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PubMedDate not supplied

Dose efficiency and the effects of resolution and noise on detail perceptibility in radiographic magnification.

The detail signal-to-noise ratio model of radiographic imaging is quantitatively analyzed in terms of its accuracy in describing observer threshold perceptibility of radiographic detail. The model is found to adequately describe the effects of magnification, scatter radiation, and system resolution on observer threshold perceptibility. However, it is shown that the model does not apply in screen/film radiography for very low contrasts and high scatter conditions due to insufficient optical density contrast. The dose-to-information conversion efficiency of a radiographic imaging system is defined and the effects of magnification, scatter, resolution, image processing, detector efficiency, grids, patient table support, field size, and geometry on the dose efficiency of the imaging system are investigated.

Models, Theoretical↗
PubMed2005

Unimolecular rate constants for HX or DX elimination (X = F, Cl) from chemically activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl: threshold energies for HF and HCl elimination.

Vibrationally activated CF(3)CH(2)CH(2)Cl molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF(3)CH(2) and CH(2)Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF(3) and CH(2)CH(2)Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF(3)CH(2)CH(2)Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH(3)CH(2)CH(2)Cl and CD(3)CD(2)CH(2)Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination of C(2)H(5) (or C(2)D(5)) radicals with CH(2)Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF(3)CH(2)CH(2)Cl and CH(3)CH(2)CH(2)Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E(0), of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF(3)CH(2)CH(2)Cl and 54 kcal mol(-1) for HCl elimination from CH(3)CH(2)CH(2)Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF(3)CH(2)CH(3,) CF(3)CH(2)CF(3), CH(3)CH(2)CH(2)F, CH(3)CHFCH(3), and CH(3)CF(2)CH(3). In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C(2)H(5)Cl, C(2)H(5)F, and CH(3)CF(3).

Journal Article↗
PubMed2001

Maximizing effective audibility in hearing aid fitting.

OBJECTIVE: This paper examines why more audibility is not always better than less audibility if hearing-impaired people are to best understand speech. DESIGN: We used speech perception data from 14 normally hearing and 40 hearing-impaired people to quantify the contribution of audibility to speech intelligibility. The quantification revealed that the effectiveness of audibility decreased with hearing loss, and the decrement was greater at high frequencies than at lower frequencies. To apply the Speech Intelligibility Index (SII) model to predict speech intelligibility for hearing-impaired people, we modified the model to take account of effective audibility rather than physical audibility. RESULTS: The modified SII model provided an adequate description of speech performance of people with a wide range of hearing threshold levels. We applied the model to the evaluation of two prescriptions for a sloping audiogram at prescribed levels and at equated loudness levels to demonstrate the necessity of considering loudness and effective audibility in prescribing amplification. Effective audibility is defined as audibility corrected for the effects of level distortion and hearing loss desensitization, and this paper proposes a method of estimating effective audibility from hearing threshold level at different frequencies. CONCLUSIONS: The practical implication of considering effective audibility in prescribing hearing aids is that for a given listening level, less gain is provided at frequencies where the hearing is most impaired to allow more gain at frequencies where audibility is most useful. In developing the NAL-NL1 prescription for nonlinear hearing aids, we adopted the modified SII model together with a loudness model to derive optimal gain-frequency response characteristics that maximize predicted speech intelligibility for people with different degrees of hearing losses.

Audiometry, Speech↗
PubMed2026

Unveiling the BMI Risk Threshold for Osteoarthritis: Multi-Database Causal and Nonlinear Evidence.

OBJECTIVE: To characterize the nonlinear relationship between BMI and osteoarthritis (OA), and to identify BMI thresholds that inform precise prevention strategies. METHODS: This multi-database study integrated Global burden of disease 2021, National Health and Nutrition Examination Survey 2007-2018, and Genome-Wide Association Studies. A generalized additive model was performed to visualize the BMI-OA relationship, adjusting for multiple confounders. We applied segmented logistic regression models to identify potential threshold effects and used Mendelian randomization to estimate the causal effects of BMI on OA subtypes. RESULTS: From 1990 to 2021, the age-standardized prevalence and years lived with disability rates for OA were highest in regions with high SDI. OA prevalence rose nonlinearly with BMI, with breakpoints at 24.00 and 41.58 kg/m2. Each unit increase in BMI was associated with higher odds of OA between 24.00 and 41.58 kg/m2 (OR = 1.022, 95% CI: 1.003-1.041) and above 41.58 kg/m2 (OR = 1.055, 95% CI: 1.022-1.090). Women and individuals aged ≥ 45 years exhibited a higher susceptibility to knee osteoarthritis. BMI was causally associated with knee osteoarthritis (OR = 1.63, 95% CI 1.50-1.77) and hip osteoarthritis (OR = 1.54, 95% CI 1.40-1.70). CONCLUSIONS: These findings suggest that OA risk awareness and weight-management strategies should begin before BMI reaches the high range, particularly among individuals with BMI exceeding 24.00 kg/m2.

Humans↗
PubMed2003

Comparison of thermal threshold tests to assess small nerve fiber function: limits vs. levels.

OBJECTIVE: The method of limits (MLI) and the method of levels (MLE) are psychophysical stimulus procedures most commonly applied to quantify warm and cold sensation thresholds in humans. This paper evaluates basic methodological properties of both methods and investigates the correspondence between the method's results. METHODS: Warm sensation threshold was measured in 20 healthy participants using the psychophysical MLIs and MLEs. Two differently shaped kind of levels stimuli were used with triangular (TRIANG) and trapezoid (TRAP) temperature-time profile. RESULTS: A linear model of temperature response, based on threshold level-crossing, quantifies sensation threshold, independent of the MLI inherent 'reaction-time' artifact. It results from modeling MLI responses to warm stimuli with different rates of temperature change. The model also quantifies the reaction-time delay in the physiological system from thermal stimulus presentation until manual response. This study shows that using the reaction-time independent MLE, TRAP should preferably be used for optimal quantification of sensation threshold. CONCLUSIONS: Statistical testing shows that model-based MLI threshold equals MLE threshold provided MLE TRAP stimuli are used. Recommendations for optimal MLI and MLE stimulus configurations and properties are given in relation to application of quantitative sensory testing.

Adult↗
PubMed1993

An epidemiological model for HIV/AIDS with proportional recruitment.

A model for HIV transmission is formulated for a homosexual population of varying size, with recruitment into the susceptible class proportional to the active population size and with stages of progression to AIDS. Analysis of this model includes identifying the threshold that determines whether the disease dies out or proportions remain endemic and establishing criteria that determine whether the population size decays asymptotically exponentially to zero or grows asymptotically exponentially to infinity. In an analogous heterosexual model, the conservation of heterosexual contacts is shown to imply that this two-sex model reduces to the one-sex model.

Female↗
PubMed1988

Analytical threshold and stability results on age-structured epidemic models with vaccination.

This paper examines mathematical models for common childhood diseases such as measles and rubella and in particular the use of such models to predict whether or not an epidemic pattern of regular recurrent disease incidence will occur. We use age-structured compartmental models which divide the population amongst whom the disease is spreading into classes and use partial differential equations to model the spread of the disease. This paper is particularly concerned with an analytical investigation of the effects of different types of vaccination schemes. We examine possible equilibria and determine the stability of small oscillations about these equilibria. The results are important in predicting the long-term overall level of incidence of disease, in designing immunisation programs and in describing the variations of the incidence of disease about this equilibrium level.

Age Factors↗
PubMed1994

Analysis of stereothresholds for stimuli below 2.5 c/deg.

We analyze published data on disparity detection thresholds for a wide range of conditions. This type of detection changes behavior dramatically at the spatial frequency of 2.5 c/deg; above this frequency threshold remains constant while below it threshold grows at a uniform rate. Many other types of threshold, such as upper disparity limits for depth perception and threshold amplitudes for stereo and monocular motion, show similar behavior. These data lead to the postulate that there are no foveal stereo channels peaking below 2.5 c/deg, so that foveal stimuli in the whole range below 2.5 c/deg are processed by a single channel tuned to this frequency. Consequently, disparity detection thresholds at frequencies below this frequency are controlled by the single parameter of effective contrast in the 2.5 c/deg channel, whose output depends jointly on the contrast and spatial frequency of the stimuli. We develop this idea to explain the relations between spatial and contrast tuning functions for disparity thresholds. To validate our conclusions, we describe an experiment with difference-of-Gaussian stimuli over a range of interocular widths and contrast differences. For a dichoptic width ratio of 2:1, the dichoptic contrast ratio required to minimize disparity detection thresholds was 1:4, just as predicted by the model.

Contrast Sensitivity↗
PubMed1995

Decreased inhibition to lateral superior olive neurons in young and aged Sprague-Dawley rats.

Lateral superior olive (LSO) neurons in young and aged Sprague-Dawley rats have functional properties consistent with a limited contralateral inhibition, which is markedly different from other animals. An unusually low proportion of LSO cells (36/113) exhibited contralateral inhibition (and ipsilateral excitation, IE), while over 25% of LSO units exhibited excitatory responses to contralateral stimuli. Inhibition of most IE LSO neurons was evident only when the contralateral intensity was greater than the ipsilateral intensity, resulting in a marked shift in sensitivity to interaural intensity differences (IID). The firing rate of IE neurons was also affected more by a change in intensity of ipsilateral compared to contralateral stimuli. The shift in the IID sensitivity and the relative decrease in effectiveness of contralaterally driven inhibition in Sprague-Dawley rat LSO neurons could be due to decreased inhibitory inputs from the MNTB principal cells, increased contralateral excitatory effects and/or increased ipsilateral excitatory effects. Age-related decreases in the numbers of MNTB neurons observed anatomically is not reflected in a change in LSO function. The Sprague-Dawley rat may be a useful model for the effect of reduced inhibition in the superior olivary complex on auditory behavior.

Acoustic Stimulation↗
PubMed1998

ErbB-2 protein in sera and tumors of breast cancer patients.

We compared levels of erbB-2 oncoprotein among three groups: Group I included 60 asymptomatic women; Group II had 51 women with benign breast biopsies; and Group III had 67 women with node-negative breast cancer. Serological levels of erbB-2 protein were measured in all participants; tumor levels were measured for Groups II and III. Forty-three percent of usable tumors (25/58), including three of seven lobular tumors, were erbB-2 positive. Tumor and blood oncoprotein levels were unrelated. Blood levels, however, were positively related to tumor volume, but only when the tumor had both a ductal carcinoma in situ (DCIS) component and an invasive component, suggesting a role for erbB-2 protein in progression of DCIS to invasive carcinoma. In Groups I and II serological levels of erbB-2 protein were directly related to age, and inversely related to having had a live birth. Therefore, a model that determined the threshold levels of serological erbB-2 positivity in Group III included age and nulliparity as independent variables. Only three of the 67 women (4.5%) in Group III were positive for serological erbB-2. In a multivariate model, with serological erbB-2 as the dependent variable, and in which the independent variables included Study Group, there was a statistical trend for younger women, in which Group III had the highest serological levels of erbB-2, followed by Group II, and then Group I. In women who were over the age of 50 years the trend was reversed; i.e., levels of erbB-2 tended to be lowest in Group III, followed by Group II, and finally Group I.

Age Factors↗
PubMed1998

Empirical therapy versus diagnostic tests in gastroesophageal reflux disease: a medical decision analysis.

Our objective was to describe the conditions that determine the costs of empirical therapy in gastroesophageal reflux disease (GERD). Our design was a threshold analysis using a decision tree. The costs of medications were estimated from the average wholesale prices. The costs of diagnostic procedures were expressed as the sum of physician and facility costs. A decision tree was modeled to calculate the threshold probability of GERD, for which empirical therapy became the preferred management strategy. Bayes' formula was used to transform the sensitivity and specificity of various symptoms and the joint occurrence of multiple symptoms into disease probabilities. The decision in favor of empirical therapy is influenced by four factors: the probability of GERD, the duration or costs of GERD therapy, the costs of erroneous empirical therapy in patients with diagnosis other than GERD, and the costs of diagnostic procedures. In general, the expected benefit of saving the costs of a diagnostic procedure outweighs the costs of occasional erroneous empirical therapy. However, if surgical therapy is considered or antisecretory therapy is administered for a time period of 10 or more years, diagnostic confirmation of GERD should be sought. In the long run, the failure to differentiate between peptic ulcer and GERD results in the highest cost associated with erroneous empirical therapy. In patients with multiple characteristic symptoms of GERD, the diagnosis can be ascertained with sufficient confidence to warrant empirical therapy.

Cost of Illness↗
PubMedDate not supplied

Optimization of input patterns and neuronal properties to evoke motor neuron synchronization.

The study used a computational approach to identify combinations of synaptic input timing and strength superimposed on a variety of active dendritic conductances that could evoke similar levels of motor unit synchronization in model motor neurons. Two motor neurons with low recruitment thresholds but different passive properties were modeled using GENESIS software. The timing and strength of synaptic inputs and the density of dendritic ion channels were optimized with a genetic algorithm to produce a set of target discharge times. The target times were taken from experimental recordings made in a human subject and had the synchronization characteristics that are commonly observed in hand muscles. The main finding was that the two parameters with the highest association to output synchrony were the ratio of inward-to-outward ionic conductances (r = 0.344; P = 0.003) and the degree of correlation in inhibitory inputs (r = 0.306; P = 0.009). Variation in the amount of correlation in the excitatory input was not positively correlated with variation in output synchrony. Further, the variability in discharge rate of the model neurons was positively correlated with the density of N -type calcium channels in the dendritic compartments (r = 0.727; P < 0.001 and r = 0.533; P < 0.001 for the two cells). This result suggests that the experimentally observed correlation between discharge variability and synchronization is caused by an increase in fast inward ionic conductances in the dendrites. Given the moderate level of correlation between output synchrony and each of the model parameters, especially at moderate levels of synchrony (E < 0.09 and CIS < 1.0), the results suggest caution in ascribing mechanisms to observations of motor unit synchronization.

Action Potentials↗
PubMed1993

The kindling model of alcohol dependence: similar persistent reduction in seizure threshold to pentylenetetrazol in animals receiving chronic ethanol or chronic pentylenetetrazol.

Rats on a chronic intermittent ethanol (CIE) regimen showed a persistent reduction in seizure threshold to the convulsant drug pentylenetetrazol (PTZ). CIE rats were given ethanol by intubation on an alternate day schedule and tested at selected intervals for seizure threshold with PTZ. A significant reduction in seizure threshold, a sign of withdrawal, was observed 20 hr after the first dose. The severity of withdrawal intensified on repetition of the ethanol administration and depression-hyperexcitability cycle, with the seizure threshold reaching a maximum decrease after 12 doses and remaining reduced up to 60 doses. The reduction in seizure threshold persisted for at least 40 days of no alcohol following the 60th dose. The long-lasting decrease in seizure threshold following CIE treatment resembled the "kindling" phenomenon produced by chronic administration of PTZ (25 mg/kg, 3 times/week). The CIE rats developed, in addition, a tolerance to the anticonvulsant action of ethanol, which occurred well after the decrease in PTZ seizure threshold, and a tolerance to the hypothermic effect of ethanol, which developed rapidly. PTZ kindled rats that had never been exposed to ethanol also exhibited tolerance to the hypothermic effect of ethanol. We propose that kindling contributes to the mechanism of the development of dependence on central nervous system depressants like benzodiazepines, barbiturates, and alcohol, drugs that act on the gamma-aminobutyric acid-A receptor chloride ion channel complex. Repeated episodes of depression and withdrawal hyperexcitability are postulated to produce kindling during the repeated withdrawal episodes.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcohol Withdrawal Delirium↗
PubMed1980

Detection of temporally uncertain signals.

The effects of signal uncertainty on detection performance were measured using a new procedure that allows precise specification of the initial temporal uncertainty. Five different models of the detection process, two assuming a continous representation of the sensory input and three assuming a discrete representation, were compared with the obtained data. The effects of varying signal uncertainty (the number of potential signal intervals was one, five, or ten) had little effect on detection performance. The one-parameter form of the choice model can be rejected without hesitation. The continuous Gaussian model and the symmetric two-state model are significantly different from the data. The high threshold and sophisticated two-state models provide accurate descriptions of the data that cannot be rejected on statistical grounds.

Acoustic Stimulation↗