Wave2000 Example #2:
Porosity effects and material characterization.
The figure below is a single "slice" of a micro-computed tomographic image
of a cylinder of human calcaneal (heel) trabecular bone. The bone trabeculae
(with thicknesses on the order of 100 microns) are represented by the light
regions and the marrow spaces are represented by the black regions. (The bone
imaging data is courtesy of Dr. John Kinney of Lawrence Livermore Laboratories,
Livermore, CA.)
Use Wave2000 to study highly scattering structures. The ultrasound "movie" below
is comprised of 3 images of an ultrasound wave propagating through the trabecular
bone slice shown in the previous figure. As may be seen, there is a large amount
of scattering of the 1 MHz ultrasound pulse (produced by a 1 cm source transducer
located on the left side of the object), in what may be described as a "mode
stirring" phenomenon. A longitudinal receiver (also not shown) is configured
in "through transmission mode," i.e., is located "coaxially" with respect to
the source on the right side of the sample. Note that Wave2000 displays only
the boundaries between disparate materials in an interrogated object during
a simulation, so that the propagating wave can be visualized.
A variety of source waveforms can be utilized in Wave2000 simulations. The
plot below shows the ultrasound source waveform transmitted into the bone by
a transducer on the left side of the sample. Wave2000 has a set of source waveform
types that may be used in a simulation. In addition, users may incorporate
their own waveforms (e.g., data from a particular transducer in their lab)
by using a Wave2000 "source file option."
Through transmission measurements are simple to define in Wave2000. The
plot below shows the ultrasound waveform measured by a through transmission
receiver on the right side of the bone. The received waveform displays characteristics
often observed in "real world" measurements. For example, there appears to
be phase cancellation distortion towards the end of the main pulse. Also, there
is significant amount of signal energy received after the main pulse comes
through, showing a remarkable similarity to reported observations.
Wave2000 has "built-in" analysis routines. Although Wave2000
allows the user to plot and save receiver measurements in an ASCII file (for
subsequent processing by a user's own custom software), Wave2000 also comes
with a small set of "built-in" analysis routines. The one shown below is a "transit
time" velocity calculation which allows the user to compute the apparent velocity
of the ultrasound wave measured at a specified receiver, with respect to a
specified source. It also displays several velocities based on distinct "arrival
time" criteria. Not shown but also included in the Wave2000 Analysis features
is the ability to compute the frequency-dependent attenuation of a receiver
waveform with respect to a given source waveform.
Wave2000 can be used in practically any materials characterization problem. The
user may explore how, for example, porosity and architecture (structure) affect
the propagation (e.g., velocity and attenuation) of ultrasound. In addition,
Wave2000 can be used to test a new idea for material characterization (e.g.,
a novel positioning of a receiver on an interrogated object). One can also
explore how changing the material properties of the constituents making up
the interrogated object affects the propagation. For example, in this bone
assessment application, it is of interest to examine the effects of replacing
blood by water or fat or void in the marrow spaces of the bone sample. Such
modifications are simple to do with Wave2000.
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