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.

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.