A Waveguide Based Acoustic Microscope

M. L. Peterson, S. Srinath and J. Murphy

A new instrument is presented which is capable of high resolution acoustic imaging at relatively low frequencies. The approach results in increased complexity of the signal processing required and reduced throughput of the instrument. However, these disadvantages are amply compensated by the ability to create velocity scan images of materials with either high attenuation or low material velocities. These measurements are not possible using traditional acoustic microscopes.

Initial performance of the new instrument is demonstrated using thin samples of shim materials to show that acceptable spatial resolution and highly accurate time delay measurements are possible. An application is then shown for use of the instrument to evaluate subchondral sclerosis in horse bones.  It has been hypothesized that changes in the elastic modulus may be associated with fatigue induced microdamage. The modulus change may further represent bone damage which precedes the development of microcracking. Thin samples are used to allow complementary microradiography to be performed on the bone slices. Because of the low material velocity, surface wave interference methods ( so called V(z) curves) are not well suited for use in
some bone samples. The thickness of the samples eliminates the potential for the samples to be evaluated using pulse-echo time delay measurements. The new instrument is thus unique in the ability to create velocity scans of these samples.