Variation In Arrival Time Research Articles

Evaluation of an instrument to improve PET timing alignment

Purpose In order to increase the simplicity and accuracy of performing the time alignment on a positron emission tomography (PET) scanner, a new generation timing alignment probe has been developed. Methods A timing alignment probe containing a plastic scintillator with an embedded sodium-22 source which is optically coupled to a fast photomultiplier tube (PMT) is described and tested. When a positron is ejected from the radioactive atom’s nucleus, its kinetic energy is absorbed in and can be detected as a light flash from the scintillator. This is used as the reference time for each atom’s positron decay. It is only after the positron slows that it can combine with an electron, forming positronium after which the 511 keV annihilation photons will be created, possibly traveling to the PET detectors. In practice, the probe is placed in the center of the scanner’s field of view and connected to the coincidence circuit. Since the delay between an annihilation photon’s detection and positron detection is almost identical (the lifetime of positronium in a solid is extremely short, and the gamma rays’ path lengths are equal with the probe in the center of the scanner), the probe’s signal provides a fixed reference time to which the response of individual crystals in the PET detectors can be compared. We present an evaluation of the performance of this probe. We first investigated the intrinsic performance of the time-alignment probe comparing its timing resolution with two barium fluoride crystals in coincidence. We then investigated the timing performance of the probe in coincidence with various individual scintillation crystals and with detectors from two commercial PET scanners. Results The best full-width at half-maximum (FWHM) timing resolution of the probe was found when in coincidence with BaF 2 at 400 ps. The common commercial scintillator lutetium oxy-orthosilicate (LSO) was tested and its FWHM was 510 ps. When testing the crystal arrays used in two commercial block detectors it was found that there are significant, systematic timing delays among the crystals. In the Siemens HiRez ® detector the average time difference between the positron detection and annihilation photon detection is 500 ps with a standard deviation of 115 ps. In the Siemens Focus ® detector the average was 1000 ps and a standard deviation of 400 ps. The reasons for the variation in apparent arrival times among the crystals appear to be due to the electronic readout rather than light transport from the crystals to the PMT. Conclusion Many PET scanners use a global time offset for each detector, and these variations in apparent time delay from individual crystals in the detector would have a significant impact if these detectors were employed in a time-of-flight (ToF) PET scanner.

Arrival time variations of pulses in shallow water and low-frequency acoustical underwater positioning.

Variations in acoustic arrival time in shallow water are presented to establish a method for long range underwater acoustical positioning systems (UWPSs) with possible applications for underwater vehicles. At the present time, there are different methods used for UWPS for only short distances (about a few hundred meters), relating the sound signals to distance based on ocean physical properties. It is possible to use arrival times of high‐frequency signals (up to 100 kHz) radiated by a set of reference sound sources (such as 3–4), placed at different points in an area to determine the position. However, the main problem for long range positioning (i.e., operation area of several hundred to thousand kilometers) is significant spatial and temporal variabilities of waveguide environmental properties affecting the signal travel time. To establish limits for low‐frequency acoustical UWPS, here we use recent field observations in the SW06 experiment to estimate signal arrival time variations caused by different hydrodynamic conditions observed simultaneously during acoustic transmissions. Based on preliminary mapping of the operation area, and the subsequent optimization, the accuracy related to the waveguide dynamics is presented. [Work supported by ONR321OA and CRDF.

The possibility of using a vertical array for estimating the delays of sound pulses at multimegameter ranges

The majority of schemes used for acoustic monitoring of temperature fields in the ocean are based on measuring the variations of the so-called ray arrival times, i.e., the travel times of sound pulses along different ray trajectories connecting a source and a receiver. The solution of the inverse problem is considerably hindered by the fact that, in the case of signal detection by a point receiver, it is possible to resolve only the sound pulses propagating along steep rays. To a large extent, this is caused by the phenomenon of ray chaos, which is fully developed at distances of about one thousand of kilometers. The present study shows that the use of a vertical receiving array provides an opportunity to loosen this restriction. An appropriate space-time processing procedure is proposed. The procedure is based on the characteristics of ray arrival distribution in the time-depth plane that remain stable even under the conditions of ray chaos.

Multipath reflection from surface waves

A tank experiment was conducted at Scripps Institution of Oceanography to measure reflection of underwater sound from surface waves. Short pulses at a nominal 200 kHz were transmitted beneath surface waves of wavelength 0.7 m to a receiver at 1.2 m range. The surface wave crests act as curved mirrors for underwater sound and lead to focussing and caustics in the surface reflected pulses. The locations of the foci and caustics move steadily as the wave progresses and lead to rapid variation of amplitude, phase and arrival time of the received pulses. Wavefront modeling has been used to calculate theoretical waveforms for the measured surface wave shape. The theory shows there are typically three distinct reflected eigenrays beneath a wave crest and they interfere to give rapid variation of the received signal. The theory gives good agreement with the details of the time dependent interference of the surface reflected pulses. [Work supported by ONR.]

Pooling Restaurant Reservations to Increase Service Efficiency

This article examines whether restaurant reservations should be locked to specific tables at the time the reservation is made or whether the reservations should be pooled and assigned to tables in real time. In two motivating studies, the authors find that there is a lack of consensus in the restaurant industry on handling reservations. Contrary to what might be expected based on research that shows the benefits of resource pooling in other contexts, a survey of 425 restaurants indicated that more than 80% lock reservations to tables. In two simulation studies, the authors determine that pooling reservations enables a 15-minute reduction in table turn times more than 15% of the time, which consequently increases service efficiency and enables a restaurant to serve more customers during peak periods. Pooling had the most consistent advantage with higher customer service levels, larger restaurants, customers who arrive late, and larger variation in customer arrival time.

Optimum assignment of trains to platforms under partial schedule compliance

The paper develops a linear mixed integer programming formulation for allocating platforms optimally to trains arriving at a busy multi-platform station. The formulation does not assume that all trains arrive as per schedule, in fact it assumes that the exact arrival times of trains are known shortly (an hour or so) before the actual arrivals of the trains. Such variation in arrival times often necessitates delaying of trains (on the entry tracks) due to non-availability of platforms; these delays may also cause queuing up of trains on the tracks. While determining the optimum allocation the formulation takes into account the inconvenience caused due to (i) delay, (ii) allocation of non-preferred platforms (some platforms may be preferred for some trains – as is the case in India), and (iii) last minute reassignment of platforms. The constraints ensure that all physical and safety related restrictions are satisfied. Various problems developed from the schedule of arrivals at a busy station in India are also solved and the results analyzed.

Getting to grips with jitter

The word 'jitter' in the communications context has come to mean any random or periodic variation in timing of a regular event from its intended or nominal timing. On the one hand, jitter can refer to arrival time variations for IP packets, or, on the other, to variations on clock signals in digital systems, as well as many other things. A big problem with jitter is just measuring it.

Diffraction induced space–time splitting effects in ultra-short pulse propagation

Diffraction induced space–time splitting effects are studied in the propagation of ultra-short optical pulses by an optimized numerical simulation technique, avoiding approximations used in exact calculations. Point-wise detection as well as detection in a two-plus-one dimension of a pulse are simulated. Effects of diffraction on the spectral and spatial–temporal features of pulse evolution are analysed for a rotationally nonsymmetric pulse. Delays in the arrival of the portions of the pulse travelling off-axis are visualized with accurate estimation of pulse arrival time and amplitude variations. Subtle effects of pulse splitting and recombination in space and time both in near and far field are brought out clearly.

Variation of the Time to Aortic Enhancement of Fixed-Duration Versus Fixed-Rate Injection Protocols

The objective of the study was to clarify whether a fixed-duration injection protocol is useful in determining the optimal scan delay time without the need for a bolus-tracking technique. Three hundred eighteen patients underwent a helical CT examination using a bolus-tracking technique. All the examinations were performed after administering a nonionic contrast medium (300 or 370 mg I/mL; 2 mL/kg of body weight for patients weighing < or = 75 kg, 150 mL for those weighing > 75 kg). The patients were assigned to one of three groups according to the injection protocol. The injection rate was alternated to 3 or 4 mL/sec in group 1. The injection duration was 38 or 47 sec in groups 2 and 3, respectively. The aortic arrival time and the 100-H threshold time in each patient were measured. The mean values and the variations in the aortic arrival time and 100-H threshold time according to the injection protocols and the contrast media were compared. The mean variations (+/- SD) of aortic arrival times and 100-H thresholds in group 2 (aortic arrival time = 16.1 +/- 2.7 sec, 100-H threshold time = 19.6 +/- 2.9 sec) were smaller than in groups 1 (16.3 +/- 3.0 sec and 19.9 +/- 3.7 sec, respectively) and 3 (16.8 +/- 3.5 sec and 20.4 +/- 4.1 sec, respectively). However, the range of aortic arrival times and 100-H threshold times was more than 10 sec for all groups. The mean aortic arrival time and 100-H threshold time for all patients were 16.5 and 20.0 sec, respectively, and did not vary significantly with the injection protocol and concentration of contrast medium. The individual variations of the aortic arrival and 100-H threshold times can be reduced using a fixed-duration injection technique, but there are still substantial variations. Therefore, a bolus-tracking technique is recommended for optimal timing of arterial phase scanning.

Model-based end-to-end available bandwidth inference using queueing analysis

End-to-end available bandwidth estimation between Internet hosts is important to understand network congestion and enhance the performance of Quality-of-Service (QoS) demanding applications. In this paper, we investigate model-based available bandwidth measurement via the use of an active probing stream. A general end-to-end measurement framework, which unifies the current research approaches and highlights insights for measurement practice, is proposed. Within this framework, the end-to-end available bandwidth is inferred based on the measurement of the performance metrics of an active probing stream. We study two probing streams: Poisson and periodic probing. Of particular interest to our investigations is the Squared Coefficient of Variation (SCV) of the inter-probing packet arrival time at the receiver. The performance comparison of the available bandwidth measurements based on loss models and delay models indicates that the delay-based measurement exhibits many advantages over the loss-based measurements, such as accuracy, overhead and robustness. We conducted a comparison study between the proposed SCV-based probing scheme, namely, SCVProbe, and Pathload using ns-2 simulation in terms of probing accuracy, convergence time and overhead. Our evaluation results indicate that SCVProbe achieves similar or even better measurement accuracy than Pathload with much less probing time and smaller overhead.

Imaging and suppressing near-receiver scattered surface waves

When traveling through a complex overburden, upcoming seismic body waves can be disturbed by scattering from local heterogeneities. Currently, surface-consistent static and amplitude corrections correct for rapid variations in arrival times and amplitudes of a reflector, but these methods impose strong assumptions on the near-surface model. Observations on synthetic and laboratory experiments of near-surface scattering with densely sampled data suggest that removing noise from near-receiver scattering requires multichannel approaches rather than single-channel, near-surface corrections. In this paper we develop a wavefield-based imaging method to suppress surface waves scattered directly beneath the receivers. Using an integral-equation formulation, we account for near-surface heterogeneities by a surface impedance function. This impedance function is used to model scattered surface waves, excited by upcoming wavefronts. The final step in our algorithm is to subtract the scattered surface waves. We successfully apply this method to laboratory data of scattered surface waves, excited and monitored with a noncontacting acquisition system.

Evaluation of postponement structures to accommodate mass customization

In order to meet increasing customer demands for more diverse product offerings, firms are revising their supply chain structures to accommodate mass customization. The revised structures often involve delaying the delivery of the products until after the customer orders arrive, termed time postponement (TP), or delaying the differentiation of the products until later production stages, termed form postponement (FP). We develop models representing possible implementations of the TP and FP structures and compare their performance in total supply chain cost and expected customer waiting times. We find that once the number of different products increases above some threshold level, the TP structure is preferred under both performance metrics. For the most general model, a numerical experiment was designed to investigate how different factors affect the performance of the TP and FP structures. Through this experiment we show that higher arrival time and process time variations make the FP structure more favorable while increases in the number of products and higher interest rates make the TP structure more favorable. We also offer guidance to managers using either structure on where to allocate resources for performance improvement. For example, to improve the customer waiting times under the FP structure, increasing the coverage of the generic component and reducing the number of products provide larger benefits than reducing the variability of the arrival and process times.

Annual variation in arrival and departure times of carrion insects at carcasses: implications for succession studies in forensic entomology

A succession of insect species associate with decaying bodies, and because of the relatively predictable arrival and departure times of many species, this process is routinely used to estimate minimum post-mortem interval. Corpse fauna are compared with baseline data on succession rates, which are usually taken from decomposing animal carcasses. Baseline data are traditionally collected over a single year only; however, annual differences in weather patterns and insect population parameters probably alter succession rates. Surprisingly, this potential source of error has not previously been examined. Therefore, a two-year succession experiment was performed to investigate annual variation in succession rates. Sets of five still-born piglet carcasses were placed in a forest each season of both years. Six insects that occur year-round were selected: adults of Aleochara brachialis Jekel, Anotylus spp., Creophilus erythrocephalus Fabricius (Staphylinidae: Coleoptera), Ptomaphila lacrymosa Schreibers (Silphidae: Coleoptera), Pseudonemadus spp. (Leiodidae: Coleoptera), and larvae of Calliphora spp. (Calliphoridae: Diptera). Arrival and departure weeks to and from carcasses were measured in each season of both years. Two-way ANOVAs revealed significant season and year interactions on the arrival weeks of Anotylus spp., C. erythrocephalus, and Pseudonemadus spp., and significant differences in seasonal and yearly arrival times for A. brachialis, and P. lacrymosa. For larvae of Calliphora spp., there was a significant seasonal effect on arrival times. A significant interaction between season and year affected the departure weeks of all taxa. These results show that error in the estimates of minimum post-mortem interval can result if baseline succession data are collected only over one year.

The outer turbulence scale in the interstellar plasma

The structure function for phase fluctuations on spatial scales from 106 to 1017 m is constructed using data on diffractive and refractive scintillation of pulsars, scattering angles, variations in pulse arrival times, and differences in dispersion measures observed for close pairs of pulsars in globular clusters. For distances R>1 kpc (a sample of pulsars with DM≥30 pc/cm3), the fluctuations in the interstellar electron density on scales from 106 to 1014 m are well described by a Kolmogorov spectrum with index 11/3. Analysis of variations in the dispersion measures for close pairs of pulsars in globular clusters indicates an outer turbulence scale of L0=1015 m. The relative level of turbulent fluctuations is determined for the interstellar plasma, and the important role of turbulence in the energy balance is demonstrated.

Linear detection of optical polarization mode dispersion by arrival time modulation

Light with a low-speed polarization modulation suffers arrival time variations due to polarization mode dispersion. These can be detected by integration of the voltage-controlled oscillator input signal in the clock recovery phase-locked loop. This scheme is demonstrated in a 40-Gbit/s non-return-to-zero optical transmission experiment.

Polarization mode dispersion detected by arrival time measurement of polarization-scrambled light

Polarization mode dispersion (PMD) limits optical fiber capacity. PMD compensators usually minimize the associated eye closure. This measure scales with the square of the differential group delay (DGD) and makes it difficult to detect low DGDs. However, light with a low-speed polarization modulation suffers arrival time variations, in the presence of PMD, that are proportional to the DGD. These are detected by integrating the voltage-controlled oscillator (VCO) input signal of the clock recovery phase-locked loop (PLL). This novel method has been demonstrated for 40 Gb/s nonreturn-to-zero (NRZ) and for 2/spl times/40 Gb/s return-to-zero (RZ) polarization division multiplex transmission. PMD detection sensitivities range between 2 ps and 84 fs.

A Simple Algorithm for Sequentially Incorporating Gravity Observations in Seismic Traveltime Tomography

The geologic structure of the Earth's upper crust can be revealed by modeling variation in seismic arrival times and in potential field measurements. We demonstrate a simple method for sequentially satisfying seismic traveltime and observed gravity residuals in an iterative 3-D inversion. The algorithm is portable to any seismic analysis method that uses a gridded representation of velocity structure. Our technique calculates the gravity anomaly resulting from a velocity model by converting to density with Gardner's rule. The residual between calculated and observed gravity is minimized by weighted adjustments to the model velocity-depth gradient where the gradient is steepest and where seismic coverage is least. The adjustments are scaled by the sign and magnitude of the gravity residuals, and a smoothing step is performed to minimize vertical streaking. The adjusted model is then used as a starting model in the next seismic traveltime iteration. The process is repeated until one velocity model can simultaneously satisfy both the gravity anomaly and seismic traveltime observations within acceptable misfits. We test our algorithm with data gathered in the Puget Lowland of Washington state, USA (Seismic Hazards Investigation in Puget Sound [SHIPS] experiment). We perform resolution tests with synthetic traveltime and gravity observations calculated with a checkerboard velocity model using the SHIPS experiment geometry, and show that the addition of gravity significantly enhances resolution. We calculate a new velocity model for the region using SHIPS traveltimes and observed gravity, and show examples where correlation between surface geology and modeled subsurface velocity structure is enhanced.

84 fs PMD detection sensitivity in 2 × 40 Gbit/s RZ polarisation multiplex transmission experiment

Interchannel phase modulation of polarisation-multiplexed optical signals results in pulse arrival time variations in the presence of polarisation mode dispersion (PMD). Integration of the voltage controlled oscillator (VCO) input signal in the clock recovery phase locked loop (PLL) allows detection of these. 150 and 84 fs PMD detection sensitivities are demonstrated for 4.8 and 38.4 µs measurement intervals, respectively.

Optimization of contrast-enhanced MR angiography of the hands with a timing bolus and elliptically reordered 3D pulse sequence.

Our objective was to optimize bolus administration and sequence setting in gadolinium-enhanced magnetic resonance (MR) angiography of the hands. Elliptically reordered three-dimensional (3D) spoiled gradient-echo sequence with non-slab-selective radio frequency excitation was optimized according to the measurements of arterial and venous time-signal curves in 21 patients. Great variations in bolus arrival time and arterio-venous transit time could be observed. In most patients high-quality arterial depiction could be obtained with minor venous contamination. Contrast-to-noise, spatial resolution, and selective arterial filling is still a challenge for 3D MR angiography of the hand but can be optimized using Gadolinium-BOPTA and a dedicated pulse sequence setting with exact bolus timing.

In situ estimation of sediment sound speed and critical angle

Understanding the basic physics of sound penetration into ocean sediments is essential for the design of sonar systems that can detect, localize, classify, and identify buried objects. In this regard the sound speed of the sediment is a crucial parameter as the ratio of sound speed at the water-sediment interface determines the critical angle. Sediment sound speed is typically measured from core samples using high frequency (100's of kHz) pulsed travel time measurements. Earlier experimental work on subcritical penetration into sandy sediments has suggested that the effective sound speed in the 2-20 kHz range is significantly lower than the core measurement results. Simulations using Biot theory for propagation in porous media confirmed that sandy sediments may be highly dispersive in the range 1-100 kHz for the type of sand in which the experiments were performed. Here it is shown that a direct and robust estimate of the critical angle, and therefore the sediment sound speed, at the lower frequencies can be achieved by analyzing the grazing angle dependence of the phase delays observed on a buried array. A parametric source with secondary frequencies in the 2-16 kHz range was directed toward a sandy bottom similar to the one investigated in the earlier study. An array of 14 hydrophones was used to measure penetrated field. The critical angle was estimated by analyzing the variations of signal arrival times versus frequency, burial depth, and grazing angle. Matching the results with classical transmission theory yielded a sound speed estimate in the sand of 1626 m/s in the frequency range 2-5 kHz, again significantly lower the 1720 m/s estimated from the cores at 200 kHz. However, as described here, this dispersion is consistent with the predictions of the Biot theory for this type of sand.