proceedings - International Tissue Elasticity Conference

087 IMPROVING SHEAR VELOCITY ESTIMATION ACCURACY BY DETECTING POTENTIAL FALSE
PEAKS IN TISSUE DISPLACEMENT RESPONSE.
Liexiang Fan 1 , Paul Freiburger 1 .
1 Siemens Medical Solutions USA, Ultrasound Division, 22010 SE 51 st Street, Issaquah, WA, USA.
Background: It has been proven feasible to detect tissue shear velocity by monitoring the shear wave
arrival times at multiple locations after acoustic radiation force impulse (ARFI) excitation [1]. In [1], it is
also demonstrated that accurate results can be achieved by using the peak of the tissue displacement
response to track the wave front propagation. When we applied this method, we observed the appearance
of two peaks in the displacement response in both phantom and in–vivo data. In some cases, the highest
peak was a false indicator of the wave front propagation. A false peak detected before the true peak
resulted in overestimation of the shear velocity, and a false peak detected after the true peak resulted in
underestimation of the shear velocity. There are many possible causes for a second peak which will not
be addressed in this work. Instead, we developed a method which can identify a false peak. This has
enabled improved shear velocity estimation.
Aims: The aim of this work is to identify the true peak in the tissue displacement response after ARFI
excitation in order to prevent over/under–estimation of shear velocity.
Methods: A set of displacements over time are generated within a region on interest (ROI) after ARFI
excitation. A temporal displacement profile is generated from the median value of the displacements
calculated within the ROI and is used as a reference. The highest peak is assumed to be the reference
peak reflecting the average arriving time for the ROI. For each individual profile, the peak closest to the
reference peak is taken as the true peak when two peaks are observed.
Results: Figure 1a shows tissue displacement responses at locations 0.83mm, 2.17mm, 3.5mm and
4.17 mm away from the center of the ARFI excitation. At location 4.17mm, there are two peaks, and the
highest peak is a false peak. Figure 1b shows the results for peak occurrence times at 25 locations evenly
distributed between 1.33mm to 5.33 mm from the center of the ARFI excitation pulse. The ‘x’ points are
the peak occurrence times before the proposed method is applied, the ‘o’ points are the result after the
proposed method is applied.
Conclusions: This work demonstrates the improvement of shear velocity detection when two peaks
appear in the tissue displacement response after ARFI excitation.
Acknowledgements: We would like to thank Duke University, especially Drs. K. Nightingale and M. Palmeri for their
technical communication on shear velocity estimation using ARFI.
References:
[1] Nightingale, KR et al. Shear Wave Velocity Estimation Using Acoustic Radiation Force Impulse Excitation in Liver
in–vivo. IEEE Ultrasonic Symposium, Vancouver, Canada, October, 2006.
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(a) (b)
Figure 1: Tissue displacement response at four locations (a) and the shear wave travel time of 25 locations (b) after
ARFI excitation.
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