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Results for “Monte Carlo Method”

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PubMed2005

A Monte Carlo analysis technique applied to cerebral perfusion SPECT scans.

OBJECTIVE: Voxel-based techniques are used to provide objective analyses of SPECT scans. The aim of this study was to develop a voxel-based analysis technique that uses a Monte Carlo method of statistical inference and assess this analysis technique for application to cerebral perfusion SPECT scans. METHODS: Assessment of the validity of this non-parametric, Monte Carlo method of statistical inference has been performed for a range of study designs, image characteristics and analysis parameters using phantom SPECT and Gaussian images. The Monte Carlo method of statistical inference and the voxel-based analysis technique were clinically evaluated for the analysis of individual cerebral perfusion SPECT scans using control subject data. In addition, a comparison has been performed with an existing analysis package that uses a theoretical parametric method of statistical inference (statistical parametric mapping). RESULTS: The Monte Carlo method was found to provide accurate statistical inference for phantom SPECT and Gaussian images independent of degrees of freedom, acquired counts, image smoothness and voxel significance level threshold. The clinical evaluation of the analysis of individual cerebral perfusion SPECT scans demonstrated satisfactory statistical inference and characterization of perfusion deficits. CONCLUSION: An analysis method incorporating a Monte Carlo method of statistical inference has been successfully applied for the analysis of cerebral perfusion SPECT scans.

Algorithms↗

PubMed2002

Advances in space radiation shielding codes.

Early space radiation shield code development relied on Monte Carlo methods and made important contributions to the space program. Monte Carlo methods have resorted to restricted one-dimensional problems leading to imperfect representation of appropriate boundary conditions. Even so, intensive computational requirements resulted and shield evaluation was made near the end of the design process. Resolving shielding issues usually had a negative impact on the design. Improved spacecraft shield design requires early entry of radiation constraints into the design process to maximize performance and minimize costs. As a result, we have been investigating high-speed computational procedures to allow shield analysis from the preliminary concept to the final design. For the last few decades, we have pursued deterministic solutions of the Boltzmann equation allowing field mapping within the International Space Station (ISS) in tens of minutes using standard Finite Element Method (FEM) geometry common to engineering design methods. A single ray trace in such geometry requires 14 milliseconds and limits application of Monte Carlo methods to such engineering models. A potential means of improving the Monte Carlo efficiency in coupling to spacecraft geometry is given.

Cosmic Radiation↗

PubMed2002

[Application of Monte Carlo sampling method in iodized salt monitoring].

OBJECTIVE: To explore a sampling method which could reflect iodine deficiency disorders (IDD) status at provincial level and discover risk areas with non-iodized salt problem. METHOD: Baseline data of Iodized salt from Gansu and Fujian provinces were analyzed with Monte Carlo method both at county and prefecture levels respectively. True positive rate and false positive rate were also calculated. RESULTS: With data from 7 - 8 villages or 4 - 5 townships counties at risk could be discovered. The true positive rate was around 80% and false positive rate was around 20%. At prefecture level, when randomly selecting and checking 3 counties, the samples would satisfy the discovery of all the risk areas with non-iodized salt problem. CONCLUSIONS: We suggested that the sampling method of iodized salt investigation in national IDD surveillance as follows: to randomly choose 3 counties at each prefecture, 4 townships at each county, 2 villages at each township and 10 salt samples by household survey. The coverage rate of iodized salt in a province could be calculated by post-weighted method with population number.

China↗

PubMed2005

Argon and nitrogen adsorption in disordered nanoporous carbons: simulation and experiment.

We report experimental measurements of the isosteric heats of adsorption for argon and nitrogen in two microporous saccharose-based carbons, using a Tian-Calvet microcalorimeter. These data are used to test recently developed molecular models of these carbons, obtained by a constrained reverse Monte Carlo method. Grand canonical Monte Carlo simulation is used to calculate the adsorption isotherms and isosteric heats for these systems, and the results for the latter are compared to the experimental data. For argon, excellent quantitative agreement is obtained over the entire range of pore filling. In the case of nitrogen, very good agreement is obtained over the range of coverage 0.25 < or = gamma/gamma 0 < or = 0.85, but discrepancies are observed at lower and higher coverages. The discrepancy at low coverage may be due to the presence of oxygenated groups on the pore surfaces, which are not taken into account in the model. The differences at high coverage are believed to arise from the presence of a few mesopores, which again are not included in the model. Pair correlation functions (argon-carbon and argon-argon) are determined from the simulations and are discussed as a function of pore filling. Snapshots of the simulations are presented and provide a picture of the pore filling process.

Journal Article↗

PubMed2005

Evaluation of accuracy in activity calculations for the conjugate view method from Monte Carlo simulated scintillation camera images using experimental data in an anthropomorphic phantom.

UNLABELLED: Activity determination from scintillation camera images using the conjugate view method may be inaccurate because of variation in scattered radiation from adjacent organs and activity from overlapping tissues. The aim of this study was to simulate patient scintillation camera images and from these evaluate the accuracy of 2 correction methods. The contribution from overlapping tissue activity was also calculated for some organs. METHODS: Biokinetic data for (99m)Tc-sestamibi obtained in rats was used as input to simulate scintillation camera images with a voxel-based computer phantom using the Monte Carlo method. The organ activity was calculated using the conjugate view method with either the effective attenuation coefficient method or scatter correction using the triple-energy window (TEW) method combined with attenuation correction with a transmission factor image. Images were simulated with activity in organs one by one to evaluate the accuracy of the 2 correction methods and to evaluate the activity contribution from activity in adjacent or overlapping tissues. To allow comparison with the clinical situation, the total activity distribution from the animal study was used to simulate scintillation camera images at different points in time and the calculated activity was compared with both the input data and some patient data from the literature. RESULTS: The combination of scatter and attenuation correction gave the most accurate calculated activity, +/-10% of the true activity from the images with activity in one organ at a time. In the images similar to the clinical situation, the kidney activity was overestimated up to a factor of 34, mainly because of excretion of activity through the intestines. CONCLUSION: The scatter correction using the TEW method in combination with attenuation correction with the measured transmission factor resulted in the most accurate activity determination of the methods used. This study also shows that organ activity data calculated from scintillation camera images may be overestimated by >90% because of activity in overlapping tissues.

Animals↗

PubMed2004

Quantum Monte Carlo for electronic excitations of free-base porphyrin.

Accurate calculations of allowed and nonallowed transitions in porphyrin are reported. Using the quantum Monte Carlo method in the diffusion Monte Carlo variant, the vertical transition between the ground state singlet and the second excited state singlet as well as the adiabatic transition between the ground state and the lowest triplet state have been computed for this 162-electron system. The present theoretical results are compared to experiment and to results of other theoretical methods. The diffusion Monte Carlo energy differences are found to be in excellent agreement with experiment.

Computer Simulation↗

PubMed2005

Density guided importance sampling: application to a reduced model of protein folding.

MOTIVATION: Monte Carlo methods are the most effective means of exploring the energy landscapes of protein folding. The rugged topography of folding energy landscapes causes sampling inefficiencies however, particularly at low, physiological temperatures. RESULTS: A hybrid Monte Carlo method, termed density guided importance sampling (DGIS), is presented that overcomes these sampling inefficiencies. The method is shown to be highly accurate and efficient in determining Boltzmann weighted structural metrics of a discrete off-lattice protein model. In comparison to the Metropolis Monte Carlo method, and the hybrid Monte Carlo methods, jump-walking, smart-walking and replica-exchange, the DGIS method is shown to be more efficient, requiring no parameter optimization. The method guides the simulation towards under-sampled regions of the energy spectrum and recognizes when equilibrium has been reached, avoiding arbitrary and excessively long simulation times. AVAILABILITY: Fortran code available from authors upon request. CONTACT: m.j.parker@leeds.ac.uk.

Algorithms↗

PubMedDate not supplied

Treatment planning dosimetric parameters for a (90)Y coil source used in intravascular brachytherapy.

BACKGROUND: (90)Y coil sources have been used in animal and clinical trials for treatment of restenosis in intravascular brachytherapy (IVBT). This study aims to determine the American Association of Physicists in Medicine (AAPM) Task Group-60 (TG-60) dosimetric quantities in regions surrounding the balloon wall for use in treatment planning computer systems. METHODS: The Monte Carlo method was used to determine the dose distribution, using MCNP4B2 code. The coil source was modeled by a hollow cylinder of 2.9 cm length centered in a balloon (2.5 mm diameter) filled with carbon dioxide (CO(2)) at 5 atm. Scoring voxels consisted of contiguous annular disk shells with 0.1 mm spacing in the radial direction and 0.2 mm spacing in the longitudinal direction. The scoring region ranges from the center of the source to 1.0 cm in the longitudinal direction, and from 0.13 to 1 cm in the radial direction. In the plane containing the source axis, the Monte Carlo-generated doses in rectilinear coordinates are converted to polar coordinates. RESULTS: The dose rate of the source is provided in both Cartesian and polar coordinates. The dose rate constant [D(r(0),theta(0))], anisotropy function [F(r,theta)], and radial dose function [g(r)] were generated from these values and listed in tabular format. At shallow angles and longer distances from the source center, large values of the anisotropy function resulted, deviating two orders of magnitude from unity. CONCLUSIONS The doses given to the intima, media, and adventitia are very crucial quantities in IVBT. The calculated TG-60 dosimetric quantities, used commonly in conventional brachytherapy applications, provide a means for the user to determine the three-dimensional dose surrounding the balloon catheter. These parameters can be used in future treatment planning system for IVBT. We also discuss the need to develop a new formalism specific to longer sources used in IVBT.

Angioplasty, Balloon, Coronary↗

PubMed1997

Optimal parameters for laser treatment of leg telangiectasia.

BACKGROUND AND OBJECTIVE: The optimal parameters for treatment of leg telangiectasia with a pulsed laser are investigated. STUDY DESIGN/MATERIALS AND METHODS: The Monte Carlo method is used to calculate the light penetration and absorption in the epidermis, dermis, and the ectatic blood vessel. Calculations are made for different diameters and depths of the vessel in the dermis. In addition, the oxygen saturation of the blood vessel, the laser beam diameter, and the laser irradiation profile is varied. RESULTS: It is found that for vessels with diameters between 0.3 mm and 0.5 mm wavelengths about 600 nm are optimal to achieve selective photothermolysis for the considered damage model. It is also shown that an elliptical laser beam area has advantages compared to a circular beam area for treatment of leg telangiectasia. CONCLUSIONS: We recommend the treatment of leg telangiectasia with dye laser wavelengths longer than the normally used 577 nm and 585 nm.

Blood Vessels↗

PubMed2004

Adaptive integration method for Monte Carlo simulations.

We present an adaptive sampling method for computing free energies, radial distribution functions, and potentials of mean force. The method is characterized by simplicity and accuracy, with the added advantage that the data are obtained in terms of quasicontinuous functions. The method is illustrated and tested with simulations on a high density fluid, including a stringent consistency test involving an unusual thermodynamic cycle that highlights its advantages.

Algorithms↗

PubMed2005

A Monte Carlo study of intersource effects in dome-type applicators loaded with LDR Cs-137 sources.

In this study, the dose rate distributions produced by low dose rate Cs-137 sources loaded in afterloadable dome applicators are studied using the Monte Carlo method. Dose differences between Monte Carlo results and calculations done using the superposition principle are within 1-3% in front of the applicator and between 3 and 10% near and along the longitudinal source axis. Consequently, the real doses to lateral vaginal wall, rectum and bladder are very close to the doses estimated applying the superposition principle, while the dose to the vaginal cuff has been overestimated by up to 10%.

Brachytherapy↗

PubMed1999

Exploring the conformation of bilirubins with natural and unnatural analogues: use of positional and bridged isomers of bilirubin IXalpha.

Unlike bilirubin IXalpha (1), the isomers bilirubin IXdelta (2) and neobilirubin IXbeta (3) do not require conjugation with glucuronic acid in order to be excreted. A conformational analysis employing an optimized Monte Carlo method and a mixed Monte Carlo stochastic dynamics reveals that isomer 2 exhibits a structure more closed than the well known 'ridge-tile' conformation of 1. The change in the position of both propionic acid chains causes the loss of at least four hydrogen bonds. On the other hand, the change in the configuration of the distal dipyrrinone and the blockage of the lactamic nitrogen by the presence of a bridge in isomer 3 results in an open and more elongated structure, where the chance of hydrogen bond formation in this region is obliterated. The resulting molecular models for these compounds are consistent with 1H NM R, UV-vis, and TLC data.

Animals↗

PubMed2006

Hidden process models for animal population dynamics.

Hidden process models are a conceptually useful and practical way to simultaneously account for process variation in animal population dynamics and measurement errors in observations and estimates made on the population. Process variation, which can be both demographic and environmental, is modeled by linking a series of stochastic and deterministic subprocesses that characterize processes such as birth, survival, maturation, and movement. Observations of the population can be modeled as functions of true abundance with realistic probability distributions to describe observation or estimation error. Computer-intensive procedures, such as sequential Monte Carlo methods or Markov chain Monte Carlo, condition on the observed data to yield estimates of both the underlying true population abundances and the unknown population dynamics parameters. Formulation and fitting of a hidden process model are demonstrated for Sacramento River winter-run chinook salmon (Oncorhynchus tshawytsha).

Animals↗

PubMed2002

Benchmarking of the dose planning method (DPM) Monte Carlo code using electron beams from a racetrack microtron.

A comprehensive set of measurements and calculations has been conducted to investigate the accuracy of the Dose Planning Method (DPM) Monte Carlo code for dose calculations from 10 and 50 MeV scanned electron beams produced from a racetrack microtron. Central axis depth dose measurements and a series of profile scans at various depths were acquired in a water phantom using a Scanditronix type RK ion chamber. Source spatial distributions for the Monte Carlo calculations were reconstructed from in-air ion chamber measurements carried out across the two-dimensional beam profile at 100 cm downstream from the source. The in-air spatial distributions were found to have full width at half maximum of 4.7 and 1.3 cm, at 100 cm from the source, for the 10 and 50 MeV beams, respectively. Energy spectra for the 10 and 50 MeV beams were determined by simulating the components of the microtron treatment head using the code MCNP4B. DPM calculations are on average within +/- 2% agreement with measurement for all depth dose and profile comparisons conducted in this study. The accuracy of the DPM code illustrated in this work suggests that DPM may be used as a valuable tool for electron beam dose calculations.

Algorithms↗

PubMed1996

Monte Carlo exact methods used for analysing interethnic unions in Great Britain.

The value of using exact rather than asymptotic tests to measure intermarriage in the United Kingdom is examined. "We develop Markov chain Monte Carlo methods for estimating the exact conditional p-value and the exact distribution of the residuals, for quasi-independence and quasi-symmetry. These methods are used to analyse a sparse 10 x 10 symmetric table of interethnic unions, extracted from the 1% household sample of anonymized records from the 1991 U.K. census. With the exception of Pakistani/White and Other Asian/White unions, there is no evidence against quasi-symmetry. We conclude that, with these exceptions, there are no gender differences in the affinities between ethnic groups."

Culture↗

PubMed2003

Condensate statistics in one-dimensional interacting Bose gases: exact results.

Recently, a quantum Monte Carlo method alternative to the path integral Monte Carlo method was developed for solving the N-boson problem; it is based on the stochastic evolution of classical fields. Here we apply it to obtain exact results for the occupation statistics of the condensate mode in a weakly interacting trapped one-dimensional Bose gas. The temperature is varied across the critical region down to temperatures lower than the trap level spacing. We also derive the condensate statistics in the Bogoliubov theory: this reproduces the exact results at low temperature and explains the suppression of odd numbers of noncondensed particles at T approximately 0.

Journal Article↗

PubMed2003

[Estimation of disease-specific costs in a dataset of health insurance claims and its validation using simulation data].

PURPOSES: To estimate disease-specific costs in a dataset of health insurance claims with multiple diagnoses with known aggregate cost per claim and unknown disease-specific cost of each diagnosis using PDM (Proportional Disease Magnitude) method, validate its accuracy using simulation data with Monte Carlo method and improve its accuracy by developing an adjustment formula. METHODS: Developed simulation data with pre-assigned disease-specific costs, applied PDM method using arithmetic means of per-diem-per-disease cost as magnitude, validated its accuracy by observing the correlation between estimates by PDM method and known disease-specific costs and formulated an adjustment formula to improve accuracy. The reproducibility of the findings was assessed using Monte Carlo method by repeating the same procedures. RESULTS: The observed arithmetic means of per-diem-per-disease cost did not match well with actual values resulting in unsatisfactory accuracy. However, when the observed means were adjusted with a formula in which the observed mean is multiplied by (observed mean/overall mean) in the power of 2, PDM method yielded an accurate estimate of disease-specific cost. The accuracy was reproduced by Monte Carlo method with 0.9 or above R square value and slope of regression line in 76, 56 out of 100 iterations respectively. CONCLUSIONS: PDM method proved to be an objective, reproducible and accurate method for estimation of disease-specific costs of health insurance claims.

Insurance Claim Reporting↗

PubMed2001

The use of cylindrical coordinates for treatment planning parameters of an elongated 192Ir source.

PURPOSE: The doses given to the intima, media, and adventitia are very crucial quantities in intravascular brachytherapy. To facilitate accurate computerized treatment planning calculations, we have determined dose distributions in away-and-along table format around an 192Ir wire source and developed pertinent dosimetric parameters in cylindrical coordinates. METHODS AND MATERIALS: The Monte Carlo method (MCNP4C code) was used to calculate the dose distributions for the AngioRad 192Ir wire source (model SL-77HS, Interventional Therapies). The calculations were carried out for photon, beta, and electron (conversion and Auger) contributions for radial distances from 0.03 to 2.0 cm with 0.01-cm increments, and up to 2.24 cm from the source center in the longitudinal direction with 0.04-cm resolution. Dose rate values are determined in away-and-along format (cylindrical coordinates) and then converted to spherical coordinate format. Dosimetric parameters, such as the geometry factor, G(r, theta), and anisotropy function, F(r, theta), are generated in both cylindrical (R, Z, phi) and spherical (r, theta, phi) coordinates. The use of a cylindrical coordinate system for treatment planning parameters is proposed as a more suitable approach for accurate calculations. RESULTS: The photon contribution to dose varies nearly inversely with radial distance (from the source center) along the perpendicular bisector with 0.199 x 10(-3) cGy U(-1) s(-1) (0.802 cGy Ci(-1) s(-1)) at 1 cm. The beta and electron contributions start at very high values of about 35.5 x 10(-3) cGy U(-1) s(-1) and 11.0 x 10(-3) cGy U(-1) s(-1), respectively, at 0.03 cm and fall off exponentially to negligible amount near 0.2 cm. The total dose rate at 0.2 cm is 1.428 x 10(-3) cGy U(-1) s(-1) (5.754 cGy Ci(-1) s(-1)). The radial dose function, g(R), is nearly unity between 0.2 cm and 2 cm. Due to the beta and electron dose contributions, g(R) increases steeply to 5.5 as radial distance decreases from 0.2 cm down to 0.03 cm. The F(R, Z) values are close to unity for the majority of the region of interest. In contrast, F(r, theta) experiences a steep rise as shallow angles are approached (closer to the source), related to the beta dose contributions. Accurate treatment planning calculations would be possible with linear interpolation of F(R, Z), but difficult with F(r, theta) in the spherical coordinate system and the original normalization point as recommended in the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) formalism. CONCLUSION: The AngioRad 192Ir wire source, model SL-77HS, was completely characterized dosimetrically using Monte Carlo methods. The use of cylindrical coordinates and a modified anisotropy function normalization point for dosimetric parameters of an elongated 192Ir source is more suitable for accurate computerized treatment planning calculations in intravascular brachytherapy.

Algorithms↗