Christopher M. W. Daft

The elastic microstructure of various tissues

Previous work has indicated that a modified Quate-Lemons scanning acoustic microscope (SAM) is capable of measuring the acoustic propagation properties of sections of biological tissue. The lens is excited by an impulse, rather than a tone burst, and the undemodulated returning signal from the tissue is recorded. The variations in received signal with time are used to deduce the sound speed, attenuation, impedance, and section thickness. In this article, the technique is applied to various types of tissue, and the variations in acoustic propagation properties are computed. Conventional tone burst SAM images at 425 MHz are compared with the time resolved data in order to elucidate the contrast mechanisms. The effects of varying the frequency and position of the focal plane on the tone burst images are interpreted in the light of the broadband results.

A quantitative acoustic microscope with multiple detection modes

An acoustic microscope that permits operation with both toneburst and impulse excitation of the lens is presented. Either mode can be selected and combined with mechanical scanning in any direction. In the impulse-excited mode, the specular and Rayleigh signals from the sample are resolved in time, and analysis is performed to obtain surface wave propagation parameters. The power of the simultaneous application of these techniques is illustrated by measurements on specimens of intact and fractured glass and duraluminum. Reflection and transmission coefficients for a crack are measured, and conclusions are drawn about V(z) processing. These results are significant because the images of cracks produced by the conventional toneburst scanning acoustic microphone (SAM) tend to be complex. Diffraction from the tips of cracks is observed in the microscope.<<ETX>>

Acoustic Microscopy of Old and New Materials

The large number of papers on acoustic microscopy in this symposium testifies both to the range of techniques available to the user, and to the range of applications that are being found. Imaging is now routinely possible at frequencies ranging from close to those used in conventional nondestructive testing and medical ultrasound to 2GHz 1 2 , with contrast arising from scattering by internal defects or factors affecting surface wave propagation. Quantitative measurements can be made by time resolved impulse measurements 3 or by the V(z) technique with larger pulses. For very accurate measurements the line focus beam technique has become well established 4 , offering unprecedented capabilities especially for surface wave measurements on anisotropic materials. In a growing number of disciplines it is being discovered that the acoustic microscope can provide new information, specifically about the elastic properties of specimens and how acoustic waves interact with them. This paper offers a small selection of applications, ranging from natural materials in the form of rocks and teeth, to new man-made materials in the form of ceramics and hard-metals.

Applications of neural networks to ultrasound tomography

Ultrasound tomography holds promise in the area of medical diagnosis but is limited by the inadequacies of current reconstruction algorithms. An alternative method using neural networks is presented. The theory begins with X-ray tomography and is extended to ultrasound. After theoretical relevance is introduced, several experiments are discussed to illustrate the effectiveness of neural networks. The model was a circular cylinder with acoustic properties of tissue, insonated by a line source at 2 MHz. The transducers were arranged in a ring surrounding the cylinder, with one being the transmitter. The experiments involved varying the acoustic speed and the radius of the cylinder. In both cases, the neural network was able to generalize to parameters other than the ones used during the training.<<ETX>>

In-vivo fetal ultrasound exposimetry

An instrument has been developed to measure the acoustic pressure field during an obstetric ultrasound examination. This permits a more accurate assessment of possible bioeffects since the pressure is sensed at or near the site of the organs under investigation. The ultrasonic field is sampled using a calibrated seven-element linear array hydrophone of PVDF transducers, which is placed as close as possible to the ovary, embryo, or fetus, using a vaginal approach. The RF signals from the hydrophone are digitized at 50 Ms/s, and the maximum amplitude waveform received in the examination is recorded. The output of the clinical B-scanner is calibrated by the hydrophone in a water bath. From the hydrophone measurements, the in-vivo I/sub SPTA/, I/sub SPTP/, and I/sub SPPA/ can be computed. Further analysis allows the frequency-dependent tissue attenuation to be assessed.<<ETX>>

Wideband acoustic microscopy of tissue

A scanning acoustic microscope (SAM) has been used to measure the elastic properties of tissue with a resolution of around 8 mu m. This is achieved by broadband excitation of the acoustic lens, and the recording of an undemodulated returning signal. A method of analyzing this information to yield sound velocity, acoustic impedance, section thickness, and acoustic attenuation is described. Results from a sample of skin tissue are presented and compared with data from a computer simulation of the experiment.<<ETX>>

Frequency dependence of tissue attenuation measured by acoustic microscopy

The authors have developed a method of acoustic time-domain reflectometry that allows the elastic properties of a histological section to be measured with a resolution of around 8 mu m. Experimental data obtained using this technique have been processed so that the frequency dependence of the tissue attenuation is revealed. Comparisons are also made between scanning acoustic microscope (SAM) and SLAM data for similar tissue samples. Observations are made on the lower leg muscles of an adult ICR mouse. An analysis is made of a toneburst SAM image obtained with the lens focused on the surface of the microscope slide, at a frequency of 425 MHz and a field of view of 425 mu m.<<ETX>>