Ultrasound pulse-echo reflection from test object cylindrical reflectors

Characterization of high-frequency, single-element focused transducers with wire target and hydrophone

In this paper, a wire-target technique was used for lateral beam profile measurements for a single-element, focused transducers in the very high-frequency range (35-60 MHz). Two wire targets made from 9-cm long tungsten wires with diameters of 8 microm and 20 microm were used as the pulse-echo targets to measure the lateral beam profiles at the focal plane of two single-element, focused transducers, a spherically focused 40 MHz transducer and a lens-focused in-house lithium niobate (LiNbO3) 60 MHz transducer. For comparison, measurements on the same transducers were performed by three small-aperture hydrophones with geometrical diameters varying from 37 microm to 150 microm. Tomographic reconstruction of the acoustic field from the spherically focused transducer also was conducted. Results obtained with the wire-target technique are comparable to those obtained with small-aperture hydrophones in characterizing lateral radiation patterns of a single-element, focused transducer in the high-frequency range (35-60 MHz). However, the wire-target method may overestimate pulse length because of the additional attenuation caused by the return path. Compared to small-aperture hydrophones, the wire-target technique is simpler and more cost effective. Its major advantage, however, is in the frequency range above 100 MHz in which commercial hydrophones are not yet available.

Variable density linear acoustic inverse problem

The linear acoustic inverse problem is solved simultaneously for density (rho) and compressibility (kappa) using the basic ideas of diffraction tomography (DT). The key to solving this problem is to utilize frequency diversity to obtain the required independent measurements. The receivers are assumed to be in the far field of the object, and plane wave incidence is also assumed. The Born approximation is used to arrive at a relationship between the measured pressure field and two terms related to the spatial Fourier transform of the two unknowns, rho and kappa. The term involving compressibility corresponds to monopole scattering and that for density to dipole scattering. Measurements at several frequencies are used and a least squares problem is solved to reconstruct rho and kappa at the same time. It is observed that the low spatial frequencies in the spectra of rho and kappa produce inaccuracies in the results. Hence, a regularization method is devised to remove this problem. Several results are shown. Low contrast objects for which the above analysis holds are used to show that good reconstructions are obtained for both density and compressibility after regularization is applied.