KEYWORDS: Elastography, Ultrasonography, Near field optics, Wave propagation, Super resolution, Near field, Tissues, Microscopes, Medical imaging, Magnetic resonance imaging
When a wave field is measured within a propagative medium, it is widely accepted that the resulting image resolution depends on the measuring point density, and no longer on the wavelength. Indeed, in-situ measurements allow the near-field details needed for super-resolution to be retrieved. Rarely studied in elastography, this is supported here by experiments in ultrasounds and optics. A passive elastography imaging of two inclusions in a tissue mimicking phantom is shown with a resolution down to 1/45th of a shear wavelength.
We recently proposed an alternative elasticity measurement technique based on elastic wave propagation within a single cell. At this scale, waves at a frequency rarely reached in the field of elastography (15kHz), are captured using an ultrafast camera and a microscope. This technique is based on the local measurement of the speed of a shear wave, a type of elastic wave. By assuming an infinite and homogeneous elastic medium with respect to the wavelength, the shear modulus μ (elasticity) is estimated. These latter assumptions are discussed through experiments conducted in controlled elastic solids at micro-scale. The conclusion is that wave guide effects as well as viscosity are crucial for quantitative mapping of elasticity.
We recently proposed an alternative elasticity measurement technique based on elastic wave propagation within a single cell. Waves are capture using an ultrafast camera and a microscope. This technique is based on the local measurement of the speed vs of a shear wave, a type of elastic wave. By assuming an infinite and homogeneous elastic medium with respect to the wavelength, the shear modulus μ (elasticity) is estimated. These latter assumptions are discussed through experiments conducted in controlled elastic solids. The conclusion is that wave guide effects as well as viscosity are crucial for quantitative mapping of elasticity.
Background and motivation -
Conventional Optical Coherence Elastography (OCE) methods consist in launching controlled shear waves in tissues, and measuring their propagation speed using an ultrafast imaging system. However, the use of external shear sources limits transfer to clinical practice, especially for ophthalmic applications. Here, we propose a totally passive OCE method for ocular tissues based on time-reversal of the natural vibrations.
Methods -
Experiments were first conducted on a tissue-mimicking phantom containing a stiff inclusion. Pulsatile motions were reproduced by stimulating the phantom surface with two piezoelectric actuators excited asynchronously at low frequencies (50-500 Hz). The resulting random displacements were tracked at 190 frames/sec using spectral-domain optical coherence tomography (SD-OCT), with a 10x5µm² resolution over a 3x2mm² field-of-view (lateral x depth). The shear wavefield was numerically refocused (i.e. time-reversed) at each pixel using noise-correlation algorithms. The focal spot size yields the shear wavelength. Results were validated by comparison with shear wave speed measurements obtained from conventional active OCE. In vivo tests were then conducted on anesthetized rats.
Results -
The stiff inclusion of the phantom was delineated on the wavelength map with a wavelength ratio between the inclusion and the background (1.6) consistent with the speed ratio (1.7). This validates the wavelength measurements. In vivo, natural shear waves were detected in the eye and wavelength maps of the anterior segment showed a clear elastic contrast between the cornea, the sclera and the iris.
Conclusion -
We validated the time-reversal approach for passive elastography using SD-OCT imaging at low frame-rate. This method could accelerate the clinical transfer of ocular elastography.
Optical coherence tomography (OCT) can map the stiffness of biological tissue by imaging mechanical perturbations (shear waves) propagating in the tissue. Most shear wave elastography (SWE) techniques rely on active shear sources to generate controlled displacements that are tracked at ultrafast imaging rates. Here, we propose a noise-correlation approach to retrieve stiffness information from the imaging of diffuse displacement fields using low-frame rate spectral-domain OCT. We demonstrated the method on tissue-mimicking phantoms and validated the results by comparison with classic ultrafast SWE. Then we investigated the in vivo feasibility on the eye of an anesthetized rat by applying noise correlation to naturally occurring displacements. The results suggest a great potential for passive elastography based on the detection of natural pulsatile motions using conventional spectral-domain OCT systems. This would facilitate the transfer of OCT-elastography to clinical practice, in particular, in ophthalmology or dermatology.
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