2012 Proceedings - International Tissue Elasticity Conference

049 SHEAR WAVE SPEED AND DISPERSION MEASUREMENT USING INTERFERENCE PATTERNS
FROM A SPECIALLY DESIGNED CHIRP SIGNAL.
Zaegyoo Hah 1 , Alexander Partin 1 , Kevin J. Parker 1 .
1 Electrical and Computer Engineering Department, University of Rochester, Rochester, NY, USA.
Background: There is a growing need to measure not just shear elasticity (or shear speed) but shear
speed dispersion. For example, there is a big concern about nonalcoholic fatty liver disease (NAFLD), a
major cause of chronic liver disease. It is understood that increasing amounts of fat in the normal liver
will increase the dispersion (that is, the frequency dependence or slope) of the shear wave speed [1].
Normally, the dispersion measurements are done separately at several selected frequencies within a
range. Shear speeds are then calculated at each frequency, and regressions of the data result in the
shear speed dispersion [2]. However, there are inherent problems with this method: signal–to–noise ratio
(SNR), measurement time and safety. Therefore, to optimize measurement time and SNR, it would be
advantageous to use chirps sweeping over a frequency range.
Aims: A measurement procedure using a chirp scanning over a range of frequency is introduced. Also
analysis has been performed to provide a systematic way of measuring the shear speed and dispersion.
Methods: Some biomaterials including a 10% gelatin phantom, a fatty phantom with 10% gelatin and
30% safflower oil, and a fatty phantom with 15% gelatin and 30% castor oil are chosen for this study. All
the test biomaterials are assumed to be homogeneous. The biomaterial is vibrated by two mechanical
sources (Brüel & Kjaer, Model 2706, Naerum, Denmark) on both ends of the material such that an
interference pattern is generated inside the medium. A chirp signal, instead of a fixed frequency signal, is
used to drive the sources from 100Hz to 300Hz. The medium is scanned by an ultrasound imaging
system (GE Healthcare, Logiq9, Milwaukee, WI, USA), and the movie is saved. The average motion slice
image (Figure 1) is dependent on the chirp: the motion slice has a hyperbolic shape as shown in Figure
2a for a linear chirp. A special chirp is designed to produce the fan-shaped motion slice in Figure 2b,
thereby providing the advantage of easy smoothing and image processing. Further analysis is performed
to provide a closed form solution for the shear speed and dispersion. Measurement results are compared
to those obtained by single frequency measurements.
Results: The safflower oil phantom was measured to have (3.5m/s, ∙10 2 -4 m/s/Hz) of shear speed and
dispersion respectively, while the castor oil and the gelatin phantom showed (4.4m/s, 6∙10 -4 m/s/Hz) and
(3.0m/s, 3∙10-6 m/s/Hz). These results were compared with the conventional methods using either phase
map or waveform fitting. Single frequency measurements for the safflower oil, for example, showed some
fluctuations: 3.43, 3.62, 3.53, 3.52m/s at 110, 130, 150 and 200Hz respectively.
Conclusions: The developed method of shear speed and dispersion measurement provides advantages
over the conventional methods. It is much faster, more robust and easily implemented. The techniques
will be further applied to other biomaterials and the liver.
References:
[1] CT Barry, B Mills, Z Hah, RA Mooney, CK Ryan, DJ Rubens, KJ Parker: Shear Wave Dispersion Measures Liver
Steatosis. Ultrasound Med Biol, 38(2), pp. 175–182, 2012.
[2] Z. Hah, A. Partin, C.T. Barry, R.A. Mooney, D.J. Rubens, K.J. Parker: Dispersion and Shear–Wave Velocity in a
Mouse–Liver Model using Crawling Waves. Ultrasonic Imaging and Tissue Characterization Symposium, Rosslyn,
VA, 11–13 June, 2012.
Figure 1 Figure 2a Figure 2b
indicates Presenter 119