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PubMed1993

Factors influencing measurement of protein reflection coefficient by filtered volume technique.

In isolated perfused organs, the protein reflection coefficient (sigma) can be estimated by comparing increases in hematocrit (Hct) and protein concentration (CP) during transvascular fluid filtration. In this study, we developed an equation for sigma to examine the potential influences of perfusate leak, evaporation, and hemolysis-induced changes in red blood cell volume and perfusate water. We also performed experiments in isolated ferret lungs to quantitate the magnitude of these potential sources of error and the effects of free hemoglobin on measurements of CP. These studies demonstrated that 1) perfusate leak does not cause an error because its effects on changes in Hct and CP counteract each other; 2) evaporation causes an overestimation of sigma, but in our experiments this effect was small; 3) hemolysis-induced changes in red blood cell and perfusate water volumes may cause an over- or underestimation of sigma, but these effects are small; 4) overestimations of CP due to increasing free perfusate hemoglobin concentration can cause substantial overestimations of sigma; and 5) values of sigma calculated from previous equations and from our equation were virtually identical, suggesting that the assumptions necessary for the previous equations were not significant sources of error. In agreement with previous workers, we conclude that the most important potential source of error is hemolysis-induced increases in free perfusate hemoglobin.

Animals↗
PubMedDate not supplied

Assessment of blood pressure in hemodialysis patients.

It is self-evident that accurate measurement of blood pressure (BP) is essential for the diagnosis and treatment of hypertension. Patients on hemodialysis typically do not have their BP measured under standardized conditions, a source of error in the assessment of their BP. However, their are some unique sources of error involving interdialytic weight gain, occurrence of sleep apnea and consequent nocturnal hypertension, inability to take BP in both arms in patients who have hemodialysis angioaccess in the arm, and the white coat effect in these patients as well. Precise measurement of BP in hemodialysis patients requires interdialytic ambulatory BP monitoring. However, when ambulatory BP monitoring is not possible, BP obtained in the dialysis unit can be used in a qualitative sense for prediction of hypertension in these patients. A 2-week average predialysis BP of greater than 150/85 mmHg or a postdialysis BP of greater than 130/75 mmHg has at least 80% sensitivity in diagnosing hypertension. Specificity of at least 80% can be achieved if predialysis BP of greater than 160/90 mmHg or postdialysis BP of greater than 140/80 mmHg are used. However, poor agreement between hemodialysis unit BP and ambulatory BP precludes their use for the precise prediction of BP. Improving measurement techniques in the dialysis unit, averaging multiple BP values, using 20-minute postdialysis readings, or home BP monitoring can improve BP determination when interdialytic BP monitoring is not possible.

Blood Pressure Monitoring, Ambulatory↗
PubMed1981

A priori considerations when using laboratory determinations in cost-effectiveness and clinical decision analyses.

The incorporation of laboratory determinations into cost-effectiveness and clinical decision analyses can be weakened by failure to consider various sources of error in the generation and utilization of laboratory data. To improve these analyses, three phases in the process that converts a patient specimen into patient treatment are identified. Phase I examines quality of methodology and laboratory performance and identifies possible errors in generation of the analyte value. Phase II considers the process by which this value is compared to normal values; possible errors involved in normal value selection and the problem of false positives and false negatives are discussed. Phase III involves the interpretation of the values; possible errors in this phase are discussed. Recommendations are made to explicitly identify sources of error in all phases and thus strengthen decision analysis involving laboratory data.

Clinical Laboratory Techniques↗
PubMed1989

Quantitating error in blood flow measurements with radioactive microspheres.

Accurate determination of the reproducibility of measurements using the microsphere technique is important in assessing differences in blood flow to different organs or regions within organs, as well as changes in perfusion under various experimental conditions. The sources of error of the technique are briefly reviewed. In addition, we derived a method for combining quantifiable sources of error into a single estimate that was evaluated experimentally by simultaneously injecting eight or nine sets of microspheres (each with a different radionuclide label) into four anesthetized dogs. Each nuclide was used to calculate blood flow in 145-190 myocardial regions. We compared each flow determination (using a single nuclide label) with a weighted mean for the piece (based on the remaining nuclides). The difference was defined as "measured" error. In all, there were a total of 5,975 flow observations. We compared measured error with theoretical estimates based on the Poisson error of radioactive disintegration and microsphere entrapment, nuclide separation error, and reference flow error. We found that combined estimates based on these sources completely accounted for measured error in the relative distribution of microspheres. In addition, our estimates of the error in measuring absolute flows (which were established using microsphere reference samples) slightly, but significantly, underestimated measured error in absolute flow.

Animals↗
PubMed1991

Moving dipole inverse solutions using MEGs measured on a plane over the head.

Since magnetoencephalograms (MEGs) are measurements of the magnetic field produced in the air around the head by electrical sources in the brain, it is possible to measure MEGs on some regular surface over the head such as a plane. Such measurements are easier to make than traditional measurements at a fixed distance from the head. This paper presents results of computer modeling studies of source localization errors caused by using MEGs measured on a plane over the head. It was found that non-spherical head shape does not have a greater effect on localization accuracy for measurements on a plane than for traditional fixed-distance measurements. The source localization errors were less than 1 cm for both types of measurement for sources in the cortical region of the brain. Localization errors were found to increase for sources at greater depth in the brain, but the errors using measurements on a plane were not found to be significantly larger than those using traditional measurements. Hence, because of the ease with which measurements on a plane can be made, more wide-spread use of such measurements should be considered.

Biophysical Phenomena↗
PubMed1981

Some methodologic problems encountered in occupational health research.

The sources of error encountered in occupational health epidemiology present unique and difficult problems. Knowledge of these possible sources of error is essential for the proper design of studies interpretation of study results. This article describes important sources of bias in occupational health research including healthy worker selection, information bias and the differential distribution of workplace health services. The paper emphasizes that reference groups must be chosen with extreme care in order to avoid potential problems of healthy worker selection. In addition, knowledge of the process by which diseases are diagnosed is important in planning appropriate study designs.

Epidemiologic Methods↗
PubMed1997

The influence of skull-conductivity misspecification on inverse source localization in realistically shaped finite element head models.

The electric conductivities of different tissues are important parameters of the head model and their precise knowledge appears to be a prerequisite for the localization of electric sources within the brain. To estimate the error in source localization due to errors in assumed conductivity values, parameter variations on skull conductivities are examined. The skull conductivity was varied in a wide range and, in a second part of this paper, the effect of a nonhomogeneous skull conductivity was examined. An error in conductivity of lower than 20% appears to be acceptable for fine finite element head models with average discretization errors down to 3 mm. Nonhomogeneous skull conductivities, e.g., sutures, yield important mislocalizations especially in the vincinty of electrodes and should be modeled.

Brain↗
PubMedDate not supplied

Biological monitoring of occupational exposure to toxic chemicals. Collection, processing, and storage of specimens.

Exposure to at least 100 different chemicals may be estimated on an individual basis from their concentrations in blood or urine. The present document reviews sources of error in the collection, processing and storage of specimens for this biological monitoring. Physiological factors cause variation in the concentration of chemicals in the body fluids. Distribution of water depends on posture. Exercise and meals cause changes in blood constituents. The urine output varies and, thus, the concentrations of dissolved chemicals change. Many toxic chemicals show short half times in the blood; thus, their concentrations depend on the timing of the specimen collection. Skin absorption may result in dramatically different chemical concentrations in different parts of the circulation. The stability of chemicals in the collected specimens is generally limited: chemical deterioration, adsorption, precipitation, and evaporation are the main causes of losses. For many chemicals, especially for trace elements, contamination of the specimen is the overwhelmingly most important source of error. As the range of the chemicals measured is wide, the relative importance of the sources of error is different for different chemicals. Information on most chemicals is at present very limited. Thus, before commencing a program on biological exposure monitoring, it is advisable to search the optimal conditions for specimen collection, processing, and storage.

Adsorption↗
PubMed2002

Analysis of mechanical sources of patient alignment errors in radiation therapy.

Certain radiation treatments, such as conformal and intensity modulated treatments, involve isocentric treatment fields delivered using multiple angles or continuous angulation of the gantry, collimator and table. At our institution, treatments involving three angles (gantry, collimator, and table) can, if uncorrected, exhibit misalignments of 2 mm or more on premarked field centers and borders on the patient surface during the initial setup on a linear accelerator (linac), even though the linac operates within allowable mechanical tolerances. This paper is an analysis of three principal mechanical sources of patient alignment errors observed on linacs: (i) errors in table and gantry angle, (ii) displacement of gantry rotational axis during gantry rotation, and (iii) displacement between collimator and table rotational axes. On patient surfaces, these small, systematic mechanical errors can each be expected to produce misalignments of up to 1.5 mm, increasing to over 2 mm with nearly horizontal fields delivered at nonzero table angles onto highly oblique patient surfaces. For the underlying target volumes, the mechanical errors can, in combination, be expected to produce target volume misalignments of up to 1 mm on newly installed linacs and 3 mm on older linacs. Thus, 1 mm appears to be a mechanical limit on the positional precision of radiation treatments.

Artifacts↗
PubMed1987

Parathyroid imaging: comparison of 201Tl-99mTc subtraction scintigraphy, computed tomography and ultrasonography.

From 1982 to 1985, twenty-nine patients with suspected hyperparathyroidism were examined using 201Tl-99mTc subtraction scintigraphy (Tl-Tc), computed tomography (CT) and ultrasonography (US). For diagnosing neoplasm (adenoma or cancer), the sensitivities of the three procedures were 80 per cent or more, with no statistically significant differences. For diagnosing hyperplasia of the parathyroid glands, CT scan had the highest sensitivity (47 per cent). The most frequent source of error was minimally enlarged glands, weighing less than 500 mg. The second highest source of error was thyroid nodules, such as adenomatous goiter or cancer. Serum calcium and c-PTH levels were significantly higher in those with a parathyroid neoplasm than in those with hyperplasia (p less than 0.01, p less than 0.05, respectively). We concluded that hyperplasia is less easy to detect than neoplasm, and CT scan is superior to Tl-Tc or US scan for localizing hyperplasia.

Humans↗
PubMed1980

Analysis of errors in parameter estimation with application to physiological systems.

The accuracy of parameter estimation applied to physiological systems is analyzed. The method of analysis is applicable to procedures utilizing minimization of squared output error and a nonlinear dynamic system model. Three major sources of estimation error are described: 1) measurement error, 2) modeling error, and 3) optimization error. Measurement errors affect values used for the system output, the model input, and nonestimated parameters of the model. Modeling errors are due to failure to adequately describe the structure of the system and to numerical errors that occur in the digital computer solution of the model equations. Linearization by use of Taylor series expansions in the region of the nominal solution is used to obtain an expression for the covariance matrix of the parameter estimates in terms of the covariance matrix of each error source. The analysis is applied to the example of cardiac output estimation from respiratory measurements. The results demonstrate that an analysis of system identifiability is not sufficient to ensure usable estimates and that systematic error analysis is essential for assessing the usefulness of parameter estimation techniques.

Cardiac Output↗
PubMedDate not supplied

[Accuracy of the spatial localization of the sources of brain bioelectrical activity in a model homogeneous unrestricted environment].

The possibility is considered of use of the model of homogeneous unlimited medium (HUM) for localization of sources of brain bioelectrical activity at recording of electric potentials on its surface. It is shown that when the recording electrodes are arranged in accordance with 10-20 system, the source localization error does not exceed 10% of the head radius practically in any position of the source. A significant dependence is revealed of the source localization error on the concrete electrodes system used in experiment. It demands careful treatment of the available general recommendations on correction of the position of the source found by HUM model without consideration of the concrete arrangement of recording electrodes.

Brain↗
PubMed2005

Quantification of long chain polyunsaturated fatty acids by gas chromatography. Evaluation of factors affecting accuracy.

The accurate and reproducible analysis of long-chain polyunsaturated fatty acids (PUFA) is of growing importance. Especially for labeling purposes, clear guidelines are needed in order to achieve optimum accuracy. Since calibration standards cannot be used for method validation due to the instability of PUFAs, there is no direct way to check for the absence of systematic errors. In this study the sources of error that weaken the accuracy were evaluated using theoretical considerations and calibration standards with corrected composition. It was demonstrated that the key role for optimum accuracy lies in the optimization of the split injection system. Even when following the instructions outlined in the official methods of the American Oil Chemist's Society (AOCS), systematic errors of more than 7% can arise. Clear guidelines regarding system calibration and selection of appropriate internal standards (IS) can improve precision and accuracy significantly.

Chromatography, Gas↗
PubMed2004

Comparing air dispersion model predictions with measured concentrations of VOCs in urban communities.

Air concentrations of nine volatile organic compounds were measured over 48-h periods at 23 locations in three communities in the Minneapolis-St. Paul metropolitan area. Concentrations at the same times and locations were modeled using a standard regulatory air dispersion model (ISCST3). The goal of the study was to evaluate model performance by comparing predictions with measurements using linear regression and estimates of bias. The modeling, done with mobile and area source emissions resolved to the census tract level and characterized as model area sources, represents an improvement over large-scale airtoxics modeling analyses done to date. Despite the resolved spatial scale, the model did not fully capture the spatial resolution in concentrations in an area with a sharp gradient in emissions. In a census tract with a major highway at one end of the tract (i.e., uneven distribution of emissions within the tract), model predictions atthe opposite end of the tract overestimated measured concentrations. This shortcoming was seen for pollutants emitted mainly by mobile sources (benzene, ethylbenzene, toluene, and xylenes). We suggest that major highways would be better characterized as line sources. The model also failed to fully capture the temporal variability in concentrations, which was expected since the emissions inventory comprised annual average values. Based on our evaluation metrics, model performance was best for pollutants emitted mainly from mobile sources and poorest for pollutants emitted mainlyfrom area sources. Important sources of error appeared to be the source characterization (especially location) and emissions quantification. We expect that enhancements in the emissions inventory would give the greatest improvement in results. As anticipated for a Gaussian plume model, performance was dramatically better when compared to measurements that were not matched in space or time. Despite the limitations of our analysis, we found thatthe regulatory air dispersion model was generally able to predict space and time matched 48-h average ambient concentrations of VOC species within a factor of 2 on average, results that meet regulatory model acceptance criteria.

Air Movements↗
PubMed1983

Energy expenditure from doubly labeled water: some fundamental considerations in humans.

The isotopic loading dose and metabolic period for the measurement of energy expenditure in humans by the doubly labeled water method were predicted by a propagation of error analysis. Factors considered for sources of error were analytical errors in the mass spectrometric determination of isotopic enrichments, biological variation in the isotopic enrichments, uncertainties in total body water (TBW), evaporative water loss, and metabolic fuel. The predicted optimal isotopic loading doses were 0.3 g H2 18O/kg TBW and 0.12 g 2H2O/kg TBW for all subjects except neonates in which they were 0.4 g H2 18O/kg TBW and 0.16 g 2H2O/kg TBW. The optimal metabolic periods were 3 to 14 days in neonates and children, and 5 to 28 days in adults. The theoretical coefficient of variation of the doubly labeled water method for the measurement of energy expenditure is between 4 and 8%.

Adolescent↗
PubMed2004

Super-resolution reconstruction of compressed video using transform-domain statistics.

Considerable attention has been directed to the problem of producing high-resolution video and still images from multiple low-resolution images. This multiframe reconstruction, also known as super-resolution reconstruction, is beginning to be applied to compressed video. Super-resolution techniques that have been designed for raw (i.e., uncompressed) video may not be effective when applied to compressed video because they do not incorporate the compression process into their models. The compression process introduces quantization error, which is the dominant source of error in some cases. In this paper, we propose a stochastic framework where quantization information as well as other statistical information about additive noise and image prior can be utilized effectively.

Algorithms↗
PubMed1996

Sensing and tachyarrhythmia detection problems in implantable cardioverter defibrillators.

Sensing of cardiac activity and detection of tachyarrhythmias in implantable cardioverter defibrillators (ICDs) are complex functions and errors occur. Sources of sensing-detection errors include the variable nature of intracardiac electrograms, the occasional inability of automatically adjusting signal amplifiers to cope with this variability, problems with sensing leads, inappropriate programming, and limitations of tachyarrhythmia detection algorithms, which are optimized to avoid underdetection of ventricular tachyarrhythmias. Current ICDs vary considerably in details of sensing and detection function, programmability, and diagnostic data, so that a through knowledge of each device is necessary to diagnose and correct these problems. Stored intracardiac electrograms and/or marker channels available in most of these devices have contributed much to our understanding of sensing-detection errors. Undersensing of individual signals, most frequently due to signal variability and/or inability of the amplifier to adjust adequately, can lead to delay or failure of tachyarrhythmia detection. Delay or failure of tachyarrhythmia detection can also occur if algorithms to enhance specificity, such as sudden onset or rate stability, are utilized. Oversensing of T waves or noise can lead to false detection; however, the most common cause of false detection is the inability of current detection algorithms to distinguish supraventricular from ventricular tachyarrhythmias. New algorithms that incorporate atrial sensing, electrogram morphology analysis, or hemodynamic monitoring may result in improved detection accuracy of ICDs in the future.

Defibrillators, Implantable↗