Liu, Hsiao‐Chuan

Abstract Objective. Measuring waves induced with acoustic radiation force (ARF) in arteries has been studied over the last decade. To date, it remains a challenge to quantitatively assess the local arterial biomechanical properties. The cylindrical shape and waveguide behavior of waves propagating in the arterial wall pose complexities to determining the mechanical properties of the artery. Approach . In this paper, an artery-mimicking tube in water is examined utilizing three-dimensional measurements. The cross-section of the tube is measured while a transducer is translated over 41 different positions along the length of the tube. Motion in the radial direction is calculated using two components of motion which are measured from the two orthogonal views of the cross-section. This enables more accurate estimation of motion along the circumference of tube. Main results . The results provide more information to categorize the motion in tube wall into two types of responses: a transient response and a steady state response. The transient response is caused by ARF application and the waves travel along the length of the tube for a relatively short period of time. This corresponds to the axial and circumferential propagating waves. The two circumferential waves travel along the circumference of tube in CW (clockwise) and CCW (counter-clockwise) direction and result in a standing wave. By using a directional filter, the two waves were successfully separated, and their propagation was more clearly visualized. As a steady state response, a circumferential mode is generated showing a symmetric motion (i.e. the proximal and distal walls move in the opposite direction) following the transient response. Significance. This study provides a more comprehensive understanding of the waves produced in an artery-mimicking tube with ARF application, which will provide opportunities for improving measurement of arterial mechanical properties.

Continuous imaging of internal organs over days could provide crucial information about health and diseases and enable insights into developmental biology. We report a bioadhesive ultrasound (BAUS) device that consists of a thin and rigid ultrasound probe robustly adhered to the skin via a couplant made of a soft, tough, antidehydrating, and bioadhesive hydrogel-elastomer hybrid. The BAUS device provides 48 hours of continuous imaging of diverse internal organs, including blood vessels, muscle, heart, gastrointestinal tract, diaphragm, and lung. The BAUS device could enable diagnostic and monitoring tools for various diseases.

The structure and function of soft tissues are intimately connected. Some soft tissues are organized in such a way that they have material properties that are anisotropic, that is, the measured properties depend on which direction they are measured. Some such tissues are the brain, kidney, arteries, and cardiac and skeletal muscle. We will provide a brief overview of work targeted at characterization of the anisotropic mechanical properties in kidney and cardiac and skeletal muscles. In many cases, these tissues can be assumed to be transversely isotropic, which allows for simplification in estimating their mechanical properties. The use of propagating shear waves can be used to determine these mechanical properties. As a comparison example, we will show data from characterization of renal tissue with ultrasound shear wave elastography (SWE) and optical coherence elastography. We will also provide an overview of evaluation of skeletal muscle mechanical properties using shear wave elastography techniques and use of model fitting to estimate mechanical properties. Special care must be taken in evaluation of such anisotropic tissues to ensure that measurements are standardized across practices and studies.

Ischemic stroke results from thrombus becoming lodged in a vessel in the cerebrovascular circulation. The constituents of the thrombus can vary widely and have direct effects on the mechanical properties of the thrombus. Thrombectomy procedures could potentially be improved if the mechanical properties of the thrombus could be determined prior to treatment. In the study, we developed a new non-contact method associated with Rayleigh wave propagations for evaluating mechanical properties of human clotted blood, referred to as ultrasound-mediated optical coherence elastography (UOCE). Due to clot mechanical properties being associated with fibrin and platelet content, eight different clot analogues with varying fibrin/platelets concentrations, from 0.6% (control group) to 19.0%, were produced, and the clot analog in each concentration was repeatedly measured five times. A highly focused transducer with a center frequency of 7.5 MHz was used to generate acoustic radiation forces exerted on the surface of clotted blood to induce Rayleigh wave propagations. The transducer was driven by a 50 dB radio frequency amplifier a 4 ms pulse which was repeated at a rate of 19 Hz. The optical coherence tomography (OCT) acquisition was set to 50 lateral positions and 4000 A-scans for each lateral position at a 76 kHz scan rate for total acquisition time of 52.6 ms. The results demonstrated that the Rayleigh wave group velocities positively correlated with the concentrations of fibrin/platelets. The group velocities ranged from 0.78 ± 0.02 m/s for 0.6% fibrin/platelets and increased to 2.42 ± 0.06 m/s at a fibrin/platelets concentration of 19.0%. There are statistically significant difference ( p <0.05) between control group and all clot analogues with a fibrin/platelets concentration of greater than 2.3%. In conclusion, we propose UOCE as a novel approach capable of evaluating mechanical properties of human blood clots based on Rayleigh wave propagation. This sensitive method can be used for the evaluation of the mechanical properties of human blood clot analogues and thrombectomy samples.

We report a technique, named optical coherence viscometry (OCV), to measure the viscosity of Newtonian fluids in a noncontact manner. According to linear wave theory with small amplitudes, capillary waves are associated with fluid mechanical properties. To perform this measurement and avoid the overdamped effects of capillary waves in viscous fluids, transient acoustic radiation force was applied to generate capillary waves. Within a very limited field-of-view using optical coherence tomography, wave motion acquired in the time domain was analyzed using Fourier methods to study the wave velocity dispersion and attenuation relationships for capillary waves, which can reduce the fluid quantity drastically into tissue culture scale. We measure the viscosities of water, water–glycerol solutions with three concentrations, and biological plasma using the proposed OCV and compare the experimental results to theoretical calculations. OCV is sensitive to wave perturbations and can be a promising technique for measuring the viscosity of biological fluids and could be applied in future applications for measurements for lipid membranes in cell biology and tissue engineering investigation.

Mechanical properties in tissues are an important indicator because they are associated with disease states. One of the well-known excitation sources in optical coherence elastography (OCE) to determine mechanical properties is acoustic radiation force (ARF); however, a complicated focusing alignment cannot be avoided. Another excitation source is a piezoelectric (PZT) stack to obtain strain images via compression, which can affect the intrinsic mechanical properties of tissues in tissue engineering. In this study, we report a new technique called two-dimensional (2D) dynamic vibration OCE (DV-OCE) to evaluate 2D wave velocities without tedious focusing alignment procedures and is a non-contact method with respect to the samples. The three-dimensional (3D) Fourier transform was utilized to transfer the traveling waves (x, y, t) into 3D k-space (kx, ky, f). A spatial 2D wavenumber filter and multi-angle directional filter were employed to decompose the waves with omni-directional components into four individual traveling directions. The 2D local wave velocity algorithm was used to calculate a 2D wave velocity map. Six materials, two homogeneous phantoms with 10 mm thickness, two homogeneous phantoms with 2 mm thickness, one heterogeneous phantom with 2 mm diameter inclusion and an ex vivo porcine kidney, were examined in this study. In addition, the ARF-OCE was used to evaluate wave velocities for comparison. Numerical simulations were performed to validate the proposed 2D dynamic vibration OCE technique. We demonstrate that the experimental results were in a good agreement with the results from ARF-OCE (transient OCE) and numerical simulations. Our proposed 2D dynamic vibration OCE could potentially pave the way for mechanical evaluation in tissue engineering and for laboratory translation with easy-to-setup and contactless advantages.

AbstractEmbolectomy is one of the emergency procedures performed to remove emboli. Assessing the composition of human blood clots is an important diagnostic factor and could provide guidance for an appropriate treatment strategy for interventional physicians. Immunostaining has been used to identity compositions of clots as a gold‐standard procedure, but it is time‐consuming and cannot be performed in situ. Here, we proposed that the optical attenuation coefficient of optical coherence tomography (OCT) can be a reliable indicator as a new imaging modality to differentiate clot compositions. Fifteen human blood clots with multiple red blood cell (RBC) compositions from 21% to 95% were prepared using healthy human whole blood. A homogeneous gelatin phantom experiment and numerical simulation based on the Lambert–Beer's law were examined to verify the validity of the attenuation coefficient estimation. The results displayed that optical attenuation coefficients were strongly correlated with RBC compositions. We reported that attenuation coefficients could be a promising biomarker to guide the choice of an appropriate interventional device in a clinical setting and assist in characterizing blood clots.