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PubMedDate not supplied

A derivation of Batho's correction factor for heterogeneities.

Batho's correction factor for dose in a heterogeneous, layered medium is derived from the tissue-air ratio method (TARM). The reason why the Batho factor is superior to the TARM factor at low energy is ascribed to the fact that it accounts for the distribution of the scatter-generating matter along the centerline. The poor behavior of the Batho factor at high energies is explained as a consequence of the lack of electron equilibrium at appreciable depth below the surface.

Radiotherapy Dosage↗

PubMed2005

Evaluation of tonometric correction factors.

PURPOSE: To investigate the efficacy of currently available correction factors in correcting intraocular pressure (IOP) measurements for the errors induced by the normal variations in corneal structural characteristics. MATERIALS AND METHODS: Central corneal thickness (CCT) and corneal radius of curvature were measured on 324 individuals (175 normal: group 1 and 149 had either open angle glaucoma or ocular hypertension: group 2). IOP was measured in all normal subjects with the Goldmann applanation tonometer and the highest recorded IOP was obtained from patient charts for subjects with either open angle glaucoma or ocular hypertension. Regression analysis was performed on IOP, CCT, and corneal radius of curvature. The corrected IOP was also calculated using the models proposed by Ehlers and Orssengo and Pye. Linear regression analysis was used to calculate the residual association between corneal parameters and corrected IOP. RESULTS: There was a significant positive correlation between IOP measured using Goldmann applanation tonometer and the CCT in both groups. There was no significant correlation between corneal radius of curvature and IOP in either group. There was a significant negative correlation in both the groups between CCT and corrected IOP calculated using the models of Ehlers and Orssengo and Pye. This indicates that the Ehlers and Orssengo and Pye models may significantly overestimate the effect of CCT on IOP measurement. CONCLUSION: The effect of CCT and IOP as observed in the present study and by other studies in literature is less than predicted by both the Ehlers formula and the Orssengo and Pye model. Correcting IOP for the effect of CCT using these models could be erroneous and lead to overcorrection of IOP, thus resulting in erroneously low corrected IOP eyes with thicker cornea and erroneously high corrected IOP in eyes with thinner cornea.

Adult↗

PubMedDate not supplied

Displacement correction factor for fast-neutron dosimetry in a tissue-equivalent phantom.

The displacement correction factor to be used for analysis of fast-neutron dosimetric measurements using air-filled EG and G tissue-equivalent ion chambers in a tissue-equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35-MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC-17 (1.0-cm3) ion chamber, and is 0.989 for the EG and G IC-18 (0.1-cm3 ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

Fast Neutrons↗

PubMed2006

The value of Seasonal Correction Factors in assessing the health risk from domestic radon: a case study in Northamptonshire, UK.

Following an intensive survey of domestic radon levels in the United Kingdom (UK), the former National Radiological Protection Board (NRPB), now the Radiation Protection Division of the Health Protection Agency (HPA-RPD), established a measurement protocol and promulgated Seasonal Correction Factors applicable to the country as a whole. Radon levels in the domestic built environment are assumed to vary systematically and repeatably during the year, being generally higher in winter. The Seasonal Correction Factors therefore comprise a series of numerical multipliers, which convert a 1-month or 3-month radon concentration measurement, commencing in any month of the year, to an effective annual mean radon concentration. In a recent project undertaken to assess the utility of short-term exposures in quantifying domestic radon levels, a comparative assessment of a number of integrating detector types was undertaken, with radon levels in 34 houses on common geology monitored over a 12-month period using dose-integrating track-etch detectors exposed in pairs (one upstairs, one downstairs) at 1-month and 3-month resolution. Seasonal variability of radon concentrations departed significantly from that expected on the basis of the HPA-RPD Seasonal Correction Factor set, with year-end discontinuities at both 1-month and 3-month measurement resolutions. Following this study, monitoring with electrets was continued in four properties, with weekly radon concentration data now available for a total duration in excess of three and a half years. Analysis of this data has permitted the derivation of reliable local Seasonal Correction Factors. Overall, these are significantly lower than those recommended by HPA-RPD, but are comparable with other results from the UK and from abroad, particularly those that recognise geological diversity and are consequently prepared on a regional rather than a national basis. This finding calls into question the validity of using nationally aggregated Seasonal Correction Factors, especially for shorter exposures, and the universal applicability of these corrections is discussed in detail.

Air Pollution, Indoor↗

PubMed2001

Electron fluence correction factors for various materials in clinical electron beams.

Relative to solid water, electron fluence correction factors at the depth of dose maximum in bone, lung, aluminum, and copper for nominal electron beam energies of 9 MeV and 15 MeV of the Clinac 18 accelerator have been determined experimentally and by Monte Carlo calculation. Thermoluminescent dosimeters were used to measure depth doses in these materials. The measured relative dose at dmax in the various materials versus that of solid water, when irradiated with the same number of monitor units, has been used to calculate the ratio of electron fluence for the various materials to that of solid water. The beams of the Clinac 18 were fully characterized using the EGS4/BEAM system. EGSnrc with the relativistic spin option turned on was used to optimize the primary electron energy at the exit window, and to calculate depth doses in the five phantom materials using the optimized phase-space data. Normalizing all depth doses to the dose maximum in solid water stopping power ratio corrected, measured depth doses and calculated depth doses differ by less than +/- 1% at the depth of dose maximum and by less than 4% elsewhere. Monte Carlo calculated ratios of doses in each material to dose in LiF were used to convert the TLD measurements at the dose maximum into dose at the center of the TLD in the phantom material. Fluence perturbation correction factors for a LiF TLD at the depth of dose maximum deduced from these calculations amount to less than 1% for 0.15 mm thick TLDs in low Z materials and are between 1% and 3% for TLDs in Al and Cu phantoms. Electron fluence ratios of the studied materials relative to solid water vary between 0.83+/-0.01 and 1.55+/-0.02 for materials varying in density from 0.27 g/cm3 (lung) to 8.96 g/cm3 (Cu). The difference in electron fluence ratios derived from measurements and calculations ranges from -1.6% to +0.2% at 9 MeV and from -1.9% to +0.2% at 15 MeV and is not significant at the 1sigma level. Excluding the data for Cu, electron fluence correction factors for open electron beams are approximately proportional to the electron density of the phantom material and only weakly dependent on electron beam energy.

Electrons↗

PubMed1996

Dependence of overall correction factor of a cylindrical ionization chamber on field size and depth in medium-energy x-ray beams.

In this paper we examine the depth and field size dependence of the overall correction factor kch for in-phantom dose determinations in orthovoltage x-ray beams. The overall correction factor is considered to be composed of three contributions, i.e., (1) a contribution from the angular dependence of the chamber response free-in-air, derived based on the measured directional response of the NE2571 for different energies combined with Monte Carlo calculations; (2) a displacement effect and (3) a stem effect, both calculated using the Monte Carlo method for different field sizes and depths. The results show a variation of, at most, 2.2% at the lowest photon energies (29.8-keV average photon energy) when going from 2 cm to 5 cm for a small circular 20-cm2 field. In the medium-energy range (> or = 100 kV), variations are limited to, at most, 1.5% for 120 kV-150 kV when comparing the most extreme variations in field size and depth (i.e., 2-cm depth; 20-cm2 area compared to 5 cm depth; 200-cm2 area). Depth variations most importantly affect the overall correction factor by hardening of the photon fluence spectrum, whereas field diameter variations affect the factor by increase or decrease of contributions of photon scattering. The work shows that taking into account the uncertainties adopted in the recent review of data and methods recommended in the IAEA code of practice, the dependence of the overall correction factor on depth and field size is insignificant for the radiation qualities between 100 kV (HVL 0.17-mm Cu, average energy: 52 keV) and 280 kV (HVL 3.41-mm Cu, average energy: 144 keV).

Calorimetry↗

PubMed2000

Wall correction factors for calibration of plane-parallel ionization chambers with high-energy photon beams.

Most dosimetry protocols recommend that calibration of plane-parallel ionization chambers be performed in an electron beam of sufficiently high energy by comparison with cylindrical chambers. For various plane-parallel chambers, the 1997 IAEA TRS-381 protocol includes an overall perturbation factor pQ for electron beams, a wall correction factor p(wall) for a 60Co beam and the product of two wall corrections k(att)k(m) for 60Co in-air calibration. The recommended values of p(wall) for plane-parallel chambers, however, are limited to certain phantom materials and a 60Co beam, and are not given for other phantom materials and x-ray beams. In this work, the p(wall) values of the commercially available NACP, PTW/Markus and PTW/Roos plane-parallel chambers in a solid water phantom have been determined with 60Co and 4 and 10 MV photon beams. The k(att)k(m) values for the NACP and PTW/Markus chambers have also been obtained. The wall correction factors p(wall) and k(att)k(m) have been determined by intercomparison with a calibrated Farmer chamber. The average value of p(wall) for these plane-parallel chambers was 1.005 +/- 0.1% (1 SD) for 60Co beams and 1.007 +/- 0.2% (1 SD) for both 4 MV and 10 MV photons. The k(att)k(m) values for the NACP and PTW/Markus chambers were about 1.5% lower than other published data.

Calibration↗

PubMed2000

Entrance dose measurements for in-vivo diode dosimetry: Comparison of correction factors for two types of commercial silicon diode detectors.

Silicon diode dosimeters have been used routinely for in-vivo dosimetry. Despite their popularity, an appropriate implementation of an in-vivo dosimetry program using diode detectors remains a challenge for clinical physicists. One common approach is to relate the diode readout to the entrance dose, that is, dose to the reference depth of maximum dose such as d(max) for the 10x10 cm(2) field. Various correction factors are needed in order to properly infer the entrance dose from the diode readout, depending on field sizes, target-to-surface distances (TSD), and accessories (such as wedges and compensate filters). In some clinical practices, however, no correction factor is used. In this case, a diode-dosimeter-based in-vivo dosimetry program may not serve the purpose effectively; that is, to provide an overall check of the dosimetry procedure. In this paper, we provide a formula to relate the diode readout to the entrance dose. Correction factors for TSD, field size, and wedges used in this formula are also clearly defined. Two types of commercial diode detectors, ISORAD (n-type) and the newly available QED (p-type) (Sun Nuclear Corporation), are studied. We compared correction factors for TSDs, field sizes, and wedges. Our results are consistent with the theory of radiation damage of silicon diodes. Radiation damage has been shown to be more serious for n-type than for p-type detectors. In general, both types of diode dosimeters require correction factors depending on beam energy, TSD, field size, and wedge. The magnitudes of corrections for QED (p-type) diodes are smaller than ISORAD detectors.

Humans↗

PubMed1997

Computer model generated density correction factors for gamma spectroscopy counting.

Using the calibration curve of a single reference source to infer activity levels in samples of different bulk density and/or elemental composition may yield inaccurate results by a gamma spectroscopy system. These inaccuracies are magnified when counting low energy photons, which interact primarily through the photoelectric effect. There have been numerous methods described to empirically derive density correction factors for various samples. An alternate solution is to theoretically derive density correction factors using a computer model. The computer model generated density correction factors for material such as sand, ilmenite, and polyester are in close agreement with published empirically derived density correction factors for these same materials.

Americium↗

PubMed2001

Experimental p(wall) and p(cel) correction factors for ionization chambers in low-energy clinical proton beams.

Current dosimetry protocols for clinical protons using ionization chambers do not take into account ionization chamber-dependent perturbation correction factors. In the present investigation, the relative response of 17 cylindrical ionization chambers was evaluated at three proton beam qualities: at two points in a modulated beam and one point in a non-modulated beam, all with an incident energy of 75 MeV. Thirteen of the ionization chambers had a Farmer-type geometry but consisted of different combinations of wall and central electrode materials. All ionization chambers were calibrated in terms of air kerma as well as in terms of absorbed dose to water in a 60Co beam. The relative response of the ionization chambers was compared with results of Monte Carlo simulations of proton and secondary electron transport in the phantom and the ionization chamber geometry. The results of the measurements for cylindrical ionization chambers show relative perturbation effects that are limited to 0.5-1%, resulting in perturbation correction factors that are larger than unity compared with an NE2571 ionization chamber. The experimental relative wall and total perturbation correction factors agree with Monte Carlo calculated values, indicating that the differences between the responses of different ionization chambers are due to secondary electron effects. This conclusion is supported by the comparison of our results with those from other investigators after re-analysis of their data. The central electrode perturbation correction factor for an aluminium electrode in a Farmer-type geometry was found to be unity within the experimental uncertainties.

Electrons↗

PubMed1976

Electronic platelet counts with the Coulter counter. Reassessment of a correction factor.

Platelet counts are determined on the Coulter electronic counter by counting the diluted platelet-rich plasma obtained by sedimentation or centrifugation of whole blood. In calculating the whole-blood platelet count, an empirical correction factor for platelet-free plasma trapped by sedimented erythrocytes has been recommended, and a widely-distributed circular slide rule calculator incorporates the correction factor. In this study, visual and electronic platelet counts were compared in 100 specimens with counts ranging from 10 to 1,100 X 10(3) per mul and hematocrits ranging from 17.5 to 48.5%. Platelet-rich plasma samples prepared by a centrifugation method (Plateletfuge) gave machine counts in close agreement with those of samples prepared by sedimentation. Whole-blood platelet counts determined with the circular calculator were consistently lower than visual counts, with an average difference of -17%. The electronic counts were recalculated after elimination of the correction factor, and agreement then improved to an average difference of only +1.6%. The correction factor for trapped platelet-free plasma leads to erroneously low values and should not be used.

Blood Cell Count↗

PubMedDate not supplied

[In vivo dosimetry. Assessment of exit dose correction factors].

INTRODUCTION: In vivo dosimetry allows to verify dose delivering accuracy in radiotherapy treatments. Exit dose measurements add more information about delivered dose than entrance dose evaluations. MATERIALS AND METHODS: Commercial semiconductor diodes are used for exit dose measurements. The diodes are calibrated by comparison with an ionization chamber at a reference condition. Diode reading was compared with the dose measured by the ionization chamber at the exit point. The exit point is defined as the point on the central axis of the beam, at a distance equal to the maximum dose from the exit surface of a homogeneous water-like phantom. As clinical irradiation conditions are always different from reference conditions, exit dose correction factors have been investigated as a function of phantom thickness, field size at the isocenter, source-surface distance, wedge and tray. Measurements have been performed by irradiating a set of p-type semiconductor detectors with 6 MV photon beam (four diodes--mod. EDP10--Scanditronix) and 18 MV photon beam (three diodes--mod. EDP20--Scanditronix) from a Clinac 1800 linear accelerator (Varian, Palo Alto, CA, USA). RESULTS: The most relevant exit dose correction factors are related to field size and phantom thickness for 6 MV photons. The variation of these factors as a function of field size may be greater than 1% with a standard deviation of the same order. On the contrary, the correction factors for field, thickness and tray photons are negligible for 18 MV. CONCLUSIONS: Applying exit dose correction factors may require a great effort, particularly when many silicon diodes must be used. The actual effectiveness of each calibration factor is evaluated through the statistical analysis of experimental data. In this way, the usefulness of correction factor calculation, as depending from both experimental conditions and diode responses, can be derived from its effects on the exit dose value.

Radiotherapy Dosage↗

PubMedDate not supplied

Variation in the lung inhomogeneity correction factor with beam energy. Clinical implications.

In order to determine the magnitude of the dosimetry error introduced by failing to correct for increased transmission through lung tissue in treating thoracic malignancies, measurements in a phantom were taken using different field sizes, inhomogeneity thicknesses and photon qualities. The results indicate that the error introduced by neglecting the inhomogeneity correction is greatest at lower photon energies, smaller field sizes and greater thickness of inhomogeneity. Correction factors to account for the lung inhomogeneity were obtained from phantom measurements and were compared with those calculated using the tissue-air ratio and Batho-Young algorithms; correlation coefficients describing the relationship between measured and calculated values exceeded 0.995. The calculated values tended to overestimate the correction factor and differed most from the measured correction factors at lower energies, smaller field sizes, and greater inhomogeneity thicknesses. The importance of these results in clinical radiation therapy is discussed.

Cobalt Radioisotopes↗

PubMed1995

Aminoglycoside dosing weight correction factors for patients of various body sizes.

Prior investigations have suggested the use of a dosing weight correction factor of ideal body weight (IBW) plus 40% excess body weight (EBW, where EBW = total body weight [TBW] - IBW) to determine the weight to use for aminoglycoside dosing in morbidly obese (TBW/IBW ratio, > 2) patients. Little data are available to provide dosing information for underweight or moderately obese patients. We investigated aminoglycoside pharmacokinetics in 1,708 patients receiving gentamicin and tobramycin. Patients were stratified into underaverage-weight or overweight weight categories based on both TBW/IBW ratio and body mass index (weight/height2 ratio), which has been shown to correlate with physiologic estimates of body fat. Regression analyses revealed that the TBW/IBW ratio predicts the volume of distribution. Dosing weight correction factors to give equivalent predicted peak aminoglycoside concentrations with a 2-mg/kg loading dose are 1.13 times the TBW for underweight patients and 0.43 times the EBW plus IBW for overweight patients. There were no large differences between the dosing weight correction factors derived from IBW- and body mass index-based classification systems. These data generate useful aminoglycoside dosing weight equations for both underweight and overweight patients.

Adult↗

PubMed1999

Monte Carlo calculations of the ionization chamber wall correction factors for 192Ir and 60Co gamma rays and 250 kV x-rays for use in calibration of 192Ir HDR brachytherapy sources.

As in the method for the calibration of 192Ir high-dose-rate (HDR) brachytherapy sources, the ionization chamber wall correction factor A(w), is needed for 192Ir and 60Co gamma rays and 250 kV x-rays. This factor takes into account the variation in chamber response due to the attenuation of the photon beam in the chamber wall and build-up cap and the contribution of scattered photons. Monte Carlo calculations were performed using the EGS4 code system with the PRESTA algorithm, to calculate the A(w) factor for 51 commercial ionization chambers and build-up caps exposed to the typical energy spectrum of 192Ir and 60Co gamma rays and 250 kV x-rays. The calculated A(w) correction factors for 192Ir and 60Co sources and 250 kV x-rays agree very well to within 0.1% with published experimental data (the statistical uncertainty is less than 0.1% of the calculated correction factor value). For the 192Ir sources, A(w) varies from 0.973 to 0.993 and for the 250 kV x-rays the minimum value of A(w) for all chambers studied is 0.983. The calculated A(w) correction factors can be used to calculate the air kerma calibration factor of HDR brachytherapy sources, when interpolative methods are considered, contributing to the reduction in the overall uncertainties in the calibration procedure.

Brachytherapy↗

PubMed2003

Calculation of the Pitot tube correction factor for Newtonian and non-Newtonian fluids.

This paper presents the numerical investigation performed to calculate the correction factor for Pitot tubes. The purely viscous non-Newtonian fluids with the power-law model constitutive equation were considered. It was shown that the power-law index, the Reynolds number, and the distance between the impact and static tubes have a major influence on the Pitot tube correction factor. The problem was solved for a wide range of these parameters. It was shown that employing Bernoulli's equation could lead to large errors, which depend on the magnitude of the kinetic energy and energy friction loss terms. A neural network model was used to correlate the correction factor of a Pitot tube as a function of these three parameters. This correlation is valid for most Newtonian, pseudoplastic, and dilatant fluids at low Reynolds number.

Algorithms↗

PubMed2004

Improved scatterer property estimates from ultrasound backscatter for small gate lengths using a gate-edge correction factor.

Backscattered rf signals used to construct conventional ultrasound B-mode images contain frequency-dependent information that can be examined through the backscattered power spectrum. The backscattered power spectrum is found by taking the magnitude squared of the Fourier transform of a gated time segment corresponding to a region in the scattering volume. When a time segment is gated, the edges of the gated regions change the frequency content of the backscattered power spectrum due to truncating of the waveform. Tapered windows, like the Hanning window, and longer gate lengths reduce the relative contribution of the gate-edge effects. A new gate-edge correction factor was developed that partially accounted for the edge effects. The gate-edge correction factor gave more accurate estimates of scatterer properties at small gate lengths compared to conventional windowing functions. The gate-edge correction factor gave estimates of scatterer properties within 5% of actual values at very small gate lengths (less than 5 spatial pulse lengths) in both simulations and from measurements on glass-bead phantoms. While the gate-edge correction factor gave higher accuracy of estimates at smaller gate lengths, the precision of estimates was not improved at small gate lengths over conventional windowing functions.

Algorithms↗

PubMedDate not supplied

[Simplified method of obtaining the correction factor for calculating ventricular volumes].

For the accurate measurement of ventricular volume it is required the obtainment of a correction factor for the magnification caused by the non-parallel X-rays and the "pincushion" distortion, which causes more magnification in the periphery than in the center of the fluoroscopic field. The Kasser and Kennedy method is based in the attainment of the relation between the actual and projected dimensions of a micrometrically calibrated grid filmed at the distance measured between the intensifier tube and the mid-thoracic line during the ventriculography. This technique is very accurate but expensive and troublesome. With the simpler catheter method it is obtained the relation between the projected and actual linear dimensions of the ventriculographic catheter inmediately before ventriculography. This study was aimed to compare the accuracy of the catheter method and two other proposed methods, in which the relation between the projected and actual dimensions were obtained by filming one central coin or five arranged through the fluoroscopic field at the distance measured between the tube and the patients during the ventriculography. These four correction factors were obtained in 15 patients undergoing a diagnostic cardiac catheterization. The catheter method showed a poor correlation with the grid method (r = 0.34), while both coin methods showed a high correlation with the grid method (r = 0.93 and 0.92, respectively). It is concluded that the catheter method is inaccurate and therefore is unwise to use it for the estimation of ventricular volumes. Because the peripheric distortion is clinically unimportant, the central coin methods is proposed as a simplified method for obtaining the magnification correction factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac Catheterization↗