2012 Proceedings - International Tissue Elasticity Conference

075 SHEAR WAVE GENERATION FOR ELASTICITY IMAGING VIA MODE CONVERSION FROM
LONGITUDINAL WAVES AT ELASTICITY BOUNDARY.
K. Nii 1 , K. Okubo 1 , N. Tagawa 1 , S. Yagi 2 .
1 Tokyo Metropolitan University, 6–6 Asahigaoka, Hino, Tokyo, JAPAN; 2 Meisei University, 2–1–1
Hodokubo, Hino, Tokyo, JAPAN.
Background: The latest real–time elastography systems have a tendency to use shear waves propagating
tumor [1]. As an influential method using shear waves, acoustic radiation force impulse (ARFI) imaging
has been proposed for quantitative measurement of tissue elasticity [2]. However, the potential for effects
on the living body of ARFI are a cause for concern, and various examinations of those are strongly
demanded, for example as a subject of rise in temperature by pulses with long duration [3].
Aims: We are aiming to establish a method for generating shear waves in the body with low risk for the
living body using a mode conversion of the longitudinal pulse transmitted as a source of vibration. As a
fundamental research for this concept, in this study, we evaluate the characteristics of the mode
conversion mechanism at the elasticity boundary through numerical simulations.
Methods: In this study, we use the PZFlex, a standard FEM simulator for ultrasound propagation, to
examine the state of shear wave generation due to mode conversion. We model a layered elasticity boundary
using soft tissue with an elastic modulus of 1.0kPa, which is placed on the input side of longitudinal waves
with the width of 20mm, and hard tissue with various values of elastic modulus. The density of all the
tissues is fixed as 1000g/cm 3 . The attenuation coefficient of longitudinal waves is 0.5dB/cm/MHz for soft
tissue and 0.7dB/cm/MHz for hard tissue. To make it easy to detect the mode conversion, the attenuation
coefficient of shear waves is 0.1dB/cm/MHz for all the tissues, which may be smaller than the standard
value of normal tissue. By varying the elastic modulus of the hard tissue, we simulate the shear wave
generation via the mode conversion and compare the strength of the shear waves propagating in hard
tissue and those in soft tissue. From our initial evaluation, it was determined that a frequency of 3MHz is
desirable due to the balance of attenuation and directivity for the layered simulation model used in this
study. In actuality, this frequency should be determined by part of the body being interrogated.
Results: In the simulations, a linear array transducer with 32 elements was assumed for transmitting
the longitudinal waves. The results of the strength of the shear waves measured near the elasticity
boundary, which is proportional to the amplitude of the longitudinal waves, are shown in Table I. It was
also found that the strength of shear wave depends on the pulse width of the longitudinal wave. This
indicates that continuous waves are suitable for the mode conversion. Additionally, as shown in Figure 1,
we confirm that the shear waves are generated at the elasticity boundary of a circular tumor with the
radius of 5mm and propagate in the tumor region for a long time, with repeating reflections at the
boundary. The material parameters are the same as those of the layered model, i.e. the circular tumor
corresponds to the hard tissue in the layered model.
Conclusions: We confirmed that the mode conversion certainly occurs and suitable shear waves can be
generated. However, the frequency determined and used in this study may be too high to measure the
velocity of the shear wave because of the high attenuation characteristics disregarded in this study. In
future, we will construct a mode conversion method by which sufficiently low frequency shear waves are
generated from high frequency longitudinal waves using various frequency conversion techniques.
Elastic modulus of
4.0 9.0 16.0 25.0
hard tissue
Strength of shear
0.041 0.047 0.047 0.048
wave in soft tissue
Strength of shear
0.276 0.573 1.013 1.566
wave in hard tissue
Table I: Strength of shear waves propagating in soft
tissue and hard tissue (kPa). Figure 1: Shear wave propagation in a circular tumor. Time
References:
duration of the sequence of images is 1.12ms.
[1] Yagi et al.: Analysis of Transient Shear Wave Generation for Real–Time Elastography. IEEE Int. Ultrasonics Symp.,
pp. 1356–1359, 2005.
[2] M.L. Palmeri et al.: Quantifying Hepatic Shear Modulus In Vivo using Acoustic Radiation Force. Ultrasound Med. Biol.
pp. 546–558, 2008.
[3] B.A. Herman et al.: Models and Regulatory Considerations for Transient Temperature Rise During Diagnostic
Ultrasound Pulses. Ultrasound Med. Biol., pp. 1217–1224, 2002.
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