Safe and accurate imaging of implants for Epilepsy treatments using a parallel transmit RF coil array.

Project summary: Patients with epilepsy are treated with surgery to remove brain areas responsible for seizure generation. As part of this process the implantation of intracranial EEG electrodes is often required. Where surgery is not possible a significant proportion of patients will have Vagal Nerve Stimulators (VNS) implanted. In both cases imaging with MRI could improve the outcomes of these procedures but is limited by the risks associated with implant imaging.  The radiofrequency (RF) fields used for MRI can electrically couple to the implant and lead to a risk of local heating and tissue damage. Recent advances in MRI technology allow for much greater control of RF fields owing to the use of parallel transmit RF coils that can be used to increase image quality while minimizing implant – RF coil interactions.  This research project aims to determine the modes of operation of current and newly designed parallel transmit radiofrequency coil arrays that could significantly reduce the risk of implant heating and ensure safe operation.
 
Project description:  In this project we will first determine how to image patients with implanted VNS systems safely using parallel transmit technology. VNS systems are implanted in the neck and torso with a lead connecting the site of nerve stimulation and the stimulator typically sited under the clavicle. Owing to the relatively simple geometry and constancy of implantation this is anticipated to be relatively straight forward. A secondary aim of this first part of the project will be to test the operation of the device exposed to magnetic gradient fields aiming to test the operation of the device both in ‘on’ and ‘off’ modes.

Intracranial electroencephalography (EEG) is used in the surgical assessment of patients with severe, drug resistant epilepsy to help identify the seizure onset zone. Electrodes are typically composed of 30-40cm wires with multiple electrode contacts that record signals within the brain. Typically, multiple electrodes are implanted in a wide range of configurations that are tailored to each patient. MRI post-implantation can be a useful way of verifying anatomical targeting of the electrode without potential confounding effects of brain spatial shifts that can occur during surgery (and limit the accuracy of localization of electrodes based on pre-implantation MRI and post-implantation CT). Further, the ability to study patients using MRI with intracranial electrodes offers the opportunity to better understand epileptic activity and potential therapeutics such as electrical stimulation [1]. 

During MRI, Radio Frequency magnetic and electrical fields are produced that can couple to the metallic implanted electrodes and lead to a risk of local heating and tissue damage. The exact risk is a complex problem defined by the complex and patient specific implant and RF coil geometry. Importantly, because of this variability it can be treated as a ‘worse case’, where the findings of this project are likely to be generalizable to most other elongated brain implants. This interaction with the RF fields also locally degrades MRI image quality that relies on a uniform RF magnetic field.

This research project aims to investigate the design of modes of operation utilizing parallel transmit coils at 3T/123MHz and determine how they can be generalized to limit risks of local heating for a wide range of implant configurations. Key to this work will be defining a range of transmit coil designs and investigating the applicability of the RF fields they generate for this purpose in addition to understanding how critical parallel transmit channel number is to establishing safe operational modes. This simulation will be testing with temperature measurements using MRI and temperature probes along with B1-field measurements.

Objective 1) Determine the modes of operation of existing RF coil architectures that minimize interaction with typical VNS configurations that minimize coupling and maximize B1 magnetic field performance.

Objective 2) Test a device in-situ in the MRI scanner to determine potential heating and verify stimulator output under a range of potential imaging conditions.

Objective 3) Define the typical range of intracranial electrode implantations based on data from multiple sites (where strategies can differ). Create a ‘basis set’ of simplified models that cover the range of likely implantations.   

Objective 4) Investigate the potential safety of RF coil architectures already in development and determine modes of operation that minimize interaction.

Objective 5) Test the best candidate modes experimentally using implantations that characterize the range of those likely to be encountered. Investigate factors related to the implantation that can be computationally optimized to reduce implantation heating interactions while maintaining other implantation requirements (e.g. coverage and safe electrode trajectories). 

References:

[1] Vulliemoz S., Carmichael D.W., Rosenkranz K., Diehl B., Rodionov R., Walker M.C,, McEvoy A.W., Lemieux L. Simultaneous intracranial EEG and fMRI of interictal epileptic discharges in humans. NeuroImage, 2011. 54(1): p. 182-190.

[2] Guerin B., Angelone L.M., Dougherty D., Wald L.L. Parallel transmission to reduce absorbed power around deep brain stimulation devices in MRI: impact of number and arrangement of transmit channels. Magn. Reson Med. 2019; 00:1-13.

​