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PubMed2006

Evaluation of factors affecting CGMS calibration.

BACKGROUND: The optimal number/timing of calibrations entered into the CGMS (Medtronic MiniMed, Northridge, CA) continuous glucose monitoring system have not been previously described. METHODS: Fifty subjects with Type 1 diabetes mellitus (10-18 years old) were hospitalized in a clinical research center for approximately 24 h on two separate days. CGMS and OneTouch Ultra meter (LifeScan, Milpitas, CA) data were obtained. The CGMS was retrospectively recalibrated using the Ultra data varying the number and timing of calibrations. Resulting CGMS values were compared against laboratory reference values. RESULTS: There was a modest improvement in accuracy with increasing number of calibrations. The median relative absolute deviation (RAD) was 14%, 15%, 13%, and 13% when using three, four, five, and seven calibration values, respectively (P < 0.001). Corresponding percentages of CGMS-reference pairs meeting the International Organisation for Standardisation criteria were 66%, 67%, 71%, and 72% (P < 0.001). Nighttime accuracy improved when daytime calibrations (pre-lunch and pre-dinner) were removed leaving only two calibrations at 9 p.m. and 6 a.m. (median difference, -2 vs. -9 mg/dL, P < 0.001; median RAD, 12% vs. 15%, P = 0.001). Accuracy was better on visits where the average absolute rate of glucose change at the times of calibration was lower. On visits with average absolute rates <0.5, 0.5 to <1.0, 1.0 to <1.5, and >or=1.5 mg/dL/min, median RAD values were 13% versus 14% versus 17% versus 19%, respectively (P = 0.05). CONCLUSIONS: Although accuracy is slightly improved with more calibrations, the timing of the calibrations appears more important. Modifying the algorithm to put less weight on daytime calibrations for nighttime values and calibrating during times of relative glucose stability may have greater impact on accuracy.

Adolescent↗

PubMed2004

Variation in calibration of hand-held ultraviolet (UV) meters for psoralen plus UVA and narrow-band UVB phototherapy.

BACKGROUND: Phototherapy units should regularly use hand-held ultraviolet (UV) meters to assess the output of treatment lamps, and these meters should be accurately calibrated. Several medical physics departments in the U.K. can calibrate UV meters traceable to national standards, but there is concern that there may be disagreement among departments. In particular, there may be difficulty in calibration for narrow-band UVB phototherapy lamps (TL-01). OBJECTIVES: To ascertain the level of agreement in UV meter calibration at expert centres in the U.K., and to survey methodology at these centres, consider sources of errors and to make recommendations on calibration methods. METHODS: The same UV meter with two detectors (for UVA and UVB) was calibrated by seven medical physics departments. A questionnaire on methods was also distributed and measured spectral outputs from each centre were examined. RESULTS: The calibration factors for the meter varied by +/- 18% for the UVA detector and by +/- 60% for the UVB detector (2 standard deviations). Six centres performed calibration using a spectroradiometer and one centre used a reference meter method. The spectra of lamps used for calibration were similar. For the spectroradiometric methods there were some differences in methodology and instrumentation that may account for the differences in calibration factors. CONCLUSIONS: UV meter calibration in the U.K. shows unacceptable variability, particularly for TL-01 lamps. An accuracy of around of +/- 10% would be clinically acceptable and should be technically achievable.

Calibration↗

PubMed2000

Isoamyl nitrite depolymerized heparin as a universal calibrator for heparins and low molecular weight heparins.

Because the subcutaneous absorption of heparin depends on its molecular weight, high performance size exclusion chromatography methods, among others, have been developed for its determination. Because heparin consists of polymer chains of varying lengths, a large number of calibrators, 19 in our work, are required for these molecular weight analyses. Because the preparation of that many calibrants is both time-consuming and expensive, other methods requiring fewer calibrants are being sought. In pursuit of these aims, a heparinase-degraded heparin has been accepted as the first international reference preparation low molecular weight heparin for molecular weight calibration. We have previously proposed another calibrant, the Heparin Mass Calibrator. In this article, we introduce another calibrant, called the Heparin Molecular Mass Calibrant. It offers the advantage of a much wider availability because it is a commercial product. In addition to having a superior refractive index chromatography run profile, when used in combination with two Narrow Range Calibrators with molecular weights of 34.4 and 18.4 kDa, this new calibrant covers both the higher and the lower molecular weight ranges. Besides, ultraviolet detection is not needed for molecular weight calculations. Therefore, it is proposed that its feasibility as a universal reference calibrator for both heparins and low molecular weight heparins be considered.

Amyl Nitrite↗

PubMed2006

New methodology to obtain a calibration model for noninvasive near-infrared blood glucose monitoring.

This paper reports new methodology to obtain a calibration model for noninvasive blood glucose monitoring using diffuse reflectance near-infrared (NIR) spectroscopy. Conventional studies of noninvasive blood glucose monitoring with NIR spectroscopy use a calibration model developed by in vivo experimental data sets. In order to create a calibration model, we have used a numerical simulation of light propagation in skin tissue to obtain simulated NIR diffuse reflectance spectra. The numerical simulation method enables us to design parameters affecting the prediction of blood glucose levels and their variation ranges for a data set to create a calibration model using multivariate analysis without any in vivo experiments in advance. By designing the parameters and their variation ranges appropriately, we can prevent a calibration model from chance temporal correlations that are often observed in conventional studies using NIR spectroscopy. The calibration model (regression coefficient vector) obtained by the numerical simulation has a characteristic positive peak at the wavelength around 1600 nm. This characteristic feature of the regression coefficient vector is very similar to those obtained by our previous in vitro and in vivo experimental studies. This positive peak at around 1600 nm also corresponds to the characteristic absorption band of glucose. The present study has reinforced that the characteristic absorbance of glucose at around 1600 nm is useful to predict the blood glucose level by diffuse reflectance NIR spectroscopy. We have validated this new calibration methodology using in vivo experiments. As a result, we obtained a coefficient of determination, r2, of 0.87 and a standard error of prediction (SEP) of 12.3 mg/dL between the predicted blood glucose levels and the reference blood glucose levels for all the experiments we have conducted. These results of in vivo experiments indicate that if the parameters and their vibration ranges are appropriately taken into account in a numerical simulation, the new calibration methodology provides us with a very good calibration model that can predict blood glucose levels with small errors without conducting any experiments in advance to create a calibration model for each individual patient. This new calibration methodology using numerical simulation has promising potential for NIR spectroscopy, especially for noninvasive blood glucose monitoring.

Algorithms↗

PubMed2003

On the development and comparative evaluation of an ultrasound B-mode probe calibration method.

OBJECTIVE: Precise transducer calibration is an essential prerequisite for reliable surface registration based on ultrasound B-mode imaging devices. The clinical usage of a novel B-mode transducer calibration technique was evaluated and its attainable calibration precision assessed. MATERIALS AND METHODS: The Three Wire Method and the Cambridge Calibration Method were used as reference techniques to compare the efficiency, calibration precision and spatial requirements of the different techniques. A total of 20 calibration trials were performed using each technique and were statistically evaluated for accuracy and speed. RESULTS: The mean error characterizing the calibration precision of the Three Wire Method was 3.2 mm, obtained in a phantom with a volume of 14 x 10(6) mm(3) in 18.48 min. The Cambridge method resulted in a mean calibration error of 2.2 mm, but required a larger phantom with a volume of 35 x 10(6) mm3 to be used for a duration of 9.30 min. The proposed method yielded an average calibration error of 1.9 mm and was performed, on average, in 2 min using a phantom with a size smaller than 1 x 10(6) mm3. CONCLUSIONS: The suggested calibration method offers decreased time and space while retaining an equivalent calibration precision when compared to established reference methods.

Calibration↗

PubMed1997

Radioassay of yttrium-90 radiation using the radionuclide dose calibrator.

UNLABELLED: Yttrium-90 is used in radioimmunotherapy because of its favorable physical half-life and energetic pure beta emissions. However, it is often necessary to standardize 90Y sources to establish a dose calibrator dial setting for accurate calibration of clinical doses of 90Y preparations. METHODS: A solution of 90YCl3 containing 2.81 kBq/ml (by supplier's calibration) was prepared by serial dilution In 0.05 M HCl. Ten 100-microliters aliquots of this solution were counted in a Packard liquid scintillation analyzer; the mean radioactivity in becquerels was determined and used to evaluate dial settings 48 x 10,775 x 70 and 775 x 100 on a radionuclide dose calibrator for 90Y measurements. The dose calibrator response was also studied on 90Y sources at varying solution volumes in plastic and glass containers. RESULTS: Calibrator readings of 90Y sources in glass and plastic vials and plastic syringes were accurate at either dial setting 48 x 10 (commonly used by many 90Y laboratories) or 775 x 70. Measurements of 1.15 and 3.03 GBq (31 and 82 mCi, respectively) calibrated 90Y sources in either vial were -3.0 and +4.3%, respectively, at dial-setting 775 x 70 and -4.0 and +9.0% at 48 x 10. Yttrium-90 sources in plastic syringes gave higher readings than those in glass vials, therefore, requiring a container correction factor for accurate dose assay. Measurements of 90YCl3 shipments from four suppliers over a 3-yr period demonstrated concurring calibration measurements at both 775 x 70 and 48 x 10 settings for shipments from all suppliers. The dose calibrator response to 90Y radiation was linear within a 1-333 kBq range in a constant sample volume of 580 microliters. CONCLUSION: This work demonstrates the validity of using the 48 x 10 dial-factor combination on the standard radionuclide dose calibrator for calibration of 90Y radiopharmaceuticals.

Calibration↗

PubMed2006

Cross calibration of DXA as part of an equipment replacement program.

The purpose of this study was to develop cross calibrations when replacing three dual-energy X-ray absorptiometers (GE Lunar DPXL [Madison, WI], DPXL, GE Lunar Expert [Madison, WI], Expert, Hologic QDR2000 [Waltham, MA]) with two new GE Lunar Prodigy instruments. Subjects previously scanned on the Expert or QDR2000 were transferred to Prodigy 1 and those previously scanned on the DPXL to Prodigy 2. A cohort of subjects was recalled for each old instrument, and approximately 20 subjects had lumbar spine and hip scans on each old instrument and the appropriate new instrument. An in vitro calibration was carried out using a Bona Fide Phantom (Bio-Imaging Technologies, Inc., Newtown, PA). Calibrations were fitted using a standardized principal components method. A Bland and Altman plot was used to calculate the mean difference and limits of agreement between instruments. Standardized bone mineral density (BMD) was also used to calibrate the Hologic to Prodigy 1. There was good agreement between instruments from the same manufacturer. As expected, BMD measured on the Prodigy was about 15% higher than the Hologic. Using standardized BMD to cross calibrate gave a mean difference of 3% at the lumbar spine. The limits of agreement following calibration are clinically significant, so it is not possible to apply a calibration to an individual subject for trending purposes, as the error is similar to the expected annual change in BMD, but can be used for cross calibration in clinical trials. The in vivo calibration gave better agreement than using standardized BMD. The phantom calibration was close to the in vivo calibrations at the spine, but not in some hip regions. When introducing a new instrument, a new baseline BMD has to be obtained for each subject.

Absorptiometry, Photon↗

PubMed2005

Computerized measurement of retinal blood vessel calibre: description, validation and use to determine the influence of ageing and hypertension.

OBJECTIVE: To validate a computer-based method for measuring the calibre of retinal blood vessels, and use it to determine the effects of ageing and arterial hypertension on the calibres of these vessels and on their ratio. METHODS: Digital eye fundus images covering a 50 degrees field and centred on the optic disc were obtained using a 540 nm filter. The boundaries of blood vessels crossing a series of circles concentric with the optic disc were located by an image analysis program; the calibres of vessels crossing the circles perpendicularly were determined automatically, the average arteriole and average vein calibres were calculated, and the arteriovenous ratio (AVR) was calculated as the ratio of these averages. The within-operator, between-operator and within-eye reliability of this method was investigated using images of 30 or 40 eyes; the effects of ageing on average vessel calibres and AVR using both eyes of each of 120 normotensive volunteers aged 10-69 years (60 males, 60 females); and the effects of arterial hypertension using a group of 54 hypertensive patients aged 50.9 +/- 13.9 years. RESULTS: Within-operator, between-operator and within-eye correlation coefficients for AVR were all better than 0.95, and the corresponding coefficients of variation were all better than 3%. The results of Bland Altman approach show a very good agreement. There were no significant differences between right and left eyes or between the sexes in either the normotensive or the hypertensive group. In the normotensive group, vein calibre was almost constant (111 +/- 6 microm), but arteriole calibre and AVR fell significantly from 96 +/- 6 microm and 0.870 +/- 0.046, respectively, in the second decade of life to 85 +/- 4 microm and 0.761 +/- 0.044 in the seventh decade. Arterial hypertension was not associated with changes in vein calibre, but was associated with decreases in arteriole calibre (from 91 +/- 7 microm among normotensive individuals to 84 +/- 2 microm) and AVR (from 0.816 +/- 0.056 to 0.755 +/- 0.027). Age significantly modulated the AVR-reducing influence of hypertension (P = 0.003). CONCLUSION: The proposed method of measuring retinal blood vessel calibres is reliable and precise (especially for the AVR). In this study, its results confirmed that increasing age and arterial hypertension are both associated with reductions in retinal arteriole calibre and AVR.

Adolescent↗

PubMed1999

Local calibration of international normalised ratio improves between laboratory agreement: results from the UK National External Quality Assessment Scheme. UK NEQAS (Blood Coagulation) Steering Committee.

In the present study we have performed local calibration of International Normalised Ratio (INR) measurement systems in a large series of laboratories. We assigned INRs to five lyophilised plasma calibrants, one prepared from normal plasma and four using plasma from warfarinised patients, using different International Reference Preparations for Thromboplastin. These five calibrants, and two lyophilised test plasmas were analysed by 349 centres using 60 different thromboplastin instrument combinations. Plasma calibrants were assigned INRs using the WHO reference thromboplastin RBT-90 or the European reference thromboplastin CRM 149R. Each participating centre determined PTs of the calibrants with their local system. These PTs were then used to construct a local calibration graph relating PT to INR. The PTs of test plasmas were converted directly into INR using the local calibration model and into INR using the conventional method. The overall medians of conventionally derived INRs of two test plasmas analysed in 349 centres were 2.50 and 3.10, compared to 2.47 and 3.04 after local calibration where RBT-90 was employed to assign INRs to calibrants. Use of CRM 149R to assign INRs to calibrants led to a significant (p<0.0001) increase in INR to 2.7 and 3.36 respectively. When results were grouped according to the thromboplastin employed, agreement between results with different reagents was improved by local calibration. There was a significant reduction (p<0.01) in the spread of results in different centres as indicated by a reduction in coefficient of variation.

Blood Coagulation↗

PubMed1997

Two-point calibration procedure of the forced oscillation technique.

The forced oscillation technique is usually calibrated by loading the measuring device with a known impedance. A correction function is calculated, relating the measured and reference impedances at each frequency. However, this one point calibration procedure does not account for transducer asymmetry. A procedure has previously been presented to circumvent this problem: in addition to one known reference impedance, the calibration was repeated with the system occluded (infinite impedance). The aim of the present study was to evaluate a variant of this procedure, in which instead of resorting to an extreme condition imposing high requirements on the flow measuring system, two reference loads of 4 and 50 hPal-1 s were measured, thus covering the range of impedances observed in children and infants (a two-point procedure). The calibration procedure was performed with these two impedances and evaluated with a third impedance of approximately 17 hPal-1 s. The results of three calibration procedures were compared: one-point, two-point and a previously reported calibration procedure. Impedances consisted of sintered glass and mesh wire screens mounted in glass or polyvinyl tubes. For low impedance values, in the range of 4 to 17 hPal-1 s, measured and predicted values were similar for the three calibration procedures at frequencies from 4-52 Hz, although with the one point calibration procedure there was some underestimation above 44 Hz. With the highest load, especially above 32 Hz, marked discrepancies between measured and predicted values were observed with the one-point calibration procedure and the previously reported calibration procedure. Under these circumstances the two-point procedure is preferred.

Calibration↗

PubMed1999

Comparison of calibration procedures for 192Ir high-dose-rate brachytherapy sources.

PURPOSE: To compare the efficacy of different calibration procedures for 192Ir high-dose-rate (HDR) brachytherapy sources and to determine their suitability in clinical practice. In addition the manufacturer's calibration is compared with our experimental measurements so that the accuracy of the source strength on the manufacturer certificate which is supplied with each new 192Ir source can be accessed. METHODS AND MATERIALS: We compared three types of calibration system: well-type chambers (HDR-1000 and SDS), cylindrical phantom, and plate phantom. The total number of measurements we obtained was 365. The number of sources used for the calibration procedure comparison was 20 and the number used for comparison with the manufacturer's calibration was 46. This study was made during the period 1989-1997. Also, Physikalisch-Technische Bundesanstalt (PTB) calibrated one of our sources using their PTB protocol so that the results could be compared with our own. RESULTS: The sensitivity of each system on scattering from the room walls was studied. It was found that different minimum lateral distances from the walls were required for the different systems tested: 15 cm and 25 cm for the well-type chambers, 75 cm for the cylindrical phantom, and 13 cm for the plate phantom. The minimum thickness required to reach phantom scattering saturation for the plate phantom setup is 24 cm. The influence of the applicator material used in the calibration setup was found to be 1.7% for the stainless steel dosimetry applicator compared to the plastic 5F applicator. The accuracy of source positioning within the applicator can lead to dosimetric errors of +/-1.2% for the radial distance of 8.0 cm used with both solid phantoms. The change in the response for both well-type chambers was only 0.1% for changes in the source position within +/-7.5 mm around the response peak. Good agreement was found between all dosimetry systems included in our study. Taking the HDR-1000 well-type chamber results as a reference, we observed percentage root mean square (RMS) values of 0.11% for the SDS well-type chamber, 0.44% for the cylindrical, and 0.60% for the plate phantom setup. A comparison of our results using the cylindrical phantom with those of the manufacturer showed a percentage RMS value of 3.3% with a percentage fractional error range of -13.0% to +6.0%. The comparison of our calibration results with those of PTB gave deviations less than 0.4% for all systems. CONCLUSIONS: Our results have shown that with careful use of all calibration system protocols an accurate determination of source strength can be obtained. However, the manufacturer's calibration is not accurate enough on its own, and it should be mandatory for clinics to always measure the source strength of newly delivered 192Ir brachytherapy sources. The influence of the applicator material, metal or plastic, should always be taken into account.

Brachytherapy↗

PubMed2003

Multivariate calibration standardization across instruments for the determination of glucose by Fourier transform near-infrared spectrometry.

The transfer of multivariate calibration models is investigated between a primary (A) and two secondary Fourier transform near-infrared (near-IR) spectrometers (B, C). The application studied in this work is the use of bands in the near-IR combination region of 5000-4000 cm(-)(1) to determine physiological levels of glucose in a buffered aqueous matrix containing varying levels of alanine, ascorbate, lactate, triacetin, and urea. The three spectrometers are used to measure 80 samples produced through a randomized experimental design that minimizes correlations between the component concentrations and between the concentrations of glucose and water. Direct standardization (DS), piecewise direct standardization (PDS), and guided model reoptimization (GMR) are evaluated for use in transferring partial least-squares calibration models developed with the spectra of 64 samples from the primary instrument to the prediction of glucose concentrations in 16 prediction samples measured with each secondary spectrometer. The three algorithms are evaluated as a function of the number of standardization samples used in transferring the calibration models. Performance criteria for judging the success of the calibration transfer are established as the standard error of prediction (SEP) for internal calibration models built with the spectra of the 64 calibration samples collected with each secondary spectrometer. These SEP values are 1.51 and 1.14 mM for spectrometers B and C, respectively. When calibration standardization is applied, the GMR algorithm is observed to outperform DS and PDS. With spectrometer C, the calibration transfer is highly successful, producing an SEP value of 1.07 mM. However, an SEP of 2.96 mM indicates unsuccessful calibration standardization with spectrometer B. This failure is attributed to differences in the variance structure of the spectra collected with spectrometers A and B. Diagnostic procedures are presented for use with the GMR algorithm that forecasts the successful calibration transfer with spectrometer C and the unsatisfactory results with spectrometer B.

Algorithms↗

PubMed2004

Cross-calibration of ionization chambers in proton and carbon beams.

The calibration coefficients of a parallel plate ionization chamber are examined by comparing the coefficients obtained through three methods: a calculation from a 60Co calibration coefficient, N(D, omega, 60Co), a cross-calibration of a parallel plate ionization chamber using a cylindrical ionization chamber at the plateau region of a mono-energetic beam and a cross-calibration of the chamber using a cylindrical chamber at the middle of the SOBP of the therapeutic beams. This paper also examines reference conditions for determining absorbed dose to water in the cases of therapeutic carbon and proton beams. In the dose calibration procedure recommended by IAEA, irradiation fields should be larger than 10 cm in diameter and the water phantom should extend by at least 5 cm beyond each side of the field. These recommendations are experimentally verified for proton and carbon beams. For proton beams, the calibration coefficients obtained by these three methods approximately agreed. For carbon beams, the calibration coefficients obtained by the second method were about 1.0% larger than those obtained by the third method, and the calibration coefficients obtained by cross-calibration using 290 MeV/u beams were 0.5% lower than those obtained using 400 MeV/u beams. The calibration coefficient obtained by the first method agreed roughly with the results obtained by SOBP beams.

Calibration↗

PubMed2003

Automated, real-time calibration of the respiratory inductance plethysmograph and its application in newborn infants.

Respiratory inductive plethysmography (RIP) is widely used in infants, children and adults. The technique is well accepted as it provides important qualitative information on the pattern of breathing, although its ability to record volume accurately was questioned due to calibration uncertainties. Existing calibration methods require two-position calibration, or patient cooperation in performing various breathing manoeuvres, or prolonged calibration paradigms. The disadvantages from calibration difficulties are even more pronounced in infants. We present a computer system that is capable of performing a single-posture, real-time RIP calibration during natural breathing and is suitable for use in newborns. The calibration algorithm is based on interactive, point-by-point calculations of maximal correlation between airflow at the mouth, Vao, and summed differentiated RIP signals. The quantities are calculated interactively at every sample point, and the process continues until stable results are reached and convergence criteria met. A graphic user interface was developed to assist in the rapid implementation and ease of use. Validation schemes were evaluated in 33 newborn infants against actual Vao. Calibration factors were obtained within 21 +/- 11 s with a mean correlation coefficient of 0.97 +/- 0.03. All RIP-derived values were similar to actual airflow signals, with error values ranging from 0.4 +/- 3.0% for respiratory rate to 1.8 +/- 7.3% for tidal volume. Calibration was found to be stable and reliable for up to 3.5 h and in changing sleep states. It is concluded that the new single-posture real-time RIP calibration system is safe and simple to use, and also quick, accurate and stable. The system was found to be suitable for use in newborns during natural breathing while asleep.

Automation↗

PubMed2001

Isotopic analogues as internal standards for quantitative analyses of drugs and metabolites by GC-MS--nonlinear calibration approaches.

In order to achieve accurate quantitation of drugs and metabolites (analytes) in complex matrices, 2H- (and less commonly 13C-) labeled analogues of the analytes are now routinely adapted as the internal standards (IS) using linear calibration models to fit data generated by selected ion monitoring gas chromatography-mass spectrometry (GC-MS) protocols. In this study, the effects of cross-contribution (contribution of the IS to the intensity of the ion designated for the analyte and vice versa) on the linearity of the calibration data are examined. Nonlinear approaches that may address this problem are also studied. Two ion pairs (one with least and one with significant cross-contribution) from each of the following analyte/IS pairs are used as the exemplar systems for this study: butalbital/13C4-butalbital, butalbital/2H5-butalbital, secobarbital/13C4-secobarbital, and secobarbital/2H5-secobarbital. Analyte/IS ion intensity ratios of a series of standard solutions are correlated with the analyte/IS concentration ratios using one-point, multiple-point (unweighted and weighted) linear, and hyperbolic functions. The one-point calibration approach produces excellent calibration results in treating data derived from ion pairs with no significant cross contribution. In cases where significant cross-contribution exists, results derived from the one-point approach show, as expected, significant deviations at both ends of the concentration range. With the cross-contribution phenomenon accounted for, the hyperbolic calibration model is clearly more effective in fitting calibration data at both the lower and higher analyte concentration ends, thus significantly lowering the detection limit and extending the calibration range to a higher level. However, the calibration range cannot be extended indefinitely. At the low concentration end, noise-to-signal ratio and the cross-contribution of the IS to the intensity of the ion designated for the analyte, however insignificant, will incrementally reduce the quality of the observed ion intensity and intensity ratio data. At the high concentration end, detection saturation and the cross-contribution of the analyte to the intensity of the ion designated for the IS, however insignificant, will incrementally decrease the "slope" of the calibration curve. Thus, acceptable sensitivity (increase in analyte/IS ion-pair intensity ratio per unit increase in analyte concentration) of the calibration curve will become the limiting factor.

Algorithms↗

PubMedDate not supplied

Prediction of aortoiliac stent graft length: comparison of a semiautomated computed tomography angiography method and calibrated aortography.

OBJECTIVE: The aim of this study is to compare multislice computed tomography (MSCT) in combination with a newly developed semiautomated software program with calibrated aortography in patients who are scheduled for endovascular aortic stent graft placement. METHODS: From November 2000 until December 2001, seven patients with an abdominal aortic aneurysm (AAA) underwent both calibrated aortography and MSCT for preoperative endovascular stent graft planning. Both studies were performed within 14 days. Further, length measurements were performed with a semiautomated computerized tomographic angiography (CTA) calibration method and a conventional calibrated aortography technique using three differently configured tubes with variable tortuosity. The AAA length measurements of the semiautomated CTA calibration method and the calibrated aortography were compared. RESULTS: Statistical analysis included linear regression analysis and revealed a probability value of 0.000381 and an r2 value of 0.93. Using phantoms, it is proven by the authors that the accuracy of the semiautomated CTA calibration method increases with increasing tortuosity when compared with the conventional calibrated aortography technique. CONCLUSIONS: Our preliminary results show that the semiautomated CTA calibration method has a potentially advantageous role in preoperative stent graft planning regarding the aortic length measurements and seems to be more accurate than calibrated aortography, especially in extremely tortuous vessels. Further studies have to be performed, however.

Aged↗

PubMed1993

Calibration of ion chambers for use in mammography.

There is at present no UK calibration service for ion chambers for mammography, where X-ray beams are produced from tubes having molybdenum targets and filters. This paper reports calibrations against a radiotherapy secondary standard (calibrated for beams from tungsten targets with aluminium filters) using beams from both types of target and filter. Two examples of the Radcal mammography dosimeter were found to have calibration factors which varied by less than 1% in molybdenum target beams from 30 to 40 kV. Differences between calibrations using the two types of X-ray beam did not exceed about 2%. All calibration factors were within about +/- 2% of 1.0. Errors are thought to be within +/- 3%. The results of an independent calibration of one of these dosimeters against a similar chamber calibrated by CEC are also reported. Calibrations of this kind can only be temporary expedients until adequate calibration facilities for mammography beams become available, but are nevertheless useful.

Calibration↗

PubMed2005

External Quality Assessment in The Netherlands: time to introduce commutable survey specimens. Lessons from the Dutch "Calibration 2000" project.

The performance of suitable secondary reference material for the use of trueness control of six routinely measured clinical enzymes in the Dutch External Quality Assessment (EQA) scheme is described. The reference material of choice was selected using the split-patient-sample between-field method (twin study) design as described in an earlier study of the Calibration 2000 project in The Netherlands. This material, which was proven to be commutable for all wet chemistry systems, was implemented as the national enzyme calibrator. It consisted of a cryo-protected lyophilised serum with additions of recombinant human enzymes. Various batches of the frozen version of this material without cryo-protection additive, called native EQA samples, were used in the general EQA scheme for performance evaluation. The results of Calibration 2000 calibrated and non-Calibration 2000 calibrated laboratories were compared for both the regular (spiked with non-human enzymes) and native EQA samples in terms of precision and bias with established reference method values for the native samples. The regular samples showed mean between-laboratory CV ranges for all six enzymes involved (low-high) of 5.5-10.3% for the non-calibrated users vs. 4.6-10.8% for the calibrated users. For the native samples these respective ranges were 5.2-9.9% vs. 2.2-4.9%. Without exception, the group of Calibration 2000 calibrated users showed the lowest bias against the reference method values. Regular EQA samples (spiked with non-human enzymes) showed poorer performance than native samples and are not suitable for accuracy assessment purposes, the main aim of EQA schemes. Native samples that are commutable should be used for trueness control in current EQA schemes.

Calibration↗