Radiofrequency-Induced Heating of a Deep Brain Stimulator Lead inside TEM Cells and inside a 3T MR-Scanner

Sammanfattning: The use of non-ionizing radiation in the magnetic resonance imaging (MRI) made it a safer diagnostic technique in comparison to the X-ray imaging method. MRI can also produce soft tissue images with a very high contrast without contrast agent which is another advantage that made MRI an important imaging technique for studying the mechanism of the deep brain stimulation (DBS) and for targeting the desired regions in the brain that should be stimulated. For these and other advantages, the number of MRI examinations have increased hugely around the world including Sweden. Despite the consideration that MRI is a safe modality, it is not free from risks and hazards. The radiofrequency (RF)-induced heating of the tissues and the metallic implants is one of the safety concerns in MRI which certainly includes patients with DBS implant.The purpose of this project, motivated by the dangers accompanying the RF-induced heating in implantable DBS lead, is to investigate the effect of the exposure to the RF fields in MRI on these leads. After temperature measurements were made, the observations were focused on the amount of temperature increase, and the time required for the temperature to increase and then to decrease to its initial value. Some factors that could affect the lead temperature were studied and that includes the effect of the lead configuration, the lead surrounding medium, the exposure level, and the orientation of the lead coil with respect to the RF field.The result showed that the temperature of the lead (placed in air) increased more but slower when the lead was formed as a coil than when it was randomly configured. It was also showed that the lead coil temperature rise was higher and faster when the coil was placed in air than when it was immersed in deionized water or in saline. The lead coil temperature rise was higher, but slower when the coil was immersed in saline compared to deionized water. Also, exposure level affected the temperature rise such that the higher exposure level showed higher and faster temperature rise of the lead coil. When the lead coil was placed in air and oriented perpendicular to the strongest magnetic field component, its temperature increased higher and faster. On the other hand, the results when the lead coil was immersed in deionized water or in saline showed a deviation from when it was placed in air such that two magnetic field components had the same effect on the lead coil temperature. The time required for the temperature to decrease to its initial value, after the end of the exposure, depended on the magnitude of the RF magnetic flux density, orientation of the lead coil with respect to the RF magnetic field, and the lead surrounding medium. The stronger RF magnetic field is, the longer time is for the temperature to decrease. Consequently, when the lead coil was directed perpendicularly to the strongest component of the RF magnetic field, it took longer time for the temperature to decrease. The time for the temperature to decrease was longer when the lead coil was immersed in water (deionized or saline) than when the lead coil was placed in air. It also took longer time for the temperature to decrease when the lead coil was immersed in saline than in deionized water.