Biomedical subjects

Moshe Rosenfeld

Publications and source records attributed to Moshe Rosenfeld.

11 recordsLinked to original sources

PubMed2006

The hemodynamics of the Berlin pulsatile VAD and the role of its MHV configuration.

The 3D flow in a model of the Berlin ventricular assist device (VAD) chamber with monoleaflet valves placed in S-shape conduits was simulated numerically. The blood flow dynamics were described in terms of flow patterns, velocity, pressure, and shear stress. The hemodynamic properties and the VAD's potential risk for thrombosis were evaluated in terms of mixing and washout properties, and global estimations of platelet level of activation (LOA). In order to evaluate the role of valves on the flow in the chamber, the flow in a model with bileaflet valves in straight conduits was simulated and compared with the original case. The results showed that in both models a large rotating flow was developed in the chamber during filling. This vortex filled the entire chamber and moved constantly up to the peak ejection phase, resulting in relatively low shear stress (up to 0.4 Pa) and no lasting stagnation regions. Significant shear stresses were found near the valves with higher values near the outlet valve in both models. The configuration of valves and conduits had a large effect on VAD washout and mixing properties, with advantage to the bileaflet model. However, since the bileaflet valves exhibited higher shear stresses, higher LOA were found for the bileaflet model.

Animals↗

PubMed2006

Numerical model of flow in a sac-type ventricular assist device.

This article addresses the growing need for comprehensive tools to investigate the hemodynamics of ventricular assist devices (VADs) in general and sac-type VADs in particular. Numerical simulations can be very helpful in these efforts. However, full simulation of flow inside sac-type VADs poses several key problems, among them simulation of the mechanical heart valves and calculation of the motion of flexible walls. We present a simplified three-dimensional (3D) numerical model of a sac-VAD chamber. The walls in the simplified model are defined to move according to experimental measurements, and the valves are modeled in fully open or fully closed positions. The model is validated by comparison to a fully coupled fluid-structure interaction numerical simulation and to experimental measurements using continuous digital particle image velocimetry. Our results demonstrate that the flexible wall motion is sensitive to changes in pressure distribution inside the chamber. However, small variations in wall motion do not significantly affect the global features of flow inside the chamber. Therefore, the simplified model can be used to predict the 3D time-dependent flow field in the VAD.

Heart-Assist Devices↗

PubMed2005

Physical stresses at the air-wall interface of the human nasal cavity during breathing.

The nose is the front line defender of the respiratory system and is rich with mechanoreceptors, thermoreceptors, and nerve endings. A time-dependent computational model of transport through nasal models of a healthy human has been used to analyze the fields of physical stresses that may develop at the air-wall interface of the nasal mucosa. Simulations during quiet breathing revealed wall shear stresses as high as 0.3 Pa in the noselike model and 1.5 Pa in the anatomical model. These values are of the same order of those known to exist in uniform large arteries. The distribution of temperature near the nasal wall at peak inspiration is similar to that of wall shear stresses. The lowest temperatures occur in the vicinity of high stresses due to the narrow passageway in these locations. Time and spatial gradients of these stresses may have functional effects on nasal sensation of airflow and may play a role in the well-being of nasal breathing.

Biomechanical Phenomena↗

PubMed2005

Contactless bio-impedance monitoring technique for brain cryosurgery in a 3D head model.

A contactless induced-current bio-impedance system for monitoring brain cryosurgery procedure was modeled and numerically simulated, where the excitation coil was also performing as the measuring, or pick-up coil. A segmented three-dimensional (3D) MRI database was used for building the volume conductor geometry, and the numerical finite-volume method was employed for solving the forward problem for calculating the scalar potential distribution and the second-order voltage change on the pick-up coil. Several coil configurations were considered, varying in their relative positioning to the 3D head model. For each case, the sensitivity of the measured voltage change on the excitation coil to the volume of a frozen lesion was calculated. The highest sensitivity (1.1 x 10(-5) relative voltage change per mm3 of frozen tissue) was obtained for a coil arrangement where its closest segment to the volume conductor is at the maximum distance away from the frozen region position. The simulated system signal-to-carrier ratio was O(10(-8)).

Brain↗

PubMed2005

The air-conditioning capacity of the human nose.

The nose is the front line defender of the respiratory system. Unsteady simulations in three-dimensional models have been developed to study transport patterns in the human nose and its overall air-conditioning capacity. The results suggested that the healthy nose can efficiently provide about 90% of the heat and the water fluxes required to condition the ambient inspired air to near alveolar conditions in a variety of environmental conditions and independent of variations in internal structural components. The anatomical replica of the human nose showed the best performance and was able to provide 92% of the heating and 96% of the moisture needed to condition the inspired air to alveolar conditions. A detailed analysis explored the relative contribution of endonasal structural components to the air-conditioning process. During a moderate breathing effort, about 11% reduction in the efficacy of nasal air-conditioning capacity was observed.

Air↗

PubMed2005

Induced current bio-impedance technique for monitoring cryosurgery procedure in a two-dimensional head model using generalized coordinate systems.

In the noninvasive bio-impedance technique, small amplitude currents are applied to the body and the developing potentials on its surface are measured. This noninvasive technique is used to monitor physiological and pathological processes, which alter the values or the spatial distribution of the electrical impedance inside the human body. A possible application of the bio-impedance technique is monitoring brain cryosurgery procedure--a surgical technique that employs freezing to destroy undesirable tissues. A numerical solver was developed to evaluate the ability of an induced-current bio-impedance system to monitor the growth of the frozen tissue inside the head in simulation. The forward-problem bio-impedance solver, which is based on the finite volume method in generalized two-dimensional (2-D) coordinate systems, was validated by a comparison to a known analytical solution for body-fitted and Cartesian meshing grids. The sensitivity of the developed surface potential to the ice-ball area was examined using a 2-D head model geometry, and was found to range between 0.8 x 10(-2) and 1.68 x 10(-2) (relative potential difference/mm2), depending on the relative positioning of the excitation coil and the head. The maximal sensitivity was achieved when the coil was located at the geometrical center of the model.

Animals↗

PubMed2004

Induced current electrical impedance tomography system: experimental results and numerical simulations.

In electrical impedance tomography (EIT), measurements of developed surface potentials due to applied currents are used for the reconstruction of the conductivity distribution. Practical implementation of EIT systems is known to be problematic due to the high sensitivity to noise of such systems, leading to a poor imaging quality. In the present study, the performance of an induced current EIT (ICEIT) system, where eddy current is applied using magnetic induction, was studied by comparing the voltage measurements to simulated data, and examining the imaging quality with respect to simulated reconstructions for several phantom configurations. A 3-coil, 32-electrode ICEIT system was built, and an iterative modified Newton-Raphson algorithm was developed for the solution of the inverse problem. The RMS norm between the simulated and the experimental voltages was found to be 0.08 +/- 0.05 mV (<3%). Two regularization methods were implemented and compared: the Marquardt regularization and the Laplacian regularization (a bounded second-derivative regularization). While the Laplacian regularization method was found to be preferred for simulated data, it resulted in distinctive spatial artifacts for measured data. The experimental reconstructed images were found to be indicative of the angular positioning of the conductivity perturbations, though the radial sensitivity was low, especially when using the Marquardt regularization method.

Algorithms↗

PubMed2003

Induced-current electrical impedance tomography: a 2-D theoretical simulation.

A reconstruction algorithm, based on the modified Newton-Raphson algorithm, was developed for induced-current electrical impedance tomography and studied in theoretical two-dimensional geometry representing a human thorax. The finite-volume method was applied for the discretization of the physical domain, resulting in a symbolic representation of the Jacobian matrix, which is accurate and fast to construct. Several system configurations, differing in the number of excitation coils and electrodes, were simulated, and the performance in thoracic imaging was studied. It was found that a six-coil system shows a significant 40% improvement of conductivity values reconstruction over the three-coil system (an error of 2.06 omega(-1) compared with 3.44 omega(-1)). A number of 32 electrodes was found to be sufficient, being the smallest number of electrodes to still provide a reasonable performance (only 4.2% degradation in average conductivity error compared with the maximum possible 106-electrode system).

Algorithms↗

PubMed2002

Induced current impedance technique for monitoring brain cryosurgery in a two-dimensional model of the head.

A fast and robust finite volume solver of the two-dimensional induced current electrical impedance forward problem was developed. The numerical solver was validated by comparison with an existing analytical solution for a symmetrical geometry case, showing an accuracy of 0.07%. The solver was used to theoretically examine the sensitivity of the induced current impedance technique for the medical procedure of monitoring brain cryosurgery. The simulation was performed using a two-dimensional approximation of otherwise realistic geometry model of the head with different ice-ball sizes, simulating the expansion of the frozen lesion. The sensitivity of the scalp potential to the ice-ball size was found to be 53 x 10(-4) (relative scalp potential mm(-2)).

Biomedical Engineering↗

PubMed2002

Unsteady effects on the flow across tilting disk valves.

The present study simulates numerically the flow across two-dimensional tilting disk models of mechanical heart valves. The time-dependent Navier-Stokes equations are solved to assess the importance of unsteady effects in the fully open position of the valve. Flow cases with steady or physiological inflow conditions and with fixed or moving valves are solved. The simulations lead into mixed conclusions. It is obvious that steady inflow cases that account for vortex shedding only cannot model realistic physiological cases. In cases with imposed physiological inflow, the details of the flow field for fixed and moving valves might differ in the fully open position as well, although the gross features are quite similar. The fixed valve case consistently results in safe estimations of several critical quantities such as the axial force, the maximal shear stress on the valve, or the transvalvular pressure drop. Thus, fixed valve simulations can provide useful information for the design of prosthetic heart valves, as long as the properties in the fully open position only are sought.

Biomechanical Phenomena↗

PubMedDate not supplied

A three-dimensional numerical fluid dynamic model of antigen-antibody surface adsorption on piezoelectric immunosensors.

A piezoelectric crystal is a unit that changes its frequency in parallel with a change in its mass. This characteristic is exploited in designing flow cell-based immunosensors for detecting the concentration of antibodies in liquid samples. In the present study, computational fluid dynamic techniques are used to optimize the antigen-antibody binding process on an electrode surface placed on the base of a conical flow cell. The geometry optimization of the flow cell was determined to minimize the test time. This time is needed for the electrode to be saturated by the antibody, a process that requires the maximization of the adsorption rate and be accomplished by increasing the shear rate in the vicinity of the electrode. To validate the numerical model and to determine its parameters, experiments were carried out using an identical flow cell. In the experiments, the system did not reach saturation within an acceptable time frame, therefore, the model parameters were determined based on the unsaturated state. The experimental results confirmed the applicability of numerical simulations in predicting the effect of changing the inlet section area of the flow cells, proving the computational model to be very valuable in designing immunosensors based on flow cells.

Adsorption↗