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At least 235 records · Page 13Linked to original sources

PubMed1998

Methodological aspects of the use of a calibrator in in vivo microdialysis-further development of the retrodialysis method.

PURPOSE: To investigate the performance of two alternative retrodialysis recovery methods and to describe the influence of different recoveries on the reliability in estimating unbound extracellular concentrations of morphine. METHODS: Unbound concentrations of morphine in striatum and in blood were determined by microdialysis after a 10 min i.v. infusion in freely moving rats. In vivo recovery of morphine was determined by morphine itself, retrodialysis by drug, and by the calibrator nalorphine, retrodialysis by calibrator. RESULTS: The low calibrator recovery in striatum (8.6%) resulted in large variability in the estimation of unbound extracellular concentrations when retrodialysis by calibrator was used. In blood, where the recovery was higher (36%), the variability was smaller. Also, when retrodialysis by drug was used, the variability remained low. This difference is caused by the propagation of errors in the way retrodialysis recovery is determined. Therefore, calibrator recovery values > or =20% are preferable in concentration estimations using retrodialysis by calibrator. CONCLUSIONS: When no time-dependent change in recovery is observed, retrodialysis by drug determined before the systemic administration is the best method. The calibrator is valuable as a quality control during the experiment.

Animals↗

PubMed2000

Respiratory inductance plethysmography in healthy infants: a comparison of three calibration methods.

Respiratory inductance plethysmography (RIP) measures respiration from body surface movements. Various techniques have been proposed for calibration in order that RIP may be used quantitatively. These include calculation of the proportionality constant of ribcage to abdominal volume change (K). The aims of this study were to 1) establish whether a fixed value of K could be used for calibration, and 2) compare this technique with multiple linear regression (MLR) and qualitative diagnostic calibration (QDC) in normal healthy infants. Recordings of pneumotachograph (PNT) flow and RIP were made during quiet (QS) and active sleep (AS) in 12 infants. The first 5 min in a sleep state were used to calculate calibration factors, which were applied to subsequent validation data. The absolute percentage error between RIP and PNT tidal volumes was calculated. The percentage error was similar over a wide range of K during QS. However, K became more critical when breathing was out of phase. A standard for K of 0.5 was chosen. There was good agreement between calibration methods during QS and AS. In the first minute following calibration during QS, the mean absolute errors were 3.5, 4.1 and 5.3% for MLR, QDC and fixed K respectively. The equivalent errors in AS were 11.5, 13.1 and 13.7% respectively. The simple fixed ratio method can be used to measure tidal volume with similar accuracy to multiple linear regression and qualitative diagnostic calibration in healthy unsedated sleeping infants, although it remains to be validated in other groups of infants, such as those with respiratory disease.

Calibration↗

PubMed1998

Effects of calibration and automatic greyscale adjustment on detectability of simulated bone lesions using a storage phosphor system.

OBJECTIVES: To analyse the interaction of calibration, automatic greyscale adjustment and X-ray exposure on the evaluation of simulated bone lesions with the Digora (Orion Corporation Soredex, Helsinki, Finland) storage phosphor system. METHODS: Exposures were made under five conditions based on the exposure time for Ektaspeed Plus film: A-calibration exposure 100%, exposure 100%; B-calibration exposure 100%, exposure 50%; C-calibration exposure 100%, exposure 10%; D-calibration exposure 50%, exposure 50%; E-calibration exposure 10%, exposure 10%. Series of images of artificial bone lesions were created each representing one of the five conditions, with or without automatic greyscale adjustment and including one of two lesion sizes. The series were evaluated by seven expert radiologists. Results were analysed by means of MANOVA statistics. RESULTS: The large lesions were evaluated significantly better than the small lesions (P < 0.011). The two 10% exposure conditions were evaluated significantly worse than the other three (P < 0.029) which had comparable results. Automatic greyscale adjustment had no significant effect (P > 0.253). CONCLUSIONS: The Digora should be calibrated for the highest exposure to be used to optimize the wide latitude. Automatic greyscale adjustment is not useful for the detection of simulated bone lesions.

Bone Diseases↗

PubMed2003

Preparation of a purified fibrinogen calibration material for Clauss method and turbidimetric immunoassay possessing biological activity and antigenicity.

Fibrinogen plays a major role in basic coagulation tests such as prothrombin time (PT), activated partial thromboplastin time (APTT) and thrombin time (TT). These show high interlaboratory variation because of inaccurate instrumental calibration. The same is true of the fibrinogen assay, despite its being a quantitative assay. Most medical laboratories use automated coagulometers and commercially available calibration materials (calibrators) to obtain an accurate fibrinogen value, but, when checked, calibrators have been found to deviate from the assigned value. The Japan Society of Laboratory Medicine (JSLM) has identified the need for a reliable plasma fibrinogen standard. To enhance the reliability of calibrator fibrinogen values and thereby remedy the poor precision and accuracy of plasma fibrinogen testing, we undertook the preparation of a standard calibration material applicable to both the Clauss method and turbidimetric immunoassay (TIA). The calibrator was prepared from fresh human plasma by glycine precipitation and virus inactivation followed by affinity chromatography to remove contaminated plasminogen. In the resulting product, clottable fibrinogen accounted for 95% of total protein and within-run precision showed a CV of less than 1.8%. We believe the preparation will become a candidate material for laboratory and manufacturer use in Japan.

Blood Coagulation Tests↗

PubMed1998

Comparison of two methods of therapy level calibration at 60Co gamma beams.

The accuracy and traceability of the calibration of radiotherapy dosimeters is of great concern to those involved in the delivery of radiotherapy. It has been proposed that calibration should be carried out directly in terms of absorbed dose to water, instead of using the conventional and widely applied quantity of air kerma. In this study, the faithfulness in disseminating standards of both air kerma and absorbed dose to water were evaluated, through comparison of both types of calibration for three types of commonly used radiotherapy dosimeters at 60Co gamma beams at a few secondary and primary standard dosimetry laboratories (SSDLs and PSDLs). A supplementary aim was to demonstrate the impact which the change in the method of calibration would have on clinical dose measurements at the reference point. Within the estimated uncertainties, both the air kerma and absorbed dose to water calibration factors obtained at different laboratories were regarded as consistent. As might be expected, between the SSDLs traceable to the same PSDL the observed differences were smaller (less than 0.5%) than between PSDLs or SSDLs traceable to different PSDLs (up to 1.5%). This can mainly be attributed to the reported differences between the primary standards. The calibration factors obtained by the two methods differed by up to about 1.5% depending on the primary standards involved and on the parameters of calculation used for 60Co gamma radiation. It is concluded that this discrepancy should be settled before the new method of calibration at 60Co gamma beams in terms of absorbed dose to water is taken into routine use.

Air↗

PubMed2003

Calibration and validation of a quality assurance system for 90Sr/90Y radiation source trains.

A quality assurance system (OPTIDOS, PTW-Freiburg) developed for dose rate verification of 90Sr/90Y radiation source trains (RSTs) was calibrated and validated. These source trains are used in the 5-F-BetaCath system (Novoste Corp.) for the treatment of endovascular diseases. The calibration factor of the OPTIDOS system was obtained empirically and is valid for 90Sr/90Y dose rate measurements at the specification point which is located at 2 mm distance from the source axis. A total of 187 OPTIDOS dose rate verifications of the 5-F-BetaCath system were performed in different hospitals. The histogram of the deviation between the manufacturer's dose rate specification and the dose rate measured using the OPTIDOS dosimetry system is Gaussian shaped with +/- 3% relative width and a mean shift of about +2% with respect to the corresponding dose rate specification. Additionally, 128 OPTIDOS dose rate verifications of the new jacketed RST (3.5-F-BetaCath, Novoste Corp.) were performed using the same calibration factor as derived for the 5-F-BetaCath system. Distribution of the deviation between the certified and the measured dose rate is nearly identical in comparison to the histogram of the 5-F-BetaCath system. The mean value of the deviations is shifted by -1.5% with respect to the certified dose rate. In order to compare the results of the calibrated OPTIDOS dosimetry system with a standard measuring method, separate dose rate measurements were performed using electron accelerator calibrated radiochromic films in which calibration is traceable to PTB (Physikalisch Technische Bundesanstalt, Germany). Deviation between both the methods is less than 3.1%. These results confirm that the calibrated OPTIDOS dosimetry system can be considered suitable for quality assurance of both types of RST used in the BetaCath systems.

Beta Particles↗

PubMed2005

Variable transformation of calibration equations for radiation dosimetry.

For radiation dosimetry, dosimetric equipment must be calibrated by using known doses. The calibration is done to determine an equation that relates the absorbed dose to a physically measurable quantity. Since the calibration equation is accompanied by unavoidable uncertainties, the doses estimated with such equations suffer from inherent uncertainties. We presented mathematical formulation of the calibration when the calibration relation is either linear or nonlinear. We also derived equations for the uncertainty of the estimated dose as a function of the uncertainties of the parameters in the equations and the measured physical quantity. We showed that a dosimeter with a linear calibration equation with zero dose-offset enables us to perform relative dosimetry without calibration data. Furthermore, a linear equation justifies useful data manipulations such as rescaling the dose and changing the dose-offset for comparing dose distributions. Considering that some dosimeters exhibit linear response with a large dose-offset or often nonlinear response, we proposed variable transformations of the measured physical quantity, namely, linear- and log-transformation methods. The proposed methods were tested with Kodak X-Omat V radiographic film and BANG polymer gel dosimeter. We demonstrated that the variable transformation methods could lead to linear equations with zero dose-offset and could reduce the uncertainty of the estimated dose.

Algorithms↗

PubMed2003

Continuous glucose monitoring with glucose sensors: calibration and assessment criteria.

Continuous glucose monitoring (CM) by means of minimally invasive or noninvasive glucose sensors can help to further optimize metabolic control in patients with diabetes without need for frequent capillary blood glucose measurements. Most glucose sensors measure glucose concentration in the interstitial fluid (ISF). Because of the varying conditions in this compartment, a general in vitro calibration ( = factory calibration) by the manufacturer appears not to be possible. Therefore, calibration of the sensor signal must be performed by the patient himself repeatedly. The calibration procedure can be performed by means of conventional capillary blood glucose measurements in order to transform the sensor signals obtained from the specific compartment (e.g., ISF) into "blood" glucose values. A number of aspects can influence the validity of this procedure. The relationship between changes in blood glucose and in ISF glucose, in both time and concentration dimensions, is not well understood, especially during dynamic changes. The physical lag time, which critically depends on the glucose sensor technology used, can also introduce a systematic calibration error. After the first calibration, usually performed some hours after the application of a given glucose sensor, recalibration at certain intervals is necessary. Therefore, patients should critically assess the values displayed by a CM system. In the case of implausible glucose values they should verify the results by means of a conventional capillary glucose measurement. Up to now there is no consensus on assessment criteria to be used for evaluation of CM system performance and calibration quality. Existing methods of displaying CM values against corresponding reference values, including linear regression analysis and error grid analysis, as well as numeric criteria such as System Error, Predicted Error Sum of Squares (in %), and Mean Absolute Deviation are not generally applicable to CM. It appears as if they do not allow sufficient description of data obtained with CM systems. There is a pressing need to develop novel adequate assessment criteria enabling a better characterization of CM system performance. If these were used uniformly by all manufacturers and scientists assessing CM systems, this would allow a fair comparison of the performance of different systems.

Blood Glucose↗

PubMed2001

Dose variance associated with calibration and administration of radiopharmaceuticals.

Variation in the amounts of radioactivity that is associated with dose calibration and administration of gamma-emitting radioactive drugs was studied. Health systems use radionuclide dose calibrators when they need to assay drugs for radioactivity. However, the radioactive drugs commonly used for therapeutic or diagnostic purposes are compounded and assayed in or administered from containers that differ from those containing the standard reference materials (SRMs). SRMs for four radionuclides--technetium 99m, indium 111, thallium 201, and iodine 131--were drawn up into vials and syringe-needle assemblies of volumes and sizes represented in clinical practice. In each sample, the amount of radioactivity was calculated and compared with values obtained from three dose calibrators. In addition, over a four-month period, 101 samples of technetium Tc 99m medronate injection with a desired activity of 20 mCi were prepared in syringes; the radioactive dose in each syringe was calibrated for administration to a patient at a specific time that day. The amounts of radioactivity at the time of preparation and the time of administration, the amount remaining in the syringe and needle after administration, and the amount reported as administered were recorded. Measurement with the dose calibrators of the SRMs in containers supplied by the National Institute of Standards and Technology showed radioactivity within 10% of the labeled amount, the percentage of variation regulations allow. Measurements of the SRMs in syringe-needle assemblies were within 10% for technetium 99m and thallium 201, 9-16% for iodine 131, and 15-26% for indium 111. The individual doses of technetium 99m medronate injection were, on average, administered on time, but doses were administered up to 75 minutes before and 107 minutes after the calibration time. The mean +/- S.D. amount administered was 19.0 +/- 1.34 mCi. The mean +/- S.D. amount reported administered was 20 +/- 0.24 mCi. How radiopharmaceuticals were dose calibrated and administered influenced the actual dose available to patients.

Calibration↗

PubMed2005

Less invasive determination of cardiac output from the arterial pressure by aortic diameter-calibrated pulse contour.

BACKGROUND: Cardiac output by modelflow pulse contour method can be monitored quantitatively and continuously only after an initial calibration, to adapt the model to an individual patient. The modelflow method computes beat-to-beat cardiac output (COmf) from the radial artery pressure, by simulating a three-element model of aortic impedance with post-mortem data from human aortas. METHODS: In our improved version of modelflow (COmfc) we adapted this model to a real time measure of the aortic cross-sectional area (CSA) of the descending aorta just above the diaphragm, measured by a new transoesophageal echo device (HemoSonic 100). COmf and COmfc were compared with thermodilution cardiac output (COtd) in 24 patients in the intensive care unit. Each thermodilution value was the mean of four measurements equally spread over the ventilatory cycle. RESULTS: Least squares regression of COtd vs COmf gave y=1.09x[95% confidence interval (CI) 0.96-1.22], R2=0.15, and of COtd vs COmfc resulted in y=1.02x(95% CI 0.96-1.08), R2=0.69. The limits of agreement of the un-calibrated COmf were -3.53 to 2.79, bias=0.37 litre min(-1) and of the diameter-calibrated method COmfc, -1.48 to 1.32, bias=-0.08 litre min(-1). The coefficient of variation for the difference between methods decreased from 28 (un-calibrated) to 12% after diameter-calibration. CONCLUSIONS: After diameter-calibration, the improved modelflow pulse contour method reliably estimates cardiac output without the need of a calibration with thermodilution, leading to a less invasive cardiac output monitoring method.

Adult↗

PubMed2001

Calibration of a 7.6 cm x 7.6 cm (3 inch x 3 inch) sodium iodide gamma ray spectrometer for air kerma rate.

An experimental procedure is described for converting a gamma ray spectral measurement from a 7.6 cm x 7.6 cm (3 inch x 3 inch) sodium iodide (NaI) detector to air kerma rate. The calibration procedure involves measuring the energy deposited in the detector using 10 radioactive sources of known activity covering an energy range from 60 keV to 1,836 keV. For each of the 10 sources, gamma ray spectra were measured with the source at different angles to the detector axis. The total energy deposited in the detector for the ten sources was confirmed by Monte Carlo calculations. The spectra measured at different angles were combined to produce a spectrum that would represent a homogeneous semi-infinite source of radiation. The resultant spectrum was then subdivided into 10 energy regions. Based on the known air kerma rates due to the sources, a calibration coefficient was calculated for each of the 10 energy regions. These calibration coefficients could then be used to convert the energy deposited in the 10 regions of an unknown spectrum to air kerma rate. The calibration procedure was confirmed by comparing the results from the detector with those from calibrated collimated beams of 137Cs and 60Co. A comparison of measurements using a calibrated pressurised ionisation chamber with those from a similar Nal spectrometer in Finland provided additional confirmation of the calibration procedure.

Air↗

PubMed2007

The study of new calibration features in the Harshaw TLD system.

The Harshaw TLD system has three key calibration procedures: the Reader, the Dosemeter and the Algorithm. These functions must be properly calibrated for the system to achieve the optimum results. For the conventional reader and dosemeter calibration, Harshaw TLD recommends a pre-fade and a post-fade of 24-48 h when calibrating the system for LiF:Mg,Ti type dosemeter. It is also recommended that keeping the fade time consistent is important to maintain the quality of the system performance. In recent years, new calibration features have been introduced into the Harshaw TLD models 6600 and 8800 operating systems. These new features are Auto Calibration, Auto QC and Auto Blank, and they give the user the ability to set up the clear-expose-read process to be performed automatically in a sequence for each dosemeter. This saves processing time and keeps the fade time the same. However, since the fade time is near zero, will it affect the TLD system calibration factors? What should the user expect? This paper presents a study of the effect of Auto Calibration/Auto QC to the TLD operation.

Body Burden↗

PubMed2000

Enhanced standardization of the International Normalized Ratio through the use of plasma calibrants: a concise review.

The INR was introduced in 1983 as a means of standardizing the prothrombin time test and improving management of patients receiving warfarin-based oral anticoagulant therapy. Despite the eventual widespread use of the INR system, unacceptable levels of inter-laboratory variation persist. The use of certified plasma calibrants has been studied as a means to reducing INR variation. This review examines the pre-analytical and analytical variables contributing to INR problems. Also, the findings of several multicenter studies in which plasma calibrants were deployed are presented. Issues such as the number of calibrants used and their composition, calibrant format, plasma citrate concentration, reference thromboplastins used, methods of calibrant certification and alternative techniques for local system calibration are examined. The development of consensus guidelines on the use of plasma calibrants for INR standardization is recommended.

Calibration↗

PubMed2003

Coronary calcium quantification using various calibration phantoms and scoring thresholds.

RATIONALE AND OBJECTIVES: To compare scoring threshold and calibration method-dependent accuracy and variability of coronary calcium measurements by multidetector computed tomography (MDCT). METHODS: Ninety-five subjects were scanned with MDCT. We calculated Agatston score and volume score. Mineral mass (MM) was calculated using patient-based and scanner-based calibration methods. Accuracy of calibration was validated using artificial calcium cylinders. RESULTS: Patient-based and scanner-based calibration permitted accurate quantification of artificial calcium cylinders (bias: 0 mg and -2 mg). In the subjects, the mean relative difference of MM measurements performed at 90 and 130 Hounsfield units threshold (59%) was lower than for Agatston score (94%) and volume score (109%; P < 0.05). Patient-based and scanner-based calibration yielded systematically different MM measurements (bias: 22%). CONCLUSIONS: MM lowers threshold-dependent variability of coronary calcium measurements. Patient-based and scanner-based calibration allows accurate calcium quantification ex vivo but reveal systematic differences in subjects. Patient-based calibration may better account for subject size and composition.

Aged↗

PubMed2001

Calibrated single-plunge bipolar electrode array for mapping myocardial vector fields in three dimensions during high-voltage transthoracic defibrillation.

Mapping of the myocardial scalar electric potential during defibrillation is normally performed with unipolar electrodes connected to voltage dividers and a global potential reference. Unfortunately, vector potential gradients that are calculated from these data tend to exhibit a high sensitivity to measurement errors. This paper presents a calibrated single-plunge bipolar electrode array (EA) that avoids the error sensitivity of unipolar electrodes. The EA is triaxial, uses a local potential reference, and simultaneously measures all three components of the myocardial electric field vector. An electrode spacing of approximately 500 microm allows the EA to be direct-coupled to high-input-impedance, isolated, differential amplifiers and eliminates the need for voltage dividers. Calibration is performed with an electrolytic tank in which an accurately measured, uniform electric field is produced. For each EA, unique calibration matrices are determined which transform potential difference readings from the EA to orthogonal components of the electric field vector. Elements of the matrices are evaluated by least squares multiple regression analysis of data recorded during rotation of the electric field. The design of the electrolytic tank and electrode holder allows the electric field vector to be rotated globally with respect to the electrode axes. The calibration technique corrects for both field perturbation by the plunge electrode body and deviations from orthogonality of the electrode axes. A unique feature of this technique is that it eliminates the need for mechanical measurement of the electrode spacing. During calibration, only angular settings and voltages are recorded. For this study, ten EAs were calibrated and their root-mean-square (rms) errors evaluated. The mean of the vector magnitude rms errors over the set of ten EAs was 0.40% and the standard deviation 0.07%. Calibrated EAs were also tested for multisite mapping in four dogs during high-voltage transthoracic shocks.

Animals↗

PubMed2004

International multicenter international sensitivity index (ISI) calibration of a new human tissue factor thromboplastin reagent derived from cultured human cells.

The international sensitivity index (ISI) of the first working standard of Simplastin HTF, a new human tissue factor thromboplastin derived from cultured human cells, has been assessed in a calibration exercise in two Canadian and five European laboratories. Calibrations against international reference preparations (IRP) were performed for the manual method and six types of automated coagulometers that cover the majority of clotting endpoint principles in routine use. The ISI was method-dependent and varied between 1.03 and 1.29 when calibrated against rTF/95 (human IRP). The ISI was also dependent on the route of calibration. Compared with calibration against rTF/95, the ISIs obtained by calibration against RBT/90 (rabbit IRP) were on average 4.4% higher (P < 0.005). Considering the principle of 'like vs. like', the ISIs obtained by calibration against rTF/95 should be preferred.

Blood Coagulation Tests↗

PubMed1994

Stepwise analysis of the calibration procedure of an accelerometer-based pacemaker.

UNLABELLED: The rate modulation of Relay 294-03 is individually adjusted during a 3-minute calibration test corresponding to the collection of moderate acceleration signals (MAL), which are related to a programmable moderate pacing rate (PR). Those rate/acceleration values form a calibration point in the flat part of the rate response curve (RRC). To help in a more clinically oriented analysis of this feature, the stimulator was first calibrated while strapped on a volunteer who remained seated (low MAL), walked (medium MAL), or jogged (high MAL). The device was then mechanically submitted to three calibrated to and fro movements corresponding to the sensitivity spectrum of the sensor. Each movement was repeated ten times to test the influence of the slope (1-10). RESULTS: (1) One-way ANOVA significant effect of the MAL recorded during the calibration test on subsequent rate modulation (P = 0.0001); (2) The sensor amplifier gain was inversely related to the MAL: high MAL induced lower amplifier gain and lower PR for identical accelerations; (3) Lower amplifier gain allowed to discriminate highest acceleration magnitudes before reaching an overflow of the sensor; (4) The maximum achievable PR increased not only with the programmed slope, but also with decreasing MAL (P = 0.0055): a low MAL shifts to the left calibration point located on the part of the RRC and makes the last steep part of the RRC start earlier, thus leading to higher maximum achievable PR. In conclusion the calibration procedure is crucial not only in defining a moderate acceleration intensity but also in determining the sensor amplifier gain and the maximum achievable PR.

Calibration↗

PubMed1992

6R instrumented spatial linkages for anatomical joint motion measurement--Part 2: Calibration.

The six-revolute-joint instrumented spatial linkage (6R ISL) is often the measurement system of choice for monitoring motion of anatomical joints. However, due to tolerances of the linkage parameters, the system may not be as accurate as desired. A calibration algorithm and associated calibration device have been developed to refine the initial measurements of the ISL's mechanical and electrical parameters so that the measurement of six-degree-of-freedom motion will be most accurate within the workspace of the anatomical joint. The algorithm adjusts the magnitudes of selected linkage parameters to reduce the squared differences between the six known and calculated anatomical position parameters at all the calibration positions. Weighting is permitted so as to obtain a linkage parameter set that is specialized for measuring certain anatomical position parameters. Output of the algorithm includes estimates of the measuring system accuracy. For a particular knee-motion-measuring ISL and calibration device, several interdependent design parameter relationships have been identified. These interdependent relationships are due to the configuration of the ISL and calibration device, the number of calibration positions, and the limited resolution of the devices that monitor the position of the linkage joints. It is shown that if interdependence is not eliminated, then the resulting ISL parameter set will not be accurate in measuring motion outside of the calibration positions, even though these positions are within the ISL workspace.

Algorithms↗