Measurement of the acoustic reflectance function based on Lamb wave using high f-number transducers

Ultrasonic reflectivity using a V(z) technique is a powerful characterization method in acoustic microscopy to measure the elastic properties of materials. Conventional techniques generally use a low f-number with high frequency; however, to measure the reflectance function of the highly attenuative material, a low frequency is essential. In this study, the transducer-pair method based on Lamb waves is used to measure the reflectance function of a highly attenuative material. The results demonstrate the feasibility of the proposed method using a commercial ultrasound transducer with high f-number.

Increasing the transmission efficiency of transcranial ultrasound using a dual-mode conversion technique based on Lamb waves

Transcranial focused ultrasound (FUS) is a noninvasive treatment for brain tumors and neuromodulation. Based on normal incidence, conventional FUS techniques use a focused or an array of ultrasonic transducers to overcome the attenuation and absorption of ultrasound in the skull; however, this remains the main limitation of using FUS. A dual-mode conversion technique based on Lamb waves is proposed to achieve high transmission efficiency. This concept was validated using the finite element analysis (FEA) and experiments based on changes in the incident angle. Aluminum, plexiglass, and a human skull were used as materials with different attenuations. The transmission loss was calculated for each material, and the results were compared with the reflectance function of the Lamb waves. Oblique incidence based on dual-mode conversion exhibited a better transmission efficiency than that of a normal incidence for all of the specimens. The total transmission losses for the materials were 13.7, 15.46, and 3.91 dB less than those associated with the normal incidence. A wedge transducer was designed and fabricated to implement the proposed method. The results demonstrated the potential applicability of the dual-mode conversion technique for the human skull.

Patterned Interference Radiation Force for Transcranial Neuromodulation

Compared with the conventional method of transcranial focused ultrasound stimulation using a single transducer or a focused beam, the compression and tensile forces are generated from the high-pressure gradient of a standing wave that can generate increased stimulation. We experimentally verified a neuromodulation system using patterned interference radiation force (PIRF) and propose a method for obtaining the magnitude of the radiation force, which is considered the main factor influencing ultrasound neuromodulation. The radiation forces generated using a single focused transducer and a standing wave created via two focused transducers were compared using simulations. Radiation force was calculated based on the relationship between the acoustic pressure, radiation force and time-averaged second-order pressure obtained using an acoustic streaming simulation. The presence of the radiation force was verified by measuring the time-averaged second-order pressure generated due to the radiation force, by using a glass tube.

Applications of Capacitive Micromachined Ultrasonic Transducers: A Comprehensive Review

Capacitive micromachined ultrasonic transducer (CMUT) was introduced as an alternative to the piezoelectric thick-film-based transducers in medical imaging applications. Gradually, CMUTs have been investigated in almost all the applications in acoustics due to their superior transduction properties. CMOS compatible process flow and limitless possibilities of miniaturization made CMUT a preferred choice for the ultrasound industry. This article comprehensively reviews all the applications in which CMUT was used until now. Such a complete review of the practical applications of CMUT has not been reported elsewhere. A topicwise presentation approach is adopted, and wherever possible, the necessary details of the device properties and experimental niceties were briefly covered.

Miniaturized ultrasound imaging probes enabled by CMUT arrays with integrated frontend electronic circuits

Capacitive micromachined ultrasonic transducer (CMUT) arrays are conveniently integrated with frontend integrated circuits either monolithically or in a hybrid multichip form. This integration helps with reducing the number of active data processing channels for 2D arrays. This approach also preserves the signal integrity for arrays with small elements. Therefore CMUT arrays integrated with electronic circuits are most suitable to implement miniaturized probes required for many intravascular, intracardiac, and endoscopic applications. This paper presents examples of miniaturized CMUT probes utilizing 1D, 2D, and ring arrays with integrated electronics.

Modeling and Measuring the Effects of Mutual Impedance on Multi-Cell CMUT Configurations

This paper presents a numerical method for calculating the frequency response of a CMUT with a large number of cells. In a multi-cell configuration, commonly found in CMUTs, each cell is affected by the acoustic loading from neighboring cells. Thus, for an accurate model of a multi-cell CMUT element it is better to consider the mutual acoustic impedance instead of the acoustic impedance of a single cell only. We calculate the velocity of every cell (plate movement) simultaneously, with the mutual impedance effects taken into account. The model predicts that the cells exhibit different frequency responses, based on their locations in the element. We used a laser interferometer to validate the model by measuring the displacement response of a CMUT immersed in vegetable oil. The device has 169 circular cells (single crystal silicon plates, 500 nm thick, 21 μm radii) placed in a hexagonal cell arrangement. The measurement results agree well with the numerical results. The computation time of our method is significantly shorter than finite element based calculations. Our model can be used for finding optimized cell configurations for CMUTs utilized in various applications such as medical imaging and therapeutic treatment.

Microfluidic sonicator for real-time disruption of eukaryotic cells and bacterial spores for DNA analysis

Biologic agent screening is a three-step process: lysis of host cell membranes or walls to release their DNA, polymerase chain reaction to amplify the genetic material and screening for distinguishing genetic signatures. Macrofluidic devices commonly use sonication as a lysis method. Here, we present a piezoelectric microfluidic minisonicator and test its performance. Eukaryotic human leukemia HL-60 cells and Bacillus subtilis bacterial spores were lysed as they passed through a microfluidic channel at 50 microL/min and 5 microL/min, respectively, in the absence of any chemical denaturants, enzymes or microparticles. We used fluorescence-activated cell sorting and hematocytometry to measure 80% lysis of HL-60 cells after 3 s of sonication. Real-time polymerase chain reaction indicated 50% lysis of B. subtilis spores with 30 s of sonication. Advantages of the minisonicator over macrofluidic implementations include a small sample volume (2.5 microL), reduced energy consumption and compatibility with other microfluidic blocks. These features make this device an attractive option for "lab-on-a-chip" and portable applications.

Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays

In this paper, we report on the characterization of 1-D arrays of capacitive micromachined ultrasonic transducers (cMUT). A 275- x 5600-micron 1-D CMUT array element is experimentally characterized, and the results are found to be in agreement with theoretical predictions. As a receiver, the transducer has a 0.28-fm/square root of Hz displacement sensitivity, and, as a transmitter, it produces 5 kPa/V of output pressure at the transducer surface at 3 MHz with a DC bias of 35 V. The transducer has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging. The radiation pattern of isolated single elements, as well as those of array elements are measured, and two major sources of acoustical cross talk are identified. A weakly dispersive non-leaky interface wave (Stoneley wave) is observed to be propagating at the silicon substrate-fluid interface at a speed close to the speed of sound in the fluid. This wave causes internal reflections, spurious resonance, and radiation from the edges of the silicon substrate. The large lateral component of the particle velocity generated by the membranes at the edge of the cMUT array elements is found to be the source of this interface wave. Lowest order Lamb waves in the silicon substrate are also found to contribute to the cross talk between elements. These waves are excited at the edges of individual vibrating membranes, where they are anchored to the substrate, and result in a narrowing of the beam profile of the array elements. Several methods, such as trench isolation and wafer thinning, are proposed and implemented to modify the acoustical cross coupling between array elements.

Theory and analysis of electrode size optimization for capacitive microfabricated ultrasonic transducers

Theoretical analysis and computer simulations of capacitive microfabricated ultrasonic transducers indicate that device performance can be optimized through judicious patterning of electrodes. The conceptual basis of the analysis is that electrostatic force should be applied only where it is most effective, such as at the center of a circular membrane. If breakdown mechanisms are ignored, an infinitesimally small electrode with an infinite bias voltage results in the optimal transducer. A more realistic design example compares the 3-dB bandwidths of a fully metalized transducer and a partially metalized transducer, each tuned with a lossless Butterworth network. It is found that the bandwidth of the optimally metalized device is twice that of the fully metalized device.

Simulation and experimental characterization of a 2-D capacitive micromachined ultrasonic transducer array element

In this letter, a 400-mumx400-mum 2-D capacitive micromachined ultrasonic transducer (cMUT) array element is experimentally characterised, and the results are found to be in good agreement with theoretical predictions. As a receiver, the transducer has a 1.8x10(-7) nm/ radical(Hz) displacement sensitivity, and, as a transmitter, it produces 16.4 kPa/V of output pressure at the transducer surface at 3 MHz. The transducer also has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging.

Surface micromachined capacitive ultrasonic transducers

The current state of novel technology, surface microfabricated ultrasonic transducers, is reported. Experiments demonstrating both air and water transmission are presented. Air-coupled longitudinal wave transmission through aluminum is demonstrated, implying a 110 dB dynamic range for transducers at 2.3 MHz in air. Water transmission experiments from 1 to 20 MHz are performed, with a measured 60 dB SNR at 3 MHz. A theoretical model is proposed that agrees well with observed transducer behavior. Most significantly, the model is used to demonstrate that microfabricated ultrasonic transducers constitute an attractive alternative to piezoelectric transducers in many applications.

Image processing for a scanning acoustic microscope that measures amplitude and phase

Several image-processing techniques for a low-frequency (3 to 10 MHz) scanning acoustic microscope (SAM) that measures amplitude and phase are described. This microscope is capable of measuring both the amplitude and phase of the reflected and transmitted signals, in contrast with most earlier implementations that only measure the amplitude. By measuring phase, the authors can carry out quantitative nondestructive evaluation (NDE) and image processing that cannot be done with amplitude or phase alone. The effective 2-D point spread function of the microscope is modified by spatial filtering of the digitized complex images. In various images, the transverse resolution is improved by about 20%, aberration of images of subsurface features is corrected, and surface features are numerically defocused. The last process is used to remove the obscuring effect of surface roughness from images of subsurface features.

High-frequency acousto-optic mode locker for picosecond pulse generation

We modeled, designed, and built a 500-MHz acousto-optic mode locker with a diffraction efficiency of 28% per 1 W drive power. The transducer is zinc oxide sputtered onto a sapphire substrate. A new figure of merit is defined for the mode-locker design, which indicates that sapphire is a good substrate material. Pulse widths of less than 10 psec with an average power of 150 mW were achieved from a 500-MHz pulse-rate, diode-pumped, cw mode-locked Nd:YLF laser using a pump power of 700 mW.

Detachable 400-MHz acousto-optic phase modulator for a single-mode optical fiber

A single-mode-fiber phase modulator was constructed by contacting the fiber with a lapped glass capillary tube. The capillary's inner surface provides a long, effectively semicircular contact region to the fiber, allowing throughput of acoustic waves launched from a thin-film ZnO transducer fabricated directly onto the capillary's other lapped face. The device operated at a center frequency of 416 MHz with a FWHM bandwidth of 14 MHz. The maximum phase shift was 0.033 rad/ radicalmw, with a largest measured value of 1.2 rad at 1.3-W input electrical power.

Design and implementation of mixed-mode transducers

In order to design a mixed-mode transducer with high efficiency and broad bandwidth for both longitudinal and shear wave modes, a theory is developed to determine the properties of this transducer with arbitrary acoustic loads at both ends of the piezoelectric element. Several Y-cut lithium niobate (LiNbO(3)) transducers were made on both [110] single-crystal bismuth germanium oxide (Bi(12)GeO(20)) and fused quartz. The piezoelectric plates were attached to indium bonding and later polished to operate in the 100-MHz frequency range. The experimental data of round-trip insertion loss for both longitudinal and shear modes showed an excellent agreement with theoretical predictions.

Lens design for acoustic microscopy

Design criteria for acoustic microscope lenses are examined with respect to their intended application. Aside from buffer rod material and F-number, the factors influencing the lens design are the critical angle for surface wave excitation, lens illumination, and leak rate of the surface wave on the sample. It is found that the design criteria are different for surface and subsurface examination and that for different applications and materials, different lenses are required for optimum imaging performance. A formalism for evaluating the performance of an acoustic microscope by considering its response in the time domain, both theoretically and experimentally, is presented.

Tunable optical filter in fiber-optic form

Light can be coupled between the two principal polarizations of birefringent fiber by using a traveling acoustic wave to produce a spatially periodic stress in the fiber. For a fixed acoustic frequency, maximum coupling occurs when the input optical wavelength is such that the beat length of the fiber equals the acoustic wavelength. By changing the acoustic frequency, the wavelength at which peak coupling occurs can be tuned. A prototype device has a passband 5 nm wide with a peak optical wavelength that can be tuned from 570 to 630 nm by changing the acoustic frequency from 2.85 to 2.55 MHz.

Switchable fiber-optic tap using the acousto-optic Bragg interaction

A new type of electronically switchable optical-fiber tap using acousto-optic Bragg diffraction is demonstrated. An acoustic transducer on a wedge launches an acoustic beam through a Hertzian contact into a D-shaped optical fiber, diffracting light out through the side of the fiber. The bandwidth of the tap is greater than 1 GHz centered at 3.5 GHz, with a tap efficiency of 0.01% per watt of rf power. The tap does not damage the fiber and is completely reversible, so its location on the fiber can easily be adjusted.