GU2 SXH LINTRODUCTIONThe applications of ultrasound in inhomogeneous media are becoming increasingly widespread.with measurements of attenuation and scattering

The theory of an ultrasonic calorimeter is outlined which shows that the device may be used to measure attenuation in penetrable media. With an auxiliary radiation force device the calorimeter can be used in principle to separate the absorption part of the attenuation in inhomogeneous media such as biological tissues. The design of a practical system is described and test measurements reported. The critical features are seen to be the thermocouple probes, the digitization circuitry, and the geometry of the design.

The use of Biot theory for modelling ultrasonic wave propagation in porous media involves the definition of a ‘critical frequency’ above which both fast and slow compressional waves will, in principle, propagate. Critical frequencies have been evaluated for healthy and osteoporotic cancellous bone filled with water or marrow, using data from the literature. The range of pore sizes in bone gives rise to a critical frequency band rather than a single critical frequency, the mean of which is lower for osteoporotic bone than normal bone. However, the critical frequency is a theoretical concept and previous researchers considered a more realistic ‘viscous frequency’ above which both fast and slow waves may be experimentally observed. Viscous frequencies in bone are found to be several orders of magnitude greater than calculated critical frequencies. Whereas two waves may well be observed at all ultrasonic frequencies for water-filled cancellous bone at 20 °C, it is probable megahertz frequencies would be needed for observation of two waves in vivo.

Formulae for differential and total scattering cross-sections per unit volume of a medium have been derived using the von Kármán correlation function of medium inhomogeneities. The scattering cross-sections have been studied in terms of their dependence on the mean scale of inhomogeneities in comparison to the wavelength, and also on the parameter ν of the von Kármán correlation function. It has been shown that the effective medium acoustic refractive index does not depend on the form of its fluctuations correlation function if the mean scale of inhomogeneities is large in comparison to the wavelength of acoustic waves, and if the Bourret approximation for the effective wave number operator is used.

A lack of knowledge of the detailed behavior of the actual transducers used in a particular application can provide an indeterminate limitation on the sensitivity or the accuracy of the measurements being made. In principle, knowledge of the distribution of the surface vibration of the transducer permits the irradiating field at any point in the region of interest to be determined. Previous reports of surface vibration distribution determination have suffered from a lack of verification. The present work reports measurements made on four transducers (two with ceramic elements and two with PVdF elements of an approximate diameter of 10 mm at frequencies between 2.25 and 3.0 MHz) using two independent techniques. Measurements were made of the fields in transverse planes at four different distances from each transducer. The PTB measurements employed optical diffraction tomography while those from Guildford used scanned miniature (0.25- or 0.5-mm-diam PVdF) hydrophones to measure both amplitude and phase. Excellent agreement was found between the measurements from the two techniques, including absolute pressure levels, confirming that the vibration distribution of a transmitting transducer can be reliably determined although it requires great technical care and, at present, is time consuming.

The use of ultrasound for determining the elastic constants of materials is a well-established science for homogeneous materials such as metals. However, its extension to anisotropic, inhomogeneous materials such as wood has proved more problematic. Wood is modelled as an orthorhombic material with the influence of inhomogeneities generally being neglected. For this paper the potential influence of inhomogeneities on waves propagating in the radial direction was considered. Within ring density and ultrasonic velocity measurements were made. A model for ultrasound propagation in the radial direction was then constructed which treats the annual ring structure in the radial direction as a layered structure and predicts the occurrence of stop bands in the frequency domain. Evidence for the existence of such stop bands is considered.

We report two studies on the design of training schedules for medical-image readers. Experiment 1 required subjects to learn to classify ultrasound images of six different phantoms; one group of subjects trained on images that varied on just two of the four possible dimensions of variation on each day, while the other group trained on images that varied on all four dimensions of variation each day. Performance improved on each day, and learning transferred to novel stimuli shown on successive days. Initially performance was worse for the high-variation group, but by the final session they had reached the same level as the restricted variation group. Within-category similarity increased after training and inter-category similarity decreased. In the second experiment, the stimuli were x-rays of perspex blocks with holes drilled in one of five possible locations. The subject's task was to search for the image- feature produced by the hole (a dark spot). One group of subjects judged 'easy' images for the first four days, and then switched to judgments of 'difficult' holes on the final day, while a second group underwent the reverse order. Although both groups improved, the group that had trained on the easy stimuli showed positive transfer to the more difficult stimuli, but the group that trained on the difficult stimuli showed no transfer to the easy stimuli.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Measurements of ultrasonic velocity and attenuation on a suspension of sub-millimetre polystyrene spheres in a solution of aqueous sodium chloride were carried out over a frequency range of 4-12 MHz, for solid volume fractions varying between 0 and 0.5. A method of predicting the volume fraction at which the single scattering approximation becomes unphysical due to multiple scattering has been proposed for these measurements. A comparison of the experimental data with the theoretical predictions of single scattering and two multiple scattering approaches has been made. Good agreement was obtained between the experimental and theoretical approaches, the predictions of the iterative multiple scattering model most closely approximating the experimental results.

A conventional clinical ultrasonic B-scanner has been successfully used to image preserved (canned) peaches, with a view to assessing the method's potential for detecting stone pits embedded in the peach flesh. In addition, it is suggested from the pictures obtained that the grey scale texture arising from the peach flesh may be indicative of variations in the degree of ripeness of the peach prior to preservation. The acoustic velocity and attenuation in the peaches appear to be close to those of soft tissues, and preliminary laboratory measurements confirm this.

The feasibility of using ultrasonic techniques to detect fatigue damage in metal-matrix composites at an early stage has been investigated theoretically. The lack of direct independent experimental evidence for such damage required damage mechanisms and levels to be postulated in a somewhat arbitrary fashion. Estimates have been made of the effect of matrix porosity, microcrack formation, particle debonding and dislocation generation on ultrasonic velocity, attenuation and backscattering. It would seem that velocity measurements provide the best opportunity for detecting low levels of damage. Initial calculations suggest that surface wave velocity measurements and acoustoelastic constant measurements may also prove useful.

The problem of ultrasonic backscattering of longitudinal, ultrasonic waves from a single debonded fibre is examined in two seperate theoretical models. In the first model the debond is assumed to be an air filled crack, whilst in the second the matrix and fibre are assumed to remain incontact but tangential motion at the interface (slip) is allowed. In addition experimental measuremnets are made on a scale model of a debonded metal wire in an epoxy resin matrix.

Fine resolution narrow-band pressure amplitude distribution measurements have been performed using a hydrophone in the field of a weakly focused bowl transducer radiating into castor oil and 1% and 2% by volume suspensions of 320-μm polystyrene beads in castor oil over the frequency range 1.0 to 2.5 MHz at 20 °C. The axial distributions permitted determination of the excess attenuation due to scattering in the suspensions as a function of frequency. The frequency range included a resonance. The excess attenuation measured was compared with the theoretical predictions of Waterman and Truell [J. Math. Phys. 2, 512–537 (1961)] and good agreement was obtained. The values of the excess attenuation obtained were used to predict axial pressure amplitude distributions in the suspension. Although the excess attenuation was determined from far-field measurements, the agreement between the predicted axial distributions and those measured experimentally extended well into the near field. Lateral distributions were measured at seven frequencies at the position of the geometrical center of curvature of the source and at the position of the true focus in the 1% suspension. Comparison with similar scans in castor oil alone revealed no detectable defocusing due to the presence of the scatterers.