Range Of Pulse Repetition Frequencies Research Articles

Focused ultrasound pulse repetition frequency impacts bubble activity and tracer delivery during blood-brain barrier opening

The impact of pulse repetition frequency (PRF) on microbubble activity and blood-brain barrier (BBB) opening, for a fixed total number of pulses, is unclear. We studied bubble response to a range of PRFs (0.125, 0.25, 0.5, 1, and 2 Hz) by monitoring bubble emissions during focused ultrasound (FUS) sonications of the rat brain (274.3 kHz). When a location was treated with a test PRF and a control PRF of 1 Hz, at the same pressure, the change in the harmonic amplitude compared to the control decreased with increasing PRF, with a median change of 73.8% at 0.125 Hz and −38.3% at 2 Hz. Significant changes were not observed with repeated sonications at the same PRF. Furthermore, between PRFs of 0.25 and 1 Hz, no difference was observed in the threshold for broadband emissions, which are linked to the potential for tissue damage. Fluorescence imaging was used to estimate the concentration of Trypan Blue (TB) dye following a fixed-pressure 75-pulse exposure for PRFs of 1 and 0.25 Hz in rats. A 0.25 Hz PRF led to a 68.2% increase in the mean concentration measured at the target, with a 53.9% increase in the mean harmonic sum compared with a 1 Hz PRF. Finally, a harmonic emissions-based controller at a PRF of 0.25 Hz yielded similar TB delivery, with fewer instances of petechiae observed through histology, compared to the same controller at 1 Hz. These results may be adapted to improve the clinical safety margin of FUS-mediated BBB opening and to improve the sensitivity to detecting small harmonic signals from cavitating microbubbles.

Experimental Investigation of Excitation Strategies for Erosion by Cavitation Histotripsy

Abstract Cavitation histotripsy has been applied to the disintegration on the surface of soft tissue in a well-controlled manner. Its performance was assumed to be determined by the acoustic pressure alone. Long pulse duration with low pulse repetition frequency (PRF) can also successfully generate erosion. This study was designed to investigate the excitation strategies for cavitation histotripsy-induced erosion. The erosion area and volumes produced by cavitation histotripsy on the alginate gel phantom using single-frequency, dual-frequency, and two pulsed excitations at the same power output at the PRF of 1 Hz and 200 Hz were compared. Dual-frequency excitation can improve the erosion at all PRFs, while pulsed excitations decrease it at the PRF of 200 Hz. Using both pulsed and dual-frequency excitations has more erosion areas than using single-frequency at a PRF of 1 Hz. In comparison, although the induced erosion areas using the pulsed excitations are larger than those of single-frequency at the PRF of 200 Hz, the erosion volumes are much lower than those of dual-frequency excitation. It suggests that a sufficient long pulse duration is another important factor for the performance of cavitation histotripsy. Dual-frequency excitation or amplitude modulation by the low-frequency sinusoidal envelope can achieve more erosion than that produced by single-frequency excitation at the same power output in a wide range of PRFs.

Experimental Investigation on Microsecond Pulsed Plasma Supported Biogas Combustion

Abstract This paper evaluates the effect of a microsecond pulsed plasma (MPP) on the stabilization and emission characteristics of non-premixed biogas/air flames with various CO2 contents. The MPP is generated by a unique DC-pulsed power generator providing high voltage (HV) pulses over a wide range of pulse repetition frequencies (PRFs). The burner configuration is made up of two concentric tubes in which a swirler is placed inside the annular part, ensuring the oxidizer's rotation. The central tube delivers the fuel through an injector placed close to the burner exit. Electrical diagnostics, including voltage, were performed. OH* chemiluminescence measurements were done to describe the structure and stability of the flame. Results showed that plasma generated by microsecond HV pulses can improve flame stability. In this regard, the distribution of key active species in the burner was studied via optical emission spectroscopy (OES). The results revealed that the pulsed plasma generates chemically active species such as excited N2*, CH*, OH* molecules, and H* and O* atoms, thereby improving flame stability. The dependence of the emitted species intensities on plasma parameters was investigated in detail. It is demonstrated that MPP can drastically enhance the dynamic flame stability of swirling non-premixed biogas flames, especially at lean operating conditions. In addition, NOx and CO emissions were studied over a wide range of pulse repetition frequencies. It is seen that the pulsed plasma increases NOx emission slightly and significantly reduces CO concentration in the flue gases.

Investigating the impact of skull vibrations on motor responses to focused ultrasound neuromodulation in small rodents and methods to mitigate them

Objective. Focused ultrasound (FUS) neuromodulation non-invasively alters brain activity, likely via acoustic radiation force with dynamics of the pulse repetition frequency (PRF). PRF impacts neuromodulation as well as indirect auditory activation, a confound linked to skull vibrations. This study aimed to minimize these vibrations, by adjusting and randomizing PRF, and determine their impact on FUS-induced motor activity. We hypothesized that: the skull would vibrate most at a specific PRF; randomizing PRF would reduce skull vibrations without affecting motor responses; and FUS would yield motor activity while actuator-induced skull vibrations would not. Approach. Three objectives were studied in parallel using C57Bl/6 mice (n = number of mice used per objective). First, skull vibration amplitude, measured as a percentage of maximum amplitude per treatment, was recorded via contact microphone over a range of PRFs to assess the PRF-dependency of skull vibrations (n = 19). Vibrations were then compared between random and fixed PRFs (n = 15). Lastly, motor responses were compared between fixed 1.5 kHz PRF FUS, random PRF FUS, air-puff stimulation, sham stimulation, and vibration induction via piezoelectric actuator (n = 30). Main Results. The study found amplitude peaked at 1.51 kHz (88.1 ± 11.5%), significantly higher than at 0.54 kHz (75.5 ± 15.1%; p = 0.0149). Random PRF reduced amplitude by 4.2% (p = 0.0181). Motor response rates to actuator-induced skull vibrations at the PRF (5.73 ± 6.96%) and its third harmonic (22.9 ± 22.7%) were not significantly different than sham (14.1 ± 11.6%), but lower than FUS (70.2 ± 16.3%; p < 0.0001). Significance. Based on these results, PRF near 0.5 kHz may best avoid skull vibrations, while random PRF could be utilized to slightly reduce vibration amplitude. The results also suggested that skull vibrations likely do not significantly impact motor responses to FUS neuromodulation.

Spaceborne Radar Sensor Architecture for Debris Detection and Tracking

This article delves into a novel spaceborne radar payload transceiver for augmenting space-based monitoring capabilities for near-Earth space situational awareness. The sensor complex data hypercube structure and sensor timing hierarchies in surveillance mode are addressed along with a suitable low pulse repetition frequency range and range-rate search with a pause while scan collection strategy. A bespoke active electronically scanned array-based Pulse Doppler Radar sensor in the Ka-band is then defined in order to cope with the significant Doppler stress characterizing a burst of echoes from hypervelocity debris targets. Before feeding the onboard Bayesian tracker, the complex data hypercube is processed by means of a Doppler filter bank including pulse compression in cascade with a constant false alarm rate-like block.

MnCl2 laser with pulse repetition frequency up to 125 kHz

This paper considers the operating parameters of 24 cm3 active volume MnCl2-laser in a wide range of PRF (pulse repetition frequency). The maximum average power reaches 1 W at 17.6 kHz PRF and 0.2% efficiency. The laser generation on manganese metal at 125 kHz PRF has been obtained for the first time. The pumping conditions under which the output power of the visible spectral range starts exceeding the output power at IR lines have been determined. In particular, the electric field strength is calculated for the given GDT (gas discharge tube) parameters and buffer gas pressure. The dependence between the output power and pumping power is shown to be linear. The condition provides the change in IR and VIS output power relation is determined.

Microbubble Radiation Force-Induced Translation in Plane-Wave Versus Focused Transmission Modes.

Due to the primary radiation force, microbubble displacement has been observed previously in the focal region of single-element and array ultrasound probes. This effect has been harnessed to increase the contact between the microbubbles and targeted endothelium for drug delivery and ultrasound molecular imaging. In this study, microbubble displacements associated with plane-wave (PW) transmission are thoroughly investigated and compared to those obtained in focused-wave (FW) transmission over a range of pulse repetition frequencies, burst lengths (BLs), peak negative pressures, and transmission frequencies. In PW mode, the displacements, depending upon the experimental conditions, are in some cases consistently higher (e.g., by 28%, when the longest BL was used at PRF = 4 kHz), and the axial displacements are spatially more uniform compared to FW mode. Statistical analysis on the measured displacements reveals a slightly different frequency dependence of statistical quantities compared to transient peak microbubble displacements, which may suggest the need to consider the size range within the tested microbubble population.

밀리미터파(Ka 밴드) 복합모드 탐색기용 고출력 펄스형 진행파관 증폭기(TWTA) 설계 및 제작

The traveling wave tube amplifier (TWTA), which can be applied to the Ka-band millimeter-wave multi-mode seeker, consists of an high voltage power supply(HVPS), a grid modulator, a command and control, and an RF assembly. We designed a power supply that generates a −17.9 kV high voltage by synchronizing the pulse repetition frequency(PRF) and power supply switching frequency(i.e. synchronization frequency), and a high-speed grid-switching modulator for RF pulse modulation. The TWTA, which is fabricated through miniaturization with a volume of 3.18 L, has high pulse switching characteristics of up to 18.5 ns. The maximum rise/fall time of the grid on/bias signal and peak power is more than 564.9 W. Moreover, an excellent spurious performance of −68.4 dBc or less was confirmed within the range of PRF and PRF/2.

Beamforming in SAR satellites for nadir echo suppression

Here, nadir echo suppression (NES) in synthetic aperture radar (SAR) satellites is studied in detail. Moreover, advantages of suppressing the nadir echo in space domain, by beamforming, rather than in time domain, which imposes stringent restrictions on radar timing, are addressed. Performing NES in time domain restricts the swath width, as well as the selection of the pulse repetition frequency (PRF). It is shown that the spatial NES could increase the swath width by ~25%. It also increases the freedom in PRF selection, which is useful in multiple-swath imaging modes and high-resolution wide-swath satellites. Furthermore, it is useful in confining the operating PRF range of SAR satellites, which offers several advantages, such as better thermal stability, more predictability of power consumption, and efficient use of downlink bandwidth. Simulation results show the effectiveness of beamforming-based NES under different situations, namely different off-nadir angles, and land types. Additionally, the conventional formula used for calculation of range ambiguity to signal ratio (RASR) neglects the nadir echo, so it could not be used in the performance assessment. Therefore, the authors derive a generalised formulation for RASR and prove that it converges to the conventional formula at medium-to-high off-nadir angles.

The effect of nanosecond pulsed high frequency discharges on the temperature evolution of ignition kernels

Nanosecond pulsed high frequency discharges (NPHFD) have attracted interest as a means of enhancing ignition probability and ignition kernel growth rates. Previous studies have shown that increasing the pulse repetition frequency (PRF) of a NPHFD burst can result in enhanced probability of ignition relative to lower values of PRF. The cause of this enhanced ignition probability is not well understood, in part, because the temperature evolution of ignition kernels have not been measured. In this work, an inverse deconvolution technique was developed, evaluated, and used to determine the temporal evolution of the temperature and volume of ignition kernels initiated by NPHFD and single pulse discharges. Radiation intensity measurements of ignition kernels were compared to modeled values to determine path-averaged temperatures. Comparisons of values derived using the deconvolution technique and measured temperatures above a McKenna burner typically agreed within 4%. When the technique was applied to ignition kernels initiated by NPHFD over a range of PRF, it was observed that increasing the PRF of a discharge sequence increased the initial rate of change of the temperature as compared to lower PRF. Additionally, kernels generated with high PRF discharges achieved higher initial peak temperatures than those generated by lower PRF discharges. A relaxation in temperature was observed following the peak, which was followed by a subsequent temperature increase in all cases generated with PRF > 10 kHz. PRF above 5 kHz result in larger initial kernel volumes and volume rates of change compared to those observed for lower PRF values. This is attributed to pulse-to-pulse coupling during the energy deposition process. The observed relationship between kernel temperature, volume, and PRF help to explain ignition probability measurements reported previously. Findings from the effort illustrate how the rate of energy deposition through pulse-to-pulse coupling can significantly enhance ignition through elevated temperatures.

A PSF-Shape-Based Beamforming Strategy for Robust 2D Motion Estimation in Ultrafast Data

This paper presents a framework for motion estimation in ultrafast ultrasound data. It describes a novel approach for determining the sampling grid for ultrafast data based on the system’s point-spread-function (PSF). As a consequence, the cross-correlation functions (CCF) used in the speckle tracking (ST) algorithm will have circular-shaped peaks, which can be interpolated using a 2D interpolation method to estimate subsample displacements. Carotid artery wall motion and parabolic blood flow simulations together with rotating disk experiments using a Verasonics Vantage 256 are used for performance evaluation. Zero-degree plane wave data were acquired using an ATL L5-12 (fc = 9 MHz) transducer for a range of pulse repetition frequencies (PRFs), resulting in 0–600 µm inter-frame displacements. The proposed methodology was compared to data beamformed on a conventionally spaced grid, combined with the commonly used 1D parabolic interpolation. The PSF-shape-based beamforming grid combined with 2D cubic interpolation showed the most accurate and stable performance with respect to the full range of inter-frame displacements, both for the assessment of blood flow and vessel wall dynamics. The proposed methodology can be used as a protocolled way to beamform ultrafast data and obtain accurate estimates of tissue motion.

Modified Multichannel Reconstruction Method of SAR With Highly Nonuniform Spatial Sampling

The traditional filter bank algorithm (FBA) for a multichannel synthetic aperture radar (SAR) system can provide an unambiguous recovery. However, the FBA may result in a seriously signal-to-noise ratio (SNR) loss or might even fail to reconstruct the spectrum if there is highly nonuniform sampling in azimuth. It is the general case because pulse repetition frequency (PRF) cannot be selected flexibly for spaceborne imaging system. In this paper, a new algorithm for processing highly nonuniform sampled SAR data to improve the SNR is presented. The proposed method is based on the operation of ambiguity-index screening (AIS) and the minimum mean square error (MMSE) criterion. It can be utilized to reconstruct the spectrum even in the presence of highly nonuniform sampling or coinciding sampling. The approach can significantly improve the SNR especially for the highly nonuniform sampling, and the azimuth ambiguity-to-signal ratio obtained by the proposed method is superior to the traditional one for a wide range of PRF. The proposed algorithm is validated by the simulated data and the real two-channel radar raw data acquired by C-band airborne SAR system with a bandwidth of 210 MHz, which is designed by the Department of Space Microwave Remote Sensing System, Institute of Electronics, Chinese Academy of Sciences.

Effects of pulsation frequency and energy deposition on ignition using nanosecond repetitively pulsed discharges

The effects of pulse repetition frequency (PRF) and energy deposition was investigated using nanosecond repetitively pulsed discharges in a constant volume combustion chamber. Quiescent lean propane-air mixtures (equivalence ratio of 0.7) at 0.2MPa were ignited using fixed total energy deposition and number of pulses across the range of PRF from 2 to 90kHz. Individual pulse energies were measured by electrical probes, and the ignition time was quantified through the use of high-frame-rate intensified imaging of the chemiluminescence. For fixed inter-electrode gap and constant total energy, the ignition time was minimized at a certain pulsation frequency, with higher PRF being detrimental and resulting in longer ignition delay times. Decreasing the total energy deposited in the mixture shifted the minimum ignition time to higher PRF. On the contrary, increasing the gap distance at constant total energy allowed for the same minimum ignition time to be achieved at significantly lower PRF. The minimum ignition delay times for fixed inter-electrode gap distance were similar for a given amount of mean energy deposited per unit time. The effect of energy deposition, PRF, and inter-electrode gap was attributed to the competition between characteristic recirculation time from the discharge-induced flowfield and the inter-pulse time.

High-speed CuBr brightness amplifier beam profile

This paper addresses the experimental study of the beam profile of the CuBr brightness amplifier operating at a wide range of pulse repetition frequencies. The use of a medium-size gas discharge tube (2cm) ensures the operation of the brightness amplifier both at typical PRFs (520kHz) and at higher PRFs (up to 100kHz), either with or without HBr additive. The effect of the active additive on the beam profile is demonstrated. The testing results on kinetic modeling of radial processes in the laser (brightness amplifier) plasma are also discussed.

SU‐G‐JeP4‐01: An Assessment of a Microsoft Kinect V2 Sensor for Voluntary Breath‐Hold Monitoring in Radiotherapy

Purpose:To determine whether the Microsoft Kinect Version 2 (Kinect v2), a commercial off‐the‐shelf (COTS) depth sensors designed for entertainment purposes, were robust to the radiotherapy treatment environment and could be suitable for monitoring of voluntary breath‐hold compliance. This could complement current visual monitoring techniques, and be useful for heart sparing left breast radiotherapy.Methods:In‐house software to control Kinect v2 sensors, and capture output information, was developed using the free Microsoft software development kit, and the Cinder creative coding C++ library. Each sensor was used with a 12m USB 3.0 active cable. A solid water block was used as the object. The depth accuracy and precision of the sensors was evaluated by comparing Kinect reported distance to the object with a precision laser measurement across a distance range of 0.6m to 2.0 m. The object was positioned on a high‐precision programmable motion platform and moved in two programmed motion patterns and Kinect reported distance logged. Robustness to the radiation environment was tested by repeating all measurements with a linear accelerator operating over a range of pulse repetition frequencies (6Hz to 400Hz) and dose rates 50 to 1500 monitor units (MU) per minute.Results:The complex, consistent relationship between true and measured distance was unaffected by the radiation environment, as was the ability to detect motion. Sensor precision was &lt; 1 mm and the accuracy between 1.3 mm and 1.8 mm when a distance correction was applied. Both motion patterns were tracked successfully with a root mean squared error (RMSE) of 1.4 and 1.1 mm respectively.Conclusion:Kinect v2 sensors are capable of tracking pre‐programmed motion patterns with an accuracy &lt;2 mm and appear robust to the radiotherapy treatment environment. A clinical trial using Kinect v2 sensor for monitoring voluntary breath hold has ethical approval and is open to recruitment.The authors are supported by a National Institute of Health Research (NIHR) Career Development Fellowship (CDF‐2013‐06‐005). Microsoft Corporation donated three sensors. The views expressed in this publication are those of the author(s) and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health.

ULA-OP 256: A 256-Channel Open Scanner for Development and Real-Time Implementation of New Ultrasound Methods.

Open scanners offer an increasing support to the ultrasound researchers who are involved in the experimental test of novel methods. Each system presents specific performance in terms of number of channels, flexibility, processing power, data storage capability, and overall dimensions. This paper reports the design criteria and hardware/software implementation details of a new 256-channel ultrasound advanced open platform. This system is organized in a modular architecture, including multiple front-end boards, interconnected by a high-speed (80 Gb/s) ring, capable of finely controlling all transmit (TX) and receive (RX) signals. High flexibility and processing power (equivalent to 2500 GFLOP) are guaranteed by the possibility of individually programming multiple digital signal processors and field programmable gate arrays. Eighty GB of on-board memory are available for the storage of prebeamforming, postbeamforming, and baseband data. The use of latest generation devices allowed to integrate all needed electronics in a small size ( 34 cm ×30 cm ×26 cm). The system implements a multiline beamformer that allows obtaining images of 96 lines by 2048 depths at a frame rate of 720 Hz (expandable to 3000 Hz). The multiline beamforming capability is also exploited to implement a real-time vector Doppler scheme in which a single TX and two independent RX apertures are simultaneously used to maintain the analysis over a full pulse repetition frequency range.

声光调Q CO2激光器的输出特性

A compact CO2 laser with short pulse duration,high peak power and high repetition was developed by using an Acousto-optic(AO) Q-switch.The operating principle of the AO Q-switch was analyzed,and the theoretical model of CO2 laser was introduced based on the small-signal gain and saturation intensity.Then the optimal numerical solution of the transmission for an output mirror was given by this model.Based on the compact CO2 laser in our laboratory,the experiments to verify the theoretical result were performed.The optimal coupling output factor gained by the theory and experiments is 39%.The variable law of laser output performance with pulse repetition frequency was researched.Results show that the peak power of the laser will fall down when the pulse repetition frequency is greater than 1 kHz,which is related to the radiation life of CO2 molecular upper energy level,and is limited by the heat effect of AO Q-switch.The range of pulse repetition frequency can be adjusted from 1 Hz to 100 kHz.When the pulse repetition frequency is 1 kHz,the measured pulse duration is 156 ns and the peak power is 10 kW.The laser has good stabilization,and is suitable for a light source used in laser-matter interaction.

Azimuth Phase Center Adaptation on Transmit for High-Resolution Wide-Swath SAR Imaging

Synthetic aperture radar (SAR) systems with multiple receive channels allow for high-resolution wide-swath imaging thus overcoming a fundamental limitation of conventional single-aperture SAR. By using multiple apertures in azimuth, additional samples are received for each transmitted pulse. This allows for a reduced pulse repetition frequency (PRF) thereby enabling a wider swath. However, a nonoptimum PRF is associated with a nonuniform sample spacing in azimuth and needs to be compensated by a multichannel reconstruction algorithm. For strong deviations from the optimum PRF, the inverse character of such an algorithm might result in a degraded performance. This can be overcome by an innovative advanced transmit antenna architecture which allows for a pulse-to-pulse shift of the phase center. Such an antenna enables the adaptive adjustment of the system's phase center positions to the respective PRF, thereby ensuring constant performance over a clearly extended PRF range. In particular, in combination with conventional multichannel processing strategies, this technique represents the next step toward a fully active multiple-input multiple-output (MIMO) SAR and has a great potential for future systems.

High Power, High Quality Phase Conjugate Vanadate Lasers

We presented high quality, high power pico-second output from a diode-side-pumped Nd: YVO4 laser system with a photorefractive phase conjugate mirror. Peak output powers in the range of 2.8-6.8MW were achieved at a pulse repetition frequency range of 0.33-1.0MHz. The corresponding average powers of 18-26W were obtained.

MW ps pulse generation at sub-MHz repetition rates from a phase conjugate Nd:YVO_4 bounce amplifier

We demonstrated high-repetition-rate (sub-MHz) MW pulse generation by combining a picosecond phase conjugate laser system based on a diode-side-pumped Nd:YVO(4) bounce amplifier with a pulse selector based on a RbTiOPO(4) electro-optical modulator. Peak output powers in the range of 2.8-6.8 MW at a pulse repetition frequency range of 0.33-1.0 MHz were achieved at an extraction efficiency of 34-35%.