Time-shift estimation and focusing through distributed aberrationusing multirow arrays

The effects of element height on time-shift estimation and transmit focus compensation are demonstrated experimentally. Multirow ultrasonic transducer arrays were emulated by combining adjacent elements of a 3.0-MHz, 0.6-mm pitch, two-dimensional array to define larger virtual elements. Pulse-echo data were acquired through tissue-mimicking distributed aberrators, and time-shift maps estimated from those data were used for transmit focus compensation. Compensated beams formed by arrays with fine row pitches were similar, but focus restoration was significantly less effective for "1.75-D" arrays with a coarse row pitch. For example, when focus compensation was derived from strongly aberrated random scattering data [70-ns nominal rms arrival time fluctuation with 7 mm FWHM (full-width at half-maximum) correlation length], the mean -20 dB lateral beamwidths were 5.2 mm for f/2.0 arrays with 0.6- and 1.8-mm row pitches and 9.5 mm for an f /2.0 array with 5.4-mm pitch. Time-shift maps estimated from random scattering data acquired with 5.4-mm pitch arrays included large discontinuities caused by low correlation of signals received on vertically and diagonally adjacent emulated elements. The results indicate that multirow arrays designed for use with aberration correction should have element dimensions much less than 75% of the correlation length of the aberration and perhaps as small as 25 to 30% of the correlation length     

Comparison of techniques for in vivo attenuation measurements

The attenuation of an ultrasound pulse within tissue can be estimated from either the amplitude decay or the frequency downshift of returning echoes. The authors compare the results of both analyses as applied to ultrasound B scan echoes from the livers of 49 individuals. The amplitude decay of the backscattered signal Fourier components with depth was used to calculate attenuation coefficients. In addition, the frequency downshift of the same backscattered signals was estimated using both zero-crossing and spectral centroid methods. The results show that the frequency-domain estimators yield consistently higher attenuation coefficients, with higher variability compared to the amplitude decay method. Explanations for the apparent bias and variability of the frequency-shift estimators include both the assumptions regarding tissue and signal which may not be met in practice and the effects of low-frequency electronic noise on spectral estimates     

Improved degradation stability of blue-green II-VI light-emitting diodes with excluded nitrogen-doped ZnSe-based layers

Degradation characteristics of p-i-n BeZnSe/Zn(Be)CdSe light-emitting diodes were investigated. Undoped short-period superlattices, which provide efficient hole transport from the p ⁺-BeTe:N near-contact region (hole injector) into the active region, were used instead of the p-doped BeZnSe:N emitter. It is demonstrated that this makes it possible to considerably lengthen the operating life of the light-emitting diodes at highest direct current densities (～4.5 kA/cm²) at room temperature.     

Visual-spatial abilities of pilots.

US Air Force pilots and control Ss participated in 5 experiments, each of which assessed a different type of visual-spatial ability. Although pilots judged metric spatial relations better than did nonpilots, they did not judge categorical spatial relations better than did nonpilots. Pilots mentally rotated objects better than did nonpilots, but pilots did not extrapolate motion, scan images, or extract visual features from objects obscured by visual noise better than did nonpilots. The results imply that efficient use of specific processing subsystems is especially important for, and characteristic of, pilots. The possible neuropsychological bases for the enhanced abilities and their susceptibility to change are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparisons of lesion detectability in ultrasound images acquired using time-shift compensation and spatial compounding

The effects of aberration, time-shift compensation, and spatial compounding on the discrimination of positive-contrast lesions in ultrasound b-scan images are investigated using a two-dimensional (2-D) array system and tissue-mimicking phantoms. Images were acquired within an 8.8 x 12-mm2 field of view centered on one of four statistically similar 4-mm diameter spherical lesions. Each lesion was imaged in four planes offset by successive 45 degree rotations about the central scan line. Images of the lesions were acquired using conventional geometric focusing through a water path, geometric focusing through a 35-mm thick distributed aberration phantom, and time-shift compensated transmit and receive focusing through the aberration phantom. The views of each lesion were averaged to form sets of water path, aberrated, and time-shift compensated 4:1 compound images and 16:1 compound images. The contrast ratio and detectability index of each image were computed to assess lesion differentiation. In the presence of aberration representative of breast or abdominal wall tissue, time-shift compensation provided statistically significant improvements of contrast ratio but did not consistently affect the detectability index, and spatial compounding significantly increased the detectability index but did not alter the contrast ratio. Time-shift compensation and spatial compounding thus provide complementary benefits to lesion detection.     

Flaw identification from time and frequency features of ultrasonicwaveforms

Time and frequency features have been used with classification algorithms to distinguish between ultrasonic echoes from flaws in pipes and ultrasonic echoes from various geometric configurations of weld root and counterbore. Waveforms containing reflections from known geometries and from flaws were obtained and sets of features were defined using a k-nearest neighbor approach to separate waveforms into classes. Two independent databases containing various flaws and pipe geometries were used to determine these feature sets. From these databases, optimal feature sets were found to separate counterbore waveforms from crack waveforms. Optimal feature sets were also found to distinguish between waveforms from counterbore, waveforms containing both counterbore and root echoes, and waveforms from flaws. The best feature sets used with the classifier algorithms could separate waveforms from the same database with accuracy in the 92-97% range and with high confidence., Another database was obtained from pipe structures in a nuclear power plant to provide a field test of the method. When applied to this database, the same classifier algorithms and feature sets used with the other databases either resulted in a comparable percentage of correct decisions, but with low confidence, or could not classify anywhere from 79 to 88% of the waveforms. Spatial parameters based on averaging feature vectors in axial and circumferential directions were also defined and used for classification. These classifications had higher accuracy but lower confidence levels than the classifications based on individual waveforms     

Renormalization of the band gap in highly photoexcited type-II ZnSe/BeTe structures

For the type-II ZnSe/BeTe heterostructures, a large (～0.1 eV) red shift of the edge of interband recombination in the ZnSe layers is observed at high densities of spatially separated photoexcited electrons and holes (～10¹³ cm^?2). The observed magnitude of renormalization of the band gap exceeds the magnitudes predicted by the multiparticle theory for dense type-I electron-hole systems at the same concentrations of two-dimensional charge carriers. Numerical calculations show that macroscopic electric fields induced by separated charges have a profound effect on the energy of direct transitions in type-II structures, resulting in an additional decrease in the energy of the transitions. In wide structures, where the ZnSe layer thickness is ? 15 nm, the renormalization effect is less pronounced. This is attributed to incomplete spatial separation of photoexcited charge carriers in the case of profound band bending and, thus, to the less-pronounced effect of electric fields.     

Room temperature stimulated emission from a CdTe/CdMgTesuperlattice laser structure in the red spectral range

We have investigated the optical response of a red emitting CdTe/CdMgTe laser structure after optical excitation. For the first time, stimulated emission has been observed in this material system at room temperature, demonstrating the potential for the development of CdTe/CdMgTe laser structures in the visible spectral range. An analysis of our experimental data yields a threshold carrier density of about 3.5·10¹² cm^-2 and a net gain coefficient of about 95 cm^-1 at T=300 K     

Propagation and backpropagation for ultrasonic wavefront design

Wave backpropagation is a concept that can be used to calculate the excitation signals for an array with programmable transmit waveforms to produce a specified field that has no significant evanescent wave components. This concept can also be used to find the field at a distance away from an aperture based on measurements made in the aperture. For a uniform medium, three methods exist for the calculation of wave propagation and backpropagation: the diffraction integral method, the angular spectrum method, and the shift-and-add method. The boundary conditions that are usually implicitly assumed by these methods are analyzed, and the relationship between these methods are explored. The application of the angular spectrum method to other kinds of boundary conditions is discussed, as is the relationship between wave backpropagation, phase conjugation, and the time-reversal mirror. Wave backpropagation is used, as an example, to calculate the excitation signals for a ring transducer to produce a specified pulsatile plane wave with a limited spatial extent.