Safe and accurate imaging with intracranial implants using a parallel transmit RF coil array

Project reference: SIE_25 
First supervisor: Ozlem Ipek

Second supervisor: David Carmichael 

Start date: October 2020  

Project summary: Intracranial electroencephalography (EEG) is used in the surgical assessment of patients with severe, drug resistant epilepsy to help identify the seizure onset zone. Intracranial depth electrodes are used to record the electrical activity of the brain, targeting areas thought to produce seizures. Anatomical targeting of the depth electrodes needs to be verified by post-operative imaging of the implanted depth electrodes by MRI. However, the radiofrequency (RF) fields used for MRI can electrically couple to the metallic implanted electrode and lead to an excessive RF current on the electrode producing a risk of local heating and tissue damage. Therefore, current MRI safety guidelines severely constraint the MRI protocol parameters for patients with implanted depth electrodes. This research project aims to design and build a new parallel transmit radiofrequency coil array that could  significantly reduce the risk of implant heating while increasing the accuracy of imaging the human brain for epilepsy patients with implanted electrodes.  
Project description: 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 postimplantation 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.   
Currently, imaging of intracranial electrodes in MRI for clinical and research purposes is severely limited by both the potential safety risks and image quality.  

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.  
Parallel transmit RF technology has the ability to shape the RF fields and recent work has shown that this can be used produce fields that minimize implant interactions for electrodes with simpler geometric variability.  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.  
Based on this work a prototype parallel transmit coil will be built and tested with realistic test objects to verify that the simulation based safety improvements are realized in-practice. This will use testing with temperature measurements using MRI and temperature probes along with B1-field measurements.  
Objective 1) Define the typical range of 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 2) Design a set of possible RF coil architectures with scalable channel count. The coil architectures will be based on loop and dipole antenna design using transmit-receive concept. 
Objective 3) Determine the modes of operation of the RF coil architecture with factors design and channel count that minimize coupling and maximize B1 magnetic field performance for the range of implantations.  
Objective 4) Develop and test a prototype RF coil to the design yielding the best simulated performance.

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