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Results for “Error minimization”

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At least 523 records · Page 29Linked to original sources

PubMed2001

Multi-isocenter stereotactic radiotherapy: implications for target dose distributions of systematic and random localization errors.

PURPOSE: This investigation examined the effect of alignment and localization errors on dose distributions in stereotactic radiotherapy (SRT) with arced circular fields. In particular, it was desired to determine the effect of systematic and random localization errors on multi-isocenter treatments. METHODS AND MATERIALS: A research version of the FastPlan system from Surgical Navigation Technologies was used to generate a series of SRT plans of varying complexity. These plans were used to examine the influence of random setup errors by recalculating dose distributions with successive setup errors convolved into the off-axis ratio data tables used in the dose calculation. The influence of systematic errors was investigated by displacing isocenters from their planned positions. RESULTS: For single-isocenter plans, it is found that the influences of setup error are strongly dependent on the size of the target volume, with minimum doses decreasing most significantly with increasing random and systematic alignment error. For multi-isocenter plans, similar variations in target dose are encountered, with this result benefiting from the conventional method of prescribing to a lower isodose value for multi-isocenter treatments relative to single-isocenter treatments. CONCLUSIONS: It is recommended that the systematic errors associated with target localization in SRT be tracked via a thorough quality assurance program, and that random setup errors be minimized by use of a sufficiently robust relocation system. These errors should also be accounted for by incorporating corrections into the treatment planning algorithm or, alternatively, by inclusion of sufficient margins in target definition.

Algorithms↗

PubMed2003

Improving mass measurement of coronary artery calcification using threshold correction and thin collimation in multi-detector row computed tomography: in vitro experiment.

RATIONALE AND OBJECTIVES: This in vitro study was designed to improve the accuracy of coronary calcium mass measurement from computed tomography (CT) images by developing threshold-based calcium CT number correction and thin-slice spiral techniques. MATERIALS AND METHODS: A cardiac CT phantom containing simulated calcified cylinders of known calcium density was scanned with sequential 4 x 2.5-, spiral 16 x 1.5-, and spiral 16 x 0.75-mm collimation on 4- and 16-detector row CT scanners. The images obtained from the spiral scans were reconstructed in various slice widths. The calcified cylinders were imaged and their mean CT number and size were measured at thresholds ranging from 50 to 390 Hounsfield unit. The calcium mass measured was compared with the actual value to determine errors, and threshold-based correction factors were derived to minimize the errors. RESULTS: The minimum amount of measurable calcium in 1-mm cylinder was 0.3 mg at the 16 X 1.5-mm protocol and 0.2 mg at the 16 x 0.75-mm protocol. Compared with 2.5-mm sequential protocol, thin-slice spiral protocols yielded a higher radiation exposure and lower or similar image noise levels. The error in calcium mass after correction was significantly smaller than that in measured mass (P < .0001) and was consistent between the imaging protocols (P = .49). The accuracy of mass measurements was clearly improved by using thin-slice imaging protocols especially in 200-mg/cm3 calcium density (P < .0001). CONCLUSION: The accuracy of calcium mass CT measurement can be improved by threshold-based calcium CT number correction and thin collimation spiral techniques.

Calcinosis↗

PubMed1981

The effect of response bias on the odds ratio.

The effect of response bias on odds ratio results was determined based on data from a population-based cardiovascular disease survey. The study subjects consisted of 5000 adult residents of a predominantly white, upper-middle class community. Information from 60% of the 1100 non-respondents was obtained by telephone. Consistent patterns of participation associated with risk factors and diseases under study were found. A simple error term was developed to convert the odds ratio for respondents to the odds ratio for the target population using individual cell response rates. This error term demonstrates that the response patterns found tended to minimize the error in odds ratio calculations for respondents. Only by obtaining relevant information on non-respondents can investigators accurately estimate response bias and its effects on the odds ratio.

Adult↗

PubMed2002

Phase-stepping interferometry: methods for reducing errors caused by camera nonlinearities.

Phase errors that arise in phase-stepping interferometry are discussed. Investigations were performed by use of a Twyman-Green interferometer equipped with a compensation plate with a variable and servo-controlled tilt angle. With this instrument, phase-stepping errors can be reduced to a negligible level. There are, however, phase errors that are caused by camera nonlinearities. Two methods for minimizing these errors are presented. The first method is based on the simple idea that the interference intensity at the output of a two-beam interferometer has an exact cosine shape. The camera signals were monitored as a function of the tilt angle of the compensation plate, and the deviation from the cosine form was used to produce a correction. The second method is based on the idea that, under specific conditions, errors of an average of two phase measurements may compensate for each other. Numerical calculations were performed and give evidence of this hypothesis. Each method, the signal-correction and the averaging method, drastically reduces errors in evaluation of phases. The combination of both methods is a powerful tool that allows precise phase data to be obtained with an uncertainty, in the range lambda/2000 approximately 0.3 nm, that is caused mainly by signal noise.

Journal Article↗

PubMed1992

Incomplete suppression of hepatic glucose production in non-insulin dependent diabetes mellitus measured with [6,6-2H2]glucose enriched glucose infusion during hyperinsulinaemic euglycaemic clamps.

We have minimized methodological errors in the isotope dilution technique by using stable isotope, [6,6-2H2]glucose, thus avoiding the problem of contamination of tritiated glucose tracers and, by maintaining a constant plasma tracer enrichment have reduced error due to mixing transients. Using these modifications we have calculated hepatic glucose production in 20 patients with non-insulin-dependent diabetes mellitus during low (1 mU kg-1 min-1) and high (8 mU kg-1 min-1) dose insulin infusions. Mean fasting hepatic glucose production was 14.2 +/- 0.8 mumol kg-1 min-1. This suppressed by only 68% to 4.6 +/- 0.8 mumol kg-1 min-1 during the low-dose insulin infusion (plasma insulin 0.85 +/- 0.05 nmol l-1) and did not suppress further during the high-dose insulin infusion (plasma insulin 14.55 +/- 0.83 nmol l-1). Hepatic glucose production was significantly higher than zero throughout the study. Thus, we have found that minimization of known errors in the isotope dilution technique results in physiologically plausible and significantly positive values for hepatic glucose production indicating that the liver is resistant to insulin in patients with non-insulin-dependent diabetes mellitus.

Adult↗

PubMed2005

Contribution of reference frames for movement planning in peripersonal space representation.

The principal goal of our study is to gain an insight into the representation of peripersonal space. Two different experiments were conducted in this study. In the first experiment, subjects were asked to represent principal anatomical reference planes by drawing ellipses in the sagittal, frontal and horizontal planes. The three-dimensional hand-drawing movements, which were achieved with and without visual guidance, were considered as the expression of a cognitive process per se: the peripersonal space representation for action. We measured errors in the spatial orientation of ellipses with regard to the requested reference planes. For ellipses drawn without visual guidance, with eyes open and eyes closed, orientation errors were related to the reference planes. Errors were minimal for sagittal and maximal for horizontal plane. These disparities in errors were considerably reduced when subjects drew using a visual guide. These findings imply that different planes are centrally represented, and are characterized, by different errors when subjects use a body-centered frame for performing the movement and suggest that the representation of peripersonal space may be anisotropic. However, this representation can be modified when subjects use an environment-centered reference frame to produce the movement. In the second experiment, subjects were instructed to represent, with eyes open and eyes closed, sagittal, frontal and horizontal planes by pointing to virtual targets located in these planes. Disparities in orientation errors measured for pointing were similar to those found for drawing, implying that the sensorimotor representation of reference planes was not constrained by the type of motor tasks. Moreover, arm postures measured at pointing endpoints and at comparable spatial locations in drawing are strongly correlated. These results suggest that similar patterns of errors and arm posture correlation, for drawing and pointing, can be the consequence of using a common space representation and reference frame. These findings are consistent with the assumption of an anisotropic action-related representation of peripersonal space when the movement is performed in a body-centered frame.

Adult↗

PubMed2004

Determination of the isoelectric point of proteins by capillary isoelectric focusing.

Different ways of determining isoelectric points (pI) of proteins in capillary isoelectric focusing are reviewed here. Due to the impossibility of direct pH measurements in the liquid phase, such assessments have to rely on the use of pI markers. Different types of pI markers have been described: dyes, fluorescently labelled peptides, sets of proteins of known pI values. It appears that, perhaps, the best system is a set of 16 synthetic peptides, trimers to hexamers, made to contain each a Trp residue for easy detection at 280 nm. By a careful blend of acidic (Asp, Glu), mildly basic, with pK around neutrality (His), and basic (Lys, Arg) amino acids, it is possible to obtain a series of pI markers with pI values quite evenly distributed along the pH scale, possessing good buffering capacity and conductivity around their pI values and thus focusing as sharp peaks. Another approach to pI determination is the monitoring of the current during mobilization: this allows, with the aid of known pI markers, to calibrate the system with a pI/current graph. Pitfalls and common errors in pI determinations are reviewed here and guidelines given for minimizing such errors in pI estimation.

Chemical Phenomena↗

PubMed1998

Linac scalpel radiosurgery at the University of Florida.

Optimal implementation of stereotactic radiosurgery requires multidisciplinary input from neurosurgeons, radiation oncologists, and physicists. Clinical processes of most importance to the physics staff include stereotactic imaging, treatment planning, and radiation delivery. Careful attention to each of these details helps to ensure the quality of the overall process. Here we provide a practical review of the clinical processes involved in linac scalpel radiosurgery. The linac scalpel system is a linear-accelerator-based radiosurgery system that was developed at the University of Florida. It has been used at the University of Florida to treat more than 1000 patients since 1988. The aim of the linac scalpel system is to minimize all possible uncertainties in imaging and treatment delivery. Once these errors are minimized, truly conformal treatment plans can be generated and delivered with confidence, allowing clinicians to focus solely on the patient's problem. By following practical examples of this well established system, many pitfalls in the clinical process can be avoided.

Cerebral Angiography↗

PubMed1977

Strong consistency of least-squares estimates in regression models.

A general theorem on the limiting behavior of certain weighted sums of i.i.d. random variables is obtained. This theorem is then applied to prove the strong consistency of least-squares estimates in linear and nonlinear regression models with i.i.d. errors under minimal assumptions on the design and weak moment conditions on the errors.

Journal Article↗

PubMed1992

Biomarkers in occupational cancer epidemiology: considerations in study design.

Epidemiologic studies of occupational groups have been central to the identification of human carcinogens. The incorporation of a biochemical component into occupational studies of cancer can expand the possibilities for identifying human carcinogens and for understanding the disease process. Two epidemiologic studies of occupation and cancer which include evaluation of biomarkers are described. The association of acetylator phenotype with bladder cancer risk was studied in benzidine-exposed workers. The association of benzene-related leukopenia with leukemia is being studied in benzene-exposed workers. These investigations illustrate issues in the use of biomarkers in epidemiologic studies of cancer risk. Such studies require the identification and characterization of the population at risk. Disease susceptibility factors are amenable for inclusion in these studies and can be statistically modeled as exposure-effect modifiers. Biomarkers of exposure are mainly of importance in short-term longitudinal and cross-sectional studies of exposure and intermediate outcomes and for validation of other data sources. Several sources of error can affect the results of molecular epidemiologic studies. Aside from minimizing laboratory error, consideration must be given in the design and execution of these studies to potential problems in subject selection and field collection of biologic samples and other relevant data.

Benzene↗

PubMed2003

A hybrid 3D watershed algorithm incorporating gradient cues and object models for automatic segmentation of nuclei in confocal image stacks.

BACKGROUND: Automated segmentation of fluorescently-labeled cell nuclei in 3D confocal microscope images is essential to many studies involving morphological and functional analysis. A common source of segmentation error is tight clustering of nuclei. There is a compelling need to minimize these errors for constructing highly automated scoring systems. METHODS: A combination of two approaches is presented. First, an improved distance transform combining intensity gradients and geometric distance is used for the watershed step. Second, an explicit mathematical model for the anatomic characteristics of cell nuclei such as size and shape measures is incorporated. This model is constructed automatically from the data. Deliberate initial over-segmentation of the image data is performed, followed by statistical model-based merging. A confidence score is computed for each detected nucleus, measuring how well the nucleus fits the model. This is used in combination with the intensity gradient to control the merge decisions. RESULTS: Experimental validation on a set of rodent brain cell images showed 97% concordance with the human observer and significant improvement over prior methods. CONCLUSIONS: Combining a gradient-weighted distance transform with a richer morphometric model significantly improves the accuracy of automated segmentation and FISH analysis.

Algorithms↗

PubMed1979

Gross errors made by routine blood sampling from two sites using a tourniquet applied at different positions.

Tourniquet application in routine blood sampling procedures may induce considerable haemoconcentration and thereby augment the concentration of serum protein and protein-bound substances. To evaluate this effect serum protein, calcium and magnesium were measured in 31 healthy persons before and after 3-min venous stasis induced by a standardized tourniquet. Four sites of tourniquet and two sampling sites on the arm were compared. The concentration of measured serum constituents rose after the stasis period (5--13% rise in serum protein), the changes being similar at all tourniquet positions. However, blood sampled from a cubital arm vein showed significantly larger haemoconcentration than samples obtained simultaneously more distally on the arm. Taking three samples in a series, the serum concentration of protein, calcium and magnesium rose from tube to tube (5--10% change). The study demonstrates that neither use of a standardized tourniquet nor release of the tourniquet before blood sampling eliminates the errors inherent in routine venepuncture. Adjustment to a constant serum protein level minimizes these errors.

Adolescent↗

PubMed1992

Determination of glucose and ethanol effective diffusion coefficients in Ca-alginate gel.

Glucose and ethanol diffusion coefficients in 2% Ca-alginate gel were measured using the experimental technique based on solute diffusion into or out of gel beads in a well-stirred solution. The aim of the study was to make the measurements under typical conditions found in alcoholic fermentations, such as the concentrations of glucose (100 g l-1) and ethanol (50 g l-1), the simultaneous counter-diffusion of glucose and ethanol, and the presence of cells in the gel beads at a level of 10(9) cells g-1 of beads. Previously, an evaluation of the error associated with the methodology used indicated how the experimental procedure would minimize the error. The individual measurement of glucose and ethanol coefficients in 2% Ca-alginate with no cells gave values of 5.1 and 9.6 x 10(-6) cm2 s-1, respectively, which are lower than those in water. When the effect of counter-diffusion was investigated, both coefficients decreased: glucose by 14% and ethanol by 28%. When cells were incorporated into the beads, only the ethanol coefficient decreased significantly, while the glucose coefficient apparently increased its value to 6.9 10(-6) cm2 s-1.

Alginates↗

PubMed2006

Head movement control in visually guided tasks: postural goal and optimality.

This work investigates the control of horizontal head movements in the context of unconstrained visually guided head and arm/finger aiming tasks. In a first experiment, the head was free to move while gaze was directed at randomly presented eccentric targets distributed horizontally (0 degrees-120 degrees) at eye level. In a second experiment, the horizontal head orientation was constrained to predetermined positions (0 degrees, 15 degrees, 30 degrees, 45 degrees or 60 degrees rightward) while the right index finger aimed at targets with the arm fully extended. Kinematics of head movements in gaze displacements exhibits an initial component weakly correlated with target position, followed by multiple corrections. Since the eyes are assumed to already be aimed at the target when the corrections occur, it is suggested that one goal of head movement control is to achieve a desired final orientation (posture). This hypothesis is supported by results from the second experiment that reveal an association between eye/head orientation angles and errors exhibited in the visuo-spatial representation of the environment. The minimization of error then underlies the control of head movement as a postural response optimized for a given target and task condition.

Adult↗

PubMed2005

Competitive spiking and indirect entropy minimization of rate code: efficient search for hidden components.

Our motivation, which originates from the psychological and physiological evidences of component-based representations in the brain, is to find neural methods that can efficiently search for structures. Here, an architecture made of coupled parallel working reconstruction subnetworks is presented. Each subnetwork utilizes non-negativity constraint on the generative weights and on the internal representation. 'Spikes' are generated within subnetworks via winner take all mechanism. Memory components are modified in order to directly minimize the reconstruction error and to indirectly minimize the entropy of the spike rate distribution, via a combination of a stochastic gradient search and a novel tuning method. This tuning dynamically changes the learning rate: the higher the entropy of the spike rate, the higher the learning rate of the gradient search in the subnetworks. This method effectively reduces the search space and increases the escape probability from high entropy local minima. We demonstrate that one subnetwork can develop localized and oriented components. Coupled networks can discover and sort components into the subnetworks; a problem subject to combinatorial explosion. Synergy between spike code and rate code is discussed.

Action Potentials↗

PubMed2001

Screening and surveillance for complications related to gastroesophageal reflux disease.

Gastroesophageal reflux disease (GERD) and columnar-lined esophagus with intestinal metaplasia (Barrett's esophagus) are the major recognized risk factors for adenocarcinoma of the esophagus. The American College of Gastroenterology recommends that patients with long-standing GERD symptoms (particularly those 50 years of age or older) undergo endoscopic screening to identify Barrett's esophagus and that those patients who have Barrett's esophagus undergo regular endoscopic surveillance. These recommendations are made with the expectation that screening and surveillance will decrease mortality from esophageal cancer, although this association is unclear. Nonetheless, retrospective studies have shown that endoscopic surveillance can detect some early, curable neoplasms in patients with Barrett's esophagus. Dysplasia in Barrett's esophagus is widely regarded as the precursor of invasive malignancy. Although grading dysplastic changes is largely subjective, dysplasia remains the most appropriate biomarker for clinical evaluation of Barrett's esophagus. Flow-cytometric and p53 abnormalities may be earlier and more specific markers for cancer development, but application of these abnormalities is not yet recommended for clinical practice. Endoscopic surveillance also is adversely affected by biopsy sampling error. Techniques that may minimize biopsy sampling error include chromoendoscopy, endosonography, optical coherence tomography, and fluorescence detection techniques. Further studies are needed to clearly define the role of these techniques in surveillance, and none is practical for routine clinical use at this time. Although not specifically recommended, experimental ablative therapies, such as photodynamic therapy, can be considered by physicians for their patients with high-grade dysplasia in Barrett's esophagus, if they are provided as part of an established, approved research protocol.

Adenocarcinoma↗

PubMed2002

Application of the 80/20 rule in safeguarding the use of high-alert medications.

Critical care units are busy, complicated settings where the margins of error are narrow and the challenges to patient safety are ever present. Applying the 80/20 rule, front-line nurses can reduce medication errors by focusing on the safe use of "high-alert" medications. There are three primary principles that practitioners can use for safeguarding against medication errors that may result from high-alert drugs. These include: reducing or eliminating the possibility of errors, making errors visible, and minimizing the consequences of errors. These principles constitute a framework of safety that guides the development of proactive error reduction strategies.

Critical Care↗

PubMed2000

End points of planar reaching movements are disrupted by small force pulses: an evaluation of the hypothesis of equifinality.

A single force pulse was applied unexpectedly to the arms of five normal human subjects during nonvisually guided planar reaching movements of 10-cm amplitude. The pulse was applied by a powered manipulandum in a direction perpendicular to the motion of the hand, which gripped the manipulandum via a handle at the beginning, at the middle, or toward the end the movement. It was small and brief (10 N, 10 ms), so that it was barely perceptible. We found that the end points of the perturbed motions were systematically different from those of the unperturbed movements. This difference, dubbed "terminal error," averaged 14.4 +/- 9.8% (mean +/- SD) of the movement distance. The terminal error was not necessarily in the direction of the perturbation, although it was affected by it, and it did not decrease significantly with practice. For example, while perturbations involving elbow extension resulted in a statistically significant shift in mean end-point and target-acquisition frequency, the flexion perturbations were not clearly affected. We argue that this error distribution is inconsistent with the "equilibrium point hypothesis" (EPH), which predicts minimal terminal error is determined primarily by the variance in the command signal itself, a property referred to as "equifinality." This property reputedly derives from the "spring-like" properties of muscle and is enhanced by reflexes. To ensure that terminal errors were not due to mid-course voluntary corrections, we only accepted trials in which the final position was already established before such a voluntary response to the perturbation could have begun, that is, in a time interval shorter than the minimum reaction time (RT) for that subject. This RT was estimated for each subject in supplementary experiments in which the subject was instructed to move to a new target if perturbed and to the old target if no perturbation was detected. These RT movements were found to either stop or slow greatly at the original target, then re-accelerate to the new one. The average latency of this second motion was used to estimate the voluntary RT for each subject (316 ms mean). Additionally, we found that the hand neither exerted target-oriented force against the handle nor drifted toward the desired end point just before coming to rest, making it unlikely that the mechanical properties of the manipulandum prevented the hand from reaching its intended target.

Adaptation, Physiological↗