A Real-Time Energy Monitoring System for an MRI Hybrid Ablation System.

The MRI hybrid ablation system is an approach to use the MR (magnetic resonance) scanner's radiofrequency amplifier itself as power source for ablation. Hereby, an electrode is connected to the MR internal radiofrequency amplifier. An average RF power is provided through a train of short RF pulses, which is sufficient to thermally destroy tissue. However, ablation with too high power values can cause tissue carbonizations, thus impeding the ablation procedure. Therefore, monitoring of the energy and the power absorbed inside the tissue is necessary. For this purpose, a measurement system was designed to measure the energy applied to the tissue when using the concept of an MRI hybrid ablation system. The system consists of a dual-directional coupler, RF-to-RMS sensors, and a microcontroller. The gradient calculation of the measured energy curve provides information about the absorbed RF power in the tissue. Validation measurements of the system were performed and compared with measurements from the MR-internal power measurement system. The energy monitoring system was also tested in an ablation experiment with ex-vivo animal tissue using the MRI hybrid ablation system. The measurements showed that the applied RF power can be monitored in real-time. It has been shown that the mean RF power absorbed in the patient decreased during an ablation procedure due to an occurring impedance mismatch and tissue changes. In further work, the influence of the monitoring system on the quality of the MR images should be investigated. Clinical relevance- This paper demonstrates an energy monitoring system for an RF ablation system, which can be used inside an MR environment.

Comparison study of reconstruction algorithms for volumetric necrosis maps from 2D multi-slice GRE thermometry images.

Cancer is a disease which requires a significant amount of careful medical attention. For minimally-invasive thermal ablation procedures, the monitoring of heat distribution is one of the biggest challenges. In this work, three approaches for volumetric heat map reconstruction (Delauney triangulation, minimum volume enclosing ellipsoids (MVEE) and splines) are presented based on uniformly distributed 2D MRI phase images rotated around the applicator's main axis. We compare them with our previous temperature interpolation method with respect to accuracy, robustness and adaptability. All approaches are evaluated during MWA treatment on the same data sets consisting of 13 ex vivo bio protein phantoms, including six phantoms with simulated heat sink effects. Regarding accuracy, the DSC similarity results show a strong trend towards the MVEE ([Formula: see text]) and the splines ([Formula: see text]) method compared to the Delauney triangulation ([Formula: see text]) or the temperature interpolation ([Formula: see text]). Robustness is increased for all three approaches and the adaptability shows a significant trend towards the initial interpolation method and the splines. To overcome local inhomogeneities in the acquired data, the use of adaptive simulations should be considered in the future. In addition, the transfer to in vivo animal experiments should be considered to test for clinical applicability.

3D-Printed Floating Cable Traps for MRI guided Microwave Ablation.

Magnetic Resonance Imaging (MRI) guided Microwave Ablation (MWA) allows for real-time therapy monitoring with MRI-thermometry. The MWA generator emits Radio Frequency (RF) interference, which can limit the accuracy of therapy monitoring. The image quality is enhanced by Floating Cable Traps (FCTs) that are used to attenuate common mode currents on supply lines between a MWA generator, and its ablation applicator. The effect of an FCT on the Signal to Noise Ratio (SNR), and changes in the MRI spectrum are discussed in this paper. The application of FCT can bring significant improvements in both, the MRI spectrum and the SNR.Floating Cable Traps are user-friendly. FCT enable coaxial cables to reduce interferences emitted in MRI guided interventions. It is used to selectively attenuate frequencies in the MRI's range. This can increase the image's Signal to Noise Ratio.

Design and Implementation of a Test Procedure for the Evaluation of Interference Coupling in Magnetic Resonance Imaging.

External therapy devices in the shielded room of a magnetic resonance tomograph (MRT) can cause radio frequency (RF) imaging artifacts, which renders the image useless for diagnosis or guiding the procedure. At present, there is no standard procedure to evaluate their conformity with MR imaging.The aim of this paper is to adapt an already existing procedure from the electromagnetic compatibility (EMC), the reverberation chamber (RVC), to evaluate interferences in the magnetic resonance (MR) environment. For this purpose, a test rig was developed which is adapted to the special conditions of the MRI environment. In addition, the suitability of this procedure will be demonstrated in first measurements. The results show that the method can trace and evaluate RF interference of therapy devices. Moreover, the shielded cabin of an MRI system is suitable to perform such measurements.

Design of a System for Magnetic-Resonance-Guided Irreversible Electroporation.

Irreversible electroporation (IRE) is a non-thermal tumor ablation method where strong electrical fields between at least two electrodes are used and can be seen as an alternative to thermal ablation techniques. The therapy outcome directly dependents on the position of the electrodes. Real-time monitoring of the IRE by magnetic resonance imaging (MRI) would allow to detect unwanted electrode displacement and to apply visualization methods for the ablation area. This requires that the IRE generator does not significantly interfere with the MRI. Currently, there is no IRE generator available designed for MRI-guided IRE.This paper presents an IRE system specifically developed for use in an MRI environment. The system is initially tested with a standard IRE sequence and then the interference between a clinical 3 T MRI device and the IRE system is investigated using a noise measurement and the signal-to-noise ratio (SNR) of images acquired with a gradient echo (GRE) sequence. The results show, that although the SNR of the images decrease by maximal 36 % when the IRE system is switched on, image quality does not visibly degrade. Hence, MRI-guided IRE is feasible with the proposed system.Clinical relevance- This paper demonstrates the possibility of MRI-guided IRE with only minor image degradation when the IRE system is used in parallel with MRI imaging.

Distributed Capacitors for MR-Receive-Coils: Theory and Method.

MR coils are a crucial part in the receiving chain of an MRI. Their characteristics determine the signal-to-noise-ratio (SNR) as well as the quality of the illumination of the volume-of-interest (VOI), which is significantly reduced as the circumference of the conductor is comparable in size to the wavelength.A well-known countermeasure to this is the use of distributed capacitors on the circumference of conductor loops. Although this measure is mentioned in numerous works, there is no systematic framework to correctly determine the values of these capacitors. In this work a systematic framework for the analysis of distributed capacitors on conductor loops is established. This is achieved by a four-pole representation of the circular loop, which allows for a eigen-mode analysis to determine the correct values.Based on the results, an experimental method for determining the values is derived and validated in workbench measurements. This provides, for the first time, an easy-to-use method for determining the correct values of distributed capacitors.

Analytical Model of a "Split-Coil" for Implementation of novel Type of Receive Coil in Magnetic Resonance Imaging.

In this work implementing MR receive coils from overlapping traces is investigated. Such a configuration is known from Microstrip transmission line (MTL) coils, which are basically used in Magnetic resonance - ultra high field (MR-UHF) imaging as TX-RX volume coils. Applications at lower field strengths are less common, because the required electrical length is more difficult to satisfy as the frequency decreases. Overlapping traces are already known for lower field strengths like 1.5T or 3T. Such configurations provide the ability of reducing the number of lumped on such a coil becoming more flexible. To investigate such a flexible coil the overlap is extended to much larger degree and it will be shown that this setup can be modeled as classical transmission line. An analytical model is developed and verified with simulations providing accurate calculations of the input impedance. This allows a reliable derivation of its parameters, which simplifies implementation of such coils.

Simulation of SAR Induced Heating in Infants undergoing 1.5 T Magnetic Resonance Imaging.

RF absorption in patients undergoing MRI procedures poses a major safety risk due to resulting heating in the tissue. In order to stay below permitted temperature limits the SAR has to be quantified and limited. Based on the model of an infant inside a birdcage coil we have investigated the SAR distribution in the body at 1.5T. Thermal simulations could thus be performed to establish a relationship between the limitations of SAR and temperature. Results show a thermal hotspot in the neck region caused by high local absorption. The temperature limits in this local area were exceeded after 7min of excitation within regulatory SAR limits. For a long-term exposure critical organs in the body's core also undergo thermal stress beyond limitations. This indicates the need for constraints in regard to long MR procedures to consider the temporal aspect of heating.Clinical Relevance-This work establishes a relationship between SAR and temperature in infants undergoing MRI and shows potential risks of long-term procedures due to induced thermal stress.

Setup of an Ablation Magnetic Resonance Imaging Hybrid System: Using MR Imaging Sequences to Destroy Tissue.

The need of external ablation generators complicates the setup of magnetic resonance (MR) guided interventions, e. g. due to inserting devices with ferrite components into the MR room or because of image distortions due to RF interferences. By using the power provided from the MR internal power amplifier, it is possible to avoid external ablation generators. Using imaging sequences with a high duty cycle, a sufficient mean power value can be generated to destroy tissue. In this paper, it has been shown that it is possible to destroy tissue with such an ablation MRI hybrid system. Simulations were done to calculate the specific absorption rate (SAR) generated by an ablation electrode at the Larmor frequency for a 3 T MR device. The SAR values were then compared with ablation experiments performed inside an MR device with protein phantoms.

Wireless video transmission into the MRI magnet room: implementation and evaluation at 1.5T, 3T and 7T.

Purpose To analyze the interference between a wireless high definition multimedia interface (WHDMI) and magnetic resonance imaging (MRI) image quality at 1.5T, 3T and 7T. Materials and methods A wireless video transmission system (WVTS) consisting of a WHDMI and a projector was used to transmit and display a video stream into the magnet room. MR image quality was analyzed at 1.5T, 3T and 7T. Signal-to-noise-ratio (SNR¯) $(\overline {{\rm{SNR}}} )$ and radio frequency (RF)-noise spectrum were measured at three transmitter positions (A: inside the cabin, B: in front of the waveguide and C: in the control room). WVTS system functionality tests included measurements of reliability, delay and image quality. Results With the WVTS mean SNR¯ $\overline {{\rm{SNR}}} $ values significantly decreased in comparison to the reference for all positions and fieldstrenghts, while the spectra's baseline is elevated at 1.5T and 3T. Peaks related to continuous wave interferences are apparent at all field strenghts. For WHDMI alone mean SNR¯ $\overline {{\rm{SNR}}} $ values were stable without significant differences to the reference. No elevation of the spectra's baseline could be observed. Functionality measurements confirmed high connection reliability with stable image quality and no delays for all field strengths. Conclusion We conclude that wireless transmission of video streams into the MRI magnet room is feasible at all field strengths without hampering image quality.