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At least 289 records · Page 16Linked to original sources

PubMed2002

Improved method for isochromatic demodulation by RGB calibration.

The red-blue-green (RGB) calibration technique consists in constructing an a priori calibration table of the isochromatic retardation versus the triplet of RGB values obtained with a RGB CCD camera. In this way a lookup table (LUT) is built in which the entry is the corresponding RGB triplet and the output is the given retardation. This calibration (a radiometric quantity) depends on the geometric and chromatic parameters of the setup. Once the calibration is performed, the isochromatic retardation at a given point of the sample is computed as the one that minimizes the Euclidean distance between the measured RGB triplet and the triplets stored in the LUT. We present an enhanced RGB calibration algorithm for isochromatic fringe pattern demodulation. We have improved the standard demodulation algorithm used in RGB calibration by changing the Euclidean cost function to a regularized one in which the fidelity term corresponds to the Euclidean distance between RGB triplets; the regularizing term forces piecewise continuity for the isochromatic retardation. Additionally we have implemented a selective search in the RGB calibration LUT. We have tested the algorithm with simulated as well as real photoelastic data with good results.

Journal Article↗

PubMed2006

Facility for spectral irradiance and radiance responsivity calibrations using uniform sources.

Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology by using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with combined standard uncertainties at the 0.1% level. However, uncertainties increase dramatically when measuring an instrument's spectral irradiance or radiance responsivity. We describe what we believe to be a new laser-based facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching or exceeding those available for spectral power responsivity calibrations. In SIRCUS, the emission from high-power, tunable lasers is introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high-radiant-flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity and aperture area measurement, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responses are directly traceable to primary national radiometric scales result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations of less than 0.1%. The details of the facility and its effect on primary national radiometric scales are discussed.

Journal Article↗

PubMedDate not supplied

Modified calibration procedures for a Yankee Environmental System UVB-1 biometer based on spectral measurements with a brewer spectrophotometer.

The calibration of the erythemal irradiance measured by a Yankee Environmental System (YES) UVB-1 biometer is presented using two methods of calibration with a wide range of experimental solar zenith angles (SZAs) and ozone values. The calibration is performed through simultaneous spectral measurements by a calibrated double-monochromator Brewer MK-III spectrophotometer at "El Arenosillo" station, located in southwestern Spain. Because the range of spectral measurements of the Brewer spectrophotometer is 290-363 nm, a previously validated radiative transfer model was used to account for the erythemal contribution between 363 and 400 nm. Both methods are recommended by the World Meteorological Organization and we present and discuss here a wide range of results and features given by modified procedures applied to these two general methods. As is well established, the calibration factor for this type of radiometric system is dependent on atmospheric conditions, the most important of which are the ozone content and the SZA. Although the first method is insensitive to these two factors, we analyze this behavior in terms of the range used for the SZA and the use of two different mathematical approaches for its determination. The second method shows the dependence on SZA and ozone content and, thus, a polynomial as a function of SZA or a matrix including SZA and ozone content were determined as general calibration factors for the UV radiometric system. We must note that the angular responses of the YES radiometer and Brewer spectroradiometer have not been considered, because of the difficulty in correcting them. The results show in detail the advantages and drawbacks (and the corresponding associated error) given by the different approaches used for the determination of these calibration coefficients.

Journal Article↗

PubMed1989

The "true" prevalence of obesity. A comparison of objective weight and height measures versus self-reported and calibrated data.

A regression model that describes the relationship between body mass index (BMI, kg/m2) based on self-reported weight and height, and body mass index based on objective measurements, makes it possible to calibrate the BMI based on self-reports. Calibrated data on prevalence of obesity are much higher than those calculated from self-reports. The aim of the present study was to evaluate which estimates of prevalence of obesity agreed better with objective values, those based on self-reported data or those based on calibrated BMIs. Results from two different representative samples of the same Swedish rural county were compared. Objective values were based on measurements of 2190 men and 1511 women (25-65 years of age) in 1977. Self-reports from 148 men and 147 women (25-64 years of age) were derived from the ULF-study, 1980-81 (Survey on Living Conditions) and then calibrated. The comparison demonstrated that calibrated data, compared with self-reports, are much closer to objective values. There was a striking agreement between objective (14.0%) and calibrated (13.6%) estimates of prevalence of women with BMI greater than 30.0 kg/m2, in contrast to the under-estimation by self-reports (8.1%). The calibration method was reliable for the evaluation of the "true" prevalence of obesity in population studies.

Adult↗

PubMedDate not supplied

Respiratory inductance plethysmography: calibration techniques, their validation and the effects of posture.

Respiratory inductance plethysmography (RIP) is used to measure ventilation from two measurements of body surface movements (rib-cage and abdomen) via the application of volume-motion (V-M) coefficients. The correct derivation of both V-M coefficients (calibration) is necessary because there are considerable spontaneous variations in relative contributions from these two compartments even during resting breathing. In order to fully test a calibration, deliberate changes in rib-cage (RC) to abdominal (AB) contribution must be made. We used this approach to test two single-posture calibration techniques, multiple linear regression (MLR) and isovolume (ISV). Ten normal subjects and nine patients with chronic airway obstruction (CAWO) were studied using quiet breathing throughout. We also studied the effects of changing posture on the constancy of the V-M coefficients. MLR proved a little more accurate (p = 0.03) in deriving the V-M coefficients than ISV in normal subjects, and ISV consistently underestimated the AB V-M coefficient relative to RC. No difference between the two techniques existed in patients with CAWO. Both MLR and ISV calibrations failed to give acceptable calibrations in some patients. When MLR calibration was used, a deliberate 20% change in relative compartmental contribution (RC-AB) induced mean errors in RIP estimations of tidal volume of 3.5 and 9.5% in normal subjects and patients respectively. When there were no deliberate changes in relative contribution, the 95% confidence limits of individual tidal volume estimates were +/- 6.6 and +/- 12% in normal subjects and patients respectively. MLR calibration provides a statistical estimate of its quality at the time of V-M coefficient derivation.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1982

Development of a correction factor for Xe-133 vials for use with a dose calibrator.

Manufacturers of dose calibrators who gave calibration settings for various radionuclides sometimes do not specify the type of radionuclide container the calibration is for. The container, moreover, may not be of the same type as those a user might purchase. When these factors are not considered, the activity administered to the patient may be significantly different from that intended. An experiment is described in which calibration factors are determined for measurement of Xe-133 activity in vials in a dose calibrator. This was accomplished by transferring the Xe-133 from the commercial vials to standard NBS calibration ampuls. Based on ten such transfers, the resulting correction factor for the dose calibrator was 1.22.

Drug Labeling↗

PubMed1981

Quality control of dose calibrators.

Radionuclide dose calibrators are instruments that require regular checks and proper usage to ensure that they are operating correctly. The responsibility for the accuracy of the activity and the resultant estimate of the dose delivered to the patient rests with the persons administering the radiopharmaceutical. Hence, it is essential that accurate measurements of radioactivity be provided by the dose calibrator in use in every nuclear medicine department. The routine testing of the calibrator would ensure the user that the overall characteristics of the instrument are within acceptable limits. The quality control of two dose calibrators was carried out elaborately and their performance was evaluated from various aspects of quality control such as geometry effect, instrument linearity, precision test, instrument accuracy etc. It was found that one of the calibrators was having non-linear response resulting in an error of 18% from the decay-predicted activity. It also showed a saturation effect beyond 310 mCi. Hence, it needed repair or recalibration. However, the other calibrator was working satisfactorily. This study also helped us in realizing certain operational pitfalls of the dose calibrators.

Dose-Response Relationship, Radiation↗

PubMed2001

Integrated self-calibration via electrokinetic solvent proportioning for microfluidic immunoassays.

On-board generation of a set of calibration standards was demonstrated within a microfluidic device designed to perform immunoassay. Electrokinetic flow was used to proportionally mix the antibody (Ab) to bovine serum albumin (BSA) and a diluting buffer, to provide varying Ab concentrations for downstream mixing with fluorescently labeled BSA (BSA*). Mixing ratios were determined from electrical impedance modeling of the fluidic network using P-SPICE software, and peak heights for the labeled species were analyzed relative to the concentration calculated from the model. For dilution and separation of fluorescently labeled amino acids, a linear calibration curve was obtained for mixing ratios of 0.118 to 7.46. A linear calibration curve was obtained for the immunoassay calibration using dilution ratios between 0.197 and 5.077. Deviations were observed at larger extremes, possibly due to leakage effects at intersections. Peak height reproducibility was +/- 3% for the immunoassay, using diluted monoclonal Ab in mouse ascites fluid as the analyte. Recovery for on-chip calibration was 92 +/- 6% versus calibrants prepared off-chip, indicating a small bias.

Amino Acids↗

PubMed2005

Four well-constrained calibration points from the vertebrate fossil record for molecular clock estimates.

Recent controversy about the use of the vertebrate fossil record for external calibration of molecular clocks centers on two issues, the number of dates used for calibration and the reliability of the fossil calibration date. Viewing matters from a palaeontological perspective, we propose three qualitative, phylogenetic criteria that can be used within a comparative framework for the selection of well-constrained calibration dates from the vertebrate fossil record. On the basis of these criteria, we identify three highly suitable new fossil calibration dates for molecular clock estimates: the lungfish-tetrapod split (between 419 and 408 Ma), the bird-crocodile split (between 251 and 243 Ma), and the alligator-caiman split (between 71 and 66 Ma). Together with our previously suggested bird-lizard split (between 252 and 257 Ma), these four fossil dates span a range of very different ages. They are, in our opinion, more suitable for molecular clock calibration than the traditionally used mammal-bird split, which is less well constrained. We plea for closer interactions between paleontologists and molecular biologists in studying the timescale of vertebrate evolution.

Animals↗

PubMed2004

Real-time update of calibration model for better monitoring of batch processes using spectroscopy.

In order to reduce the large calibration matrix usually required for calibrating multiwavelength optical sensors, a simple algorithm based on the addition in process of new standards is proposed. A small calibration model, based on 14 standards, is periodically updated by spectra collected on-line during fermentation operation. Concentrations related to these spectra are reconciled into best-estimated values, by considering carbon and oxygen balances. Using this method, fructose, acetate, and gluconacetan were monitored during batch fermentations of Gluconacetobacter xylinus 12281 using mid-infrared spectroscopy. It is shown that this algorithm compensates for noncalibrated events such as production or consumption of by-products. The standard error of prediction (SEP) values were 0.99, 0.10, and 0.90 g/L for fructose, acetate, and gluconacetan, respectively. By contrast, without an updating of the calibration model, the SEP values were 2.46, 0.92, and 1.04 g/L for fructose, acetate, and gluconacetan, respectively. Using only 14 standards, it was therefore possible to approach the performance of an 88-standard-based calibration model having SEP values of 1.11, 0.37, and 0.79 g/L for fructose, acetate, and gluconacetan, respectively. Therefore, the proposed algorithm is a valuable approach to reduce the calibration time of multiwavelength optical sensors.

Acetates↗

PubMed2001

Suitability of the Cordis Stabilizer marker guide wire for quantitative coronary angiography calibration: an in vitro and in vivo study.

Catheters usually are used for calibration purposes in quantitative coronary angiography (QCA). The systematic and random errors in these calibration factors (CFs) are dependent on the size and quality of the catheters and limited by out-of-plane magnification (OPM). Theoretically, a guide wire with evenly spaced marker bands would solve many of these potential problems. For this reason, we tested the Cordis Stabilizer marker wire, featuring 10 radiopaque platinum marker bands 15 mm apart, in in vitro and in vivo studies. To assess the effect of foreshortening, wires were positioned in a tube phantom; a centimeter grid was used as the gold standard. Radiographic images were acquired at 5-inch and 7-inch image-intensifier sizes, 512(2) and 1,024(2) matrix sizes and angulations from 0 degrees to 70 degrees in steps of 10 degrees. It was concluded that the relative errors in CFs are less than 7% if the foreshortening angles remain less than 20 degrees. In DICOM images of 15 patients, 65 measurements were taken after calibration on an 8F catheter and on a guide wire positioned in the coronary lesion. In all but two cases, the wire CFs were larger than the catheter CFs (relative difference, 24.7 +/- 19.6%). The measurements were divided into four groups: (I) no apparent OPM or foreshortening (n = 7), (II) only OPM (n = 4), (III) only foreshortening (n = 10), and (IV) the combination of both (n = 44). In group I (no OPM or foreshortening) the QCA results were similar using the guide wire or catheter as the calibration device (relative CF difference, 2.9% only). In group III the diameters were overestimated using the guide wire (obstruction diameter difference, 0.22 +/- 0.11 mm; reference diameter difference, 0.35 +/- 0.06 mm). For only OPM (group II) and the combination of OPM and foreshortening (group IV), the lesion length was underestimated on average by 2.4 mm using the catheter instead of the guide wire. In conclusion, if accurate assessment of the lesion length is important, the marker wire should be used for calibration purposes. For vessel diameter measurements, the conventional catheter calibration approach is the method of choice.

Angiography, Digital Subtraction↗

PubMed2002

Self-calibrating parallel imaging with automatic coil sensitivity extraction.

Calibration of the spatial sensitivity functions of coil arrays is a crucial element in parallel magnetic resonance imaging (PMRI). The most common approach has been to measure coil sensitivities directly using one or more low-resolution images acquired before or after accelerated data acquisition. However, since it is difficult to ensure that the patient and coil array will be in exactly the same positions during both calibration scans and accelerated imaging, this approach can introduce sensitivity miscalibration errors into PMRI reconstructions. This work shows that it is possible to extract sensitivity calibration images directly from a fully sampled central region of a variable-density k-space acquisition. These images have all the features of traditional PMRI sensitivity calibrations and therefore may be used for any PMRI reconstruction technique without modification. Because these calibration data are acquired simultaneously with the data to be reconstructed, errors due to sensitivity miscalibration are eliminated. In vivo implementations of self-calibrating parallel imaging using a flexible coil array are demonstrated in abdominal imaging and in real-time cardiac imaging studies.

Abdomen↗

PubMed2005

Inherently self-calibrating non-Cartesian parallel imaging.

The use of self-calibrating techniques in parallel magnetic resonance imaging eliminates the need for coil sensitivity calibration scans and avoids potential mismatches between calibration scans and subsequent accelerated acquisitions (e.g., as a result of patient motion). Most examples of self-calibrating Cartesian parallel imaging techniques have required the use of modified k-space trajectories that are densely sampled at the center and more sparsely sampled in the periphery. However, spiral and radial trajectories offer inherent self-calibrating characteristics because of their densely sampled center. At no additional cost in acquisition time and with no modification in scanning protocols, in vivo coil sensitivity maps may be extracted from the densely sampled central region of k-space. This work demonstrates the feasibility of self-calibrated spiral and radial parallel imaging using a previously described iterative non-Cartesian sensitivity encoding algorithm.

Algorithms↗

PubMed2004

A modified internal lock-mass method for calibration of the product ions derived from sustained off-resonance irradiation collision-induced dissociation using a Fourier transform mass spectrometer.

A modified internal lock-mass calibration method is introduced for improving the mass measurement accuracy of the product ion spectra derived from sustained off-resonance irradiation collision-induced dissociation (SORI-CID) Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. This method involves an initial external calibration of the Fourier transform mass spectrometer to obtain the initial A- and B-terms for the equation (f(i) = A/(m/z)(i) + B). The A-term is adjusted by using an empirical relationship between the up-shift of the A-term and the pulse-gas duration, whereas the B-term is adjusted by using the mass of the unfragmented precursor ion from the SORI-CID mass spectrum of the unknown sample as internal lock-mass. These adjusted A- and B-terms are then used to provide exact mass SORI-CID calibration for the unknown sample. The modified internal lock-mass method achieved average mass measurement accuracy of approximately 3 ppm which is significantly better than that of the conventional internal lock-mass calibration ( approximately 9 ppm) and is approaching that of the internal calibration ( approximately 2 ppm) and requires no addition of internal calibrant or instrumental modifications.

Angiotensin II↗

PubMed1994

Standard gases used to calibrate anesthetic vapor analyzers: are they stable?

OBJECTIVE: Gas cylinders containing volatile anesthetic vapors often are used to calibrate anesthetic agent analyzers. Differences seen between manufacturers in device calibration may be the result of error in the preparation of these calibration standards or drift in their concentration. METHODS: We measured the stability of 29 calibration gases in 2-L aluminum cylinders over a 15-month period. We also measured the stability of 18 gases in 1-L aluminum mini bottles. RESULTS: We found an average change of less than 0.02 vol% in the concentration of volatile anesthetic vapors for the 2-L aluminum cylinders containing 1.0 vol%. The maximum change was 0.04 vol%. For the 2-L cylinders, the manufacturer's certified concentration was 0.03 to 0.09 vol% higher than the concentration we measured on receipt. For the mini bottles, we found an average change of less than 0.04 vol% during a 6-month period; the maximum change was 0.06 vol%. The maximum change in 12 months was 0.14 vol%. CONCLUSIONS: Our results indicate that calibration gases containing volatile anesthetic vapors appear to be stable when stored in suitable cylinders. Aluminum cylinders sealed with a stop-cock seem to be suitable. In contrast, mini bottles seem to be less stable, probably due to the sealing construction. The difference of up to 0.09 vol% between our measurement of vapor concentration and the manufacturer's certified concentration may result from adsorption on surfaces in the cylinders after preparation or may reflect differences in calibration technique.

Anesthetics↗

PubMed2003

[Calibration of a room air gas monitor with certified reference gases].

Photo-acoustic infrared spectrometry is considered to be the gold standard for on-line measurement of anesthetic waste gas in room air. For maintenance of the precision of the measurements, the manufacturer recommends calibration of the gas monitor monitor every 3-12 months. We investigated whether the use of reference gases with analysis certificate could serve as a feasible alternative to commercial recalibration. We connected a multi-gas monitor type1302 (Bruel & Kjaer, Naerum, Denmark) to compressed air bottles containing reference gases with analysis certificate. Using a T-piece with a flow-meter, we avoided the entry of room air during the calibration phase. Highly purified nitrogen was used for zero calibration. The reference concentrations for desflurane, enflurane, halothane, isoflurane, and sevoflurane ranged from 41.6-51.1 ml/m(3) (ppm) in synthetic air. Since there is an overlap of the infrared absorption spectra of volatile anesthetics with alcohol used in operating rooms, we performed a cross-compensation with iso-propanol (107.0 ppm). A two-point calibration was performed for N(2)O (96.2 and 979.0 ppm), followed by cross-compensation with CO(2). Nafion tubes were used in order to avoid erroneous measurements due to molecular relaxation phenomena. The deviation of the measurement values ranged initially from 0-2.0% and increased to up to 4.9% after 18 months. For N(2)O, the corresponding values were 4.2% and 2.7%, respectively. Thus, our calibration procedure using certified reference gases yielded precise measurements with low deterioration over 18 months. It seems to be advantageous that the precision can be determined whenever deemed necessary. This allows for an individual decision, when the gas monitor needs to be calibrated again. The costs for reference gases and working time as well as logistic aspects such as storage and expiration dates must be individually balanced against the costs for commercial recalibration.

2-Propanol↗

PubMed1998

Comparison and cross-calibration of DXA systems: ODX-240 and Sophos L-XRA versus Hologic QDR-4500, for spinal bone mineral measurement. Translation of a reference database.

Replacement of dual-energy X-ray densitometry equipment may be necessary in time as a result of upgrading systems or new equipment. The lack of standardization in bone mineral density (BMD) measurements is known. Standardization efforts have been made for several years by the European Union under its organization COMAC-BME (Comité d'Actions Concertés-BioMedical Engineering) and by the International Committee for Standards in Bone Measurement. Cross-calibration is generally considered to be the result of linear regression between the measurements obtained with two densitometers. A major disadvantage of the regression method is the noncompatibility of the two formulae of calibration (Y versus X and X versus Y). In this study we considered cross-calibration in terms of a structural model that produced circular equations when, for example, three systems were cross-calibrated. Cross-calibration in this study was calculated from the measurement of the lumbar BMD of a population of 204 patients, with Hologic QDR-4500, ODX-240 and Sophos L-XRA systems. In vitro accuracy and short-term reproducibility of the three systems were studied. Using the structural calibration equation we transformed a reference database for L2-4 BMD obtained from a population of 983 French females, aged 11-47 years, on an ODX-240 to a reference database for a Hologic QDR-4500. A new young adult reference was obtained and consequently a new evaluation of the T-score for the Hologic QDR-4500.

Absorptiometry, Photon↗

PubMed2001

Preparation of calibration materials for microanalysis of Ti minerals by direct fusion of synthetic and natural materials: experience with LA-ICP-MS analysis of some important minor and trace elements in ilmenite and rutile.

Calibration materials for microanalysis of Ti minerals have been prepared by direct fusion of synthetic and natural materials by resistance heating in high-purity graphite electrodes. Synthetic materials were FeTiO3 and TiO2 reagents doped with minor and trace elements; CRMs for ilmenite, rutile, and a Ti-rich magnetite were used as natural materials. Problems occurred during fusion of Fe2O3-rich materials, because at atmospheric pressure Fe2O3 decomposes into Fe3O4 and O2 at 1462 degrees C. An alternative fusion technique under pressure was tested, but the resulting materials were characterized by extensive segregation and development of separate phases. Fe2O3-rich materials were therefore fused below this temperature, resulting in a form of sintering, without conversion of the materials into amorphous glasses. The fused materials were studied by optical microscopy and EPMA, and tested as calibration materials by inductively coupled plasma mass spectrometry, equipped with laser ablation for sample introduction (LA-ICP-MS). It was demonstrated that calibration curves based on materials of rutile composition, within normal analytical uncertainty, generally coincide with calibration curves based on materials of ilmenite composition. It is, therefore, concluded that LA-ICP-MS analysis of Ti minerals can with advantage be based exclusively on calibration materials prepared for rutile, thereby avoiding the special fusion problems related to oxide mixtures of ilmenite composition. It is documented that sintered materials were in good overall agreement with homogeneous glass materials, an observation that indicates that in other situations also sintered mineral concentrates might be a useful alternative for instrument calibration, e.g. as alternative to pressed powders.

Calibration↗