Toward soft-tissue elastography using digital holography to monitor surface acoustic waves

Quantifying viscosity and elasticity using holographic imaging by Rayleigh wave dispersion

Viscoelasticity is an important diagnostic parameter to investigate physiological dysfunctions in biological tissues. This Letter reports the quantification of viscoelastic parameters by Rayleigh wave tracing on the surface of tissue-mimicking phantoms using holographic imaging. The Rayleigh wave is induced by an electromechanical actuator on the surface of oil-in-gelatin phantoms and a biological tissue sample followed by holographic imaging and reconstruction of the wave. The frequency-dependent velocity dispersion is fitted to a Voigt model for the quantification of viscous and elastic moduli. The viscoelastic parameters calculated by the proposed method are validated by comparing the results from a conventional mechanical rheometer.

Surface wave elastography using high speed full-field optical interferometry

The assessment of mechanical stiffness is an essential diagnostic tool for investigating the biomechanical properties of biological tissues. Surface wave elastography (SWE) is an emerging technique to quantify elastic properties of tissues in clinical diagnosis. High-speed optical imaging combined with SWE has enormous potential in quantifying the elastic properties of tissues at microscale resolutions. In this study, we implement surface wave elastography using high-speed optical interferometry to characterize the elastic properties of tissue-mimicking phantoms andex-vivonative caprine liver tissue by imaging the surface wave induced by an electromechanical actuator. The sinusoidal mechanical excitations ranging from 120 Hz to 1.2 kHz on the surface of tissues are captured using a high-speed camera with a frame rate of 4 kHz at micrometer resolutions. The surface wavefront reconstruction is performed using a phase-shifting algorithm and linear regression is used to calculate the surface wave velocity. The mechanical stiffness estimated from the optical system is compared with the results of mechanical compression testing measurements. The results from this multimodal platform combining optical interferometry and vibrational spectroscopy using SWE are highly promising towards a non-invasive or minimally invasive imaging forin-vivoandex-vivomechanical characterization of tissues with future clinical applications.

Full-field passive elastography using digital holography

Off-axis digital holography is an imaging technique that allows direct measurement of phase and amplitude from one image. We utilize this technique to capture displacements induced by a diffuse shear wave field with high sensitivity. A noise-correlation-based algorithm is then used to measure mechanical properties of samples. This approach enables full-field quantitative passive elastography without the need of contact or a synchronized source of a mechanical wave. This passive elastography method is first validated on agarose test samples mimicking biological tissues, and first results on an ex vivo biological sample are presented.

Mechanical test study in composites using digital holographic interferometry and optical coherence tomography simultaneously

A dual optical configuration to inspect the internal and external mechanical response of a composite specimen is presented. The inspection simultaneously uses two equally aligned optical techniques, digital holographic interferometry and Fourier domain optical coherence tomography, to retrieve surface and internal data, respectively. The sample under study is a composite specimen of poly-methyl-methacrylate reinforced with metallic particles. Two different sets of samples are analyzed to compare their mechanical behavior. A homemade, fully controlled testing machine is used to apply a controlled compression load while each technique registers an image. In this form, the surface and internal optical phase measurements are correlated to the same compression value for comparison purposes. Results for each technique are directly presented as simultaneous displacement maps, and a discussion and conclusion of this proposed dual method of inspection are presented.

Optical elastography: tracking surface waves with digital image correlation

Elastography consists in evaluating the propagation speed of waves into a tissue to estimate its stiffness. Usually this method is based on Ultrasounds, magnetic resonance imaging or optical coherent tomography. This paper proposes a simple optic method using ultrafast cameras. Based on digital image correlation (DIC), the tracking of elastic surface wave from white light intensity pattern, allows estimating the propagation speed as an indicator of the tissue local stiffness. Two configurations are presented: (1) 2D imaging of a flat phantom surface with a single camera and (2) 3D imaging of a curved phantom surface with two cameras. As a feasibility study of the first configuration, surface wave speed was measured on isotropic and anisotropic phantoms. Comparisons with ultrasound methods fully validate this approach. Although more sophisticated, the second configuration account for propagation distortions caused by locally curved topology. Triangulation techniques used to retrieve local topology are named stereo-correlation in the field of biomechanics. Stereo-elastography is thus proposed to determine tissue local elasticity from any soft tissue surface wave.

Soft tissue elastography via shearing interferometry

Early detection of cancer can significantly increase the survival chances of patients. Palpation is a traditional method in order to detect cancer; however, in minimally invasive surgery the surgeon is deprived of the sense of touch. We demonstrate how shearing elastography can recover elastic parameters and furthermore can be used to localize stiffness imhomogenities even if hidden underneath the surface. Furthermore, the influence of size and depth of the stiffness imhomogenities on the detection accuracy and localization is investigated.

Comparative study of shear wave-based elastography techniques in optical coherence tomography

We compare five optical coherence elastography techniques able to estimate the shear speed of waves generated by one and two sources of excitation. The first two techniques make use of one piezoelectric actuator in order to produce a continuous shear wave propagation or a tone-burst propagation (TBP) of 400 Hz over a gelatin tissue-mimicking phantom. The remaining techniques utilize a second actuator located on the opposite side of the region of interest in order to create three types of interference patterns: crawling waves, swept crawling waves, and standing waves, depending on the selection of the frequency difference between the two actuators. We evaluated accuracy, contrast to noise ratio, resolution, and acquisition time for each technique during experiments. Numerical simulations were also performed in order to support the experimental findings. Results suggest that in the presence of strong internal reflections, single source methods are more accurate and less variable when compared to the two-actuator methods. In particular, TBP reports the best performance with an accuracy error < 4.1 % . Finally, the TBP was tested in a fresh chicken tibialis anterior muscle with a localized thermally ablated lesion in order to evaluate its performance in biological tissue.

Assessing the effects of riboflavin/UV-A crosslinking on porcine corneal mechanical anisotropy with optical coherence elastography

In this work we utilize optical coherence elastography (OCE) to assess the effects of UV-A/riboflavin corneal collagen crosslinking (CXL) on the mechanical anisotropy of in situ porcine corneas at various intraocular pressures (IOP). There was a distinct meridian of increased Young's modulus in all samples, and the mechanical anisotropy increased as a function of IOP and also after CXL. The presented noncontact OCE technique was able to quantify the Young's modulus and elastic anisotropy of the cornea and their changes as a function of IOP and CXL, opening new avenues of research for evaluating the effects of CXL on corneal biomechanical properties.

Non-contact single shot elastography using line field low coherence holography

Optical elastic wave imaging is a powerful technique that can quantify local biomechanical properties of tissues. However, typically long acquisition times make this technique unfeasible for clinical use. Here, we demonstrate non-contact single shot elastographic holography using a line-field interferometer integrated with an air-pulse delivery system. The propagation of the air-pulse induced elastic wave was imaged in real time, and required a single excitation for a line-scan measurement. Results on tissue-mimicking phantoms and chicken breast muscle demonstrated the feasibility of this technique for accurate assessment of tissue biomechanical properties with an acquisition time of a few milliseconds using parallel acquisition.

Water jet indentation for local elasticity measurements of soft materials

We present a novel elastography method for soft materials (100Pa-100kPa) based on indentation by a μm-sized water jet. We show that the jet creates a localized deformation ("cavity") of the material that can be easily visualized. We study experimentally how cavity width and depth depend on jet speed, height, incidence angle and sample elasticity. We describe how to calibrate the indenter using gels of known stiffness. We then demonstrate that the indenter yields quantitative elasticity values within 10% of those measured by shear rheometry. We corroborate our experimental findings with fluid-solid finite-element simulations that quantitatively predict the cavity profile and fluid flow lines. The water jet indenter permits in situ local stiffness measurements of 2D or 3D gels used for cell culture in physiological buffer, is able to assess stiffness heterogeneities with a lateral resolution in the range 50-500μm (at the tissue scale) and can be assembled at low cost with standard material from a biology laboratory. We therefore believe it will become a valuable method to measure the stiffness of a wide range of soft, synthetic or biological materials.

Multi-point vibrometer based on high-speed digital in-line holography

This paper describes a digital holographic setup based on in-line holography and a high-speed recording to get a multipoint vibrometer. The use of a high-speed sensor leads to specificities that enable the in-line configuration to be used. The case of transient vibrations is investigated through a full simulation of the holographic process. The simulation shows that the first instants are critical since distortion may occur, resulting in errors in the phase measurement. Experimental results are provided by exciting an aluminum beam with a transient signal. A comparison with the velocity measured by a pointwise vibrometer is provided. Frequency response functions are extracted and the experimental results confirm the ability of the method to provide full-field contactless measurements at the high-speed time scale evolution of the vibration.

Evaluation of surface acoustic waves on the human skin using quasi-time-averaged digital Fresnel holograms

This paper proposes a first attempt to visualize and analyze the vibrations induced by a bone-conduction device and propagating at the surface of the skin of a human face. The method is based on a new approach in a so-called quasi-time-averaging regime, resulting in the retrieval of the vibration amplitude and phase from a sequence of digital Fresnel holograms recorded with a high image rate. The design of the algorithm depends on the ratio between the exposure time and the vibration period. The results show the propagation of vibrations at the skin surface, and quantitative analysis is achieved by the proposed approach.

Elastography of soft materials and tissues by holographic imaging of surface acoustic waves

We use optical interferometry to capture coherent surface acoustic waves for elastographic imaging. An inverse method is employed to convert multi-frequency data into an elastic depth profile. Using this method, we image elastic properties over a 55 mm range with <5 mm resolution. For relevance to breast cancer detection, we employ a tissue phantom with a tumor-like inclusion. Holographic elastography is also shown to be well-behaved in ex vivo tissue, revealing the subsurface position of a bone. Because digital holography can assess waves over a wide surface area, this constitutes a flexible new platform for large volume and non-invasive elastography.