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PubMed1991

Measurement systems calibration: microcomputer implementation.

Measurement systems used in the collection and processing of laboratory data must be calibrated periodically to obtain accurate results. Because calibration factors can change over time or may be reset to optimize measurements for specific tests, care must be taken to assure that calibration factors and data are aligned correctly. Users should be able to process current data or re-process older data using appropriate calibration factors. The alignment of calibration factors and data should occur in a simple, automatic and transparent way. This document describes one approach to calibration procedures and computer programs used to collect, process, document, measure and display laboratory data. The examples are from our neurophysiology laboratory, where investigators study the mammalian spinal cord and peripheral neuromuscular system. Typical calibration problems, some workable solutions, and computer programs (described in pseudocode) are presented.

Calibration↗

PubMed1991

Calibration and validation of linearity in chromatographic biopharmaceutical analysis.

Calibration in chromatographic biopharmaceutical analysis is a major determinate of method performance and many methods have been proposed to evaluate an appropriate calibration model, to determine the linear range and to evaluate the goodness of fit. Ten chromatographic bioanalytical methods have been evaluated in this work by observation of concentration-response curves, linearity plots, calculation of concentration residuals, correlation coefficients and lack of fit analysis. These methods were applied to univariant linear regression, weighted regression, polynomial regression and power fit models in order to determine the most appropriate way to establish and evaluate calibration functions. It was found that weighted linear regression provided the most appropriate calibration function for eight of the 10 methods studied, whereas unweighted regression and the power fit model proved appropriate for one each of the other two methods. The choice of calibration function was best accomplished through observation of calculated concentration residuals. Linearity and sensitivity plots were of little value for assessment of linearity through the selected calibration range if conventional (+/- 5%) tolerance limits are employed. Validation of the calibration model can be accomplished by demonstrating the concentration residuals and the slope of the log concentration-log response plots are within reasonable tolerance limits or by lack of fit analysis. Correlation coefficients were demonstrated to be of little value for this purpose and the quadratic approach to linearity validation was in disagreement with other methods in four of the 10 methods evaluated.

Calibration↗

PubMedDate not supplied

Linear regression analysis and its application to multivariate chromatographic calibration for the quantitative analysis of two-component mixtures.

Multivariate chromatographic calibration technique was developed for the quantitative analysis of binary mixtures enalapril maleate (EA) and hydrochlorothiazide (HCT) in tablets in the presence of losartan potassium (LST). The mathematical algorithm of multivariate chromatographic calibration technique is based on the use of the linear regression equations constructed using relationship between concentration and peak area at the five-wavelength set. The algorithm of this mathematical calibration model having a simple mathematical content was briefly described. This approach is a powerful mathematical tool for an optimum chromatographic multivariate calibration and elimination of fluctuations coming from instrumental and experimental conditions. This multivariate chromatographic calibration contains reduction of multivariate linear regression functions to univariate data set. The validation of model was carried out by analyzing various synthetic binary mixtures and using the standard addition technique. Developed calibration technique was applied to the analysis of the real pharmaceutical tablets containing EA and HCT. The obtained results were compared with those obtained by classical HPLC method. It was observed that the proposed multivariate chromatographic calibration gives better results than classical HPLC.

Calibration↗

PubMed2005

Calibration of a Hall effect displacement measurement system for complex motion analysis using a neural network.

Biomechanics studies often require the analysis of position and orientation. Although a variety of transducer and camera systems can be utilized, a common inexpensive alternative is the Hall effect sensor. Hall effect sensors have been used extensively for one-dimensional position analysis but their non-linear behavior and cross-talk effects make them difficult to calibrate for effective and accurate two- and three-dimensional position and orientation analysis. The aim of this study was to develop and calibrate a displacement measurement system for a hydraulic-actuation joystick used for repetitive motion analysis of heavy equipment operators. The system utilizes an array of four Hall effect sensors that are all active during any joystick movement. This built-in redundancy allows the calibration to utilize fully connected feed forward neural networks in conjunction with a Microscribe 3D digitizer. A fully connected feed forward neural network with one hidden layer containing five neurons was developed. Results indicate that the ability of the neural network to accurately predict the x, y and z coordinates of the joystick handle was good with r(2) values of 0.98 and higher. The calibration technique was found to be equally as accurate when used on data collected 5 days after the initial calibration, indicating the system is robust and stable enough to not require calibration every time the joystick is used. This calibration system allowed an infinite number of joystick orientations and positions to be found within the range of joystick motion.

Algorithms↗

PubMed2005

Effects of calibration on L-THIA GIS runoff and pollutant estimation.

Urbanization can result in alteration of a watershed's hydrologic response and water quality. To simulate hydrologic and water quality impacts of land use changes, the Long-Term Hydrologic Impact Assessment (L-THIA) system has been used. The L-THIA system estimates pollutant loading based on direct runoff quantity and land use based pollutant coefficients. The accurate estimation of direct runoff is important in assessing water quality impacts of land use changes. An automated program was developed to calibrate the L-THIA model using the millions of curve number (CN) combinations associated with land uses and hydrologic soil groups. L-THIA calibration for the Little Eagle Creek (LEC) watershed near Indianapolis, Indiana was performed using land use data for 1991 and daily rainfall data for six months of 1991 (January 1-June 30) to minimize errors associated with use of different temporal land use data and rainfall data. For the calibration period, the Nash-Sutcliffe coefficient was 0.60 for estimated and observed direct runoff. The calibrated CN values were used for validation of the model for the same year (July 1-December 31), and the Nash-Sutcliffe coefficient was 0.60 for estimated and observed direct runoff. The Nash-Sutcliffe coefficient was 0.52 for January 1, 1991 to December 31, 1991 using uncalibrated CN values. As shown in this study, the use of better input parameters for the L-THIA model can improve accuracy. The effects on direct runoff and pollutant estimation of the calibrated CN values in the L-THIA model were investigated for the LEC. Following calibration, the estimated average annual direct runoff for the LEC watershed increased by 34%, total nitrogen by 24%, total phosphorus by 22%, and total lead by 43%. This study demonstrates that the L-THIA model should be calibrated and validated prior to application in a particular watershed to more accurately assess the effects of land use changes on hydrology and water quality.

Calibration↗

PubMed2005

Evaluation of basic algorithms for transferring quantitative multivariate calibrations between scanning grating and FT NIR spectrometers.

A key issue in near infrared spectroscopy (NIR) is the possibility to use calibrations generated on one instrument for predictions on others. A number of methods for calibration transfer have been proposed, but which method to choose is typically not straightforward. An evaluation of a number of methods for transferring quantitative calibrations between different instruments was carried out on near infrared diffuse-reflectance data from a pharmaceutical formulation. Six instruments were included in the study, five of which were scanning grating instruments, both with and without fibre-optic probe configuration, and one of which was a Fourier-transform instrument, equipped with a fibre-optic probe. The results show that it is possible to transfer calibrations between different instruments, provided that a structured procedure is used. Simple techniques for calibration transfer, such as slope/bias correction on the predicted results, as well as standard normal variate transformation and local centring of the raw spectra, gave considerably lower prediction errors on transfer than did standardisation with a certified diffuse-reflectance standard, or direct transfer without any transfer function. Notably, including more than one instrument in the calibration also improved the prediction ability of the models on calibration transfer. No significant differences in wavelength scale were found when a certified diffuse-reflectance wavelength standard was measured on the instruments studied. Nor did simulated wavelength scale differences below +/-0.3 nm cause any significant change in the prediction errors.

Algorithms↗

PubMed2006

Transfer of NIR calibrations for pharmaceutical formulations between different instruments.

In order to evaluate how well existing techniques for transferring NIR calibrations perform for solid pharmaceutical formulations, a study on four assays of active ingredients was undertaken. The study included two configurations of dispersive NIR instruments and one Fourier transform (FT) instrument. Three methods for calibration transfer: slope/bias correction, local centring and piecewise direct standardisation (PDS), were tested and evaluated. Our conclusions are that the calibration transfer methods tested can perform equally well. It was shown that it is possible to transfer calibrations between instruments of different configurations or even of different types, without loosing the prediction ability of the calibration. To achieve a good calibration transfer, a larger variation in the content of the active ingredient in the samples and more samples are needed for the slope and bias correction method compared to the local centring method. For PDS to be a successful calibration transfer method, an optimisation of the number of transfer samples and how they are selected together with various factors specific for this method is needed. Local centring is the preferred transfer method as its performance is excellent yet it is simple to perform, no optimisation is needed, only a few transfer samples are required and the transfer samples do not have to vary in their content of the active ingredient.

Calibration↗

PubMed2005

Confhusius: a robust and fully automatic calibration method for 3D freehand ultrasound.

This paper describes a new robust and fully automatic method for calibration of three-dimensional (3D) freehand ultrasound called Confhusius (CalibratiON for FreeHand UltraSound Imaging USage). 3D Freehand ultrasound consists in mounting a position sensor on a standard probe. The echographic B-scans can be localized in 3D and compounded into a volume. However, especially for quantitative use, this process dramatically requires a calibration procedure that determines its accuracy and usefulness. Calibration aims at determining the transformation (translations, rotations, scaling) between the coordinates system of the echographic images and the coordinate system of the localization system. To calibrate, we acquire images of a phantom whose 3D geometrical properties are known. We propose a robust and fully automatic calibration method based on the Hough transform and robust estimators. Experiments have been done with synthetic and real sequences, and this calibration method is shown to be easy to perform, accurate, automatic and fast enough for clinical use.

Calibration↗

PubMed1998

Interassay calibration as a major contribution to the comparability of results in clinical enzymology.

OBJECTIVE: Factors contributing to the applicability of interassay calibration of methods measuring enzyme catalytic activities are described. Also discussed are the properties essential for such a material. Similarity of specificity for the methods to be calibrated as well as commutability between the material(s) intended to be used as calibrator are the main criteria to be satisfied. RESULT: Several examples demonstrated that interassay calibration is feasible but a multi-enzyme calibrator with a wide commutability for the most popular methods remains to be developed. This is the project of the IFCC Working Group on Calibrators in Clinical Enzymology (WG-CCE). Several experimental data are also presented that indicate that the temperature at which the reaction is carried out is not a limiting factor in the implementation of interassay calibration in clinical enzymology.

Calibration↗

PubMed2002

Alternative calibration approaches to compensate the effect of co-extracted matrix components in liquid chromatography-electrospray ionisation tandem mass spectrometry analysis of pesticide residues in plant materials.

The aim of this study was to evaluate the applicability of different calibration approaches in a multi- and single-residue analysis of modern pesticides in plant matrices using liquid chromatography-electrospray mass spectrometry (HPLC-ESI-MS). In the first set of experiments the determination of eight pesticides representing different groups of polar/unstable pesticides (carbamates, benzimidazoles, azoles, benzoylphenylurea) in apple samples was performed. The trueness and precision of data obtained by using: (i) external solvent standard calibration, (ii) external matrix-matched standard calibration and (iii) echo-peak internal standard calibration was compared. The last mentioned method is a novel technique providing the possibility to inject internal standard of the same identity as a target analyte, so that its retention time is close to the analyte from the sample. According to expectation, when using external standard solvent calibration the results were under- or overestimated due to suppression or enhancement of analyte's signal by matrix components. On the other hand with the use of matrix-matched calibration accurate data were obtained. With echo-peak technique accurate results comparable to those obtained by matrix calibration were obtained for six out of eight pesticides. In the second set of experiment we used the echo-peak technique to overcome the problem with the response instability in the analysis of chlormequat in pear concentrate samples. As an internal standard method the echo-peak technique provided the possibility of monitoring of signal decrease during the analytical sequence and to compensate this decrease by relating sample peak area relatively to this internal standard.

Calibration↗

PubMed2001

A fast calibration method for 3-D tracking of ultrasound images using a spatial localizer.

We have developed a fast calibration method for computing the position and orientation of 2-D ultrasound (US) images in 3-D space where a position sensor is mounted on the US probe. This calibration is required in the fields of 3-D ultrasound and registration of ultrasound with other imaging modalities. Most of the existing calibration methods require a complex and tedious experimental procedure. Our method is simple and it is based on a custom-built phantom. Thirty N-fiducials (markers in the shape of the letter "N") embedded in the phantom provide the basis for our calibration procedure. We calibrated a 3.5-MHz sector phased-array probe with a magnetic position sensor, and we studied the accuracy and precision of our method. A typical calibration procedure requires approximately 2 min. We conclude that we can achieve accurate and precise calibration using a single US image, provided that a large number (approximately ten) of N-fiducials are captured within the US image, enabling a representative sampling of the imaging plane.

Calibration↗

PubMed1997

HDR source calibration methods and discrepancies.

PURPOSE: To report the calibration discrepancies observed in HDR sources and our methods of calibration of a new source. METHODS AND MATERIALS: 21 sources have been calibrated since March 1991 to February 1996 and discrepancies are reported in comparison to the certificate value. The calibration methodology used for two different systems: the NEL Farmer system and reentrant well chamber (HDR-1000) are described. A lead shield insert developed for use with the well chamber for timer linearity measurement is described. RESULTS: Eleven sources had discrepancy from 0 to 1%, 7 sources had discrepancy from 1 to 5%, and 3 sources had discrepancy from 5 to 10%. The maximum discrepancy observed was -8.04 % for one source. Our procedure of dealing with situations when discrepancy exceeds the range of +/-5% value is discussed. CONCLUSION: (a) The discrepancies that we observed in our study are compared with other users and found to be within the range of values observed. (b) Due to the easy set up procedure, less prone to error in the use of well chamber and with the use of a new lead insert, we conclude in our case that, the well system can be used for monthly calibration of HDR source and timer linearity measurements. (c) Because the Farmer system has a first generation calibration factor, obtained by interpolation of two calibration energies from ADCL, we prefer to use the Farmer system for determining the activity of the new HDR source.

Brachytherapy↗

PubMed2002

A self consistent normalized calibration protocol for three dimensional magnetic resonance gel dosimetry.

In a clinical setting, mixed and inconsistent results have been reported using Magnetic Resonance Relaxation imaging of irradiated aqueous polymeric gels as a three-dimensional dosimeter, for dose verification of conformal radiation therapy. The problems are attributed to the difficulty of identifying an accurate dose calibration protocol for each delivered gel at the radiation site in a clinical setting. While careful calibration is done at the gel manufacturing site in a controlled laboratory setting, there is no guarantee that the dose sensitivity of the gels remains invariant upon delivery, irradiation, magnetic resonance imaging and storage at the clinical site. In this study, we have compared three different dose calibration protocols on aqueous polymeric gels for a variety of irradiation scenarios done in a clinical setting. After acquiring the three-dimensional proton relaxation maps of the irradiated gels, the dose distributions were generated using the off-site manufacturer provided calibration curve (Cal-1), the on-site external tube gel calibration (Cal-2) and the new on-site internal normalized gel calibration (Cal-3) protocols. These experimental dose distributions were compared with the theoretical dose distributions generated by treatment-planning systems. We observed that the experimental dose distributions generated from the Cal-1 and Cal-2 protocols were off by 10% to 40% and up to 200% above the predicted maximum dose, respectively. On the other hand, the experimental dose distributions generated from the Cal-3 protocol matched reasonably well with the theoretical dose distributions to within 10% difference. Our result suggests that an independent on-site normalized internal calibration must be performed for each batch of gel dosimeters at the time of MR relaxation imaging in order to account for the variations in dose sensitivity caused by various uncontrollable conditions in a clinical setting such as oxygen contamination, temperature changes and shelf life of the delivered gel between manufacturing and MR acquisitions.

Calibration↗

PubMed2001

Calibration of plane parallel ionization chambers: a new service for secondary standard dosimetry laboratories?

This paper presents the results of calibration of PTW Markus and NACP ionization chambers following the procedures outlined in the TRS 381 International Atomic Energy Agency code of practice. According to this dosimetry protocol, calibration of plane parallel chambers follows 2 methods. The first method uses a high-energy electron beam and consists of comparing the plane parallel chamber with a cylindrical chamber whose N(D,air) calibration factor, traceable to cobalt-60 (60Co), is known, while the second method applies to a 60Co gamma beam whose air kerma rate is known at the calibration point. As the second method is generally applied in Secondary Standard Dosimetry Laboratories, the consistency of calibrations free in-air and in a water phantom has been studied. A close agreement is shown between the 2 methods (the calibration factors differ by at most 0.98%). These results lead to the conclusion that either of the 2 calibration methods can be used, provided that the correction factors given by the TRS 381 code of practice are applied.

Calibration↗

PubMed2006

Oscillometric blood pressure measurement used for calibration of the arterial tonometry method contributes significantly to error.

BACKGROUND AND OBJECTIVE: We have investigated the accuracy of the Colin tonometry module, a non-invasive alternative to intra-arterial blood pressure (BP) measurement. The claims of previous publications range between good agreement to significant discrepancies between the Colin tonometry and intra-arterial methods. The Colin method relies on calibration of the tonometry module using an oscillometric method. Our aim was to look at the effect of this calibration on the accuracy. METHODS: Ten conscious patients who already had an indwelling arterial catheter were studied. The Colin monitor was set to perform ten calibration cycles first with the oscillometric cuff on the same arm as the tonometry module and secondly on the opposite arm. Simultaneous recording of BP wave forms from the intra-arterial BP transducer and the Colin monitor allowed comparison of beat-by-beat systolic, diastolic and mean BP. RESULTS: There was considerable inter- and intra-patient variation. In the worst case, the error range was 41 mmHg over ten calibrations. Analysis of variance showed that contra- and ipsilateral calibrations gave a significantly different bias while the multiple calibrations accounted for a significant proportion of the variability in systolic BP error. CONCLUSIONS: The Colin Tonometry method is not accurate enough to be used with confidence in clinical practice. The main reason for this is its reliance on an oscillometric method for calibration of the tonometry module. Single BP measurements, using either manual or semiautomatic instruments may vary considerably from the 'true' BP due to short-term perturbations of BP.

Blood Pressure↗

PubMed2004

Principal component analysis applied to Fourier transform infrared spectroscopy for the design of calibration sets for glycerol prediction models in wine and for the detection and classification of outlier samples.

Principal component analysis (PCA) was used to identify the main sources of variation in the Fourier transform infrared (FT-IR) spectra of 329 wines of various styles. The FT-IR spectra were gathered using a specialized WineScan instrument. The main sources of variation included the reducing sugar and alcohol content of the samples, as well as the stage of fermentation and the maturation period of the wines. The implications of the variation between the different wine styles for the design of calibration models with accurate predictive abilities were investigated using glycerol calibration in wine as a model system. PCA enabled the identification and interpretation of samples that were poorly predicted by the calibration models, as well as the detection of individual samples in the sample set that had atypical spectra (i.e., outlier samples). The Soft Independent Modeling of Class Analogy (SIMCA) approach was used to establish a model for the classification of the outlier samples. A glycerol calibration for wine was developed (reducing sugar content < 30 g/L, alcohol > 8% v/v) with satisfactory predictive ability (SEP = 0.40 g/L). The RPD value (ratio of the standard deviation of the data to the standard error of prediction) was 5.6, indicating that the calibration is suitable for quantification purposes. A calibration for glycerol in special late harvest and noble late harvest wines (RS 31-147 g/L, alcohol > 11.6% v/v) with a prediction error SECV = 0.65 g/L, was also established. This study yielded an analytical strategy that combined the careful design of calibration sets with measures that facilitated the early detection and interpretation of poorly predicted samples and outlier samples in a sample set. The strategy provided a powerful means of quality control, which is necessary for the generation of accurate prediction data and therefore for the successful implementation of FT-IR in the routine analytical laboratory.

Calibration↗

PubMed2000

A prototype device for standardized calibration of pulse oximeters.

OBJECTIVE: To develop and test a method for standardized calibration of pulse oximeters. METHODS: A novel pulse oximeter calibration technique capable of simulating the behavior of real patients is discussed. It is based on an artificial finger with a variable spectral-resolved light attenuator in conjunction with an extensive clinical database of time-resolved optical transmission spectra of patients fingers in the wavelength range 600-1000 nm. The arterial oxygen saturation of the patients at the time of recording was derived by analyzing a corresponding blood sample with a CO-oximeter. These spectra are used to compute the modulation of the light attenuator which is attached to the artificial finger. This calibration method was tested by arbitrarily playing back recorded spectra to pulse oximeters and comparing their display to the value they displayed when the spectra were recorded. RESULTS: We were able to demonstrate that the calibrator could generate physiological signals which are accepted by a pulse oximeter. We also present some experience of playing back recorded patient spectra. The mean difference between the original reading of the pulse oximeters and the display when attached to the calibrator is 1.2 saturation points (displayed oxygen saturation SpO2) with a standard deviation of 1.9 saturation points. CONCLUSIONS: The tests have shown the capabilities of a spectral light modulator for use as a possible calibration standard for pulse oximeters. If some improvements of the current prototype can be achieved we conclude from the experience with the device that this novel concept for the calibration of pulse oximeters is feasible and that it could become an important tool for assessing the performance of pulse oximeters.

Calibration↗

PubMed1999

The calibration and validation of a phase-modulated near-infrared cerebral oximeter.

OBJECTIVE: This study was undertaken to compare the cerebral oxygenation measured by an experimental phase-modulated near-infrared (NIR) spectroscopy system with capillary saturation estimated from jugular venous oxygen saturation. METHODS: Jugular venous catheters were placed in 30 patients undergoing carotid endarterectomy and 194 measurements of venous oxygen saturation were obtained intra operatively. Simultaneous measurement of optical path length at 754, 785, and 816 nm was performed using a phase-modulated near-infrared spectroscopy system. Optical calibration was performed using both an optical bench and a scattering mold. Hemoglobin saturation was calculated from NIR measurements using equations derived from diffusion theory. Capillary saturation was calculated from the arterial and venous saturations. RESULTS: Jugular venous saturations ranged from 41 to 92%. When calibrated using the optical bench, the NIR estimates of hemoglobin saturation deviated from estimated capillary values by an average of 2.6% bias and 4.3% deviation. No systematic bias was noted. NIR values derived from mold calibration were less accurate and precise (4.6% bias and 6.9% deviation.) Use of the initial venous sample as an in vivo calibration improved the accuracy of the mold calibration but did not alter the performance of the bench calibration. CONCLUSIONS: Under the conditions tested, an experimental phase-modulated near-infrared spectroscopy system calibrated using an optical bench agreed with capillary saturation estimated from jugular venous samples. Further work is necessary to demonstrate valid performance of the system under other conditions.

Aged↗