Medical Ultrasound Transducers Research Articles

Exploring dynamics, stability, and bifurcation in a coupled damped oscillator with piezoelectric energy harvester

In this work, we analyze and investigate the mathematical modeling of a dynamical system connected to a piezoelectric device. Piezoelectric transducers are established as highly effective energy harvesting (EH) devices, often employed in real-world mechanical systems. The structure of the dynamical model contains a damped Duffing oscillator acting as the major component, which is connected to an unstretched pendulum and simultaneously to the piezoelectric harvester. To derive the governing equations of motion (EOM), Lagrange’s equations are applied based on the system’s generalized coordinates, in which they are analytically solved using the multiple scales (MS) perturbation method up to the second approximation. Moreover, the solutions achieved are compared with numerical ones to increase transparency and demonstrate the accuracy of the approximate solutions. In addition, comprehensive graphical representations have been produced to investigate the nonlinear stability of the modulation equations. Phase portraits, bifurcation diagrams, and Lyapunov spectra are displayed to depict various system behaviors, alongside Poincaré maps for deeper understanding. The various stability ranges are also explored and discussed. In conclusion, mechanical vibrations are transformed into electricity by the piezoelectric transducer connected to the dynamic model, which has widespread applications and includes crystal oscillators, medical ultrasound, gas igniters, and displacement transducers.

Thermal cycle stability and microstructure of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ferroelectric single crystals have been commercially available as important components in medical ultrasound transducers due to their excellent piezoelectric and electromechanical coupling performance. The variation in piezoelectric and dielectric properties of PMN-PT single crystals with ambient temperature is an important application indicator. In this work, the PMN-PT single crystals after direct current poling (DCP) and alternating current poling (ACP) were subjected to the cyclic thermal treatment process. The thermal cycling stability and microstructural changes in PMN-PT single crystals were investigated. The ACP single crystals exhibit a higher dielectric constant ε33T/ε0 (6500–7600) and piezoelectric coefficient d33 (2100–2500 pC N−1) compared to the DCP single crystals (ε33T/ε0 of 4100–5000, d33 of 1200–1300 pC N−1). Under thermal cycling at 60 °C, the DCP and ACP single crystals exhibit good thermal cycling stability after 150 cycles. Microstructural observations show that the domain structure of the DCP single crystals exhibits “staggered domain walls, inhomogeneous domain size, variety of domain structure,” while the relatively homogeneous stripe-like domains were observed in the ACP single crystals. After thermal cycling, new fine striped domains appear in both the DCP and ACP single crystals due to the instability of rotated polarization, but the piezoelectric and dielectric properties are not greatly affected. This work provides an intensive understanding of the effects of thermal cycling on the domain structure, which is useful for applications.

Development of a wideband medical ultrasonic linear array transducer with a double-sided FPCB

Medical ultrasound transducers are being used to acquire medical image information from a patient. In order to acquire high-quality images, studies have been conducted to improve the performance of the transducer. Among them, the high acoustic impedance de-matching layer is effective in improving the sensitivity and bandwidth of the transducer using the backward reflected wave. First, the performance trend of the transducer according to the thickness of the acoustic de-matching layer was analyzed through theoretical calculation and finite element method. It was confirmed that the sensitivity and bandwidth of the transducer increased by adjusting the time delay of the backward reflected wave according to the thickness of the acoustic de-matching layer. Based on the analyzed results, a new structure of a transducer applied with a double-sided FPCB was devised. The time delay of the backward reflected wave was simply controlled by changing the thickness of the polyimide in the FPCB. The bandwidth of the transducer can be increased by changing the thickness of the polyimide in the FPCB. Also, a prototype of a transducer having a designed structure was manufactured and its performance was measured, and it was confirmed that the designed structure was effective in increasing the bandwidth of the transducer.

Dramatical improvement in temperature stability of ZnO modified PNN-PZT ceramics via synergistic effect of doping and composite

In this work, doping and composite are simultaneously utilized to improve the Tm of 0.55 Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3 ceramic by adding ZnO (PNN-PZT:xZnO). It is found that Zn ions prefer to substitute for Ni ions, making Ni ions spontaneously crystallize at grain boundaries in the form of an oxide. Moreover, limited by the solid solubility, superfluous ZnO particles also stay at grain boundaries with the increasing ZnO content. Thus, triple contribution of ions substitution, local stress fields and electric fields coexist in the PNN-PZT:xZnO ceramics with high ZnO content, dramatically improving the Tm from 102 °C to 171 °C. The excellent comprehensive electromechanical performance is obtained in the sample with x = 0.05: d33 ∼ 824 pC/N, d33* ∼1059 p.m./V, Tm ∼140 °C, Ec ∼5.2 kV/cm, Qm ∼87. As the ceramic is heated to 125 °C, its k33 and d33 values still remain ∼67% and ∼80% of the corresponding room temperature values as evidenced by in-situ temperature stability measurement. Our results indicate that PNN-PZT:xZnO ceramics are the most promising candidates to develop high-performance medical ultrasound transducers for imaging, meanwhile, provide a fresh design strategy to optimize the performance of the piezoelectric ceramics.

Development of Polymer-Ceramic-Metal Graded Acoustic Matching Layers via Cold Sintering.

A family of three phase, polymer-ceramic-metal (Poly-cer-met) electrically conducting composites was developed via cold sintering for acoustic matching application in medical ultrasound transducers. A range of acoustic impedance ( Z ) between MRayl with low attenuation (<3.5 dB/mm, measured at 10 MHz) was achieved in composites of zinc oxide, silver, and in thermoplastic polymers like Ultem polyetherimide (PEI) or polytetrafluoroethylene (PTFE) at sintering pressure less than 50 MPa and temperature of 150 °C. Densities exceeding 95% were achieved, with resistivities less than 1 Ω -cm. The acoustic velocity was homogeneous across the part (variations <5%). The acoustic velocities exceeded 2500 m/s for Z above 12 MRayl. The experimentally measured acoustic impedance of ZnO/Ag/PEI composites was observed to be in close agreement with the theoretical logarithmic model developed for different volume fractions of individual phases at the percolation limit for Ag. Thus, the acoustic properties of this family of matching layers (MLs) can be predicted to a good approximation before experimental realization. Additionally, a non-conducting low Z (5 MRayl MRayl) with acoustic velocities exceeding 2000 m/s was achieved using hydrozincite as the ceramic component. Scaling of the composites to 2'' diameter was demonstrated. A -6 dB bandwidth greater than 85% was measured for a three ML ultrasound transducer, fabricated using a single cold sintered layer ( Z = 19 MRayl) and two other commercial layers in the stack. Finally, a co-cold sintered graded prototype consisting of three tape-casted formulations corresponding to Z = 5 , 9, and 19 MRayl, while still retaining the correct distributions of the components was demonstrated.

A rapid method for pressure-mapping ultrasound fields using a moving hydrophone

Medical ultrasound transducers are commonly characterized in well-controlled water tanks using miniature pressure-calibrated hydrophones. Spatial beam dimensions of the ultrasound field can be quantified by positioning hydrophones in a 2D plane where acoustic waveforms are acquired at closely spaced locations. However, even when using automated computer-controlled positioning, this technique is time consuming if the hydrophone is repositioned and allowed to come to rest at each location where pressure measurements are desired. Here we present a novel technique where acoustic waveforms are acquired while the hydrophone is in motion. The transducer under test is pulsed at a rate which, depending on the hydrophone speed, determines the spatial resolution of the pressure map. Applying this technique results in substantial time savings and allows larger beam areas to be more accurately mapped. Limitations, such as translation stage motor noise and pulse length, are discussed and examples are presented which demonstrate the ability to rapidly characterize a variety of ultrasound transducer types.

Optimized integrated design of a high-frequency medical ultrasound transducer with genetic algorithm

Designing efficient acoustic stack and elements for high-frequency (HF) medical ultrasound (US) transducers involves various interrelated parameters. So far, optimizing spatial resolution and acoustic field intensity simultaneously has been a daunting task in the area of HF medical US imaging. Here, we introduce optimized design for a 50-MHz US probe for skin tissue imaging. We have developed an efficient design and simulation approach using Krimholtz, Leedom and Matthaei (KLM) equivalent circuit model and spatial impulse response method by means of Field II software. These KLM model and Field II software are integrated, and a GA algorithm is used to optimize the design of the US transducer to obtain the best imaging performance. As a result, a 50-MHz single element probe is effectively optimized with 5 mm acoustic focal length, 72 upmu {text{m}} lateral, and 42 upmu {text{m}} axial imaging resolution, with an enhancement in imaging resolution over the conventionally designed and simulated probe by 10%. This work has the potential to benefit many applications that require a fast, high-resolution and strong US focus in skin imaging.

Classification of Medical Ultrasound Transducer Using Neural Network

Diagnostic medical ultrasound makes a significant contribution to patient care and is increasingly used in a variety of clinical settings by many different professionals with varying technical backgrounds (e.g. hepatocellular carcinoma). Therefore, the importance of ultrasound quality control is not only necessary for patient and operator safety but is also essential for maintaining the performance of the equipment to the highest-level achievable and it is required by various regulatory and accrediting agencies. Ultrasound image degradation originates primarily from transducer defects and potentially undermines reliable image interpretation. The ultrasound probe in-air reverberation pattern is used in routine quality assurance. We produce a quantitative quality control based on in-air reverberation images. They are easily generated for any probe independent of the level of expertise of the operator. The results are available to the sonographer prior to clinical use and transducer status can be remotely monitored with trend analysis over time. The method presents a scheme for the classification of normal functioning and defect transducers Region of Interest selected ROIs of 65 probes based on texture analysis that automatically detects in-air reverberation regions and recognizes them as normal functioning and defect transducers. However, feature selection is done by Twin Support Vector Machine. The accuracy of these features in distinguishing normal functioning and defect transducers has been evaluated by artificial neural network, and linear support vector machine algorithms classifiers. From the analysis of results, it was found that artificial neural network classifier gave an overall classification accuracy of 100% with 100% sensitivity. The results show that it is feasible to identify defect transducers based on texture features extracted from in-air reverberation ultrasound images. This method is shown to be useful for increased accuracy and increased speed for classification of functioning and defect transducers for improving the quality assurance of ultrasound.

Parylene coating for 13 MHz 1-3 composite transducer performance enhancement

1-3 piezoelectric composite material is widely applied in medical ultrasound transducers. The acoustic impedance mismatching still exists between composite materials and human tissue, this mismatching limits the performance of the 1-3 composite transducer. In this work, Parylene coating technique is employed to enhance the performance of a 1-3 piezoelectric composite transducer whose center frequency is 13 MHz. Theory and simulation analysis is carried out to estimation and optimize the influence of the Parylene coating layer. Pulse- echo experiment is performed to verify the hypothesis of theory and simulation. The experiment results confirm that the performances of the transducer are effectively enhanced by Parylene coating and the optimum thickness of the coating Parylene layer is about 39.5 μm. Compared to the uncoated 1-3 composite transducer, the echo amplitude of 39.5 μm Parylene coated transducer increases 98.8%, the value of −6 dB pulse width decreases 39.5%, the value of −20 dB pulse width decreases 22.7%, the bandwidth increases 51.9%, and the center frequency is not affected. These results prove the feasibility and effectiveness of the Parylene coating layer in transducer performance enhancement.

The effective coupling coefficient for a completed PIN-PMN-PT array

The computation of the electromechanical coupling coefficient (EMCC) of a fully assembled medical ultrasound transducer array is directly computed with closed form expressions. The Levenberg-Marquardt non-linear regression algorithm (LMA) is employed to help confirm the EMCC calculated prediction (kEFF) and provide statistical insights. The complex electrical impedance spectra of a 1–3 composite array with two matching layers operating at a 3.75 MHz center frequency using PIN-PMN-PT single crystal material is measured in air both before and after oven heating at 160 °C for 15 min. The oven heating produces changes in the EMCC of −4.9%, clamped dielectric constant of −11%, and effective transducer longitudinal velocity of −2.5%. Utilizing the pre- and post-heating array impedance data, the calculated EMCC values from the new closed form expressions agree well with the complete KLM model based LMA, and also exhibit approximately one tenth the error as compared to the formulas for a flat, unloaded transducer.

Acoustic microbubble trapping for enhanced targeted drug delivery

Systemic drug administration can result in low-efficiency therapeutic delivery (less than 1% to solid tumors) and potential side-effects by subjecting all tissues to toxic agents. Recently, targeted drug delivery and localized payload release has been a key goal of biomedical research. By combining ultrasound contrast agents (UCAs) with a therapeutic payload, promises potential for an enhanced therapeutic outcome. We have proposed a ultrasonic trap technique that is able to accumulate large populations of UCAs for enhanced localized release of a therapeutic agent. Sequences for UCA manipulation, fast imaging, and UCA fragmentation are interleaved with a same linear array medical ultrasound transducer. Once UCA clusters are halted in the trap within flow channels through custom-designed beam profiles, destruction pulses are transmitted to deliver therapeutic payload and simultaneous fast imaging depicts the flow dynamics. Verification of the simultaneous operation of trapping, destruction and imaging sequences is performed with a tissue-mimicking flow phantom and microscopy, respectively. Optical imaging clarifies UCAs can still be retained within the low pressure trap during the fast switch between different excitation schemes.

Two reduced-order approaches for characterizing the acoustic output of high-power medical ultrasound transducers

An approach that combines numerical modeling with measurements is gaining acceptance for field characterization in medical ultrasound. The general method is suitable for accurate simulation of the fields radiated by therapeutic transducers in both water and tissue. Here, three characterization methods are compared. Simulations based on the 3D Westervelt equation with a boundary condition determined from acoustic holography measurements are used as the most accurate benchmark method. Two simplified methods are based on an axially symmetric nonlinear parabolic formulation, either the KZK equation or its wide-angle extension. Various approaches for setting a boundary condition to the parabolic models are presented and discussed. Simulation results obtained with the proposed methods are compared for a typical therapeutic array and a strongly focused single-element transducer, with validation measurements recorded by a fiber optic hydrophone at the focus at increasing acoustic outputs. It is shown that the wide-angle parabolic model is more accurate than the KZK model in governing diffraction effects in the nearfields of the focused beams. However, both methods give accurate results in the focal zone, even at very high outputs when shocks are present. [Work supported by and RSF 14-12-00974, P01 DK043881, NIH EB7643, and NSBRI through NASA 9-58.]

A Wideband High-Voltage Power Amplifier Post-Linearizer for Medical Ultrasound Transducers

The medical ultrasound transducer is a principal component in ultrasound systems, as it significantly influences system performance. The high-voltage power amplifier (HVPA) is the key ultrasound transmitter component and interfaces with the medical ultrasound transducer. Therefore, the performance of the HVPA critically affects the echo signal quality of the ultrasound transducer. As they are inherently non-linear devices, harmonic distortion of echo signals generated by the ultrasound transducers would critically deteriorate the echo signal quality in ultrasound systems. The primary aim of this research is to integrate a novel post-linearizer into the HVPA to suppress harmonic distortion in medical ultrasound transducers. Moreover, this study is based on the assumption that linearizing the HVPA could reduce the harmonic distortion components of the echo signals. The developed HVPA with post-linearizer was tested in an ultrasound microscopy system in order to demonstrate the harmonic suppression capability on the echo signal generated by the ultrasound transducer. When 10 MHz, 5-cycle, 26 dBm input power was sent to the HVPA with and without the post-linearizer circuits, the measured second-, third-, fourth- and fifth- harmonic distortions of a 10 MHz transducer with the post-linearizer (−13.11 dB, −10.81 dB, −16.33 dB, and −22.78 dB) were suppressed to a greater degree than those of the same transducer without the post-linearizer (−4.58 dB, −8.87 dB, −10.51 dB, and −15.31 dB).. Therefore, we conclude that the addition of the post-linearizer to the HVPA is a potentially useful electronic technique for improving echo signal quality in medical ultrasound transducers.

Development of an Estimation Instrument of Acoustic Lens Properties for Medical Ultrasound Transducers

In medical ultrasound transducers, the transmission mode (pass-through) approach has been used to estimate the characteristics of the acoustic lens. However, it is difficult to measure the acoustic lens properties with high precision because of human, systemic, or mechanical measurement errors. In this paper, we propose a low-cost estimation instrument for acoustic lens properties connected with a customized database. In the instrument, three-axis and one-axis transmitting and material fixtures accurately align the transmitting and receiving transducers separately. Through the developed instrument, we obtained a precise standard deviation of the attenuation coefficient and velocity of the acoustic lens material of 0.05 dB/cm and 2.62 m/s, respectively. Additionally, the simultaneous alignment between the fixtures is controllable with developed programs, thus generating very accurate information of the acoustic lens about the testing ultrasound transducer. In our instrument, the database could support users in managing the result data efficiently. User programs developed using LabVIEW provide the capability to obtain precise values of the attenuation coefficient and velocity, which represent the fundamental material characteristics of the acoustic lens of the medical ultrasound transducers. The developed review program of the customized database can also search the acoustic lens information and store the experimental results.

Improving the Performance of a 1-D Ultrasound Transducer Array by Subdicing.

In medical ultrasound transducer design, the geometry of the individual elements is crucial since it affects the vibration mode of each element and its radiation impedance. For a fixed frequency, optimal vibration (i.e., uniform surface motion) can be achieved by designing elements with very small width-to-thickness ratios. However, for optimal radiation impedance (i.e., highest radiated power), the width should be as large as possible. This leads to a contradiction that can be solved by subdicing wide elements. To systematically examine the effect of subdicing on the performance of a 1-D ultrasound transducer array, we applied finite-element simulations. We investigated the influence of subdicing on the radiation impedance, on the time and frequency response, and on the directivity of linear arrays with variable element widths. We also studied the effect of varying the depth of the subdicing cut. The results show that, for elements having a width greater than 0.6 times the wavelength, subdicing improves the performance compared with that of nonsubdiced elements: the emitted pressure may be increased up to a factor of three, the ringing time may be reduced by up to 50%, the bandwidth increased by up to 77%, and the sidelobes reduced by up to 13 dB. Moreover, this simulation study shows that all these improvements can already be achieved by subdicing the elements to a depth of 70% of the total element thickness. Thus, subdicing can improve important transducer parameters and, therefore, help in achieving images with improved signal-to-noise ratio and improved resolution.

Tutorial on the underwater or medical ultrasound transducer

Tutorial on the underwater or medical ultrasound transducer

Ultrasound Transducers

Diagnostic medical ultrasound transducers have evolved through the years and have contributed significantly to improved patient care. This article discusses the history and types of transducers and the elements that have changed over time. There has been a sharp transition from natural to human-made elements and from one to many in a single transducer. Ergonomics also now plays a role in transducer design and will continue to do so; the grip, weight, and size of transducers are in the forefront of design considerations. The evolution of transducers has changed not only how well we visualize anatomy and what anatomy we see but also how the patient’s care is managed. Different, new, and emerging technologies certainly will continue to be identified within the sonography community.

Effect of the angular aperture of medical ultrasound transducers on the parameters of nonlinear ultrasound field with shocks at the focus

Certain modern applications of high-intensity focused ultrasound (HIFU) in medicine use the nonlinear effect of shock front formation in the focal waveform. However, an important problem remains unsolved: determination of transducer parameters that provide the given pressure levels of the shock wave field at the focus required for a specific application. In this paper, simulations based on the Khokhlov-Zabolotskaya equation are performed to test and confirm the hypothesis that angular aperture of the transducer is the main parameter that determines the characteristic amplitude of the shock front and corresponding values for the peak positive and negative pressures at the focus. A criterion for formation of a developed shock in the acoustic waveform, as well as a method for determining its amplitude is proposed. Quantitative dependences of the amplitude of the developed shock and the peak pressures in the wave profile on the angular aperture of the transducer are calculated. The effects of saturation and the range of changes of the shock waveform parameters at the focus are analyzed for a typical HIFU transducer.

Preparation and characterization of lead zirconate titanate thick films prepared by chemical solution deposition for MEMS applications

Lead zirconate titanate (PbZrxTi1−xO3, PZT) films are widely used in silicon based microelectromechanical systems (MEMS) due to their high piezoelectric and electromechanical coupling coefficient. Si3N4 is often needed as a masking layer to protect Si during wet etching. In this paper, the preparation and characterization of Pb(Zr0.52Ti0.48)O3 thick films deposited on Pt/Ti/Si3N4/SiO2/Si substrate by chemical solution deposition were studied. PZT films with a thickness of 2μm were spin-coated on the substrates and pyrolyzed at 350°C, and then annealed at 650°C. PZT thick films exhibit highly (100) preferential orientation. The dielectric constant and loss tangent of the films are 2370 and 0.046 at 1kHz, respectively. The remnant polarization (Pr) and the coercive fields (Ec) are 32μC/cm2 and 56kV/cm, respectively. PZT films on patterned Si wafers have been studied for the applications on biosensors and medical ultrasound transducers.

An equivalent source model for simulating high intensity focused ultrasound fields using a nonlinear parabolic equation

The nonlinear parabolic Khokhlov-Zabolotskaya (KZ) equation is commonly used for simulation of nonlinear focused fields generated by medical ultrasound transducers. Axially symmetric codes can be easily implemented on personal computers with simulation times on the order of tens of minutes even for the most extensive simulations of shock formation conditions. However, model accuracy can be limited for high intensity focused ultrasound sources with large convergence angles. In previous studies, it was shown that nonlinear parabolic modeling can provide good matching with measurements and full diffraction modeling in the focal region for both single element piston sources and multi-element arrays. Boundary conditions for the modeling were adjusted by varying the aperture of an equivalent planar source with a uniform pressure magnitude and a phase distribution for parabolic focusing. With this approach, there were slight discrepancies in the lateral pressure distributions in the focal plane. Here, it is proposed that an additional variation in the position of the equivalent source enables matching both axial and lateral distributions of the real HIFU sources with higher accuracy. Several examples are presented to compare modeling and measurements of the fields generated by laboratory and clinical HIFU transducers. [Work supported by RSF 14-12-00974 and NIH EB007643.]