Showing papers on "Pulse duration published in 1996"

Pulse wave analysis.

Abstract: PULSE WAVE ANALYSIS IN HISTORICAL TIMES: Interpretation of the arterial pulse has been an important part of the medical examination from ancient times. Graphic methods for clinical pulse wave recording were introduced by Marey in Paris and by Mahomed in London last century. Mahomed showed how such recordings could be used to detect asymptomatic hypertension, and used them to chart the natural history of essential hypertension and to distinguish between this condition and chronic nephritis. Interest in arterial pulse analysis, as applied by Mahomed, lapsed with the introduction of the cuff sphygmomanometer 100 years ago. MODERN PULSE WAVE ANALYSIS: Analysis of the arterial pulse is now regaining favour as limitations of the cuff sphygmomanometer are better recognized (including the ability only to measure extremes of the pulse in the brachial artery). In addition, high-fidelity tonometers have been introduced for very accurate, non-invasive measurement of arterial pulse contour, and there is now a better understanding of arterial hemodynamics, and appreciation of disease and aging effects in humans. It is now possible to record the pulse wave accurately in the radial or carotid artery, to synthesize the ascending aortic pulse waveform, to identify systolic and diastolic periods and to generate indices of ventricular-vascular interaction previously only possible with invasive arterial catheterization. Pressure pulse wave analysis now permits more accurate diagnosis and more logical therapy than was ever possible in the past.

Observation of laser assisted photoelectric effect and femtosecond high order harmonic radiation.

Abstract: We report the first observation of laser-induced free-free transitions in the primary photoelectron spectra of gaseous helium ionized by ultrashort soft x-ray pulses. Measured transition amplitudes are well described by projecting the initial electronic state onto a Volkov wavefunction. Additionally, we report the first direct measurements on the temporal duration of femtosecond high order harmonic radiation. The harmonic pulse duration is observed to exhibit strong dependencies upon both laser intensity and position of the generating medium relative to the laser focus.

System and method for multisite steering of cardiac stimuli

Abstract: A system and corresponding method for steering stimulus pulses to one or more selected cardiac sites are disclosed. An apparatus and corresponding method permit improved capture and easy adjustment of the stimulus pulse when the threshold level changes, without simply increasing stimulus power or replacing the implanted lead. Substantially concurrent pulses may be delivered across one, two or more anode electrodes and a corresponding common or cathode electrode, where the pulse amplitude, pulse duration or pulse phase characteristics of each individual component pulse are adjusted to permit the resulting composite pulse to be steered or directed towards a desired target cardiac site. Different embodiments of the apparatus and corresponding method include those which permit the steering of pacing pulses in the left atrium or in multiple chambers, use of an improved single lead system having electrodes floating in the atrium, steering of atrial pulses to provide an enhancement of a VDD system, steering of atrial or ventricular defibrillation stimuli, and steering stimuli for arrhythmia prevention. In yet other embodiments, the steered pulse system is combined with capture detection and automatic adjustment of the steering parameters.

Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps

Abstract: Laser‐induced ablation has been extended down to a pulse duration of 20 fs generated by a Ti sapphire laser system at a wavelength of 780 nm. Barium aluminum borosilicate glass with an extremely high glass transformation temperature (∼600 °C) served as target material. The most significant observation was a substantial decrease of the ablation threshold fluence at pulse durations below 100 fs. All results indicate a dominant role of multiphoton absorption in addition to collisional ionization in this time domain.

Laser Skin Resurfacing

Abstract: The zone of thermal damage can be reduced by applying the principle of selective photo thermolysis, 8 which states that selective heating can be achieved by preferential laser light absorption and heat production in the target chromophore with heat being localized to the target by a pulse duration shorter than or equal to the thermal relaxation time (time for the target to cool by 63%) of the chromophore. The thermal relaxation time for the 20 to 30 μm thickness of water-containing tissue that absorbs the CO 2 laser light is less than 1 millisecond. 9 To limit collateral thermal damage, the CO 2 laser has to be pulsed or scanned across the tissue with a tissue dwell time less than or equal to the thermal relaxation time (1 millisecond). In addition, sufficient energy has to be delivered in that short time to achievetissue vaporization with a single pulse or

Laser-induced optical breakdown on hard and soft tissues and its dependence on the pulse duration: experiment and model

Abstract: Threshold values of laser-induced optical breakdown (LIOB) on the surface of human corneal tissues, human enamel, and bovine brain tissues are presented. The data are obtained by using a regeneratively amplified Nd:YLF laser and a multistage dye laser system, respectively. The measured decrease in threshold fluence at shorter pulse durations is in good agreement with the authors' theoretical model.

16-fs, 1-microJ ultraviolet pulses generated by third-harmonic conversion in air.

Abstract: We describe a simple method for generating sub-20-fs ultraviolet light pulses with useful average powers, using a kilohertz Ti:sapphire laser system. By focusing a 22-fs, 1-mJ laser pulse in air, we obtain ultraviolet pulses with an energy of 1 microJ and at a wavelength of 266 nm and with an average power of 1 mW. The pulse duration of the ultraviolet pulses was measured to be 16 fs with frequency-resolved optical gating.

Effect of pulse duration on bubble formation and laser‐induced pressure waves during holmium laser ablation

Abstract: Background and Objective One concern during laser ablation of tissue is the mechanical injury that may be induced in tissue in the vicinity of the ablation site. This injury is primarily due to rapid bubble expansion and collapse or due to laser-induced pressure waves. In this study, the effect of laser pulse duration on the thermodynamics of bubble formation and accompanying acoustic pressure wave generation has been investigated. Study Design/Materials and Methods Q-switched holium:YAG laser pulses (pulse duration 500 ns, pulse energy 14 mJ) and free-running holmium:YAG laser pulses (pulse duration 100–1,100 μs, pulse energy 200 mJ) were delivered in water and tissue phantoms via a 200- and 400-μm fiber, respectively. The tissue phantoms consisted of polyacrylamide gels with varying mechanical strengths. Bubble formation was recorded with a fast flash photography setup, while acoustic transients were measured with a needle hydrophone. Results It was observed that, as the pulse length was increased the bubble shape changed from almost spherical for Q-switched pulses to a more elongated cylinder shape for longer pulse durations. The bubble expansion velocity was larger for shorter pulse durations. Only the Q-switched pulse induced a measurable thermo-elastic expansion wave. All pulses that induced bubble formation generated pressure waves upon collapse of the bubble in gels as well as in water. However, the magnitude of the pressure wave depended strongly on the size and geometry of the induced bubble. Conclusion The magnitude of the collapse pressure wave decreased as laser pulse duration increased. Hence it may be possible to reduce collateral mechanical tissue damage by stretching the holmium laser pulse. © 1996 Wiley-Liss, Inc.

Method for the laser treatment of subsurface blood vessels

Abstract: The invention provides a method for selectively destroying blood vessels contained at a selected depth and in a selective area of a patients dermis by positioning a laser so that light from the laser will impinge on the selected area of the dermis and operating the laser to deliver pulse light to the area, which light has a wavelength between 700 nm and 1100 nm, with each pulse delivering a fluence at the surface above the area being treated of between 5 joules per square centimeter and 100 joules per square centimeter, and each pulse having a pulse duration of between 0.2 milliseconds and 100 milliseconds.

Stochastic Resonance in Chemistry. 1. The Belousov−Zhabotinsky Reaction

Abstract: We demonstrate the phenomenon of stochastic resonance in a nonlinear chemical reaction. The term “stochastic resonance” (SR) denotes the detection of a weak periodic signal in a noisy system displaying a threshold. If the sum of the periodic signal and the noise amplitude crosses the threshold, an output pulse is triggered. At an optimal noise amplitude the distribution of pulse intervals as well as the signal-to-noise ratio will pass through a maximum. In the continuously stirred tank reactor (CSTR) experiments we superimpose a periodic flow rate signal on an excitable focal steady state located near a Hopf bifurcation in the Belousov−Zhabotinsky (BZ) reaction. In the Showalter−Noyes−Bar-Eli model of this reaction we vary the perturbation frequency and amplitude as well as the pulse length of the applied noise to elaborate the optimal conditions for stochastic resonance to occur in the model.

Effects of CO2 laser pulse duration in ablation and residual thermal damage: implications for skin resurfacing.

Abstract: Background and Objectives Resurfacing with the CO2 laser is rapidly gaining acceptance for skin rejuvenation. Advances in CO2 laser and scanning technology allow for precise tissue removal with minimal thermal damage. High energy CO2 laser pulses have been widely used effectively to smooth the surface of facial skin; however, pulse duration effects on ablation and thermal damage have not been systematically studied over the millisecond region (0.25–10 ms). Study Design/Materials and Methods This study characterizes the ablation threshold, heat of ablation, and residual thermal damage in skin resulting from CO2 laser pulses with a Gaussian beam profile. Mass loss from fresh pig skin was measured with an analytical balance, and residual thermal damage was determined through histology. Results Pulse durations >1 ms were associated with higher ablation thresholds and localized increased thermal damage. Conclusions Our results show that although pulse duration is an important determinant in ablation and thermal damage, irradiance is more critical as an independent parameter in predicting the effects of CO2 laser pulses. © 1996 Wiley-Liss, Inc.

High-power linear amplification using periodically updated amplitude and phase correction values

Abstract: In a high-power transmitter, an input complex signal is multiplied in a complex multiplier by control signals. The output complex signal from the multiplier is converted to a high frequency signal and amplified by a power amplifier for transmission. The amplitude of the input complex signal is detected to access a memory where amplitude and phase correction values are stored. During a read mode of the memory, a set of amplitude and phase correction values is specified by the detected amplitude and supplied to the complex amplifier as the control signals. During a write mode of the memory, a set of amplitude and phase correction values is specified by a delayed version of the detected amplitude and rewritten with a set of new amplitude and phase correction values. The amplified high frequency signal is down-converted to a low frequency complex signal. The nonlinearity of the power amplifier is determined from a delayed version of the input complex signal and the down-converted complex signal and the new amplitude and phase correction values are produced from the detected nonlineariry and delayed versions of the amplitude and phase correction values which were supplied to the complex multiplier. At intervals, the memory is switched from the read mode to the write mode for updating its contents.

Experimental investigation of ultrashort pulse laser-induced breakdown thresholds in aqueous media

Abstract: Laser-induced breakdown (LIE) thresholds are determined for pulse durations of 2.4 ps, 400 fs, and 100 fs at 580 mn in high purity water, saline and tap water. The dependence of LIE irradiance thresholds on pulse duration, optical wavelength, and focal volume is examined, and the experimental data obtained is compared with a theoretical model. The slopes of the probability curves calculated are compared with mechanisms for LIE, namely avalanche ionization, multiphoton initiated avalanche ionization, and multiphoton ionization. Lastly, the dependence of the peak breakdown electric field on pulse duration and focal volume is empirically determined and compared with previous work.

Construction of a two-photon microscope and optimisation of illumination pulse duration.

Abstract: The construction of a two-photon/confocal microscope system is described in detail. For two-photon illumination, a Ti:sapphire modelocked laser generating 62-fs pulses at 715 nm was used. The effect of the optical train on illumination pulse width was examined and the observed increase in pulse duration was almost completely removed by the addition/adjustment of a prism compressor system. The imaging capabilities of the two-photon microscope are demonstrated and it is shown that the imaging performance of the two-photon microscope is similar to that of a conventional confocal microscope. With two-photon illumination, the resolution (full width at half-maximum intensity) was 0.42 microM (x-y) and 0.81 microM axially, while with single-photon illumination (at 488 nm in the same instrument with a confocal pinhole detector) the resolution was 0.3 microM (x-y) and 0.75 microM axially. The results are discussed with regard to the general problem of femtosecond pulse distortion in an optical system and a simple procedure for optimal pulse restoration is described.

Effect of laser pulse repetition rate and pulse duration on mast cell number and degranulation.

Abstract: Background and Objective Mast cell activation by low-level laser therapy (LLLT), leading to degranulation and the release of mediators, may be one of the mechanisms by which LLLT can accelerate tissue repair in mammals. The objective of this work, part of an investigation to determine the optimum parameters for increasing mast cell number and degranulation in injured skin, was to determine the effect of different pulsing frequencies of LLLT. Study Design/Materials and Methods Partial-thickness wounds in anaesthetized adult male Wistar rats were irradiated immediately after injury with monochromatic coherent light (wavelength 820 nm) pulsed at either 2.5, 20, 292, or 20,000 Hz at an average power density of 800 mW/cm2 for 27 seconds; the energy density was 21.6 J/cm2. The effects on mast cell number and degranulation were assessed 2 hours post-treatment by counting the numbers of intact and degranulated mast cells in Carnoy-fixed, toluidine blue-stained, sections of irradiated and sham-irradiated wounds. Results The total number of mast cells was increased significantly (P 0.05). However, although the number of degranulated mast cells was higher in all laser-treated wounds, in comparison with the sham-irradiated group, only the 20 Hz (pulse duration 45 ms) and 292 Hz (pulse duration 3 ms) frequencies were significantly effective (P<0.05). Conclusion Increase in mast cell number is not pulsing frequency dependent, whereas degranulation is. © 1996 Wiley-Liss, Inc.

Measurement of the duration of high harmonic pulses

Abstract: Summary form only given. High harmonics are generated during the ionization of an atomic gas and therefore the probability of ionization, the number of unionized atoms, and the phase matching effects will impose a natural temporal envelope on the harmonic generation. We made some simulations which showed the maximal, minimal, and averaged possible values of the intensity of the harmonic as a function of the angular frequency detuning multiplied by the length of the harmonic pulse. From the envelope of the maximum signal we can extract the high harmonic pulse duration.

Experimental evaluation of input-output models of motoneuron discharge

Abstract: 1. We measured the modulation of the background firing rate of cat spinal motoneurons produced by simulated, repetitive excitatory postsynaptic potentials (EPSPs) to test the accuracy of several proposed motoneuron input-output functions. Rhythmic discharge was elicited in the motoneurons by injecting suprathreshold current steps 1-1.5 s in duration. On alternate trials, trains of short (0.5-5 ms) current pulses were superimposed on the current steps to stimulate the effects of trains of individual EPSPs. The increase in firing rate (delta F) due to the addition of the pulses was calculated as the difference in motoneuron discharge rate between trials with and without the superimposed pulse trains. 2. In the same motoneurons, we were able to study the effects of changes in pulse frequency, duration, and amplitude, as well as changes in the background discharge rate. A sublinear relationship between pulse rate and delta F was observed, with delta F rising relatively steeply with increasing pulse frequency at low pulse rates and saturating at high pulse rates. A similarly shaped relation was observed between delta F and pulse duration. In contrast, delta F generally increased in a greater than linear fashion with increasing pulse amplitude. 3. In previous studies we demonstrated that when a relatively constant synaptic input is produced by high-frequency synaptic activity, delta F is approximately equal to the product of the net synaptic current reaching the soma and the slope of the motoneuron9s steady-state frequency-current (f-I) relation. In the present study, this input-output function consistently underestimated the observed delta F, particularly for low input rates, indicating that the transient current pulses are more effective in modulating motoneuron discharge than an equivalent amount of constant current. 4. Other investigators have proposed input-output functions derived from the relation between synaptic potential amplitude and the magnitude of the peak of a cross correlogram compiled from the discharge of the pre- and postsynaptic neurons. These functions consistently overestimated the observed delta F, particularly for high pulse rates. This overestimation may result in part from the fact that the effects of a synaptic potential (or current pulse) on postsynaptic discharge probability also include a period of decreased firing probability. Moreover, the cross correlation function may depend on the arrival rate of synaptic potentials (or current pulses). 5. Another proposed input-output function based on a simple threshold-crossing model of the motoneuron with a fixed spike threshold predicts firing rates that were often close to the observed delta F. However, the model did not reproduce the observed relations between delta F and input pulse rate or pulse duration. 6. The deficiencies of the basic threshold-crossing model may arise from the fact that it does not incorporate variations in membrane conductance and firing threshold that occur in real motoneurons. A more complete motoneuron model that incorporates both of these features was able to replicate the observed delta Fs associated with changes in input pulse frequency and duration.

Laser drilling of advanced materials: Effects of peak power, pulse format, and wavelength

Abstract: The quality of laser‐drilled holes is significantly influenced by the laser peak power, pulse format, and wavelength. Three advanced materials are used to demonstrate the importance of choosing the correct laser parameters for a specific material. The materials are: (1) intermetallic single crystal nickel aluminide (NiAl) alloy; (2) N5, a single crystal nickel‐based superalloy; and (3) a silicon carbide (SiC) ceramic matrix composite (CMC). The laser peak power is varied in terms of the pulse duration and format. For low peak powers, long pulse format (pulse lengths on the order of ms), conventional and also shaped‐pulse (burst) commercial rod Nd:YAG laser systems were used. For high peak powers, a short pulse format was adopted for the laser used, namely a cw‐pumped Q‐switched or modelocked/Q‐switched Nd:YAG oscillator followed by a cw‐pumped multipass amplifier using a face‐pumped, total internal reflection Nd:YAG slab as the gain medium. Finally, a KrF excimer laser operating at a 248 nm wavelength was used. The excimer wavelength was chosen to be close to the Nd:YAG fourth harmonic.

Heavy ion and proton induced single event transients in comparators

Abstract: This paper presents a display of heavy-ion- and proton-induced single event transients for three comparators. The transient vital signs are serious: low LET threshold, very high voltage amplitude and extended pulse duration (microsecs.).

Sub-10-fs operation of Kerr-lens mode-locked lasers.

Abstract: We present results of a three-dimensional model of a Kerr-lens mode-locked titanium-doped sapphire laser in the sub-10-fs regime of operation. We find that space–time focusing of the pulse within the laser crystal imposes a limitation on the pulse duration as a function of the crystal length. Thus good compensation of the net group-velocity dispersion of the cavity is not sufficient for successful sub-10-fs operation of the laser. The dependence of the pulse duration on the intracavity dispersion is also presented, which demonstrates that pulses as short as 6 fs can be generated directly from the laser.

Thermal Latency in Radiofrequency Ablation

Abstract: Background Progression of unintentionally induced atrioventricular delay is occasionally observed directly after termination of radiofrequency delivery in the vicinity of the atrioventricular node. We postulated that the application of a radiofrequency pulse may result in a tissue temperature rise that continues after the pulse. Methods and Results Using the thigh muscle preparation, 5-, 10-, 20-, and 30-second pulses were applied at 30 to 40 W via a standard 4-mm tip electrode with 10-g contact pressure. Forty-one undisturbed pulses were delivered while recording intramural temperatures at 2-, 4-, and 7-mm depth. Maximal “thermal latency” was observed with the shortest pulse duration and at greatest depth. With 5-second applications, tissue temperature at 7-mm depth peaked 11.6 seconds after termination of radiofrequency delivery and stayed above end-of-pulse value as long as 34.5 seconds after the pulse. The additional rise in tissue temperature was 2.9°C. If only recordings within the lesion border zon...

Phase-modulated binomial RF pulses for fast spectrally-selective musculoskeletal imaging.

Abstract: Two fat-suppressed three-dimensional gradient echo pulse sequences, FLASH and DESS (Double Echo in Steady-State), that have significantly reduced scan time compared with conventional chemical shift fat-suppression sequences are presented. This fat-suppression technique is based on selectively exciting water spins using a time-optimized binomial RF pulse pair at the water resonance frequency with a null in the excitation profile at the fat frequency. To minimize the total pulse length, the delay between the binomial components of the RF pulse is decreased from a standard jump-return implementation. To maintain the proper null frequency, the phase of the second RF pulse is shifted such that it returns the fat resonance back to its initial z position while further tipping the water spins. Nonselective phase-modulated RF pulse pairs can be implemented in 1.20 ms at 1.0 T, significantly reducing the minimum TR.

Temporal evolution of soft x-ray pulse emitted from aluminum plasma produced by a pair of Ti:sapphire laser pulses

Abstract: The temporal and spectral evolution of soft x‐ray pulses (40–100 A) emitted from Al plasma produced by a pair of femtosecond Ti:sapphire laser pulses at normal incidence was studied. Both the soft x‐ray emission and the pulse duration increased with increase in the scale length of the preformed plasma. Prepulse enhanced soft x‐ray emission about 100 times with a pulse duration of 100–130 ps. A spectrally resolved time history revealed that the emission at shorter wavelengths started and decayed relatively more quickly, and emissions at longer wavelengths built up and decayed more slowly.

Group-velocity-dispersion-compensated propagation of short pulses in dispersive media

Abstract: An optical scheme based on a single angularly dispersive element is proposed for canceling group-velocity dispersion for pulse propagation in dispersive media. It is demonstrated experimentally that a pulse duration of 100 fs is preserved throughout the travel of a 248-nm-pulse through a 75-mm-long fused-silica slab.

A long-pulse 300 kev electron gun with a plasma cathode for high-pressure gas lasers

Abstract: The design and main features of a plasma cathode electron gun for high‐pressure gas lasers are discussed. The mesh plasma cathode in combination with a low‐pressure gas discharge was used for the formation of a large cross‐section (55×4 cm2) electron beam with emission current densities up to 1.7 A/cm2, accelerating voltages up to 300 kV, and a pulse length of 20 μs (full width at half maximum).

Responses of the phototransduction cascade to dim light.

Abstract: The biochemistry of visual excitation is kinetically explored by measuring the activity of the cGMP phosphodiesterase (PDE) at light levels that activate only a few tens of rhodopsin molecules per rod. At 23 degrees C and in the presence of ATP, the pulse of PDE activity lasts 4 s (full width at half maximum). Complementing the rod outer segments (ROS) with rhodopsin kinase (RK) and arrestin or its splice variant p44 does not significantly shorten the pulse. But when the ROS are washed, the duration of the signal doubles. Adding either arrestin or p44 back to washed ROS approximately restores the pulse width to its initial value, with p44 being 10 times more efficient than arrestin. This supports the idea that, in vivo, capping of phosphorylated R* is mostly done by p44. When myristoylated (14:0) recoverin is added to unwashed ROS, the pulse duration and amplitude increase by about 50% if the free calcium is 500 nM. This effect increases further if the calcium is raised to 1 microM. Whenever R* deactivation is changed--when RK is exogenously enriched or when ATP is omitted from the buffer--there is no impact on the rising slope of the PDE pulse but only on its amplitude and duration. We explain this effect as due to the unequal competition between transducin and RK for R*. The kinetic model issued from this idea fits the data well, and its prediction that enrichment with transducin should lengthen the PDE pulse is successfully validated.

Analytical solution of non-Fourier temperature response in a finite medium under laser-pulse heating

Abstract: A temperature wave solution predicted by the hyperbolic heat equation is developed for a finite medium exposed to a surface heat flux of laser with an actual temporal profile. By using the analytical solution, the temperature response, the propagation and the reflection of the temperature wave due to such heat pulse are investigated for different pulse duration, thickness of the medium, and energy absorption depth.