2012 Proceedings - International Tissue Elasticity Conference

029 IN VIVO TIME HARMONIC MULTIPLE FREQUENCY ELASTOGRAPHY OF HUMAN LIVER.
H Tzschätzsch 1 , J Braun 1 , T Fischer 1 , R Klaua 2 , M Schultz 2 , I Sack 1 .
1 Charité–Univeritätsmedizin Berlin, Berlin, GERMANY; 2 GAMPT mbH, Merseburg,
Sachsen–Anhalt, GERMANY.
Background: Noninvasive measurement of tissue stiffness by elastography is increasingly used for the staging
of hepatic fibrosis. In particular, the shear–wave based Fibroscan technique has achieved a remarkably high
precision for the diagnosis of high grades of fibrosis [1]. The diagnosis of low grades of liver fibrosis remains a
major challenge since therapies should be initiated at early stages of the disease. Magnetic resonance
elastography (MRE) has been proven most sensitive to early fibrosis [2,3]. In extension to single–frequency
time–harmonic MRE, broad–band MRE based on four mechanical excitation frequencies showed an excellent
diagnostic power [4]. However, to date MRE has only limited availability and suffers from long scan times.
Aims: We developed a one–dimensional time–harmonic ultrasound elastography (USE) system which
adopts the mechanical principle of multifrequency MRE of liver. Due to rapid data acquisition, USE is
capable of measuring superimposed multifrequency vibrations in a fraction of the time needed for MRE.
The feasibility of the method is demonstrated in healthy volunteers. Results are compared to findings of
the literature made by multifrequency MRE [5].
Methods: Three healthy volunteers (male, ages: 26, 35 and 42 years) were scanned after written informed
consent was obtained. Superimposed time–harmonic mechanical stimulation of seven frequencies (30–60Hz
with 5Hz increments) was achieved by a cradle actuator beneath the right lateral costal arch mounted to a
commercial loudspeaker (Figure 1a). The spectral vibration output of the controller is shown in Figure 1b. In
addition, the vibration response measured in the liver from the lateral–anterior intercostal position is shown.
The mechanical vibrations were captured by a custom designed A–line ultrasonic device, capable of 50MHz
data sampling and 1kHz pulse repetition frequency, which was equipped with a single–element 2MHz
transducer probe (GAMPT, Merseburg, Germany). Motion estimation was based on a windowed (1.1mm width)
phase root seeking algorithm over 10cm depth. The speed of propagating shear waves was detected by a
phase gradient method after applying a spatial bandpass filter to the temporal Fourier–transformed vibration
signals at each single drive frequency. Total signal acquisition time was one second. The experiment was
repeated twenty times in expiration for each volunteer to account for variability in probe positioning.
Results: Group mean shear–wave speed values are plotted in Figure 1c. The apparent dispersion of the
wave speed with drive frequency is due to the well–known viscoelastic behavior of liver tissue. The elastic
and Kelvin–Voigt (K–V) models were used for fitting experimental wave speed values. The elastic model
yields a shear modulus μ=1.93±0.46kPa while the K–V model yields μ=0.64±0.24kPa and η=28±1Pa s.
MRE literature values taken from [5] are also shown. Fitting the elastic model yields μ=2.07±0.05 and the
K–V model yields μ=1.54±0.23kPa and η=25±5Pa s.
Conclusions: Multifrequency USE is capable to measure the dispersion of the shear wave speed in a total
of 5 minutes. Remarkably, the obtained viscoelastic parameters agree well with the values measured by
multifrequency MRE which needs about half an hour measure time. A limitation of the method is the
high rate of failed scans (ca. 50%) characterized by noise and compression artifacts since probe
repositioning and repetitive scans prolong the measure time.
References:
[1] Ziol et al.: C. Hepatology, 41(1), pp. 48–54, 2005.
[2] Yin et al.: Clin Gastroenterol Hepatol, 5(10), pp. 1207–1213, 2007.
[3] Huwart et al.: Radiology, 245(2), pp. 458–466, 2007.
[4] Asbach et al.: Radiology, 257(1), pp. 80–86, 2010.
[5] Klatt et al.: Physics in Medicine and Biology, 52, pp. 7281–7294, 2007.
Figure 1: (a) Experimental
setup; (b) Spectrum of
vibration input and
tissue oscillation; (c)
Experimental data
and model of MRE
and USE.
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