Ultrasonic Beam Profiles Research Articles

Feasibility Study for a High-Frequency Flexible Ultrasonic Cuff for High-Precision Vagus Nerve Ultrasound Neuromodulation.

In the emerging research field of bioelectronic medicine, it has been indicated that neuromodulation of the vagus nerve (VN) has the potential to treat various conditions such as epilepsy, depression, and autoimmune diseases. In order to reduce side effects, as well as to increase the effectiveness of the delivered therapy, sub-fascicle stimulation specificity is required. In the electrical domain, increasing spatial selectivity can only be achieved using invasive and potentially damaging approaches like compressive forces or nerve penetration. To avoid these invasive methods while obtaining a high spatial selectivity, a 2-mm diameter extraneural cuff-shaped proof-of-concept design with integrated lead zirconate titanate (PZT) based ultrasound (US) transducers is proposed in this article. For the development of the proposed concept, wafer-level microfabrication techniques are employed. Moreover, acoustic measurements are performed on the device, in order to characterize the ultrasonic beam profiles of the integrated PZT-based US transducers. A focal spot size of around [Formula: see text] is measured for the proposed cuff. Moreover, the curvature of the device leads to constructive interference of the US waves originating from multiple PZT-based US transducers, which in turn leads to an increase of 45% in focal pressure compared to the focal pressure of a single PZT-based US transducer. Integrating PZT-based US transducers in an extraneural cuff-shaped design has the potential to achieve high-precision US neuromodulation of the VN without requiring intraneural implantation.

Ultrasonic Bessel beam generation from radial modes of piezoelectric discs

We present comprehensive analytical and experimental investigations on ultrasonic Bessel beam generation from radial modes of piezoelectric disc transducers. The Bessel vibration pattern of the radial modes was experimentally measured using Laser Doppler Vibrometry and was found to be in very good agreement with those obtained from numerical simulations. Ultrasonic beam profiles from the first four radial modes of the piezoelectric disc were measured using a hydrophone in a water tank. The results obtained from the experimental scans were compared to the predictions from an analytical model and were found to be in very good agreement. Also, the Bessel beams generated from the radial modes (except the first) of the piezoelectric discs were found to have narrow beam width for the central lobe compared to those from an ideal piston source of same size. The proposed approach of using radial modes for Bessel beam generation finds applications in imaging, acoustic particle manipulation and trapping, and acousto-optics.

Radial modes of laterally stiffened piezoelectric disc transducers for ultrasonic collimated beam generation

We numerically investigate the resonance and vibration characteristics of radial modes of laterally stiffened piezoelectric disc transducers. Lateral stiffening is modelled using a spring and vibration characteristics of the piezo-disc are investigated with increasing lateral stiffness. It is found that the resonant frequency response of the radial modes follows an asymptotic behaviour approaching that of a clamped disc with increasing lateral stiffness. The radial mode vibration pattern of the discs is also found to be affected by lateral stiffness. While the vibration pattern of a free disc corresponds to a Bessel function, laterally stiffened discs show edge-effects where they depart from the Bessel-like behaviour. In addition, a fully clamped piezo-disc is found to have an extra side-lobe when compared to a free disc. Ultrasonic beam profiles generated from radial modes of laterally stiffened discs are numerically investigated. It is found that the free piezo-disc generates a Bessel beam that has multiple side-lobes. Increasing the lateral stiffness results in a significant reduction of side-lobes in the beam profile. This technique of generating a collimated beam with side-lobe reduction finds significant applications in imaging through concrete, drilling mud, and other highly attenuating materials.

Low-frequency ultrasonic Bessel-like collimated beam generation from radial modes of piezoelectric transducers

We present a very simple approach to generate a collimated ultrasonic beam that exploits the natural Bessel-like vibration pattern of the radial modes of a piezoelectric disc with lateral clamping. This eliminates the need for the conventional annular Bessel pattern of the electrodes with individual electrode excitation on the piezo-disc, thus simplifying the transducer design. Numerical and experimental studies are carried out to investigate the Bessel-like vibration patterns of these radial modes showing an excellent agreement between these two studies. Measured ultrasonic beam-profiles in water from the radial modes confirm the profile to be a Bessel beam. Collimated beam generation from radial modes is investigated using a coupled electromechanical finite-element model. It is found that clamping the lateral edges of piezoelectric transducers results in a high-degree of collimation with practically no side-lobes similar to a parametric array beam. Ultrasonic beam-profile measurements in water with both free and clamped piezoelectric transducer are presented. The collimated beam generation using the present technique of using the laterally clamped radial modes finds significant applications in low-frequency imaging through highly attenuating materials.

Accurate Estimation of Carotid Luminal Surface Roughness Using Ultrasonic Radio-Frequency Echo

It would be useful to measure the minute surface roughness of the carotid arterial wall to detect the early stage of atherosclerosis. In conventional ultrasonography, the axial resolution of a B-mode image depends on the ultrasonic wavelength of 150 µm at 10 MHz because a B-mode image is constructed using the amplitude of the radio-frequency (RF) echo. Therefore, the surface roughness caused by atherosclerosis in an early stage cannot be measured using a conventional B-mode image obtained by ultrasonography because the roughness is 10–20 µm. We have realized accurate transcutaneous estimation of such a minute surface profile using the lateral motion of the carotid arterial wall, which is estimated by block matching of received ultrasonic signals. However, the width of the region where the surface profile is estimated depends on the magnitude of the lateral displacement of the carotid arterial wall (i.e., if the lateral displacement of the arterial wall is 1 mm, the surface profile is estimated in a region of 1 mm in width). In this study, the width was increased by combining surface profiles estimated using several ultrasonic beams. In the present study, we first measured a fine wire, whose diameter was 13 µm, using ultrasonic equipment to obtain an ultrasonic beam profile for determination of the optimal kernel size for block matching based on the correlation between RF echoes. Second, we estimated the lateral displacement and surface profile of a phantom, which had a saw tooth profile on its surface, and compared the surface profile measured by ultrasound with that measured by a laser profilometer. Finally, we estimated the lateral displacement and surface roughness of the carotid arterial wall of three healthy subjects (24-, 23-, and 23-year-old males) using the proposed method.

Ultrasonic beam propagation in highly anisotropic materials simulated by multi-Gaussian beams

The necessity of nondestructively inspecting fiber-reinforced composites, austenitic steels, and other inherently anisotropic materials has stimulated considerable interest in developing beam models for anisotropic media. The properties of slowness surface play a key role in the beam models based on the paraxial approximation. In this paper, we apply a modular muiti-Gaussian beam (MMGB) model to study the effects of material anisotropy on ultrasonic beam profile. It is shown that the anisotropic effects of beam skew and excess beam divergence enter into the MMGB model through parameters defining the slope and curvature of the slowness surface. The overall beam profile is found when the quasilongitudinal (qL) beam propagates in the symmetry plane of a transversely isotropic gr/ep composite. Simulation results are presented to illustrate the effects of these parameters on ultrasonic beam diffraction and beam skew. The MMGB calculations are also checked by comparing the anisotropy factor and beam skew angle with other analytical solutions.

Optimal design of EMAT transmitters

A three-part finite element model is developed that characterizes the ultrasonic pulse produced by an electromagnetic acoustic transducer (EMAT). The model represents several significant improvements over previously published works, as follows: (a) spatial inhomogeneities in the magnetic flux density are calculated and then incorporated in the determination of body forces, (b) an improved model of the electromagnetic induction phenomenon is formulated, allowing a more accurate evaluation of the ultrasonic pulse launched by an EMAT transmitter and (c) results from the model are compared directly with experimental measurements, yielding discrepancies of the order of 15% in the amplitude of the ultrasonic pulse. The new model is used to optimize the design of the EMAT system. In particular, a parametric study was conducted on the effects of varying an EMAT's magnet-to-coil width ratio. For the EMAT configuration considered, significant improvements can be achieved in the ultrasonic beam amplitude and profile by increasing the ratio to about 1.2; further increases in magnet dimensions yield only marginal improvements in the ultrasonic beam, at the cost of excessive EMAT size.

Extrinsic and intrinsic fiber optic Sagnac ultrasound sensors

A compact, dual-probe, fiberized Sagnac ultrasound sensor (SUS) capable of detecting ultrasonic waves of various types is described. The main advantages of the proposed device are (1) the ability to detect ultrasonic waves on the surface (extrinsic mode) as well as in the interior of structures (intrinsic mode); (2) improved SNR, which has been achieved using an optical frequency shifting technique for biasing to quadrature and for elimination of parasitic interference between the desired sampling beams and other nonessential beams in the interferometer; (3) the ability to detect ultrasonic signals on rough surfaces by the use of an electromechanical speckle-hunting technique (in the extrinsic mode); (4) dual-probe configuration; and (5) directional sensitivity to ultrasound when the system is operated in the intrinsic mode or on a polished surface in the extrinsic mode. Several applications of the SUS for nondestructive characterization of material properties and testing of structures for flaws are presented. Bulk longitudinal waves, Rayleigh waves and Lamb waves are measured using the SUS to (1) characterize piezoelectric ultrasonic transducer beam profiles, (2) study the scattering of ultrasonic waves by flaws, (3) identify and size surface breaking flaws in aircraft wheel components, and (4) monitor in real time the cure process of an epoxy resin.

A parametric study of ultrasonic beam profiles for a linear phased array transducer

A numerical simulation model is presented to investigate the influences of design parameters of linear phased array transducers on beam focusing and steering performance. The characteristic of ultrasonic beam profiles has been simulated on the basis of the Huygen's superposition principle. For the simulation, a linear phased array is considered as the composition of finite number of elements separated by equidistance. Individual elements are considered as two-dimensional point sources. The waves generated from piezoelectric elements are considered as simplified transient ultrasonic waves that are constructed with the cosine function enveloped with a Hanning window. The characteristic of ultrasonic wave propagation into a medium from the phased array transducer is described. The effects of the number, the interelement spacing, steering angle, the focal length, and frequency bandwidth of the piezoelectric elements on beam directivity and ultrasonic pressure field in a linear phased array transducer are systematically discussed.

Analysis of the Ultrasonic Beam Profile Due to Variation of the Inter-Element Spacing for the Phased Array Transducer

The phased array transducer has two distinct advantages. One is rapid scanning comparing with the conventional mechanical or manual scanning system. Therefore, output image is represented in real-time. The other is the dynamic focusing and the dynamic steering of ultrasonic beam. Only the delay times that are controlled electrically are used to focus and to steer beam without any lenses or wedges. In this study, the profile of the ultrasonic beam for the phased array transducer has been simulated in the Huygens principle with varying the inter-element spacing of the linear phased array transducer. From the result of this study, it was found that the ultrasonic beam spread wider as the inter-element spacing was decreased. However, the focusing effect was improved, even when the number of the element was not big. In addition, there was grating lobes that are constructed when the inter-element spacing is more than half wavelength. However, this grating lobe has lower amplitude than the main lobe.

Acoustic fields of concave cylindrical transducers

The calculation of ultrasonic beam profiles from concave cylindrical sources is carried out, using the diffraction impulse response approach. Both transient and continuous wave radiation and receiving patterns can be derived with this method. Analytic expressions are given for the impulse response, in certain conditions, and numerical integration methods are applied for solving the problem at points of divergence. Experimental results obtained in water are presented for comparison.

Ultrasonic beam profiles and beam propagation in an austenitic weld using a theoretical ray tracing model

A model for ultrasonic wave propagation in anisotropic and inhomogeneous materials is applied to the case of ultrasonic inspection of an austenitic V-butt weld manufactured by the downhand Manual Metal Arc technique. We examine the propagation behaviour of waves within the weld region and, in addition, model beam divergence behaviour. From this work we predict directions of low inspection sensitivity and also identify regions of material to which no ultrasound penetrates. The relative merits of the three different wave modes are examined, showing clearly the advantages of horizontally polarized shear waves for austenitic steel inspection. Vertically polarized shear waves are shown to be the least effective for such inspections. We discuss the relevance of this work to the ultrasonic non-destructive testing of austenitic steel components, concluding that care is needed over the choice of wave modes and angles, to ensure sensitive inspection of the whole weld material.

Device for testing ultrasonic beam profiles

A device for testing the profile of an ultrasonic beam comprising a block of solid material permitting propagation of said beam therethrough without substantial scattering. A plurality of discrete particles having surfaces interfacing with said solid material are located in and are generally uniformly distributed throughout a substantially planar region in said block. The test device is scanned with an ultrasonic beam and an image of the beam profile is generated by reflection of the beam by the particles.

Back reflection of ultrasonic waves from a liquid-solid interface

The problem of back reflection of ultrasound from a liquid-solid interface is considered analytically. In experiments it has been observed that the back reflected energy radiates from the interface at the Rayleigh critical angle, irrespective of the incident angle. Mathematically, this radiation corresponds to the complex leaky Rayleigh wave pole of the reflection coefficient. The back-reflected amplitude is proportional to the Fourier transform of the incident field on the interface evaluated at the pole wavenumber. The transforms are calculated for several different ultrasonic beam profiles. The beams are characterized by the field on the transmitting transducer. A Gaussian profile is shown to produce a very small back reflection. Similar results hold for a beam composed of an array of Gaussian beams. This beam has the useful property that as limiting cases it reduces to a simple Gaussian beam and a regular SINC beam. Other beam profiles are examined which give a larger effect than these. The role that the radiation function of the transmitter plays in the back reflection is discussed.

Sonographic tissue texture: influence of transducer focusing pattern.

The ultrasonic beam profile pattern is not accurately known for most transducers in clinical use. A simple scan of an easily available phantom permits the clinician to determine the focusing pattern of his own transducer and equipment. Disregarding other operator-controllable parameters such as the time-gain compensation, output, and system sensitivity, the true echo pattern of a tissue is not accurately represented until the distance of the region of narrowest beam focus is reached. The fine speckled echo pattern commonly seen in uniform organs near the transducer is largely artifactual due to interference phenomena in the near field of the acoustic beam. Closest approximation of true texture is not displayed until near the depth at which the narrowest focus occurs. Beyond that depth, broad, coarser patterns can be expected.