly better when the frequency dependence of Q is taken into account, and in the mantle, frequency dependence lies close to laboratory values. Inverting these measurements for spatial variations in Q reveals a highly attenuating wedge, with Q 500, assuming frequency dependence. Comparison with P results shows that attenuation in bulk modulus is negligible within the low-Q wedge, as expected for thermally activated attenuation mechanisms. Bulk attenuation is significant in the overlying crust and subducting plate, indicating that Q must be controlled by other processes. The shallowest part of the wedge shows little attenuation, as expected for a cold viscous nose that is not involved in wedge corner flow. Overall, the spatial pattern of Q beneath Alaska is qualitatively similar to other subduction zones, although the highest wedge attenuation is about a factor of 2 lower. The Q values imply that temperatures exceed 1200°C in the wedge, on the basis of recent laboratory-based calibrations for dry peridotite. These temperatures are 100–150°C colder than we infer beneath Japan or the Andes, possibly explaining the absence of arc volcanism in central Alaska.

Small-size sonic crystals with strong attenuation bands in the audible frequency range

Abstract: Recent experiments have proved that sonic crystals containing locally resonant structures exhibit strong sound attenuation bands at frequencies about two orders of magnitude smaller than predicted by Bragg’s theory. The effect is well reproduced here by means of two-dimensional numerical simulations of the elastic wave propagation in a 13 cm slab of locally resonant sonic crystals. Three strong attenuation bands are found in the frequency range from 0.3 to 6.0 kHz. A heuristic model is proposed, which allows one to predict the resonance frequencies in good agreement with the numerical simulations.

A random walk model of wave propagation

Abstract: This paper shows that a reasonably accurate description of propagation loss in small urban cells can be obtained with a simple stochastic model based on the theory of random walks, that accounts for only two parameters: the amount of clutter and the amount of absorption in the environment. Despite the simplifications of the model, the derived analytical solution correctly describes the smooth transition of power attenuation from an inverse square law with the distance to the transmitter, to an exponential attenuation as this distance is increased - as it is observed in practice. Our analysis suggests using a simple exponential path loss formula as an alternative to the empirical formulas that are often used for prediction. Results are validated by comparison with experimental data collected in a small urban cell.

Low loss (1.7 dB/km) hollow core photonic bandgap fiber

Abstract: We report a silica hollow core photonic bandgap fiber with a minimum attenuation of 1.72 dB/km at 1565 nm wavelength and discuss the scaling of attenuation with wavelength and the minimum loss wavelength in this type of fiber.

Temperature dependence of ultrasonic propagation speed and attenuation in excised canine liver tissue measured using transmitted and reflected pulses.

Abstract: Previous reported data from our laboratory demonstrated the temperature dependence of propagation speed and attenuation of canine tissue in vitro at discrete temperatures ranging from 25 to 95 °C. However, concerns were raised regarding heating the same tissue specimen over the entire temperature range, a process that may introduce irreversible and, presumably, cumulative tissue degradation. In this paper propagation speed and attenuation vs temperature are measured using multiple groups of samples, each group heated to a different temperature. Sample thicknesses are measured directly using a technique that uses both transmitted and reflected ultrasound pulses. Results obtained using 3 and 5 MHz center frequencies demonstrate a propagation speed elevation of around 20 m/s in the 22–60 °C range, and a decrease of 15 m/s in the 60–90 °C range, in agreement with previous results where the same specimens were subjected to the entire temperature range. However, sound speed results reported here are slightly hi...

Predicting macrobending loss for large-mode area photonic crystal fibers.

Abstract: We report on an easy-to-evaluate expression for the prediction of the bend-loss for a large mode area photonic crystal fiber (PCF) with a triangular air-hole lattice. The expression is based on a recently proposed formulation of the V-parameter for a PCF and contains no free parameters. The validity of the expression is verified experimentally for varying fiber parameters as well as bend radius. The typical deviation between the position of the measured and the predicted bend loss edge is within measurement uncertainty.

Optical-CT gel-dosimetry II: Optical artifacts and geometrical distortion

Abstract: There is a clear need for technology that enables accurate, high-resolution, three-dimensional (3D) measurement of intricate dose distributions associated with modern radiation treatments. A potential candidate has emerged in the form of water-equivalent "3D gel dosimetry" utilizing optical-computed-tomography (optical-CT). In a previous paper we presented basic physical characterization of an in-house prototype optical-CT scanning system. The present paper builds on that work by investigating sources of optical artifacts and geometric distortion in optical-CT scanning. Improvements in scanner design are described. Correction strategies were developed to compensate for reflection and refraction, imperfections in the water-bath, signal drift, and other effects. Refraction and reflection were identified as the principal factors causing inaccurate reconstruction of absolute attenuation coefficients. A correction specific to a given flask was developed utilizing prescans of the flask when filled with water-bath fluid, thereby isolating the refractive and reflective components for that flask. Residual artifacts were corrected by fitting a theoretical model to the well-behaved portion of these prescans and extrapolating to regions of lost data, enabling reconstruction of absolute optical-CT attenuation coefficients to within 4% of corresponding spectrophotometer values. Needle phantoms are introduced to quantify geometric distortion under a range of conditions. Radial distortion of reconstructed needle positions was reduced to < 0.3 mm (0.27% of the field of view) through adjustment of the water-bath refractive index. Geometric distortion in polymer gel due to radiation-induced refractive index changes was found to be negligible under the conditions examined. The influence of scattered light on reconstructed attenuation coefficients was investigated by repeat optical-CT scans while varying the aperture of a scatter-rejecting collimator. Significant depression of reconstructed attenuation coefficients was observed, particularly under conditions of poor scatter rejection collimation. The general conclusion is that the first-generation optical-CT technique can be made insensitive to geometrical distortion, but can be susceptible to scatter effects. For accurate reconstruction of absolute attenuation coefficients, correction strategies are essential.

Photon mass attenuation coefficients, effective atomic numbers and electron densities of some thermoluminescent dosimetric compounds

Abstract: Photon mass attenuation coefficients of some thermoluminescent dosimetric (TLD) compounds, such as LiF, CaCO3, CaSO4, CaSO4.2H2O, SrSO4, CdSO4, BaSO4, C4H6BaO4 and 3CdSO4.8H2O were determined at 279.2, 320.07, 514.0, 661.6, 1115.5, 1173.2 and 1332.5 keV in a well-collimated narrow beam good geometry set-up using a high resolution, hyper pure germanium detector. The attenuation coefficient data were then used to compute the effective atomic number and the electron density of TLD compounds. The interpolation of total attenuation cross-sections of photons of energyE in elements of atomic numberZ was performed using the logarithmic regression analysis of the data measured by the authors and reported earlier. The best-fit coefficients so obtained in the photon energy range of 279.2 to 320.07 keV, 514.0 to 661.6 keV and 1115.5 to 1332.5 keV by a piece-wise interpolation method were then used to find the effective atomic number and electron density of the compounds. These values are found to be in agreement with other available published values.

The global short-period wavefield modelled with a Monte Carlo seismic phonon method

Abstract: SUMMARY At high frequencies (∼1 Hz), much of the seismic energy arriving at teleseismic distances is not found in the main phases (e.g. P, PP, S, etc.) but is contained in the extended coda that follows these arrivals. This coda results from scattering off small-scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation. Most analyses of seismic coda to date have concentrated on S-wave coda generated from lithospheric scattering for events recorded at local and regional distances. Here, we examine the globally averaged vertical-component, 1-Hz wavefield (>10 ◦ range) for earthquakes recorded in the IRIS FARM archive from 1990 to 1999. We apply an envelopefunction stacking technique to image the average time‐distance behavior of the wavefield for both shallow (≤50 km) and deep (≥500 km) earthquakes. Unlike regional records, our images are dominated by P and P coda owing to the large effect of attenuation on PP and S at high frequencies. Modelling our results is complicated by the need to include a variety of ray paths, the likely contributions of multiple scattering and the possible importance of P-to-S and S-to-P scattering. We adopt a stochastic, particle-based approach in which millions of seismic phonons are randomly sprayed from the source and tracked through the Earth. Each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer. Discontinuities are modelled by treating the energy normalized reflection and transmission coefficients as probabilities. Scattering probabilities and scattering angles are computed in a similar fashion, assuming random velocity and density perturbations characterized by an exponential autocorrelation function. Intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of traveltime. We find that most scattering occurs in the lithosphere and upper mantle, as previous results have indicated, but that some lower-mantle scattering is likely also required. A model with 3 to 4 per cent rms velocity heterogeneity at 4-km scale length in the upper mantle and 0.5 per cent rms velocity heterogeneity at 8-km scale length in the lower mantle (with intrinsic attenuation of Q α = 450 above 200 km depth and Q α = 2500 below 200 km) provides a reasonable fit to both the shallow- and deep-earthquake observations, although many trade-offs exist between the scale length, depth extent and strength of the heterogeneity.

Attenuation of Strong Ground Motion in Peninsular India

Abstract: Earthquakes in India occur in the plate boundary region of the Himalayas as well as in the intraplate region of Peninsular India (PI). Devastating events have occurred in PI in the recent past, which is a warning about the possibility of such earthquakes in future also. But very limited recorded data is available about ground motion in PI for engineers to rely upon. The present paper, after a review of available data, develops an attenuation relationship based on a statistically simulated seismological model.

Apparatus for measuring concentration of light-absorbing substance in blood

Abstract: An apparatus for measuring a concentration of a light-absorbing substance in blood is disclosed. A light emitter emits light beams to irradiate a living tissue, each of the light beams being associated with one wavelength which is absorbed by the blood. A first instrument measures first intensities of the light beams, which are to be incident on the living tissue. A second instrument measures second intensities of the light beams, which are transmitted through the living tissue. A first calculator calculates an attenuation variation ratio, which is a ratio of attenuation variations of the respective light beams due to variation of a volume of the blood caused by pulsation, based on the second intensities of the light beams. A second calculator calculates the concentration based on the first intensities, the second intensities, and the attenuation variation ratio.

Guided wave propagation in an elastic hollow cylinder coated with a viscoelastic material

Abstract: The propagation of ultrasonic guided waves in an elastic hollow cylinder with a viscoelastic coating is studied. The principle motivation is to provide tools for performing a guided wave, nondestructive inspection of piping and tubing with viscoelastic coatings. The theoretical boundary value problem is solved that describes the guided wave propagation in these structures for the purpose of finding the guided wave modes that propagate with little or no attenuation. The model uses the global matrix technique to generate the dispersion equation for the longitudinal modes of a system of an arbitrary number of perfectly bonded hollow cylinders with traction-free outer surfaces. A numerical solution of the dispersion equation produces the phase velocity and attenuation dispersion curves that describe the nature of the guided wave propagation. The attenuation dispersion curves show some guided wave modes that propagate with little or no attenuation in the coated structures of interest. The wave structure is examined for two of the modes to verify that the boundary conditions are satisfied and to explain their attenuation behavior. Experimental results are produced using an array of transducers positioned circumferentially around the pipe to evaluate the accuracy of the numerical solution.

Guided wave propagation in three-layer pavement structures

Abstract: A study on guided waves in a layered half-space with large velocity contrasts and a decreasing velocity with depth is presented. Multiple mode dispersion curves are calculated in the complex wave number domain, taking into consideration the attenuation caused by leakage into the underlying half-space. The excitability of the modes by a vertical point force on the surface is also calculated. Results show that the measurable wave field at the surface of a pavement structure is dominated by leaky quasi-Lamb waves in the top and second layers. The fundamental antisymmetric mode of vibration is the dominating mode generated in the stiff top layer. This mode drives the complete system and continuity across the boundaries generates higher order modes in the embedded second layer. The interaction of leaky Lamb waves in the first two layers results in large variations in the excitability and the attenuation, so that only the waves corresponding to certain portions of the dispersion curves are measurable remote from the source at the pavement surface. It is concluded that these portions of dispersion curves can be individually resolved in practice, by using multichannel processing techniques. This holds the potential for a refined nondestructive testing technique for pavements.

Sound attenuation in lined bends

Abstract: In the present paper we are concerned with sound propagation and attenuation in two- or three-dimensional lined bends. First it is shown that the effect of locally reacting absorbing materials at the walls of a waveguide can easily be taken into account in the multimodal formulation proposed in earlier papers by the authors, and, for bends, algebraic solutions are carried out for the acoustic field and scattering properties. Then a study of the sound attenuation in lined bends is given using the multimodal formulation and the properties of such waveguides are shown and discussed, in particular, the presence of a plateau of attenuation at high frequencies and a whispering gallery effect that occurs in bends.

Ultrasound attenuation in normal and spontaneously degenerated articular cartilage.

Abstract: High-frequency ultrasound (US) measurements may provide means for the quantification of articular cartilage quality. Bovine patellar cartilage samples (n = 32) at various degenerative stages were studied using US attenuation measurements in the 5- to 9-MHz frequency range. The results were compared with the histologic, biochemical and mechanical parameters obtained for the same samples, to identify which structural or functional factors could be related to the attenuation and its variations. Attenuation, as calculated in the frequency or time domain, correlated significantly with the histologic tissue integrity (i.e., Mankin score, Spearman r = -0.576 or -0.571, p < 0.01), but the slope of attenuation vs. frequency was not related to Mankin score. Ultrasound speed was, however, the most sensitive indicator of Mankin score (r = -0.755, p < 0.01). Cartilage quality index (CQI), a combination of structural and functional parameters, correlated significantly with the attenuation or speed (r = -0.655 or -0.872, p < 0.01). Our results suggest that US attenuation and speed may be suited for the diagnostics of cartilage degeneration.

A Method to Remove Artifacts in Attenuation-Corrected Myocardial Perfusion SPECT Introduced by Misalignment Between Emission Scan and CT-Derived Attenuation Maps

Abstract: Nonuniform soft-tissue attenuation affects the diagnostic accuracy of SPECT in myocardial perfusion imaging. The attenuation map required for attenuation correction can be acquired using x-ray tomography (CT). Frequent findings in attenuation-corrected images are defects in the apical and anterior myocardial wall. We assume that these are artifacts produced by misalignment of SPECT images and the attenuation map. Methods: One hundred forty patients underwent myocardial perfusion imaging with 99mTc-methoxyisobutylisonitrile. Twenty-seven of 140 showed pronounced defects in the apical or anterior wall only after CT-based attenuation correction. SPECT and corresponding CT slices were examined for misalignment in the ventrodorsal direction (y-direction) visually and by threshold-based delineation of the body surface. Mismatched studies were realigned and image reconstruction and analysis were redone. The effect of the correction was assessed visually and by semiquantitative analysis based on a 20-segment model using 4D-MSPECT. Results: In 15 of 27 patients, the improved coregistration led to smaller and less-pronounced defects in the regions mentioned. In 6 of 27 patients, former defects were judged as normal. No improvement was seen in only 4 patients. In these 4 subjects, the mismatch in the y-direction was

A method for estimating the physical and acoustic properties of the sea bed using chirp sonar data

Abstract: This paper proposes a method, based on the Biot model, for estimating the physical and acoustic properties of surficial ocean sediments from normal incidence reflection data acquired by a chirp sonar. The inversion method estimates sediment porosity from reflection coefficient measurements and, using the estimated porosity and the measured change in fast wave attenuation with frequency, estimates the permeability of the top sediment layer. The spectral ratio of echoes from the interface at the base of the upper sediment layer and from the sediment-water interface provides a measure of the change in attenuation with frequency. Given the porosity and permeability estimates, the Kozeny-Carman equation provides the mean grain size and the inversion method yields the acoustic properties of top sediment layer. The inversion technique is tested using chirp sonar data collected at the 1999 Sediment Acoustics Experiment (SAX-99) site. Remote estimates of porosity, grain size, and permeability agree with direct measurements of those properties.