Design considerations for piezoelectric polymer ultrasound transducers

Much work has been published on the design of ultrasound transducers using piezoelectric ceramics, but a great deal of this work does not apply when using the piezoelectric polymers because of their unique electrical and mechanical properties. The purpose of this paper is to review and present new insight into seven important considerations for the design of active piezoelectric polymer ultrasound transducers: piezoelectric polymer materials selection, transducer construction and packaging requirements, materials characterization and modeling, film thickness and active area design, electroding selection, backing material design, and front protection/matching layer design. Besides reviewing these design considerations, this paper also presents new insight into the design of active piezoelectric polymer ultrasonic transducers. The design and fabrication of an immersible ultrasonic transducer, which has no adhesive layer between the active element and backing layer, is included. The transducer features direct deposition of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer onto an insulated aluminum backing substrate. Pulse-echo tests indicated a minimum insertion loss of 37 dB and -6 dB bandwidth of 9.8 to 22 MHz (71%). The use of polymer wear-protection/quarter-wave matching layers is also discussed. Test results on a P(VDF-TrFE) transducer showed that a Mylar/sup TM/ front layer provided a slight increase in pulse-echo amplitude of 15% (or 1.2 dB) and an increase in -6 dB pulse-echo fractional bandwidth from 86 to 95%. Theoretical derivations are reported for optimizing the active area of the piezoelectric polymer element for maximum power transfer at resonance. These derivations are extended to the special case for a low profile (i.e., thin) shielded transducer. A method for modeling the non-linear loading effects of a commercial pulser-receiver is also included.     

Analysis and design of focused interdigital transducers

Focused interdigital transducers (FIDTs) can generate surface acoustic wave (SAW) with high intensity and high beamwidth compression ratio. Owing to these features, they are very suitable to be used as the sources of microacoustic channels or waveguides in the near future. The focusing properties of FIDTs are dominated solely by their geometric shapes. Therefore, to obtain optimal performance, it is essential to analyze the FIDTs with a variety of geometric shapes. However, among the existing studies concerning the diffraction of FIDTs, a detailed analysis and design of FIDTs is still in paucity. In this paper, we adopted the exact angular spectrum of plane wave theory (ASoW) to calculate the amplitude fields of FIDTs on Y-Z lithium niobate (LiNbO3) with the shape as a concentric circular arc and the concentric wave surface. Based on the calculation results, we discussed the variations of the amplitude fields induced by changing number of pairs, degree of arc, and geometric focal length. In addition, the focusing properties of FIDTs on the (100)-oriented GaAs substrate were also analyzed and discussed. We also summarized the guiderules for designing a FIDT via four important factors. It is worth noting that the results of this study provide an important basis for designing various FIDTs to fit the desired applications.     

聚焦换能器离散发射阵列的辐射力计算

为了对医学超声中使用的聚焦换能器离散阵列的辐射力进行估算,文章中应用几何声学的方法,推导了聚焦换能器离散发射阵列超声作用于吸收靶上的辐射力公式,并分别给出了活塞阵元与聚焦阵元的辐射力公式。将该几何声学公式与瑞利积分结果比较显示,当声束倾斜角θi<30°,半孔径角α<15°时,误差不超过4.5%。这种结果说明了这种方法可以用来有效地估算离散阵列聚焦换能器的辐射力。     

Ultrasound characteristics of focused axisymmetrically curved surface transducers

A high-speed method for exactly calculating the time-dependent pressure of the radiated acoustic field from pulse-excited axisymmetrically curved surface or lens transducers using the Rayleigh surface integral is presented. The time-dependent pressure of a medical lens transducer with its outer surface shaped like a revolving arc is calculated as an example. Convolution-algorithm procedures are used to study the acoustic radiation field for various forms of exciting pulses. In the calculations, an integral representation of the impulse response function is reduced to a one-dimensional integration. With the help of the triangle criterion for identification and the principle of one-dimensional constriction, the calculation can be scanned over the segmented ranges of integration to carry out the numerical analysis. Ultrasound characteristics of the transducers are calculated, and their relationship to the structure of the transducers is examined.     

Verification of the Westervelt equation for focused transducers

This study investigates the validity of the Westervelt equation for focused transducers. The angular spectrum method is employed to analyze the second-harmonic acoustic field under the weakly nonlinear approximation. Although it is well known that the Westervelt equation is accurate for the case of quasi-plane waves, the present work demonstrates accurate solution for the highly focused case of a spherically-curved ultrasound transducer having an aperture angle of 80°. It is further found that the solution error is inversely dependent on the nonlinearity coefficient.     

超声阵列换能器设计及声场模拟

聚焦超声采用的聚焦方式有许多种,电子聚焦是其中之一,与其它聚焦方式相比,电子聚焦技术有很多优点,然而一些相互矛盾的条件对其设计有很大影响。通过用快速算法对阵元声场、以及频率对场的影响进行模拟,对阵列设计中的几个主要考虑方面进行了分析,提供了一般设计思路。采用伪逆矩阵算法,对变迹技术和多焦点技术进行计算机模拟,为焦域控制提供了新的思路,结果表明：将电子聚焦阵列换能器用于高强度聚焦超声治疗设备,比其它     

1.5-D high intensity focused ultrasound array for non-invasive prostate cancer surgery

The aim of this study is to demonstrate the feasibility of a new spherically curved 1.5-D phased array for the treatment of localized prostatic cancer. The device is designed to conform to the Ablatherm(R) machine (EDAP-Technomed. France), a commercially available machine in which high intensity focused ultrasound (HIFU) treatment for prostate cancer is administered transrectally. It uses high intensity electronically focused ultrasound to steer a beam along two axes, allowing enough depth to be reached to treat large prostates and eliminating two degrees of mechanical movement. Through computer simulation, it was determined that a curved 1.5-D configuration offered the optimal design. Two configurations were then proposed, and their ability to steer a beam within a target volume centered on the geometric focus of the transducer was simulated. An eight-element prototype was constructed to test the piezo-composite material and its electro-acoustical efficiency. Then, an array was constructed, and a multichannel amplifier and control system were added, to permit remote operation. Acoustical and electrical measurements were made to verify performance. Finally, the 1.5-D array was tested in vitro on samples of pig liver to confirm the ability to induce lesions     

聚焦超声的辐射力计算与高强度聚焦超声功率测量实验

本文研究聚焦超声场的辐射力计算，应用几何声学方法，推导了聚焦超声作用于测试靶上的辐射力的通用公式，讨论全反射靶和全吸收靶上的辐射力，最后给出了应用辐射力法测量高强度聚焦超声装置的声功率的实例，其结果与量热法测量的声功率接近，偏差不大于３％。     

聚焦超声的辐射力计算与高强度聚焦超声功率测量实验

研究聚焦超声场的辐射力计算。应用几何声学方法,推导了聚焦超声作用于测试靶上的辐射力通用公式,讨论了全反射靶和全吸收靶上的辐射力。最后给出了应用辐射力法测量高强度聚焦超声装置的声功率的实例,其结果与良热法测得的声功率接近,偏差不大于3%。     

Optoelectronic transmitters for medical ultrasound transducers

Optoelectronics and fiber optics can be used to miniaturize and improve the flexibility of the transducer cable and transducer handle of medical diagnostic ultrasound scanners. The reduction in size has gained importance as 2-D array transducers with up to 1000 independent channels become accepted to improve diagnostic ultrasound images. The authors describe the analysis, design, fabrication and testing of a prototype silicon photoconductive semiconductor switch (PCSS). The monolithic silicon PCSS was used in combination with an infrared semiconductor diode laser with a fiber optic “pigtail” to shock excite and burst excite a 2-D array transducer element resonant at 2.5 MHz. Optically controlled voltage, current, and ultrasound pulses are compared to those from conventional electronic shock excitation and narrow band Doppler pulses. The optically triggered ultrasound pulse for single shock excitation produced 30 V spikes at the 2-D array element with a fall time of 200 nsec and a rise time of 2 μsec with a peak current through the transducer element of 34 mA. An optically produced burst of eight pulses at a frequency of 2.5 MHz produced 11 V spikes at the transducer with a fall time under 100 nsec and a rise time of approximately 300 nsec. The peak current per pulse was 25 mA through the transducer element. These results show the feasibility of applying optoelectronic technology to replace conventional electronic transmitter technology     

High frequency piezoelectric MEMS ultrasound transducers

High-frequency ultrasound array transducers using piezoelectric thin films on larger structures are being developed for high-resolution imaging systems. The increase in resolution is achieved by a simultaneous increase in operating frequency (30 MHz to about 1 GHz) and close coupling of the electronic circuitry. Two different processing methods were explored to fabricate array transducers. In one implementation, a xylophone bar transducer was prototyped, using thin film PbZr(0.52)Ti(0.48)O(3) (PZT) as the active piezoelectric layer. In the other, the piezoelectric transducer was prepared by mist deposition of PZT films over electroplated Ni posts. Because the PZT films are excited through the film thickness, the drive voltages of these transducers are low, and close coupling of the electronic circuitry is possible. A complementary metal-oxidesemiconductor (CMOS) transceiver chip for a 16-element array was fabricated in 0.35-microm process technology. The ultrasound front-end chip contains beam-forming electronics, receiver circuitry, and analog-to-digital converters with 3-Kbyte on-chip buffer memory.     

Modeling and optimization of high-frequency ultrasound transducers

Obtaining an accurate transducer model for a high-frequency transducer can be troublesome using traditional models, such as the KLM model, since it is often difficult to measure precisely the piezoelectric, dielectric, and mechanical properties of the transducer. This paper describes an alternative method of modeling transducers using network theory. The network theory model for a transducer is determined from a measurement of the transducer impedance in water and the pulse-echo response of the system for a given electrical source and load. A discussion of how this model can be used to optimize the design of an electrical matching circuit is given. This method is illustrated by designing a two-element transmission line matching circuit for a miniature 53 MHz transducer. Excellent agreement between the network model prediction and the experimental response is obtained     

Design and characterization of dual-curvature 1.5-dimensional high-intensity focused ultrasound phased-array transducer

A dual-curvature focused ultrasound phased-array transducer with a symmetric control has been developed for noninvasive ablative treatment of tumors. The 1.5-D array was constructed in-house and the electro-acoustic conversion efficiency was measured to be approximately 65%. In vitro experiments demonstrated that the array uses 256 independent elements to achieve 2-D wide-range high-intensity electronic focusing.     

Simulation of field characteristics of the focused axisymmetrically curved surface transducers

The acoustical fields produced by a variety of focused axisymmetrically curved surface transducers with large aperture have been investigated on the basis of Fresnel-Kirchhoff diffraction theory. The interrelation of ultrasound characteristics with some structural aspects of the transducers has been calculated and compared. The forms of resulting intensity maxima and their resulting suitability for surgical lesion of tissues are compared through parameters such as “lesioning capability” and “focussing efficiency.” Although only modest changes in intensity distribution are possible, some may have applications in making lesions for the control of focal cancer and other pathologies     

Characterization of spherically focused transducers using an ultrasonic measurement model approach

A new and computationally efficient method is developed for characterizing a spherically focused, ultrasonic transducer (and its accompanying test system). Procedures for determining the probe's effective radius, effective focal length, and system efficiency factor are described. Predicted responses that make use of these effective parameters are shown to correspond very well to measured responses for a number of different transducers.     

Design and fabrication of PZN-7%PT single crystal high frequency angled needle ultrasound transducers

A high-frequency angled needle ultrasound transducer with an aperture size of 0.4 x 0.56 mm2 was fabricated using a lead zinc niobate-lead titanate (PZN- 7%PT) single crystal as the active piezoelectric material. The single crystal was bonded to a conductive silver particle matching layer and a conductive epoxy backing material through direct contact curing. A parylene outer matching layer was formed by vapor deposition. Angled needle probe configuration was achieved by dicing at 45 degrees to the single crystal poling direction to satisfy a clinical request for blood flow measurement in the posterior portion of the eye. The electrical impedance magnitude and phase of the transducer were 42 Omega and -63 degrees , respectively. The measured center frequency and the fractional bandwidth at -6 dB were 43 MHz and 45%, respectively. The two-way insertion loss was approximately 17 dB. Wire phantom imaging using fabricated PZN-7%PT single crystal transducers was obtained and spatial resolutions were assessed.     

Evaluation of accuracy of a theoretical model for predicting thenecrosed tissue volume during focused ultrasound surgery

The concept of thermal dose as a predictor for the size of the necrosed tissue volume during high-intensity focussed ultrasound surgery was tested. The sensitivity of the predicted lesion size to the uncertainties in the iso-dose constant, attenuation coefficient, and thermal dose threshold of necrosis was studied. The predicted lesion size appears to be independent of attenuation at some high attenuation values and certain depth in tissue. Thus, for a given target depth, a proper selection of frequency could minimize the lesion size variability due to uncertainty in the tissue attenuation. The predicted lesion size was less dependent on the uncertainties in the iso-dose constant and thermal dose of necrosis. The predictions of the model were compared with experimental data in rabbit muscle, and experimental data in cat and rat brain measured by others. The agreement was found to be good in most of the experiments. Similarly, the model was found to predict well the trends of increasing power and pulse duration     

Characterization of high-frequency, single-element focused transducers with wire target and hydrophone

In this paper, a wire-target technique was used for lateral beam profile measurements for a single-element, focused transducers in the very high-frequency range (35-60 MHz). Two wire targets made from 9-cm long tungsten wires with diameters of 8 microm and 20 microm were used as the pulse-echo targets to measure the lateral beam profiles at the focal plane of two single-element, focused transducers, a spherically focused 40 MHz transducer and a lens-focused in-house lithium niobate (LiNbO3) 60 MHz transducer. For comparison, measurements on the same transducers were performed by three small-aperture hydrophones with geometrical diameters varying from 37 microm to 150 microm. Tomographic reconstruction of the acoustic field from the spherically focused transducer also was conducted. Results obtained with the wire-target technique are comparable to those obtained with small-aperture hydrophones in characterizing lateral radiation patterns of a single-element, focused transducer in the high-frequency range (35-60 MHz). However, the wire-target method may overestimate pulse length because of the additional attenuation caused by the return path. Compared to small-aperture hydrophones, the wire-target technique is simpler and more cost effective. Its major advantage, however, is in the frequency range above 100 MHz in which commercial hydrophones are not yet available.     

Multifrequency ultrasound transducers for conformal interstitial thermal therapy

Control over the pattern of thermal damage generated by interstitial ultrasound heating applicators can be enhanced by changing the ultrasound frequency during heating. The ability to change transmission frequency from a single transducer through the use of high impedance front layers was investigated in this study. The transmission spectrum of multifrequency transducers was calculated using the KLM equivalent circuit model and verified with experimental measurements on prototype transducers. The addition of a quarter-wavelength thick PZT (unpoled) front layer enabled the transmission of ultrasound at two discrete frequencies, 4.7 and 9.7 MHz, from a transducer with an original resonant frequency of 8.4 MHz. Three frequency transmission at 3.3, 8.4, and 10.8 MHz was possible for a transducer with a half-wavelength thick front layer. Calculations of the predicted thermal lesion size at each transmission frequency indicated that the depth of thermal lesion could be varied by a factor of 1.6 for the quarter-wavelength front layer. Heating experiments performed in excised liver tissue with a dual-frequency applicator confirmed this ability to control the shape of thermal lesions during heating to generate a desired geometry. Practical interstitial Designs that enable the generation of shaped thermal lesions are feasible.     

高强度聚焦超声换能器的新型设计

采用高强度聚焦超声（HIFU）治疗局部肿瘤，如何增强对病变区域的辐照效果，而尽量减少对健康区域的辐照损伤是一个很重要的问题。文章基于目前常用的凹球面自聚焦换能器，提出了解决上述问题的方案，设计了新型的换能器，对其工作方式及声场特性进行了研究。结果表明：采用多换能器轮流发射的方式进行治疗是一种行之有效的解决方案。