Pulse Repetition Time Research Articles

Staggered-PRT Sequences for Doppler Weather Radars. Part II: Ground Clutter Mitigation on the NEXRAD Network Using the CLEAN-AP Filter

AbstractThe staggered–pulse repetition time (SPRT) technique has been shown to be effective at mitigating range and velocity ambiguities; however, mitigation of ground clutter contamination for SPRT sequences has proven to be more challenging. Using the properties of the autocorrelation spectral density, the Clutter Environment Analysis using Adaptive Processing (CLEAN-AP) filter is extended to SPRT sequences for its implementation on the U.S. Next Generation Weather Radar (NEXRAD) network. The performance of the CLEAN-AP filter for SPRT sequences is characterized and illustrated with simulations and real data. The study shows that the proposed ground clutter filter meets NEXRAD operational performance requirements for ground clutter mitigation.

Optimization of inhomogeneous magnetization transfer (ihMT) MRI contrast for preclinical studies using dipolar relaxation time (T1D ) filtering.

A pulsed inhomogeneous magnetization transfer (ihMT)-prepared fast imaging sequence was implemented at 11.75T for preclinical studies on mouse central nervous system. A strategy based on filtering the ihMT signal originating from short dipolar relaxation time (T1D ) components is proposed. It involves increasing the repetition time of consecutive radiofrequency (RF) pulses of the dual saturation and allows improved signal specificity for long T1D myelinated structures. Furthermore, frequency offset, power and timing saturation parameters were adjusted to optimize the ihMT sensitivity. The optimization of the ihMT sensitivity, whilst preserving the strong specificity for the long T1D component of myelinated tissues, allowed measurements of ihMT ratios on the order of 4-5% in white matter (WM), 2.5% in gray matter (GM) and 1-1.3% in muscle. This led to high relative ihMT contrasts between myelinated tissues and others (~3-4 between WM and muscle, and ≥2 between GM and muscle). Conversely, higher ihMT ratios (~6-7% in WM) could be obtained using minimal T1D filtering achieved with short saturation pulse repetition time or cosine-modulated pulses for the dual-frequency saturation. This study represents a first stage in the process of validating ihMT as a myelin biomarker by providing optimized ihMT preclinical sequences, directly transposable and applicable to other preclinical magnetic fields and scanners. Finally, ihMT ratios measured in various central nervous system areas are provided for future reference.

A Transition Arm Modular Multilevel Universal Pulse-Waveform Generator for Electroporation Applications

High-voltage (HV) pulses are used in electroporation to subject pulsed electric field (PEF) onto a sample under treatment. Pulse-waveform shape, voltage magnitude, pulse duration, and pulse repetition rate are the basic controllable variables required for particular PEF application. In practice, a custom-made pulse generator is dedicated for each PEF application with limited flexibility in changing these variables. In this paper, a universal pulse-waveform generator (UPG) is proposed, where the controller software algorithm can manipulate a basic generated multilevel pulse waveform to emulate many different PEF pulse waveforms. The commonly used PEF HV pulse waveforms can be generated as bipolar or monopolar with controllable pulse durations, repetition times, and voltage magnitudes. The UPG has the ability to generate multilevel pulses that have controllable dv/dt, which allows reduction of the electromagnetic interference generated by the converter. The UPG topology is based on half-bridge modular multilevel converter (HB-MMC) cells forming two transition arms in conjunction with two bistate arms, together creating an Hbridge. The HB-MMC cell capacitors provide a controllable energy source which charge from the dc input supply and discharge across the load, while the two bistate arms allow charging the HB-MMC cell capacitors. Hence, the UPG topology offers modularity, redundancy, and scalability. The HB-MMC individual cell capacitance is low and the cell voltages are balanced by employing the sorting and rotating algorithm used in conventional HB-MMC topologies for HV dc transmission applications. The viability of the proposed UPG converter is validated by MATLAB/Simulink simulation and scaled-down experimentation.

An Improved Clutter Suppression Method for Weather Radars Using Multiple Pulse Repetition Time Technique

This paper describes the implementation of an improved clutter suppression method for the multiple pulse repetition time (PRT) technique based on simulated radar data. The suppression method is constructed using maximum likelihood methodology in time domain and is called parametric time domain method (PTDM). The procedure relies on the assumption that precipitation and clutter signal spectra follow a Gaussian functional form. The multiple interleaved pulse repetition frequencies (PRFs) that are used in this work are set to four PRFs (952, 833, 667, and 513 Hz). Based on radar simulation, it is shown that the new method can provide accurate retrieval of Doppler velocity even in the case of strong clutter contamination. The obtained velocity is nearly unbiased for all the range of Nyquist velocity interval. Also, the performance of the method is illustrated on simulated radar data for plan position indicator (PPI) scan. Compared with staggered 2-PRT transmission schemes with PTDM, the proposed method presents better estimation accuracy under certain clutter situations.

Staggered-PRT Sequences for Doppler Weather Radars. Part I: Spectral Analysis Using the Autocorrelation Spectral Density

AbstractThe autocorrelation spectral density (ASD) was introduced as a generalization of the classical periodogram-based power spectral density (PSD) and as an alternative tool for spectral analysis of uniformly sampled weather radar signals. In this paper, the ASD is applied to staggered pulse repetition time (PRT) sequences and is related to both the PSD and the ASD of the underlying uniform-PRT sequence. An unbiased autocorrelation estimator based on the ASD is introduced for use with staggered-PRT sequences when spectral processing is required. Finally, the strengths and limitations of the ASD for spectral analysis of staggered-PRT sequences are illustrated using simulated and real data.

Mating behavior of Psammotettix alienus (Hemiptera: Cicadellidae).

The Wheat dwarf virus, the causal agent of the wheat dwarf disease, is transmitted by leafhoppers from the genus Psammotettix and currently the main protection strategy is based on the use of insecticide treatments. Sustainable management strategies for insect vectors should include methods that are targeted to disrupt reproductive behavior and here we investigated the mating behavior of Psammotettix alineus (Dahlbom 1850) in order to determine the role of vibrational signals in intra-specific communication and pair formation. Both genders spontaneously emit species- and sex-specific calling songs that consisted of regularly repeated pulse trains and differ primarily in pulse train duration and pulse repetition time. Females preferred the conspecific male calling song. After a coordinated exchange of pulse trains, the male approached the stationary female. During the close range courtship and also immediately prior to copulatory attempts distinct male vibrational signals associated with wing flapping and wing vibrations were recorded from the substrate. In the presence of a receptive female, competing males emitted vibrational signals most likely aimed to interfere with male-female interaction. Mated females regained sexual receptivity after they laid eggs. Although results suggest that the viruliferous status of insects may have an effect on vibrational songs, our current results did not reveal a significant effect of virus on leafhopper performance in mating behavior. However, this study also suggests, that detailed understanding of plant-vector-virus interactions relevant for vector mating behavior is essential for trying new approaches in developing future control practices against plant viruses transmitted by insect vectors.

The effect of ultrasound with acoustic radiation force on rabbit lung tissue: a preliminary study.

Acoustic radiation force (ARF) elastography has recently become available. The previous animal studies have revealed lung injuries induced by diagnostic ultrasound, but the effects on the lung resulting from exposure to ultrasound with ARF are unknown. This study aimed to assess the risk of lung injury associated with ultrasound with ARF. A focused 2.5-MHz transducer that emits ultrasound with ARF was used. A rabbit was anesthetized, and the transducer was placed in the right subcostal region. Exposure settings of mechanical index (MI) 0.80, pulse duration 10ms, pulse repetition time 5s, and exposure time 150s were applied. One red spot (7×6mm) was observed on the surface of the right lung corresponding to the area of exposure. Alveolar hemorrhage was observed microscopically. This lesion was visible across a range of 20-170μm in depth from the pleural surface. The first example of lung hemorrhage induced by ultrasound with ARF was observed in this study. This observation suggests the possibility of lung injury in humans when ARF elastography is applied with the transducer directed toward the lung. Further studies are needed to determine the safety of this modality.

Assessing the Effectiveness of Ramp-Up During Sonar Operations Using Exposure Models.

Ramp-up procedures are used to mitigate the impact of sound on marine mammals. Sound exposure models combined with observations of marine mammals responding to sound can be used to assess the effectiveness of ramp-up procedures. We found that ramp-up procedures before full-level sonar operations can reduce the risk of hearing threshold shifts with marine mammals, but their effectiveness depends strongly on the responsiveness of the animals. In this paper, we investigated the effect of sonar parameters (source level, pulse-repetition time, ship speed) on sound exposure by using a simple analytical model and highlight the mechanisms that limit the effectiveness of ramp-up procedures.

Improvement of accuracy of meteorological objects velocity unambiguous measurement in Doppler weather radars with staggered pulse repetition times

In this article we investigate the procedures intended for meteorological objects (MO) mean radial velocity unambiguous estimation in pulse Doppler weather radars with constant and staggered pulse repetition times (PRT). It has been demonstrated that the fulfillment of the requirements to the unambiguous velocity measurement range by means of PRT staggering can be accompanied by the undesirable increase of velocity estimation errors. The causes, which yield this drawback in the known algorithms of velocity unambiguous measurement [1–4], have been determined. We demonstrate the possibility to decrease this drawback using modified estimation of the radial velocity. An influence of various factors (namely staggering principle; the width of the spectrum and the order of autoregressive process, which approximates weather echoes; training sample size; and the variant of final velocity estimation) on the accuracy of modified estimation of radial velocity has been investigated. Reasonable parameters of modified procedure and scheme for its practical implementation have been proven.

An Improved M-PRT Technique for Spectral Analysis of Weather Radar Observations

The exploitation of Doppler radars for weather observations is strongly constrained by the well-known range–velocity dilemma. To overcome the range and velocity ambiguities, dual and triple staggered pulse-repetition time (PRT) techniques are commonly used in Doppler radar systems. Today, a triple-PRT (3-PRT) scheme is operational in France. These techniques imply nonuniform sampling of the weather signal, inducing multiple replicas in the Doppler spectrum. The situation is particularly complicated for short-wavelength radars, where larger extension factors of the unambiguous Nyquist interval are needed. To overcome these difficulties, a novel technique called OptM-PRT is proposed. It mainly consists in optimizing the transmission scheme based on multiple pulse repetition time, so that the corresponding autocorrelation function is well filled. The Doppler spectrum is therefore reconstructed with much less ambiguities, from the computation of the autocorrelation function of radar signal and its Fourier transform. Considering both 3-PRT and Opt9-PRT schemes, the magnitude and Doppler velocity of radar returns in rain are simulated for different spectral widths, with and without elimination of the spectral lines of ground clutter. When the ground clutter is filtered out, the 3-PRT is found to better reproduce the Doppler velocity, whereas the Opt9-PRT better restitutes the magnitude of the signal. In the presence of noise, the Opt9-PRT scheme produces the best result for both the magnitude and velocity. The 3- and Opt9-PRT techniques have been applied to the C-band Doppler radar operating in Bourges, France. The experimental results show that Opt9-PRT efficiently reconstructs the Doppler spectrum of rain echoes.

WE-AB-BRB-09: Real Time In Vivo Scintillating Fiber Array Detector for Medical LINACS

Purpose: An in vivo transmission scintillation fiber detector was developed to monitor patient treatment in real time for the enhancement of patient safety and treatment accuracy. The detector system is capable of monitoring each pulse from a medical LINAC during treatment to determine the dose delivered as treatment progresses. Methods: The detector system consists of 60 parallel scintillating fibers coupled to fast data processing optoelectronics that can monitor the beam fluence in real time. Each 2.5mm2 square fiber is aligned with an MLC leaf pair and is long enough to capture a 40cm field. The fibers are embedded within a water equivalent polymer substrate that is secured in the LINAC accessory tray. The fibers are coupled to high speed photosensors and front end amplifiers that filter noise and pass each pulse to a high speed analog-to-digital converter. The system components are capable of detecting pulse repetition times shorter than what is delivered by a medical LINAC to ensure true real time data acquisition. Results: The system was able to capture and record the signal from each linac pulse and display the information in real time with no pulse pile up. It was found that the fiber array attenuates 2.65% of the beam which can easily be compensated for in treatment planning. The fibers responded linearly with dose, are independent of clinical beam energies, and are independent of dose rate. Calibration of the system was performed as a function of beam energy, beam size, dose rate, and monitor units to optimize beam fluence error detection. Conclusion: The detector system presented provides true real time in vivo beam monitoring to enhance patient safety and treatment delivery accuracy. Furthermore, the detector can be used for current patient specific QA.

Polarimetric Variables Retrieval with Clutter Suppression for Staggered PRT Sequences

AbstractThis paper presents a procedure to filter ground clutter from dual-polarized staggered pulse repetition time (PRT) radar data in simultaneous and alternating transmission modes for polarimetric variables retrieval. The filter is designed in the time domain so that polarimetric variables such as the differential phase () and the copolar correlation coefficient () can be estimated directly from clutter-filtered time series data using a conventional method. In the case of the simultaneous mode, a single filter is used for both channels to maintain the signal correlation after filtering. For the alternating mode, because the polarizations are transmitted in different waveforms, two separate filters are required. However, the filters are designed so that the responses of the filters to the signals are identical within the extended Doppler range. Based on radar simulation, it is shown that the method can provide accurate retrieval of polarimetric variables even in the case of strong clutter contamination. Also, the performance of the method is illustrated on dual-polarized staggered PRT ⅔ data from the NASA dual-frequency dual-polarized Doppler radar (D3R).

Detection of subnanotesla oscillatory magnetic fields using MRI.

Direct mapping of neuronal currents using MRI would have fundamental impacts on brain functional imaging. Previous reports indicated that the stimulus-induced rotary saturation (SIRS) mechanism had the best potential of direct detection of neural oscillations; however, it lacked the high-sensitivity level needed. In this study, a novel strategy is proposed in an effort to improve the detection sensitivity. In our modified SIRS sequence, an external oscillatory magnetic field is used as the excitation pulse in place of the standard 90-degree excitation pulse. This approach could potentially lead to tens or even hundreds times of enhancement in the detection sensitivity for low field signals. It also helps to lower the physiological noise, allows for shorter pulse repetition time, and is less affected by the blood oxygen level. We demonstrate that a 100-Hz oscillatory magnetic field with magnitude as low as 0.25 nanotesla generated in a current loop can be robustly detected using a 3-Tesla MRI scanner. The modified SIRS sequence offers higher detection sensitivity as well as several additional advantages. These promising results suggest that the direct detection of neural oscillation might be within the grasp of the current MRI technology.

On the Alternate Transmission Mode for Polarimetric Phased Array Weather Radar

AbstractPulse-to-pulse switching of polarizations (alternate transmission mode) is considered for polarimetric phased array radar (PAR). It is argued that the performance of the radar in terms of data quality should match or exceed the achieved standards of the Weather Surveillance Radar-1988 Doppler (WSR-88D). It turns out that the most stringent demand on the radar concerns the surveillance scan at the lowest elevations wherein the polarimetric variables are free of overlaid echoes, while ground clutter is significantly reduced. The scan uses a long pulse repetition time that has repercussion on the standard errors of the polarimetric variables and hence the choice of polarimetric mode. Herein the dwell time of this scan serves as a benchmark for comparisons of the accuracy of estimates. Because weather PAR should provide useful information at low signal-to-noise ratios (SNR) as low as those measured by the WSR-88D, the statistics of polarimetric variables, known at high SNR, is extended to low SNRs. It follows that the alternate mode would not match the performance of the simultaneous mode in the surveillance scans on the WSR-88D. Quasi-simultaneous transmission and reception of horizontally polarized and vertically polarized waves is discussed as a cost-effective alternative.

Pulsed‐CO2 Laser Beam Photothermal Technology Combined with Conjugated Gold Nanoparticles for the Selective Elimination of Surface Escherichia Coli K12 from Fresh Fruits

AbstractsPostharvest methods for microbial decontamination of fresh produce are limited, and chemical treatment is less popular with growing concerns over toxic residues. A photothermal guiding system was developed with a pulsed‐CO2 laser and adjustable two‐ZnSe lens beam expander. The system was optimized to ensure uniform radiation of the sample with respect to pulse width (PW) and repetition time (RT) without damaging the food samples. Conjugated gold nanoparticles (GNPs) added to fruit surfaces tested the selective photothermal nanotherapy under laser radiation for enhancing treatment.Apple peel samples (1 cm2) contaminated with Escherichia coli K12 were treated at varying PW and RT for 60, 120 and 180 s. Inactivation of E. coli K12 significantly (P < 0.05) increased as the PW increased at constant RT. Also, the inactivation increased as both RT decreased and PW increased, and with the addition of the GNP. Scanning electron microscopy analysis showed structural damage of E. coli and GNP with minimal damage to fruit surfaces.Practical ApplicationsThis unique laser system could provide rapid, energy‐efficient and selective inactivation of food surface contaminants through the use of conjugated gold nanoparticles. The system's likely application is to ensure food safety in numerous segments of the U.S. food industry; it would be especially useful for producers of fresh food products. Photothermal nanotherapy provides selective heating to a localized area around the targeted bacteria without damaging the food, which makes it particularly applicable for heat‐sensitive foods, such as fruits and leafy green vegetables, and could be a desirable alternative to chemical treatments. It also has the potential to be successfully applied to liquid food samples such as water, fruit juices and other transparent liquids.

Efficient generation of T2*-weighted contrast by interslice echo-shifting for human functional and anatomical imaging at 9.4 Tesla.

Standard gradient-echo sequences are often prohibitively slow for T2*-weighted imaging as long echo times prolong the repetition time of the sequence. Echo-shifting offers a way out of this dilemma by allowing an echo time that exceeds the repetition time. The purpose of this work is to present a gradient-echo sequence that is optimized for multislice T2*-weighted imaging applications by combining echo-shifting with an interleaved slice excitation order. This combined approach offers two major advantages: First, it combines the advantages of both concepts, that is, echo time and pulse repetition time can be significantly increased without affecting scan time. Second, there is no echo-shifting related signal loss associated with this concept as only a single radiofrequency pulse is applied per pulse repetition time and slice. A 9.4 Tesla high-resolution T2*-weighted anatomical brain scan of the proposed sequence is compared to a standard gradient-echo. Furthermore, results from 9.4 Tesla blood oxygen level dependent functional magnetic resonance imaging experiments with an in-plane resolution of 0.8 × 0.8 mm(2) are presented. The proposed sequence allows for efficient generation of T2*-weighted contrast by combining echo-shifting with an interleaved slice excitation order.

Eliminating thermal effects in z-scan measurements of thin PTCDA films.

We investigate the two-photon absorption (TPA) and nonlinear refraction of a micrometer thick 3,4,9,10-perylentetracarboxyl-dianhydride (PTCDA) film using z-scans with tightly focused 100 fs laser pulses. The PTCDA film was grown by organic molecular beam deposition on a Pyrex substrate. To study the influence of sample heating, the pulse repetition rate was varied between 4 MHz and 50 kHz with an acousto-optic pulse selector. We find that thermal effects diminish for pulse repetition times longer than 5 and 0.75 µs when using a 10x or 20x microscope lens, respectively, resulting in a TPA coefficient of 6 cm/GW and a nonlinear refractive index of 1.2 x 10⁻¹³ cm²/W at a wavelength of 820 nm.

S. cerevisiae fermentation activity after moderate pulsed electric field pre-treatments

The batch fermentation process, inoculated by Pulsed Electric Field (PEF) treated wine yeasts (Saccharomyces cerevisiae Actiflore F33), was studied. PEF treatment was applied to the aqueous yeast suspensions ([Y]=0.012g/L) at the electric field strengths of E=100 and 6000V/cm using the same treatment protocol (number of pulses n=1000, pulse duration ti=100μs, and pulse repetition time Δt=100ms). Electrical conductivity was increasing during and after the PEF treatment, which reflected cell electroporation. Then, fermentation was run for 150h in an incubator (30°C) with synchronic agitation. Electro-stimulation was revealing itself by the improvement of fermentation characteristics, and thus increased yeast metabolism. At the end of the lag phase (t=40h), fructose consumption in samples with electrically activated inoculum exceeded that of the control samples by ≈2.33 times for E=100V/cm and by ≈3.98 for E=6000V/cm. At the end of the log phase (120h of fermentation), ≈30% mass reduction was reached in samples with PEF-treated inocula (E=6000V/cm), whereas the same mass reduction of the control sample required approximately 20 extra hours of fermentation.

Strategies for improved 3D small‐tip fast recovery imaging

PurposeSmall‐tip fast recovery (STFR) imaging is a recently proposed steady‐state sequence that has similar image contrast as balanced steady‐state free precession but has the potential to simultaneously remove banding artifacts and transient fluctuation. STFR relies on a “tip‐up” radiofrequency (RF) pulse tailored to the accumulated phase during the free precession (data acquisition) interval, designed to bring spins back to the longitudinal axis, thereby preserving transverse magnetization as longitudinal magnetization for the next pulse repetition time. We recently proposed an RF‐spoiled STFR sequence suitable for thin slab imaging, however, in many applications, e.g., functional magnetic resonance imaging or isotropic‐resolution structural imaging, three‐dimensional (3D) steady‐state imaging is desirable. Unfortunately, 3D STFR imaging is challenging due to the need for 3D tailored RF pulses. Here, we propose new strategies for improved 3D STFR imaging, based on (i) unspoiled imaging, and (ii) joint design of nonslice‐selective tip‐down/tip‐up RF pulses.Theory and MethodsWe derive an analytic signal model for the proposed unspoiled STFR sequence, and propose two strategies for designing the 3D tailored tip‐down/tip‐up RF pulses. We validate the analytic results using phantom and in vivo imaging experiments.ResultsOur analytic model and imaging experiments demonstrate that the proposed unspoiled STFR sequence is less sensitive to tip‐up excitation error compared to the corresponding spoiled sequence, and may, therefore, be an attractive candidate for 3D imaging. The proposed “joint” RF pulse design method, in which we formulate the tip‐down/tip‐up RF pulse design task as a magnitude least squares problem, produces modest improvement over a simpler “Separate” design approach. Using the proposed unspoiled sequence and joint RF pulse design, we demonstrate proof‐of‐principle 3D STFR brain images with balanced steady‐state free precession‐like signal properties but with reduced banding.ConclusionUsing the proposed unspoiled sequence and joint RF pulse design, STFR brain images in a 3D region of interest with balanced steady‐state free precession‐like signal properties but with reduced banding can be obtained. Magn Reson Med 72:389–398, 2014. © 2013 Wiley Periodicals, Inc.

A GAUSSIAN MONOCYCLE PULSE GENERATOR/MODULATOR FOR UWB RADIOS APPLICATIONS

In this paper, we present, at transistor level, a very low complexity Gaussian monocycle pulse generator/modulator for DPSK low voltage and low power tunable ultra-wideband (UWB) applications. The pulse generator, simulated in a SMIC 0.13 μm CMOS technology, provides a tunable, both in pulse width duration and repetition time, Gaussian monocycle compliant with the sub-GHz ETSI band and suitable to be used in UWB radio applications. The preliminary IC simulation results show a pulse duration from 1 ns up to 20 ns with good pulse symmetry and with the DPSK modulation capability. The circuit power consumption is about 1 mW from a 1.8 V power supply.