Calculated Correction Factors Research Articles

Re-evaluation of the BIPM international standard for air kerma in 60Co gamma radiation

A re-evaluation of the BIPM standard for air kerma in 60Co radiation has been made. The changes to the air-kerma rate determination arise from four sources: (i) the results of Monte Carlo calculations of correction factors for the standard; (ii) a re-evaluation of the correction factor for saturation; (iii) a new evaluation of the air volume of the standard using an experimental chamber of variable volume; (iv) the adoption of a new reference beam at the BIPM. The combined effect of these changes is an increase in the BIPM determination of air-kerma rate by the factor 1.0054 and a reduction of the relative standard uncertainty of this determination to 1.5 parts in 103. A full uncertainty budget is presented. The new standard is effective from 1 November 2007.

Calculation of thermal neutron self-shielding correction factors for aqueous bulk sample prompt gamma neutron activation analysis using the MCNP code

In this work thermal neutron self-shielding in aqueous bulk samples containing neutron absorbing materials is studied using bulk sample prompt gamma neutron activation analysis (BSPGNAA) with the MCNP code. The code was used to perform three dimensional simulations of a neutron source, neutron detector and sample of various material compositions. The MCNP model was validated against experimental measurements of the neutron flux performed using a BF 3 detector. Simulations were performed to predict thermal neutron self-shielding in aqueous bulk samples containing neutron absorbing solutes. In practice, the MCNP calculations are combined with experimental measurements of the relative thermal neutron flux over the sample’s surface, with respect to a reference water sample, to derive the thermal neutron self-shielding within the sample. The proposed methodology can be used for the determination of the elemental concentration of unknown aqueous samples by BSPGNAA where knowledge of the average thermal neutron flux within the sample volume is required.

SU-FF-T-142: Determination of Correction Factors for a 2D Diode Array Measurements in MV Photon Beams

Purpose: To evaluate the accuracy of MapCheck measurements for range of field‐sizes, SSDs and depths. Method and Materials: Central axis measurements for various collimator settings (3×3cm2 to 40×40cm2), SSDs (80cm, 90cm, 100cm and 110cm) and depths (5cm, 10cm, 20cm) for 6 and 15 MV‐photon beams were performed using MapCheck and PTW MP3 system (0.125cc IC). Correction factor was determined as the ratio of IC to MapCheck measurements.Backscattering effect was assessed by comparing the MapCheck measurements with and without 10cm thick water‐equivalent backscattering material. Phantom scattering effect was evaluated by taking the ratio of MapCheck with backscattering material to the IC measurements under electron equilibrium conditions. This was also compared to the theoretical calculation using ratios of phantom scatter factor between an infinite large and 30 by 30 cm2 phantom. For 15MV, additional effect due to heterogeneity structure in the Mapcheck needs to be added. Results: The total correction factor for 6MV‐photon beams was up to 9.5%, mostly due to the lack of phantom scattering in MapCheck measurements when the field‐sizes larger than 30cm. The result was consistent with theoretical analysis with phantom scatter alone. The backscatter correction factor varied by 3% and increased with increasing field‐size. For 15MV, the total correction factor was up to 8%. The difference between the measured correction factor and theoretical calculation is attributed to heterogeneity factor, about 2% and is a function of field‐size only. Conclusion: We have determined correction factors for MapCheck which is a function of the scaled field‐size and depth only and can be mostly explained by lack of phantom scattering for large fields. A theoretical model is provided for general case when the solid phantom used is not 30×30 cm2.

A method for calculation of true coincidence summing correction factors for extended sources

In gamma-ray spectrometry, true coincidence summing correction factors for an extended sample can be calculated from full-energy-peak and total efficiencies as if the sample were a point source, if the so-called linear-to-square- (LS) curve, introduced by Blaauw and Gelsema, is known and properly applied. A method is described for obtaining the efficiencies and the corresponding LS-curve for an arbitrary cylindrical sample from calibration measurements in a reference geometry. The approach is aimed at the analysis of samples measured on p-type HPGe detectors in environmental gamma-ray spectrometry and was successfully verified against experimental data.

Establishment of Illuminance Scale at UME with an Accurately Calibrated Radiometer

Traceability in illuminance measurements at the National Metrology Institute of Turkey (TUBITAK-UME) was established in 2003 with a detector-based realization. The new measurement technique was developed for the determination of illuminance responsivity and upgrating of the illuminance scale. The unit of the illuminance responsivity, in A/lx, was measured with an expanded uncertainty of 0.2% (k = 2) by supplying using the developed scanning technique for the calculation of color correction factor. The surface of a radiometer was scanned using a double-monochromator facility upgraded with an x-y scanning system. The illuminance responsivity as a function of bandpass and temperature were also investigated in this study.

Real-time Quantitative RT-PCR Shows Variable, Assay-dependent Sensitivity to Formalin Fixation: Implications for Direct Comparison of Transcript Levels in Paraffin-embedded Tissues

Real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) is a versatile tool for precise quantification of gene expression. Formalin-fixed and paraffin-embedded (FFPE) tissue is well suited for qRT-PCR, if RNA extraction is optimized and small amplicon sizes are used. However, little is known whether individual assays may show variable sensitivity to fixation. This is of great importance, if a direct comparison of different transcripts is performed within the same sample, such as for mRNA splice variants. We established a cell culture model to test for and quantify differences in performance of individual qRT-PCR assays on FFPE as compared with fresh material, using TaqMan methodology. RNA was isolated from 7 different cell lines either directly or after preparation of a FFPE cell block. RNA from both sources was reverse transcribed and gene expression quantified using 13 different TaqMan assays. All assays allowed highly reproducible target quantification, using both fresh and FFPE-derived cDNA. The 13 assays showed an average Ct difference of 3.2 between fresh and FFPE cells, if identical amounts of cDNA were used as template. However, the Ct shifts varied from 1.8 to 5.1 for individual assays, indicating variable resistance to fixation. These Ct shift differences were statistically highly significant in 27/78 (35%) of all possible combinations of assays. Because the Ct shift remained constant for each assay, they could be used for calculation of correction factors which rendered FFPE-derived expression data highly comparable to those obtained from fresh material, and as a consequence among each other. Thus, a standardized assessment of qRT-PCR assay efficiencies in FFPE allows for precise intraindividual comparison of mRNA species, such as splice variants with different biologic functions, in archival tissues.

TU‐C‐224C‐07: A Comprehensive Dosimetric Protocol for the Cyberknife Radiosurgery System

Purpose: To propose dosimetric guidelines specifically designed for the Cyberknife radiosurgery system. Non‐availability of 10×10cm2 field and use of small circular collimators (5mm to 60mm) pose serious problems, that have been faced in this study by means of 8 different detectors and Monte Carlo simulation. This work is oriented to measurement of total scatter factors (Sc,p) and to reference dosimetry, though indications will also be given in view of a comprehensive guideline. Method and Materials: PTW PinPoint 31014, Exradin A16 and T14P microchambers, TN 502RDM micromosfet, PTW 30008 diode and TM60003 diamond, MD55 and EBT radiochromic films were used to measure Sc,p. Monte Carlo simulations (BEAMnrc) were used to produce phase space descriptions at the exit plane of each collimator, to calculate: 1) theoretical Sc,p values in water, and 2) correction factors to be applied to Sc,p as measured by 5 detectors (PinPoint, A16, T14P, diode, diamond), obtained by simulating shape and chemical composition of each detector. BEAMnrc was also used to calculate stopping power ratios and chamber correction factors for the Cyberknife linac, to decide whether values of kQ from the IAEA398 protocol could be applied without using a 10×10cm2 field. Results: Sc,p of the 5mm collimator as measured by simulated detectors averaged 0.653 − 9%+14%. Variation for larger collimators was smaller. After Monte Carlo correction, Sc,p of the 5mm collimator became 0.686 −2%+1%. Pure Monte Carlo calculation gave Sc,p=0.715 +/−1%. Calculation of correction factors showed that kQ values for the investigated chambers could be chosen when using IAEA398, introducing +/−0.2% uncertainty. Conclusion: Pure Monte Carlo calculation gave higher values of Sc,p compared to Monte Carlo‐corrected measurement. The latter is to be preferred because correction factors are less sensitive to beam parameters than pure calculation of Sc,p. For determination of Sc,p use of microchambers and Monte Carlo correction is recommended.

Calculations of the corrections for the true summation of cascade γ rays on the basis of statistical simulation using evaluated nuclear data

A method of calculating the corrections to the true summation of the γ radiation for arbitrary radionuclides for measuring point and volume sources using scintillation and ultrapure germanium detectors is described. The calculations are based on the Monte Carlo method using ENSDF evaluated data on the structure of atomic nuclei. An experimental check made using IAEA test spectra confirms that the calculations of the correction factors are reliable. The method is integrated into the γ spectrometric packages LSRM-2000 and AkWin, where the corrections for the ture summation are used for developing a library.

Monte Carlo calculations of calibration and geometry correction factors of a radionuclide calibrator

The activity of radioactive pharmaceuticals administered to patients in nuclear medicine is usually determined using well-type high-pressure ionization chambers. For the Bqmeter chamber (Consortium BQM, Czech Republic) a Monte Carlo model was created using the MCNP4C2 code. Basic chamber characteristics for two sample containers of various geometry (a vial and an ampoule) were calculated and compared with measurements. As the pharmaceuticals are often measured in various syringes, the chamber response for samples in syringes was also studied.

Interpretation of the TRADE In-Pile source multiplication experiments

Within the framework of the neutronic characterization of the TRIGA RC-1 reactor in support to the TRADE (TRiga Accelerator Driven Experiment) program, the interpretation of the subcriticality level measurements performed in static regime during the TRADE In-Pile experimental program is presented. Different levels of subcriticality have been measured using the MSA (Modified Source Approximated) method by the insertion of a standard fixed radioactive source into different core positions. Starting from a reference configuration, fuel elements were removed: control rods were moved outward as required for the coupling experiments envisioned with the proton accelerator and fission chambers were inserted in order to measure subcritical count rates. A neutron-physics analysis based on the modified formulation of the source multiplication method (MSM) has been carried out, which requires the systematic solution for each experimental configuration of the homogeneous, both in the forward and adjoint forms, and inhomogeneous Boltzmann equations. By means of such a methodology calculated correction factors to be applied to the MSA measured reactivities were produced in order to take into account spatial and energetic effects creating changes in the detector efficiencies and effective source with respect to the calibration configuration. The methodology presented has been tested against a large number of experimental states. The measurements have underlined the sensitivity of the MSA measured reactivities to core geometry changes and control rod perturbations; the efficiency of MSM factors to dramatically correct for this sensitivity is underlined, making of this technique a relevant methodology in view of the incoming US RACE program to be performed in TRIGA reactors.

Cross-camera comparison of SPECT measurements of a 3-D anthropomorphic basal ganglia phantom

SPECT examinations of neurotransmitter systems in the brain have to be comparable between centres to generate a comprehensive data pool, e.g. for multicentre studies. Equipment-specific effects on quantitative evaluations and corresponding methods for compensation, however, have been insufficiently examined. Previous studies have shown that quantitative results may vary significantly according to the imaging equipment used, thereby affecting clinical interpretation of the data. The aim of this study was to determine correction factors for common camera/collimator combinations based on standardised measurements of an anthropomorphic 3D basal ganglia phantom to compensate for the effects of different SPECT camera/collimator equipment. The latter may serve as a model for human studies of the dopaminergic system. The striatum and background chambers of a commercially available phantom (RSD Alderson) were filled with various( 123)I concentrations encompassing specific striatum/background ratios from 0.6 to 16.1. This setup was imaged with the following four camera/collimator combinations: Siemens Multispect 3 fitted with LEHR and( 123)I parallel-hole collimators, Siemens ECAM with LEHR parallel-hole collimators and Philips Prism 3,000 fitted with LEHR fanbeam collimators, using standardised protocols for acquisition and reconstruction. All scans were automatically co-registered to a SPECT template of the phantom and quantified using a 3D volume of interest (VOI) map based on a CT scan of the phantom. All striatal/background ratios calculated by SPECT were compared with the true ratios calculated from the measurements in a well counter. Regression analyses were performed and recovery correction factors between measured and true ratios determined. The relation between true and measured ratios could be sufficiently described by a linear regression for each camera/collimator combination without relevant improvement when using second-order polynomial regression models. The recovery correction factors and standard errors were 2.04+/-0.04 for the Philips Prism 3,000, 2.67+/-0.03 for the Siemens Multispect 3/LEHR parallel-hole collimators, 2.15+/-0.03 for the Siemens Multispect 3/(123)I collimators and 2.81+/-0.03 for the Siemens ECAM. Percentage recovery ranged from 36% to 49%. Measurements of a 3D basal ganglia phantom with various imaging devices revealed linear correlations between measured and true striatal/background ratios. Based on these findings, adjustment of quantitative results between different equipment seems possible, provided that acquisition, reconstruction and evaluation are adequately standardised. The use of identical evaluation methods in phantom and patient studies (comparable shape, size and location of the VOIs) might allow transfer of the calculated correction factors from phantom to studies of the dopaminergic system in patients.

Calculation of total efficiencies of extended samples for HPGe detectors

Calculation of coincidence summing correction factors in gamma-ray spectrometry can be carried out by making use of full-energy-peak and total efficiencies for a given sample geometry and detector setup. In the case of extended samples, the contribution of gamma-rays that scatter in the sample itself to the total efficiency needs to be taken into account. A method is described for calculating this contribution based on calibration measurements with point sources positioned on the detector housing. The approach is aimed at environmental gamma-ray spectrometry analysis of cylindrical samples on p-type HPGe detectors and is verified against full Monte Carlo calculations of total efficiencies with the GEANT code.

How many branchiopod crustacean species are there? Quantifying the components of underestimation

ABSTRACTAims For the crustacean class Branchiopoda, we estimated global species richness, assessed the reliability of relative diversity estimates among higher taxa and made predictions about the kinds of species remaining to be described.Location Global.Methods We calculated correction factors to account for three different sources of diversity underestimation, analysed taxonomic patterns of species descriptions over time and tested possible correlates of description date.Results Our work suggests that there are c. 2.1 times more branchiopod species in nature than the c. 1180 currently described. The largest correction factor was at the scale of single biogeographical regions, mainly attributed to poor knowledge of species diversity within described morphospecies. Relative diversity estimates among major taxonomic groups are apparently quite reliable, with diversity differences greater than 1.5‐fold robust to the heterogeneity of the taxonomic process. Current knowledge of branchiopod diversity is biased towards species with large geographical ranges, whereas body size and higher taxonomic group were only weak predictors of description date.Main conclusions A proper understanding of species boundaries in nature, which is greatly aided by molecular studies, is key in accurately estimating species richness for invertebrate groups. However, we suggest that while many species remain to be described, numbers of presently described species are still useful for macroevolutionary studies of diversification when interpreted with caution.

Efeito da idade da vaca e da data juliana de nascimento sobre o ganho médio diário de bezerros de corte no período pré-desmame

Estudaram-se os efeitos da idade da vaca ao parto (IDV) e da data juliana de nascimento (DJN) sobre o ganho médio diário no período pré-desmame de bezerros de corte e determinaram-se os respectivos fatores de correção (FC), utilizando 463.971 e 39.220 dados das raças Nelore e Tabapuã, respectivamente. O modelo estatístico continha os efeitos de IDV, modelada por um polinômio segmentado quadrático-quadrático com um nó aos sete anos e de DJN do bezerro, modelada por um polinômio segmentado com três segmentos quadráticos e dois nós, aos 185 e 295 dias. Tanto a IDV como a DJN influenciaram significativamente o ganho médio diário dos bezerros no período pré-desmame. Para os dois efeitos foram encontradas diferenças entre as duas raças. Para a IDV, os melhores ganhos ocorreram aos seis anos para a raça Tabapuã e aos nove anos para a raça Nelore. Quanto ao efeito da DJN, os melhores ganhos ocorreram no inverno para ambas as raças. Os FC foram calculados por raça, sendo os da IDV calculados separadamente para machos e fêmeas e as do DJN por estação. A determinação de fatores de correção deverá possibilitar melhor precisão da seleção, reduzindo os efeitos não genéticos que concorrem para a variação dessa característica.

Predictive value of clinical risk assessment tools and guidelines for 10-year coronary heart disease risk in practice-based primary care

Summary Background: Risk for developing myocardial infarction derived from risk tables in primary care subjects in Switzerland may over- or un- derestimate risk. Coronary calcifications may improve the performance of risk tables. Methods: We used coronary calcium score percentiles >50 (CS% >50) as a surrogate marker for 10-year myocardial infarction risk in a prospective cross-sectional monocenter study. CS% >50 was compared to several risk charts, was used to reclassify subjects in the intermediate risk category assessed by Fram- ingham risk scores (FRS), and was used to cal- culate a cohort specific correction factor for FRS and PROCAM. Results from risk charts were entered into the Bayes formula as the pretest risk estimates. Subjects in our cohort were 100 primary care patients with no known history of cardiovascular disease randomly selected from three primary care practices. Results: The sensitivity of FRS to detect CS% >50 was 47%, and specificity was 85%. NCEP III and Swiss guidelines had sensitivi- ties of 53% and 67%, respectively (p = ns), and specificities of 66% and 67% (p 50-derived post-test probabilities shifted 16 subjects (76%) into the low-risk cat- egory and 5 subjects (24%) into the high-risk category. Cohort-specific correction factors were 0.68 for FRS and 0.64 for PROCAM. Conclusions: For our Swiss German co- hort, FRS and PROCAM tended to overesti- mate risk. A biological risk marker (coronary calcifications) may allow improvement of risk prediction in primary care subjects with inter- mediate risk and also helps in the calculation of cohort-specific correction factors.

Estimation of Right Ventricular Mass by Two-dimensional Echocardiography

This report describes two original echocardiographic approaches to measure right ventricular (RV) mass (RVM). In the bullet formula (5/24 pi D1 D2 L), where D1 and D2 are short axes and L the log axis, the RVM is obtained by subtracting the cavity volume from the RV total volume and subsequently multiplying the difference by myocardium density. The second method uses 3 endocardium segments measured at: (1) short axis plane of the aortic valve and left atrium (b1); (2) short axis plane at the midpoint between the tricuspid valve annulus and the apex (b2); and (3) 4-chamber view (h). Those segment lengths are applying in the formula A = [(b1 + b2)/2] x h. The result is multiplied by the wall thickness and by myocardium density. Both formulas were primarily tested in 30 mongrel dogs and have shown good correlation with the true mass ( r = 0.869 with the segments formula and r = 0.819 with the bullet formula). The same method was used in 20 human patients before heart transplant with similar results ( r = 0.810 with the segments formula and r = 0.836 with the bullet formula). The RVM can be satisfactorily estimated by 2-dimensional echocardiography. The linear regression between the calculated mass (using the smoothest and thinner myocardium thickness) and the actual mass may provide the correction factor for the RVM calculation. Two echocardiographic methods were used to measure right ventricular mass. One of them used a bullet formula variant (5/24 pi D1 D2 L). The second method used 3 endocardium segments measured in 3 2-dimensional echocardiographic planes (short axis of aortic valve and left ventricle, and 4-chamber view), and applied in the formula A = [(b1 + b2)/2] x h. Both formulas have shown good correlation with the true mass in 30 mongrel dogs ( r = 0.869 with the segments formula and r = 0.819 with the bullet formula) and in 20 human patients before heart transplant ( r = 0.810 and r = 0.836, respectively).

Paparamborde: a software dedicated to quantitative mapping of biological samples using scanning transmission ion microscopy

Paparamborde, Programme d’Analyse des PArticules RAlenties dans la Matière de BORDEaux, a software dedicated to the scanning transmission ion microscopy (STIM) analysis of biological samples was developed. The programme includes the reconstruction of the median and/or mean energy loss image of the sample, as well as accurate calculation of sample thickness performed over the whole or a part of the scanned surface. The calculation is based on the conversion of transmitted particle energy into areal density of matter (μg/cm2) and uses SRIM code for the stopping power calculation. A new routine for the calculation of X-ray absorption correction factors (XCF), as needed for PIXE quantification of trace element, has been added in the latest version of Paparamborde, allowing the thickness heterogeneity of sample to be properly taken into account in the calculation of XCF.

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Tropical forest tree mortality, recruitment and turnover rates: calculation, interpretation and comparison when census intervals vary

Summary Mathematical proofs show that rate estimates, for example of mortality and recruitment, will decrease with increasing census interval when obtained from censuses of non‐homogeneous populations. This census interval effect could be confounding or perhaps even driving conclusions from comparative studies involving such rate estimates. We quantify this artefact for tropical forest trees, develop correction methods and re‐assess some previously published conclusions about forest dynamics. Mortality rates of > 50 species at each of seven sites in Africa, Latin America, Asia and Australia were used as subpopulations to simulate stand‐level mortality rates in a heterogeneous population when census intervals varied: all sites showed decreasing stand mortality rates with increasing census interval length. Stand‐level mortality rates from 14 multicensus long‐term forest plots from Africa, Latin America, Asia and Australia also showed that, on average, mortality rates decreased with increasing census interval length. Mortality, recruitment or turnover rates with differing census interval lengths can be compared using the mean rate of decline from the 14 long‐term plots to standardize estimates to a common census length using λcorr = λ × t0.08, where λ is the rate and t is time between censuses in years. This simple general correction should reduce the bias associated with census interval variation, where it is unavoidable. Re‐analysis of published results shows that the pan‐tropical increase in stem turnover rates over the late 20th century cannot be attributed to combining data with differing census intervals. In addition, after correction, Old World tropical forests do not have significantly lower turnover rates than New World sites, as previously reported. Our pan‐tropical best estimate adjusted stem turnover rate is 1.81 ± 0.16% a−1 (mean ± 95% CI, n = 65). As differing census intervals affect comparisons of mortality, recruitment and turnover rates, and can lead to erroneous conclusions, standardized field methods, the calculation of local correction factors at sites where adequate data are available, or the use of our general standardizing formula to take account of sample intervals, are to be recommended.

Robust incremental compensation of the light attenuation with depth in 3D fluorescence microscopy.

Summary Fluorescent signal intensities from confocal laser scanning microscopes (CLSM) suffer from several distortions inherent to the method. Namely, layers which lie deeper within the specimen are relatively dark due to absorption and scattering of both excitation and fluorescent light, photobleaching and/or other factors. Because of these effects, a quantitative analysis of images is not always possible without correction. Under certain assumptions, the decay of intensities can be estimated and used for a partial depth intensity correction. In this paper we propose an original robust incremental method for compensating the attenuation of intensity signals. Most previous correction methods are more or less empirical and based on fitting a decreasing parametric function to the section mean intensity curve computed by summing all pixel values in each section. The fitted curve is then used for the calculation of correction factors for each section and a new compensated sections series is computed. However, these methods do not perfectly correct the images. Hence, the algorithm we propose for the automatic correction of intensities relies on robust estimation, which automatically ignores pixels where measurements deviate from the decay model. It is based on techniques adopted from the computer vision literature for image motion estimation. The resulting algorithm is used to correct volumes acquired in CLSM. An implementation of such a restoration filter is discussed and examples of successful restorations are given.