Minimum Integration Time Research Articles

Geometric and dosimetric verification of step‐and‐shoot modulated fields with a new fast and high resolution beam imaging system

A technique for geometric and dosimetric pretreatment verification of step‐and‐shoot intensity modulated radiotherapy treatments (IMRT) using a beam imaging system (BIS) made up of a charge‐coupled device (CCD) digital camera optically coupled with a metal‐plate/phosphor screen is described. Some physical properties of BIS were investigated in order to demonstrate its capability to perform measurements with a high spatial resolution and a high sampling rate. High‐speed imaging, with a minimum charge integration time on the CCD of 120 ms, can be performed. The study of the signal‐to‐noise ratio as a function of sampling time is presented. In‐plane and cross‐line pixel size was measured to be , which agrees within 0.5% of the manufacturer value of 0.366 mm. Spatial linearity results are very good and there are no detectable image distortions on whole detector area. A software routine was written to automatically extract positions of the collimator leaves from the images of the field shaped by the multileaf collimator (MLC) and also to compare them with the coordinates from the treatment planning system (TPS), thus directly testing both the MLC positioning and the treatment parameters transfer from TPS to the linear accelerator in a fast and precise way. The dosimetric capabilities (characteristics) of the imaging device for photon beams with energies of 6 and 15 MV were studied. Additional plexiglass buildup layers, depending on x‐ray energy, were needed to reach maximum efficiency. The energy dependence of the BIS response versus dose and dose rate was found to be linear over a wide range. Relative output factors of BIS as a function of field size, compared with values measured with an ionization chamber, were in good accord for smaller field sizes but showed differences up to 4% for all the energies at the respective buildup depth for bigger fields. Square field profiles at water‐equivalent buildup depths, extracted from BIS maps, are compared with the corresponding scans performed with a diode detector. Disagreement is always shown in the regions outside the field penumbra (tails) and near the field edges only for field sizes due to the metal/phosphor screen higher sensitivity to low energy scattering x‐rays. A straightforward correction method for the “tails effect” was developed and then generalized to MLC‐shaped fields. In order to demonstrate the validity of this procedure, the comparison between the two‐dimensional (2D) dose distributions of a triangle MLC‐shaped field and of two simple IMRT fields created by the superimposition of five segments resulting from BIS images and the dose distribution of the same fields achieved by film, was measured and reported. The gamma index method, introduced by Low et al. (1998), was used for 2D dose distributions analysis. Agreements are good for both 6 and 15 MV energies. The described technique provides a lower time‐expensive mean to verify geometric and dosimetric accuracy of the treatment delivered in IMRT with the use of a high resolution beam imaging system and homemade software tools.

Improved charge-coupled device detectors for high-speed, charge exchange spectroscopy studies on the DIII-D tokamak

Charge exchange spectroscopy is one of the key ion diagnostics on the DIII-D tokamak. It allows determination of ion temperature, poloidal and toroidal velocity, impurity density, and radial electric field Er throughout the plasma. For the 2003 experimental campaign, we replaced the intensified photodiode array detectors on the central portion of the DIII-D charge exchange spectroscopy system with advanced charge–coupled device (CCD) detectors mounted on faster (f/4.7) Czerny–Turner spectrometers equipped with toroidal mirrors. The CCD detectors are improved versions of the ones installed on our edge system in 1999. The combination improved the photoelectron signal level by about a factor of 20 and the signal to noise by a factor of 2–8, depending on the absolute signal level. The new cameras also allow shorter minimum integration times while archiving to PC memory: 0.552 ms for the slower, lower-read noise (15 e) readout mode and 0.274 ms in the faster, higher-read noise (30 e) mode.

Focus on Spectroscopy

Focus on Spectroscopy

Detection and direction-discrimination of diotic and dichotic ramp modulations in amplitude and phase.

When the source of a tone moves with respect to a listener's ears, dichotic (or interaural) phase and amplitude modulations (PM and AM) are produced. Two experiments investigated the psychophysical characteristics of dichotic linear ramp modulations in phase and amplitude, and compared them with the psychophysics of diotic PM and AM. In experiment 1, subjects were substantially more sensitive to dichotic PM than diotic PM, but AM sensitivity was equivalent in the dichotic and diotic conditions. Thresholds for discriminating modulation direction were smaller than detection thresholds for dichotic AM, and both diotic AM and PM. Dichotic PM discrimination thresholds were similar to detection thresholds. In experiment 2, the effects of ramp duration were examined. Sensitivity to dichotic AM and PM, and diotic AM increased as duration was increased from 20 ms to 200 ms. The functions relating sensitivity to ramp duration differed across the stimuli; sensitivity to dichotic PM increased more rapidly than sensitivity to dichotic or diotic AM. This was also reflected in shorter time-constants and minimum integration times for dichotic PM detection. These findings support the hypothesis that the analysis of dichotic PM and AM rely on separate mechanisms.

Temporal modulation transfer functions in auditory receptor fibres of the locust ( Locusta migratoria L.).

The temporal resolution of auditory receptors of locusts was investigated by applying noise stimuli with sinusoidal amplitude modulations and by computing temporal modulation transfer functions. These transfer functions showed mostly bandpass characteristics, which are rarely found in other species at the level of receptors. From the upper cut-off frequencies of the modulation transfer functions the minimum integration times were calculated. Minimum integration times showed no significant correlation to the receptor spike rates but depended strongly on the body temperature. At 20 degrees C the average minimum integration time was 1.7 ms, dropping to 0.95 ms at 30 degrees C. The values found in this study correspond well to the range of minimum integration times found in birds and mammals. Gap detection is another standard paradigm to investigate temporal resolution. In locusts and other grasshoppers application of this paradigm yielded values of the minimum detectable gap widths that are approximately twice as large than the minimum integration times reported here.

Pixel array detectors for time resolved radiography (invited)

Intense x-ray sources coupled with efficient, high-speed x-ray imagers are opening new possibilities of high-speed time resolved experiments. The silicon pixel array detector (PAD) is an extremely flexible technology which is currently being developed as a fast imager. We describe the architecture of the Cornell PAD, which is capable of operating with submicrosecond frame times. This 100×92 pixel prototype PAD consists of a pixelated silicon diode layer, for direct conversion of the x rays to charge carriers, and a corresponding pixellated complementary metal–oxide–semiconductor electronics layer, for processing and storage of the generated charge. Each pixel diode is solder bump bonded to its own pixel electronics consisting of a charge integration amplifier, an array of eight storage capacitors and an output amplifier. This architecture allows eight complete frames to be stored in rapid succession, with a minimum integration time of 150 ns per frame and an interframe deadtime of 600 ns. We describe the application of the PAD to capture an x-radiograph movie of the mass-density distribution of the spray plume from internal combustion engine fuel injectors.

Temporal modulation transfer functions in the barn owl (Tyto alba).

Barn owls (Tyto alba) have evolved several specializations in their auditory system to achieve the high sensory acuity required for prey capture, including superior processing of interaural time differences and phase coding in the auditory periphery. Here, we tested whether barn owls are capable of high temporal resolution that may be a prerequisite for the accuracy in binaural processing. Temporal resolution was measured psychoacoustically and demonstrated in temporal modulation transfer functions. Four barn owls were trained in an operant task with food reward to detect sinusoidal amplitude modulations within an 800-ms gated white-noise burst or 800-ms periods of modulation in continuous white noise (spectrum levels of -5 dB and 15 dB SPL). Within the range of tested amplitude modulation frequencies from 5 Hz to 1280 Hz, barn owls' detection thresholds were lowest at 10-20 Hz. This sensitivity corresponds to an intensity-difference limen of between 0.9 dB and 1.4 dB. For all conditions, temporal modulation transfer functions showed band-pass characteristics with a high-frequency cutoff in the range of 37 Hz to 92 Hz, corresponding to minimum integration times of 4.3 ms and 1.7 ms, respectively. In summary, these data indicate a temporal resolution in the owl's auditory system that is good, but not unusual, compared to other vertebrates.

Improved charge coupled device detectors for the edge charge exchange spectroscopy system on the DIII-D tokamak

Charge exchange spectroscopy is one of the key ion diagnostics on the DIII-D tokamak. It allows measurement of impurity densities, toroidal and poloidal rotation speeds, ion temperatures, and the radial electric field. For the 2000 experimental campaign, we have replaced the intensified photodiode array detectors on the edge portion of the system with advanced charge-coupled device (CCD) detectors mounted on faster (f/4.7) Czerny–Turner spectrometers equipped with toroidal mirrors. The combination has improved the photoelectron signal level by about a factor of 20 and the signal to noise by a factor of 2–8, depending on the absolute signal level and readout mode. A major portion of the signal level improvement comes from the improved quantum efficiency of the back-illuminated, thinned CCD detector (70% to 85% quantum efficiency for the CCD versus 10% for the image intensifier) with the remainder coming from the faster spectrometer. The CCD camera also allows shorter minimum integration times: 0.33 ms while archiving to computer memory and 0.15 ms using temporary storage on the CCD chip.

Atmospheric temperature profiling in the presence of clouds with a pure rotational Raman lidar by use of an interference-filter-based polychromator

A lidar polychromator design for the measurement of atmospheric temperature profiles in the presence of clouds with the rotational Raman method is presented. The design utilizes multicavity interference filters mounted sequentially at small angles of incidence. Characteristics of this design are high signal efficiency and adjustable center wavelengths of the filters combined with a stable and relatively simple experimental setup. High suppression of the elastic backscatter signal in the rotational Raman detection channels allows temperature measurements independent of the presence of thin clouds or aerosol layers; no influence of particle scattering on the lidar temperature profile was observed in clouds with a backscatter ratio of at least 45. The minimum integration time needed for temperature profiling with a statistical temperature error of +/-1 K at, e.g., 20-km height and 960-m height resolution is 1.5 h.

Radon detection system, design, test and performance

A portable radon detection system (α-Inin) has been designed and constructed for using it in adverse environmental conditions where humidity, temperature and chemical vaporous are present. The minimum integration time is in periods of 15 min during 41 days. A 12 V battery and a photovoltaic module allow the α-Inin autonomy in field measurements. Data is collected by means of a laptop computer where data processing and α-Inin programming are carried out. α-Inin performance was simultaneously tested in a controlled radon chamber, together with a commercial α-Meter.

Characterization and preliminary results of the collective Thomson scattering system on FTU tokamak

A collective Thomson scattering (CTS) experiment is now installed on FTU aimed to measure the plasma ion temperature and test the theory of CTS. A high power 140 GHz gyrotron beam is focalized into the plasma center by means of a circular corrugated transmission line and an in-vessel optical system to a radius of 0.025 m. The scattered radiation is collected at 90° scattering angle by a symmetric in-vessel optical system and a low-power quasioptical transmission line which collects the scattered radiation on a superheterodyne receiver. Both in-vessel optical systems are placed in a vertical port. The receiver is based on 32 channels, 1.2 GHz global bandwidth, with a measured noise temperature Trec=1 eV and minimum integration time of 1 ms. The antenna decoupling measurement resulted about 60 dB, while the attenuation introduced by the collecting system is evaluated in 5 dB. The angle of the scattering wavevector and the magnetic field directions is 25°. The experiment is run at a magnetic field B=7.4–7.7 T, i.e., the gyrotron frequency is well below the first electron cyclotron emission (ECE) harmonic, so the ECE plasma background temperature is low and it is measured TECE=70–50 eV, respectively. The detected polarization is the ordinary mode. In this experiment the calculated signal is of the order of the ECE noise level. Indeed in a series of dedicated FTU runs, CTS signals with signal-to-noise consistent with calculation (made using standard formulas) were found. Some features of the detected scattered spectra are reported, as well as plasma radiation measurements in various plasma conditions including lower hybrid rf heating.

A fast charge coupled device detector for charge exchange recombination spectroscopy on the DIII-D Tokamak

Charge exchange recombination (CER) spectroscopy has become a standard diagnostic for Tokamaks. CER measurements have been used to determine spatially and temporally resolved ion temperature, toroidal and poloidal ion rotation speed, impurity density, and radial electric field. Knowledge of the spatial profile and temporal evolution of the electric field shear in the plasma edge is crucial to understanding the physics of the L to H transition. High speed CER measurements are also valuable for edge localized mode studies. Since the 0.52 ms minimum time resolution of our present system is barely adequate to study the time evolution of these phenomena, we have developed a new charge coupled device (CCD) detector system with about a factor of 2 better time resolution. In addition, our existing system detects sufficient photons to utilize the shortest time resolution only under exceptional conditions. The new CCD detector has a quantum efficiency of about 0.65, which is a factor of 7 better than our previous image intensifier-silicon photodiode detector systems. We have also equipped the new system with spectrometers of lower f/number. This combination should allow more routine operation at the minimum integration time, as well as improving data quality for measurements in the divertor-relevant region outside of the separatrix. Construction details, benchmark data, and initial Tokamak measurements for the new system will be presented.

A fast charge coupled device detector for charge exchange recombination spectroscopy on the DIII-D Tokamak (abstract)

Charge exchange recombination (CER) spectroscopy has become a standard diagnostic for Tokamaks. CER measurements have been used to determine spatially and temporally resolved ion temperature, toroidal and poloidal ion rotation speed, impurity density, and radial electric field. Knowledge of the spatial profile and temporal evolution of the electric field shear in the plasma edge is crucial to understanding the physics of the L to H transition. High speed CER measurements are also valuable for edge localized mode studies. Since the 0.52 ms minimum time resolution of our present system is barely adequate to study the time evolution of these phenomena, we have developed a new charge coupled device (CCD) detector system with about a factor of 2 better time resolution. In addition, our existing system detects sufficient photons to utilize the shortest time resolution only under exceptional conditions. The new CCD detector has a quantum efficiency of about 0.65, which is a factor of 7 better than our previous image intensifier-silicon photodiode detector systems. We have also equipped the new system with spectrometers of lower f/number. This combination should allow more routine operation at the minimum integration time, as well as improving data quality for measurements in the divertor-relevant region outside of the separatrix. Construction details, benchmark data, and initial Tokamak measurements for the new system will be presented.

Temporal modulation transfer functions in the European starling ( Sturnus vulgaris): II. Responses of auditory-nerve fibres

The temporal resolution of cochlear-nerve fibres in the European starling was determined with sinusoidally amplitude-modulated noise stimuli similar to those previously used in a psychoacoustic study in this species (Klump and Okanoya, 1991). Temporal modulation transfer curves (TMTFs) were constructed for cochlear afferents allowing a direct comparison with the starling's behavioural performance. On average, the neuron's detection of modulation was less sensitive than that obtained in the behavioural experiments, although the most sensitive cells approached the values determined psychophysically. The shapes of the neural TMTFs generally resembled low-pass or band-pass filter functions, and the shapes of the averaged neural functions were very similar to those obtained in the behavioural study for two different types of stimuli (gated and continuous carrier). Minimum integration times calculated from the upper cut-off frequency of the neural TMTFs had a median of 0.97 ms with a range of 0.25 to 15.9 ms. The relations between the minimum integration times and the tuning characteristics of the cells (tuning curve bandwidth, Q 10 dB-value, high- and low-frequency slopes of the tuning curves) are discussed. Finally, we compare the TMTF data recorded in the starling auditory nerve with data from neurophysiological and behavioural observations on temporal resolution using other experimental paradigms in this and other vertebrate species.

Evolution of Anuran Chorus Behavior: Neural and Behavioral Constraints

This article provides values for two fundamental time constraints (minimum integration time and temporal integration time constant) of the peripheral auditory system of the Neotropical arboreal frog, Eleutherodactylus coqui. The minimum integration time (MinIT) is a common measure of temporal resolution, or the auditory system's ability to distinguish two events exclusively on the basis of temporal cues. In the case of E. coqui, calls separated in time by less than the MinIT (0.5 ms on the average) cannot be resolved by the auditory system. This represents one limit that any chorusing model must consider if it uses gaps as cues for the initiation of calling by participant males. The mean temporal integration time constants for auditory nerve fiber groups in this species vary between 190 and 270 ms. These values reflect the time over which the ear can integrate energy, and therefore they provide an upper limit to useful communication signal duration with respect to enhanced detectability in background no...

Minimum audible movement angle in the horizontal plane as a function of stimulus frequency and bandwidth, source azimuth, and velocity.

Minimum audible movement angles (MAMAs) were measured in the horizontal plane for four normal-hearing adult subjects in a darkened anechoic chamber. On each trial, a single stimulus was presented, and the subject had to say whether it came from a stationary loudspeaker or from a loudspeaker that was moving at a constant angular velocity around him. Thresholds were established by adaptively varying stimulus duration. In experiment 1, MAMAs were measured as a function of center frequency (500-5000 Hz), velocity (10 degrees-180 degrees/s), and direction of motion (left versus right). There was no effect of direction of motion. MAMAs increased with velocity, from an average of 8.8 degrees of arc for a target moving at 10 degrees/s to an average of 20.2 degrees of arc for a target moving at 180 degrees/s. MAMAs were higher for a 3000-Hz tone than for tones of lower or higher frequencies, as has been previously reported [D. R. Perrott and J. Tucker, J. Acoust. Soc. Am. 83, 1522-1527 (1988)]. In experiment 2, minimum audible angles (MAAs) were measured with sequentially presented stationary tone pulses (500-5000 Hz), and were shown to exhibit the same dependence on signal frequency that the MAMAs showed (average MAA at 3000 Hz: 8.4 degrees; average MAA at the other frequencies: 3.4 degrees). In experiment 3, MAMAs and MAAs were measured as a function of stimulus bandwidth (centered at 3000 Hz) and listening azimuth (0 degrees vs 60 degrees). Average MAAs decreased monotonically as stimulus bandwidth increased from 0 Hz to wideband (from 8.4 degrees to 1.2 degrees at 0 degrees azimuth; from 11.3 degrees to 1.5 degrees at 60 degrees azimuth). As in experiment 1, MAMAs increased with stimulus velocity, from values comparable to the MAAs for the slowest-velocity (10 degrees/s) targets to 70 degrees of arc or more in the poorest condition (third-octave band of noise presented at a velocity of 180 degrees/s and an azimuth of 60 degrees). MAMAs obtained in the slower-velocity conditions depended in the same way on stimulus bandwidth and listening azimuth that MAAs depended on these variables. In no case was the MAMA ever smaller than the MAA. It is hypothesized that a minimum integration time is required to achieve optimal performance in a dynamic spatial resolution task. Average estimates of this minimum time based on the current data vary from 336 ms (for targets presented at midline) to 1116 ms (for narrow-band targets presented at 60 degrees azimuth).(ABSTRACT TRUNCATED AT 400 WORDS)

Temporal resolution in frog auditory-nerve fibers.

Sinusoidally amplitude-modulated (SAM) noise was monaurally presented to the neotropical frog, Eleutherodactylus coqui, while recording intracellularly from auditory-nerve fibers. Neuronal phase locking was measured to the SAM noise envelope in the form of a period histogram. The modulation depth was changed (in 10% steps) until the threshold modulation depth was determined. This was repeated for various modulation frequencies (20-1200 Hz) and different levels of SAM noise (34-64 dB/Hz). From these data, temporal modulation transfer functions (TMTFs) were produced and minimum integration time (MIT) for each auditory fiber was calculated. The median MIT was 0.42 ms (lower quartile 0.32, upper quartile 0.68 ms). A noise level-dependent effect was noted on the shape of the TMTF as well as the minimum integration time. The latter results may be explained as a loss in spectral resolution with increasing noise level, which is consistent with the correlation that was found between minimum integration time and bandwidth.

Regulation of the flow rate of liquid-metal coolants on experimental stands

Systems for automatic regulation of the flow rate of alkali metals, based on the series ENIV, VIN, and TsLIN three-phase electromagnetic pumps with a pumping rate of 0.5-200 m/sup 3/ per hour, were evaluated. The stability of each system was investigated by the method of undamped oscillations. The possibility of employing the analog temperature regulators VRT-2, RPA-T, and R113 was assessed. The functions performed by the most suitable automatic regulation unit, the RPA-T, were described. The limiting period of flow rate oscillations with a maximum gain of the RPA-T in alkali metal regulation systems equaled about 0.5 sec and the minimum integration time of the RPA-T was an order of magnitude longer than the optimal interval. Use of the systems on experimental stands enabled raising the quality of the studies and expanding the zone of servicing of the facilities by the same personnel.

Phase effects in masking related to dispersion in the inner ear.

Phase effects in masking experiments using multitone maskers are usually associated with strong variations in the masker envelope. In this article, psychoacoustic experiments with such maskers that lead to phase-dependent threshold variations of up to 20 dB, although the phase transformation leaves the envelope unchanged, are described. However, after filtering the maskers with a realistic basilar membrane model, the envelopes are different owing to the models phase-dispersive properties. Comparison of model outputs with the experimental results reveals a strong correlation between the two for a wide range of parameters, provided one makes the additional assumption that the ear has a minimum integration time of a few milliseconds.

Detection and discrimination of simulated motion of auditory targets in the horizontal plane.

Three experiments investigated subjects' ability to detect and discriminate the simulated horizontal motion of auditory targets in an anechoic environment. "Moving" stimuli were produced by dynamic application of stereophonic balancing algorithms to a two-loudspeaker system with a 30 degree separation. All stimuli were 500-Hz tones. In experiment 1, subjects had to discriminate a left-to-right moving stimulus from a stationary stimulus pulsed for the same duration (300 or 600 ms). For both durations, minimum audible "movement" angles ("MAMA's") were on the order of 5 degrees for stimuli presented at 0 degrees azimuth (straight ahead), and increased to greater than 30 degrees for stimuli presented at +/- 90 degrees azimuth. Experiment 2 further investigated MAMA's at 0 degrees azimuth, employing two different procedures to track threshold: holding stimulus duration constant (at 100-600 ms) while varying velocity; or holding the velocity constant (at 22 degrees-360 degrees/s) while varying duration. Results from the two procedures agreed with each other and with the MAMA's determined by Perrott and Musicant for actually moving sound sources [J. Acoust. Soc. Am. 62, 1463-1466 (1977b)]: As stimulus duration decreased below 100-150 ms, the MAMA's increased sharply from 5 degrees-20 degrees or more, indicating that there is some minimum integration time required for subjects to perform optimally in an auditory spatial resolution task. Experiment 3 determined differential "velocity" thresholds employing simulated reference velocities of 0 degrees-150 degrees/s and stimulus durations of 150-600 ms. As with experiments 1 and 2, the data are more easily summarized by considering angular distance than velocity: For a given "extent of movement" of a reference target, about 4 degrees-10 degrees additional extent is required for threshold discrimination between two "moving" targets, more or less independently of stimulus duration or reference velocity. These data suggest that for the range of simulated velocities employed in these experiments, subjects respond to spatial changes--not velocity per se--when presented with a "motion" detection or discrimination task.