Effects Of Pulse Repetition Frequency Research Articles

Effects of pulse repetition frequency on bubble cloud characteristics and ablation in single-cycle histotripsy

Objective. Histotripsy is a cavitation-based ultrasound ablation method in development for multiple clinical applications. This work investigates the effects of pulse repetition frequency (PRF) on bubble cloud characteristics and ablative capabilities for histotripsy using single-cycle pulsing methods. Approach. Bubble clouds produced by a 500 kHz histotripsy system at PRFs from 0.1 to 1000 Hz were visualized using high-speed optical imaging in 1% agarose tissue phantoms at peak negative pressures, p-, of 2–36 MPa. Main results. Results showed a decrease in the cavitation cloud threshold with increasing PRF, ranging from 26.7 ± 0.5 MPa at 0.1 Hz to 15.0 ± 1.9 MPa at 1000 Hz. Bubble cloud analysis showed cavitation clouds generated at low PRFs (0.1–1 Hz) were characterized by consistently dense bubble clouds (41.7 ± 2.8 bubbles mm−2 at 0.1 Hz), that closely matched regions of the focus above the histotripsy intrinsic threshold. Bubble clouds formed at higher PRFs measured lower cloud densities (23.1 ± 4.0 bubbles mm−2 at 1000 Hz), with the lowest density measured for 10 Hz (8.8 ± 4.1 bubbles mm−2). Furthermore, higher PRFs showed increased pulse-to-pulse correlation, characteristic of cavitation memory effects; however, bubble clouds still filled the entire volume of the focus due to their initial density and enhanced bubble expansion from the restimulation of residual nuclei at the higher PRFs. Histotripsy ablation assessed through lesion analysis in red blood cell (RBC) phantoms showed higher PRFs generated lesions with lower adherence to the initial focal region compared to low PRF ablations; however, no trend of decreasing ablation efficiency with PRF was observed, with similar efficiencies observed for all the PRFs tested in this study. Significance. Notably, this result is different than what has previously been shown for shock-scattering histotripsy, which has shown decreased ablation efficiencies at higher PRFs. Overall, this study demonstrates the essential effects of PRF on single-cycle histotripsy procedures that should be considered to help guide future histotripsy pulsing strategies.

Thermoacoustic stabilization of a sequential combustor with ultra-low-power nanosecond repetitively pulsed discharges

This study demonstrates the stabilization of a sequential combustor with Nanosecond Repetitively Pulsed Discharges (NRPD). Sequential combustors offer advantages such as enhanced fuel flexibility and broader operational range compared to the conventional combustors. However, thermoacoustic instabilities limit their potential. While passive control strategies like Helmholtz dampers have been utilized, active control methods have been hindered by the lack of robust actuators for harsh conditions with sufficient control authority. This study demonstrates successful suppression of instabilities using NRPD in a lab-scale atmospheric sequential combustor, even at low plasma power (1.1 W, about 1.5×10−3 percent of thermal power of 73.4 kW). We also examine the effect of pulse repetition frequency and plasma generator voltage on NO emissions and the sequential flame topology. Notably, higher plasma power can further enhance suppression, albeit with a slight increase in NO emissions. However, the highest plasma power in our study (81 W, 1.1×10−1 percent of flame thermal power) shows a slight improvement in acoustic suppression while significantly elevating NO emissions. Intriguingly, specific plasma parameter combinations can excite another acoustic mode, necessitating further exploration for optimized control. This pioneering study highlights the potential of ultra-low-power plasma-based thermoacoustic control in sequential combustors, paving the way for enhanced stability and performance.

Numerical modeling of plasma assisted ignition of CH4/O2/He mixture by the nanosecond repetitive pulsed surface dielectric barrier discharge

A nanosecond repetitive pulsed surface dielectric barrier discharge (NRP-SDBD) is a promising method for generation of non-equilibrium plasma with more kinetic activity. This paper presents numerical studies on the effects of pulse repetition frequency (PRF) on low temperature ignition of lean and stoichiometric CH4/O2/He mixture in NRP-SDBD plasma. A skeletal plasma kinetics mechanism is primary developed and calibrated, to mitigate the computationalloads of modeling SDBD discharge by the two-dimensional plasma solver PASSKEy coupled with Poisson's equation. The discharge current was well predicted, meanwhile the sequential and spatial reduced electric field (E/N) is revealed as the ionization wave propagates forward. Special attention is placed on time-evolution of electric field strength used as inputs in simulation of plasma assisted ignition (PAI) conducted by zero-dimensional ZDPlasKin-CHEMKIN code. The principle of PAI on lean and stoichiometric CH4/O2/He mixture via varied PRF is analyzed in details from view of plasma kinetics and chemical kinetics. Ultimately, the important role of plasma active species O, O(1D), O2(a1Δg) on promoting ignition chain reactions is highlighted. It is also concluded that high PRF is beneficial for the accumulation of short lifetime species such as CH4(v13) and O(1D), which participate in the formation of important intermediate CH3.

Effects of pulse repetition frequency on bubble cloud characteristics and ablation for single-cycle histotripsy

Histotripsy is a cavitation-based focused ultrasound ablation method in development for multiple clinical applications. This work investigates the effects of pulse repetition frequency (PRF) on histotripsy bubble cloud characteristics and ablative capabilities for single cycle histotripsy. Bubble clouds produced by a 500 kHz histotripsy system at PRF’s from 0.1 to 1000 Hz were visualized using high speed imaging in 1% agarose phantoms. Cloud images were analyzed to determine bubble cloud density (bubbles/mm2) and pulse-to-pulse bubble correlation. Ablation was assessed through lesion analysis in red blood cell (RBC) phantoms. Results showed cavitation clouds generated at low PRF were characterized by consistently dense bubble clouds (49.6 ± 7.7 bubbles/mm2, 1 Hz) that closely matched regions above the histotripsy intrinsic threshold. Bubble clouds formed at high PRF had significantly lower cloud density (20.0 ± 8.3 bubbles/mm2, 1000 Hz). In addition, bubbles in higher PRF clouds had significantly increased pulse-to-pulse correlation, characteristic of what has been reported as the cavitation memory effect. Results from RBC ablation showed that higher PRF generated lesions had lower adherence to the focal region and less repeatability compared to low PRF ablations. This study demonstrates essential differences in bubble cloud characteristics that will help guide future histotripsy pulsing strategies.

Characterization of time-resolved emission of N2(C) in an atmospheric pressure nanosecond pulsed air-plasma using streak spectroscopy

A new optical emission spectroscopy (OES) system using a streak camera (streak-OES) is developed to obtain highly time-resolved OES measurements of nanosecond-duration pulsed air-plasmas at atmospheric pressure. The newly developed system is used to measure the time evolution of the emission of N2(C), a critical species in the two-step mechanism of rapid heating characteristic of nanosecond pulsed discharges. Time resolution of less than 400 ps is achieved in the current system, a significant improvement over conventional OES measurements with ICCD cameras or photomultiplier tubes. In addition, as long as the driving voltage pulse lies within the streak camera sweep window, the entire plasma emission spectrum for a single discharge is acquired. This capability is in contrast to prior studies where the time-dependent emission is obtained by plotting measurements at single steps from separate discharges. The effect of pulse repetition frequency (PRF) on the N2(C) decay is investigated by recording emission spectra for single discharges at 5–100 kHz. The streak images clearly show that the decay time of the N2(C–B, 0–0) transition increases as the PRF is increased. The emission decay exhibits two distinct phases with significantly different decay rates, consistent with prior measurements. The 1/e lifetimes of the two decay regimes are found using exponential fits to the emission decay curves. In addition, joint statistics relating the decay times and energy deposited in the plasma were also obtained.

Experimental investigation of reattachment behavior of turbulent lifted diffusion jet flames induced by repetitive DC electric pulse discharges with conditional PIV

ABSTRACTAmong the recently prevailing flame stabilization enhancement by electric field and plasma, repetitive DC electric pulse discharge is applied to a turbulent lifted diffusion jet flame to improve the reattachment in this study. The main objectove of this study is to characterize experimentally the stability and the reattachment behaviors of lifted flames under the electric pulse effect with a proposed electrode configuration. The effects of pulse repetition frequency (PRF) and voltage polarity on flame are investigated. The reattachment velocity increases by increasing PRF. The time history of flame base trace and absolute flame speed with positive-voltage-pulse are illustrated. The electric corona discharge is observed during upstream propagation process, and its role in flame acceleration is discussed. In addition, 2D-PIV measurement conditioned by high speed sequence images is conducted. Finally, the conditionally instantaneous flame propagation speed is enhanced and exceeds 3 times that of stoichiometric laminar flame speed, which leads to lifted flame reattachment.

Reduction of flame development time in nanosecond pulsed high frequency discharge ignition of flowing mixtures

The effects of discharge and flow parameters on ignition kernel development time are explored in flowing methane–air mixtures. A nanosecond pulsed high frequency discharge in a pin-to-pin configuration is used as the ignition source, providing 2.9 ± 0.23 mJ/pulse. The effects of pulse repetition frequency (PRF) in the range of 10–300 kHz, number of pulses in the range of 1–50 (≈2.9–145 mJ), equivalence ratio in the range of 0.55–0.65, gap distance in the range of 0.5–2.5 mm, and flow velocity in the range of 2.5–10 m/s are explored. For all conditions, the ignition events are in the “fully-coupled” regime, in which high ignition probability is achieved and locally extinguished ignition kernels are avoided. It is found that reducing the PRF reduces the kernel development time for fixed total energy deposition due to an increased volume of unburned mixture exposed to the discharge. Increasing the number of pulses at a given PRF also decreases the kernel development time, again by increasing the volume of gas exposed to the discharge. The equivalence ratio only has an effect on the kernel growth rate after the discharge, with identical kernel areas measured at all equivalence ratios while the discharge is active. Increasing gap distance decreases the kernel development time by providing a larger initial kernel volume as well as reducing heat and radical quenching at the electrode surfaces. Finally, the flow velocity has an effect on the kernel growth rate at velocities greater than 5 m/s, with larger flow velocities resulting in shorter kernel development time. This is due to the competing rates of self-propagating flame expansion and kernel growth due to convection past the discharge region. The combined effects of all of the above parameters on the kernel area after the discharge are summarized in a correlation equation, which predicts the trends in kernel growth rate based on an estimated plasma area defined by the discharge duration, flow velocity, and gap distance.

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.

Effect of pulse repetition frequency of high-intensity focused ultrasound on in vitro thrombolysis

Vascular occlusion by the thrombi is the main reason for ischemic stroke and deep vein thrombosis. High-intensity focused ultrasound (HIFU) and histotripsy or microtripsy pulses can effectively dissolve the blood clot with no use of thrombolytic agent and ultrasound contrast agent (microbubbles). In this study, HIFU bursts at the same duty cycle (2%) but varied pulse repetition frequency (PRF) from 1Hz to 1000Hz were delivered to in vitro porcine blood clot for 30s. Thrombolysis efficiency initially increases slightly with the PRF, 86.4±10.3%, 89.9±11.9, and 92.9±12.8% at the PRF of 1Hz, 10Hz, and 100Hz, respectively, without significant difference (p>0.05), but then drops dramatically to 37.9±6.9% at the PRF of 1000Hz (p<0.05). The particle size in the supernatant of dissolution is 547.1±129.5nm, which suggests the disruption of thrombi into the subcellular level. Thrombi motion during HIFU exposure shows violent motion and significant curling at the low PRF, rotation about its axis with occasional curling at the moderate PRF, and localized vibration at the high PRF due to the generation of acoustic radiation force and streaming. Quantitative analysis of recorded motion shows the axial displacement decreases with the PRF of delivered HIFU bursts, from 3.9±1.5mm at 1Hz to 0.7±0.4mm at 1000Hz. Bubble cavitation during HIFU exposure to the blood clot was also monitored. The increase of PRF led to the increase of inertial cavitation but the decrease of stable cavitation. In summary, the PRF of delivered HIFU bursts at the same output energy has a significant effect on the thrombi motion, bubble cavitation activities, and subsequently thrombolysis efficiencies.

Cell Electrosensitization Exists Only in Certain Electroporation Buffers.

Electroporation-induced cell sensitization was described as the occurrence of a delayed hypersensitivity to electric pulses caused by pretreating cells with electric pulses. It was achieved by increasing the duration of the electroporation treatment at the same cumulative energy input. It could be exploited in electroporation-based treatments such as electrochemotherapy and tissue ablation with irreversible electroporation. The mechanisms responsible for cell sensitization, however, have not yet been identified. We investigated cell sensitization dynamics in five different electroporation buffers. We split a pulse train into two trains varying the delay between them and measured the propidium uptake by fluorescence microscopy. By fitting the first-order model to the experimental results, we determined the uptake due to each train (i.e. the first and the second) and the corresponding resealing constant. Cell sensitization was observed in the growth medium but not in other tested buffers. The effect of pulse repetition frequency, cell size change, cytoskeleton disruption and calcium influx do not adequately explain cell sensitization. Based on our results, we can conclude that cell sensitization is a sum of several processes and is buffer dependent. Further research is needed to determine its generality and to identify underlying mechanisms.

Noninvasive thrombolysis using microtripsy: a parameter study.

Histotripsy fractionates soft tissue by well-controlled acoustic cavitation using microsecond-long, high-intensity ultrasound pulses. The feasibility of using histotripsy as a noninvasive, drug-free, and image-guided thrombolysis method has been shown previously. A new histotripsy approach, termed microtripsy, has recently been investigated for the thrombolysis application to improve treatment accuracy and avoid potential vessel damage. In this study, we investigated the effects of pulse repetition frequency (PRF) on microtripsy thrombolysis. Microtripsy thrombolysis treatments using different PRFs (5, 50, and 100 Hz) and doses (20, 50, and 100 pulses) were performed on blood clots in an in vitro vessel flow model. To quantitatively evaluate the microtripsy thrombolysis effect, the location of focal cavitation, the incident rate of pre-focal cavitation on the vessel wall, the size and location of the resulting flow channel, and the generated clot debris particles were measured. The results demonstrated that focal cavitation was always well confined in the vessel lumen without contacting the vessel wall for all PRFs. Pre-focal cavitation on the front vessel wall was never observed at 5Hz PRF, but occasionally observed at PRFs of 50 Hz (1.2%) and 100 Hz (5.4%). However, the observed pre-focal cavitation was weak and did not significantly affect the focal cavitation. Results further demonstrated that, although the extent of clot fractionation per pulse was the highest at 5 Hz PRF at the beginning of treatment (<20 pulses), 100 Hz PRF generated the largest flow channels with a much shorter treatment time. Finally, results showed fewer large debris particles were generated at a higher PRF. Overall, the results of this study suggest that a higher PRF (50 or 100 Hz) may be a better choice for microtripsy thrombolysis to use clinically due to the larger resulting flow channel, shorter treatment time, and smaller debris particles.

ICCD Imaging of the Transition From Uncoupled to Coupled Mode in a Plasma Source for Biomedical and Materials Treatment Applications

Intense and energetic atmospheric plasma was achieved by jet-to-jet coupling using an array of plasma jets arranged in an axisymmetric structure. To highlight the effects of pulse repetition frequency, peak voltage and mass flow rate on the transition from a coupled mode to an uncoupled one, qualitative results coming from iCCD imaging of a nanopulsed Gatling machine gun-like plasma source will be shown. Moreover, results on surface modifications and bacterial inactivation will be presented.

Effect of pulse repetition frequency on microcalcification detection in color flow imaging

The twinkling sign is a phenomenon involving rapidly alternating color pixels behind a stationary strongly reflecting medium where an acoustic shadow is expected in the Doppler mode. However, the mechanism of the occurrence of such a sign is not yet clarified. In this study, to clarify the mechanism, we investigated effect of pulse repetition frequency (PRF) on echo signals from a glass bead (diameter 973 µm) in a soft-tissue-mimicking phantom, and observed the appearance pattern of the twinkling sign of the same glass bead. It was observed that the variance of the IQ signal and the appearance frequency of the twinkling sign decreased when PRF increased.

Effect of pulse repetition frequency and scan step size on the dimensions of the lesions formed in agar by HIFU histotripsy

Histotripsy uses high-intensity focused ultrasound pulses at low duty cycle to generate energetic bubble clouds inside tissue to fractionate a region. As a potential tumor treatment modality, this cavitation-based non-invasive technique has the advantages of easy monitoring and sharp borders. Aiming at therapy efficiency, we experimentally investigated the effects of pulse repetition frequency (PRF) and lateral scan step size on the dimensions of lesions formed through HIFU histotripsy in agar mimicking tissue in terms of mechanical (not acoustical) properties. The single-element spherically focused source (1.1MHz, 6.34cm focal length, f/1) was excited to reach the peak compressional and rarefactional pressures of ∼102 and 17MPa, respectively. A targeted rectangular block of 4.5mm wide (lateral) and 6mm deep (axial) was scanned in a raster pattern with a constant axial step size of 3mm. The lateral step size was varied between 375, 750, 1500, 2250 and 4500μm. Pulses at each treatment location consisted of 5000 20-cycle sine wave tone bursts with the PRF of 167, 333 or 1000Hz. Results suggested that the bubble activity region could extend beyond the −3dB region and that refining the lateral scan mesh and/or increasing PRF enlarged the lesion extent. The 1500μm–333Hz and the 1500μm–1kHz conditions were in a more favorable position to be viewed as optimal with regard to lesion volume generation rate, bubble activity region width, and the potential for thermal damage.

Assessment of Ultrasound Histotripsy-Induced Damage to Ex Vivo Porcine Muscle

Cavitation-based histotripsy uses high-intensity focused ultrasound pulses at a low duty cycle to generate energetic bubble clouds inside tissue to fractionate cells and is a potential noninvasive tumor treatment modality. Aiming at determining therapy efficiency, we experimentally investigated the effects of pulse repetition frequency and lateral scan step size on the degree of damage of histotripsy-induced lesions in porcine muscle tissue. A single-element spherically focused source (1.1 MHz, 6.34-cm focal length, f/1) was excited to reach the peak compressional and rarefactional pressures of approximately 102 and 17 MPa, respectively. A targeted square of 9 mm wide (lateral to focal plane) was scanned in a raster pattern with the step sizes of 375, 750, 1500, 2250, and 4500 μm. Pulses at each treatment location consisted of 5000 20-cycle sine wave tone bursts with a pulse repetition frequency of 167, 333, or 1000 Hz. Histopathologic examination and image processing were performed to evaluate the tissue damage for each experimental condition. Skeletal myofiber damage was successfully created with our 7 exposure conditions. Three scales for muscle damage were identified through performing quad-tree decomposition to photomicrograph images and then relating decomposition with lesion homogeneity. Using a finer scan step size promoted the lesion homogeneity. Selection of the optimal condition does not depend solely on the comparison of tissue damage. Given the uncertainty on which of the 3 scales for tissue damage allows muscle repair, 2 conditions were identified as optimal: the 1500 μm-333 Hz condition for scale 3 (related to mild damage) and the 750 μm-333 Hz condition for scale 1 (related to severe damage).

Optimization of graffiti removal on natural stone by means of high repetition rate UV laser

The use of laser for graffiti removal is a promising alternative to conventional cleaning methods, though irradiation parameters must be carefully selected in order to achieve the effective cleaning without damaging the substrate, especially when referring to natural stone. From a practical point of view, once a safe working window is selected, it is necessary to determine the irradiation conditions to remove large paint areas, with minimal time consumption. The aim of this paper is to present a systematic procedure to select the optimum parameters for graffiti removal by means of the 3rd harmonic of a high repetition rate nanosecond Nd:YVO4 laser. Ablation thresholds of four spray paint colors were determined and the effect of pulse repetition frequency, beam diameter and line scan separation was analyzed, obtaining a set of values which optimize the ablation process.

Acousto-optic Q-switched laser performances of Er3+:Yb3+:LuAl3(BO3)4 crystal at 1.5–1.6 \mu m

Diode-pumped acousto-optic Q-switched pulse laser at 1.5–1.6 \mu m is obtained in an Er3+:Yb3+:LuAl3(BO3)4 crystal. Single-pulse laser operation with slope efficiency of 14% and threshold of approximately 100 mW is realized at an average absorbed pump power of 314 mW and repetition frequency of 20 kHz. Output pulse energy is 67 \mu J. The effects of pulse repetition frequency, absorbed pump power, and duty cycle on the wavelength and pulse profile of the Q-switched Er3+:Yb3+:LuAl3(BO3)4 laser are also investigated.

Pulse Repetition Frequency Effect on Nanosecond-Pulse Diffuse Discharge in Atmospheric-Pressure Air With a Point-to-Plane Gap

Diffuse discharge excited by nanosecond-pulse discharge in atmospheric air has a promising application. Unipolar repetitive pulses with 25-ns rise time and 40-ns duration are used to produce the diffuse discharge in a point-to-plane gap in atmospheric air. The effect of pulse repetition frequency (PRF) on the discharge is investigated under two gap spacings. The discharge images show that both the PRF and air gap spacing are responsible for the diffuse discharge mode and the transition to a filamentary discharge.

The relationship of acoustic emission and pulse-repetition frequency in the detection of gas body stability and cell death

The effect of pulse-repetition frequency (PRF) and number of exposures on membrane damage and subsequent death of contrast agent-attached phagocytic cells was examined. Phagocytic cells of a mouse macrophage cell line were grown as monolayers on thin Mylar sheets. Optison® microbubbles were attached to these cells by incubation. Focused ultrasound exposures ( P r = 2 MPa) were implemented at a frequency of 2.25 MHz with 46 cycle pulses and clinically relevant PRFs of 1 kHz, 100 Hz, 10 Hz, 1 Hz and 0.1 Hz in a degassed water bath. A 1-MHz receive transducer measured the scattered signal. The frequency spectrum was normalized to a control spectrum from linear scatterers. Photomicrographs of the cell monolayer were made before and after exposure, and a dye exclusion test (Trypan blue) was used to find the percentage of blue-stained cells indicating cell death, which was then related to acoustic emission. For 10 acoustic pulses and a high prerinse gas body concentration, there was less cell death and correspondingly lower change in the acoustic emissions at a PRF of 1 kHz than with PRFs of 100 Hz, 10 Hz, 1 Hz and 0.1 Hz ( p < 0.001). The reduced effect at high PRF may be indicative of some evolution of the shelled microbubble that requires significant total exposure duration (> 10 ms, but < 100 ms). (E-mail: ssamuel@umich.edu)

The effect of pulse repetition frequency on the uptake into electropermeabilized cells in vitro with possible applications in electrochemotherapy

Electrochemotherapy is a technique where electric pulses in combination with chemotherapeutic agents are applied to tumor cells. In general, patients find electrochemotherapy tolerable, in spite of unpleasant sensations associated with contraction of muscles located beneath or in the vicinity of the electrodes. These contractions are due to the intensity of the electric pulses required for effective electropermeabilization of tumor cell membranes. Since a train of eight electric pulses with repetition frequency of 1 Hz is usually applied to the tumors, each pulse in the train excites underlying nerves and provokes muscle contractions. Therefore, for patients involved in electrochemotherapy, the use of pulses with repetition frequency higher than the frequency of tetanic contraction would represent reduced number of muscle contractions and associated unpleasant sensations. Our results of the uptake of Lucifer Yellow into electropermeabilized cells in vitro show that with increased repetition frequency the uptake stays at similar levels even at frequencies up to 8.3 kHz. On the basis of these results the possibilities for the clinical use of pulses with high repetition frequency in electrochemotherapy are considered.