• Bel-Brunon, A.
• Abdul-Ghafour, T.
• Zorgani, A.
• Catheline, S.
• Lescanne, M.

Abstract

Transient elastography is a well-developed modality for mapping soft tissue elasticity. It consists in imaging the propagation of a shear wave inside a tissue. Under the assumption of incompressibility, the shear modulus µ is proportional to the shear wave speed Cs following the relationship µ = ρCs^2 , ρ is the tissue density. Several imaging modalities can be used to image the displacement resulting from a shear wave: ultrasound, Magnetic Resonance elastography (MRE) or Optical Coherence Tomography (OCT). These methods allow observing the shear wave propagation inside the tissue to evaluate its speed and therefore, determine its elasticity. However, this observation is unidirectional and in a single plane. Investigating anisotropy therefore requires multiple measurements and is subjected to the noise of the reconstruction. The relationship between the shear wave speed and the shear modulus is also valid for surface waves, which can be tracked using Digital Image Correlation (DIC). In this work we report a feasibility study to use an optical based method to assess the surface wave speed on a soft tissue. This consists in imaging the propagation of an actively induced surface wave at the surface of a controlled soft tissue (phantom). Three cases have been tested: 1. isotropic phantom, 2. anisotropic phantom and 3. dome-like shaped phantom. Phantoms were made of gelatin or PVA. Graphite (Sigma Aldrich, St Louis, MO, USA) was added to the mixture to create the random pattern necessary for DIC; however it was not sufficient for the anisotropic phantom, which was therefore covered with a white paint random pattern. For cases 1 and 2, a x2 telecentric lens was mounted on an ultrafast digital camera; the field of view was 50x32 mm 2. For case 3, two ultrafast digital cameras were used with 60mm lens to capture images for stereo-DIC. The acquisition frequency was 4000 Hz for an impulse of 1000Hz of the point-like source. Displacement fields were computed using ViC3D software and ranged between 0.05 +/-0.005 mm (case 3) and 0.5 +/-0.0008 mm (cases 1 and 2). The obtained surface wave speed was in the order of 2 m/s, which was expected. DIC and stereo-DIC allowed quantifying the phantom anisotropy as well as the wave speed on a curved surface using a single test. This could be a new method to characterize tissues in-vivo, especially sub-surface phenomena.

Topics

• density
• impedance spectroscopy
• surface
• tomography
• anisotropic
• elasticity
• random
• isotropic