Count Loss Research Articles

The rising world of flow cytometric analysis of pollen grains.

Key terms flow cytometry; genome size; plant sciences; pollen grains; pulse analysis FLOW cytometry is nowadays recognized as a vital tool in plant sciences, with more and more applications in both basic and applied research (1). Due to the unique characteristics of plant material, i.e., complex three-dimensional tissues with the presence of rigid cell walls, specific methods had to be developed to produce suspensions of single particles of interest (2). These particles of interest include, nuclei, mitochondria, chloroplasts, and chromosomes, with most of the applications being focused around the estimation of nuclear DNA content (either in absolute or relative amounts), leading to increasing impacts in the fields of plant breeding and population biology. One of the few single particles produced by plants is the pollen grain. Still, most works focused on studying pollen grains have involved the extraction of nuclei or sperm cells, rather than using the intact pollen. This is mostly due to the autofluorescence and/or specific staining of the pollen exine, and to the particle size limitations of some flow cytometers (3). To add to this, isolation of high-quality nuclei from pollen grains is often difficult, at least in comparison with somatic tissues. Together, these difficulties have led to a very minute number of publications involving flow cytometric analysis of pollen grains in the last 25 years.

Characterization of a Small Animal PET Detector Block Incorporating a Digital Photon Counter Array

A Small Animal PET detector block made with a Dual Layer Offset crystal array with 1.27 mm wide LYSO crystals on a Philips PDC3200-22-44 Digital Photon Counter (DPC) array was characterized while operating near room temperature. Crystal map peak to valley ratio, energy resolution, and timing resolution were characterized as a function of various device settings of the DPC and temperature. In addition, rates of count loss due to the phenomena of incomplete neighbor logic and dark-readout deadtime were measured. Device settings of interest were: Trigger scheme-defining the threshold of when a DPC will generate a timestamp and enter a readout cycle, inhibit fraction-the fraction of noisy cells which are disabled, and RTL-refresh-a setting which reduces the probability of the DPC being triggered from dark noise. At 15°C, peak to valley ratios were measured to be around 11, and energy resolution around 11.5% regardless of device settings. Timing resolution ranged from near 300 ps to 1.5 ns. Count loss from dark readout deadtime was insignificant compared to incomplete neighbor logic, which ranged from as high as 95% to 5% of coincidences. It was found that trade-offs had to be made between timing resolution and count loss. With the most optimal device settings for small animal PET, a timing resolution of 1.4 ns and coincidence losses of 5% were achieved. At these settings, the detector block had little sensitivity to a 5°C temperature fluctuation.

Markerless attenuation correction for carotid MRI surface receiver coils in combined PET/MR imaging

The purpose of the study was to evaluate the effect of attenuation of MR coils on quantitative carotid PET/MR exams. Additionally, an automated attenuation correction method for flexible carotid MR coils was developed and evaluated.The attenuation of the carotid coil was measured by imaging a uniform water phantom injected with 37 MBq of 18F-FDG in a combined PET/MR scanner for 24 min with and without the coil. In the same session, an ultra-short echo time (UTE) image of the coil on top of the phantom was acquired. Using a combination of rigid and non-rigid registration, a CT-based attenuation map was registered to the UTE image of the coil for attenuation and scatter correction. After phantom validation, the effect of the carotid coil attenuation and the attenuation correction method were evaluated in five subjects.Phantom studies indicated that the overall loss of PET counts due to the coil was 6.3% with local region-of-interest (ROI) errors reaching up to 18.8%. Our registration method to correct for attenuation from the coil decreased the global error and local error (ROI) to 0.8% and 3.8%, respectively. The proposed registration method accurately captured the location and shape of the coil with a maximum spatial error of 2.6 mm. Quantitative analysis in human studies correlated with the phantom findings, but was dependent on the size of the ROI used in the analysis.MR coils result in significant error in PET quantification and thus attenuation correction is needed. The proposed strategy provides an operator-free method for attenuation and scatter correction for a flexible MRI carotid surface coil for routine clinical use.

Uncertainty of nuclear counting

Nuclear counting is affected by pulse pileup and system dead time, which induce rate-related count loss and alter the statistical properties of the counting process. Fundamental equations are presented to predict deviations from Poisson statistics due to non-random count loss in nuclear counters and spectrometers. Throughput and dispersion of counts are studied for systems with pileup, extending and non-extending dead time, before and also after compensation for count loss. Equations are provided for random fractions of the output events, applicable to spectrometry applications. Methods for loss compensation are discussed, including inversion of the throughput equation, live-time counting and loss-free counting. Secondary effects in live-time counting are addressed: residual interference from pileup in systems with imposed dead times and errors due to varying count rate when measuring short-lived radionuclides.

Scaling Limits of MEMS Beam-Steering Switches for Data Center Networks

Transparent optical circuit switching can improve the aggregate bandwidth, scalability, and cost of data center networks provided, it can meet the performance requirements on switching speed, port count, and optical efficiency. Here, we examine the theoretical scaling limits of transparent nonblocking optical switches based on MEMS electrostatic tilt mirror devices. Using physical optics and electromechanics, we present a first principles analysis of how the response speeds of a set of canonical devices scale as a function of switch port count, crosstalk, and insertion loss. Our model indicates that the optimal actuator design (parallel plate versus vertically offset comb) and actuation method (digital versus analog) changes as a function of switch port count. It also suggests that conventional switch topologies do not allow a favorable tradeoff between switching speed and optical efficiency or crosstalk. However, high switching speeds can be achieved by multistage switch architectures such as the two examples we describe, a multiport wavelength switch and a wavelength-independent space switch.

Performance assessment of a NaI(Tl) gamma counter for PET applications with methods for improved quantitative accuracy and greater standardization.

BackgroundAlthough NaI(Tl) gamma counters play an important role in many quantitative positron emission tomography (PET) protocols, their calibration for positron-emitting samples has not been standardized across imaging sites. In this study, we characterized the operational range of a gamma counter specifically for positron-emitting radionuclides, and we assessed the role of traceable 68Ge/68Ga sources for standardizing system calibration.MethodsA NaI(Tl) gamma counter was characterized with respect to count rate performance, adequacy of detector shielding, system stability, and sample volume effects using positron-emitting radionuclides (409- to 613-keV energy window). System efficiency was measured using 18F and compared with corresponding data obtained using a long-lived 68Ge/68Ga source that was implicitly traceable to a national standard.ResultsOne percent count loss was measured at 450 × 103 counts per minute. Penetration of the detector shielding by 511-keV photons gave rise to a negligible background count rate. System stability tests showed a coefficient of variation of 0.13% over 100 days. For a sample volume of 4 mL, the efficiencies relative to those at 0.1 mL were 0.96, 0.94, 0.91, 0.78, and 0.72 for 11C, 18F, 125I, 99mTc, and 51Cr, respectively. The efficiency of a traceable 68Ge/68Ga source was 30.1% ± 0.07% and was found to be in close agreement with the efficiency for 18F after consideration of the different positron fractions.ConclusionsLong-lived 68Ge/68Ga reference sources, implicitly traceable to a national metrology institute, can aid standardization of gamma counter calibration for 18F. A characteristic feature of positron emitters meant that accurate calibration could be maintained over a wide range of sample volumes by using a narrow energy window centered on the 511-keV peak.

Measurement of Linear Attenuation Coefficient of Tc-99m using Planar Gamma Camera Image

Introduction: Attenuation is one of the most important factors which have an impact on the accuracy on quantitation of activity when using gamma camera image in a medium. Due to attenuation a number of photons are lost and there is also loss of counts which otherwise would have been included in the images. The purpose of this study is to estimate the linear attenuation coefficient for Tc-99m with water filled phantom using conjugate view method and transmission method and compare the obtained measurements with the reference values. Materials and Methods: To determine the linear attenuation coefficient of radionuclide Tc-99m using conjugate view method, the sensitivity of the gamma camera was determined. 30 MBq of Tc-99m was taken which was measured in dose calibrator and gamma camera images were acquired. Then the sensitivity was calculated as CPS/MBq. A Jaszczak phantom was filled up with water and a small sized cylindrical container filled with activity of 30 MBq Tc-99m liquid was placed inside the Jaszczak phantom centrally. The phantom was scanned on a dual head gamma camera and the images were taken and counts from the images were estimated. After estimation of all values, the linear attenuation coefficient was calculated using conjugate view formula. The linear attenuation coefficient was also measured from the transmission imaging. The transmission factor (TF) was determined by measuring the ratio of count rates obtained with Tc-99m under gamma camera, with and without the phantom. Results: The linear attenuation coefficient was calculated 0.1467 ± 0.0057 from conjugate view method and 0.1483 ± 0.0023 from transmission measurements. Both results were found within ±5% of the established value (0.15 cm-1). Conclusions: Correction for attenuation is required for the accurate estimation of activity. For conjugate view method, the attenuation coefficient was measured with a known activity of Tc-99m. The attenuation coefficient was also estimated by transmission measurements. The accuracy of the activity measurement is highly dependent on the accuracy of the attenuation coefficient used in the calculation. DOI: http://dx.doi.org/10.3329/bjnm.v17i1.22493 Bangladesh J. Nuclear Med. 17(1): 61-66, January 2014

Characterizing energy dependence and count rate performance of a dual scintillator fiber-optic detector for computed tomography.

Kilovoltage (kV) x-rays pose a significant challenge for radiation dosimetry. In the kV energy range, even small differences in material composition can result in significant variations in the absorbed energy between soft tissue and the detector. In addition, the use of electronic systems in light detection has demonstrated measurement losses at high photon fluence rates incident to the detector. This study investigated the feasibility of using a novel dual scintillator detector and whether its response to changes in beam energy from scatter and hardening is readily quantified. The detector incorporates a tissue-equivalent plastic scintillator and a gadolinium oxysulfide scintillator, which has a higher sensitivity to scatter x-rays. The detector was constructed by coupling two scintillators: (1) small cylindrical plastic scintillator, 500 μm in diameter and 2 mm in length, and (2) 100 micron sheet of gadolinium oxysulfide 500 μm in diameter, each to a 2 m long optical fiber, which acts as a light guide to transmit scintillation photons from the sensitive element to a photomultiplier tube. Count rate linearity data were obtained from a wide range of exposure rates delivered from a radiological x-ray tube by adjusting the tube current. The data were fitted to a nonparalyzable dead time model to characterize the time response. The true counting rate was related to the reference free air dose air rate measured with a 0.6 cm(3) Radcal(®) thimble chamber as described in AAPM Report No. 111. Secondary electron and photon spectra were evaluated using Monte Carlo techniques to analyze ionization quenching and photon energy-absorption characteristics from free-in-air and in phantom measurements. The depth/energy dependence of the detector was characterized using a computed tomography dose index QA phantom consisting of nested adult head and body segments. The phantom provided up to 32 cm of acrylic with a compatible 0.6 cm(3) calibrated ionization chamber to measure the reference air kerma. Each detector exhibited counting losses of 5% when irradiated at a dose rate of 26.3 mGy/s (Gadolinium) and 324.3 mGy/s (plastic). The dead time of the gadolinium oxysulfide detector was determined to be 48 ns, while the dead time of the plastic scintillating detector was unable to accurately be calculated due to poor counting statistics from low detected count rates. Noticeable depth/energy dependence was observed for the plastic scintillator for depths greater than 16 cm of acrylic that was not present for measurements using the gadolinium oxysulfide scintillator, leading us to believe that quenching may play a larger role in the depth dependence of the plastic scintillator than the incident x-ray energy spectrum. When properly corrected for dead time effects, the energy response of the gadolinium oxysulfide scintillator is consistent with the plastic scintillator. Using the integrated dual detector method was superior to each detector individually as the depth-dependent measure of dose was correctable to less than 8% between 100 and 135 kV. The dual scintillator fiber-optic detector accommodates a methodology for energy dependent corrections of the plastic scintillator, improving the overall accuracy of the dosimeter across the range of diagnostic energies.

Binomial distribution of Poisson statistics and tracks overlapping probability to estimate total tracks count with low uncertainty

In the solid state nuclear track detectors of chemically etched type, nuclear tracks with center-to-center neighborhood of distance shorter than two times the radius of tracks will emerge as overlapping tracks. Track overlapping in this type of detectors causes tracks count losses and it becomes rather severe in high track densities. Therefore, tracks counting in this condition should include a correction factor for count losses of different tracks overlapping orders since a number of overlapping tracks may be counted as one track. Another aspect of the problem is the cases where imaging the whole area of the detector and counting all tracks are not possible. In these conditions a statistical generalization method is desired to be applicable in counting a segmented area of the detector and the results can be generalized to the whole surface of the detector. Also there is a challenge in counting the tracks in densely overlapped tracks because not sufficient geometrical or contextual information are available. It this paper we present a statistical counting method which gives the user a relation between the tracks overlapping probabilities on a segmented area of the detector surface and the total number of tracks. To apply the proposed method one can estimate the total number of tracks on a solid state detector of arbitrary shape and dimensions by approximating the tracks averaged area, whole detector surface area and some orders of tracks overlapping probabilities. It will be shown that this method is applicable in high and ultra high density tracks images and the count loss error can be enervated using a statistical generalization approach.

A revised monitor source method for practical deadtime count loss compensation in clinical planar and SPECT studies

The aim of the study is to verify the fundamental assumption in the monitor source method, i.e. uniform fractional count loss across the field of view (FOV), and to introduce a revised monitor source method for SPECT deadtime correction that minimally interferes with the clinical protocol. SPECT images of non-uniform phantoms (4GBq 99mTc) with and without monitor sources (2 × 20MBq 99mTc) attached to each detector were acquired nine times over 48 h in the photopeak energy window and the scatter energy window. Fractional count loss uniformity across the FOV was evaluated by correlating count rates in different regions of interest on projection images at different deadtime loss levels. The correction factors were calculated as the ratios of monitor source count rates with and without the phantom. Such factors were applied to the phantom images acquired without the monitor sources. The counting efficiency (count rate per unit activity) of the camera was calculated as a function of activity in the FOV both prior to and after the deadtime count-loss correction. The deadtime correction effectiveness was assessed by the independence of the efficiency on the activity in the FOV. Methods to interpolate the projection deadtime loss, based on limited projections, were also investigated. The fractional deadtime count loss was uniform across the FOV (r > 0.99). After the deadtime correction, the efficiency was largely independent of the activity in the FOV. The median and maximum absolute errors after the deadtime count loss correction were ≤1% and ~2%, respectively. Measured deadtime loss from five views per detector can be used to estimate deadtime count loss with errors ≤1% for all SPECT projections. The revised monitor source method can effectively correct planar and SPECT deadtime loss. Sparse sampling of the projection deadtime loss allows the acquisition of high monitor source counts with minimal time added while preserving the entire useful FOV.

Cache agent-based geocasting in VANETs

Geocast routing is one of the most investigated choices for researchers of VANETs due to group of vehicles sharing geographical region on the road. Most existing geocast protocols are based on flooding technique. They suffer from high packet duplication, hop-to-hop disconnection, hop count, packet loss and low packet delivery. In this paper, a cache agent-based geocasting CAG protocol is proposed. Re-caching and full radio range transmissions integrated with connectivity assurance algorithm CAA are the two additional concepts of CAG. The performance of CAG has been compared with caching and transmission range control CTRC and directional flooding D-Flooding protocols. CAG outperforms CTRC and D-Flooding in terms of packet delivery, packet duplication, hop-to-hop disconnection, hop count, and packet loss.

Clinical Outcomes of Nd-YAG Laser Membranotomy in Retained Host Corneal Membrane after Keratoplasty

Purpose: To evaluate the clinical efficacy and shortcomings of neodymium-doped yttrium aluminum garnet (Nd-YAG) laser membranotomy in inadvertently retained host membrane. Methods: Among 742 patients who underwent penetrating keratoplasty (PKP) and deep anterior lamellar keratoplasty (DALK) surgery at Seoul St. Mary’s Hospital between January 2007 and May 2013 by a single surgeon, 10 patients had a thin, opaque membrane in the anterior chamber observed under slit lamp examination and both a subjective decrease in visual acuity and decrease i best corrected visual acuity. A single surgeon performed membranotomy using the Nd-YAG laser at 4.9 months after graft surgery. In this study we compared the differences in visual acuity, endothelial cell count and correlations between distance from donor endothelium and retained host membrane and endothelial loss before and after the graft surgery. Results: Patients who had Nd-YAG laser membranotomy performed on the retained host membrane showed significant improvements in visual acuity (p = 0.039). Donor endothelial cell count was significantly reduced 1 month after Nd-YAG laser. The average distance between donor endothelium and retained host membrane was 712.0 ± 217.5 μm. The distance and the decreased donor endothelial cell count were not statistically correlated (R = 0.39, p = 0.072). There were no significant complications after the laser membranotomy. Conclusions: Inadvertently retained host membrane forms close to the donor endothelium as identified on optical coherence tomography (OCT) images. Nd-YAG laser membranotomy significantly affected clear vision but endothelial cell count loss was also observed. J Korean Ophthalmol Soc 2015;56(5):664-671

A Dual-Channel Current Source Driver for Complementary Switches

In this paper, a dual-channel current source driver is presented as an efficient means of driving complementary switches. The driver transfers charge between the gates of the power devices through a single magnetic core with a discontinuous current. This promotes lower conduction loss than continuous current resonant gate drivers (RGDs), and lower component count or conduction loss than existing discontinuous current RGD applied to complementary switches. In addition, controlled switching speed is achieved through nonzero inductor current resulting in lower current stress, and faster switching compared to some pulse RGD circuits. There is good correlation between the analysis and experimental results of a 1.8-MHz prototype.

Determination of U–Pb Ages for Young Zircons using Laser Ablation‐ICP‐Mass Spectrometry Coupled with an Ion Detection Attenuator Device

We have developed new analytical procedures to measure precise and accurate 238U–206Pb and 235U–207Pb ages for young (~ 1 Ma) zircons using laser ablation‐ICP‐mass spectrometry. For young zircons, both careful correction for the background counts and analysis of very small Pb/U ratios (i.e., 206Pb/238U < 0.00016 and 207Pb/235U < 0.0001 for 1 Ma zircons) are highly desired. For the correction of the background, the contribution of the background signal intensities for the analytes, especially for the residual signal intensities for 206Pb and 207Pb, was defined through laser ablation of synthesised zircons (ablation blank) containing negligible Pb. The measured signal intensities for 202Hg, 206Pb and 207Pb signals obtained by the ablation blank were slightly higher than those obtained by data acquisition without laser ablation (gas blank). For the wider dynamic range measurements on Pb/U ratios, an attenuator device for the ion detection system was employed to extend the capability to monitor high‐intensity signals (i.e., > 3 Mcps). Through the attenuator device, the ion currents were reduced to 1/450 of the signal intensity without the attenuator. Because the switching time for the attenuator was shorter than 1 ms, signal intensities for only specific isotopes could be reduced. With attenuation of the 238U signal, counting statistics on 206Pb and 207Pb isotopes could be improved and counting loss on the 238U signal could be minimised. To demonstrate the reliability of this new analytical technique, 238U–206Pb and 235U–207Pb ages for three young zircon samples (collected from Osaka Group Pink Volcanic Ash, Kirigamine and Bishop Tuff) were measured. The data presented here demonstrate clearly that the present technique could become a major analytical tool for in situ U–Pb age determination of young zircons (~ 1 Ma).

Energy dispersive CdTe and CdZnTe detectors for spectral clinical CT and NDT applications

We are developing room temperature compound semiconductor detectors for applications in energy-resolved high-flux single x-ray photon-counting spectral computed tomography (CT), including functional imaging with nanoparticle contrast agents for medical applications and non-destructive testing (NDT) for security applications. Energy-resolved photon-counting can provide reduced patient dose through optimal energy weighting for a particular imaging task in CT, functional contrast enhancement through spectroscopic imaging of metal nanoparticles in CT, and compositional analysis through multiple basis function material decomposition in CT and NDT. These applications produce high input count rates from an x-ray generator delivered to the detector. Therefore, in order to achieve energy-resolved single photon counting in these applications, a high output count rate (OCR) for an energy-dispersive detector must be achieved at the required spatial resolution and across the required dynamic range for the application. The required performance in terms of the OCR, spatial resolution, and dynamic range must be obtained with sufficient field of view (FOV) for the application thus requiring the tiling of pixel arrays and scanning techniques. Room temperature cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) compound semiconductors, operating as direct conversion x-ray sensors, can provide the required speed when connected to application specific integrated circuits (ASICs) operating at fast peaking times with multiple fixed thresholds per pixel provided the sensors are designed for rapid signal formation across the x-ray energy ranges of the application at the required energy and spatial resolutions, and at a sufficiently high detective quantum efficiency (DQE). We have developed high-flux energy-resolved photon-counting x-ray imaging array sensors using pixellated CdTe and CdZnTe semiconductors optimized for clinical CT and security NDT. We have also fabricated high-flux ASICs with a two dimensional (2D) array of inputs for readout from the sensors. The sensors are guard ring free and have a 2D array of pixels and can be tiled in 2D while preserving pixel pitch. The 2D ASICs have four energy bins with a linear energy response across sufficient dynamic range for clinical CT and some NDT applications. The ASICs can also be tiled in 2D and are designed to fit within the active area of the sensors. We have measured several important performance parameters including: the output count rate (OCR) in excess of 20 million counts per second per square mm with a minimum loss of counts due to pulse pile-up, an energy resolution of 7keV full width at half-maximum (FWHM) across the entire dynamic range, and a noise floor about 20keV. This is achieved by directly interconnecting the ASIC inputs to the pixels of the CdZnTe sensors incurring very little input capacitance to the ASICs. We present measurements of the performance of the CdTe and CdZnTe sensors including the OCR, FWHM energy resolution, noise floor, as well as the temporal stability and uniformity under the rapidly varying high flux expected in CT and NDT applications.

Evaluation of a compact, high-resolution SPECT detector based on digital silicon photomultipliers

MicroSPECT is one of the main functional imaging techniques used in the preclinical setting. Even though high-resolution images can be obtained with currently available systems, their sensitivity is often quite low due to the use of multi-pinhole collimation. This results in long acquisition times and hampers dynamic imaging. However, it has already been shown that this limited sensitivity can be overcome using high-resolution detectors. In this article, we therefore investigated the use of a digital photon counter (DPC) in combination with a 2 mm thick monolithic LYSO crystal for SPECT imaging. These light sensors contain arrays of avalanche photodiodes whose signals are directly digitised. The DPCs have the advantage that they are very compact, have a high intrinsic resolution, are MR compatible and allow disabling cells with a high dark count rate. In order to investigate the influence of the temperature dependent dark count rate on the detector performance, we compared it at 3 °C and 18 °C.At 3 °C, we observed an energy resolution of 28.8% and an intrinsic spatial resolution of 0.48 mm. Furthermore, the count rate at 10% loss is 60 kcps. Next, we looked at the event loss at 18 °C caused by the higher dark count rate and found a 5% loss compared to the 3 °C measurements. At this higher temperature the energy resolution becomes 29.2% and the intrinsic spatial resolution decreases to 0.52 mm. Due to the 5% count loss, the count rate at 10% loss increases to 63 kcps. A small degradation of the detector performance is thus observed at 18 °C.These results show the usefulness of this detector for SPECT imaging together with its excellent intrinsic spatial resolution. A drawback of the detector is its low, spatially varying energy resolution. Even though the detection efficiency and intrinsic spatial resolution are better at 3 °C, results are still acceptable at 18 °C.

Optimized light sharing for high-resolution TOF PET detector based on digital silicon photomultipliers

The majority of current whole-body PET scanners are based on pixelated scintillator arrays with a transverse pixel size of 4 mm. However, recent studies have shown that decreasing the pixel size to 2 mm can significantly improve image spatial resolution. In this study, the performance of Digital Photon Counter (DPC) from Philips Digital Photon Counting (PDPC) was evaluated to determine their potential for high-resolution whole-body time of flight (TOF) PET scanners. Two detector configurations were evaluated. First, the DPC3200-44-22 DPC array was coupled to a LYSO block of 15 × 15 2 × 2 × 22 mm3 pixels through a 1 mm thick light guide. Due to light sharing among the dies neighbour logic of the DPC was used. In a second setup the same DPC was coupled directly to a scalable 4 × 4 LYSO matrix of 1.9 × 1.9 × 22 mm3 crystals with a dedicated reflector arrangement allowing for controlled light sharing patterns inside the matrix. With the first approach an average energy resolution of 14.5% and an average CRT of 376 ps were achieved. For the second configuration an average energy resolution of 11% and an average CRT of 295 ps were achieved. Our studies show that the DPC is a suitable photosensor for a high-resolution TOF-PET detector. The dedicated reflector arrangement allows one to achieve better performances than the light guide approach. The count loss, caused by dark counts, is overcome by fitting the matrix size to the size of DPC single die.

Comparison of power losses, current and voltage stresses of semiconductors in voltage source transformerless multilevel inverters

The power losses, current and voltage stresses in semiconductor devices in voltage source transformerless multilevel inverters (VSTMLIs) topologies are unequal, depending on the switching states, the duty ratio, the levels of output voltage, the load power factor and the depth of modulation index. Non-uniform power loss profiles mean both unequal junction temperature and power rating in semiconductors. In this study, a comparison of power losses, current and voltage stresses in semiconductors of VSTMLIs is performed. Initially, the structural characteristics and operating principles of dominant VSTMLIs with sinusoidal pulse width modulation are analysed. Then, modifications of existing VSTMLIs and simplified multilevel topologies are proposed, which achieve a further reduction in switch count and power losses in semiconductors. The proposed modified circuitries are suitable for converting existing four-level VSTMLI topologies into five-level ones. This conversion is desirable, when a zero output voltage level under no-load conditions or low modulation indices should be obtained. Power losses in semiconductors are calculated analytically and then confirmed by simulation. A comparative presentation of profiles of power losses, current and voltage stresses in semiconductors of the examined VSTMLIs is analytically carried out. The obtained results are valuable for sizing and selecting the semiconductors in VSTMLIs.

Heavy Charged Particle Nuclear Track Counting Statistics and Count Loss Estimation in High Density Track Images

In this paper statistical error of count loss in track counting of intensely irradiated solid state nuclear track detectors is studied. Track counting statistics for the chemically etched solid state nuclear track detectors as one of the most commonly used passive detectors are studied in this paper for the counting errors occurred in high densities of nuclear tracks which correspond to high accumulative doses of heavy charged particles, e.g. environmental Alphas of Radon and its daughters. Co-occurrences of two or more particles in close spatial positions cause overlapping of nuclear tracks observable after the etching process and this issue influences the accuracy of counting process. In case of higher densities of nuclear tracks, the overlapping tracks in the track detector correspond to the dead time behavior of paralyzable model in active detectors which leads to miscounting of receiving pulses. However, for both models binomial distribution of Poisson statistics can be taken into account to obtain a relation between the true number of incident particles, those that have formed a nuclear track, and the measured count of detected objects which include both singular tracks and overlapped tracks. This modeling is accredited due to the randomness in the nature of irradiation. It will be shown that in high density nuclear tracks images, where there are significant numbers of overlapping tracks, using the statistical correction of a true form is mandatory. Otherwise the measurement system is not reliable.

Age at antiretroviral therapy initiation predicts immune recovery, death, and loss to follow-up among HIV-infected adults in urban Zambia.

We analyzed the association of age at antiretroviral therapy (ART) initiation with CD4(+) T cell count recovery, death, and loss to follow-up (LTFU) among HIV-infected adults in Zambia. We compared baseline characteristics of patients by sex and age at ART initiation [categorized as 16-29 years, 30-39 years, 40-49 years, 50-59 years, and 60 years and older]. We used the medication possession ratio to assess adherence and analysis of covariance to measure the adjusted change in CD4(+) T cell count during ART. Using Cox proportional hazard regression, we examined the association of age with death and LTFU. In a secondary analysis, we repeated models with age as a continuous variable. Among 92,130 HIV-infected adults who initiated ART, the median age was 34 years and 6,281 (6.8%) were aged ≥50 years. Compared with 16-29 year olds, 40-49 year olds (-46 cells/mm(3)), 50-59 year olds (-53 cells/mm(3)), and 60+ year olds (-60 cells/mm(3)) had reduced CD4(+) T cell gains during ART. The adjusted hazard ratio (AHR) for death was increased for individuals aged ≥40 years (AHR 1.25 for 40-49 year olds, 1.56 for 50-59 year olds, and 2.97 for 60+ year olds). Adherence and retention in care were poorest among 16-29 year olds but similar in other groups. As a continuous variable, a 5-year increase in age predicted reduced CD4(+) T cell count recovery and increased risk of death. Increased age at ART initiation was associated with poorer clinical outcomes, while age <30 years was associated with a higher likelihood of being lost to follow-up. HIV treatment guidelines should consider age-specific recommendations.