Directional laser generation and detection of ultrasound with arrays of optical fibers

Some aspects of the generation and detection of ultrasound with laser beams are presented. The objective of the study is (1) to present some experimental results on the generation of ultrasound with an array of optical fibers, (2) to discuss the possibility of generating directional surface waves with acousto-optic scanning in a very narrow frequency band, thus increasing the signal-to-noise ratio, and (3) to discuss the feasibility of the directional detection of ultrasound by using an array of optical fibers as a receiver, also with the goal of increasing the signal-to-noise ratio.     

Binary code design for high-frequency ultrasound

This paper proposes an approach to designing binary codes suitable for high-frequency applications of coded excitation in medical ultrasound. For a high-frequency ultrasound system, transmitting well-designed binary codes with a low sampling ratio (i.e., the bit rate divided by the transducer center frequency) is a practical way to improve the signal-to-noise ratio (SNR) because the challenge of implementing arbitrary-waveform generators for transmitting nonbinary codes increases with the frequency and the switching speed of square-wave pulsers are limited. One conventional approach designs codes using a base sequence that modulates wideband sequences up to the transducer passband. Because a major portion of codes is excluded as a candidate, codes designed using this approach typically need long compression filters for restoring the axial resolution, and they do not improve the SNR efficiently. In contrast, the approach proposed here searches all the codes that match the transducer passband; hence, the resultant codes exhibit better performance. The technique was tested using a bit rate of 50 MHz and a sampling ratio of 2. For a transducer with an ideal Gaussian frequency response with a center frequency of 25 MHz and a -6 dB bandwidth of 15 MHz, the SNR for the same side-lobe extent was 1 to 6 dB higher for the codes designed using the proposed approach compared with those designed using the conventional approach. When a real transducer response with a center frequency of 26.4 MHz and a one-way -6 dB bandwidth of 20.7 MHz was considered, the codes designed using the proposed approach were superior by 0.5 to 5 dB. Therefore, our approach is better than the conventional approach for designing binary codes for high-frequency ultrasound, with the results indicating that the moderate bit rate of 50 MHz will suffice when the ultrasonic center frequency is 25 MHz.     

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     

Development of a real-time, high-frequency ultrasound digital beamformer for high-frequency linear array transducers

A real-time digital beamformer for high-frequency (>20 MHz) linear ultrasonic arrays has been developed. The system can handle up to 64-element linear array transducers and excite 16 channels and receive simultaneously at 100 MHz sampling frequency with 8-bit precision. Radio frequency (RF) signals are digitized, delayed, and summed through a real-time digital beamformer, which is implemented using a field programmable gate array (FPGA). Using fractional delay filters, fine delays as small as 2 ns can be implemented. A frame rate of 30 frames per second is achieved. Wire phantom (20 microm tungsten) images were obtained and -6 dB axial and lateral widths were measured. The results showed that, using a 30 MHz, 48-element array with a pitch of 100 microm produced a -6 dB width of 68 microm in the axial and 370 microm in the lateral direction at 6.4 mm range. Images from an excised rabbit eye sample also were acquired, and fine anatomical structures, such as the cornea and lens, were resolved.     

Microfabrication of electrode patterns for high-frequency ultrasound transducer arrays

High-frequency ultrasound is needed for medical imaging with high spatial resolution. A key issue in the development of ultrasound imaging arrays to operate at high frequencies (?30 MHz) is the need for photolithographic patterning of array electrodes. To achieve this directly on 1-3 piezocomposite, the material requires not only planar, parallel, and smooth surfaces, but also an epoxy composite filler that is resistant to chemicals, heat, and vacuum. This paper reports, first, on the surface finishing of 1-3 piezocomposite materials by lapping and polishing. Excellent surface flatness has been obtained, with an average surface roughness of materials as low as 3 nm and step heights between ceramic/polymer of ～80 nm. Subsequently, high-frequency array elements were patterned directly on top of these surfaces using a photolithography process. A 30-MHz linear array electrode pattern with 50-μm element pitch has been patterned on the lapped and polished surface of a high-frequency 1-3 piezocomposite. Excellent electrode edge definition and electrical contact to the composite were obtained. The composite has been lapped to a final thickness of ～55 μm. Good adhesion of electrodes on the piezocomposite has been achieved and electrical impedance measurements have demonstrated their basic functionality. The array was then packaged, and acoustic pulse-echo measurements were performed. These results demonstrate that direct patterning of electrodes by photolithography on 1-3 piezocomposite is feasible for fabrication of high-frequency ultrasound arrays. Furthermore, this method is more conducive to mass production than other reported array fabrication techniques.     

Design and fabrication of annular arrays for high-frequency ultrasound

The design, fabrication, and performance of miniature high-frequency annular arrays are described. A 50-MHz, 2-mm-diameter, 7-element, equal-area annular array was fabricated and tested. The array elements were defined using photolithography and the electrical contacts were made using ultrasonic wire bonding. The resulting transducer produced pulses with a -6 dB bandwidth of 52% and an insertion loss of -16 dB. A radiation pattern was collected by scanning the transducer array above the tip of a glass fiber. A -6 dB two-way beam width of 75 microns was found at f/2. The radiation pattern decreased smoothly to less than -60 dB at a distance of 550 microns.     

Chirp-coded excitation imaging with a high-frequency ultrasound annular array

High-frequency ultrasound (HFU, > 15 MHz) is an effective means of obtaining fine-resolution images of biological tissues for applications such as opthalmologic, dermatologic, and small animal imaging. HFU has two inherent drawbacks. First, HFU images have a limited depth of field (DOF) because of the short wavelength and the low fixed F-number of conventional HFU transducers. Second, HFU can be used to image only a few millimeters deep into a tissue because attenuation increases with frequency. In this study, a five-element annular array was used in conjunction with a synthetic-focusing algorithm to extend the DOF. The annular array had an aperture of 10 mm, a focal length of 31 mm, and a center frequency of 17 MHz. To increase penetration depth, 8-micros, chirp-coded signals were designed, input into an arbitrary waveform generator, and used to excite each array element. After data acquisition, the received signals were linearly filtered to restore axial resolution and increase the SNR. To compare the chirpcoded imaging method with conventional impulse imaging in terms of resolution, a 25-microm diameter wire was scanned and the -6-dB axial and lateral resolutions were computed at depths ranging from 20.5 to 40.5 mm. The results demonstrated that chirp-coded excitation did not degrade axial or lateral resolution. A tissue-mimicking phantom containing 10-microm glass beads was scanned, and backscattered signals were analyzed to evaluate SNR and penetration depth. Finally, ex vivo ophthalmic images were formed and chirpcoded images showed features that were not visible in conventional impulse images.     

A low-cost bipolar pulse generator for high-frequency ultrasound applications

A design of a low-cost bipolar pulse generator for high-frequency (HF) ultrasound applications is presented. The pulse generator can produce N cycle (1-255 cycles) bipolar pulses with center frequency over 60 MHz. The measured pulse amplitude was over 160 Vpp, and the pulse ringing was less than 0.3 Vpp (i.e., signal-to-ring ratio is 55 dB). The pulser can be used in high-frequency ultrasound Doppler and B-mode imaging applications with arrays.     

Unit-delay focusing architecture and integrated-circuit implementation for high-frequency ultrasound

High-frequency ultrasound (above 10 MHz) has been used successfully in many medical applications, including eye, skin, gastrointestinal, intravascular, and Doppler flow imaging. Most of these applications use single-element transducers, thereby imposing a tradeoff between resolution and depth of field. Fabrication difficulties and the need for high-speed electronic beamformers have prevented widespread use of arrays at high frequencies. In this paper, a unit-delay focusing architecture suitable for use with high-frequency ultrasound annular arrays is described. It uses a collection of identical, active delay cells that may be simultaneously varied to accomplish focusing. Results are presented for an analog integrated circuit intended for use with a five-element, 50-MHz planar annular array. Focusing is possible over an axial range for which the ratio of maximum to minimum f-number is 2.1. Unit-delay architectures also are described for curved annular arrays and linear arrays.     

Continuous spore disruption using radially focused, high-frequency ultrasound

We report on the development of a novel, continuous-flow, radially focused ultrasonic disruptor capable of lysing Bacillus spores in the absence of added chemical denaturants, enzymes, or microparticles. Greater than 99% disruption was achieved for Bacillus globigii spores and Escherichia coli and Bacillus subtilis vegetative cells with sample residence times of 62, 12, and 12 s, respectively. Microscopic and SEM images indicated that at equivalent power levels, the incidence of cell death or loss of viability typically exceeded the efficiency of (visible) cell lysis. However, semiquantitative PCR showed up to a 1,000-fold increase in intracellular DNA availability from ultrasonically disrupted spores, and liberated DNA was intact and available for subsequent detection.     

Improved synthetic aperture focusing technique with applications in high-frequency ultrasound imaging

Synthetic aperture focusing using a virtual source was used previously to increase the penetration and to extend the depth of focus in high-frequency ultrasonic imaging. However, the performance of synthetic aperture focusing is limited by its high sidelobes. In this paper, an adaptive weighting technique based on a focusing-quality index is introduced to suppress the sidelobes. The focusing-quality index is derived from the spatial spectrum of the scan-line data along the mechanical scan direction (i.e., the synthetic aperture direction) after focusing delays relative to the virtual source have been applied. The proposed technique is of particular value in high-frequency ultrasound in which dynamic focusing using array transducers is not yet possible. Experimental ultrasound data from a 50-MHz imaging system with a single-crystal transducer (f-number=2) are used to demonstrate the efficacy of the proposed technique on both wire targets and speckle-generating objects. An in vivo experiment also is performed on a mouse to further demonstrate the effectiveness. Both 50-MHz fundamental imaging and 50-MHz tissue harmonic imaging are tested. The results clearly demonstrate the effectiveness in sidelobe reduction and background-noise suppression for both imaging modes. The principles, experimental results, and implementation issues of the new technique are described in this paper.     

The design of protection circuitry for high-frequency ultrasound imaging systems

Transmission line lengths in the protection circuitry of a high-frequency (>20-MHz) ultrasound imaging system have an important effect on the frequency, amplitude, and bandwidth of the pulse-echo response of the system. A model that includes the transmission line lengths between the pulser, transducer, and receiver and the electromechanical properties of high-frequency transducers is used to illustrate the importance of correctly choosing these line lengths. An iterative optimization procedure for designing the protection circuitry for a broadband system is proposed. A theoretical and experimental analysis of the validity of this approach is reported for a 45-MHz PVDF transducer.     

Improvements in the reliability and element quality of parallel tetrahedral mesh generation

The paper presents a parallel tetrahedral mesh generation approach based on recursive bidivisions using triangular surfaces. Research was conducted for addressing issues concerning mesh generation reliability and element quality. A novel procedure employing local modification techniques is proposed for repairing the intersecting interdomain mesh instead of directly repeating the bidivision procedure, which improves the robustness of the complete meshing procedure significantly. In addition, a new parallel quality improvement scheme is suggested for optimizing the distributed volume meshes. The scheme is free of any communication cost and highly efficient. Finally, mesh experiments of hundreds of millions of elements are performed to demonstrate the reliability, effectiveness and efficiency of the proposed method and its potential applications to large‐scale simulations of complex aerodynamics models. Copyright © 2012 John Wiley & Sons, Ltd.     

A multifunctional, reconfigurable pulse generator for high-frequency ultrasound imaging

High-frequency (>20 MHz) ultrasound (HFUS) imaging systems have made it possible to image small structures with fine spatial resolution. They find a variety of biomedical applications in dermatology, ophthalmology, intravascular imaging, and small-animal imaging. One critical technical challenge of HFUS is to generate high-voltage, high-frequency pulsed signals to effectively excite the transducer for a high SNR. This paper presents the development of a multifunctional, reconfigurable pulse generator for HFUS imaging. The pulse generator can produce a high-voltage unipolar pulse, a bipolar pulse, or arbitrary pulses for B-mode imaging, Doppler measurement, and modulated excitation imaging. The characteristics of the pulses, such as timing, waveform, and frequency are reconfigurable by a high-speed field-programmable gate array (FPGA). Customized software was developed to interface with the FPGA through a USB connector for pulse selection, and easy, flexible, real-time pulse management. The hardware was implemented in a compact, printed circuit board (PCB)-based scheme using state-of-the-art electronics for costeffectiveness and fully digital control. Testing results show that the unipolar pulse can reach over 165 Vpp with a 6-dB bandwidth of 70 MHz, and the bipolar pulse and arbitrary pulses can reach 150 and 60 Vpp with central frequencies of 60 and 120 MHz, respectively.     

Stochastically induced hysteresis in optical carrier generation

A study is made of a mechanism for the occurrence of a noise-induced kinetic transition in a thin amorphous semiconductor wafer. Pis’ma Zh. Tekh. Fiz. 24, 1–4 (July 26, 1998)     

A novel envelope detector for high-frame rate, high-frequency ultrasound imaging

This paper proposes a novel design of envelope detectors capable of supporting a small animal cardiac imaging system requiring a temporal resolution of more than 150 frames per second. The proposed envelope detector adopts the quadrature demodulation and the lookup table (LUT) method to compute the magnitude of the complex baseband components of received echo signals. Because the direct use of the LUT method for a square root function is not feasible due to a large memory size, this paper presents a new LUT strategy dramatically reducing its size by using binary logarithmic number system (BLNS). Due to the nature of BLNS, the proposed design does not require an individual LOG-compression functional block. In the implementation using a field programmable gate array (FPGA), a total of 166.56 Kbytes memories were used for computing the magnitude of 16-bit in-phase and quadrature components instead of 4 Gbytes in the case of the direct use of the LUT method. The experimental results show that the proposed envelope detector is capable of generating LOG-compressed envelope data at every clock cycle after 32 clock cycle latency, and its maximum error is less than 0.5 (i.e., within the rounding error), compared with the arithmetic results of square root function and LOG compression.     

Optimum displacement for compound image generation in medical ultrasound

Incoherent averaging of measurements made at different look directions can be used to reduce speckle noise in medical ultrasound images. An analytic expression for the correlation between two measurements made at different spatial positions is derived. Using this expression, the optimum aperture displacement for efficient incoherent averaging (i.e. compounding) is computed and found to equal approximately one-half the aperture length.     

Robust deconvolution of high-frequency ultrasound images using higher-order spectral analysis and wavelets

Deconvolution of high-frequency (30-40 MHz) ultrasonic images of human skin was studied in vivo. Separate one-dimensional (1-D) functions for the axial and lateral profiles were first estimated using higher-order spectral methods. Subsequently, deconvolution was implemented using a regularized inverse Wiener filtering of the wavelet and scaling coefficients that were obtained after a wavelet decomposition of the RF signals. Deconvolution was first performed in the axial direction, then in the lateral direction. The methods were applied to data obtained from the skin of 16 volunteers using three different transducers. Significant improvements in both the axial and lateral resolutions were obtained in all the cases. Features such as hair follicles in the dermis and fingerprints on the surface of the finger were more clearly displayed in the processed images compared to the original images. The results indicate that the deconvolution method using higher-order spectral methods and wavelet analysis could significantly improve the quality of high-frequency ultrasonic skin images.     

Evaluation of an algorithm for semiautomated segmentation of thin tissue layers in high-frequency ultrasound images

An algorithm consisting of speckle reduction by median filtering, contrast enhancement using top- and bottom-hat morphological filters, and segmentation with a discrete dynamic contour (DDC) model was implemented for nondestructive measurements of soft tissue layer thickness. Algorithm performance was evaluated by segmenting simulated images of three-layer phantoms and high-frequency (40 MHz) ultrasound images of porcine aortic valve cusps in vitro. The simulations demonstrated the necessity of the median and morphological filtering steps and enabled testing of user-specified parameters of the morphological filters and DDC model. In the experiments, six cusps were imaged in coronary perfusion solution (CPS) then in distilled water to test the algorithm's sensitivity to changes in the dimensions of thin tissue layers. Significant increases in the thickness of the fibrosa, spongiosa, and ventricularis layers, by 53.5% (p < 0.001), 88.5% (p < 0.001), and 35.1% (p = 0.033), respectively, were observed when the specimens were submerged in water. The intraobserver coefficient of variation of repeated thickness estimates ranged from 0.044 for the fibrosa in water to 0.164 for the spongiosa in CPS. Segmentation accuracy and variability depended on the thickness and contrast of the layers, but the modest variability provides confidence in the thickness measurements.