A parallel tracking method for acoustic radiation force impulse imaging

Radiation force-based techniques have been developed by several groups for imaging the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one such method that uses commercially available scanners to generate localized radiation forces in tissue. The response of the tissue to the radiation force is determined using conventional B-mode imaging pulses to track micron-scale displacements in tissue. Current research in ARFI imaging is focused on producing real-time images of tissue displacements and related mechanical properties. Obstacles to producing a real-time ARFI imaging modality include data acquisition, processing power, data transfer rates, heating of the transducer, and patient safety concerns. We propose a parallel receive beamforming technique to reduce transducer heating and patient acoustic exposure, and to facilitate data acquisition for real-time ARFI imaging. Custom beam sequencing was used with a commercially available scanner to track tissue displacements with parallel-receive beamforming in tissue-mimicking phantoms. Using simulations, the effects of material properties on parallel tracking are observed. Transducer and tissue heating for parallel tracking are compared to standard ARFI beam sequencing. The effects of tracking beam position and size of the tracked region are also discussed in relation to the size and temporal response of the region of applied force, and the impact on ARFI image contrast and signal-to-noise ratio are quantified.     

Image quality, tissue heating, and frame rate trade-offs in acoustic radiation force impulse imaging

The real-time application of acoustic radiation force impulse (ARFI) imaging requires both short acquisition times for a single ARFI image and repeated acquisition of these frames. Due to the high energy of pulses required to generate appreciable radiation force, however, repeated acquisitions could result in substantial transducer face and tissue heating. We describe and evaluate several novel beam sequencing schemes which, along with parallel-receive acquisition, are designed to reduce acquisition time and heating. These techniques reduce the total number of radiation force impulses needed to generate an image and minimize the time between successive impulses. We present qualitative and quantitative analyses of the trade-offs in image quality resulting from the acquisition schemes. Results indicate that these techniques yield a significant improvement in frame rate with only moderate decreases in image quality. Tissue and transducer face heating resulting from these schemes is assessed through finite element method modeling and thermocouple measurements. Results indicate that heating issues can be mitigated by employing ARFI acquisition sequences that utilize the highest track-to-excitation ratio possible.     

Bubble-based acoustic radiation force elasticity imaging

Acoustic radiation force is applied to bubbles generated by laser-induced optical breakdown (LIOB) to study viscoelastic properties of the surrounding medium. In this investigation, femtosecond laser pulses are focused in the volume of gelatin phantoms of different concentrations to form bubbles. A two-element confocal ultrasonic transducer generates acoustic radiation force on individual bubbles while monitoring their displacement within a viscoelastic medium. Tone burst pushes of varying duration have been applied by the outer element at 1.5 MHz. The inner element receives pulse-echo recordings at 7.44 MHz before, during, and after the excitation bursts, and crosscorrelation processing is performed offline to monitor bubble position. Maximum bubble displacements are inversely related to the Young's moduli for different gel phantoms, with a maximum bubble displacement of over 200 microm in a gel phantom with a Young's modulus of 1.7 kPa. Bubble displacements scale with the applied acoustic radiation force and displacements can be normalized to correct for differences in bubble size. Exponential time constants for bubble displacement curves are independent of bubble radius and follow a decreasing trend with the Young's modulus of the surrounding medium. These results demonstrate the potential for bubble-based acoustic radiation force methods to measure tissue viscoelastic properties.     

Acoustic radiation force impulse imaging of myocardial radiofrequency ablation: initial in vivo results

Acoustic radiation force impulse (ARFI) imaging techniques were used to monitor radiofrequency (RF) ablation of ovine cardiac tissue in vivo. Additionally, ARFI M-mode imaging methods were used to interrogate both healthy and ablated regions of myocardial tissue. Although induced cardiac lesions were not visualized well in conventional B-mode images, ARFI images of ablation procedures allowed determination of lesion location, shape, and relative size through time. The ARFI M-mode images were capable of distinguishing differences in behavior through the cardiac cycle between healthy and damaged tissue regions. As conventional sonography is often used to guide ablation catheters, ARFI imaging, which requires no additional equipment, may be a convenient modality for monitoring lesion formation in vivo.     

The role of viscosity in the impulse diffraction field of elastic waves induced by the acoustic radiation force

Several ultrasound-based techniques for the estimation of soft tissue elasticity are currently being investigated. Most of them study the medium response to dynamic excitations. Such responses are usually modeled in a purely elastic medium using a Green's function solution of the motion equation. However, elasticity by itself is not necessarily a discriminant parameter for malignancy diagnosis. Modeling viscous properties of tissues could also be of great interest for tumor characterization. We report in this paper an explicit derivation of the Green's function in a viscous and elastic medium taking into account shear, bulk, and coupling waves. From this theoretical calculation, 3D simulations of mechanical waves in viscoelastic soft tissues are presented. The relevance of the viscoelastic Green's function is validated by comparing simulations with experimental data. The experiments were conducted using the supersonic shear imaging (SSI) technique which dynamically and remotely excites tissues using acoustic radiation force. We show that transient shear waves generated with SSI are modeled very precisely by the Green's function formalism. The combined influences of out-of-plane diffraction, beam shape, and shear viscosity on the shape of transient waves are carefully studied as they represent a major issue in ultrasound-based viscoelasticity imaging techniques.     

Lower-limb vascular imaging with acoustic radiation force elastography: Demonstration of in vivo feasibility

Acoustic radiation force impulse (ARFI) imaging characterizes the mechanical properties of tissue by measuring displacement resulting from applied ultrasonic radiation force. In this paper, we describe the current status of ARFI imaging for lower-limb vascular applications and present results from both tissue-mimicking phantoms and in vivo experiments. Initial experiments were performed on vascular phantoms constructed with polyvinyl alcohol for basic evaluation of the modality. Multilayer vessels and vessels with compliant occlusions of varying plaque load were evaluated with ARFI imaging techniques. Phantom layers and plaque are well resolved in the ARFI images, with higher contrast than B-mode, demonstrating the ability of ARFI imaging to identify regions of different mechanical properties. Healthy human subjects and those with diagnosed lower-limb peripheral arterial disease were imaged. Proximal and distal vascular walls are well visualized in ARFI images, with higher mean contrast than corresponding B-mode images. ARFI images reveal information not observed by conventional ultrasound and lend confidence to the feasibility of using ARFI imaging during lower-limb vascular workup.     

Liquid lens using acoustic radiation force

A liquid lens is proposed that uses acoustic radiation force with no mechanical moving parts. It consists of a cylindrical acrylic cell filled with two immiscible liquids (degassed water and silicone oil) and a concave ultrasound transducer. The focal point of the transducer is located on the oil-water interface, which functions as a lens. The acoustic radiation force is generated when there is a difference in the acoustic energy densities of different media. An acoustic standing wave was generated in the axial direction of the lens and the variation of the shape of the oil-water interface was observed by optical coherence tomography (OCT). The lens profile can be rapidly changed by varying the acoustic radiation force from the transducer. The kinematic viscosity of silicone oil was optimized to minimize the response times of the lens. Response times of 40 and 80 ms when switching ultrasonic radiation on and off were obtained with a kinematic viscosity of 200 cSt. The path of a laser beam transmitted through the lens was calculated by ray-tracing simulations based on the experimental results obtained by OCT. The transmitted laser beam could be focused by applying an input voltage. The liquid lens could be operated as a variable-focus lens by varying the input voltage.     

Direct measurement of acoustic impedance in liquids by a new pulse echo technique

For the first time a novel experimental technique has been used for direct evaluation of acoustic impedance of liquids. Till date researchers have been using ultrasonic velocity and density measurements to compute acoustic impedance and thermodynamic parameters, which are then used to investigate the nature of molecular interactions of solvent molecules in binary, ternary and quaternary liquid mixtures. Evaluation of acoustic impedance directly is a step in minimizing the cumbersome process of measuring velocities and densities separately of solutions having different mole fraction concentration. Acoustic impedances of ten pure polar and non polar solvents have been determined at different temperatures of 298K, 303K and 308K. This is further coupled with inline simultaneous determination of ultrasonic velocities of the solvents. In order to establish the new method, results have been discussed by comparing the calculated ultrasonic velocity from the experimental acoustic impedance, ultrasonic velocity determination by delay time of flight and the literature values of ultrasonic velocity of the pure solvents.     

A novel motion compensation algorithm for acoustic radiation force elastography

A novel method of physiological motion compensation for use with radiation force elasticity imaging has been developed. The method utilizes a priori information from finite element method models of the response of soft tissue to impulsive radiation force to isolate physiological motion artifacts from radiation force-induced displacement fields. The new algorithmis evaluated in a series of clinically realistic imaging scenarios, and its performance is compared to that achieved with previously described motion compensation algorithms. Though not without limitations, the new model-based motion compensation algorithm performs favorably in many circumstances and may be a logical choice for use with in vivo abdominal imaging.     

Advanced radiation measurement techniques in diagnostic radiology and molecular imaging

This paper reports some technological advances recently achieved in the fields of micro-CT and small animal PET instrumentation. It highlights a balance between image-quality improvement and dose reduction. Most of the recent accomplishments in these fields are due to the use of novel imaging sensors such as CMOS-based X-ray detectors and silicon photomultipliers (SiPM). Some of the research projects carried out at the University of Pisa for the development of such advanced radiation imaging technology are also described.     

Dynamic mechanical response of elastic spherical inclusions to impulsive acoustic radiation force excitation

Acoustic radiation force impulse imaging has been used clinically to study the dynamic response of lesions relative to their background material to focused, impulsive acoustic radiation force excitations through the generation of dynamic displacement field images. Dynamic displacement data are typically displayed as a set of parametric images, including displacement immediately after excitation, maximum displacement, time to peak displacement, and recovery time from peak displacement. To date, however, no definitive trends have been established between these parametric images and the tissues' mechanical properties. This work demonstrates that displacement magnitude, time to peak displacement, and recovery time are all inversely related to the Young's modulus in homogeneous elastic media. Experimentally, pulse repetition frequency during displacement tracking limits stiffness resolution using the time to peak displacement parameter. The excitation pulse duration also impacts the time to peak parameter, with longer pulses reducing the inertial effects present during impulsive excitations. Material density affects tissue dynamics, but is not expected to play a significant role in biological tissues. The presence of an elastic spherical inclusion in the imaged medium significantly alters the tissue dynamics in response to impulsive, focused acoustic radiation force excitations. Times to peak displacement for excitations within and outside an elastic inclusion are still indicative of local material stiffness; however, recovery times are altered due to the reflection and transmission of shear waves at the inclusion boundaries. These shear wave interactions cause stiffer inclusions to appear to be displaced longer than the more compliant background material. The magnitude of shear waves reflected at elastic lesion boundaries is dependent on the stiffness contrast between the inclusion and the background material, and the stiffness and size of the inclusion dictate when shear wave reflections within the lesion will interfere with one another. Jitter and bias associated with the ultrasonic displacement tracking also impact the estimation of a tissue's dynamic response to acoustic radiation force excitation.     

Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere

The acoustic radiation force of Langevin type resulting from the interaction of a high-order Bessel beam with a rigid immovable sphere in an ideal fluid is theoretically investigated. The analysis is based on applying the generalized Rayleigh series used in the near-field acoustic scattering problem to calculate the force. With appropriate selection of specific Bessel beam parameters, results for the rigid sphere unexpectedly reveal a negative radiation force caused by the Lagrangean energy density. Specifically, the negative force on the rigid sphere arises when the kinematic energy density is larger than the potential energy density. This condition provides an impetus for further designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.     

A finite-element method model of soft tissue response to impulsive acoustic radiation force

Several groups are studying acoustic radiation force and its ability to image the mechanical properties of tissue. Acoustic radiation force impulse (ARFI) imaging is one modality using standard diagnostic ultrasound scanners to generate localized, impulsive, acoustic radiation forces in tissue. The dynamic response of tissue is measured via conventional ultrasonic speckle-tracking methods and provides information about the mechanical properties of tissue. A finite-element method (FEM) model has been developed that simulates the dynamic response of tissues, with and without spherical inclusions, to an impulsive acoustic radiation force excitation from a linear array transducer. These FEM models were validated with calibrated phantoms. Shear wave speed, and therefore elasticity, dictates tissue relaxation following ARFI excitation, but Poisson's ratio and density do not significantly alter tissue relaxation rates. Increased acoustic attenuation in tissue increases the relative amount of tissue displacement in the near field compared with the focal depth, but relaxation rates are not altered. Applications of this model include improving image quality, and distilling material and structural information from tissue's dynamic response to ARFI excitation. Future work on these models includes incorporation of viscous material properties and modeling the ultrasonic tracking of displaced scatterers.     

Development and characterization of a vitreous mimicking material for radiation force imaging

In many medical ultrasound applications tissue-mimicking phantoms are of fundamental importance for the performance of controlled experiments. Traditionally, such phantoms have been constructed using gelatin and agar gels. Although the use of these materials has become standard, few alternative materials have not been fully explored. In this paper, we present a protocol developed in our laboratory that reliably produces very soft, acrylamide-based phantoms that can mimic both acoustical and mechanical characteristics of the vitreous body of the eye. Following the described protocol, a series of phantoms were constructed ranging in acrylamide concentration from 1.60% to 1.70%. Measurements across the series yielded attenuation coefficients of 0.067-0.140 dB/cm/MHz, depending on acrylamide concentration. Speed of sound ranged between 1499 and 1510 m/s, also depending on acrylamide concentration. Published values for the vitreous gel indicate an attenuation of 0.10 dB/cm/MHz and a speed of sound of 1510 m/s, making our phantoms an excellent analog of this tissue. One application of these acrylamide phantoms is to test the efficacy of the Kinetic Acoustic Vitreoretinal Examination (KAVE), a tool developed in our laboratory with the potential to aid in the diagnosis of vitreoretinal disorders. KAVE utilizes acoustic radiation force to generate small, localized displacements within the vitreous-mimicking gel. These localized displacements are quantified to yield maximum displacement, relative elasticity, and relative viscosity images. We present KAVE images of a set of four phantoms with different gel concentrations. Although B-mode and relative viscosity images exhibit no significant differences, maximum displacement, and relative elasticity images clearly differentiate gels of different concentrations. Maximum displacements ranged between 30 and 5 microns, depending on acrylamide concentration. The results presented in this paper show that soft gel phantoms can be produced in a range of elasticities not previously reported, and that these phantoms are useful for testing ultrasound instruments designed for evaluation of the vitreous gel. Furthermore, the use of acrylamide-based gels may also offer a valuable and attractive alternative for many other ultrasound applications.     

Assessing and improving acoustic radiation force image quality using a 1.5-D transducer design

A 1.5-D transducer array was proposed to improve acoustic radiation force impulse (ARFI) imaging signal-to-noise ratio (SNRARFI) and image contrast relative to a conventional 1-D array. To predict performance gains from the proposed 1.5-D transducer array, an analytical model for SNRARFI upper bound was derived. The analytical model and 1.5-D ARFI array were validated using a finite element modelbased numerical simulation framework. The analytical model demonstrated good agreement with numerical results (correlation coefficient = 0.995), and simulated lesion images yielded a significant (2.92 dB; p < 0.001) improvement in contrast-tonoise ratio when rendered using the 1.5-D ARFI array.     

Nonlinear resonant interaction between acoustic impulse and electrons in superconductors

The problem of inelastic electron interaction in a perfectly pure superconductor with a longitudinal sound impulse is considered. It is shown that the problem reduces to the equivalent one of elastic scattering by the static potential, and the sound absorption is expressed in terms of the reflection coefficient of this scattering. The classical and quantum properties of the scattering are studied and the phase region in which new excitations are created is indicated. A formula is derived that expresses the density matrix of the excitations created in terms of the exact scattering matrix of an impulse. The quasiclassical creation of excitations by a smooth-shaped impulse is investigated with regard to both overbarrier and underbarrier processes.     

On the thermal effects associated with radiation force imaging of soft tissue

Several laboratories are investigating the use of acoustic radiation force to image the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one approach that uses brief, high-intensity, focused ultrasound pulses to generate radiation force in tissue. This radiation force generates tissue displacements that are tracked using conventional correlation-based ultrasound methods. The tissue response provides a mechanism to discern mechanical properties of the tissue. The acoustic energy that is absorbed by tissue generates radiation force and tissue heating. A finite element methods model of acoustic heating has been developed that models the thermal response of different tissues during short duration radiation force application. The beam sequences and focal configurations used during ARFI imaging are modeled herein; the results of these thermal models can be extended to the heating due to absorption associated with other radiation force-based imaging modalities. ARFI-induced thermal diffusivity patterns are functions of the transducer f-number, the tissue absorption, and the temporal and spatial spacing of adjacent ARFI interrogations. Cooling time constants are on the order of several seconds. Tissue displacement due to thermal expansion is negligible for ARFI imaging. Changes in sound speed due to temperature changes can be appreciable. These thermal models demonstrate that ARFI imaging of soft tissue is safe, although thermal response must be monitored when ARFI beam sequences are being developed.     

Spectral correlation in ultrasonic pulse echo signal processing

The effects of using spectral correlation in a maximum-likelihood estimator (MLE) for backscattered energy corresponding to coherent reflectors embedded in media of microstructure scatterers is considered. The spectral autocorrelation (SAC) function is analyzed for various scatterer configurations based on the regularity of the interspacing distance between scatterers. It is shown that increased regularity gives rise to significant spectral correlation, whereas uniform distribution of scatters throughout a resolution cell results in no significant correlation between spectral components. This implies that when a true uniform distribution for the effective scatterers exists, the power spectral density (PSD) is sufficient to characterize their echoes. However, as the microstructure scatterer distribution becomes more regular, SAC terms become more significant. MLE results for 15 A-scans from stainless steel specimens with three different grain sizes indicate an average 6-dB signal-to-noise ratio (SNR) improvement in the coherent scatterer (flat-bottom hole) echo intensities for estimators using the SAC characterization as opposed to the PSD characterization.     

Reciprocity calibration of impulse responses of acoustic emission transducers

By means of reciprocity calibration in Rayleigh-wave and longitudinal-wave sound fields, frequency characteristics of amplitude and phase of absolute sensitivity of acoustic emission transducers were measured on the basis of the newly derived complex reciprocity parameters, and the impulse responses were obtained through inverse Fourier transform. Calibration results were confirmed with supplemental experiments in which the fracturing of a pencil lead was utilized for the source of elastic waves. Impulse responses of acoustic emission transducers to both the Rayleigh-wave and longitudinal-wave displacement velocities were determined by means of purely electrical measurements without the use of mechanical sound sources or reference transducers.     

Assessment of shear modulus of tissue using ultrasound radiation force acting on a spherical acoustic inhomogeneity

An ultrasound-based method to locally assess the shear modulus of a medium is reported. The proposed approach is based on the application of an impulse acoustic radiation force to an inhomogeneity in the medium and subsequent monitoring of the spatio-temporal response. In our experimental studies, a short pulse produced by a 1.5-MHz highly focused ultrasound transducer was used to initiate the motion of a rigid sphere embedded into an elastic medium. Another 25 MHz focused ultrasound transducer operating in pulse-echo mode was used to track the displacement of the sphere. The experiments were performed in gel phantoms with varying shear modulus to demonstrate the relationship between the displacement of the sphere and shear modulus of the surrounding medium. Because the magnitude of acoustic force applied to sphere depends on the acoustic material properties and, therefore, cannot be used to assess the absolute value of shear modulus, the temporal behavior of the displacement of the sphere was analyzed. The results of this study indicate that there is a strong correlation between the shear modulus of a medium and spatio-temporal characteristics of the motion of the rigid sphere embedded in this medium.