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.