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At least 109 records · Page 6Linked to original sources
PubMed2005

Analytical approach to noncircular section birdcage coil design: verification with a Cassinian oval coil.

A general analytical framework is presented for the design of birdcage radiofrequency resonators on cylindrical formers having arbitrary cross-sectional shape. The primary objective of such shapes would be to improve the sensitivity of the NMR experiment to noncircular regions of the human anatomy while maintaining field homogeneity and quadrature polarization comparable to those of standard circular birdcage coils. The shape of the corresponding radiofrequency screen, which is required to decouple the coil from the rest of the NMR system and which is key to the performance, is also provided by this methodology. The theory was tested by constructing a 3-T, quadrature, proton coil on a shape conforming to the anthropomorphic mean of the human head, namely, the oval of Cassini. Both bench tests (Q) and in vivo spectral and imaging comparisons of the Cassinian coil with an equivalently dimensioned and constructed circular birdcage coil, respectively, predicted and demonstrated in vivo an improvement in SNR of approximately 24% over the circular section coil. The experimental RF field homogeneity and quadrature performance were comparable for both coil geometries, with the circular coil being marginally superior.

Equipment Design↗
PubMed2002

Estimation of heat transfer and temperature rise in partial-body regions during MR procedures: an analytical approach with respect to safety considerations.

In order to assess thermal response to RF exposure during MR procedures at the tissue level, simple analytical solutions to the non-stationary Pennes' bio-heat equation were obtained using the Green's function approach. Two thermal models appropriate for partial-body exposure were analyzed: In the first model, the temperature field at the periphery of an idealized volume RF resonator was modeled. The analytical solution reveals that tissue response to RF heating is characterized by an equilibration time and length. Both parameters are inversely related to tissue perfusion and vary for the soft-tissues considered between 0.27-25 min and 1.5-12 mm, respectively. None of the tissues investigated increase in temperature more than 0.5 degrees C for each W/kg of power dissipated. Secondly, a homogeneous tissue solution was derived that predicts the temperature-time course to an MR examination with time-varying specific absorption rates (SAR). Since SAR limits indicated in current MR safety standards relate to running SAR averages computed over an appropriate period of time, an expression was formulated that gives an upper limit for the temperature rise averaged over the same period of time, as a function of both the upper limit of running SAR averages and the duration of the MR examination. The analysis revealed that the partial-body SAR limits indicated in the IEC standard may not guarantee under all circumstances compliance with the basic restrictions concerning temperature rise.

Body Temperature↗
PubMed1997

The relationship of social environment, social networks, and health outcomes in the Seattle Longitudinal Study: two analytical approaches.

This study examined the relation of social networks and perceived social environment to health outcomes and cost utilization over a 1-year period in a community sample of 387 (173 males, 214 females). Two analytical strategies, a variable-oriented approach and a subject-oriented approach, were used to complement each other: structural equation modeling assessed the direct relationship between social relationships and health, while cluster analysis examined how social relationship patterns were related to health outcomes. Lower levels of perceived social environment were associated with an increased number of hospital visits. For unmarried individuals, increased age was associated with greater medical usage and estimated total health care cost, while low perceived social environment was related to a greater number of doctor visits. Among married individuals, women had more frequent doctor visits, medication usage, estimated outpatient costs, and primary care visits. Married individuals with low levels of social networks had increased total health care costs, outpatient costs, and primary care visits. Cluster analysis was used to group individuals by characteristics of perceived social environment and social networks. Members of the cluster groups with greater health problems were more likely to be isolated, had the least social contact, and had lower levels of education and income. Including subject-oriented approaches is useful in complementing more popular regression methods for data analysis.

Adult↗
PubMed2002

A reactive analytical approach for the estimation of olefinic content in gasoline-range hydrocarbons by gas chromatography.

The estimation of olefinic content in conversion processes such as the etherification of olefins in fluid catalytic cracking (FCC) gasoline is essentially required. Gas chromatography (GC) is the well-established method for the quantitative analysis of olefins in etherification processes. The current state-of-the-art GC methods employing highly specific long single capillary columns such as Petrocol-DH are being used for the analysis of gasoline-range hydrocarbons. However, the method needs many standard reference samples of respective components in a complex mixture of hydrocarbons, which limits the scope of the analytical method. The alternative approach followed by this investigation is based on the reactive method of the analysis of olefins in FCC light gasoline by subjecting them to hydrogenation and estimating the olefinic content by GC comparing the gas chromatograms of the original feed and hydrogenated product using a Petrocol-DH column. A decrease in the quantity and disappearances of the peaks are assumed as olefins, and their number and total composition is calculated. In this study the bromine number method is used to estimate the olefinic content for a comparison of results with the adopted proposed methodology. The adopted methodology quantitates olefinic content in FCC light gasoline, which is comparable with reported literature values and the bromine number method. With the availability of standard reference samples of some important major reactive olefins, the adopted methodology can also give component-wise analysis as well as total olefinic content in a single step in processes such as etherification. The methodology can be also useful in reactions in which the conversion of total olefinic content is needed such as hydration, esterification, and alkylation of olefins in a complex mixture of hydrocarbons apart from the etherification of olefins in FCC gasoline.

Journal Article↗
PubMed2002

Controlling chaos in higher dimensional maps with constant feedback: an analytical approach.

We introduce two methods to control chaos in higher-dimensional discrete maps with constant feedback. It is analytically shown for a general class of function vectors that chaotic attractors can be converted into fixed point attractors. Additionally, a method to choose an appropriate constant feedback is presented. The application of these methods does not require a priori knowledge of system equations, since time series information can be used. Desired periodic orbits can be accessed by varying the constant feedback. As an example, the methods are applied to the Hénon map.

Journal Article↗
PubMed1983

Drugs and driving: a systematic analytical approach.

To collect useful epidemiological data about drug involvement in highway safety, it is essential that sensitive and specific analytical procedures be used to establish the presence of and to determine the concentrations of drugs and metabolites in samples collected from drivers. This paper describes a comprehensive and systematic screening procedure requiring 6 mL of blood, which has been used for the analysis of samples collected from injured and fatally injured drivers. The procedure uses radioimmunoassay, gas chromatography with selective detectors, and high performance liquid chromatography. Drugs and metabolites presumptively identified are then confirmed primarily using gas chromatography--chemical ionization mass spectrometry.

Accidents, Traffic↗
PubMed1990

New in vitro analytical approaches for clinical chemistry measurements in critical care.

The effective management of patients in intensive care units, operating rooms, and emergency rooms requires frequent measurement of a select group of analytes, preferably at or near the patient's bedside. Tests recognized as being essential for such management include blood gases and related variables (pH, pO2, pCO2, HCO3-, hematocrit/hemoglobin, O2 saturation), electrolytes (Na+, K+, Ca2+, Cl-), and in some cases, certain metabolites (glucose, lactate, urea, creatinine). This report describes the measurement principles, practical instrumental designs, analytical performance, and limitations of several newer electrochemical sensor-based approaches useful for in vitro determination of these species in undiluted whole-blood samples. Considerable attention is given to the most recent advances in ion-selective electrode technology as they relate to blood gas and electrolyte determinations. Similar attention is given to modern enzyme-electrode techniques, which are useful for direct measurements of metabolites in whole blood. The challenges of integrating these new analytical methods into convenient, multi-analyte, user-friendly, bedside or stat-lab instruments are also discussed.

Autoanalysis↗
PubMed2002

Heterogeneous nucleation and crowding in sickle hemoglobin: an analytic approach.

Sickle hemoglobin nucleation occurs in solution as a homogeneous process or on existing polymers in a heterogeneous process. We have developed an analytic formulation to describe the solution crowding and large nonideality that affects the heterogeneous nucleation of sickle hemoglobin by using convex particle theory. The formulation successfully fits the concentration and temperature dependence of the heterogeneous nucleation process over 14 orders of magnitude. Unlike previous approaches, however, the new formulation can also accurately describe the effects of adding nonpolymerizing agents to the solution. Without additional adjustable parameters, the model now describes the data of M. Ivanova, R. Jasuja, S. Kwong, R. W. Briehl, and F. A. Ferrone, (Biophys. J. 2000, 79:1016-1022), in which up to 50% of the sickle hemoglobin is substituted by cross-linked hemoglobin A, which does not polymerize, and which substitution causes the rates to decrease by 10(5). The success of this approach provides insight into the polymerization process: from the size-dependence of the contact energy deduced here, it also appears that various contacts of unknown origin are energetically significant in the heterogeneous nucleation process.

Calorimetry↗
PubMed2001

Electric Potential and Reaction Rates at Charged Surfaces in Asymmetric Electrolytes-An Analytic Approach.

The electric potential and reaction rates of ions hitting the chemically active surfaces of microcrystals in an asymmetric electrolyte are computed analytically. Following ideas of Debye we start by solving the Poisson-Boltzmann equations and by determining the electric potential of the transport equations. We find distinct deviations when comparing our result with the Gouy-Chapman formula. In a simple model approximating a situation in which lead and hydrogen ions can react at goethite surfaces we compute analytically the currents of ions diffusing to the surfaces of microcrystals where they undergo a chemical reaction. We compute the reaction rates that can be controlled either by chemical reactions at the surface of the microcrystals or by diffusional transport. For realistic parameters of our model we find that the diffusional transport is the rate determining step. Copyright 2001 Academic Press.

Journal Article↗
PubMed1997

Analytical approaches to the determination of pressure distribution under a plantar prominence.

INTRODUCTION:: The purpose of the current research was to develop an analytical model for the interface between the metatarsal head, surrounding soft tissue, and the shoe. Results of 'peak plantar pressures' computed from this model were compared to previous results obtained from clinical data and from a finite element (FE) model. METHODS:: An analytical model for determining the pressure distribution at the interface between the foot and shoe insole was developed based on the solution[Figure: see text] for a rigid cylinder and a single elastic layer mounted on a rigid half-space originally developed by Meijers (see Figure 1). The rigid cylinder was used to represent the metatarsal head region (including bone and soft tissue). Material properties and loading conditions were chosen to reflect those used by Lemmon et al. for 'Normal' and 'Reduced' soft tissue thickness cases. RESULTS:: Peak pressures computed from the analytical model are plotted against insole thickness in Figure 1, alongside experimental and FE model data from Lemmon et al. Correlations between analytical, FE, and experimental results for both the Normal and Reduced soft tissue cases were all highly significant (r(2) = 0.86, p < 0.01). DISCUSSION:: The analytical model produced unrealistically high peak pressures compared to the FE and experimental results, with differences of approximately one order of magnitude. This was not surprising, given that the entire metatarsal head region (soft tissue included) was modelled as a rigid cylinder. The soft tissue on the bottom of the foot, and the much greater surface area over which the loads are carried in the real case, contribute greatly to reducing these peak pressure values. However, the downward trend in peak pressure with increased insole thickness was retained. CONCLUSION:: It is anticipated that the use of more accurate and detailed models will greatly improve the initial results.

Journal Article↗
PubMed2003

Introduction of the new analytical approaches to the doping control on the XIV Winter Olympic Games.

In this paper we present introduction and development of some new analytical methods for identification of anabolic steroids, their metabolites and certain hormones, especially determination of exogenous testosterone by means of gas chromatography-mass spectrometry. Identification of central nervous stimulants and corticosteroides has been performed by high performance liquid chromatography. In desire to achieve better results, to increase strength and endurance, to sharpen reflexes and to reduce stress and anxiety athletes as well as other people use different pharmacological substances, hormones or even illicit drugs. Use of these substances without medical supervision can lead to adverse effects to one's health or even cause a death. At the same time, use of such substances means a kind of cheat that could not be accepted. This is why International Olympic Committee started at 1968 with official doping control that is permanently carried out and continuously increasing number of banned substances. Doping control demands for discover and development of new sensitive and specific methods for detection of banned substances and their metabolites in urine and blood.

Journal Article↗
PubMed1994

A decision-analytic approach to postexposure rabies prophylaxis.

The risks and benefits of postexposure rabies prophylaxis were analyzed from clinical and economic perspectives. A decision-analytic model was constructed by using probability and outcome data from the literature and the state health department. Health outcomes were measured in quality-adjusted life years. In the base case (overweight adult male), treatment is optimal when the probability of animal rabidity is greater than 1 in 2000. Sensitivity analysis showed robustness in the treatment decision; however, the incremental cost-effectiveness ratio ($140,000/quality-adjusted life year) is sensitive to the rabidity probability. Treatment is optimal from the patient's perspective; however, it may not be cost-effective when the probability of rabidity is low.

Animals↗
PubMed2005

Analytical approaches to the determination of simple biophenols in forest trees such as Acer (maple), Betula (birch), Coniferus, Eucalyptus, Juniperus (cedar), Picea (spruce) and Quercus (oak).

Analytical methods are reviewed for the determination of simple biophenols in forest trees such as Acer (maple), Betula (birch), Coniferus, Eucalyptus, Juniperus (cedar), Picea (spruce) and Quercus (oak). Data are limited but nevertheless clearly establish the critical importance of sample preparation and pre-treatment in the analysis. For example, drying methods invariably reduce the recovery of biophenols and this is illustrated by data for birch leaves where flavonoid glycosides were determined as 12.3 +/- 0.44 mg g(-1) in fresh leaves but 9.7 +/- 0.35 mg g(-1) in air-dried samples (data expressed as dry weight). Diverse sample handling procedures have been employed for recovery of biophenols. The range of biophenols and diversity of sample types precludes general procedural recommendations. Caution is necessary in selecting appropriate procedures as the high reactivity of these compounds complicates their analysis. Moreover, our experience suggests that their reactivity is very dependent on the matrix. The actual measurement is less contentious and high performance separation methods particularly liquid chromatography dominate analyses whilst coupled techniques involving electrospray ionization are becoming routine particularly for qualitative applications. Quantitative data are still the exception and are summarized for representative species that dominate the forest canopy of various habitats. Reported concentrations for simple phenols range from trace level (<0.1 microg g(-1)) to in excess of 500 microg g(-1) depending on a range of factors. Plant tissue is one of these variables but various biotic and abiotic processes such as stress are also important considerations.

Chromatography, High Pressure Liquid↗
PubMed1985

A decision analytic approach to the role of visual evoked response and cerebrospinal fluid abnormalities in the management of singular spinal sclerosis.

With the use of Bayesian decision analytic techniques we assess the role of visual evoked response (VER) and cerebrospinal fluid (CSF) examinations in the investigation of patients with suspected singular spinal sclerosis. An abnormal VER increases the probability of multiple sclerosis by 50%. An associated CSF abnormality does not affect this probability. In a setting of a negative VER, CSF abnormalities increase the probability of multiple sclerosis by 27%. On currently available data, VER should therefore be the primary investigation in patients with suspected singular spinal sclerosis. If the result is abnormal myelography may be omitted. CSF examinations are only useful if VER facilities are not available.

Bayes Theorem↗
PubMed2006

Optimal medical therapy is superior to transplantation for the treatment of class I, II, and III heart failure: a decision analytic approach.

BACKGROUND: The survival benefit of heart transplantation (HT) compared with optimal medical therapy (OMT) has never been tested. METHODS AND RESULTS: We created a decision analytic model that simulates a randomized clinical trial of OMT versus HT for each New York Heart Association (NYHA) class. The simulation calculates average life expectancy. The following assumptions were made for OMT annual mortality: class I no excess mortality from HF; class II and III based on MERIT-HF are 5.3% and 8.1%. Class IV is 12.8%, based on COPERNICUS. HT mortality rates were based on survival curves for HT 1982 to 2001. For classes I, II, and III, OMT demonstrated a life expectancy gain of 113 months (232+/-2.2 versus 119+/-2.1), 38 months (152+/-2.1 versus 114+/-2.1), and 6 months (117+/-1.8 versus 111+/-2.2), respectively, over HT. Class IV favored HT with a life expectancy gain of 26 months (107+/-2.1 versus 81+/-1.4) over OMT. Sensitivity analysis revealed if improvement in OMT decreased mortality by 38% for class IV patients, OMT and HT would have equivalent life expectancies. If improvement in HT resulted in a 7% increase in post-HT survival, OMT and HT would be equivalent for class III patients. If improvement in HT resulted in a 30% increase in post-HT survival, OMT and HT would be equivalent for class II patients. CONCLUSIONS: Our model predicts that currently, OMT is superior to HT for classes I, II, and III, but HT is superior for class IV. However, future advances in OMT or HT may change the relative benefits of these treatment modalities.

Adrenergic beta-Antagonists↗
PubMed2005

Temporal statistics of the beam-wander contribution to scintillation in ground-to-satellite optical links: an analytical approach.

The beam-wander contribution to the scintillation in a ground-to-satellite free-space optical link is one of major importance. An analytical model, based on the duality between beam wander and angle-of-arrival fluctuations, is proposed for the temporal statistics. The expression of the probability density function of the log-amplitude fluctuations is first obtained. Then, the expressions of the spatial and temporal autocovariances are also obtained. We present plots of the beam-wander contribution to the log-amplitude variance, as a function of the transmitter aperture size and the turbulence accumulated in the propagation path. We also present the angular fluctuation and log-amplitude scintillation spectrum plots for some selected cases.

Journal Article↗
PubMed1997

Novel analytical approach to monitoring advanced glycosylation end products in human serum with on-line spectrophotometric and spectrofluorometric detection in a flow system.

We proposed a simple analytical procedure for measurement of serum advanced glycosylation end products (AGEs) based on simultaneous detection of low-molecular-mass peptides and AGEs with a flow system and two detectors connected on-line: spectrophotometric for peptides (lambda = 280 nm) and spectrofluorometric for AGEs (lambda ex = 247 nm, lambda em = 440 nm). Sample pretreatment was carried out in microcentrifuge tubes: Serum (20 microL) was deproteinized with trichloroacetic acid (480 microL, 0.15 mol/L) and lipids were extracted with chloroform (100 microL). Twenty microliters of the filtered aqueous layer was injected to the flow system and the relation between fluorescence and absorption signals was measured. A peptide-derived AGE calibrator was used for calibration. Within-day and between-day CVs were 6.7% and 9.1%, respectively, at an AGE concentration corresponding approximately to that in healthy individuals. Mean results (+/-SD) in 10 healthy individuals were 10.1% +/- 1.0%, in 21 patients with diabetes without complications 18.0% +/- 6.2%, in 25 patients with complications 24.1% +/- 15.4%, and in 12 diabetic patients in end-stage renal disease 92% +/- 30%. Comparison with an ELISA procedure (x, in arbitrary units/L) yields a regression equation y = 0.713x + 1.24 (Sy [symbol: see text] x = 6777, r = 0.8477, n = 41).

Diabetes Mellitus↗
PubMed2006

An analytical approach to determine the pore shape and size of MCM-41 materials from X-ray diffraction data.

The pore shape and size of MCM-41 were studied analytically by comparing the observed powder X-ray diffraction intensities with that derived from the MCM-41 crystal structure models, with two different pore shapes, a hexagon and a circle. The powder diffraction patterns from the as-synthesized and the calcined MCM-41 were measured by a synchrotron radiation at SPring-8, Japan. The MCM-41 structure with circular and hexagonal pore shapes explains well for the as-synthesized and the calcined MCM-41 crystals, respectively. The pore size and boundary obtained by this approach agree with those obtained from an N2 gas adsorption measurement combined with the Fourier synthesized density map.

Journal Article↗