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PubMed1993

The sulcus connection. On a mode of participation of outer hair cells in cochlear mechanics.

Motile outer hair cells (OHCs) can only participate well in cochlear mechanics when one end of the hair cells is more restrained in its movements than the other. On this thought a model of the organ of Corti (OoC) is developed in which (in every cross section) the tectorial membrane (TM) is considered to consist of two stiff segments connected by a hinge. Movements of the TM then induce movements of the fluid contained in the inner spiral sulcus (ISS) and the fluid dynamics of the sulcus will play an important part in restraining the top ends of the hair cells. Since the model is meant to reflect low-level phenomena, it is conceived as linear. For simplicity, it operates in the long-wave mode. The sulcus is considered as a narrow channel that is closed at both ends. Since the sulcus fluid is an independent energy-storage element of the system, the entire cochlear model, consisting of two main fluid channels, the organ of Corti (OoC), the three-element TM and the sulcus, and equipped with motile OHCs, can be described by a fourth-order differential equation, with four boundary conditions. It is shown that the model can ideally produce response curves that reflect more than 40 dB of cochlear gain. When the actual effect of the sulcus is taken into account, however, the maximum gain is reduced to approximately 30 dB. The main reason for the reduction is that the sulcus cannot produce a large enough pressure to restrain the upper ends of the OHCs.

Acoustics↗
PubMed1994

A method for determination of frequency-dependent effective scatterer number density.

Previous researchers have shown that for low scatterer concentrations, the scatterer number density in an ultrasound pulse-echo experiment can be obtained from ratios of the fourth to the second moment of the backscattered signal. In this paper a new method is presented for determining an effective scatterer number density, which is the actual number density multiplied by a frequency-dependent factor that depends on the differential scattering cross sections of all scatterers. The method of data reduction goes beyond the work of previous authors in that, in addition to accounting for the possibility that different sets of scatterers may dominate the echo signal at different frequencies, it also explicitly retains both the temporal nature of the data acquisition and the properties of the ultrasound field in the data reduction. Tests of the method in phantoms yielded good agreement between measured values of the effective scatterer number density and number densities calculated using first principles.

Acoustics↗
PubMed1995

Physiological modeling of speech production: methods for modeling soft-tissue articulators.

As the initial components of a comprehensive physiological model of speech production, methods have been developed for modeling soft tissue structures, such as the tongue and lips, as continua. This approach provides a foundation for applying finite element methods to simulate these structures in a biomechanical model of speech production. Movements and deformations of the structures can then be computed as the solutions of a nonlinear second-order constraint system of ordinary differential equations, which is obtained from a finite element approximation of an energy rate equation. The muscle fibers in the soft tissue structures are represented as fields that specify the directions in which active and passive tensile stress is produced. The elastic behavior of the passive components is modeled using an isotropic exponential constitutive strain energy function, and the viscous stress components, by using linear viscosity. Incompressibility is maintained elementwise in the simulation using a simultaneously solved system for the computation of Lagrange multipliers. As first step towards modeling a vocal tract, a preliminary model of a tongue with eight muscles has been completed. Simulation results demonstrate the validity of the method and they support the feasibility of a physiologically based model of speech production.

Biomechanical Phenomena↗
PubMed2005

Mechanism of tissue-selective drug action in the cardiovascular system.

Analysis of the human genome project tells us that there may be as few as 3000 genes that are likely to be good drug targets. Although the number of targets is still very large, these data have been interpreted by some to mean that the pharmaceutical industry may someday run out of novel drug targets. Despite the doom and gloom of such analysis, there is considerable reason for optimism. Drugs may exhibit selectivity of action beyond that predicted by target expression alone. Drugs that act at a single molecular target may have very different pharmacology and, as a result, different therapeutic uses. Three well-characterized model systems are highlighted to illustrate this point. The first model system is exemplified by nifedipine and verapamil, both of which act on L-type calcium channels. Both drugs are used to treat hypertension, but only verapamil can be used to produce atrioventricular block in patients with atrial fibrillation. The second model system describes the therapeutic exploitation of unusual conditions that occur in the ischemic myocardium to produce drugs that are more effective for suppressing ischemia-induced arrhythmias. The third model system discusses the mechanisms through which phosphodiesterase-5 (PDE5) inhibitors act selectively to facilitate penile erection while having little effect in the non-penile vasculature that also expresses PDE5.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1997

A general model for the origin of allometric scaling laws in biology.

Allometric scaling relations, including the 3/4 power law for metabolic rates, are characteristic of all organisms and are here derived from a general model that describes how essential materials are transported through space-filling fractal networks of branching tubes. The model assumes that the energy dissipated is minimized and that the terminal tubes do not vary with body size. It provides a complete analysis of scaling relations for mammalian circulatory systems that are in agreement with data. More generally, the model predicts structural and functional properties of vertebrate cardiovascular and respiratory systems, plant vascular systems, insect tracheal tubes, and other distribution networks.

Animals↗
PubMed1988

Neural model of adaptive hand-eye coordination for single postures.

A neural network model has been developed that achieves adaptive visual-motor coordination of a multijoint arm, without a teacher. The model learns to position an arm so that it reaches a cylinder arbitrarily positioned in space. The model uses a new neural architecture and a new algorithm for modifying neural-connection strengths. Computer simulations show that the model performs with an average position error of 4% of the arm's length and with an average orientation error of 4 degrees. The model is designed to be generalized for coordinating any number of topographic sensory inputs with limbs of any number of joints.

Humans↗
PubMed2005

Heading in football. Part 2: biomechanics of ball heading and head response.

OBJECTIVES: Controversy surrounding the long term effects of repeated impacts from heading has raised awareness among the public and the medical community. However, there is little information about the human response to the impacts and what measures can be taken to alter their effect. The objective of the current study was to gain a better understanding of heading biomechanics through the implementation of a numerical model and subsequent investigation of parameters related to heading technique and ball characteristics. METHODS: A controlled laboratory study was carried out with seven active football players, aged 20-23 years who underwent medical screening and were instrumented with accelerometers mounted in bite plates and electromyographic electrodes on the major neck muscle groups. Balls were delivered at two speeds (6 m/s and 8 m/s) as the subjects demonstrated several specific heading manoeuvres. Photographic targets were tracked via high speed video to measure heading kinematics. One subject demonstrating reasonably averaged flexion-extension muscle activity phased with head acceleration data and upper torso kinematics was used to validate a biofidelic 50th percentile human numerical model with detailed representation of the head and neck. RESULTS: Heading kinematics and subject responses were used with a detailed numerical model to simulate impact biomechanics for a baseline heading scenario. Changes to heading techniques and ball characteristics which mitigated head impact response were identified. CONCLUSION: A numerical model combined with biomechanical measurement techniques is an important tool for parametric investigation of strategies to reduce head impact severity via changes in heading technique or the physical properties of the ball.

Adult↗
PubMed2006

Interactions of doctors with the pharmaceutical industry.

OBJECTIVE: To assess the opinions and practice patterns of obstetrician-gynaecologists on acceptance and use of free drug samples and other incentive items from pharmaceutical representatives. METHODS: A questionnaire was mailed in March 2003 to 397 members of the American College of Obstetricians and Gynecologists who participate in the Collaborative Ambulatory Research Network. RESULTS: The response rate was 55%. Most respondents thought it proper to accept drug samples (92%), an informational lunch (77%), an anatomical model (75%) or a well-paid consultantship (53%) from pharmaceutical representatives. A third (33%) of the respondents thought that their own decision to prescribe a drug would probably be influenced by accepting drug samples. Respondents were more likely to think the average doctor's prescribing would be influenced by acceptance of the items than theirs would be (p<0.002). Respondents who distributed drug samples to patients indicated doing so because of patients' financial need (94%) and for their convenience (76%) and less so as a result of knowledge of the efficacy of the sample product (63%). A third (34%) of respondents agreed that interactions with industry should be more strictly regulated. CONCLUSION: Obstetrician-gynaecologists largely indicated that they would act in accordance with what they think is proper regarding accepting incentive items from pharmaceutical representatives. Although accepting free drug samples was considered to be appropriate more often than any other item, samples were most commonly judged to be influential on prescribing practices. The widely accepted practice of receiving and distributing free drug samples needs to be examined more carefully.

Advertising↗
PubMed2004

Identifying and reducing errors with surgical simulation.

The major determinant of a patient's safety and outcome is the skill and judgment of the surgeon. While knowledge base and decision processing are evaluated during residency, technical skills-which are at the core of the profession-are not evaluated. Innovative state of the art simulation devices that train both surgical tasks and skills, without risk to patients, should allow for the detection and analysis of errors and "near misses". Studies have validated the use of a sophisticated endoscopic sinus surgery simulator (ES3) for training residents on a procedural basis. Assessments are proceeding as to whether the integration of a comprehensive ES3 training programme into the residency curriculum will have long term effects on surgical performance and patient outcomes. Using various otolaryngology residencies, subjects are exposed to mentored training on the ES3 as well as to minimally invasive trainers such as the MIST-VR. Technical errors are identified and quantified on the simulator and intraoperatively. Through a web based database, individual performance can be compared against a national standard. An upgraded version of the ES3 will be developed which will support patient specific anatomical models. This advance will allow study of the effects of simulated rehearsal of patient specific procedures (mission rehearsal) on patient outcomes and surgical errors during the actual procedure. The information gained from these studies will help usher in the next generation of surgical simulators that are anticipated to have significant impact on patient safety.

Computer-Assisted Instruction↗
PubMed1977

Noise reduction by radiographic magnification.

Effective noise in the rare-earth system can be substantially reduced by use of radiographic magnification technique. The basic imaging properties of the rare-earth system (Alpha 8-XM) and the medium-speed system (Par-RP) were evaluated by the Wiener spectra, modulation transfer functions, and H&D curves. The visibility of radiographic details was determined with simple test objects. Results indicate that, as the magnification increases, the effective noise in the rare-earth system is reduced, and that the visibility of small structures can be improved.

Models, Anatomic↗
PubMed1977

A modeled study for diagnosis of small anechoic masses with ultrasound.

Hemispherical holes created in the bases of uniform natural sponges were imaged in water to determine how anechoic masses fill in with echoes at increased gains. The increase in gain, from the threshold for outlining a hole, to the fill-in of the hole with echoes, was obtained for a range of hole diameters and imaging conditions. These gain changes fit on a smooth curve when plotted against the hole diameter divided by the ultrasound beam width. The main sources of fill-in, beam width and multiple reverberations, appear to behave similarly in vivo. Understanding these effects may help reduce diagnostic errors.

Cysts↗
PubMed1978

Measurement of left ventricular cast volume by computed tomography.

Angiocardiographic methods available for cardiac-chamber volume measurements are neither consistently accurate nor precise. To explore the capability of computed tomography for left ventricular volume measurement, Silastic casts of 24 normal human left ventricles were measured by a displacement method, a conventional angiocardiographic biplane volume method, and computed tomography. The displacement method used degassing to remove air trapped in the casts; displacement was measured by Archimedes' principle. Cast volumes measured by biplane methods displayed spread around the regression line, caused by the chamber's irregular shape and its variations in orientation. Computed tomographic measurements were independent of chamber orientation and significantly more accurate.

Angiocardiography↗
PubMed1984

Magnetic resonance signal intensity patterns obtained from continuous and pulsatile flow models.

The purpose of this investigation was to extend previous steady state flow studies with magnetic resonance (MR) to pulsatile flow measurements obtained with gated cardiac techniques in man with a pulsatile artificial heart device. Bovine blood and a solution of MnCl2 were studied. Correlation was made with complex and time-varying MR signals observed in the descending aorta on cardiac gated images obtained through the midthorax. MR signals from flowing fluids represent velocity distribution as shown in a velocity profile, and laminar flow is distinguished from nonlaminar. At very slow flow rates, signal intensity is lower than background. As the rate is increased, paradoxical enhancement occurs followed by loss of signal, which is complete at 7 liters per minute with fluid and 15 liters per minute with blood. These areas correspond to maximum fluid velocities of 41 and 88 cm/sec, respectively.

Animals↗
PubMed1984

Validation of a CT method for flow determination. Work in progress.

The dynamic computed tomographic (DCT) method of flow determination is based on the application of venoarterial indicator dilution curves as they reflect flow. Using a simple model consisting of a tube and a single mixing chamber, experimental parameters were optimized in a controlled manner to achieve ideal mixing for flow determination. Under these ideal conditions, and using 16 consecutive 3-second scans of an 8-mm thick section, tube diameters of 8 mm and 20 mm, and flow range of 0.7-2.7 liters per minute, the accuracy of the CT method of flow determination was good (17% error). There was a correlation coefficient of 0.85 between measured flow (FM) and calculated flow (FC), and the paired t-test for FM and FC indicated that there was no statistically significant difference (P = .25). The DCT method of flow determination has been validated in this experiment preliminary to its application to flow characterization in blood vessels in the animal model.

Blood Flow Velocity↗
PubMed1995

Scanner conformity in CT densitometry of the lungs.

PURPOSE: To quantify inter- and intrascanner conformity in computed tomographic (CT) densitometry of the lungs. MATERIALS AND METHODS: With six scanners from four manufacturers, a lung densitometry protocol with several variations was applied for performance comparison. Phantoms included water, air, and a humanoid thorax phantom equipped with a dog lung and exchangeable pseudolungs of polyethylene foam. RESULTS: All scanners produced acceptable CT numbers (Hounsfield units) for water, but some not for air. An incorrect calibration of air density affected all CT numbers at lung densities, but the error was easily correctable. Some systems were more sensitive to object size than others were. Sensitivity of CT numbers to section thickness, reconstruction filter, zoom factor, and table height was small, except for two scanners in relation to section thickness. CONCLUSION: After correction for poor air calibration, scanner conformity was acceptable when the reproducibility of lung densitometry in clinical practice was set as a reference.

Absorptiometry, Photon↗
PubMed1985

Nonuniformity of inner and outer systolic wall thickening in conscious dogs.

Transmural differences in systolic wall thickening were analyzed in 13 conscious dogs by implanting sonomicrometers to continuously measure total wall thickness (WT) and outer WT approximately half the distance through the myocardium at a closely adjacent location. Inner WT was derived by subtraction of outer WT from total WT. Outer wall measurements spanned, on average, the outer 44 +/- 10% (+/- SD) of the wall; derived inner wall measurements spanned the remaining 56 +/- 10%. At rest the fractional contribution (FC) of the outer wall to total systolic wall thickening was 29 +/- 9%, which was significantly less than the FC of the inner wall, 71 +/- 9%. These data are in good agreement with simplified modeling of a cross section of the left ventricle as two concentric rings that predicts that the FC of inner and outer halves of the wall should be approximately 67 and 33%, respectively. During treadmill exercise, the extent of both inner and outer thickening increased significantly (30 and 29%, respectively) but the relative FC of the inner and outer wall remained the same. The data indicate that systolic wall thickening is nonuniform and that this nonuniformity remains constant during the increased inotropic and chronotropic stimulation associated with exercise.

Animals↗