Kijanka, Piotr

Patients with chronic kidney disease (CKD) and renal transplant rejection are at high risk to develop advanced pathology including glomerular sclerosis and renal interstitial fibrosis. The elasticity of kidney tissues is associated with the disease progression and can be evaluated with ultrasound elastography. Understanding how tissue pathology changes the renal mechanical properties is crucial. Optical coherence elastography (OCE) might be a promising tool for evaluating renal tissues due to higher resolution. A phantom with 8% gelatin and 2% cellulose (50 mm) was fabricated to validate the ability of OCE to bulk materials and the results were confirmed by comparing the dataset obtained with an ultrasound-based Verasonics system as the ground truth. Excitation signals, composed with 4 cycles of a 400 Hz harmonic wave repeated at a rate of 38 Hz, were directly applied to the surface area with two directions and cortical regions of ex vivo porcine kidneys that had been cut open. Preliminary results showed that the velocity of surface wave on the cortical regions was near 1 m/s and corresponded with the result of shear waves from Verasonics system (1.2 m/s). Our study demonstrated that OCE has an ability to assess the mechanical properties of bulk materials and is a promising tool to evaluate mechanical properties of renal tissue with high resolution and sensitivity.

Local Phase Velocity Imaging (LPVI) is a novel method of shear wave elastography (SWE) analysis, which provides phase velocity images at multiple frequencies. The primary components of this method include using two acoustic radiation force (ARF) pushes near the edges of the field-of-view. Fourier analysis of the spatiotemporal data is conducted to obtain maps of the phase velocity at multiple frequencies. A robustness study was conducted to evaluate the effectiveness of LPVI in elastic and viscoelastic phantoms with different mechanical properties, ARF focal depths and F-numbers, and shear wave frequencies using a linear array transducer. The results showed effective phase velocity reconstructions for most scanning parameters in the phantoms evaluated. We evaluated mean and standard deviations of phase velocities in the different phantoms for regions-of-interest defined in the images. Issues were seen with shear wave signal being distorted from the incident push when using low F-numbers as well as issues with attenuating shear waves with higher F-numbers and deeper focal depths. Future directions of this work will be to evaluate the robustness of LPVI in inclusion phantoms as well as testing on a curved array transducer.

Shear wave elastography (SWE) is a method used in several clinical applications for assessment of soft tissue viscoelastic properties. SWE uses an acoustic radiation force (ARF) to produce laterally propagating shear waves to obtain the wave velocity. Here, we present a two-point shear wave attenuation coefficient measurement method without using a rheological model. The technique uses information related to the magnitude spectrum frequency shift of shear waves measured at only two lateral locations. We examined how the distance from the start of the ARF push and the distance between the two locations affected the attenuation coefficient estimates. We tested this method on digital phantom data created using local interaction simulation approach in viscoelastic media and on data acquired from phantom and ex vivo liver experiments. We compared the results from the two-point method with other two techniques used for assessing shear wave attenuation: the frequency-shift-based (FS) method and attenuation measuring ultrasound shearwave elastography. In comparison with the FS method, our technique does not assume that the shape parameter of the Gamma function is constant over a lateral distance. Tests conducted showed that the proposed method is feasible to provide robust attenuation estimates based on two measurement points in tissue-mimicking materials and ex vivo liver tissue.

Shear wave elastography is a method for quantitatively measuring the mechanical properties of soft tissues. Shear wave attenuation is high in soft tissues and increases with frequency. In a recent work, high resolution and accuracy can be reached at higher temporal frequencies. To combat the high shear wave attenuation and to obtain higher accuracy and lower variability, we propose to combine acoustic radiation force excitations that have multiple push beams and high temporal frequencies. Multiple push beams were generated with multiple focused beams and multiple intersecting unfocused beams. Measurements made with tonebursts of ultrasound with length 200 μs repeated at 500 and 1000 Hz were compared against a single toneburst. Measurements were made in commercial homogeneous and heterogeneous phantoms. Wave velocity images were reconstructed from the acquired data. The accuracy and variance were evaluated in the homogeneous phantoms. The contrast and accuracy were evaluated in the heterogeneous phantoms. In both types of phantoms, the magnitude at selected frequencies will be compared among the different excitations. The frequency bandwidth for each excitation will be compared. This work highlights that the combination of multiple push beams and excitations with high temporal frequencies can be combined for improved imaging in shear wave elastography.

Ultrasound shear wave elastography (SWE) is a promising imaging modality used for noninvasive, quantitative evaluation of tissue mechanical properties. This method uses an acoustic radiation force to produce laterally propagating shear waves that can be tracked to obtain the wave velocity. One of the ways to explore the viscoelasticity is through examining the shear wave velocity dispersion curves. In this paper, we present an alternative method to the classical two-dimensional Fourier transform (2D-FT), a two-point wavelet transformation method. We use the Morlet wavelet function as the mother wavelet to filter two shear waves at different locations. We examined how starting distance from the push and distance between the two locations affected the shear wave velocity dispersion. We tested this method on digital phantom data created using local interaction simulation approach (LISA) in viscoelastic media and on data acquired from phantom experiments. We compared results from the two-point method with the 2D-FT technique. The two-point method provided dispersion curves estimation with lower errors over a wider frequency band. Tests conducted showed that the two-point technique gives results with better accuracy in simulation results and can be used to measure phase velocity of viscoelastic materials.

Shear wave elastography (SWE) is clinically used for the measurement of soft tissue mechanical properties. Most SWE methods assume that the tissue is elastic, but soft tissues are inherently viscoelastic. The viscoelasticity can be characterized by examining phase velocity dispersion. Methods to extract the phase velocities from the spatiotemporal data, v(x,t), involve using a two-dimensional Fourier transform. The Fourier representation, V(k,f), is searched for peaks that correspond to the phase velocities. We present a method that uses the Multiple Signal Classification (MUSIC) method to provide robust estimation of the phase velocity dispersion curves. We compared results from the MUSIC method with the current approach of searching for peaks in the V(k,f) representation. We tested this method on digital phantom data created using finite element models (FEMs) in viscoelastic media excited by a simulated acoustic radiation force push from a curved linear array. We evaluated the algorithm with different levels of added noise. Additionally, we tested the methods on data acquired in viscoelastic phantoms with a Verasonics system. The MUSIC algorithm provided dispersion estimation with lower errors than the conventional peak search strategy. This method can be used for evaluation of wave velocity dispersion in viscoelastic tissues and guided wave propagation. [This work was supported in part by grant R01DK092255.]