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Results for “Measuring algorithm”

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At least 721 records · Page 40Linked to original sources

PubMed2005

A comparison of a similarity-based and a feature-based 2-D-3-D registration method for neurointerventional use.

Two-dimensional (2-D)-to-three-dimensional (3-D) registration can improve visualization which may aid minimally invasive neurointerventions. Using clinical and phantom studies, two state-of-the-art approaches to rigid registration are compared quantitatively: an intensity-based algorithm using the gradient difference similarity measure; and an iterative closest point (ICP)-based algorithm. The gradient difference approach was found to be more accurate, with an average registration accuracy of 1.7 mm for clinical data, compared to the ICP-based algorithm with an average accuracy of 2.8 mm. In phantom studies, the ICP-based algorithm proved more reliable, but with more complicated clinical data, the gradient difference algorithm was more robust. Average computation time for the ICP-based algorithm was 20 s per registration, compared with 14 min and 50 s for the gradient difference algorithm.

Algorithms↗

PubMed2003

A new framework of image reconstruction from fan beam projections.

In this paper, we present a unified framework to reconstruct images for both fan beam and cone beam projections. The important feature of our theoretical framework is that it does not depend on the classical concept of the Radon transform at all. This property allows us to directly generalize the ideas and techniques developed in this paper to the cone beam reconstruction problem. In this paper, we extract such a framework from developing a new image reconstruction scheme from fan beam projections. Our new scheme also provides us new understanding of fan beam reconstruction problem. Our main results for the fan beam reconstruction are the following: First, we derive a general reconstruction scheme, in which the data sufficiency condition is transparently revealed by the reconstruction formula. Specifically, the data sufficiency condition for an accurate reconstruction of a region of interest (ROI) is that all the lines passing through the ROI must intersect the source trajectory at least once. Second, we further simplify the general reconstruction scheme by following three major steps of our new framework: (i) using symmetries of intermediate function; (ii) handling the data redundancy; (iii) changing discrete summation over the possible focal points into an integral along the source trajectory. After these steps, we obtain a new filtered backprojection algorithm. The key characteristic of this new algorithm is to take derivative of measured data with respect to the trajectory parameter. In practice, we can trade this derivative to some other continuous functions. In the configuration of a circular source trajectory with a third generation arc/collinear detector, we demonstrate how to remove the undesirable derivative of measured projection data. It results in a new algorithm for the sequential reconstruction of a ROI with a general normalized weighting function.

Algorithms↗

PubMed2004

Algorithms for calculation of kinetic parameters from T1-weighted dynamic contrast-enhanced magnetic resonance imaging.

PURPOSE: To quantify the errors involved in calculating dynamic parameters (K(trans) and ve) from dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) scans, and to develop alternative analyses to improve accuracy or increase processing speed. MATERIALS AND METHODS: This paper presents three different ways of handling the discrete samples of the arterial input and tissue response data with increasing fidelity, with which this continuous arterial input function (AIF) is represented. Also, a new noniterative approach to parameter estimation was developed from one used previously for analysis of radioactive tracer concentrations in radioangiography. The analysis methods were tested using simulated data. RESULTS: The more sophisticated schemes for data processing give more accurate parameter estimates when data are sparsely sampled, at least for the AIF that we modeled. The noniterative algorithm is very rapid in execution, but was more susceptible to measurement errors. CONCLUSION: The improved algorithms presented should be useful when the AIF and tissue response are sparsely sampled. The noniterative approach may be suitable for semiquantitative visualization, or where the AIF and tissue response are sampled accurately and with a small time interval between samples.

Algorithms↗

PubMedDate not supplied

EM-REML estimation of covariance parameters in Gaussian mixed models for longitudinal data analysis.

This paper presents procedures for implementing the EM algorithm to compute REML estimates of variance covariance components in Gaussian mixed models for longitudinal data analysis. The class of models considered includes random coefficient factors, stationary time processes and measurement errors. The EM algorithm allows separation of the computations pertaining to parameters involved in the random coefficient factors from those pertaining to the time processes and errors. The procedures are illustrated with Pothoff and Roy's data example on growth measurements taken on 11 girls and 16 boys at four ages. Several variants and extensions are discussed.

Journal Article↗

PubMed2006

Relation between radiation-induced whole lung functional loss and regional structural changes in partial irradiated rat lung.

PURPOSE: Radiation-induced pulmonary toxicity is characterized by dose, region, and time-dependent severe changes in lung morphology and function. This study sought to determine the relation between the structural and functional changes in the irradiated rat lung at three different phases after irradiation. MATERIALS AND METHODS: Six groups of animals were irradiated to 16-22 Gy to six different lung regions, each containing 50% of the total lung volume. Before and every 2 weeks after irradiation, the breathing rate (BR) was measured, and at Weeks 8, 26, and 38 CT was performed. From the computed tomography scans, the irradiated lung tissue was delineated using a computerized algorithm. A single quantitative measure for structural change was derived from changes of the mean and standard deviation of the density within the delineated lung. Subsequently, this was correlated with the BR in the corresponding phase. RESULTS: In the mediastinal and apex region, the BR and computed tomography density changes did not correlate in any phase. After lateral irradiation, the density changes always correlated with the BR; however, in all other regions, the density changes only correlated significantly (r(2) = 0.46-0.85, p < 0.05) with the BR in Week 26. CONCLUSION: Changes in pulmonary function correlated with the structural changes in the absence of confounding heart irradiation.

Animals↗

PubMed2003

Analysis of various beamlet sizes for IMRT with 6 MV photons.

Application of intensity modulated radiation therapy (IMRT) using multileaf collimation often requires the use of small beamlets to optimize the delivered radiation distribution. Small-beam dose distribution measurements were compared to dose distributions calculated using a commercial treatment planning system that models its data acquired using measurements from relatively large fields. We wanted to evaluate only the penumbra, percent depth-dose (PDD) and output model, so we avoided dose distribution features caused by rounded leaf ends and interleaf leakage by making measurements using the secondary collimators. We used a validated radiochromic film dosimetry system to measure high-resolution dose distributions of 6 MV photon beams. A commercial treatment planning system using the finite size pencil beam (FSPB) dose calculation algorithm was commissioned using measured central axis outputs from 4.0x4.0 to 40.0x40.0 cm2 beams and radiographic-film profile measurements of a 4.0x4.0 cm2 beam at twice the depth of maximum dose (dmax). Calculated dose distributions for square fields of 0.5x0.5 cm2, and 1.0x1.0 cm2, to 6.0x6.0 cm2, in 1.0x1.0 cm2, increments were compared against radiochromic film measurements taken with the film oriented parallel to the beam central axis in a water equivalent phantom. The PDD of the smaller field sizes exhibited behavior typical of small fields, namely a decrease in dmax with decreasing field size. The FSPB accurately modeled the depth-dose and central axis output for depths deeper than the nominal dmax of 1.5 cm plus 0.5 cm. The dose distribution in the build-up and penumbra regions was not accurately modeled for depths less than 2 cm, especially for the fields of 2.0x2.0 cm2 and smaller. Using the gamma function with 2 mm and 2% criteria, the dose model was shown to accurately predict the penumbra. While for single small beams the compared dose distributions passed the gamma function criteria, the clinical appropriateness of these criteria is not clear for a composite IMRT plan. Further investigation of the cumulative impact of the observed dose discrepancies is warranted. We speculate that the observed differences in the penumbra regions arise from some energy dependent artifact in the radiographic-film profiles used for commissioning. In the future, radiochromic film based commissioning might provide a more accurate data set for dose modeling.

Calibration↗

PubMed2002

Design of angle-tolerant multivariate optical elements for chemical imaging.

Multivariate optical elements (MOEs) are multilayer optical interference coatings with arbitrary spectral profiles that are used in multivariate pattern recognition to perform the task of projecting magnitudes of special basis functions (regression vectors) out of optical spectra. Because MOEs depend on optical interference effects, their performance is sensitive to the angle of incidence of incident light. This angle dependence complicates their use in imaging applications. We report a method for the design of angle-insensitive MOEs based on modification of a previously described nonlinear optimization algorithm. This algorithm operates when the effects of deviant angles of incidence are simulated prior to optimization, which treats the angular deviation as an interferent in the measurement. To demonstrate the algorithm, a 13-layer imaging MOE (IMOE, with alternating layers of high-index Nb2O5 and low-index SiO2) for the determination of Bismarck Brown dye in mixtures of Bismarck Brown and Crystal Violet, was designed and its performance simulated. For angles of incidence that range from 42 degrees to 48 degrees, the IMOE has an average standard error of prediction (SEP) of 0.55 microM for Bismarck Brown. This compares with a SEP of 2.8 microM for a MOE designed by a fixed-angle algorithm.

Journal Article↗

PubMed2000

Oblique scaling: an algorithm to correct for a non-perpendicular camera view in tendon strain measurements.

Image distortion due to a non-perpendicular camera view introduces serious errors in tendon and ligament strain measurements when data are recorded using a single camera. These errors can be corrected with the oblique scaling algorithm using two pairs of scaling markers attached to the tendon surface. Computer simulations show that application of this algorithm reduces errors over 100 times to less than 0.06%. The method is relatively insensitive to measurement errors in the scaling marker distance but sensitive to the accuracy of alignment of the scaling markers. It can be concluded that the oblique scaling algorithm eliminates the influences of a non-perpendicular camera view in single-camera tendon and ligament strain measurements.

Algorithms↗

PubMed2006

UF6 enrichment measurements using TDLS techniques.

The objective of this work was investigation of possibility of tunable diode laser spectroscopy (TDLS) technique application for gaseous uranium hexafluoride (UF6) isotope measurement. Spectra of uranium hexafluoride gas mixture were investigated using two different Fourier Transform Spectrometers Vector 22 and Bruker 66v. Observed spectral features were identified and model spectra of different gas mixture components were developed. Optimal spectral range for measurements was determined near maximum of UF6 combination band nu1+nu3. Laboratory prototype of multi-channel instrument under consideration based on tunable diode lasers was built and algorithms were developed to measure gaseous UF6 isotopic ratios. Diode laser used operated at the wavelengths near lambda=7.68 microm. It was placed in a liquid nitrogen cooled cryostat. Three instrument channels were used for laser frequency calibration and spectra recording. Instrument was tested in measurements of real UF6 gas mixtures. Measurement accuracy was analyzed and error sources were identified. The root-mean-square random error in the 235U isotopic content is characterized by a spread of about 0.27% for quick measurements (at times less than 1 min) and 1% for periods of more than an hour. It was estimated that the measurement accuracy could be improved by at least an order of magnitude by minimizing the error sources.

Algorithms↗

PubMed1999

On-line estimation of the maximum specific growth rate of nitrifiers in activated sludge systems.

The on-line estimation of the maximum specific growth rate of autotrophic biomass is addressed in this article. A general nitrification process model, which is valid for any realistic flow pattern, is used to develop the estimation algorithm. Depending on the measurements available, two estimation equations are derived. While both require measuring the nitrification activity of the activated sludge, one requires the additional measurement of the nitrifiable nitrogen concentrations at the two ends of the bioreactor, and the other requires the nitrate nitrogen concentrations at the same locations. The algorithm also requires some stoichiometric and kinetic parameters. However, sensitivity analysis shows that the estimate is insensitive to the parameters other than the autotrophic decay rate. Compared to the existing algorithms, the algorithm developed in this article does not rely on the assumption of ideal flow pattern in the plant and does not require an error-prone estimate of the autotrophic biomass concentration. Experimental and simulation studies show that the algorithm performs well and is robust to influent variations and accidental sludge losses.

Algorithms↗

PubMed1999

Senescent changes in scotopic contrast sensitivity.

Scotopic contrast sensitivity functions (CSFs) were measured for 50 observers between the ages of 20 and 88 years. Using a maximum-likelihood, 2-alternative, temporal forced-choice threshold-estimation algorithm, scotopic CSFs were measured at 7 spatial frequencies ranging from 0.2 to 3.0 cpd, with mean retinal illuminance equated for observers at -0.85 log scotopic Trolands. For each stimulus condition, eight cycles of a horizontal sinusoidal grating were presented within +/- 1 S.D. of a 2-D Gaussian-spatial envelope and within a 1-s Gaussian-temporal envelope. Stimuli were centered on the nasal retina along the horizontal meridian 6 degrees from the fovea. Scotopic CSFs were found to be low-pass. Statistically significant age-related declines in contrast sensitivities were found for spatial frequencies at or below 1.2 cpd. There was also a statistically significant decrease in the high frequency cut-off with age (P < 0.01). An explanation of these results in terms of optical factors is rejected, while the results are consistent with age-related changes in the magnocellular pathway.

Adult↗

PubMed2003

Comparing the continuous representation of time-series expression profiles to identify differentially expressed genes.

We present a general algorithm to detect genes differentially expressed between two nonhomogeneous time-series data sets. As increasing amounts of high-throughput biological data become available, a major challenge in genomic and computational biology is to develop methods for comparing data from different experimental sources. Time-series whole-genome expression data are a particularly valuable source of information because they can describe an unfolding biological process such as the cell cycle or immune response. However, comparisons of time-series expression data sets are hindered by biological and experimental inconsistencies such as differences in sampling rate, variations in the timing of biological processes, and the lack of repeats. Our algorithm overcomes these difficulties by using a continuous representation for time-series data and combining a noise model for individual samples with a global difference measure. We introduce a corresponding statistical method for computing the significance of this differential expression measure. We used our algorithm to compare cell-cycle-dependent gene expression in wild-type and knockout yeast strains. Our algorithm identified a set of 56 differentially expressed genes, and these results were validated by using independent protein-DNA-binding data. Unlike previous methods, our algorithm was also able to identify 22 non-cell-cycle-regulated genes as differentially expressed. This set of genes is significantly correlated in a set of independent expression experiments, suggesting additional roles for the transcription factors Fkh1 and Fkh2 in controlling cellular activity in yeast.

Algorithms↗

PubMed2006

Effective diabetes care by a registered nurse following treatment algorithms in a minority population.

OBJECTIVE: To compare usual care with nurse-directed care for patients with diabetes. STUDY DESIGN: Randomized before-after trial. METHODS: Diabetic patients were randomly selected for a diabetes managed care program (DMCP), in which a specially trained registered nurse, supervised by an endocrinologist, followed detailed treatment algorithms. Process and outcome measures during the year before DMCP entry were compared with those during the first year of DMCP enrollment. RESULTS: A total of 367 patients completed a full year in the DMCP. Data from the prior year were available for 331 patients. Among a subset of Latino patients, 95% earned less than 25,000 US dollars and 73% had an education of 6th grade or less. Process measures recommended by the American Diabetes Association (ADA) were met 98% of the time during the DMCP year compared with 54% of the time during the prior year (P <.001). Mean glycosylated hemoglobin (A1C) levels fell from 9.3% to 8.7% in the year before entry into the DMCP and to 7.0% by the end of the first DMCP year (P < .001). At DMCP entry, 28% met the ADA A1C goal of <7%; 60% did so at the end of the year. Fifty-one percent met the ADA low-density lipoprotein cholesterol goal at entry into the DMCP compared with 82% at the end of the year. CONCLUSION: A nurse making clinical decisions based on detailed treatment algorithms did a better job of achieving ADA-recommended process and outcome measures than physicians providing usual care.

Adult↗

PubMed2007

Correction of compliance and resistance altered by endotracheal tube leaks and non-linear pressure/volume-relationships.

Measurements of lung compliance (C) and resistance (R) are influenced by endotracheal tube leaks (ETTL) as well as non-linear pressure/volume relationships (P/V relationship). To keep C and R reliable, we developed an algorithm to mathematically correct inspiratory and expiratory volume (V) and flow. In this study, a ventilated lung model for non-linear P/V relationship with adjustment of an increasing ETTL was studied. In addition, the recordings (airway pressure, flow, and volume) of 21 infants (median weight: 1,220 g, range: 640-2,160 g, with a median leak size of 32%, range: 24-56%) were investigated. C and R were calculated continuously from the recordings of flow, volume, and airway pressure over time according to the changing volume. A method especially developed for the analysis of non-linear pressure-volume-relationship (APVNL) was employed. C and R affected by leaks were corrected applying the newly developed mathematical algorithm and compared with measurements without leakage. C could be corrected up to a leak of 80% and R up to 55% leak at half tidal V for the model with non-linear P/V-R. C and R without leak and after leak correction did not differ significantly in all infants where the APVNL method was applied (P > 0.05).

Airway Resistance↗

PubMed2002

Measuring portal venous perfusion with contrast-enhanced CT: comparison of direct and indirect methods.

RATIONALE AND OBJECTIVES: Two algorithms can be used to measure portal venous perfusion (PVP) with contrast material-enhanced single-level liver computed tomography. The "direct" and "indirect" algorithms use data from the portal vein and aorta, respectively. This study compared PVP values obtained with direct and with indirect algorithms in a series of patients. MATERIALS AND METHODS: Both techniques were applied in 27 patients with cirrhosis (14 men and 13 women; mean age, 56.1 years +/- 9.4) and 18 control patients (seven men and 11 women; 52.8 years +/- 12.3). A single section through the liver was scanned after intravenous injection of ioversol (40-mL bolus; 320 mg of iodine per milliliter). RESULTS: Both techniques showed reduced PVP in patients with cirrhosis (0.63 for direct and 0.17 for indirect method) compared with control patients (1.06 and 0.26, respectively), but only the direct method agreed with physiologic expectations based on animal and human studies. In separating cirrhotic and control patients, the area under the receiver operating characteristic curve was significantly greater for the direct method (0.91 vs 0.78; P = .03). CONCLUSION: Both direct and indirect methods are feasible and distinguish well between cirrhotic and control patients, but the direct method is more physiologic and is preferable if portal venous data are available.

Algorithms↗

PubMed2004

Identifying projected clusters from gene expression profiles.

In microarray gene expression data, clusters may hide in certain subspaces. For example, a set of co-regulated genes may have similar expression patterns in only a subset of the samples in which certain regulating factors are present. Their expression patterns could be dissimilar when measuring in the full input space. Traditional clustering algorithms that make use of such similarity measurements may fail to identify the clusters. In recent years a number of algorithms have been proposed to identify this kind of projected clusters, but many of them rely on some critical parameters whose proper values are hard for users to determine. In this paper, a new algorithm that dynamically adjusts its internal thresholds is proposed. It has a low dependency on user parameters while allowing users to input some domain knowledge should they be available. Experimental results show that the algorithm is capable of identifying some interesting projected clusters.

Algorithms↗

PubMed1989

In vivo evaluation of a noninvasive method to measure the retinal thickness in primates.

To diagnose certain macular diseases earlier and monitor their therapy more sensitively, we are developing a noninvasive method to measure the retinal thickness. The new instrument, which is an extension of slit-lamp biomicroscopy, was used to obtain the data, which were analyzed with an algorithm to yield thickness measurements. The measurements performed in monkeys indicated that the retinal thickness can be visualized in a region extending from the optic disc to the fovea and that quantitative results can be obtained. The retinal thickness reproducibility was 6% for the same location on the same day, 15% for the same location on different days, and 12% for the same location in different eyes. The average retinal thickness in these areas was 335 microns, indicating that the reproducibility was between 20 and 50 microns. Measurements across the foveola illustrated that retinal thicknesses as low as 80 microns could be obtained.

Animals↗

PubMed1994

Bedside assessment of intravascular volume status in patients undergoing coronary bypass surgery.

BACKGROUND: Management of intravascular volume is crucial in patients after cardiopulmonary bypass as myocardial dysfunction is common. The purpose of this study was to validate a novel bedside technique for real-time assessment of intravascular volumes. METHODS: Eleven patients undergoing cardiopulmonary bypass were studied. In addition to standard monitors, a fiberoptic thermistor catheter was placed in the descending aorta and central venous injections of 10 ml ice-cold indocyanine green dye were performed. Total blood volume was measured by a standard in vitro technique. Circulating and central blood volume were calculated by using cardiac output, mean transit times, and a newly developed recursive convolution algorithm that models recirculation. Measurements were performed after induction of anesthesia and at 1, 6, and 24 h after surgery. RESULTS: A two-compartment model of the circulation was required for adequate fit of the data. We found a significant correlation between total and circulating blood volumes (r = 0.87). One hour after surgery, central blood volume was decreased by 10% (P < 0.05). At 6 and 24 h after surgery, circulating blood volumes were significantly increased by 29% and 20%, respectively (P < 0.01), although central blood volume was similar to control values. Before surgery stroke volume index correlated with circulating blood volume (r = 0.87) but not with pulmonary capillary wedge and central venous pressures. CONCLUSIONS: This study shows that bedside determinations of intravascular blood volumes are feasible and that these measurements are more indicative of intravascular volume status than are either pulmonary capillary wedge or central venous pressures in the post-cardiopulmonary bypass period. Our data also demonstrate that despite a normal central blood volume both circulating and total blood volume are significantly increased in the immediate post-cardiopulmonary bypass period.

Aged↗