Dead-time Correction Research Articles

Atom probe tomography investigation of highly-enriched 28Si crystal

As part of the Avogadro constant re-determination, the molar mass of a highly-enriched 28Si crystal is being determined using various mass-spectrometry techniques. These calculations rely on the silicon material having high chemical purity, which has been initially assessed by infrared spectrometry. APT is uniquely suited to assess isotopic purity and distribution within a sample, thus ensuring the homogeneity required for precise measurements. In this study, Atom Probe Tomography (APT) is employed to further investigate isotopic composition and elemental purity within the mono-isotopically highly-enriched 28Si crystal (AVO28) and ultra-highly-enriched 28Si (UHP) and contrasted with natural silicon isotopes samples (WASO04, PSM, and SRM990). Reliable silicon isotope ratios are obtained from standard samples through a data processing procedure which utilizes automated mass ranging, background and deadtime corrections. The 29Si/28Si isotope ratios for WASO04, PSM and SRM990 agree with published values. AVO28 and UHP analysis reveals no impurities detected above 2 µg/g, with a uniform distribution of 28Si and 29Si at the nanometer scale. AVO28 exhibits 29Si content of 41 ± 2 µg/g, with no detection of 30Si below 2 µg/g, consistent with SIMS and other international comparisons. This work demonstrates APT’s capability for impurity detection and high-precision isotope analysis, reinforcing the silicon molar mass calculations and supporting Avogadro’s constant redefinition.

Exploring the gamma counting function in PET for sample activity measurement

With the widespread availability of small animal PET scanners globally, there exists a significant opportunity to enhance their operational utility. Our research introduces a novel utilization pathway by integrating gamma-counting functionalities within PET systems. This approach leverages the shared gamma counting technology between PET systems and traditional gamma counters, enabling direct radioactivity measurement from PET-derived raw count data. We established a correlation between the raw data from all-digital PET and the radioactivity of samples, conducting experiments to assess the system's measurement capabilities. The system demonstrated high accuracy and stability across a detectable range of 25.53–1900.69 kBq in single-photon mode and 19.61–1107.04 kBq in coincidence mode, with the difference between the sample activity and the linear function being no more than 5 %. Due to the multi-channel characteristic of PET detectors, our system exhibits a tenfold higher upper detection limit than that of gamma counters, without dead-time correction. This study demonstrates that all-digital PET can directly measure radioactivity, simplifying nuclear medicine procedures by integrating gamma counting within PET systems. This strategy not only reduces the need for multiple instruments but also has the potential to improve quantitative accuracy by minimizing cross-calibration errors, thereby maximizing the operational utility of existing all-digital PET scanners.

Gamma-gamma coincidence spectroscopy analysis with cascade summing corrections in close counting geometries

Gamma-gamma coincidence spectroscopy is a highly effective technique that becomes more challenging to implement when the distance between the detector and the sample is reduced. This difficulty arises due to cascade summing, which can complicate the acquisition of efficiency curves and introduce uncertainties, especially when using common sources like 152Eu. To address this issue, a method for correcting coincidence summing in close counting geometries is presented in this study. The approach utilizes MCNPX-Polimi and MCNP6.2, along with a 152Eu source calibration source for verification and validation. Additionally, the use of high purity germanium (HPGe) detectors coupled with Lynx digital signal analyzers (DSAs) for gamma-gamma coincidence is discussed. This work also outlines the generation, processing, and handling of time-stamped list (TLIST) mode data for dead-time corrections and coincidence gating.

A novel method for accurate and real-time quantification of radiopharmaceutical activity in automated dose dispensers

In recent years, there has been a rapid growth in new technologies within nuclear medicine and PET diagnostics, particularly in the field of radiopharmaceutical dose preparation. These advancements are driven by the integration of automatic algorithms into the process. This evolution has brought about significant improvements in the efficiency and accuracy of radiopharmaceutical dose preparation. These automatic algorithms utilize state-of-the-art technologies to streamline the process and ensure precise and tailored doses for patients. The integration of these advanced technologies has revolutionized the field, allowing for more effective and personalized treatment options in nuclear medicine and PET diagnostics. By employing automatic radiopharmaceutical dose dispensers (ARDDs), radiation exposure to technologists is minimized while also mitigating the risk of potential infections for patients. Ionization chambers are currently the most used detectors to measure the activity of radiopharmaceuticals in ARDDs. However, they present certain limitations, including size constraints. This study is part of an international endeavor to develop a new approach for evaluating radiopharmaceutical activity in an ARDD named KARl 100. A new approach was used to develop a scintillation detector by linking a PVT detector to a SiPMs array consisting of four elements, each having an active surface of 6 × 6 mm with a bias voltage of 30 V. The detection system's amplified output was then fed to a single-channel analyzer called Enviro, developed by Tema Sinergie S.p.A in Italy. The counts per second (CPS) detected by the detector were studied using WinTAM software allowing for the radionuclide’s activity concentration to be read out at run-time. Three widely used medical radioisotopes, 18F, 11C and 68Ga, were used to evaluate the performance of the detector in terms of linearity and repeatability of the measurements. Half-life and linear response were evaluated over a wide range of activities with different decay schemes. Data analysis was conducted, resulting in half-lives of the 18F, 11C and 68Ga radioisotopes of 109.75 ± 0.35, 20.62 ± 0.34 and 67.82 ± 0.25 min, respectively, with calculated deviations of 0.0167%, 1.00% and 0.29% from expected values. The linear response of the detector over the entire range of desired activity concentrations was confirmed after data correction for the detector dead time. Calibration of the detector combined with the dispenser system was carried out in parallel investigations to provide the required device properties for reference applications in nuclear medicine. Furthermore, Monte Carlo simulations using FLUKA code were performed to investigate the contribution of beta particles in addition to gamma radiations to assess the influence of beta particles on the calibration factors. For better accuracy, a shielding was designed to prevent any beta particles from reaching the detector.

Impact of improved dead time correction on the quantification accuracy of a dedicated BrainPET scanner.

Quantitative values derived from PET brain images are of high interest for neuroscientific applications. Insufficient DT correction (DTC) can lead to a systematic bias of the output parameters obtained by a detailed analysis of the time activity curves (TACs). The DTC method currently used for the Siemens 3T MR BrainPET insert is global, i.e., differences in DT losses between detector blocks are not considered, leading to inaccurate DTC and, consequently, to inaccurate measurements masked by a bias. However, following careful evaluation with phantom measurements, a new block-pairwise DTC method has demonstrated a higher degree of accuracy compared to the global DTC method. Differences between the global and the block-pairwise DTC method were studied in this work by applying several radioactive tracers. We evaluated the impact on [11C]ABP688, O-(2-[18F]fluoroethyl)-L-tyrosine (FET), and [15O]H2O TACs. For [11C]ABP688, a relevant bias of between -0.0034 and -0.0053 ml/ (cm3 • min) was found in all studied brain regions for the volume of distribution (VT) when using the current global DTC method. For [18F]FET-PET, differences of up to 10% were observed in the tumor-to-brain ratio (TBRmax), these differences depend on the radial distance of the maximum from the PET isocenter. For [15O]H2O, differences between +4% and -7% were observed in the GM region. Average biases of -4.58%, -3.2%, and -1.2% for the regional cerebral blood flow (CBF (K1)), the rate constant k2, and the volume of distribution VT were observed, respectively. Conversely, in the white matter region, average biases of -4.9%, -7.0%, and 3.8% were observed for CBF (K1), k2, and VT, respectively. The bias introduced by the global DTC method leads to an overestimation in the studied quantitative parameters for all applications compared to the block-pairwise method. The observed differences between the two DTC methods are particularly relevant for research applications in neuroscientific studies as they affect the accuracy of quantitative Brain PET images.

Innovative dead-time correction and background subtraction for neutron multiplicity measurements using neural networks

The number of neutrons emitted from a nuclear reaction plays a crucial role in various fields, including nuclear theory, nuclear nonproliferation, nuclear energy and nuclear criticality safety. Accurate determination of neutron multiplicities requires the application of several corrections, with dead-time correction and background subtraction being particularly significant. These corrections become more challenging for neutron detectors with time-dependent neutron capture. In this work, we perform a comprehensive study of three existing methods used for dead-time correction and background subtraction in neutron detectors with time-dependent neutron capture. The methods were tested for dead-times in the range from 0 to 1 μs using a Monte Carlo model simulating the dead-time and background effects in the standard neutron multiplicity probability distribution of 252\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{252}$$\\end{document}Cf. The previous methods showed larger than desired uncertainty or systematic trade off. Those uncertainties prompted the development of a novel approach using neural networks trained with data from Monte Carlo simulations. The Neural Network method enabled the correction of neutron multiplicity probabilities more accurately than the other methods with fractional errors smaller than 3% for multiplicities around the peak of 252\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{252}$$\\end{document}Cf. A similar approach using neural networks could be applied to problems where the system being studied can be accurately simulated without having an accurate analytical description available. The neural network method presented in this paper can be easily expanded if multiplicities greater than 10 are expected.

Using XAS to monitor radiation damage in real time and post-analysis, and investigation of systematic errors of fluorescence XAS for Cu-bound amyloid-β.

X-ray absorption spectroscopy (XAS) is a promising technique for determining structural information from sensitive biological samples, but high-accuracy X-ray absorption fine structure (XAFS) requires corrections of systematic errors in experimental data. Low-temperature XAS and room-temperature X-ray absorption spectro-electrochemical (XAS-EC) measurements of N-truncated amyloid-β samples were collected and corrected for systematic effects such as dead time, detector efficiencies, monochromator glitches, self-absorption, radiation damage and noise at higher wavenumber (k). A new protocol was developed using extended X-ray absorption fine structure (EXAFS) data analysis for monitoring radiation damage in real time and post-analysis. The reliability of the structural determinations and consistency were validated using the XAS measurement experimental uncertainty. The correction of detector pixel efficiencies improved the fitting χ2 by 12%. An improvement of about 2.5% of the structural fitting was obtained after dead-time corrections. Normalization allowed the elimination of 90% of the monochromator glitches. The remaining glitches were manually removed. The dispersion of spectra due to self-absorption was corrected. Standard errors of experimental measurements were propagated from pointwise variance of the spectra after systematic corrections. Calculated uncertainties were used in structural refinements for obtaining precise and reliable values of structural parameters including atomic bond lengths and thermal parameters. This has permitted hypothesis testing.

The effects of instrumental deadtime on NICER timing products

Abstract The X-ray timing instrument as part of the neutron star interior composition explorer has the potential to examine the time-domain properties of compact objects in regimes not explored by previous timing instruments, due to its combination of high effective area and timing resolution. We consider the effects of instrumental deadtime at a range of effective countrates in a series of observations of the X-ray binary GX 339-4 to determine what effect deadtime has on photometric and Fourier frequency-domain products. We find that there are no significant inconsistencies across the functional detectors in the instrument, and that in the regimes where instrumental deadtime is a limiting factor on observations that previous approaches to dealing with deadtime, as applied to RXTE and other detectors, are still appropriate, and that performing deadtime corrections to light curves before creating Fourier products are not necessary at the count rates considered in our analysis.

Particle size distribution measurements under supercooled large drop conditions with Cloud Combination Probe in CARDC icing wind tunnel

Particle size distribution (PSD) of the icing cloud is one of the most important icing parameters for icing wind tunnels. However, accurate measurements of the PSD are challenging for supercooled large drop (SLD) conditions due to the wide droplet size spectrum and low number density of large droplets. In this study, PSD measurements with the Cloud Combination Probe (CCP) are comprehensively investigated in the CARDC icing wind tunnel. A method based on the CCP is developed for composite PSD calculation, and an icing cloud measurement test with 79 test conditions covering Appendix C and freezing drizzle conditions, is conducted for accuracy assessment of the CCP and reliability validation of the PSD calculation method. Based on the test, the effects of spurious particle images are examined first, followed by a quantitative assessment of the influences of particle size calculation and sample volume calculation on the PSD, medium volume diameter (MVD), and liquid water content (LWC). The results show that all PSD measurements are adversely impacted by spurious particle images, primarily caused by coincidence errors. For five typical particle size parameters, the effects of particle size calculation on the PSD are significant, with maximum relative deviations of about 110% for MVD and 63% for LWC. The influences of Depth-of-Field factors are not noticeable except for the manufacturer factor. Additionally, the dead time also dramatically impacts the PSD, resulting in a decrease in the MVD and LWC with maximum decreases of about 60% and 43%, respectively. Finally, the LWCs determined by composite PSDs show good agreement with the reference LWCs obtained by the SEA multi-element sensor, where the relative deviation is within ±10% for the majority of measured points. This demonstrates the CCP is suitable for wind tunnel measurements of PSDs under Appendix C and freezing drizzle conditions employing the PSD calculation method. To accurately calculate the PSD, spurious particle image removal and dead-time correction should be considered, and out-of-focus correction diameter and in-house factor are preferred.

Accuracy improvement of a simulation model of a radiation portal monitor by correcting dead-time effect

Thousands of radiation portal monitors (RPMs) have been deployed around the world to detect radioactive materials that are outside of regulatory control. The detection capability of the RPMs can be evaluated by conducting a field test with various radioactive materials. However, field tests are limited because the RPMs are operated 24 h a day. The Monte Carlo N-Particle (MCNP) transport code provides an alternative. To evaluate the performance of the RPM with MCNP-based simulation model, the simulation results should be experimentally validated and the measurement result for the validation should be corrected for dead time effect. In this study, an MCNP-based RPM simulation model was developed and the accuracy of the RPM simulation model was improved by correcting dead-time effect. Static and dynamic measurements were conducted and the dead-time correction for the measurement data was factored in. With the static measurement data, the simulation model was developed and the comparing results with the static measurement data show that it satisfied an average relative difference of less than 10%. The simulation model was validated with 17 dynamic measurement datasets and the validation results show that the average relative differences are also within 10% even though the experimental conditions of the static and dynamic measurements were different.

Recoil Proton Telescopes and Parallel Plate Avalanche Counters for the 235U(n,f) cross section measurement relative to H(n,n)H between 10 and 450 MeV neutron energy

With the aim of measuring the 235U(n,f) cross section at the n_TOF facility at CERN over a wide neutron energy range, a detection system consisting of two fission detectors and three detectors for neutron flux determination was realized. The neutron flux detectors are Recoil Proton Telescopes (RPTs), based on scintillators and solid state detectors, conceived to detect recoil protons from the neutron-proton elastic scattering reaction. This system, along with a fission chamber and an array of parallel plate avalanche counters for fission event detection, was installed for the measurement at the n_TOF facility in 2018, at CERN.An overview of the performances of two RPTs — especially developed for this measurement — and of theparallel plate avalanche counters are described in this article. In particular, the characterizationin terms of detection efficiency by Monte Carlo simulations and response to neutron beam, the studyof the background, dead time correction and characterization of the samples, are reported. Theresults of the present investigation show that the performances of these detectors are suitable foraccurate measurements of fission reaction cross sections in the range from 10 to 450 MeV.

Soft-Sensor Modeling of Temperature Variation in a Room under Cooling Conditions

Non-uniform temperature distributions in air-conditioned areas can reduce the energy efficiency of air conditioners and cause uncomfortable thermal sensations for occupants. Furthermore, it is impractical to use physical sensors to measure the local temperature at every position. This study developed a soft-sensing model that integrates the fundamentals of thermodynamics and transport phenomena to predict the temperature at the target position in space. Water experiments were conducted to simulate indoor conditions in an air-conditioning cooling mode. The transient temperatures of various positions were measured for model training and validation. The velocity vectors of water flow were acquired using the particle image velocimetry method. Correlation analysis of various positions was conducted to select the input variable. The soft-sensing model was developed using the multiple linear regression method. The model for the top layer was modified by the correction of dead time. The experimental results showed the temperature inhomogeneity between different layers. The temperature at each target position under two initial temperatures and two flow rates was accurately predicted with a mean absolute error within 0.69 K. Moreover, the temperature under different flow rates can be predicted with one model. Therefore, this soft-sensing model has the potential to be integrated into air-conditioning systems.

A detector block-pairwise dead time correction method for improved quantitation with a dedicated BrainPET scanner

‘Objective. Dead time correction (DTC) is an important factor in ensuring accurate quantification in PET measurements. This is currently often achieved using a global DTC method, i.e., an average DTC factor is computed. For PET scanners designed to image dedicated organs, e.g., those used in brain imaging or positron emission mammography (PEM), a substantial amount of the administered radioactivity is located outside of the PET field-of-view (FOV). This activity contributes to the dead time (DT) of the scintillation detectors. Moreover, the count rates of the individual scintillation detectors are potentially very inhomogeneous due to the specific irradiation of each detector, especially for combined MR/PET systems, where radiation shields cannot be applied. Approach: We have developed a block-pairwise DTC method for our Siemens 3T MR BrainPET insert by extending a previously published method that uses the delayed random coincidence count rate to estimate the DT in the individual scans and planes (i.e., scintillation pixel rings). The method was validated in decay experiments using phantoms with a homogenous activity concentration and with and without out-of-FOV activity. Based on a three-compartment phantom, we compared the accuracy and noise properties of the block-pairwise DTC and the global DTC method. Main results. The currently used global DTC led to a substantial positive bias in regions with high activity; the block-pairwise DTC resulted in substantially less bias. The noise level for the block-pairwise DTC was comparable to the global DTC and image reconstructions without any DTC. Finally, we tested the block-pairwise DTC with a data set obtained from volunteer measurements using the mGluR5 (metabotropic glutamate receptor subtype 5) antagonist [11C]ABP688. When the relative differences in activity concentrations obtained with global DTC and block-pairwise DTC for the ACC and the cerebellum GM were compared, the ratios differed by a factor of up to 1.4 at the beginning—when the first injection is administered as a bolus with high radioactivity. Significance. In this work, global DTC was shown to have the potential to introduce quantification bias, while better quantitation accuracy was achieved with the presented block-pairwise DTC method. The method can be implemented in all systems that use the delayed window technique and is particulary expected to improve the quantiation accuracy of dedicated brain PET scanners due to their geometry.’

Study of digitizer properties in nuclear spectroscopy experiments

In the present work a new CAEN 725S digitizer was implemented and compared to standard analog electronics in simple detector circuits, widely used in experimental nuclear physics. The first obtained results demonstrated that low-cost digital signal processing can offer a faster, improved performance regarding dead time corrections, without any significant effect in the obtained detector resolution values.

Determination of Appropriate Source-to-Detector Distance Dependence of Gamma Spectrometry System to Achieve Higher Resolution

Operation of gamma spectrometry requires intensive monitoring of gamma ray fluxes. When a radioactive intense source is placed close to a crystal detector, it becomes saturated as a result of detector dead time. On the contrary, when placed far away, there is loss of count statistics. For this reason, optimal position for placing the source has to be investigated. With the improvement in the instrumentation of radiation detection applications over the years, high count rate measurement accuracy is more crucial than ever. This is due to periodic measurement and new correction models for dead time. The aim of this study is to examine the performance of two gamma spectroscopic systems. The source detector distance dependence on dead time, peak-to-compton ratio, overall amplifier gain, FWHM, and voltage variation were investigated using American standard procedures (ANSI/IEE -325). Measurements were performed at five (5) different distances of detector cap for four (4) point sources (Co-60, Eu-152, Cs-137 and Ba-133). Source-to-detector distance at 25 cm was improved to avoid summing coincidence and dead time correction. The results obtained at 25 cm showed that the dead time was found less than 1% as compared to 4 cm. This will describe the stability of dead time. It also indicated that for both detectors the rise in biased voltage will yield a good resolution at 1332.5 KeV. This study is significant as it provides information to ensure that detectors are kept at optimal distance to achieve good dead time.

Development of 4πβ(LS)-γ digital coincidence counting system at NIM

A new 4πβ(LS)-γ digital coincidence counting (DCC) system has been developed at NIM. The system equipped with a triple-to-double coincidence ratio (TDCR) counter in the β-channel and a NaI(Tl) scintillation detector in the γ-channel. A dedicated DCC software was designed for off-line implementation of 4πβ(LS)-γ coincidence counting method. The software consists of three modules: software-based circuits module, dead-time and resolving-time correction module and efficiency extrapolation module, respectively. The performance of the newly developed 4πβ(LS)-γ DCC system was demonstrated by a comparison measurement of Co-60 solution with the conventional 4πβ(PC)-γ DCC system.

Study of LEAP® 5000 Deadtime and Precision via Silicon Pre-Sharpened-Microtip™ Standard Specimens.

Atom probe tomography (APT) is a technique that has expanded significantly in terms of adoption, dataset size, and quality during the past 15 years. The sophistication used to ensure ultimate analysis precision has not kept pace. The earliest APT datasets were small enough that deadtime and background considerations for processing mass spectrum peaks were secondary. Today, datasets can reach beyond a billion atoms so that high precision data processing procedures and corrections need to be considered to attain reliable accuracy at the parts-per-million level. This paper considers options for mass spectrum ranging, deadtime corrections, and error propagation as applied to an extrinsic-silicon standard specimen to attain agreement for silicon isotopic fraction measurements across multiple instruments, instrument types, and acquisition conditions. Precision consistent with those predicted by counting statistics is attained showing agreement in silicon isotope fraction measurements across multiple instruments, instrument platforms, and analysis conditions.

Dead time correction of Feynman sampled rates suitable for neutron coincidence counting

Neutron coincidence counting (NCC) based on shift-register logic (SRL) is extensively used by international nuclear safeguards inspectorates to verify special nuclear material inventories, particularly Pu. An alternative autocorrelation pulse counting analysis technique is founded on Feynman sampling and makes use of the well-known Feynman-Y statistic. Although the latter approach has been widely adopted by the reactor noise and sub-criticality communities it has not been used by the nonproliferation community. Both techniques may be interpreted using the same theoretical model and so should extract the same fundamental information about the detected neutron pulse train, namely the first and second reduced factorial multiplets that are determined by the joint properties of the measurement item and the surrounding external neutron detector. In the nuclear safeguards domain, a semi-empirical dead time treatment is conventionally applied to the trigger (singles) and doubles rates and, as already noted, the Feynman-Y method is not typically applied. In this article, we revisit the dead time treatment of Robba et al. (1983) for the Feynman sampling method. We show so that a dead time corrected multiplet analysis, familiar to safeguards practitioners, can be extracted. This is a first and necessary step to enabling a comparison of the relative performance of these two autocorrelation techniques on a like basis for deeply subcritical measurement items, which is the domain of concern to international nuclear safeguards inspectorates. From the analytical derivations it emerges that the Robba formalism, with minor corrections, and the commonly used NCC shift-register logic dead time correction formalism yield numerically equivalent corrections for practical purposes over the operational range of safeguards interest.

Design and Application of a β-γ Coincidence Measurement System Based on Digital Technology

In order to reduce the uncertainty of coincidence measurement, a β-γ coincidence measurement system based on digital technology is designed. The system consists of data acquisition hardware and data analysis software. It uses the data acquisition card to collect data from the detector, and then finish the dead time correction, coincidence resolving time correction and efficiency extrapolation necessary for coincidence measurement on the computer. The designed coincidence measurement system is used to measure 60Co standard source in the laboratory, and adopt two efficiency extrapolation methods to process the measurement results. The results show that the deviation of the activity of 60Co standard source measured using the designed coincidence system from the value measured using a standard device is less than 0.5%.

Activity regimes for 82Rb cardiac PET: Effects on absolute MBF and MPI

BackgroundSelection of optimal tracer activity for 82Rb PET is based on a trade-off between necessary count-statistics in the late static phase and detector saturation in the early blood-pool phase. Administered tracer activity recommended in prescribing information differs substantially from recommendations in current literature. The present study examines the effect on both absolute myocardial blood flow (MBF), myocardial flow reserve (MFR) and relative myocardial perfusion imaging (MPI) of reducing dose. MethodsForty patients were scanned twice on a PMT-based PET/CT (GE D690): At recommended activity (1110 MBq) and at either 740 or 370 MBq. MBF, MFR, total perfusion deficit (TPD) and ejection fractions (EF) were quantified. Results were compared using linear regression and Bland–Altman plots. ResultsLinear correlation between MBF at 1110 MBq at either reduced activity had an R2 > 0.98. A small bias (± 5%-9%) was observed with opposite signs for 1110/740 and 1110/370. Limits of agreement for MBF were larger for 1110/370. MFR had a lower linear correlation (R2 = 0.96), but wide limits of agreement especially for 1110/370. TPD and EF correlated well at 1110/740 (R2 = 0.96 and 0.99, respectively), but large scatter was observed for 1110/370. ConclusionReduction of the tracer activity to 740 MBq, significantly reduced dead-time correction factors, while still producing reliable static and gated images. However, despite large dead-time at 1110 MBq, no systematic bias on absolute MBF was observed compared to reduced activities.