Dead-time Correction Research Articles

Rapid workflow of mMR PET list-mode data processing using CUDA.

The purpose of this work is to provide a software solution which would enable very fast list-mode data processing offered by the massively parallel graphics processor units (GPU) while dealing efficiently with the common bottleneck of data transfers between hard disk storage, host (CPU) memory and device (GPU) memory. This software is dedicated to the PETLINK™ list-mode data format which is used by the Siemens’ hybrid Biograph Molecular MR (mMR) scanner. This software is developed as part of our platform for mMR 3D and 4D PET image reconstruction and will be freely accessible to the community. The aim is to perform real time processing (i.e. without perceivable delay) of the list-mode data during data transfers from hard disk to the CPU memory. The output of the processing includes: (1) The count-rate data (head curve: prompts, randoms and singles per second). (2) Plot of the variation of the centre of mass due to kinetics and motion (crude motion detection and quantification). (3) Projection movies (sagittal and coronal) for visual inspection for motion and data quality. (4) Crystal fan-sums for any time frame (used for randoms noise reduction). (5) Singles rate for each detector bucket (used for dead-time correction during normalisation). (6) Static and dynamic sinograms of span-1 and span-11. This workflow enables list-mode data processing synchronously with data transfer from disk to CPU memory. It opens a way for fast creation of multiple bootstrap realisations and multiple image reconstructions for a single dataset providing valuable insight into distributions of any statistic used in the development of robust image bio-markers. Also, the proposed workflow is advantageous for list-mode image reconstruction.

Accurate and precise measurement of oxygen isotopic fractions and diffusion profiles by selective attenuation of secondary ions (SASI).

The accuracy and precision of isotopic analysis in Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) relies on the appropriate reduction of the dead-time and detector saturation effects, especially when analyzing species with high ion yields or present in high concentrations. Conventional approaches to avoid these problems are based on Poisson dead-time correction and/or an overall decrease of the total secondary ion intensity by reducing the target current. This ultimately leads to poor detection limits for the minor isotopes and high uncertainties of the measured isotopic ratios. An alternative strategy consists of the attenuation of those specific secondary ions that saturate the detector, providing an effective extension of the linear dynamic range. In this work, the selective attenuation of secondary ion signals (SASI) approach is applied to the study of oxygen transport properties in electroceramic materials by isotopic labeling with stable (18)O tracer and ToF-SIMS depth profiling. The better analytical performance in terms of accuracy and precision allowed a more reliable determination of the oxygen surface exchange and diffusion coefficients while maintaining good mass resolution and limits of detection for other minor secondary ion species. This improvement is especially relevant to understand the ionic transport mechanisms and properties of solid materials, such as the parallel diffusion pathways (e.g., oxygen diffusion through bulk, grain boundary, or dislocations) in electroceramic materials with relevant applications in energy storage and conversion devices.

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.

Capture cross-section measurement at J-PARC:61Ni case.

61 Ni(n, γ ) cross-section has been measured at ANNRI, using the TOF method. ANNRI is an outstanding neutron facility based on spallation in MLF at J-PARC. In this cross-section measurement, we used HPGe detectors covering a substantial fraction of solid angle. In this paper, we focused the analysis on the capture rate to extract the corresponding cross-section. The dead-time correction, the overlapped neutrons correction and the background estimation were discussed.

Correction of dead time effects in laser-induced desorption time-of-flight mass spectrometry: Applications in atom probe tomography

Dead time effects in time-of-flight secondary ion mass spectrometry are well known and can be corrected for using Poisson statistics. Laser-induced desorption, however, introduces nonlinearity in the evaporation process resulting in highly fluctuating signals that make proper dead time correction much more challenging. Here, we propose a modified dead time correction procedure that overcomes such obstacles using data from correlated detection events from different isotopes of a single element. Provided the signals are not affected by unresolved mass interferences, this dead time correction enables us to obtain meaningful isotope ratios as demonstrated for atom probe tomography data of carbon from nanodiamonds and of silicon.

Correction of dead-time and pile-up in a detector array for constant and rapidly varying counting rates

The effect of dead-time and pile-up in counting experiments may become a significant source of uncertainty if not properly taken into account. Although analytical solutions to this problem have been proposed for simple set-ups with one or two detectors, these are limited when it comes to arrays where time correlation between the detector modules is used, and also in situations of variable counting rates. In this paper we describe the dead-time and pile-up corrections applied to the n_TOF Total Absorption Calorimeter (TAC), a 4π γ-ray detector made of 40 BaF2 modules operating at the CERN n_TOF facility. Our method is based on the simulation of the complete signal detection and event reconstruction processes and can be applied as well in the case of rapidly varying counting rates. The method is discussed in detail and then we present its successful application to the particular case of the measurement of 238U(n, γ) reactions with the TAC detector.

Data processing for the Active Particle-induced X-ray Spectrometer and initial scientific results from Chang'e-3 mission

The Active Particle-induced X-ray Spectrometer (APXS) is an important payload mounted on the Yutu rover, which is part of the Chang'e-3 mission. The scientific objective of APXS is to perform in-situ analysis of the chemical composition of lunar soil and rock samples. The radioactive sources, 55Fe and 109Cd, decay and produce α-particles and X-rays. When X-rays and α-particles interact with atoms in the surface material, they knock electrons out of their orbits, which release energy by emitting X-rays that can be measured by a silicon drift detector (SDD). The elements and their concentrations can be determined by analyzing their peak energies and intensities. APXS has analyzed both the calibration target and lunar soil once during the first lunar day and again during the second lunar day. The total detection time lasted about 266 min and more than 2000 frames of data records have been acquired. APXS has three operating modes: calibration mode, distance sensing mode and detection mode. In detection mode, work distance can be calculated from the X-ray counting rate collected by SDD. Correction for the effect of temperature has been performed to convert the channel number for each spectrum to X-ray energy. Dead time correction is used to eliminate the systematic error in quantifying the activity of an X-ray pulse in a sample and derive the real count rate. We report APXS data and initial results during the first and second lunar days for the Yutu rover. In this study, we analyze the data from the calibration target and lunar soil on the first lunar day. Seven major elements, including Mg, Al, Si, K, Ca, Ti and Fe, have been identified. Comparing the peak areas and ratios of calibration basalt and lunar soil the landing site was found to be depleted in K, and have lower Mg and Al but higher Ca, Ti, and Fe. In the future, we will obtain the elemental concentrations of lunar soil at the Chang'e-3 landing site using APXS data.

Continuous bed motion on clinical scanner: design, data correction, and reconstruction

With the addition of variable bed speed capabilities, continuous bed motion (CBM) becomes an acquisition generalization for axial sensitivity modeling and therefore a flexible tool in axial local image quality adjustment. In this paper, we describe the principles behind CBM mode planning and its influence on image reconstruction.The data correction method underwent the most changes compared to the commonly used step and shoot (S&S) mode. The CBM normalization array accommodates for activity decay, dead time correction, and the fact that various detector pairs acquire the same data for different durations. The normalization array is computed by simulating the movement of an object through the scanner, assisted by the monitoring of basic scanner acquisition parameters such as the singles rate.The sensitivity, which is the number of counts acquired per image plane, is an intrinsic part of normalization computing. Basic equations to estimate acquisition time at matched sensitivity between single speed CBM and S&S mode are presented.The CBM feature was implemented on a Siemens clinical scanner and initial studies of phantoms and patients are presented. The equivalence of single speed CBM and S&S image quality is demonstrated.

Pulse pile-up and dead time corrections for digitized signals from a BaF2 calorimeter

The pulse pile-up and associated dead time effects in an 243Am(n,γ) cross section measurement with a BaF2 Total Absorption Calorimeter at the CERN n_TOF facility have been characterized. In this case, reliable corrections of these effects are crucial because of the complex detector configuration and of the high count rate induced by the sample activity of 50 MBq. The techniques, which have been developed for offline processing of the present data, may be of general interest for the analysis of other experiments using digital acquisition systems.

Performance of digital gamma ray spectrometer for loss free counting and its application to NAA using short-lived radionuclides

A digital gamma-ray spectrometer with loss-free counting system was used for real time correction for dead time (DT) due to pulse processing in the electronics. This spectrometer was used for radioactive assay of samples having constant as well as varying DT conditions. The system was tested by measuring activity of short and medium lived nuclides namely 28Al, 52V and 128I in the DT ranges of 80–2 %. Using this spectrometer and neutron activation analysis (NAA), concentrations of Al, V, Ti, Ca, Dy and Mn were determined in some samples and reference materials.

Evaluation of dead-time corrections for post-radionuclide-therapy 177Lu quantitative imaging with low-energy high-resolution collimators

Evaluation of dead-time corrections for post-radionuclide-therapy 177Lu quantitative imaging with low-energy high-resolution collimators

Trace Element Analyses by EMP: Pb-in-Monazite and New Multipoint Background Method

University of Oregon, CAMCOR, Eugene (OR), USA Precision in electron microprobe analysis is primarily a matter of counting statistics, and involves voltage, beam current, counting time, and geometric efficiency. Besides potential issues with dead-time correction and beam damage, counting statistics do not significantly affect the accuracy of major and minor element analyses. Accuracy chiefly depends on standards and matrix correction. Additional challenges with both accuracy and high precision appear when measuring trace elements [1]: 1) Analytical precision requires the use of high intensity monochromators, optimized PHA settings, high beam current, higher voltage, and/or lengthy count time. These latter requirements can lead to beam damage, surface contamination, and internal charge effects. 2) Characterization and measurement of the background become crucial for accurate results when the peak-to-background ratio approaches 1. 3) Accurate results require correction or avoidance of peak and background interferences, even for some 2

Deadtime Correction and Hydride Evaluation for Atom-Probe Data, with Applications for Studies of Nanoscale Grains and Carbon

Deadtime Correction and Hydride Evaluation for Atom-Probe Data, with Applications for Studies of Nanoscale Grains and Carbon

Neutron capture cross section measurements for 197Au from 3.5 to 84 keV at GELINA

Cross section measurements have been performed at the time-of-flight facility GELINA to determine the average capture cross section for 197Au in the energy region between 3.5 keV and 84 keV. Prompt γ-rays, originating from neutron-induced capture events, were detected by two C6 D6 liquid scintillators. The sample was placed at about 13m distance from the neutron source. The total energy detection principle in combination with the pulse height weighting technique was applied. The energy dependence of the neutron flux was measured with a double Frisch-gridded ionization chamber based on the 10B(n,α) reaction. The data have been normalized to the well-isolated and saturated 197Au resonance at 4.9 eV. Special care was taken to reduce bias effects due to the weighting function, normalization, dead time and background corrections. The total uncertainty due to normalization, neutron flux and weighting function is 1.0%. An additional uncertainty of 0.5% results from the correction for self-shielding and multiple interaction events. Fluctuations due to resonance structures have been studied by complementary measurements at a 30m flight path station. The results reported in this work deviate systematically by more than 5% from the cross section that is recommended as a reference for astrophysical applications. They are about 2% lower compared to an evaluation of the 197Au(n, γ) cross section, which was based on a least squares fit of experimental data available in the literature prior to this work. The average capture cross section as a function of neutron energy has been parameterized in terms of average resonance parameters. Maxwellian average cross sections at different temperatures have been calculated.

The accuracy and precision of the advanced Poisson dead‐time correction and its importance for multivariate analysis of high mass resolution ToF‐SIMS data

Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) data collected in single ion counting mode suffers from dead‐time effects that lead to potentially confusing artefacts when common multivariate analysis (MVA) methods are applied to the data. These artefacts can be eliminated by applying an advanced Poisson dead‐time correction that accounts for the signal intensity in the dead‐time window preceding each time channel. Because this correction is nonlinear, it changes the noise distribution in the data. In this work, the accuracy of this dead‐time correction and the noise characteristics of the corrected data have been analysed for spectra with small numbers of primary ion pulses. A simple but accurate equation for estimating the standard deviation in the advanced dead‐time corrected data has been developed. Based on these results, a scaling procedure to enable successful MVA of advanced dead‐time corrected ToF‐SIMS data has been developed. The improvements made possible by using the advanced dead‐time correction and our recommended scaling are presented for principal components analysis of a ToF‐SIMS image of aerosol particles on polytetrafluoroethylene. Recommendations are made for using the advanced dead time correction and scaling ToF‐SIMS data in order optimize the outcomes of MVA. Copyright © 2014 John Wiley & Sons, Ltd.

SU‐E‐I‐20: Dead Time Count Loss Compensation in SPECT/CT: Projection Versus Global Correction

Purpose:To compare projection‐based versus global correction that compensate for deadtime count loss in SPECT/CT images.Methods:SPECT/CT images of an IEC phantom (2.3GBq 99mTc) with ∼10% deadtime loss containing the 37mm (uptake 3), 28 and 22mm (uptake 6) spheres were acquired using a 2 detector SPECT/CT system with 64 projections/detector and 15 s/projection. The deadtime, Ti and the true count rate, Ni at each projection, i was calculated using the monitor‐source method. Deadtime corrected SPECT were reconstructed twice: (1) with projections that were individually‐corrected for deadtime‐losses; and (2) with original projections with losses and then correcting the reconstructed SPECT images using a scaling factor equal to the inverse of the average fractional loss for 5 projections/detector. For both cases, the SPECT images were reconstructed using OSEM with attenuation and scatter corrections. The two SPECT datasets were assessed by comparing line profiles in xyplane and z‐axis, evaluating the count recoveries, and comparing ROI statistics. Higher deadtime losses (up to 50%) were also simulated to the individually corrected projections by multiplying each projection i by exp(‐a*Ni*Ti), where a is a scalar. Additionally, deadtime corrections in phantoms with different geometries and deadtime losses were also explored. The same two correction methods were carried for all these data sets.Results:Averaging the deadtime losses in 5 projections/detector suffices to recover >99% of the loss counts in most clinical cases. The line profiles (xyplane and z‐axis) and the statistics in the ROIs drawn in the SPECT images corrected using both methods showed agreement within the statistical noise. The count‐loss recoveries in the two methods also agree within >99%.Conclusion:The projection‐based and the global correction yield visually indistinguishable SPECT images. The global correction based on sparse sampling of projections losses allows for accurate SPECT deadtime loss correction while keeping the study duration reasonable.

SU‐E‐I‐21: Deformation Mapping and Shape Prediction with 3D Tumor Volume Morphing

Purpose:To compare projection‐based versus global correction that compensate for deadtime count loss in SPECT/CT images.Methods:SPECT/CT images of an IEC phantom (2.3GBq 99mTc) with ∼10% deadtime loss containing the 37mm (uptake 3), 28 and 22mm (uptake 6) spheres were acquired using a 2 detector SPECT/CT system with 64 projections/detector and 15 s/projection. The deadtime, Ti and the true count rate, Ni at each projection, i was calculated using the monitor‐source method. Deadtime corrected SPECT were reconstructed twice: (1) with projections that were individually‐corrected for deadtime‐losses; and (2) with original projections with losses and then correcting the reconstructed SPECT images using a scaling factor equal to the inverse of the average fractional loss for 5 projections/detector. For both cases, the SPECT images were reconstructed using OSEM with attenuation and scatter corrections. The two SPECT datasets were assessed by comparing line profiles in xyplane and z‐axis, evaluating the count recoveries, and comparing ROI statistics. Higher deadtime losses (up to 50%) were also simulated to the individually corrected projections by multiplying each projection i by exp(‐a*Ni*Ti), where a is a scalar. Additionally, deadtime corrections in phantoms with different geometries and deadtime losses were also explored. The same two correction methods were carried for all these data sets.Results:Averaging the deadtime losses in 5 projections/detector suffices to recover >99% of the loss counts in most clinical cases. The line profiles (xyplane and z‐axis) and the statistics in the ROIs drawn in the SPECT images corrected using both methods showed agreement within the statistical noise. The count‐loss recoveries in the two methods also agree within >99%.Conclusion:The projection‐based and the global correction yield visually indistinguishable SPECT images. The global correction based on sparse sampling of projections losses allows for accurate SPECT deadtime loss correction while keeping the study duration reasonable.

라이다 측정 거리 향상을 위한 통합 수신 시스템 개발 (아날로그방식과 광자계수방식 신호 접합)

본 연구는 에어로졸의 광학적 특성 분석을 위한 라이다 시스템의 정확성 향상을 위해 새롭게 개발한 신호 수신 시스템을 설명하고자 한다. 광주과학기술원의 라이다에 아날로그 방식과 광자계수 방식을 통합한 시스템을 활용하여 에어로졸 후방산란 신호를 동시에 관측 가능한 수신단을 개발하였다. 관측된 두 신호 결합을 위해 접합 알고리즘을 고안하였고, 신호 결합에 앞서 광자계수 방식 신호의 Pile up효과를 보정하기 위해 부동시간(Dead time)을 계산하여 보정하였다. 관측 신호 분석을 통해 아날로그 방식 신호, 광자계수 방식 신호, 접합신호 그리고 부동시간 보정에 따른 차이점을 설명하고, 최종적으로 에어로졸 후방산란계수를 산출하여 상호 비교 및 정확성 향상을 확인하였다. We upgraded to utilize a novel method for combining the analog to digital converter and photon-counting measurements for backscatter photon signal of lidar. We have and improve the standard combining method for determination of those conversion factors between analog to digital converter data and photon-counting data measurement which is conducted dead time correction. The combining method and dead time correction method presented here has been successfully applied to experimental data obtained in Gwangju, Korea.

Analytic Model for Dead Time Effect in Neutron Multiplicity Counting

Neutron multiplicity counting (NMC) measurements are often affected by the detection system dead time. Still, dead time losses are often neglected in analytic NMC models, and most of the dead time corrections are done through empirical models, experimentally fitted to the measurement system. In the present paper, we introduce a new analytic model for calculating the effect of a system dead time on the outcome of NMC. The model is subjected to two assumptions (in addition to the standard model assumptions in multiplicity counting): The first is that the dead time can be described by a paralyzable model, and the second is that the dead time effect may occur only between neutrons arriving from the same source event. The second assumption is, in fact, a restriction on the source event rate in the system and, in certain cases, may eventually be translated into a restriction on the mass of the measured sample.

The NAA method at Polytechnique Montreal: an efficient alternative way to use the k 0 NAA models

At Ecole Polytechnique Montreal, the philosophy in performing neutron activation analysis (NAA) is long-term and application oriented. Thinking long-term implies a good understanding of the fundamentals of the method, of the samples, of the tools, reactor and detectors, and there must be constant innovation that is experimentally validated with extensive measurements. Application oriented means a NAA method developed to provide users with fast, sensitive, accurate and reliable analyses for various types of materials. This philosophy dictates the manner in which the developments in the areas of NAA software, peak-area calculation, dead-time correction, detection efficiency model, k 0 and Q 0 values, neutron moderation and neutron self-shielding are carried out. This paper presents a survey of the Laboratory’s methodology, reviewing a few of its unique features such as detector efficiency calibration and sample related perturbations of the neutron activation. These features are used as examples to provide the reader with an understanding of the philosophy and the evolution of the NAA method at Ecole Polytechnique.