Acoustic Radiation Force Excitation Research Articles

Anisotropic elasticity measurements of the retina using optical coherence elastography

Anisotropic elasticity measurements of the retina are essential for retinal disease diagnosis and function assessment. Optical coherence elastography (OCE) is a high-resolution imaging technique for mapping the elasticity distribution of tissues. However, previous OCE measurements quantified the tissue elasticity in a single direction, resulting in a biased estimation of the elastic properties. In this study, we propose an OCE method with acoustic radiation force (ARF) excitation to map the retinal anisotropic elasticity in the depth and lateral directions. The axial elasticity was analyzed using the natural frequency of free vibration, and the lateral elasticity was quantified using the elastic wave velocity. After evaluating the feasibility of the OCE method on the phantoms, the anisotropic elasticity of ex vivo porcine retinas was mapped. The results show that the OCE method with ARF excitation can assess the elasticity in orthogonal directions and provide a comprehensive understanding of the elasticity of the anisotropic tissues.

Optical coherence elastography under homolateral parallel acoustic radiation force excitation for ocular elasticity quantification.

Alteration in the elastic properties of biological tissues may indicate changes in the structure and components. Acoustic radiation force optical coherence elastography (ARF-OCE) can assess the elastic properties of the ocular tissues non-invasively. However, coupling the ultrasound beam and the optical beam remains challenging. In this Letter, we proposed an OCE method incorporating homolateral parallel ARF excitation for measuring the elasticity of the ocular tissues. An acoustic-optic coupling unit was established to reflect the ultrasound beam while transmitting the light beam. The ARF excited the ocular tissue in the direction parallel to the light beam from the same side of the light beam. We demonstrated the method on the agar phantoms, the porcine cornea, and the porcine retina. The results show that the ARF-OCE method can measure the elasticity of the cornea and the retina, resulting in higher detection sensitivity and a more extensive scanning range.

Feasibility of Phase Velocity Imaging Using Multi Frequency Oscillation-Shear Wave Elastography.

To assess viscoelasticity, a pathologically relevant biomarker, shear wave elastography (SWE) generally uses phase velocity (PV) dispersion relationship generated via pulsed acoustic radiation force (ARF) excitation pulse. In this study, a multi-frequency oscillation (MFO)- excitation pulse with higher weight to higher frequencies is proposed to generate PV images via the generation of motion with energy concentrated at the target frequencies in contrast to the broadband frequency motion generated in pulsed SWE (PSWE). The feasibility of MFO-SWE to generate PV images at 100 to 1000 Hz in steps of 100 Hz was investigated by imaging 6 and 70 kPa inclusions with 6.5 and 10.4 mm diameter and ex vivo bovine liver with and without the presence of an aberration layer and chicken muscle ex vivo, and 4T1 mouse breast tumor, in vivo with comparisons to PSWE. MFO-SWE-derived CNR was statistically higher than PSWE for 6 kPa (both with and without aberration) and 70 kPa (with aberration) inclusions and derived SNR of the liver was statistically higher than PSWE at higher frequency (600-1000 Hz). Quantitatively, at 600-1000 Hz, MFO-SWE improved CNR of inclusions (without and with) aberration on an average by (8.2 and 156)% and of the tumor by 122%, respectively, and improved SNR of the liver (without and with) aberration by (20.2 and 51.5)% and of chicken muscle by 72%, respectively compared to the PSWE. These results indicate the advantages of MFO-SWE to improve PV estimation at higher frequencies which could improve viscoelasticity quantification and feature delineation.

Real-Time Nondestructive Viscosity Measurement of Soft Tissue Based on Viscoelastic Response Optical Coherence Elastography.

Viscoelasticity of the soft tissue is an important mechanical factor for disease diagnosis, biomaterials testing and fabrication. Here, we present a real-time and high-resolution viscoelastic response-optical coherence elastography (VisR-OCE) method based on acoustic radiation force (ARF) excitation and optical coherence tomography (OCT) imaging. The relationship between displacements induced by two sequential ARF loading-unloading and the relaxation time constant of the soft tissue-is established for the Kelvin-Voigt material. Through numerical simulation, the optimal experimental parameters are determined, and the influences of material parameters are evaluated. Virtual experimental results show that there is less than 4% fluctuation in the relaxation time constant values obtained when various Young's modulus and Poisson's ratios were given for simulation. The accuracy of the VisR-OCE method was validated by comparing with the tensile test. The relaxation time constant of phantoms measured by VisR-OCE differs from the tensile test result by about 3%. The proposed VisR-OCE method may provide an effective tool for quick and nondestructive viscosity testing of biological tissues.

The lens capsule significantly affects the viscoelastic properties of the lens as quantified by optical coherence elastography.

The crystalline lens is a transparent, biconvex structure that has its curvature and refractive power modulated to focus light onto the retina. This intrinsic morphological adjustment of the lens to fulfill changing visual demands is achieved by the coordinated interaction between the lens and its suspension system, which includes the lens capsule. Thus, characterizing the influence of the lens capsule on the whole lens's biomechanical properties is important for understanding the physiological process of accommodation and early diagnosis and treatment of lenticular diseases. In this study, we assessed the viscoelastic properties of the lens using phase-sensitive optical coherence elastography (PhS-OCE) coupled with acoustic radiation force (ARF) excitation. The elastic wave propagation induced by ARF excitation, which was focused on the surface of the lens, was tracked with phase-sensitive optical coherence tomography. Experiments were conducted on eight freshly excised porcine lenses before and after the capsular bag was dissected away. Results showed that the group velocity of the surface elastic wave, , in the lens with the capsule intact ( ) was significantly higher (p < 0.001) than after the capsule was removed ( ). Similarly, the viscoelastic assessment using a model that utilizes the dispersion of a surface wave showed that both Young's modulus, E, and shear viscosity coefficient, η, of the encapsulated lens ( ) were significantly higher than that of the decapsulated lens ( ). These findings, together with the geometrical change upon removal of the capsule, indicate that the capsule plays a critical role in determining the viscoelastic properties of the crystalline lens.

Nonlinear least-squares estimation of shear modulus and shear viscosity in viscoelastic media

Time-domain estimates of the shear wave speed are obtained by performing cross-correlations or by evaluating the time-to-peak for two laterally separated shear wave particle velocity waveforms, where the distance divided by the propagation time yields an approximate value for the shear wave speed. Challenges associated with these time-domain estimation approaches include increasing errors as the shear viscosity increases and the absence of an estimated value for the shear viscosity parameter which is responsible for the rapid attenuation of shear waves in soft tissue. To obtain a time-domain estimate of the shear viscosity that also provides an estimate for the shear wave speed, a nonlinear least squares estimation approach is applied to three-dimensional (3D) shear wave particle velocities computed for a shear elasticity of 1.5 kPa, shear viscosities of 1, 2, 3, and 4 Pa.s, and a realistic simulated 3D acoustic radiation force excitation. The results show cross-correlations tend to over-estimate the value of the shear wave speed, the nonlinear least squares approach tends to under-estimate the value of the shear wave speed, and the nonlinear least square approach produces more accurate estimates as the shear viscosity increases. Two-dimensional maps of each estimated parameter are also shown.

Full wave simulation of arterial response under acoustic radiation force

With the ultimate goal of estimating arterial viscoelasticity using shear wave elastography, this paper presents a practical methodology to simulate the response of a human carotid artery under acoustic radiation force (ARF). The artery is idealized as a nearly incompressible viscoelastic hollow cylinder submerged in incompressible, inviscid fluid. For this idealization, we develop a multi-step methodology for efficient computation of three-dimensional response under complex ARF excitation, while capturing the fluid-structure interaction between the arterial wall and the surrounding fluid. The specific steps include (a) performing dimensional reduction through semi-analytical finite element formulation, (b) efficient finite element discretization using traditional and recent techniques. The computational efficiency is further enhanced by utilizing (c) modal superposition, followed by, where appropriate, (d) impulse response function. In addition to developing the methodology, convergence analysis is performed for a typical arterial geometry, leading to recommendations on various discretization parameters. At the end, the computational effort is shown to be several orders of magnitude less than the traditional, fully three-dimensional analysis using finite element methods, leading to a practical yet accurate simulation of arterial response under ARF excitations.

Quantifying the efficiency of the acoustic radiation force from two-wave beating for structural resonance excitation of a thin spherical shell

The structural echoes obtained from low-frequency (∼kHz) excitation of resonant underwater targets can serve as a basis for automated target recognition. Acoustic radiation force (ARF) can provide a means to excite those structural echoes with compact transducing mechanisms as it relies on high-frequency transducers to generate locally low-frequency excitation via the classical beating effect. However, a key question is quantifying the efficiency of ARF excitation mechanisms to excite those structural echoes when compared to using direct low-frequency pulses. Here, we develop a time-domain finite element framework that compares the ARF and direct low-frequency excitation methods to excite classical resonances scenarios such as thickness resonances of a thin plate or the so-called “mid-frequency enhancement echo” for elastic spherical shells. These canonical target objects provide a straightforward procedure to investigate the ARF excitation efficiency and establishes the numerical framework to investigate more complex targets with a wider range of environmental parameters.

Age-related changes in the viscoelasticity of rabbit lens characterised by surface wave dispersion analysis

The viscoelastic properties of the young and mature rabbit lenses in situ are evaluated using wave-based optical coherence elastography (OCE). Surface waves in the crystalline lens are generated using acoustic radiation force (ARF) focused inside the eyeball. Surface-wave dispersion is measured with a phase-stabilised optical coherence tomography (OCT) system. The Young’s modulus and shear viscosity coefficient are quantified based on a Scholte wave model. The results show that both elasticity and viscosity are significantly different between the young and mature lenses. The Young’s modulus of the lenses increased with age from 7.74 ± 1.56 kPa (young) to 15.15 ± 4.52 kPa (mature), and the shear viscosity coefficient increased from 0.55 ± 0.04 Pa s (young) and 0.86 ± 0.13 Pa s (mature). It is shown that the combination of ARF excitation, OCE imaging, and dispersion analysis enables nondestructive quantification of lenticular viscoelasticity in situ and shows promise for in vivo applications.

Toward improved accuracy in shear wave elastography of arteries through controlling the arterial response to ultrasound perturbation in-silico and in phantoms

Dispersion-based inversion has been proposed as a viable direction for materials characterization of arteries, allowing clinicians to better study cardiovascular conditions using shear wave elastography. However, these methods rely on a priori knowledge of the vibrational modes dominating the propagating waves induced by acoustic radiation force excitation: differences between anticipated and real modal content are known to yield errors in the inversion. We seek to improve the accuracy of this process by modeling the artery as a fluid-immersed cylindrical waveguide and building an analytical framework to prescribe radiation force excitations that will selectively excite certain waveguide modes using ultrasound acoustic radiation force. We show that all even-numbered waveguide modes can be eliminated from the arterial response to perturbation, and confirm the efficacy of this approach with in silico tests that show that odd modes are preferentially excited. Finally, by analyzing data from phantom tests, we find a set of ultrasound focal parameters that demonstrate the viability of inducing the desired odd-mode response in experiments.

Viscoelastic response ultrasound for interrogating dystrophic muscle degeneration

Dystrophic muscles undergo repeating cycles of inflammation, necrosis, fibrosis, and fatty deposition, with associated changes in viscoelastic property. Thus, a noninvasive approach to monitoring muscle viscoelasticity is relevant to monitoring disease progression and responses to therapy. Viscoelastic Response (VisR) ultrasound is an acoustic radiation force (ARF) imaging method that executes consecutive, colocated ARF excitations and evaluates the induced on-axis tissue displacement to decipher elastic and viscous properties. Further, by rotating an asymmetrically shaped ARF excitation beam to evaluate elasticity and viscosity along and across the axis of symmetry in transversely isotropic materials, such as muscle, VisR interrogates viscoelastic anisotropy. Finally, machine learning models have been developed to enable quantitative VisR assessments of elastic and viscous moduli. VisR has been demonstrated in vivo for monitoring dystrophic muscle degeneration in boys with Duchenne muscular dystrophy (DMD). Relative to age-matched controls, boys with DMD had higher VisR-derived elasticity and viscosity in early disease stages, when inflammation is present, and higher viscosity in late disease stages after cycles of fatty deposition. Boys with DMD also had lower VisR-derived longitudinal than transverse elastic and viscous moduli, while the opposite was true for controls. VisR is a promising approach to monitoring dystrophic muscle degeneration in DMD.

Phase velocity imaging using acoustic radiation force-induced shear wave with a multi-frequency excitation pulse

The viscoelastic properties of tissues are often assessed by analyzing the phase velocity dispersion curve computed using a 2-D Fourier transform onthe single fixed duration acoustic radiation force excitation pulse (FD-ARFEP)-induced shear wave with broadband frequencies. This study proposes a multi-frequency ARFEP (MF-ARFEP) using a single imaging transducer to generate shear waves at target frequencies of 100:100:1000 Hz to reduce noise in the phase velocity images. The MF-ARFEP is generated by summing sinusoids with target frequencies and then sampled to collect tracking pulses in-between the sampled MF-ARFEP. The MF-ARFEP is validated by imaging 6 and 70 kPa inclusions in a 18 kPa background, exvivo canine liver, and in vivo 4T1 breast cancer mouse tumor with comparison to the FD-ARFEP. The median phase velocity was similar between MF-ARFEP versus FD-ARFEP. However, the CNR was on an average 1.3 times higher in MF-ARFEP versus FD-ARFEP -derived images of inclusions and the coefficient of variation was on an average 1.1 and 2.2 times lower in MF-ARFEP versus FD-ARFEP-derived images of tumor and liver. These results demonstrate the feasibility of the MF-ARFEP excitation pulse to generate phase velocity images at 100:100:1000 Hz frequencies with reduced noise compared to the FD-ARFEP. Ongoing studies entail the translation of this new viscoelasticity imaging method for the characterization of breast cancer tumors in patients.

Analysis of multiple shear wave modes in a nonlinear soft solid: Experiments and finite element simulations with a tilted acoustic radiation force

Tissue nonlinearity is conventionally measured in shear wave elastography by studying the change in wave speed caused by the tissue deformation, generally known as the acoustoelastic effect. However, these measurements have mainly focused on the excitation and detection of one specific shear mode, while it is theoretically known that the analysis of multiple wave modes offers more information about tissue material properties that can potentially be used to refine disease diagnosis. This work demonstrated proof of concept using experiments and finite element simulations in a uniaxially stretched phantom by tilting the acoustic radiation force excitation axis with respect to the material's symmetry axis. Using this unique set-up, we were able to visualize two propagating shear wave modes across the stretch direction for stretches larger than 140%. Complementary simulations were performed using material parameters determined from mechanical testing, which enabled us to convert the observed shear wave behavior into a correct representative constitutive law for the phantom material, i.e. the Isihara model. This demonstrates the potential of measuring shear wave propagation in combination with shear wave modeling in complex materials as a non-invasive alternative for mechanical testing.

Viscoelastic Response Ultrasound Derived Relative Elasticity and Relative Viscosity Reflect True Elasticity and Viscosity: In Silico and Experimental Demonstration.

Viscoelastic response (VisR) ultrasound characterizes the viscoelastic properties of tissue by fitting acoustic radiation force (ARF)-induced displacements in the region of ARF excitation to a 1-D mass-spring-damper (MSD) model. Elasticity and viscosity are calculated separately but relative to the applied ARF amplitude. We refer to these parameters as "relative elasticity (RE)" and "relative viscosity (RV)." We herein test the hypothesis that RE and RV linearly correlate to true elasticity and viscosity in tissue. VisR imaging was simulated in 144 homogeneous viscoelastic materials with varying elasticities and viscosities. Derived RE linearly correlated with material elasticity and varied by an average of 2.52% when the material viscosity changed from 0.1 to 1.3 Pa · s. Derived RV linearly correlated with material viscosity but varied by an average of 102.5% when material elasticity changed from 3.33 to 20 kPa. The effect of elasticity on RV measurement was compensated using the slope of the linear relationship between RV and natural frequency ( ωtextn ). After compensation, RV [Formula: see text] (elasticity compensated RV) linearly correlated with material viscosity and varied by less than 1.00% on average when the modeled shear elastic modulus changed from 3.3 to 20 kPa. In addition to elasticity compensation, variation in ARF amplitude over depth was compensated, yielding REDC and [Formula: see text]. REDC and [Formula: see text] successfully contrasted elastic and viscous inclusions, respectively, in three simulated phantoms. Experimentally, in the homogeneous oil-in-gelatin phantoms and excised livers, REDC linearly correlated with shear wave dispersion ultrasound vibrometry (SDUV) derived shear elastic modulus, and [Formula: see text] linearly correlated with SDUV-derived shear viscosity. In excised livers containing viscoelastic oil-in-gelatin inclusions, the inclusions were successfully contrasted from the liver background by both REDC and [Formula: see text]. These results suggest that RE and RV are relevant for qualitatively assessing the elastic and viscous properties of tissue.

Spatial localization of mechanical excitation affects spatial resolution, contrast, and contrast-to-noise ratio in acoustic radiation force optical coherence elastography.

The notion that a spatially confined mechanical excitation would produce an elastogram with high spatial resolution has motivated the development of various elastography techniques with localized mechanical excitation. However, a quantitative investigation of the effects of spatial localization of mechanical excitation on the spatial resolution of elastograms is still lacking in optical coherence elastography (OCE). Here, we experimentally investigated the effect of spatial localization of acoustic radiation force (ARF) excitation on spatial resolution, contrast, and contrast-to-noise ratio (CNR) of dynamic uniaxial strain elastograms in dynamic ARF-OCE, based on a framework for analyzing the factors that influence the quality of the elastogram at different stages of the elastography workflow. Our results show that localized ARF excitation with a smaller acoustic focal spot size produced a strain elastogram with superior spatial resolution, contrast, and CNR. Our results also suggest that the spatial extent spanned by the displacement response in the sample may connect between the spatial localization of the mechanical excitation and the resulting elastogram quality. The elastography framework and experimental approach presented here may provide a basis for the quantitative analysis of elastogram quality in OCE that can be adapted and applied to different OCE systems and applications.

Simultaneously imaging and quantifying in vivo mechanical properties of crystalline lens and cornea using optical coherence elastography with acoustic radiation force excitation.

The crystalline lens and cornea comprise the eye’s optical system for focusing light in human vision. The changes in biomechanical properties of the lens and cornea are closely associated with common diseases, including presbyopia and cataract. Currently, most in vivo elasticity studies of the anterior eye focus on the measurement of the cornea, while lens measurement remains challenging. To better understand the anterior segment of the eye, we developed an optical coherence elastography system utilizing acoustic radiation force excitation to simultaneously assess the elasticities of the crystalline lens and the cornea in vivo. A swept light source was integrated into the system to provide an enhanced imaging range that covers both the lens and the cornea. Additionally, the oblique imaging approach combined with orthogonal excitation also improved the image quality. The system was tested through first ex vivo and then in vivo experiments using a rabbit model. The elasticities of corneal and lens tissue in an excised normal whole-globe and a cold cataract model were measured to reveal that cataractous lenses have a higher Young’s modulus. Simultaneous in vivo elasticity measurements of the lens and cornea were performed in a rabbit model to demonstrate the correlations between elasticity and intraocular pressure and between elasticity and age. To the best of our knowledge, we demonstrated the first in vivo elasticity of imaging of both the lens and cornea using acoustic radiation force-optical coherence elastography, thereby providing a potential powerful clinical tool to advance ophthalmic research in disorders affecting the lens and the cornea.

Constructive shear wave interference imaging to characterize skin sclerosis

Acoustic Radiation Force Impulse (ARFI) and Shear Wave Elasticity Imaging (SWEI) have been shown to have value in quantifying the stiffness of skin in sclerotic disease processes, such as morphea and Graph-Versus-Host-Disease (GVHD). Quantifying the stiffness of sclerotic skin can have clinical value in longitudinal monitoring of disease progress and the response of the disease to therapeutic measures. Skin, however, presents some unique challenges to characterizing shear elasticity. The fibrosis in sclerotic skin can create very stiff elastic moduli, resulting in shear wave speeds that can exceed 5-6 m/s, causing (1) the displacements generated in these tissues to be small (single microns) and (2) challenges in tracking the waves as they propagate given limitations in the pulse repetition frequency of the tracking beams on commercial systems. A novel approach to measuring shear elasticity in the skin has been developed to help overcome these challenges. While traditional SWEI methods typically involve the generation of a single acoustic radiation force excitation and track the resultant shear wave at a series of spatially-offset positions, Constructive Shearwave Interference (CSI) imaging generates acoustic radiation force in a ring, and the resultant shear waves propagate towards the center of that ring, where they constructively interfere. Using the known radius of the excitation ring, the shear wave speed can be estimated by the arrival time of the shear wave front at the center of the ring, where the displacement amplitude is greatly increased through the constructive interference process. The feasibility of the CSI technology has been demonstrated in silico, and a custom acoustic radiation force excitation ring transducer has been developed with a center tracking piston to validate the CSI technology in tissue-mimicking phantoms of layered media. A clinical study in patients with chronic GVHD is being conducted to demonstrate the clinical utility of this technology and compare it to traditional SWEI methods.

Shear wave elastography with combined high spatial and temporal frequency excitations

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.

In Vivo Viscoelastic Response (VisR) Ultrasound for Characterizing Mechanical Anisotropy in Lower-Limb Skeletal Muscles of Boys with and without Duchenne Muscular Dystrophy

Our group has previously found that in silico, mechanical anisotropy may be interrogated by exciting transversely isotropic materials with geometrically asymmetric acoustic radiation force excitations and then monitoring the associated induced displacements in the region of excitation. We now translate acoustic radiation force-based anisotropy assessment to human muscle in vivo and investigate its clinical relevance to monitoring muscle degeneration in Duchenne muscular dystrophy (DMD). Clinical anisotropy assessments were performed using Viscoelastic Response ultrasound, with a degree of anisotropy reflected by the ratios of Viscoelastic Response relative elasticity (RE) or relative viscosity (RV) measured with the asymmetric radiation force oriented parallel versus perpendicular to muscle fiber alignment. In vivo results from rectus femoris and gastrocnemius muscles of boys aged ∼7.9–10.4 y indicate that RE and RV anisotropy ratios in rectus femoris muscles of boys with DMD were significantly higher than those of healthy control boys (RE: DMD = 1.51 ± 0.87, control = 0.99 ± 0.69, p = 0.04, Wilcoxon rank sum test; RV: DMD = 1.04 ± 0.71, control = 0.74 ± 0.22, p = 0.02). In the gastrocnemius muscle, only the RV anisotropy ratio was significantly higher in dystrophic than control patients (DMD = 1.23 ± 0.35, control = 0.88 ± 0.31, p = 0.04). In the dystrophic rectus femoris muscle, the RE anisotropy ratio was inversely correlated (slope = –0.03/lbf, r = –0.43, p = 0.07, Pearson correlation) with quantitative muscle testing functional output measures but was not correlated with quantitative muscle testing in the dystrophic gastrocnemius. These results suggest that Viscoelastic Response RE and RV measures reflect differences in mechanical anisotropy associated with functional impairment with dystrophic degeneration that are relevant to monitoring DMD clinically.

Acoustic Radiation Force Based Ultrasound Elasticity Imaging for Biomedical Applications.

Pathological changes in biological tissue are related to the changes in mechanical properties of biological tissue. Conventional medical screening tools such as ultrasound, magnetic resonance imaging or computed tomography have failed to produce the elastic properties of biological tissues directly. Ultrasound elasticity imaging (UEI) has been proposed as a promising imaging tool to map the elastic parameters of soft tissues for the clinical diagnosis of various diseases include prostate, liver, breast, and thyroid gland. Existing UEI-based approaches can be classified into three groups: internal physiologic excitation, external excitation, and acoustic radiation force (ARF) excitation methods. Among these methods, ARF has become one of the most popular techniques for the clinical diagnosis and treatment of disease. This paper provides comprehensive information on the recently developed ARF-based UEI techniques and instruments for biomedical applications. The mechanical properties of soft tissue, ARF and displacement estimation methods, working principle and implementation instruments for each ARF-based UEI method are discussed.