PubMed Health⌕ Search

Biomedical subjects

O Dössel

Publications and source records attributed to O Dössel.

At least 37 records · Page 2Linked to original sources
PubMed2002

Numerical calculations of switched magnetic field gradients during magnetic resonance imaging.

During magnetic resonance imaging (MRI) pulse-sequences the human body is exposed to switched magnetic gradient fields. These gradients become stronger and are switched faster for fast imaging. Effects resulting from these fields with trapezoidal waveforms are on the one hand sensory perception of induced currents and on the other hand muscular and cardiac stimulation. All three components of the current density induced by gradient pulse sequence were analysed in a high-resolution model of the human torso. The evaluation of the calculated data was performed thoroughly in the region of the heart muscle of the torso model to find out how different waveforms of the switched gradient field influence strength and direction of induced currents.

Electrocardiography↗
PubMed2002

Thermal heating of human tissue induced by electromagnetic fields of magnetic resonance imaging.

The paper presents a simulation of the transient temperature distribution in the human body caused by induced eddy currents during magnetic resonance imaging (MRI). In a first simulation the validity of the used heat conduction equation was proven using a simple example of a cool-down-process of a sphere. Thereafter the heating of a phantom model with an implanted electrode placed in a MRI-System (active body coil) was examined. The resulting increase in temperature was compared with existing measurements. Finally the implications of the heating of the tissue are discussed based on the observed experimental and numerical results.

Body Temperature Regulation↗
PubMed2002

Simulation of a birdcage and a ceramic cavity HF-resonator for high magnetic fields in magnetic resonance imaging.

The aim of this work was the 3D-simulation of a dielectric resonator for high-field-MRI. A 12-rod-bird-cage-resonator was simulated in a first step, in order to verify the capability of the commercial simulation software MAFIA to simulate homogeneous, transversal B-fields in resonators. The second step was the simulation of frequency-independent dielectric ceramic resonators for static magnetic field strengths of 7 T and 12 T (294 MHz and 504 MHz respectively). The results were compared to the measured results of a manufactured TiO2- and a Al2O3-resonator. Only minor deviations showed up. These results led to the conclusion that dielectric resonators for high field MRI can be optimised using numerical field calculation software.

Artifacts↗
PubMed2002

Development and characterisation of a ceramic HF-resonator for the MR-tomography.

Future trends in magnetic resonance imaging (MRI) lead to higher magnetic field strengths of the static magnetic fields and as an implication of that to much higher frequencies. Nowadays a common model of a send-receive coil is the birdcage resonator. However it is very difficult to find an optimal L/C-relation for the capacities and inductivities at frequencies above 300 MHz. The idea is to build a completely new send-receive resonator without discrete network elements. The solution presented in this work is the development of a fully ceramic resonator with a high dielectric constant (between 30-100) and a low loss factor (tan delta approximately 10(3)). This approach has shown to produce a stable transversal magnetic field at the desired MR-frequencies above 300 MHz.

Artifacts↗
PubMed2002

Hyperelastic description of elastomechanic properties of the heart: a new material law and its application.

Knowledge concerning passive mechanic cardiac properties is necessary to model behavior of whole hearts. Commonly, a continuum mechanics based description is chosen in conjunction with the finite element method. The aim of this work is to summarize, derive and evaluate hyperelastic material laws for inhomogeneous, anisotropic myocardium. Hence, different material laws were set up and their parameters were determined taking measurement data in literature into account. The material laws were compared from a theoretical and numerical point of view. Furthermore, the application of continuum mechanics based methods is evaluated concerning aspects of numerical solution and spatial discretisation. In further work the laws will be implemented and integrated in an existing software environment, which allows the calculation of deformations in complex geometries.

Anisotropy↗
PubMed2002

Modeling force development in the sarcomere in consideration of electromechanical coupling.

Models of the cellular force development simulate the contractive behavior of the sarcomere. In conjunction with electrophysiological models they can contribute to a better comprehension of physiology and pathologies. Aim of this study is to examine the coupling of cellular electrophysiological processes and force development. For that a graphical user interface was developed to simplify the parameterization and calculation of the models as well as to present the results graphically. A feedback mechanism is introduced to pay attention to close connections between force development and intracellular processes. On basis of various tests with different boundary conditions, new force models are developed, parameterized, validated and compared with models in literature. In future studies the results will be tested in multiple cell organization.

Actins↗
PubMed2002

Design of a system for contact-free measurement of the conductivity of biological tissue.

The electrical impedance of tissue can give important informations about the viability of the tissue. A non-invasive and contact-free measuring system using an inductive sensor is presented. Using a single pick-up coil the system is not sensitive enough for measuring the small changes in conductivity of biological tissue. Employing a gradiometer instead of a single pick-up coil can improve the resolution of the system. The developed data acquisition system is realized using four parts: A lock-in-amplifier, providing a sinusoidal signal and measuring the signal of the sensor, a power amplifier, driving the excitation coil of the sensor, the inductive sensor and a preamplifier for buffering a reference signal and amplifying the output signal of the sensor. In this paper the focus is on the hardware that was set up. Results of measurements on inhomogeneous phantoms are shown.

Amplifiers, Electronic↗
PubMed2002

[Computer models of the electrophysiological properties of the heart].

Computer models of the heart can improve the understanding of the electrophysiological processes in healthy and diseased heart. They become more and more important for detailed diagnosis of arrhythmias and for optimization of therapy. Models of myocardium cells known today are described--they are based on the properties of all relevant ion channels in the cell membrane. Then it is demonstrated, how many cells can be joined to form a cell patch and how finally the complete heart can be modelled. A simpler approach is using a so called cellular automation that allows for a significant reduction of calculation time while sacrificing some accordance to reality. Adaptive cellular automations allow for a fast simulation with acceptable accuracy. Using them some results were gained for the simulation of typical arrhythmias, in the field of validation using an animal model and for therapy planning with RF-ablation.

Arrhythmias, Cardiac↗
PubMed2001

Accelerated coronary MRA by simultaneous acquisition of multiple 3D stacks.

The implementation and first in vivo results of a novel coronary magnetic resonance angiography (MRA) protocol allowing simultaneous acquisition of multiple geometrically independent 3D imaging stacks are presented. Each imaging stack is acquired in a separate cardiac phase using an individual magnetization preparation and navigator-based gating and prospective motion correction. Each stack covers one of the main coronary vessels. Thus, an improvement of scan efficiency was achieved, which was used in this study to reduce total scan time at standard image quality. Experiments performed in healthy volunteers and in patients using a two-stack approach yielded a total scan time reduction of 50% with an image quality equivalent to standard single-stack coronary MRA.

Adult↗
PubMed1999

Monte Carlo simulations of the imaging performance of metal plate/phosphor screens used in radiotherapy.

The imaging performance of metal plate/phosphor screens which are used for the creation of portal images in radiotherapy is investigated by using Monte Carlo simulations. To this end the modulation transfer function, the noise power spectrum and the detective quantum efficiency [DQE(f)] are calculated for different metals and phosphors and different thicknesses of metal and phosphor for a range of spatial resolutions. The interaction of x-rays with the metal plate/phosphor screen is modeled with the EGS4 electron gamma shower code. Optical transport in the phosphor is modeled by simulating scattering and reabsorption events of individual optical photons. It is shown that metals with a high atomic number perform better than lighter metals in maximizing the DQE(f). It is furthermore shown that the DQE(f) for the metal plate/phosphor screen alone is nearly x-ray quantum absorption limited up to spatial frequencies of 0.4 cycles/mm. In addition, it is argued that the secondary quantum sink of optical photons imposed by the optical chain (mirror, lenses and video camera) leads to a significant degradation of the signal-to-noise ratio at spatial frequencies which are most important for successful registration of portal images. Therefore, the conclusion is that a replacement of the optical chain by a flat array of photodiodes placed directly under the phosphor will lead to a substantial improvement in image quality of portal images.

Algorithms↗