Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Correction factor”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 181 records · Page 10Linked to original sources
PubMed1980

[Applanation tonometry on contact lenses with high water content. Problems, results, correction factors].

The use of contact lenses with high water content for different therapeutic purposes has become widespread in the past four years. Permanent-wear contact lenses of this type are used in the treatment of corneal conditions which resist therapy, and also postoperatively. They cause no problems in follow-up examinations with the slit lamp but make it very difficult to measure IOP, which is frequently necessary in such cases. Therefore, we carried out invetigations with the aim of calculating correction factors for applanation tonometry on contact lenses with high water content. It was found that the power of the contact lens has a highly significant effect on the reading obtained. Simple correction tables are given which enable a sufficient degree of accuracy to be achieved in tonometry on these contact lenses.

Contact Lenses, Hydrophilic↗
PubMed1975

Cumulative radiation effect. part IV. Normalisation of fractionated and continuous therapy - area and volume correction factors.

In three previous papers, a scale of accumulated sub-tolerance radiation damage, the Cumulative Radiation Effect (CRE), was described for fractionated treatment regimes and continuous radiation therapy from both long-lived and short-lived sources. This scale provides a means of assessing and comparing the biological effects of various treatments. In this paper, it is intended to further the scope of application of the CRE-system by describing in detail the normalisation between the assessments of fractionated and continuous therapy. To do this, it is necessary to consider area and volume correction factors, whose roles are to modify the values of uncorrected CREs so that a specific biological effect, regardless of the area or volume treated, is described by a unique value of the corrected CRE. The problem arising when the CRE achieved by a treatment regime is not constant over a volume is briefly discussed and a simple hypothesis is proposed which may stimulate interest in this problem. The establishment of a normalisation procedure between fractionated treatment regimes and continuous radiation therapy, which is the primary aim of this paper, serves to integrate the concept of the CRE into a unified system of assessment of biological damage, so that a unique value of the CRE describes a specific level of biological damage regardless of the area or volume treated, or the time course of administration of the radiation employed.

Dose-Response Relationship, Radiation↗
PubMed1985

Air-ground interface correction factors for gamma emitters in air.

For isotropic point photon sources above an air-ground interface, the spectral energy fluences at the ground have been calculated by the Monte Carlo method for source energies in the range 50 keV-10 MeV. The source heights and the horizontal source-detector distances varied from 0.05 to 5 mean free paths in air. Air-ground interface correction factors have been calculated and then analytically approximated. The greatest deviations from unity were found for 100-keV sources located on the ground. Near the source, kerma free in air is enhanced by 30%, and far from the source it is reduced by a factor of 3.

Air Pollution, Radioactive↗
PubMed1987

Correction factors for gravimetric measurement of peritumoural oedema in man.

The water content of samples of normal and oedematous brain in lobectomy specimens from 16 patients with cerebral tumours has been measured by gravimetry and by wet and dry weighing. Uncorrected gravimetry underestimated the water content of oedematous peritumoural cortex by a mean of 1.17%, and of oedematous peritumoural white matter by a mean of 2.52%. Gravimetric correction equations calculated theoretically and from an animal model of serum infusion white matter oedema overestimate peritumoural white matter oedema in man, and empirical gravimetric error correction factors for oedematous peritumoural human white matter and cortex have therefore been derived. These enable gravimetry to be used to accurately determine peritumoural oedema in man.

Adult↗
PubMed1983

A nomogram for the determination of "virtual shunt" with correction factors for variations in pH, temperature and hemoglobin concentration.

Several nomograms are currently in use for determination of virtual shunt from a known FIO2 and PaO2. They are constructed for fixed standard values of pH, T and Hb. In the critically ill however those values often vary widely from the standard values. This may lead to substantial under- or overestimation of the virtual shunt. We therefore calculated correction factors resulting in a more precise value for the virtual shunt.

Body Temperature↗
PubMed1984

Correction factors for determining body exposure from forearm percutaneous absorption data.

Evaluation of toxic exposure via the skin is invariably based upon forearm percutaneous absorption data and an appropriate area multiplication factor. Published experimental data clearly show, however, substantial site variation in skin penetration such that the use of forearm data may produce an inaccurate prediction of total body exposure. Using literature results for cutaneous absorption as a function of anatomic position and accepted estimations of human body surface areas, correction factors for use with forearm penetration data have been evaluated. Calculations have been performed for an adult, a small child and a neonate. Examples of limited exposure to specific regions of the body have also been assessed relative to predictions based upon percutaneous absorption across forearm skin. It appears that the use of forearm skin penetration data alone underestimates body exposure by at least a factor of two.

Adult↗
PubMed1997

Experimental determination of fluence correction factors at depths beyond dmax for a Farmer type cylindrical ionization chamber in clinical electron beams.

Recently, it has been recommended that electron beam calibrations be performed at a new reference depth [Burns et al., Med. Phys. 23, 383 (1996)] given by dref = 0.6R50-0.1 cm, where R50 is the depth of 50% depth dose. In order to calibrate electron beams at dref with a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors Pwall and Pfl at dref are required. Using a parallel plate Holt chamber as a reference chamber, the product PwallPfl has been determined for a 6.1-mm-diameter PTW cylindrical ionization chamber at dref as a function of R50 of clinical electron beams (6 < or = nominal energy E < or = 22 MeV). Assuming that Pwall for the PTW chamber is unity in electron beams, the measured Pfl values ranged from 0.96 to 0.98 as the energy is increased. These results are in close agreement with recently reported calculated values. Determination of dref requires the knowledge of R50. A relation between I50 and R50 is given in the IAEA Protocol [TRS No. 277 (IAEA, Vieńna, 1987), pp. 1-98] for broad beams at SSD = 100 cm. It has been shown experimentally that the equation R50 = 1.029 x I50-0.063 cm, derived by Ding et al. [Med. Phys. 22, 489 (1995)] from Monte Carlo simulations of realistic clinical electron beams, can be used satisfactorily to obtain R50 from I50, where I50 is the depth of 50% ionization. The largest difference between the measured value of R50 and that calculated by using the above equation has been found to be about 1 mm at 22 MeV.

Biometry↗
PubMed1999

Correction factors and performance of a 4 degrees C sealed water calorimeter.

In the past two decades, the water calorimetry technique for determination of absorbed dose to water in several types of radiation beams has moved significantly closer to being a recognized method. In this paper we summarize the constructional details of a 4 degrees C sealed water calorimeter currently in operation at the University of Gent. This sealed water (SW) calorimeter is of the Domen type and has been improved in several aspects compared with its original design. The relevant correction factors for heat transport and for field perturbation are described. Using relative response measurements in 60Co, we experimentally verified the relative heat defect for two distinct chemical systems, using two different detection vessel arrangements. The overall 1sigma uncertainty on the absorbed dose to water at 60Co based on this system amounts to 0.7%.

Calibration↗
PubMed2005

Humidity and aggregate content correction factors for air-coupled ultrasonic evaluation of concrete.

This paper describes the use of non-contact ultrasound for the evaluation of concrete. Micromachined capacitance transducers are used to transmit ultrasonic longitudinal chirp signals through concrete samples using air as the coupling medium, and a pulse compression technique is then employed for measurement of time of flight through the sample. The effect on the ultrasonic wave speed of storing concrete samples, made with the same water/cement ratio, at different humidity levels is investigated. It is shown that there is a correlation between humidity and speed of sound, allowing a correction factor for humidity to be derived. A strong positive linear correlation between aggregate content and speed of sound was then observed; there was no obvious correlation between compressive strength and speed of sound. The results from the non-contact system are compared with that from a contact system, and conclusions drawn concerning coupling of energy into the samples.

Air↗
PubMed2001

The effect of feed intake on ileal rate of passage and apparent amino acid digestibility determined with or without correction factors in pigs.

The apparent ileal digestibilities of amino acids and rate of passage were evaluated in pigs (BW = 78.3 +/- 7.4 kg) fed a semipurified diet. The pigs were fed 1.82, 2.73, or 3.65 kg DMI/d. The highest level of feed intake was considered to be ad libitum feeding. The pigs were fed according to a 3 x 3 Latin square design and were allowed to adapt to each experimental diet for 5 d. This was followed by 1 d of continuous collection of ileal digesta and a 2nd d of continuous collection separated into six 2-h postprandial time blocks. Ytterbium chloride hexahydrate was used to determine rate of passage. The ileal digestibilities of amino acids and rate of passage were unaffected (P > 0.05) by level of feed intake. The use of correction factors to more accurately express amino acid concentrations in the diet and digesta affected (P < 0.05) the apparent ileal digestibility coefficients of some amino acids.

Amino Acids↗
PubMed1990

Limitations of ZAF correction factors in the determination of calcium/phosphorus ratios: important forensic science considerations relevant to the analysis of bone fragments using scanning electron microscopy and energy-dispersive x-ray microanalysis.

A series of calcium phosphate standards having calcium/phosphorus (Ca/P) molar ratios of 0.50, 1.00, 1.50, and 1.67, respectively, was prepared for bulk specimen analysis using scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDXA). The standards were mounted on carbon planchettes as either pure crystals or crystals embedded in epoxy resin. Ten different samples of each embedded and non-embedded standard were analyzed in a JEOL 100 CX electron microscope interfaced with a Kevex 8000 EDXA system using a lithium-drifted silicon detector and a multichannel analyzer. The Ca/P ratios were determined by calculating both net peak intensities without matrix corrections and atomic kappa-ratios using the MAGIC V computer program with ZAF correction factors for quantitative analysis. There was such extensive absorption of phosphorus X-rays in standards embedded in an epoxy matrix that the observed Ca/P ratios were statistically compatible with four different standards ranging in theoretical Ca/P ratios from 1.0 to 1.67. Although the non-embedded crystals showed a greater separation in the Ca/P ratios, both methods of preparation produced serious flaws in analysis. Direct application of the discovery of this caveat to the identification of suspected bone fragments for forensic science purposes is discussed.

Bone and Bones↗
PubMed1984

Correction factors for survey meter calibrations.

A factor is derived to correct the response of a large cylindrical air ionization chamber in the radiation field of a point-like calibration gamma-ray source. The calculated factor is compared with the observed response of commercially available instruments used for radiation safety surveys. It is concluded that proper use of the factor can improve the accuracy of survey meter calibrations when point-like standard sources are employed. Furthermore, it is shown how the design of a chamber can reduce the response error when using point-like sources for calibration.

Calibration↗
PubMed1981

The correction factors for sucrose gap measurements and their practical applications.

The distribution of extracellular and intracellular potential in the sucrose gap apparatus, previously established for a single fiber using the cable equations for a core conductor model (Jirounek and Straub, Biophys. J., 11:1, 1971), is obtained for a multifiber preparation. The exact equation is derived relating the true membrane potential change to the measured potential differences across the sucrose gap, the junction potentials between sucrose and physiological solution, the membrane potential in the sucrose region, and the electrical parameters of the preparation in each region of the sucrose gap. The extracellular potential distribution has been measured using a modified sucrose gap apparatus for the frog sciatic nerve and the rabbit vagus nerve. The results indicate a hyperpolarization of the preparations in the sucrose region, of 60--75 mV. The hyperpolarization is independent of the presence of junction potentials. The calculation of the correction terms in the equation relating the actual to the measured potential change is illustrated for the case of complete depolarization by KC1 on one side of the sucrose gap. The correction terms in the equation are given for various experimental conditions, and a number of nomographic charts are presented, by means of which the correction factors can be rapidly evaluated.

Animals↗
PubMed1998

Analytical approach to heterogeneity correction factor calculation for brachytherapy.

In brachytherapy treatment planning, the effects of tissue and applicator heterogeneities are commonly neglected due to lack of accurate, general, and fast three-dimensional (3D) dose-computational algorithms. A novel approach, based on analytical calculation of scattered photon fluxes inside and around a disk-shaped heterogeneity, has been developed for use in the three-dimensional scatter-subtraction algorithm. Specifically, our model predicts the central-ray dose distribution for a collimated photon isotropic source or brachytherapy "minibeam" in the presence of a slab of heterogeneous material. The model accounts for the lateral dimensions, location, composition, density, and thickness of the heterogeneity using precalculated scatter-to-primary ratios (SPRs) for the corresponding homogeneous problem. The model is applicable to the entire brachytherapy energy range (25 to 662 keV) and to a broad range of materials having atomic numbers of 13 to 82, densities of 2.7 g.cm-3 (Al) to 21.45 g.cm-3 (Pt) and thicknesses up to 1 mean free path. For this range of heterogeneous materials, the heterogeneity correction factors (HCFs) vary from 0.09 to 0.75. The model underestimates HCF when multiple scattering prevails and overestimates HCF when absorption dominates. However, the analytic model agrees with Monte Carlo photon transport (MCPT) benchmark calculations within 1.8% to 10% for 125I, 169Yb, 192Ir, and 137Cs for a wide variety of materials, with the exception of Ag. For 125I shielded by Ag, where the mean discrepancy can exceed 25%, the error is due to K-edge characteristic x rays originating within the heterogeneity. The proposed approach provides reductions in CPU time required of 5 x 10(4)-10(5) and 100 in comparison with direct MCPT simulation and 1D numerical integration, respectively. The limitations of model applicability, as determined by the physical properties of heterogeneity material and accuracy required, are also discussed.

Algorithms↗
PubMed2004

Is there a simple correction factor for comparing adolescent tobacco-use estimates from school- and home-based surveys?

School surveys conducted to monitor the prevalence of adolescent smoking generally yield higher estimates than home-based surveys. Our goal was to examine the consistency of this mode effect over age for different measures of adolescent smoking behavior. If consistent, it would be feasible to define a simple adjustment factor for comparing overall results across survey modes. We identified three pairs of school- and home-based surveys (two national pairs and one California pair), each pair conducted in the same year, that allowed us to compare three key measures of adolescent smoking across age: (a) Ever tried, (b) smoking in past 30 days, and (c) regular smoking. We also examined male experimentation with smokeless tobacco, for which a mode effect has not been investigated. School surveys yielded higher estimates than in-home surveys for all the tobacco-use measures examined. In general, the mode effect was greater in younger age groups. Estimates for the two modes appeared closer for current and regular smoking measures compared with the ever-tried measure. Considering report of smoking in the past 30 days, the mode effect was much greater for infrequent smoking (1-9 days) than for more frequent (10+ days) smoking. Finally, school surveys yielded much higher estimates for male smokeless tobacco use than did home surveys. Because mode effects were related to age and level of reported use, no simple correction factor for comparing across modes is feasible. Social desirability bias of self-reported behavior in the different settings may account in part for the observed mode effects.

Adolescent↗
PubMedDate not supplied

Corneal power correction factor for photorefractive keratectomy.

BACKGROUND: Studies of corneal power changes resulting from photorefractive keratectomy generally rely on keratometer or videokeratograph measurements. These instruments convert corneal radius of curvatures values to optical powers by means of the single refracting surface formula, which incorporates an index of refraction value of 1.3375. This index approximates that of the tears but not the 1.376 index of the corneal epithelium or stroma. A hypothetical optical model was used to determine the most appropriate index to be chosen with respect to corneal power calculations relative to photorefractive keratectomy. METHODS: The contribution of each refractive element in the tear lens-corneal surface to the total power of the eye was calculated in order to identify which index of refraction was most appropriate for the corneal power calculation. RESULTS: The outer tear surface has significant optical power but the tear layer as a whole has nearly zero power due to the offsetting negative power of the posterior test surface. There is no significant difference in the effective power of light leaving the corneal anterior surface when considered with or without the tear layer. Photorefractive keratectomy changes the epithelium and anterior surface of the corneal stroma, but does not affect the posterior stroma or other ocular media. Hence the refractive index for the corneal epithelium or stroma of 1.376 should be used in converting radius to optical power values. The error in assuming a corneal index of 1.3375 is a constant proportion equal to 11.4% of the corneal power reading. CONCLUSIONS: Photorefractive keratectomy presents a situation in which the actual corneal refractive index of 1.376 should be used for correct corneal radius to power conversions. This may be accomplished by changing the index value in the instrument algorithm for keratometry and videokeratography to 1.376 or by adding a correction factor of either 11.4% of the regular reading to its value or multiplying by the factor 1.114. In other applications of keratometry or videokeratography, the index 1.3375 may be more appropriate.

Cornea↗
PubMed2000

Monte Carlo study of correction factors for Spencer-Attix cavity theory at photon energies at or above 100 keV.

To develop a primary standard for 192Ir sources, the basic science on which this standard is based, i.e., Spencer-Attix cavity theory, must be established. In the present study Monte Carlo techniques are used to investigate the accuracy of this cavity theory for photons in the energy range from 20 to 1300 keV, since it is usually not applied at energies below that of 137Cs. Ma and Nahum [Phys. Med. Biol. 36, 413-428 (1991)] found that in low-energy photon beams the contribution from electrons caused by photons interacting in the cavity is substantial. For the average energy of the 192Ir spectrum they found a departure from Bragg-Gray conditions of up to 3% caused by photon interactions in the cavity. When Monte Carlo is used to calculate the response of a graphite ion chamber to an encapsulated 192Ir source it is found that it differs by less than 0.3% from the value predicted by Spencer-Attix cavity theory. Based on these Monte Carlo calculations, for cavities in graphite it is concluded that the Spencer-Attix cavity theory with delta = 10 keV is applicable within 0.5% for photon energies at 300 keV or above despite the breakdown of the assumption that there is no interaction of photons within the cavity. This means that it is possible to use a graphite ion chamber and Spencer-Attix cavity theory to calibrate an 192Ir source. It is also found that the use of delta related to the mean chord length instead of delta = 10 keV improves the agreement with Spencer-Attix cavity theory at 60Co from 0.2% to within 0.1% of unity. This is at the level of accuracy of which the Monte Carlo code EGSnrc calculates ion chamber responses. In addition, it is shown that the effects of other materials, e.g., insulators and holders, have a substantial effect on the ion chamber response and should be included in the correction factors for a primary standard of air kerma.

Air↗
PubMed1991

Analytic representation of the backscatter correction factor at the exit of high energy photon beams.

In high energy X-ray beams, the dose calculated near the exit surface under electronic equilibrium conditions is generally overestimated since it is derived from measurements performed in water with large thickness of backscattering material. The resulting error depends on a number of parameters such as beam energy, field dimensions, thickness of overlying and underlying material. We have systematically measured for four different energies and for different combinations of the above parameters, the reduction of dose due to the lack of backscatter. This correction is expressed as a multiplicative factor, called "Backscatter Correction Factor" (BCF). This BCF is larger for lower energies, larger field sizes and larger depths. The BCF has been represented by an analytical expression which involves an exponential function of the backscattering thickness and linear relationships with depth, field size and beam quality index. Using this expression, the BCF can be calculated within 0.5% for any conditions in the energy range investigated.

Humans↗