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Results for “Measuring algorithm”

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At least 1,207 records · Page 67Linked to original sources

PubMed2001

Source apportionment of fine particulate matter by clustering single-particle data: tests of receptor model accuracy.

The source apportionment accuracy of a neural network algorithm (ART-2a) is tested on the basis of its application to synthetic single-particle data generated by a source-oriented aerosol processes trajectory model that simulates particle emission, transport, and chemical reactions in the atmosphere. ART-2a successfully groups particles from the majority of sources actually present, when given complete data on ambient particle composition at monitoring sites located near the emission sources. As particles age in the atmosphere, accumulation of gas-to-particle conversion products can act to disguise the source of the primary core of the particles. When ART-2a is applied to synthetic single-particle data that are modified to simulate the biases in aerosol time-of-flight mass spectrometry (ATOFMS) measurements, best results are obtained using the ATOFMS dual ion operating mode that simultaneously yields both positive and negative ion mass spectra. The results of this study suggest that the use of continuous single-particle measurements coupled with neural network algorithms can significantly improve the time resolution of particulate matter source apportionment.

Aerosols↗

PubMed2002

Measurement of tumor oxygenation using new frequency domain phosphorometers.

Oxygen dependent quenching of phosphorescence allows for non-invasive measurements of oxygen in tissue. We have designed and constructed a novel multi-frequency instrument for measurement of phosphorescence lifetimes and developed algorithms for determining the distribution of oxygen (oxygen histogram) in the microvasculature of tissue with good temporal resolution (Vinogradov et al., 2002, Compar. Biochem. A, these proceedings). This technology, in combination with a new water soluble near infra red phosphor (Oxyphor G2), was used to examine the oxygenation of subcutaneous Q7 tumors grown on the flank of Buffalo rats and their response to giving the rats oxygen or carbogen to breathe. Phosphorescence was measured using excitation at 635 nm and emission at >700 nm (the phosphorescence maximum is near 800 nm). The excitation and collection light guides were placed on the surface of the skin of the anesthetized animals separated by approximately 0.8 cm. A 6 x 6 or 7 x 7 grid (approx. 4 cm x 4 cm) was drawn on the flank and oxygen histograms were measured in each square, providing 'images' of the oxygen distribution in the tissue. This procedure determines the tissue oxygen distribution at each position in the grid. Regions of relative hypoxia (associated with the tumor) can be readily localized and the extent of hypoxia quantitatively evaluated.

Algorithms↗

PubMed2004

Audit on nuchal translucency thickness measurements in Flanders, Belgium: a plea for methodological standardization.

OBJECTIVES: To audit nuchal translucency thickness (NT) measurements for fetal aneuploidy screening in Flanders, and to estimate the impact of small variations in NT measurement on the screening result of two first-trimester screening algorithms: maternal age + NT (Algorithm A), and maternal age + NT + pregnancy associated plasma protein-A + free beta-human chorionic gonadotropin (Algorithm B). METHODS: We used the database of first-trimester combined screening, as collected by the General Medical Laboratory AML in Antwerp, Belgium, between 1 January 2001 and 1 April 2004. Audit was performed by establishing a delta-NT distribution curve for one trainee of The Fetal Medicine Foundation (FMF) and for a group of 263 other sonographers, in comparison with the FMF reference values. Risks for fetal aneuploidy were calculated at a cut-off value of 1 : 300 for Algorithm A and 1 : 150 for Algorithm B. These risks were recalculated in both algorithms after a modeled increase of all NT values by 0.1 or 0.2 mm. RESULTS: In a total of 592 measurements performed by the FMF trainee, the 5th, 50th and 95th percentiles of delta-NT measurements were at -0.41, +0.03 and +0.68 mm, respectively. These values were close to the FMF reference values. The screen-positive rate for this set of data was 4.4% (26/592) in both algorithms. For the 12 555 measurements of the 263 other sonographers, the 5th, 50th and 95th percentiles of delta-NT were at -0.81, -0.14 and +0.73 mm, respectively, which clearly indicates underestimation of NT in the lower range. In this set of data the screen-positive rate was 3.5% for both algorithms (439/12 555 for Algorithm A and 436/12 555 for Algorithm B). Also in this group, 5% (59/1186) of negative screening results at maternal age > or = 35 years in Algorithm A became positive after a modeled 0.1-mm increase in NT, whereas this was only in 1.2% (134/11 369) of tests at maternal age < 35 years (P < 0.0001). The overall increase of screen-positive rate in Algorithm A after an NT modification of +0.1 mm was 1.2% (152/12 555), significantly more than in Algorithm B (86/12 555; 0.7%) (P < 0.0001). CONCLUSION: In Flanders, there is a systematic underestimation of NT in comparison with the FMF reference range. Attempts to change these measurements according to the FMF criteria are crucial. This will mainly influence the screening results of women at advanced maternal age and of NT-based algorithms without the use of other parameters.

Adult↗

PubMed2004

Validation of a new ultrasound method for the measurement of carotid artery intima medial thickness and plaque dimensions.

BACKGROUND: Carotid ultrasound is an accepted method for the detection of subclinical atherosclerosis. Valid methods that allow quantitation of carotid atheroma burden may be useful for stratifying risk. OBJECTIVE: To validate the results of intima medial thickness (IMT) and plaque measurements using a newly created software algorithm by comparing them with those obtained using a previously validated method. METHODS: Carotid ultrasound videotapes (n=24) were analyzed by experienced observers using a validated method and a new method. Ultrasound parameters were compared by measuring the difference +/- SD to yield indexes of accuracy and precision. Performance was also assessed using correlation and Bland-Altman analyses. RESULTS: Average IMT (n=24), plaque area (n=46), and several indexes that integrate IMT and plaque measurements were all found to be comparable with measurements obtained using the previously validated method. For example, the plaque area showed excellent accuracy and precision (-0.17+/-2.0 mm2, P=0.56), excellent correlation (r=0.98, standard error of the estimate = 2.01 mm2, P<0.001) and no evidence of bias using Bland-Altman analyses (Spearman's rho = 0.04, P=0.82). CONCLUSIONS: A new algorithm for the quantitation of carotid atheroma burden yields results that are comparable with those of a previously validated and widely used method. Availability of valid tools for measuring carotid ultrasound should facilitate the incorporation of this procedure into clinical risk stratification paradigms.

British Columbia↗

PubMed1997

Predicting survival in heart failure case and control subjects by use of fully automated methods for deriving nonlinear and conventional indices of heart rate dynamics.

BACKGROUND: Despite much recent interest in quantification of heart rate variability (HRV), the prognostic value of conventional measures of HRV and of newer indices based on nonlinear dynamics is not universally accepted. METHODS AND RESULTS: We have designed algorithms for analyzing ambulatory ECG recordings and measuring HRV without human intervention, using robust methods for obtaining time-domain measures (mean and SD of heart rate), frequency-domain measures (power in the bands of 0.001 to 0.01 Hz [VLF], 0.01 to 0.15 Hz [LF], and 0.15 to 0.5 Hz [HF] and total spectral power [TP] over all three of these bands), and measures based on nonlinear dynamics (approximate entropy [ApEn], a measure of complexity, and detrended fluctuation analysis [DFA], a measure of long-term correlations). The study population consisted of chronic congestive heart failure (CHF) case patients and sex- and age-matched control subjects in the Framingham Heart Study. After exclusion of technically inadequate studies and those with atrial fibrillation, we used these algorithms to study HRV in 2-hour ambulatory ECG recordings of 69 participants (mean age, 71.7+/-8.1 years). By use of separate Cox proportional-hazards models, the conventional measures SD (P<.01), LF (P<.01), VLF (P<.05), and TP (P<.01) and the nonlinear measure DFA (P<.05) were predictors of survival over a mean follow-up period of 1.9 years; other measures, including ApEn (P>.3), were not. In multivariable models, DFA was of borderline predictive significance (P=.06) after adjustment for the diagnosis of CHF and SD. CONCLUSIONS: These results demonstrate that HRV analysis of ambulatory ECG recordings based on fully automated methods can have prognostic value in a population-based study and that nonlinear HRV indices may contribute prognostic value to complement traditional HRV measures.

Aged↗

PubMed2001

Sleep estimation from wrist movement quantified by different actigraphic modalities.

Progress in transducer design and empirical characterization of wrist movement has led to diverse wrist activity monitors, each with its unique features and modality of operation. This study compared sleep--wake estimates from nocturnal wrist activity quantified by different motion-quantifying algorithms. Healthy young adults wore an Actillume and a Mini Motionlogger on the same wrist while nocturnal polysomnography data were recorded simultaneously in the laboratory. Activity data were analyzed with ACTION3 using scoring algorithms independently calibrated for each measurement modality. Overall, each modality yielded accurate and reliable sleep estimates relative to polysomnographic estimates (agreement rates: 91.4--96.5%, correlations for sleep duration: 0.79--0.94). Estimates derived from Actillume modalities were comparable to those of Mini Motionloggers, suggesting that the transducers of these two devices performed comparably for monitoring sleep and wakefulness. Wrist movement quantified by the Mini Motionlogger proportional-integrating mode yielded the best accuracy for detection of sleep--wake states.

Adult↗

PubMed2001

Three-dimensional optic nerve head algorithm for the detection of glaucomatous damage.

PURPOSE: To determine whether a three-dimensional optic disc algorithm could be useful in differentiating optic nerve heads (ONH) with a normal visual field from those with an abnormal visual field also when using a simultaneous stereoscopic computerised ONH analyser. METHODS: One eye was randomly chosen from 45 normals and 55 patients with glaucoma (mean deviation -7.7 +/- 9.0 dB, corrected pattern standard deviation 3.1 +/- 2.3 dB). All the subjects were examined with the Humphrey Perimeter (program 30-2) and with the Topcon Image-net X Rev-3.51 b. Using the topographic map of the system, the algorithm of the third moment or cup shape measure was applied to the numbers (the number of points ranged from 623 to 1,883 depending on the size of the disc area) that identify the heights of all the points of the optic disc surface. Findings were analysed by means of the Mann-Whitney U test and receiver operator characteristic curves. RESULTS: The sensitivity and specificity of this three-dimensional ONH algorithm applied to Topcon Image-net was 90.6% and 85.1% respectively. No difference was found between the groups for age, disc area and number of the analysed points. CONCLUSIONS: The algorithm of cup shape measure or third moment is a useful parameter to separate eyes with normal from those with abnormal visual field also when using a simultaneous stereoscopic system such as Image-net.

Algorithms↗

PubMedDate not supplied

Problem-oriented prefetching for an integrated clinical imaging workstation.

Prefetching methods have traditionally been used to restore archived images from picture archiving and communication systems to diagnostic imaging workstations prior to anticipated need, facilitating timely comparison of historical studies and patient management. The authors describe a problem-oriented prefetching scheme, detailing 1) a mechanism supporting selection of patients for prefetching via characterizations of clinical problems, using multiple data sources (picture archiving and communication systems, hospital information systems, and radiology information systems), classifying patients into cohorts on the basis of their medical conditions (e.g., lung cancer); and 2) prefetching of multimedia data (imaging, laboratory, and medical reports) from clinical databases to enable the viewing of an integrated patient record. Preliminary evaluation of the prefetching algorithm using classic information retrieval measures showed that the system had high recall (100 percent), correctly identifying and retrieving data for all patients belonging to a target cohort, but low precision (50 percent). A key finding during testing was that the recall of the system was increased through the use of multiple data sources (compared with one data source), because of better patient descriptors. Medical problems and patient cohorts were more specifically defined by combining information from heterogeneous databases.

Algorithms↗

PubMed2006

A fluctuation method to quantify in vivo fluorescence data.

Quantitative in vivo measurements are essential for developing a predictive understanding of cellular behavior. Here we present a technique that converts observed fluorescence intensities into numbers of molecules. By transiently expressing a fluorescently tagged protein and then following its dilution during growth and division, we observe asymmetric partitioning of fluorescence between daughter cells at each division. Such partition asymmetries are set by the actual numbers of proteins present, and thus provide a means to quantify fluorescence levels. We present a Bayesian algorithm that infers from such data both the fluorescence conversion factor and an estimate of the measurement error. Our algorithm works for arbitrarily sized data sets and handles consistently any missing measurements. We verify the algorithm with extensive simulation and demonstrate its application to experimental data from Escherichia coli. Our technique should provide a quantitative internal calibration to systems biology studies of both synthetic and endogenous cellular networks.

Algorithms↗

PubMedDate not supplied

Investigation of an FFT-based correlation technique for verification of radiation treatment setup.

Electronic portal imaging devices that capture an image of a patient's treatment field electronically by means of a computer data acquisition system operating in real time are becoming available. This paper reports on a study of a field verification correlation algorithm that can compare each treatment portal image to an image of the correct treatment field positioning. The algorithm requires no human intervention or analysis of the images but rather uses fast Fourier transforms to produce a correlation distribution. The position and amplitude of the correlation distribution maximum were tested as objective measures of translational and rotational differences of subject positions between pairs of images. The concept was tested by using a prototype algorithm to obtain the correlation distributions for images of an Alderson Rando head phantom. Images of the phantom setup with various errors were compared with an image of the phantom in the initial, correct treatment position. Translations, in-plane and out-of-plane rotations, and combinations of translations and rotations were studied. The algorithm accurately measured translations. The value of the correlation distribution maximum was found to be a reasonable candidate for an alignment parameter for which tolerable error thresholds might be established.

Algorithms↗

PubMed1997

Cerebral hemoglobin and optical pathlength influence near-infrared spectroscopy measurement of cerebral oxygen saturation.

Near-infrared spectroscopy (NIRS) is a noninvasive optical technique to monitor cerebral oxygen saturation at the bedside. Despite its applicability, NIRS has had limited clinical use because of concerns about accuracy, noted by intersubject variability in slope and intercept of the line between NIRS- and weighted-average arterial-cerebrovenous saturation (SMO2). This study evaluated transcranial optical pathlength and cerebral hemoglobin concentration as sources for this intersubject variability. Experiments were performed in an in vitro brain model and in piglets. Optical pathlength and cerebral hemoglobin concentration were measured by time-resolved spectroscopy (TRS). NIRS and TRS were recorded in the model, as perfusate blood saturation was varied (0%-100%) at several hemoglobin concentrations, and in piglets, as SMO2 was varied (15%-90%) before and after hemodilution. In the model, hemoglobin concentration significantly altered the NIRS versus blood saturation line slope and intercept, as well as optical pathlength. In piglets (before hemodilution), there was significant intersubject variability in NIRS versus SMO2 line slope (0.73-1.4) and intercept (-24 to 36) and in transcranial optical pathlength (13.4-16 cm) and cerebral hemoglobin concentration (0.58-1.1 g/dL). By adjusting the NIRS algorithm with optical pathlength or cerebral hemoglobin measurements, intersubject variability in slope (0.9-1.2) and intercept (-9 to 18) decreased significantly. Hemodilution significantly changed NIRS versus SMO2 line slope and intercept, as well as transcranial optical pathlength and cerebral hemoglobin concentration (before versus after hemodilution: slope 0.9 vs 0.78, intercept 13 vs 19, pathlength 13.9 vs 15.6 cm, cerebral hemoglobin 0.98 vs 0.73 g/dL). By adjusting the NIRS algorithm with the cerebral hemoglobin measurements, slope and intercept remained unchanged by hemodilution. These data indicate that intersubject variability in NIRS originates, in part, from biologic variations in transcranial optical pathlength and cerebral hemoglobin concentration. Instruments to account for these factors may improve NIRS cerebral oxygen saturation measurements.

Animals↗

PubMed2001

Evaluation of voxel-based registration of 3-D power Doppler ultrasound and 3-D magnetic resonance angiographic images of carotid arteries.

Spatial registration and fusion of ultrasound (US) images with other modalities may aid clinical interpretation. We implemented and evaluated on patient data an automated retrospective registration of magnetic resonance angiography (MRA) carotid bifurcation images with 3-D power Doppler ultrasound (PD US) and indirectly with 3-D B-mode US. Volumes were initially thresholded to reduce the uncorrelated noise signals. The registration algorithm subsequently maximized the mutual information measure between the PD US and 3-D MRA via iterative simplex search to find best "rigid body" transformation. We rated the performance of the algorithm visually on (n = 5) clinical MRA and 3-D PD US datasets. We also evaluated quantitatively the effect of thresholding, initial misalignment of the paired volumes and the reproducibility registration. We investigated the effect of image artefacts by simulation experiments. Preregistration misalignments of up to 5 mm in the transaxial plane, up to 10 mm along the axis of the carotids and up to 40 degrees resulted in 107 of 110 successful registrations, with translational and rotational errors of 0.32 mm +/- 0.3 mm and 1.6 +/- 2.1 degrees. The algorithm was not affected by missing arterial segments of up to 8 mm in length. The average registration time was 4 min. We conclude that the algorithm could be applied to 3-D US PD and MRA data for automated multimodality registration of carotid vessels without the use of fiducials.

Algorithms↗

PubMed2000

Assessing the accuracy of prediction algorithms for classification: an overview.

We provide a unified overview of methods that currently are widely used to assess the accuracy of prediction algorithms, from raw percentages, quadratic error measures and other distances, and correlation coefficients, and to information theoretic measures such as relative entropy and mutual information. We briefly discuss the advantages and disadvantages of each approach. For classification tasks, we derive new learning algorithms for the design of prediction systems by directly optimising the correlation coefficient. We observe and prove several results relating sensitivity and specificity of optimal systems. While the principles are general, we illustrate the applicability on specific problems such as protein secondary structure and signal peptide prediction.

Algorithms↗

PubMed2005

Model-based color halftoning using direct binary search.

In this paper, we develop a model-based color halftoning method using the direct binary search (DBS) algorithm. Our method strives to minimize the perceived error between the continuous tone original color image and the color halftone image. We exploit the differences in how the human viewers respond to luminance and chrominance information and use the total squared error in a luminance/chrominance based space as our metric. Starting with an initial halftone, we minimize this error metric using the DBS algorithm. Our method also incorporates a measurement based color printer dot interaction model to prevent the artifacts due to dot overlap and to improve color texture quality. We calibrate our halftoning algorithm to ensure accurate colorant distributions in resulting halftones. We present the color halftones which demonstrate the efficacy of our method.

Algorithms↗

PubMed2000

Feasibility of a novel blood noise reduction algorithm to enhance reproducibility of ultra-high-frequency intravascular ultrasound images.

BACKGROUND: Ultra-high-frequency (40- to 50-MHz) intravascular ultrasound (IVUS) improves image quality compared with conventional 20- to 30-MHz IVUS. However, as the frequency of IVUS increases, high-intensity backscatter from blood components may cause visual difficulties in discrimination between the lumen and arterial wall structure. The purpose of this study was to evaluate the effect of a novel blood noise reduction algorithm (BNR) on quantitative coronary ultrasound measurements. METHODS AND RESULTS: IVUS studies using a 40-MHz transducer were performed in 35 patients with coronary artery disease. A total of 620 gray-scale images (310 pairs) were processed with and without the BNR, and lumen cross-sectional area (CSA) was determined by 2 independent observers. With the BNR, the intraobserver and interobserver correlation coefficients for lumen CSA were significantly improved (0.85 to 0.99 and 0.80 to 0.98, respectively). In the 270 images (135 pairs) in which vessel wall measurements were possible, the BNR significantly improved the intraobserver and interobserver correlation coefficients for plaque plus media CSA (0.83 to 0.99 and 0.76 to 0.97, respectively), whereas no influence was observed for external elastic membrane CSA (1.00 to 1.00 and 0.99 to 0.99, respectively). CONCLUSIONS: This study demonstrates the feasibility of this novel algorithm to reduce blood noise, thereby enabling accurate lumen border delineation and providing reproducible measurements of both the lumen and plaque plus media CSAs. Incorporating a digital BNR may serve as an important adjunct to ultra-high-frequency IVUS imaging for improving accurate quantitative evaluation of vessel dimensions.

Algorithms↗

PubMed2004

Online prediction of self-paced hand-movements from subthalamic activity using neural networks in Parkinson's disease.

The significance of local field potential (LFP) activity in the subthalamic nucleus (STN) of patients with Parkinson's disease is unclear. Here we show that it is possible to predict self-paced hand-movements from the oscillatory nature of the STN LFP on a trial-by-trial basis. To this end we used a neural networks' classification algorithm on features representing different measures of spectral activity. Our experiments were simulated to process online LFP signal recordings collected beforehand from macroelectrodes implanted in STN. With spectral features extracted via wavelet transformation, we were able to predict a voluntary hand-movement's onset with 95% sensitivity and 77% specificity. Most predictions were made over a second in advance of the movement. We conclude that oscillatory LFP activity in STN is directly or indirectly related to processes involved in motor preparation. The ability to predict movement in real-time may open up several experimental and therapeutic possibilities.

Action Potentials↗

PubMed1999

Scatter and attenuation measurements at distances greater than 10 cm from an 192Ir source.

Most algorithms developed by various investigators for use in brachytherapy treatment planning have typically been designed to calculate the dose within a 10 cm range of a radiation source. These algorithms predict the dose well at distances < 10 cm from the source but were not developed and should not be utilized to predict the dose at distances > 10 cm. On the contrary, treatment planning systems and manual calculations will produce erroneous results when dose points > 10 cm are calculated using these algorithms. The spread in the data generated by the above algorithms is 16% at 15 cm and 42% at 20 cm. Physical measurements were performed at distances between 5 and 50 cm from a high activity 192Ir source in water. The measured data correlated well with the predicted data from 5 to 10 cm, which had a 5% spread. Beyond 10 cm the measured data fell central to the range of the predicted data, with the spread of the predicted data increasing from 5% to 80% with increasing distance from the source. The measured data was fitted with a model incorporating a buildup factor and an attenuation factor. The best fit values are in reasonable agreement with those obtained by two of the investigators, Thomason and Tripathi.

Algorithms↗

PubMed2002

Mass and thermal diffusivity algorithms: reduced algorithms for mass and thermal diffusivity determinations.

Mass and thermal diffusivity measurements conducted on Earth are prone to contamination by uncontrollable convective contributions to the overall transport. Previous studies of mass and thermal diffusivities conducted on spacecraft have demonstration the gain in precision, and lower absolute values, resulting from the reduced convective transport possible in a low-gravity environment. We have developed and extensively tested real-time techniques for diffusivity measurements, where several measurements may be obtained on a single sample. This is particularly advantageous for low gravity research were there is limited experiment time. The mass diffusivity methodology uses a cylindrical sample geometry. A radiotracer, initially located at one end of the host is used as the diffusant. The sample is positioned in a concentric isothermal radiation shield with collimation bores located at defined positions along its axis. The intensity of the radiation emitted through the collimators is measured versus time with solid-state detectors and associated energy discrimination electronics. For the mathematical algorithm that we use, only a single pair of collimation bores and detectors are necessary for single temperature measurements. However, by employing a second, offset, pair of collimation holes and radiation detectors, diffusivities can be determined at several temperatures per sample. For thermal diffusivity measurements a disk geometry is used. A heat pulse is applied in the center of the sample and the temperature response of the sample is measured at several locations. Thus, several values of the diffusivity are measured versus time. The exact analytic solution to a heat pulse in the disk geometry leads to a unique heated area and measurement locations. Knowledge of the starting time and duration of he heating pulse is not used in the data evaluation. Thus, this methodology represents an experimentally simpler and more robust scheme.

Journal Article↗