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At least 325 records · Page 18Linked to original sources

PubMed1992

A new guide for positioning of maxillary posterior denture teeth.

There is no definitive method for the arrangement of artificial teeth in complete denture construction. A new method is introduced whereby the maxillary posterior teeth have been set in approximate positions mediolaterally similar to their natural predecessors. The procedure is based on a constant relationship derived from the natural dentition. Its clinical application proved to be suitable for the arrangement of artificial teeth for complete dentures with minimal errors.

Adult↗

PubMed1991

Studies on in vitro proteolytic sensitivity of peptides inhibiting herpes simplex virus ribonucleotide reductases lead to discovery of a stable and potent inhibitor.

The nonapeptide, HSV R2-(329-337), corresponding to the subunit 2 (R2) carboxyl terminus of herpes simplex virus (HSV) ribonucleotide reductases, specifically inhibits this enzyme activity. We report here that under standard reductase assay conditions, this peptide was rapidly degraded by proteases present in the partially purified enzyme extract. The main process of proteolysis involves the successive removal of Tyr329 and Ala330, which corresponds to an aminopeptidase activity. Determination of the proteolytic susceptibility of HSV R2-(329-337) analogs showed that natural modifications which are present in the homologous varicella zoster virus (VZV) nonapeptide decreased its susceptibility to protease action 1.5-fold. Nx-acetylation, a modification known to protect peptides against aminopeptidase attacks, greatly improved the proteolytic resistance of HSV and VZV nonapeptides. Moreover, Ac-VZV R2-(298-306) exhibited a 15-fold higher potency on reductase inhibition than HSV R2-(329-337). The degradation process of HSV R2-(329-337) was partially inhibited by amastatin, bestatin, and leupeptin whereas it was completely abolished by bacitracin, suggesting a combined action of more than one aminopeptidase activity. Moreover, bacitracin protected most of these nonapeptide analogs from proteolysis, although it was less effective in preventing HSV R2-(332-337) degradation. Our results indicate that it is possible to determine, in the presence of bacitracin, the relative inhibitory potencies of HSV R2-(329-337) analogs with minimal error due to proteolytic susceptibility. Moreover, HSV R2-(329-337) modifications that were found to protect the peptide against degradation might be useful to increase its efficacy in vivo.

Amino Acid Sequence↗

PubMedDate not supplied

Calculation of intraocular lens power after corneal refractive surgery.

PURPOSE: Underestimation of required intraocular lens (IOL) power with resultant hyperopia is common in post-corneal refractive surgery eyes. A number of methods to minimize error have been proposed but most studies have been small and theoretical. METHODS: We retrospectively reviewed 34 eyes that had undergone routine phacoemulsification and IOL implantation after photorefractive keratectomy or laser in situ keratomileusis. Sixteen eyes were included in the final analysis. Using known pre- and postoperative data, four methods were used to obtain keratometric values combined with three common IOL formulae (Holladay 2, SRK/T and Hoffer Q) and Koch's published Double-K nomogram. The Double-K method was also used in conjunction with the Holladay 2 formula. Target refractions were calculated and then compared to actual postoperative results. RESULTS: The Clinical History method at the spectacle plane produced the lowest mean K-values. Shammas adjustment formula combined with the Holladay 2 and Hoffer Q produced results closest to emmetropia. The Double-K methods produced the least number of hyperopic results. Overall, all methods would have resulted in unacceptably high rates of hyperopia and deviation from target refraction. CONCLUSIONS: No method produces acceptably consistent results because modern IOL formulae were designed for presurgical eyes. Accuracy will only be improved when new IOL formulae based on the anatomy of postrefractive eyes become available. Shammas adjustment formula and regression formulae are viable alternatives especially when there is a lack of preoperative data. The Double-K methods are best suited to avoiding a hyperopic surprise.

Algorithms↗

PubMed1976

The temporal and steady-state relationships between activation of the sodium conductance and movement of the gating particles in the squid giant axon.

1. Comparisons were made between the kinetics and steady-state properties of the sodium conductance changes and of the sodium gating currents, in the squid giant axon perfused with caesium fluoride and maintained at a high membrane holding potential. The conductance measurements were made with reduced external sodium and as much electrical compensation as possible, in order to minimize errors caused by the series resistance. 2. After an initial delay of 10-150 musec whose size was a function of the holding potential and pulse amplitude, the conductance rise on depolarization followed cube law kinetics. 3. Values of the time constant taum, as defined by Hodgkin & Huxley (1952b), were determined for membrane potentials ranging between -140 and +70 mV. They lay on a nearly symmetrical bell-shaped curve with maximum (at 6-3 degrees C) of just under 500 musec at -36 mV. 4. Values of the gating current time constant tau(V) were determined over the same potential range, and found to lie on a very similar bell-shaped curve. A computed least-squares best fit gave the maximum as 460 musec, also falling at about -36 mV. 5. The midpoint of the minfinity curve lay at -34 mV, and its slope at this point was 0-0139 mV-1. Another series of measurements on intact axons gave a midpotential of -25 mV. In the perfused axons the state of the membrane was better described by the constant field equation than by gNa. Recalculation of minfinity from PNa shifted the curve about 15 mV in a positive direction.

Animals↗

PubMed1985

Calcium entry and transmitter release at voltage-clamped nerve terminals of squid.

Presynaptic and post-synaptic cells of the squid giant synapse were voltage-clamped simultaneously to study the relationship between presynaptic Ca current and transmitter-induced post-synaptic current (p.s.c.). Local Ca application was used to restrict Ca current and transmitter release to a limited region of the presynaptic terminal and thus minimize errors due to spatial heterogeneity of presynaptic membrane potential. Presynaptic terminals were depolarized by brief (3-6 ms) voltage-clamp pulses of varying amplitude to collect graded series of presynaptic Ca current and p.s.c. records. During presynaptic depolarization at 14 degrees C, Ca current activation preceded initial onset of p.s.c. (on-p.s.c.) by an interval of approximately 1 ms. The main component of on-p.s.c. followed Ca current activation by about 2 ms. The delay between a brief Ca tail current and peak response of the p.s.c. produced after pulse termination (off-p.s.c.) was also approximately 2 ms. Curves relating both Ca current and p.s.c. magnitudes to presynaptic potential were bell shaped with peaks near -10 mV, but the p.s.c. curve showed stronger voltage dependence on both sides of the peak. With very small and very large presynaptic command pulses, Ca current could be observed without measureable p.s.c. Synaptic transfer curves, plotting p.s.c. as a function of presynaptic Ca current, resembled third-power functions. On the average, p.s.c.s fit a curve representing the 2.9 power of Ca current (range 2.4-3.5 in eighteen experiments). Transfer curves consisted of two limbs: one from presynaptic pulses below -10 mV and the other from more positive pulses. These two limbs were similar and generally resembled power functions of identical exponent. It is thus likely that the third-power function accurately reflects synaptic current transfer, rather than interference from some other voltage-dependent process. Power functions fitting small-pulse and large-pulse limbs of some transfer curves had different scale coefficients, even though exponent values were the same. Consideration of synaptic transmission kinetics suggests that the voltage dependence of Ca channel opening rates can probably explain the difference in transfer curve limbs. Our experiments provide no evidence for an intrinsic voltage dependence of the transmitter release process.

Action Potentials↗

PubMed2005

Interpolating three-dimensional kinematic data using quaternion splines and hermite curves.

Kinematic interpolation is an important tool in biomechanics. The purpose of this work is to describe a method for interpolating three-dimensional kinematic data, minimizing error while maintaining ease of calculation. This method uses cubic quaternion and hermite interpolation to fill gaps between kinematic data points. Data sets with a small number of samples were extracted from a larger data set and used to validate the technique. Two additional types of interpolation were applied and then compared to the cubic quaternion interpolation. Displacement errors below 2% using the cubic quaternion method were achieved using 4% of the total samples, representing a decrease in error over the other algorithms.

Algorithms↗

PubMed2006

Highly accelerated MRI by skipped phase encoding and edge deghosting with array coil enhancement (SPEED-ACE).

The fast MRI method of skipped phase encoding and edge deghosting (SPEED) is further developed with array coil enhancement, and thus is termed SPEED-ACE. In SPEED-ACE, k space is sparsely sampled with skipped phase encoding at every Nth step using a set of receiver coils simultaneously, similar to SENSE, leading to sensitivity-weighted images with up to N layers of overlapping aliasing ghosts. The ghosted images are edge enhanced by a differential filter to yield ghosted edge maps, in which the ghost overlapping layers are greatly reduced since the sparseness of edges reduces the chance of ghost overlapping. Typical ghosted edge maps can be adequately modeled with a double-layer structure. By using data from at least three coils through least-square-error minimization, a deghosted edge map is obtained and inverse-filtered into a final deghosted image. In this way, SPEED-ACE partially samples k space with a skip size of N by using multiple receiver coils in parallel, and obtains a fairly good deghosted image with an undersampling factor of N. SPEED-ACE is not limited to the double-layer ghost model, but can be generalized to include more layers of ghosts for more flexible and improved performance. As a new parallel imaging method, SPEED-ACE was tested using in vivo data to demonstrate the possibility of achieving undersampling factors even greater than the number of receiver coils, which is so far not achievable by other parallel imaging methods.

Algorithms↗

PubMedDate not supplied

Silicon diode detectors used in radiological physics measurements. Part II: Measurement of dosimetry data for high-energy photons.

Initial calibration of a linear accelerator requires physics instruments to measure accurately central axis depth-dose and off-axis data, both in and out of the beam. These data for an 8- MeV unit were first measured using film, a Farmer 0.6-cm3 ion chamber, a 0.3-cm3 ion chamber, and a 0.1-cm3 silicon diode. Both small probes and film gave a high response compared to the Farmer probe, which has a uniform energy response. Measurements with the diode interfaced to an X-Y recorder required only a fraction of the time required with the chambers, minimizing error due to change in machine output, and permitted resolution of isodose lines in the penumbra. However, corrections required at points in depth due to nonuniform energy response of the unshielded diode were laborious. Construction of a partially shielded diode which duplicates the response of the Farmer probe eliminated the necessity for corrections, permitting rapid accumulation of a wide range of depth-dose and off-axis data.

Elementary Particles↗

PubMedDate not supplied

A scanning-slit x-ray videoabsorptiometric technique for bone mineral measurement.

An x-ray videoabsorptiometric technique was developed for measurement of bone mineral content (BMC) in vivo. The principle utility of this technique is the precise measurement of commonly fractured bones, such as the femoral neck, that are difficult to measure by other techniques because of repositioning problems. Scanning slits reduce scattered radiation and improve linearity of measurements. Heavily filtered, high-kVp beams are used to minimize errors from beam hardening, and data renormalization is employed to compensate for spatial nonuniformities of the beam and detector. Linearity of measured BMC over the range 0.8 to 5 g/cm2 is very good (r = 0.998) and compares well to single- and dual-photon absorptiometry. A 1.6% change in measured BMC is observed for a 10% change (approximately 2 cm) in tissue thickness while a 10% change in marrow type causes a 0.6%-0.8% change in BMC. Manual repositioning of a femur phantom revealed a variation of 0.84% over ten measurements when femur values were referenced to standards. A computer repositioning algorithm provides much easier identification of the region for analysis and yields comparable variation (0.9%).

Aged↗

PubMed1994

Implementation of a modified birdcage resonator for 19F/1H MRI at low fields (0.14 T).

Fluorine-19 nuclear magnetic resonance of perfluorinated blood substitute materials provides a method for determination of oxygen tension (pO2) in vivo. Use of a double resonant 19F/1H radio frequency coil allows convenient correlation between the high resolution anatomic presentation of proton images and the fluorine distribution. However, quantitative 19F measurements require an RF coil with good H1 field homogeneity over the image volume and a high quality factor (Q) to minimize errors caused by the low signal-to-noise levels available in in vivo imaging and image nonuniformities introduced by the large chemical shift of fluorocarbons. The birdcage coil design provides a high Q structure with optimum H1 field uniformity and fill factor. However, at low resonance frequencies, the inherently low inductance of the birdcage geometry requires the use of a large number of chip capacitors giving rise to unwieldy coil fabrication and increased cost. This communication describes a modification to the birdcage design that reduces the chip capacitor requirement by at least a factor of 4 for a given dimension, yet retains the essential characteristics of the birdcage design. The modified structure was tuned for double resonance at 5.7/6.0 MHz for 19F/1H magnetic resonance imaging at 0.14 T. For a coil with a length to diameter ratio of 1.67, an H1 uniformity of +/- 2% for the 19F resonance was obtained over a cylindrical region with radius approximately 0.6r (r = radius of coil) and length approximately 1.8r within the coil.

Animals↗

PubMed1995

An MR compatible flow simulator for intravascular pressure simulation.

An MR compatible flow simulator is described which generates physiologically realistic pressure and flow waveforms. The simulator is based on a servomotor-driven gear pump which produces pulsatile flow by modulation of the servomotor rotation rate. Operation of the simulator is under the control of a personal computer, which executes an iterative feedback loop to minimize errors between measured and desired pressure waveforms. The simulator is totally automatic, requiring only a few minutes of iteration to generate the desired pressure waveform. Accurate sinusoidal waveforms with frequencies up to 10 Hz have been generated using the simulator, with high-frequency contamination of the measured waveform at least 80 dB below the fundamental frequency. Aortic waveforms have been produced with realistic flow rates and pressure variations. The pump assembly is mechanically straightforward and can operate at an 8-m distance from the flow phantom to allow the device to be isolated from the MR magnet room.

Biophysical Phenomena↗

PubMed1988

Is the structure of bat echolocation calls an adaptation to the mammalian hearing system?

A common feature of most bat echolocation calls is their hyperbolalike frequency modulation. It is proposed that these calls evolved as an adaptation to the filters in the peripheral hearing system. From an analysis of 420 echolocation sounds of Eptesicus fuscus, the bandwidths of filters giving a minimal error of time-delay estimation are predicted; these could be compared to neurophysiological findings.

Adaptation, Physiological↗

PubMed1993

Passive acoustic localization of the Atlantic bottlenose dolphin using whistles and echolocation clicks.

A method for localization and tracking of calling marine mammals was tested under realistic field conditions that include noise, multipath, and arbitrarily located sensors. Experiments were performed in two locations using four and six hydrophones with captive Atlantic bottlenose dolphins (Tursiops truncatus). Acoustic signals from the animals were collected in the field using a digital acoustic data acquisition system. The data were then processed off-line to determine relative hydrophone positions and the animal locations. Accurate hydrophone position estimates are achieved by pinging sequentially from each hydrophone to all the others. A two-step least-squares algorithm is then used to determine sensor locations from the calibration data. Animal locations are determined by estimating the time differences of arrival of the dolphin signals at the different sensors. The peak of a matched filter output or the first cycle of the observed waveform is used to determine arrival time of an echolocation click. Cross correlation between hydrophones is used to determine inter-sensor time delays of whistles. Calculation of source location using the time difference of arrival measurements is done using a least-squares solution to minimize error. These preliminary experimental results based on a small set of data show that realistic trajectories for moving animals may be generated from consecutive location estimates.

Animals↗

PubMed1961

Oxygen consumption of tissues in the human lung.

A method for estimating the metabolic rate of the human pulmonary tissues is described. Six patients with far advanced pulmonary tuberculosis showed an average rate of 12 percent of the total oxygen consumption. A method for measuring the right ventricular output with an inert gas is also outlined, and the use of this approach to minimize errors in the estimation of the tissue metabolic rate is described.

Heart Ventricles↗

PubMed2001

Determination of DNA content of aquatic bacteria by flow cytometry.

The distribution of DNA among bacterioplankton and bacterial isolates was determined by flow cytometry of DAPI (4',6'-diamidino-2-phenylindole)-stained organisms. Conditions were optimized to minimize error from nonspecific staining, AT bias, DNA packing, changes in ionic strength, and differences in cell permeability. The sensitivity was sufficient to characterize the small 1- to 2-Mb-genome organisms in freshwater and seawater, as well as low-DNA cells ("dims"). The dims could be formed from laboratory cultivars; their apparent DNA content was 0.1 Mb and similar to that of many particles in seawater. Preservation with formaldehyde stabilized samples until analysis. Further permeabilization with Triton X-100 facilitated the penetration of stain into stain-resistant lithotrophs. The amount of DNA per cell determined by flow cytometry agreed with mean values obtained from spectrophotometric analyses of cultures. Correction for the DNA AT bias of the stain was made for bacterial isolates with known G+C contents. The number of chromosome copies per cell was determined with pure cultures, which allowed growth rate analyses based on cell cycle theory. The chromosome ratio was empirically related to the rate of growth, and the rate of growth was related to nutrient concentration through specific affinity theory to obtain a probe for nutrient kinetics. The chromosome size of a Marinobacter arcticus isolate was determined to be 3.0 Mb by this method. In a typical seawater sample the distribution of bacterial DNA revealed two major populations based on DNA content that were not necessarily similar to populations determined by using other stains or protocols. A mean value of 2.5 fg of DNA cell(-1) was obtained for a typical seawater sample, and 90% of the population contained more than 1.1 fg of DNA cell(-1).

Animals↗

PubMed1966

Isotopic method for estimating microbial cell volumes.

Reid, Della F. (University of Hawaii, Honolulu), and Hilmer A. Frank. Isotopic method for estimating microbial cell volumes. J. Bacteriol. 92:639-644. 1966.-A sensitive isotopic method was developed for estimating the mean cell volume in small numbers of bacterial spores by measuring directly the extracellular fluid volume when cells are present in a saturated inulin solution containing inulin-C(14) at low specific activity. The method minimizes errors due to solute adsorption to the cells and avoids errors arising from differences in cell packing. The effects of these sources of error on cell volume and intercellular space estimations made with nonpenetrating solutes are discussed.

Bacillus subtilis↗

PubMed2001

DMP3: a dynamic multilayer perceptron construction algorithm.

This paper presents DMP3 (Dynamic Multilayer Perceptron 3), a multilayer perceptron (MLP) constructive training method that constructs MLPs by incrementally adding network elements of varying complexity to the network. DMP3 differs from other MLP construction techniques in several important ways, and the motivation for these differences are given. Information gain rather than error minimization is used to guide the growth of the network, which increases the utility of newly added network elements and decreases the likelihood that a premature dead end in the growth of the network will occur. The generalization performance of DMP3 is compared with that of several other well-known machine learning and neural network learning algorithms on nine real world data sets. Simulation results show that DMP3 performs better (on average) than any of the other algorithms on the data sets tested. The main reasons for this result are discussed in detail.

Algorithms↗

PubMed2002

Community contexts of human welfare.

This chapter identifies "context minimization error" as the tendency to ignore the impact of enduring neighborhood and community contexts on human behavior. The error has adverse consequences for understanding psychological processes and efforts at social change. The chapter describes a series of theoretical models of how neighborhoods and community settings are associated with various aspects of human welfare and reviews evidence of associations of contexts with health, psychological distress, risky behaviors, psychological attitudes, and child development. It suggests that many psychological processes may play out differently in different contexts and that contextual factors interact with sociocultural characteristics of individuals in predicting outcomes. People, in turn, can shape community contexts. A more sophisticated understanding of the effects of contexts depends on more sophisticated approaches to assessing them.

Adaptation, Psychological↗