2012 Proceedings - International Tissue Elasticity Conference

Session MIP–3: Methods for Imaging Elastic Tissue Properties – III
Friday, October 5 10:30A – 12:00P
062 IMAGING TRANSVERSE ISOTROPIC ELASTIC PROPERTIES OF MUSCLE WITH
ULTRASONICALLY TRACKED RADIATION FORCE INDUCED SHEAR WAVES IN 3D.
MH Wang 1 , BC Byram 1 , ML Palmeri 1 , NC Rouze 1 , KR Nightingale 1 .
1 Duke University, Durham, NC, USA.
Background: The mechanical properties of skeletal muscle are consistent with a transverse isotropic
model of elasticity due to the parallel arrangement of thin muscle fibers. As a result, the shear wave
speed (SWS) is fastest along the direction of the fibers and slowest across the fibers. With the recent
availability of 2D matrix array ultrasound transducers, shear wave propagation can now be monitored in
a 3D volume using ultrasound. This overcomes the inherent limitation in determining the fiber
orientation, which is 3D in nature, from 2D cross–sectional images.
Aims: To determine the muscle fiber orientation and SWS along and across the fibers in 3D using
ultrasonically tracked shear waves.
Methods: Shear wave imaging was performed on freshly excised canine muscle embedded in agar. An
annular focused HIFU piston transducer (H–101, Sonic Concepts, Bothell, WA) was used to induce shear
wave propagation with acoustic radiation force. A 2.8MHz 2D matrix array transducer (4Z1C on an
SC2000 scanner, Siemens Healthcare, Ultrasound Business Unit, Mountain View, CA, USA) inserted in
the central opening of the HIFU piston was used for monitoring the resulting shear wave displacement in
3D. The shear wave propagates along the plane of symmetry of the muscle fibers. To determine the
orientation of this plane, the SWS was measured in all possible directions from a single point (the push
focus at 60mm). SWS measurements in directions oblique to the plane of propagation are associated with
high uncertainty. Therefore, the plane orientation containing SWS measurements with the lowest
measurement uncertainty was determined to be the plane of shear wave propagation. The fiber
orientation was estimated by fitting an ellipse to the SWS measured within this plane and finding the
angle of the major axis. To verify the 3D muscle fiber orientation determined from SWS measurements,
high–resolution 3D ultrasound volumes of the samples were obtained using a high frequency 14MHz
1D array (14L5 on the S2000 scanner, Siemens Healthcare) mechanically swept using a translation stage.
The 14L5 B–mode volumes were rigidly registered to the 2D matrix array data using fiducial markers
which were visible in both. To determine the fiber orientation in the 14L5 B–mode volume, 3D line
enhancement filtering was performed followed by a 3D Radon transform (RT). The angle giving the highest
variance in the 3D RT was taken as the fiber orientation.
Results: The SWS measured from a single shear wave acquisition in one muscle sample has been
analyzed to date with on–going accrual. The SWS within the plane of propagation is shown in Figure 1.
The push axis was intentionally made non–perpendicularly to the muscle fibers. The normal of the shear
wave plane of propagation, as determined from SWS measurements, was 26º from the push axis. There
was a 5.8º difference between the angle of this plane and the 3D RT fiber orientation. The difference
between the major axis of the SWS within this plane and the projection of the 3D RT fiber orientation in
this plane was 3º. The difference between the fiber orientation measured from SWS and 3D RT (red and
green arrows in Figure 1) was 6.5º. The length of the major axis (SWS along the fibers) was 5.2m/s, while
the length of the minor axis (SWS across the fibers) was 3.0m/s.
Conclusions: Muscle fiber orientation in 3D and the SWS along and across the fibers can be determined
by monitoring shear wave propagation in 3D with ultrasound.
Acknowledgements: NIH grants R01 EB–002132 and R01 CA142824.
Figure 1: Polar plot of the SWS within the plane of
propagation at the push focus and the
best–fit ellipse. The major and minor
axes of the SWS and the fiber axis from
3D RT are shown by arrows.
indicates Presenter 101