2012 Proceedings - International Tissue Elasticity Conference

Session SIP: Signal and Image Processing
Friday, October 5 8:00A – 10:00A
004 SHEAR WAVE IMAGING USING HARMONIC DISTORTION OF SINUSOIDAL EXCITATION.
Yoshiki Yamakoshi 1 , Parajuli Raj Kumar 1 , Reisen Tei 1 , Daisuke Nakai 1 , Yui Tsuihiji 1 .
1 Gunma University, 1–5–1 Tenjin–cho, Kiryu–shi , Gunma 376–8515 JAPAN.
Background: Continuous shear wave excitation (CSWE) has potential in tissue characterization, and
numerous methods have been proposed in order to reconstruct different kinds of maps [1,2]. For
example, a shear wave propagation map gives information about the mechanical structure of a medium,
such as adhesion and denaturation of tissue. However, the problem of CSWE is that the spatial
resolution is limited by wavelength of shear wave, and it is difficult to introduce high frequency shear
wave due to large attenuation in tissue.
Aims: We propose an imaging method which reconstructs phase modulation component of harmonic
distortion in continuous shear wave excitation.
Methods: A local velocity map is used in order to recover the phase modulation component of shear wave
harmonic distortion. First, the complex displacement map is calculated from an ultrasonic wave Doppler
signal by using cross–correlation of up–sampled radio–frequency (RF) signals. Then, the vibration phase
map is derived. A local velocity map is reconstructed from spatial differentiation of the vibration phase
map along the shear wave propagation direction. When there is no phase modulation in the shear wave,
the local velocity map becomes a shear wave velocity map. But if there is small amplitude phase
modulation, the phase modulation appears on the shear wave velocity map as wave–front pattern of the
harmonic component. The normal direction of the pattern shows the propagation direction, and the
interval between the peaks corresponds to the wavelength. Visibility of the pattern shows the modulation
index. In order to reconstruct the harmonic component of the shear wave, Fourier analysis method is well
known. However, in real–time image reconstruction, spectral resolution of Fourier analysis is not enough
for the harmonic component of small amplitude. The proposed method uses spatial differentiation of the
vibration phase in order to obtain the harmonic component, image of relative high signal–to–noise (S/N)
is obtained when the data acquisition time is limited.
Results: Images which are obtained for an agar gel phantom are shown in Figure 1. The vibration
frequency is 700Hz. Figure 1a is the real part of the complex displacement which shows the wave–front of
the fundamental component of the shear wave. Figure 1b shows the phase modulation component which is
reconstructed by the proposed method. Figure 1c is the real part of complex displacement which is
reconstructed for 3rd order harmonic component by Fourier analysis method. From the result, the wave–front
of small amplitude harmonic distortion, which is difficult to observe by Fourier analysis method, is
reconstructed by the proposed method. In this experiment, harmonic distortion at the vibrator head, which is
measured by 3axes accelerometer, is -47dB (3rd order). Existence of the phase modulation is also observed on
complex displacement vector space. Figure 2 shows an experimental result in vivo (muscle of upper arm). The
vibration frequency is 250Hz. Figure 2a is the shear wave amplitude map for the fundamental component.
Figures 2b and 2c are the local velocity map and the phase modulation component, respectively.
Conclusions: We proposed a visualization method of the harmonic component with small amplitude. Although
the result is preliminary, this map might be useful in observation of mechanical structure of medium.
References:
[1] Y. Yamakoshi, J. Sato, and T. Sato: Ultrasonic Imaging of Internal Vibration of Soft Tissue under Forced
Vibration, IEEE Trans. on UFFC, 37, 45–53, 1990.
[2] K. Parker, M. Doyley and D. Rubens: Imaging the Elastic Properties of Tissue: The 20 Year Perspective, Phys.
Med. Biol. 56, R1–R29, 2011.
(a) (b) (c)
(a) (b) (c)
Figure 1: Results for agar gel phantom
(a) Complex displacement spectrum map (real part).
(b) Phase modulation map by the proposed method.
(c) 3 rd order harmonic component reconstructed by Fourier
analysis method (real part of complex displacement).
Figure 2: In vivo result (Muscle of upper arm)
(a) Shear wave amplitude map for fundamental component.
(b) Local velocity map by the proposed method.
(c) Phase modulation map reconstructed by the
proposed method.
Vibration frequency is 250Hz.
indicates Presenter 93