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At least 19 recordsLinked to original sources
PubMedDate not supplied

Routine calibration of brachytherapy sources with a well-type dose calibrator using standard calibrated sources.

Brachytherapy, which refers to short-distance treatment via implanted sources, requires an accurate knowledge of the dose distribution in the implant volume. A critical step in the determination of the dose distribution involves an accurate calibration of brachytherapy source strength. Following the guidelines recommended by AAPM Task Group No. 22, a routine calibration procedure for brachytherapy sources 137-Cs, 192-Ir, and 125-I has been developed. The calibration of sources is performed with a well-type dose calibrator using standard calibrated sources obtained from the American, British, and French national laboratories. Selected characteristics of the dose calibrator are reported, and some of the problems associated with the dose calibrator are discussed. The results of a comparison of the calibrated sources from the three national laboratories are presented.

Brachytherapy↗
PubMed1995

A transmission electron microscope (TEM) calibration standard sample for all magnification, camera constant, and image/diffraction pattern rotation calibrations.

A calibration sample for transmission electron microscopy (TEM) has been developed that performs the three major instrument calibrations for a transmission electron microscope: the image magnification calibration for measurements of images, the camera constant calibration for indexing diffraction patterns, and the image/diffraction pattern rotation calibration for relating crystal directions to features in the image. This offers an improvement over commercially available calibration standards, where up to five different samples are required to perform these three calibrations. The new calibration sample consists of an electron-transparent cross-sectional TEM sample made from a molecular beam epitaxy (MBE)-grown, single-crystal semiconductor wafer. When the calibration structure is viewed in a TEM, it appears as a series of light and dark layers where the layer thicknesses are very accurately known. The calibrated thickness measurements of these light (silicon) and dark (SiGe alloy) layers are based on careful TEM measurements of the [111] lattice spacing of silicon which is visible on the calibration sample itself, and are supported by X-ray diffraction measurements. Furthermore, the layer thickness variation across the entire silicon wafer has been verified to be less than 1%, allowing all samples prepared from the same wafer to have errors in the given layer thickness values of less than 1%. As the sample is a single crystal of silicon, the calibrations requiring electron diffraction information such as the camera constant calibration and the image/diffraction pattern rotation calibration can also be performed easily and unambiguously. One single calibration sample can therefore be used to provide all three of the major TEM instrument calibrations at all magnifications and all camera lengths.

Calibration↗
PubMed2004

In-house calibration of the international sensitivity index or calibration curve for determination of the international normalized ratio.

CONTEXT: The international normalized ratio (INR) has been used since 1983 to standardize prothrombin time results for patients on oral anticoagulants. However, significant interlaboratory variations have been noted. Attempts have been made to address these differences with the use of instrument-specific International Sensitivity Index (ISI) values and in-house calibration of ISI values. OBJECTIVE: To assess the performance of laboratories using a calibration curve for INR testing. DESIGN: Attempts to improve performance of the INR include the use of instrument-specific ISI values, model-specific ISI values, in-house calibration of ISI values, and more recently, the preparation of a calibration curve. Several studies have shown an improvement in performance using these procedures. In this study of licensed laboratories performing routine coagulation testing in the Province of Ontario, Canada, the determination of the INR by a calibration curve was compared with the laboratories' usual method of assessment. These methods were subsequently analyzed by comparing the results to instrument-specific ISI, model-specific ISI, and in-house calibrators. International normalized ratios derived by both methods were analyzed for accuracy and precision. The stability of a calibration curve was also investigated. RESULTS: Performance of INR testing has improved with use of a calibration curve or in-house calibrators. CONCLUSION: The results confirm that either in-house calibrators or the calibration curve improve performance of INR testing. The calibration curve may be easier to use and appears stable up to 4 months.

Calibration↗
PubMed2005

Calibration of mass spectrometric peptide mass fingerprint data without specific external or internal calibrants.

BACKGROUND: Peptide Mass Fingerprinting (PMF) is a widely used mass spectrometry (MS) method of analysis of proteins and peptides. It relies on the comparison between experimentally determined and theoretical mass spectra. The PMF process requires calibration, usually performed with external or internal calibrants of known molecular masses. RESULTS: We have introduced two novel MS calibration methods. The first method utilises the local similarity of peptide maps generated after separation of complex protein samples by two-dimensional gel electrophoresis. It computes a multiple peak-list alignment of the data set using a modified Minimum Spanning Tree (MST) algorithm. The second method exploits the idea that hundreds of MS samples are measured in parallel on one sample support. It improves the calibration coefficients by applying a two-dimensional Thin Plate Splines (TPS) smoothing algorithm. We studied the novel calibration methods utilising data generated by three different MALDI-TOF-MS instruments. We demonstrate that a PMF data set can be calibrated without resorting to external or relying on widely occurring internal calibrants. The methods developed here were implemented in R and are part of the BioConductor package mscalib available from http://www.bioconductor.org. CONCLUSION: The MST calibration algorithm is well suited to calibrate MS spectra of protein samples resulting from two-dimensional gel electrophoretic separation. The TPS based calibration algorithm might be used to correct systematic mass measurement errors observed for large MS sample supports. As compared to other methods, our combined MS spectra calibration strategy increases the peptide/protein identification rate by an additional 5-15%.

Algorithms↗
PubMed2002

Calibration of a subcutaneous amperometric glucose sensor implanted for 7 days in diabetic patients. Part 2. Superiority of the one-point calibration method.

UNLABELLED: Calibration, i.e. the transformation in real time of the signal I(t) generated by the glucose sensor at time t into an estimation of glucose concentration G(t), represents a key issue for the development of a continuous glucose monitoring system. OBJECTIVE: To compare two calibration procedures. In the one-point calibration, which assumes that I(o) is negligible, S is simply determined as the ratio I/G, and G(t) = I(t)/S. The two-point calibration consists in the determination of a sensor sensitivity S and of a background current I(o) by plotting two values of the sensor signal versus the concomitant blood glucose concentrations. The subsequent estimation of G(t) is given by G(t) = (I(t)-I(o))/S. RESEARCH DESIGN AND METHODS: A glucose sensor was implanted in the abdominal subcutaneous tissue of nine type 1 diabetic patients during 3 (n = 2) and 7 days (n = 7). The one-point calibration was performed a posteriori either once per day before breakfast, or twice per day before breakfast and dinner, or three times per day before each meal. The two-point calibration was performed each morning during breakfast. RESULTS: The percentages of points present in zones A and B of the Clarke Error Grid were significantly higher when the system was calibrated using the one-point calibration. Use of two one-point calibrations per day before meals was virtually as accurate as three one-point calibrations. CONCLUSION: This study demonstrates the feasibility of a simple method for calibrating a continuous glucose monitoring system.

Biosensing Techniques↗
PubMed2004

Influence of wavelength-shifted calibration spectra on multivariate calibration models.

Multivariate calibration models are sensitive to wavelength shifts in calibration spectra as such disturbances are linearly independent from unshifted spectra and increase the calibration model's dimension. However, if wavelength shifts included in the calibration model are random, the predictability of the model is not improved. On the contrary, overfitting is introduced, thereby increasing the prediction error. Because calibration spectra are defined to be error free and are the only available data at that point, there is no analytical way to find out that the calibration model is erroneous. This study gives a mathematical explanation of how the model's dimension is increased by wavelength shifts and that the additional basis vectors, principal components for instance, possess derivative-shaped features. It is also demonstrated by means of an example that the reverse is not necessarily true. Hence, derivative-shaped features found in principal components are no indication of wavelength-shifted calibration spectra. A method is presented for analyzing calibration spectra for such shifts. The algorithm takes advantage of the fact that artificial shift compensations of true shifts increase the similarity, i.e., correlation, of shifted spectra with respect to the remaining, unshifted spectra. Synthetic and experimental data are used to demonstrate and assess the performance of the algorithm. It is shown that wavelength shifts in calibration spectra can be detected and corrected if a small number of spectra are disturbed. Significant improvements of the prediction errors of chemometric calibration models can be achieved by means of this shift-correction algorithm.

Acetone↗
PubMed1997

Uncertainty of calibrations at the accredited dosimetry calibration laboratories.

The American Association of Physicists in Medicine, through a subcommittee (formerly Task Group 3) of the Radiation Therapy Committee, has accredited five laboratories to perform calibrations of instruments used to calibrate therapeutic radiation beams. The role of the accredited dosimetry calibration laboratories (ADCLs) is to transfer a calibration factor from an instrument calibrated by the National Institute of Standards and Technology (NIST) to a customer's instrument. It is of importance to the subcommittee, to physicists using the services of the ADCLs, and to the ADCLs themselves, to know the uncertainty of instrument calibrations. The calibration uncertainty has been analyzed by asking the laboratories to provide information about their calibration procedures. Estimates of uncertainty by two procedures were requested: Type A are uncertainties derived as the standard deviations of repeated measurements, while type B are estimates of uncertainties obtained by other methods, again expressed as standard deviations. Data have been received describing the uncertainty of each parameter involved in calibrations, including those associated with measurements of charge, exposure time, and air density, among others. These figures were combined with the uncertainty of NIST calibrations, to arrive at an overall uncertainty which is expressed at the two-standard deviation level. For cable-connected instruments in gamma-ray and x-ray beams of HVL > 1 mm Al, the figure has an upper bound of approximately 1.2%.

Biophysical Phenomena↗
PubMed1983

Some factors affecting the calibration of radionuclide calibrators--I. 99mTc.

In the Winnipeg area a centralized radiopharmacy serves several Nuclear Medicine departments supplying all their 99mTc radiopharmaceuticals on a daily basis. It was observed that there was very poor consistency between the assays as determined by the Radiopharmacy and those determined in the various Nuclear Medicine departments. An attempt to "correct" the various calibrators using a 57Co "mock" 99mTc source resulted in an aggravation of the problem, and an investigation of relative responses of various calibrators to the "mock" relative to a 99mTc source was undertaken, together with an investigation of the influences of various environmental conditions on the calibrator function. In many Nuclear Medicine departments technologists sensitive to concerns of radiation exposure have stacked lead shielding around the calibrator as added protection. However if the calibrator is not calibrated with sufficient lead shielding, such additional shielding will increase the ionization within the chamber, resulting in an elevated assay being obtained due to the lead X-rays and back-scattered radiation. In the absence of lead shielding, similar effects, although smaller in magnitude, are observed from other environmental factors such as the proximity of a wall or a person. Thus in selecting a calibrator, one which comes from the manufacturer, with a calibration certified by the manufacturer, incorporating an adequate amount of lead shielding to prevent additional back-scatter re-entering the ionization chamber should be considered essential unless a complete on-site recalibration using accurately calibrated samples of the radionuclides in question is planned.

Calibration↗
PubMed2005

Calibration of the NPL secondary standard radionuclide calibrator for the new 10R Schott, type 1+ vials.

For many years, P6 vials have been used for the distribution of a wide range of diagnostic and therapeutic radioactive solutions. The activity measurements were performed in this geometry and, in time, the UK calibration system for nuclear medicine was based on this container as a standard. However, one major supplier of radiopharmaceuticals has replaced the P6 vial with the 10R Type 1+ Schott vial. As the dimensions of the new vial are different from those of the P6 vial and the responses of radionuclide calibrators are known to be container dependent, the need for re-calibration became apparent. Preliminary measurements made on some typical radionuclide calibrators for (125)I solution indicated a difference in response of about 10% between the two vials. The master ionisation chamber of the NPL secondary standard radionuclide calibrator has been re-calibrated and new calibration factors and volume correction factors for 10R Schott vials have been derived for the relevant medical radionuclides. The standard holder was also modified to accommodate the new larger vial. The complete list of factors and the method used to determine them is presented in this paper. The availability of these new factors will improve the quality of activity measurements in nuclear medicine, as calibration services can now be provided by NPL for the new container. These factors can also be employed for all commercial NPL secondary standard radionuclide calibrators (now known as the NPL-CRC and previously as the 671 or ISOCAL IV).

Journal Article↗
PubMed1993

Cross-calibration of liquid and solid QCT calibration standards: corrections to the UCSF normative data.

Quantitative computed tomography (QCT) has been shown to be a precise and sensitive method for evaluating spinal bone mineral density (BMD) and skeletal response to aging and therapy. Precise and accurate determination of BMD using QCT requires a calibration standard to compensate for and reduce the effects of beam-hardening artifacts and scanner drift. The first standards were based on dipotassium hydrogen phosphate (K2HPO4) solutions. Recently, several manufacturers have developed stable solid calibration standards based on calcium hydroxyapatite (CHA) in water-equivalent plastic. Due to differences in attenuating properties of the liquid and solid standards, the calibrated BMD values obtained with each system do not agree. In order to compare and interpret the results obtained on both systems, cross-calibration measurements were performed in phantoms and patients using the University of California San Francisco (UCSF) liquid standard and the Image Analysis (IA) solid standard on the UCSF GE 9800 CT scanner. From the phantom measurements, a highly linear relationship was found between the liquid- and solid-calibrated BMD values. No influence on the cross-calibration due to simulated variations in body size or vertebral fat content was seen, though a significant difference in the cross-calibration was observed between scans acquired at 80 and 140 kVp. From the patient measurements, a linear relationship between the liquid (UCSF) and solid (IA) calibrated values was derived for GE 9800 CT scanners at 80 kVp (IA = [1.15 x UCSF] - 7.32).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed2003

Calibration of pneumotachographs using a calibrated syringe.

Pneumotachograph require frequent calibration. Constant-flow methods allow polynomial calibration curves to be derived but are time consuming. The iterative syringe stroke technique is moderately efficient but results in discontinuous conductance arrays. This study investigated the derivation of first-, second-, and third-order polynomial calibration curves from 6 to 50 strokes of a calibration syringe. We used multiple linear regression to derive first-, second-, and third-order polynomial coefficients from two sets of 6-50 syringe strokes. In part A, peak flows did not exceed the specified linear range of the pneumotachograph, whereas flows in part B peaked at 160% of the maximum linear range. Conductance arrays were derived from the same data sets by using a published algorithm. Volume errors of the calibration strokes and of separate sets of 70 validation strokes (part A) and 140 validation strokes (part B) were calculated by using the polynomials and conductance arrays. Second- and third-order polynomials derived from 10 calibration strokes achieved volume variability equal to or better than conductance arrays derived from 50 strokes. We found that evaluation of conductance arrays using the calibration syringe strokes yields falsely low volume variances. We conclude that accurate polynomial curves can be derived from as few as 10 syringe strokes, and the new polynomial calibration method is substantially more time efficient than previously published conductance methods.

Calibration↗
PubMedDate not supplied

Syringe calibration factors for the NPL Secondary Standard Radionuclide Calibrator for selected medical radionuclides.

Before a radiopharmaceutical is administered to a patient, its activity needs to be accurately assayed. This is normally done via a radionuclide calibrator, using a glass vial as the calibration device. The radionuclide is then transferred to a syringe and it is now becoming common practice to re-measure the syringe and use this value as the activity administered to the patient. Due to elemental composition and geometrical differences, etc. between the glass vial and the syringe, the calibration factors are different for the two containers and this can lead to an incorrect activity being given to the patient unless a correction is applied for these differences. To reduce the uncertainty on syringe measurements, syringe calibration factors and volume correction factors for the NPL Secondary Standard Radionuclide Calibrator have been derived by NPL for several medically important radionuclides. It was found that the differences between the calibration factors for the syringes and glass vials depend on the energies of the photon emissions from the decay of the radionuclides; the lower the energy, the greater the difference. As expected, large differences were observed for 125I (70%) and only small differences for 131I. However, for radionuclides such as 99mTc and 67Ga, differences of up to 30% have been observed. This work has shown the need for the use of specifically derived syringe calibration factors as well as highlighting the complexity of the problem with regard to syringe types, procurement, etc.

Calibration↗
PubMed1994

Feasibility study of heparin mass calibrator as a GPC calibrator for heparins and low molecular weight heparins.

The proposed European Pharmacopoeial (EP) method for molecular weight determination of low molecular weight heparins (LMWHs) has been shown to have a range which is too narrow to allow for the accurate molecular weight determinations of all LMWHs. We have recently shown that a chemically degraded benzyl ester of unfractionated heparin, ITH-3, is a better calibrator using this methodology. Data is presented here which indicates that this calibrator cannot only be used to determine the molecular weights of LMWHs but also of unfractionated heparins. Therefore, it is now renamed heparin mass calibrator or HMC because of this extension of range. Weight average molecular weight, number average molecular weight, peak molecular weight, and dispersity values were calculated using the HMC calibration and a reference narrow-range 19-calibrator method for various LMWHs and unfractionated heparins. Values for the parameters calculated using the HMC calibration were found not to be significantly different from those of the reference method until the molecular weight exceeded 15.0 kDa. In contrast, the molecular weight profile obtained with the proposed EP method was significantly different from the reference method for samples > 8.0 kDa. The range exhibited by the HMC calibrator should allow it to be used for both LMWHs and unfractionated heparins.

Chromatography, High Pressure Liquid↗
PubMed1998

Guidelines for calibration of stimulus and recording parameters used in clinical electrophysiology of vision. Calibration Standard Committee of the International Society for Clinical Electrophysiology of Vision (ISCEV).

In order to perform a technically adequate clinical electrophysiological procedure it is necessary to calibrate the stimulating and recording equipment. Published standards for the electroretinogram (ERG), electro-oculogram (EOG), visual evoked potential (VEP), and guidelines for the Pattern ERG (PERG) specify stimulus and recording parameters. Yet, most commercial instruments do not provide the means for calibration of these parameters. The goal of this document is to provide guidelines for proper calibration of stimulus and recording equipment. The need for such guidelines is clear on both clinical and scientific grounds. Stimulus and amplifier characteristics have substantial effects on the peak latency and amplitude measurements that are commonly used in clinical electrophysiology. Many review articles on clinical electrophysiology emphasize the need for establishing norms for each laboratory as a function of age and gender rather than relying on published norms. However, if stimulus and recording parameters are not calibrated periodically, then these norms may actually be misleading due to changes in stimulus or recording conditions induced by aging of equipment or inadvertent change in settings. This document is divided into two major sections. The first is concerned with calibration of the visual stimulus. It begins with background technical information on the physics of light and its measurement. This is followed by protocols for measurement of the luminous intensity of flash stimuli and the mean luminance, contrast, and visual angle of pattern stimuli. The second section is concerned with calibration of electrophysiologic recording systems. It begins with a description of the characteristics of bioelectrical signals and their measurement. This is followed by protocols for measurement of electrode impedance and amplifier calibration. Although this document was prepared as guidelines for clinical electrophysiological testing, it should be noted that the techniques described are more generally applicable to studies which are dependent upon accurate measurement of luminance or electrophysiological signals.

Electrooculography↗
PubMed1998

Ionization chambers for measuring air kerma integrated over beam area. Deviations in calibration values using simplified calibration methods.

Calibrations of kerma-area product meters (KAP meters) are often performed using simplified methods. The accuracy thus obtained can be insufficient, especially when the KAP meters are used for optimizing radiological procedures. The deviations between the best available calibration factor (k) and the simplified calibration factor (ks) were measured at different clinical x-ray installations. Depending on the type of x-ray installation and calibration method, the quotient ks/k ranged from 0.83 to 1.19, reflecting the error made in practice using these methods. A simple alternative calibration method based on comparison with a KAP meter calibrated by the best available method is described. Depending on tube potential and the stability of the electrometers, the uncertainty in the calibration factor derived with this method was between 3.8% and 5.6% (at 95% confidence level).

Air↗
PubMed2026

Calibrated Prediction Intervals for Polygenic Scores: Updated Comparisons, Contextual Calibration, and Data Normalization.

Calibrated prediction intervals for polygenic scores (PGS) are essential for communicating individual-level uncertainty in genomic medicine. We present updated comparisons of two methods for constructing such intervals: CalPred, a parametric approach, and PredInterval, a non-parametric approach. Our results show that both methods can achieve calibrated coverage, although CalPred additionally requires a sufficiently large calibration set. The two methods also exhibit complementary trade-offs with respect to dataset size and risk identification. We further show that contextual calibration, as introduced in Hou et al. and followed in Shi et al., is most naturally achieved through appropriate phenotype normalization and data preprocessing. Apparent miscalibration can arise from inadequate normalization or from providing contextual information to some methods but not others. In UK Biobank, standard GWAS phenotype normalization procedures are sufficient to achieve contextual calibration for traits analyzed. In the extreme simulations of Hou et al. and Shi et al., supplying contextual covariates to PredInterval restores contextual calibration without normalization, and appropriate normalization can achieve contextual calibration without supplying covariates, while also substantially improving upstream tasks including association power and PGS accuracy. Together, these results underscore the central role of phenotype normalization and data preprocessing in GWAS analyses, including reliable uncertainty quantification for PGS.

Journal Article↗
PubMed1996

Standardizing flow cytometry: construction of a standardized fluorescence calibration plot using matching spectral calibrators.

Calibration of flow cytometers is becoming an increasingly important issue for both quality control of instrument performance and quantitation of antibody binding capacity of cells. Due to the numerous different instruments and analysis software currently available, a standardized method of calibration is necessary if interlaboratory comparison of instrument performance and antibody binding is to be achieved. This report describes a new methodology to obtain a standard calibration plot that can be derived from all instruments and from which specific instrument-independent performance parameters may be calculated that can be used to directly compare the performance and setup of these instruments. The requirements that the calibrated standards must meet are discussed, as well as the acceptable ranges proposed for the instrument-independent performance parameters. In addition, data are presented from standard calibration plots generated by different flow cytometers in numerous laboratories. The corresponding Primary Performance Parameters calculated from these plots are presented and compared. It is expected that the use of this calibration method may help standardize flow cytometric measurements and will provide instrument-independent performance parameters to monitor quality control of instruments and reagents.

Calibration↗
PubMedDate not supplied

Experimental determination of calibration settings for plastic syringes containing solutions of 90Y using commercial radionuclide calibrators.

Calibration factors for several makes (Capintec, AtomLab, and PTW) and models of commercially available re-entrant ionization chambers ("dose calibrators") were determined for solutions of 90Y using 10 ml plastic syringes that are used in the administration of the recently approved radioimmunotherapy drug Zevalin. Effects of filling volume on chamber response were studied for the anticipated prescription volume range of 3-9 ml in those syringes. A series of syringes were prepared over that range with accurately known volumes using a "standard" solution containing 90YCl3 in 1 moll(-1) HCl and approximately 50 microg of nonradioactive YCl3 per gram of solution. The respective calibration factors for the Capintec and AtomLab chambers required to give the correct activity reading were found to be "55+/-2 x 10" and 393+/-6, where the uncertainties are expanded (k = 2) uncertainties. The results indicated no significant effect of filling volume on the calibration setting to within the uncertainty on the activity measurement for all but the PTW chamber. In that case, a variation of about 5% was observed over the filling range. A direct comparison was also made between the actual 90Y Zevalin drug and the "standard" solution, with no differences in determined calibration settings observed within the uncertainties on the activity calibration.

Calibration↗