2012 Proceedings - International Tissue Elasticity Conference

116
Session MMT: Mechanical Measurement Techniques for Tissues
Friday, October 5 4:15P – 5:30P
012 SUB–VOXEL MICRO-ARCHITECTURE ASSESSMENT BY SCATTERING OF MECHANICAL
SHEAR WAVES.
S. A. Lambert 1 , S. Chatelin 1 , S. P. Nashölm 2 , L. Juge 1 , P. Garteiser 1 , L. Ter Beek 3 , V. Vilgrain 1 ,
B. E. Van Beers 1 , L. E. Bilston 4 , B. Guzina 5 , S. Holm 2 and R. Sinkus 1 .
1 University Paris Diderot, Sorbonne Paris Cité, INSERM, CRB3, UMR773, 75018, Paris, FRANCE;
2 Informatics Department, University of Oslo, NORWAY; 3 Philips Healthcare, Best, The
NETHERLANDS; 4 Neuroscience Research Australia and University of New South Wales, Sydney,
NSW, AUSTRALIA; 5 Civil Engineering Department, University of Minnesota, Minneapolis, MN, USA.
Background: Magnetic Resonance Elastography [1] (MRE) is a technique capable of noninvasively
assessing the mechanical properties of tissues. It can be hypothesized that the presence of micro–obstacles
(similar to effects leading to the apparent diffusion coefficient) changes the dispersion relation of
propagating shear waves and, hence, might influence the apparent mechanical properties of the medium
at the macroscopic scale [2]. In Diffusion Weighted Imaging, micro–structural information is lost due to
the massive averaging that occurs within the imaging voxel and can only be revealed when exploring the
tissue using different b–values. Similarly here, where the propagation of a mechanical wave enters into
the diffusive regime due to multiple scattering effects, the frequency-dependence of the mechanical
properties could allow the assessment of the sub–voxel micro–architecture.
Aims: In this study we investigate the propagation of shear waves in FEM simulations as well as in
calibrated phantoms containing accurately controlled size distributions of scattering particles.
Methods: Gel phantoms were fabricated using an agarose solution at 15g/L (BRL, Type 5510UB). In order to
create well defined scattering particle size distributions, colloidal suspensions of polystyrene microspheres
with precisely known diameter (10μm diameter, Sigma–Aldrich) and concentrations (20–1.25%) were added to
the gel before solidification. MRE was performed on a horizontal 7T imaging scanner (Pharmascan, Bruker,
Erlangen, Germany) (Figure 1). The MRE (spin echo, 0.4mm thickness, field of view of 30×30mm 2 ,
TE/TR (ms)=13–35/270–505, 8 samples for time encoding, excitation frequencies: 600–1000Hz) sequence
was acquired for the three spatial directions of motion in order to obtain volumetric images of the 3D
mechanical wave propagating inside the phantom. Data were reconstructed with an isotropic
reconstruction technique [3]. Wave propagation has been simulated only in 2D in order to void a time
consuming 3D approach using Diffpack finite element code [4]. For both numerical and experimental
approaches, power law fit was used to study the influence of micro–particle size concentration on wave
scattering frequency dependence.
Results: For the same wavelength over micro–obstacles diameter ratio ≈400), ( experimental and
numerical results are in good agreement (Figure 2). The maximum of dispersion could be explained by
scattering of mechanical wave by fractal systems [5].
Conclusions: For the first time, we demonstrated that the frequency–dependence of mechanical shear
wave diffusion can allow probing sub–voxel distributions of scattering structures and as a consequence
overcome the spatial resolution limitation relying intrinsically on the MR imaging sensitivity.
(a) (b)
(c) (d)
(a) (b)
Figure 1: Phantom and experimental set–up: (a) A flexible carbon
fiber rod transmits horizontal vibrations from a shaker
to a toothpick mechanically coupled to the sandwich Figure 2: Slopes of the power fit for (a) the elastic–shear
design phantoms (c) positioned in (b) a 7T MRI scanner
modulus (Gd) and (b) the propagation coefficient of the
(b). (d) Confocal–microscopy image of the phantom with
wave (β) as a function of microspheres concentration.
1.25% in volume of 10μm diameter microspheres.
References:
[1] Muthupillai, R: Science, p. 269, 1995. [4] Langtangen, HP: LNCSE, Vol. 2, 1999.
[2] Holm, S: JASA, pp. 542–559, 2010. [5] Teixeira, J: J. Appl. Cryst, pp. 781–785, 1988.
[3] Sinkus, R: MRI, pp. 159–165, 2005.
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