MR-tomography on patients with heart pacemakers-a numerical study

Patients having a heart pacemaker are not allowed to go to MR tomography (MRT). One of the most dangerous effects is the heating of the tissue around the electrode caused by the coupling to the RF field of the MR system. Experiments have been carried out using tanks filled with saline water and large heating has sometimes been observed. Other experiments e.g. with electrodes in the brain did not show any heating at all. In this work numerical studies have been carried out to understand the different results. In conclusion it is suggested that MRT could be possible if the "normal" geometry of the wires of a heart pacemaker is ensured and an open MR system is used.

Improvement of patient return electrodes in electrosurgery by experimental investigations and numerical field calculations

Numerical field calculations and experimental investigations were performed to examine the heating of the surface of human skin during the application of a new electrode design for the patient return electrode. The new electrode is characterised by an equipotential ring around the central electrode pads. A multi-layer thigh model was used, to which the patient return electrode and the active electrode were connected. The simulation geometry and the dielectric tissue parameters were set according to the frequency of the current. The temperature rise at the skin surface due to the flow of current was evaluated using a two-step numerical solving procedure. The results were compared with experimental thermographical measurements that yielded a mean value of maximum temperature increase of 3.4 degrees C and a maximum of 4.5 degrees C in one test case. The calculated heating patterns agreed closely with the experimental results. However, the calculated mean value in ten different numerical models of the maximum temperature increase of 12.5 K (using a thermodynamic solver) exceeded the experimental value owing to neglect of heat transport by blood flow and also because of the injection of a higher test current, as in the clinical tests. The implementation of a simple worst-case formula that could significantly simplify the numerical process led to a substantial overestimation of the mean value of the maximum skin temperature of 22.4 K and showed only restricted applicability. The application of numerical methods confirmed the experimental assertions and led to a general understanding of the observed heating effects and hotspots. Furthermore, it was possible to demonstrate the beneficial effects of the new electrode design with an equipotential ring. These include a balanced heating pattern and the absence of hotspots.

Magnetic resonance imaging with implanted neurostimulators: numerical calculation of the induced heating

Magnetic resonance imaging (MRI) is still contraindicated in patients with implanted active medical devices, as the applied radiofrequency (RF) fields can lead to significant heating of the implants and the electrodes. A head model with an implanted deep brain stimulation electrode (DBS) was exposed to a continuous RF-field similar to the excitational field used in MRI at a frequency of 64 MHz. In this study a two-step procedure for the accurate estimation of electrode-heating during MRI is presented. First the energy loss was calculated in the frequency domain during an applied RF-pulse. Then a thermodynamic algorithm taking heat transfer mechanisms into account was used. The applied method showed to be numerically stable and gave more accurate results than first calculated using a simple worst-case approximation.

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.

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.

Development of a cost-effective and MRI compatible temperature measurement system

A reliable temperature measurement system working inside a MRI-system is required in order to determine the amount of local temperature rise during application of radiofrequency fields on medical implants and thus to ensure patient safety. Hence the aim of this study was to develop a cost-effective temperature measurement system suitable for use in a MRI system to investigate this heating having mainly phantom experiments in mind. Three active temperature measurement systems were set up, the first using a PTC as the temperature sensor, the other two with platinum resistors of 100 omega and 1000 omega. Interference tests in a MRI systems were performed. It could be shown that a stable temperature measurement at a resolution of 0.1 degree C could be established.

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.

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.

Calculation of the dielectric properties of biological tissue using simple models of cell patches

The measurement of the dielectric properties of biological tissue is of increasing scientific relevance. Models for the comprehension of the dielectric properties at various frequencies have been successfully set up. However, students often have problems in understanding the effects taking place on cellular level which lead to the observed dispersion. A numerical model of a biological tissue brick composed of single cells (micron-dimensions) between two plate electrodes is presented in this study. An electrical current in a range of 1 Hz to 3 GHz was applied to the electrodes and hence to the tissue model. Using an equivalent series circuit of a resistor and a capacitor it is possible to calculate the effective equivalent dielectric properties of the whole tissue model. The results show an increasing conductivity and decreasing permittivity with increasing frequency. This corresponds to experimental results obtained with different biological tissues.

Simulation of non-contact measurement of the electrical impedance using an anatomical model

The measurement of the impedance of biological tissue can be a non-invasive method to find new data of diagnostic relevance. A system for a non-contact measurement of the electrical impedance of biological tissue is presented. The system consists of an excitation coil and two sensing coils, an upper and a lower coil. If the two sensing coils are coupled it can be used as a gradiometer coil. Numerical experiments with focus on the eddy currents in the tissue and on the detection of the small changes of the signal are carried out to calculate the fields, eddy current distributions and induced voltages. Hereby tests with different frequencies of the excitation current and different conductivities of a tissue block are used. Then the homogeneous tissue block is replaced with a fraction of the arm of an anatomical model which contains different tissue classes.

Numerical field calculation of patient return electrodes in electrosurgery

In order to examine the warming up characteristics during application of a new electrode design for a patient return electrode of an electrosurgical system numerical field calculations were performed in this study. A multi-layer thigh model was provided for this purpose, to which the patient return electrode and the active electrode were connected. The simulation geometry as well as the dielectric tissue parameters were set according to the current frequency. The heating up at the skin surface by the flowing current was evaluated. The results were compared with experimental thermographical measurements.