Conventional Short Pulse Research Articles

Coded Excitation for Ultrasonic Testing: A Review.

Originating in the early 20th century, ultrasonic testing has found increasingly extensive applications in medicine, industry, and materials science. Achieving both a high signal-to-noise ratio and high efficiency is crucial in ultrasonic testing. The former means an increase in imaging clarity as well as the detection depth, while the latter facilitates a faster refresh of the image. It is difficult to balance these two indicators with a conventional short pulse to excite the probe, so in general handling methods, these two factors have a trade-off. To solve the above problems, coded excitation (CE) can increase the pulse duration and offers great potential to improve the signal-to-noise ratio with equivalent or even higher efficiency. In this paper, we first review the fundamentals of CE, including signal modulation, signal transmission, signal reception, pulse compression, and optimization methods. Then, we introduce the application of CE in different areas of ultrasonic testing, with a focus on industrial bulk wave single-probe detection, industrial guided wave detection, industrial bulk wave phased array detection, and medical phased array imaging. Finally, we point out the advantages as well as a few future directions of CE.

Assessment of stone ablation rate using the Moses technology modes with different energy and pulse settings: An experimental study

ABSTRACT Objectives To compare lithotripsy ablation rate with the Moses modes versus conventional pulse modes when using the Holmium:Yttrium–Aluminum–Garnet (Ho:YAG) laser. Methods The Lumenis® Pulse P120H Holmium Laser System and a 365 μm Moses D/F/L fiber were used to assess stone ablation rate in conventional Short and Long Pulse as well as Moses Contact and Distance at 10 W (0.5Jx20Hz and 2Jx5Hz) and 60 W (1Jx60Hz and 2Jx30Hz). Hard and soft phantom stones were formed, and all tests were conducted in a custom experimental configuration installed in a saline-filled bath. The laser was delivered up to 3 kJ of total energy. The fragmentation pattern was assessed via photographs in each cohort. Results The time to reach the target energy was 5 min and 50 s in all 10 W and 60 W trials, respectively. In both stone types, ablation was more effective when high-power, high-energy and Moses Distance was utilized. In soft stones, the lowest ablation rate was detected in the Long Pulse modality in all power, energy and frequency settings. Overall, when dusting settings (high-frequency, low-energy) were used, a deeper single cavitation was observed rather than small cavitations. Conclusions The most effective pulse modality as evaluated via stone ablation rate depends on the stone hardness as well as energy and frequency settings. In both hard and soft stones, ablation is more effective when 60 W (2Jx30Hz) power settings and Moses Distance are used. Tailored laser settings in terms of energy and frequency could be set for each case scenario.

Flow imaging using differential Golay encoded ultrasound

In our research we present a new method of differential compression of the Golay encoded ultrasound (DCGEU) in the standard beamforming mode to visualize the slow (<1cm/s) blood mimicking fluid flow in small diameter tubes. The proposed DCGEU method is based on synthesis of several subsequent B-mode frames acquired with certain time intervals (30 ms in this study) followed by the visualization of differential beamformed radio frequency (RF) echoes, which yielded the images of the scatterers moving slowly in the vessel and suppressing the static echoes outside the vessel. In order to extract small backscattered echoes from the vessel area we took an advantage of improved sensitivity of the complementary Golay coded sequences (CGCS).The validation of the proposed DCGEU method was carried out in two stages. In the first one, we compared the flow images in small tubes with a diameter of 1 mm and 2.5 mm, reconstructed from numerically simulated acoustic data for the standard transmission of short pulses and 16-bits long CGCS signals. In the second stage of the research, the experimental data were acquired in a flow phantom with silicone tubes with an internal diameter of 1.5 mm and 4.5 mm and a fluid flow velocity of 0.9 cm/s. The experiments were carried out using preprogrammed Verasonics Vantage™ research ultrasound system equipped with ALT L12-5/50 mm MHz linear array transducer with 7.8 MHz center frequency.It was evidenced both in simulations and experiments that the DCGEU provided a good flow image along the entire length of tubing with virtually angle independent detection in comparison with the conventional short pulse interrogation.

Mutually orthogonal Golay complementary sequences in the simultaneous synthetic aperture method for medical ultrasound diagnostics. An experimental study

Complementary Golay coded sequences (CGCS) have several advantages over conventional short pulse transmitted signals. Specifically, CGCS allow the signal-to-noise ratio (SNR) to be increased. Moreover, due to matched filtering and compression, echoes resembling the short pulse waveform with substantially higher amplitude can be obtained. However, CGCS require two subsequent transmissions to obtain a single compressed signal. This decreases the data acquisition rate and the frame rate of ultrasound imaging by two-fold. To alleviate this problem, mutually orthogonal Golay complementary sequences (MOGCS) can be used. MOGCS allow the simultaneous transmission of two CGCS pairs to be implemented, yielding the acoustic data for two image frames in one data acquisition cycle.The main objective of this work was an experimental study of the most crucial parameters of the received acoustic signals, e.g. the signal-to-noise ratio (SNR), the side-lobes level (SLL) of the signal and the axial resolution, obtained from simultaneous transmission of two pairs of CGCS comprising a MOGCS set to demonstrate their feasibility of being used in ultrasonography. For this purpose, a simultaneous synthetic transmit aperture method (SSTA) was proposed. The SSTA is based on MOGCS transmission and simultaneous reconstruction of two image frames from a single data acquisition cycle. This doubles the image reconstruction rate in comparison with conventional CGCS signals.In this paper, the ultrasound data from a perfect reflector, commercial phantoms and in vivo measurements were analysed. Two 16-bit long CGCS pairs comprising the MOGCS set were programmed and transmitted using the Verasonics Vantage™ research ultrasound system equipped with a Philips ATL L7-4 linear array ultrasound probe. It was shown that the signal parameters and overall quality of reconstructed B-mode images did not deteriorate when using the MOGCS in comparison to the conventional CGCS and short pulse signals explored so far.

Ultra-Short Pulsed Laser Deposition of Oxides, Borides and Carbides of Transition Elements

Oxides, borides and carbides of the transition elements are materials of great interest from a technologic point of view. Many of these materials are used in the form of thin films, so several techniques are commonly used to deposit them. Among these techniques, Pulsed Laser Deposition (PLD) performed using ultra-short pulse lasers, mainly fs lasers, presents unique characteristics in respect to PLD performed using conventional short pulse lasers. Indeed, the films deposited using fs PLD are often nanostructured, and this technique often allows the target stoichiometry to be transferred to the films. In this work, we will review the use of ultra-short PLD in the production of films obtained from transition metal oxides, borides and carbides, evidencing the advantages offered by this technique, together with the problems arising with some of the studied systems. We conclude that even if ultra-short PLD is surely one of the most important and useful deposition techniques, it also presents limits that cannot be ignored.

A novel coding scheme and its application to ultrafast ultrasound imaging

Ultrafast ultrasound imaging using plane or diverging waves, not focused beams, has enabled quantitative assessment of biological tissue elasticity, kinematics, and hemodynamics beyond anatomical information in the past decade. However, its sonographic signal-to-noise ratio (SNR) and penetration depth are limited by insufficient energy delivery under safety limits. We hereby propose a novel coding scheme and apply it to ultrafast ultrasound imaging to increase the SNR without compromising the spatial resolution and frame rate. In our coded ultrafast ultrasound imaging scheme, each transmit is a long pulse containing N (N = 2k, k = 0, 1, 2,…) waves with short time intervals and polarity coefficients of + 1 or −1, instead of the conventional short pulse with single wave. In reception, a linear decoding scheme comprised of addition, subtraction and delay operations is devised to recover N times higher intensity backscattered signals to gain SNR of 10·log10(N). Experimental results acquired by the Verasonics Vantage system from the calibration phantom and in vivo human back muscle show that the proposed method using two transmits of N = 32 waves achieves 13.0 ± 0.5 dB SNR improvement at 40 mm depth at 4000 frames/sec, leading to better image contrast and larger penetration depth.

Effect of frequency characteristic of excitation pulse on lateral spatial resolution in coded ultrasound imaging

Recently, portable ultrasonic diagnostic equipment has frequently been used in clinical situations. The use of portable ultrasonic diagnostic equipment expands various diagnosis areas, such as remote medical diagnosis, and emergent diagnosis at disaster. It is expected that portable ultrasonic diagnostic equipment will be used more frequently in the future. To make ultrasonic diagnostic equipment portable, the number of transducer elements in an ultrasonic probe should be reduced significantly. Therefore, the transmit–receive sensitivity of the ultrasonic probe is degraded. For the improvement of the signal-to-noise ratio (SNR) of the received ultrasonic echo, coded excitation was introduced in ultrasonic imaging. Owing to pulse compression applied to the received echo signal, its SNR significantly improved without the degradation of the range spatial resolution. However, the lateral spatial resolution in coded ultrasound imaging has not been investigated in previous studies. The present study showed that the lateral resolution in coded ultrasound imaging using a typical code, 5-bit Barker code, was worse than that using a conventional short pulse. Such degradation was discussed in terms of the frequency characteristics of the impulse response of the ultrasonic transducer and the excitation pulse. Also, the Gaussian phase coherence factor was introduced as one of the methods to overcome such degradation in lateral spatial resolution.

Single laser pulse generates dual photoacoustic signals for differential contrast photoacoustic imaging

Photoacoustic sensing and imaging techniques have been studied widely to explore optical absorption contrast based on nanosecond laser illumination. In this paper, we report a long laser pulse induced dual photoacoustic (LDPA) nonlinear effect, which originates from unsatisfied stress and thermal confinements. Being different from conventional short laser pulse illumination, the proposed method utilizes a long square-profile laser pulse to induce dual photoacoustic signals. Without satisfying the stress confinement, the dual photoacoustic signals are generated following the positive and negative edges of the long laser pulse. More interestingly, the first expansion-induced photoacoustic signal exhibits positive waveform due to the initial sharp rising of temperature. On the contrary, the second contraction-induced photoacoustic signal exhibits exactly negative waveform due to the falling of temperature, as well as pulse-width-dependent signal amplitude. An analytical model is derived to describe the generation of the dual photoacoustic pulses, incorporating Gruneisen saturation and thermal diffusion effect, which is experimentally proved. Lastly, an alternate of LDPA technique using quasi-CW laser excitation is also introduced and demonstrated for both super-contrast in vitro and in vivo imaging. Compared with existing nonlinear PA techniques, the proposed LDPA nonlinear effect could enable a much broader range of potential applications.

A Single-Stage Stand-Alone Photovoltaic Energy System With High Tracking Efficiency

A single-stage flyback PV inverter with maximum power point tracking (MPPT) high-speed capability is presented in this paper. The proposed stand-alone photovoltaic energy system (SAPES) incorporates the use of hybrid MPPT to ensure peak energy harvesting under all weather conditions. The proposed hybrid MPPT method combines the conventional short current pulse (SCP) MPPT method and the perturb and observe (P&O) methods. Measurement of the offline parameter [short-circuit current of PV module ( ${I}_{\rm sc}$ )] for SCP is made on the basis of the difference between the offline parameter and the instantaneous current of PV module ( ${I}_{\rm pv}$ ). Use of a single power conversion stage is achieved by employing a modified flyback inverter operating in a discontinuous-conduction mode. The control structure of the conventional flyback inverter is also modified to incorporate the hybrid MPPT algorithm. The proposed SAPES method is tested using computer-aided simulations and real-time hardware in the loop experimentation using the dSPACE DS1104 board. The results obtained using the proposed systems are better than those for the conventional algorithms under the environmental conditions tested.

Ultrasound elasticity imaging using Golay code

Ultrasound elasticity imaging is becoming a new diagnostic tool for clinicians to detect lesions or cancers in tissues. In this paper, Golay code is applied to elasticity imaging to improve its quality. Phase-zero algorithm is employed as the displacement estimator, and the amplitude modulation correction location estimate method is used as the location estimator. We compared the performance of Golay code and the conventional short pulse in simulation method. The simulation results demonstrate that Golay code can achieve higher elastographic signal-to-noise ratio (SNRe) than the short pulse in low echo signal-to-noise ratio (eSNR) conditions, because the eSNR gain with Golay code increases the accuracy of the displacement estimates. However, in high eSNR conditions, Golay code performs worse than the short pulse, because the range sidelobe level of Golay code will decrease the SNRe and the performance of Golay code depends mainly on its range sidelobe level in high eSNR conditions. Therefore, the optimal conditions for Goaly code to be used in elasticity imaging are the low eSNR, great depth or high attenuation conditions.

Digital passband processing of wideband-modulated optical signals for enhanced underwater imaging.

Radar modulation, demodulation, and signal processing techniques have been merged with laser imaging to enhance visibility in murky underwater environments. The modulation provides a way to reject multiple scattered light that would otherwise reduce image contrast and resolution. Recent work has focused on the use of wideband modulation schemes and digital passband processing to resolve range details of an underwater scene. Use of the CLEAN algorithm has also been investigated to extract object features that are obscured by scattered light. Results from controlled laboratory experiments show an improvement in the range resolution and accuracy of underwater imagery relative to data collected with a conventional short pulse system.

Coded excitation plane wave imaging for shear wave motion detection.

Plane wave imaging has greatly advanced the field of shear wave elastography thanks to its ultrafast imaging frame rate and the large field-of-view (FOV). However, plane wave imaging also has decreased penetration due to lack of transmit focusing, which makes it challenging to use plane waves for shear wave detection in deep tissues and in obese patients. This study investigated the feasibility of implementing coded excitation in plane wave imaging for shear wave detection, with the hypothesis that coded ultrasound signals can provide superior detection penetration and shear wave SNR compared with conventional ultrasound signals. Both phase encoding (Barker code) and frequency encoding (chirp code) methods were studied. A first phantom experiment showed an approximate penetration gain of 2 to 4 cm for the coded pulses. Two subsequent phantom studies showed that all coded pulses outperformed the conventional short imaging pulse by providing superior sensitivity to small motion and robustness to weak ultrasound signals. Finally, an in vivo liver case study on an obese subject (body mass index = 40) demonstrated the feasibility of using the proposed method for in vivo applications, and showed that all coded pulses could provide higher SNR shear wave signals than the conventional short pulse. These findings indicate that by using coded excitation shear wave detection, one can benefit from the ultrafast imaging frame rate and large FOV provided by plane wave imaging while preserving good penetration and shear wave signal quality, which is essential for obtaining robust shear elasticity measurements of tissue.

Enhanced ultrasound strain imaging using chirp-coded pulse excitation

To improve the quality of ultrasound strain imaging, chirp-coded pulse excitation which can enhance echo signal-to-noise ratio (eSNR) was used. The effects of various factors on chirp-coded strain imaging were investigated. Five chirp schemes were designed to investigate the relationship of the range side lobe level (RSLL), main lobe width and energy for strain imaging. We use phase zero method with amplitude modulation correction as strain estimator. The simulation results demonstrate that elastographic signal-to-noise ratio (SNRe) for chirp pulse decreases with the RSLL and the main lobe width, and that there is a tradeoff among choosing a high-energy tapering window function, reducing the RSLL and narrowing the main lobe in designing a chirp scheme for strain imaging. Chirp pulse performs much better than conventional short pulse in low eSNR, great depth or high attenuation conditions due to the increased eSNR with it. However, in high eSNR condition, the increased eSNR with chirp pulse does not improve SNRe, and the performance of chirp pulse mainly depends on the RSLL and main lobe width. Some chirp schemes still achieve higher SNRe than short pulse in high eSNR condition, because these chirp schemes have narrower main lobe than short pulse and have very low RSLL. Chirp pulse has better lesion detectability and axial strain resolution than short pulse especially in low eSNR condition, because chirp pulse can use shorter window length to get the same SNRe that is achieved by short pulse. Within the scope of five window functions (Tukey, Lanczos, Parzen, Dolph–Chebyshev and Kaiser), we tried to find the optimal chirp scheme which is possibly the combination of chirp pulse excitation with 40% tapered Tukey window and matched compression filter. A commercial elastic phantom experiment on a freehand strain imaging system further validates the superior performance of chirp pulse.

Development of Chip Separation Technique for InGaN-Based Light Emitting Diodes

We compare the chip separation techniques for InGaN-based light emitting diode (LED), which are prepared by conventional diamond tip scribing and short pulse laser scribing with pulse width of nanosecond or femtosecond. Differently from the diamond tip scribing, the laser scribing does not suffer from the shallow scribing depth, which usually results in a failure in cleaving the wafer along the scribed line. Furthermore, the LED scribed by using a femtosecond pulse laser exhibits an 11% enhancement in the output power at 20 mA, compared to that by using a nanosecond pulse laser. This is because the femtosecond laser scribing not only eliminates particle generation, but also minimizes the effect of thermal damage on the sapphire substrate. The femtosecond laser also offers a high speed, high yield, and hence low cost scribing process due to its extremely short pulse width and deep focal length.

Numerical Investigation of Superharmonic Imaging Using Chirp Excitation

At present, along with the fundamental method of ultrasound imaging, tissue harmonic imaging based on imprinting of the second harmonic component is established as the standard method of ultrasound imaging. Recently introduced superharmonic imaging (SHI) has an edge over this standard method and has improved spatial resolution. To explore further advancement in ultrasound imaging for enhanced resolution and penetration depth. SHI is the summation of higher harmonics generated due to nonlinear tissue propagation of ultrasound. Here, the addition of third, fourth and fifth harmonics has been done for the superharmonic field. The modeling of nonlinear wave propagation is done in frequency domain on the basis of the second order operator splitting approach. The higher harmonic components are picked up using the inverse filters. The standard evaluation parameters were significantly improved with linear frequency modulation (FM) chirp. The axial level was 2.8 times higher with linear FM chirp as compared to conventional short pulse. Minor benefits such as a 5% increase in beam width and suppression of side lobes were also recorded. This numerical investigation confirms that linear FM chirp in SHI provides better resolution and penetration depth. State of the art fundamental and second harmonic ultrasound imaging techniques were also improved with linear FM chirp as compared with conventional excitation. SHI is superior to the existing diagnostic ultrasound techniques.

Characterisation of virus transport and attenuation in epikarst using short pulse and prolonged injection multi-tracer testing

Attenuation processes controlling virus fate and transport in the vadose zone of karstified systems can strongly influence groundwater quality. This research compares the breakthrough of two bacteriophage tracers (H40/1 and T7), with contrasting properties, at subsurface monitoring points following application onto an overlying composite sequence of thin organic soil and weathered limestone (epikarst). Short pulse multi-tracer test results revealed that T7 (Source concentration, Co = 1.8 × 10 6 pfu/mL) and H40/1 (Co = 5.9 × 10 6 pfu/mL) could reach sampling points 10 m below ground less than 30 min after tracer application. Contrasting deposition rates, determined from simulated tracer responses, reflected the potential of the ground to differentially attenuate viruses. Prolonged application of both T7 (Co = 2.3 × 10 4 pfu/mL) and H40/1 (Co = 1.3 × 10 5 pfu/mL) over a five hour period during a subsequent test, in which ionic strength levels observed at monitoring points rose consistently, corresponded to a rapid rise in T7 levels, followed by a gradual decline before the end of tracer injection; this reflected reaction-limited deposition in the system. T7's response contrasted with that of H40/1, whose concentration remained constant over a three hour period before declining dramatically prior to the end of tracer injection. Subsequent application of lower ionic strength tracer-free flush water generated a rapid rise in H40/1 levels and a more gradual release of T7. Results highlight the benefits of employing prolonged injection multi-tracer tests for identifying processes not apparent from conventional short pulse tests. Study findings demonstrate that despite rapid transport rates, the epikarst is capable of physicochemical filtration of viruses and their remobilization, depending on virus type and hydrochemical conditions.

Osteoblast behavior on polytetrafluoroethylene modified by long pulse, high frequency oxygen plasma immersion ion implantation

Polytetrafluoroethylene (PTFE) is a commonly used medical polymer due to its biological stability and other attractive properties such as high hardness and wear resistance. However, the low surface energy and lack of functional groups to interact with the cellular environment have severely limited its applications in bone or cartilage replacements. Plasma immersion ion implantation (PIII) is a proven effective surface modification technique. However, when conducted on polymeric substrates, conventional PIII experiments typically employ a low pulsing frequency and short pulse duration in order to avoid sample overheating, charging, and plasma sheath extension. In this paper, a long pulse, high frequency O 2 PIII process is described to modify PTFE substrates by implementing a shielded grid in the PIII equipment without these aforementioned adverse effects. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements are carried out to reveal the surface effects of PTFE after long pulse, high frequency O 2 PIII and the results are compared to those obtained from conventional short pulse, low frequency O 2 PIII, O 2 plasma immersion, and the untreated control samples. Our results show that less oxygen-containing, rougher, and more hydrophobic surfaces are produced on PTFE after long pulse, high frequency O 2 PIII compared to the other 2 treatments. Cell viability assay, ALP activity test, and real-time PCR analysis are also performed to investigate the osteoblast behavior. It is clear that all 3 surface modification techniques promote osteoblast adhesion and proliferation on the PTFE substrates. Improvements on the ALP, OPN, and ON expression of the seeded osteoblasts are also obvious. However, among these treatments, only long pulse, high frequency O 2 PIII can promote the OCN expression of osteoblasts when the incubation time is 12 days. Our data unequivocally disclose that the long pulse, high frequency O 2 PIII technique is better than the other two types of traditional plasma treatment in the development of PTFE for bone or cartilage repair.

Chromium carbide thin films deposited by ultra-short pulse laser deposition

Pulsed laser deposition performed by a laser with a pulse duration of 250 fs has been used to deposit films from a Cr 3C 2 target. Due to the different processes involved in the laser ablation when it is performed by an ultra-short pulse source instead of a conventional short pulse one, it has been possible to obtain in vacuum films containing only one type of carbide, Cr 3C 2, as shown by X-ray photoelectron spectroscopy. On the other hand, Cr 3C 2 is not the only component of the films, since a large amount of amorphous carbon is also present. The films, deposited at room temperature, are amorphous and seem to be formed by the coalescence of a large number of particles with nanometric size. The film composition can be explained in terms of thermal evaporation from particles ejected from the target.

Nonintrusive Fiber Monitoring of TDM Optical Networks

This paper introduces a new embedded nonintrusive fiber-monitoring technique for time-division-multiplexing optical networks. It allows an optical transmitter to characterize the fiber plant from reflections caused by data bursts transmitted across the network instead of dedicated test signals. The probing is performed with minimal burden on data traffic so that many measurements can be averaged to improve accuracy. The method is very suitable for embedded optical time-domain reflectometers (OTDR), which reuse a network node's optical data transmitter for OTDR excitations and embed a reflectometer inside the fiber endpoint. This paper models the OTDR with Laplace transforms, an approach previously unpursued, after which it is explained how reflections from multiple data bursts with arbitrary width can be converted into one normalized format. This new class of OTDR excites the fiber with a negative step of light instead of the conventional short pulse. The signal-to-noise ratio (SNR) for backscatter and Fresnel reflections caused by the negative step and pulse are compared theoretically. It is shown that negative-step OTDR breaks the tradeoff between excitation pulsewidth and distance resolution, has a natural separation between fiber backscatter and Fresnel reflectors, and improves the SNR of nonreflective events

Parameter optimization of pulse compression in ultrasound imaging systems with coded excitation

A linear array imaging system with coded excitation is considered, where the proposed excitation/compression scheme maximizes the signal-to-noise ratio (SNR) and minimizes sidelobes at the output of the compression filter. A pulse with linear frequency modulation (LFM) is used for coded excitation. The excitation/compression scheme is based on the fast digital mismatched filtering. The parameter optimization of the excitation/compression scheme includes (i) choice of an optimal filtering function for the mismatched filtering; (ii) choice of an optimal window function for tapering of the chirp amplitude; (iii) optimization of a chirp-to-transducer bandwidth ratio; (iv) choice of an appropriate n-bit quantizer. The simulation results show that the excitation/compression scheme can be implemented as a Dolph–Chebyshev filter including amplitude tapering of the chirp with a Lanczos window. An example of such an optimized system is given where the chirp bandwidth is chosen to be 2.5 times the transducer bandwidth and equals 6 MHz: The sidelobes are suppressed to −80 dB, for a central frequency of 4 MHz, and to −94 dB, for a central frequency of 8 MHz. The corresponding improvement of the SNR is 18 and 21 dB, respectively, when compared to a conventional short pulse imaging system. Simulation of B-mode images demonstrates the advantage of coded excitation systems of detecting regions with low contrast.