Feasibility of simultaneous intracranial EEG-fMRI in humans: A safety study |
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Authors: | David W. Carmichael John S. Thornton Roman Rodionov Rachel Thornton Andrew W. McEvoy Roger J. Ordidge Philip J. Allen Louis Lemieux |
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Affiliation: | 1. Department of Clinical and Experimental Epilepsy, UCL Institute of Neurology, London, UK;2. Lysholm Department of Neuroradiology, National Hospital for Neurology and Neurosurgery, London, UK;3. Victor Horsley Department of Neurosurgery, National Hospital for Neurology and Neurosurgery, London, UK;4. Department of Medical Physics and Bioengineering, UCL, London, UK;5. Department of Clinical Neurophysiology, National Hospital for Neurology and Neurosurgery, London, UK |
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Abstract: | In epilepsy patients who have electrodes implanted in their brains as part of their pre-surgical assessment, simultaneous intracranial EEG and fMRI (icEEG-fMRI) may provide important localising information and improve understanding of the underlying neuropathology. However, patient safety during icEEG-fMRI has not been addressed.Here the potential health hazards associated with icEEG-fMRI were evaluated theoretically and the main risks identified as: mechanical forces on electrodes from transient magnetic effects, tissue heating due to interaction with the pulsed RF fields and tissue stimulation due to interactions with the switched magnetic gradient fields. These potential hazards were examined experimentally in vitro on a Siemens 3 T Trio, 1.5 T Avanto and a GE 3 T Signa Excite scanner using a Brain Products MR compatible EEG system.No electrode flexion was observed. Temperature measurements demonstrated that heating well above guideline limits can occur. However heating could be kept within safe limits (< 1.0 °C) by using a head transmit RF coil, ensuring EEG cable placement to exit the RF coil along its central z-axis, using specific EEG cable lengths and limiting MRI sequence specific absorption rates (SARs). We found that the risk of tissue damage due to RF-induced heating is low provided implant and scanner specific SAR limits are observed with a safety margin used to account for uncertainties (e.g. in scanner-reported SAR). The observed scanner gradient switching induced current (0.08 mA) and charge density (0.2 μC/cm2) were well within safety limits (0.5 mA and 30 μC/cm2, respectively). Site-specific testing and a conservative approach to safety are required to avoid the risk of adverse events. |
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Keywords: | Safety EEG fMRI Intracranial EEG Epilepsy Simultaneous intracranial EEG-fMRI |
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