1st Supervisor: David Carmichael, King’s College London
2nd Supervisor: Dr. Özlem Ipek, King’s College London
Clinical Champion: Alexander Hammers, King’s College London
Aim of the PhD Project:
- Determine the sensitivity of advanced retrospective motion correction methods for correcting small motions in 2D and 3D MRI sequences with signal-to-noise and k-space ordering.
- Experimentally test motion correction performance on high-resolution 7T MRI datasets.
- Develop an AI based algorithm to perform the same quality data correction with the advantage of (following training) dramatically increasing its computational speed.
Project Description / Background:
Simultaneous scalp electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) acquisition allows the measurement of brain activity at high spatial-temporal resolution. MRI at 7T enables a large increase in sensitivity to fMRI signal changes. This sensitivity can be utilised to either to measure haemodynamic responses to epileptic activity much more accurately either in the spatial or temporal domain.
However there are a number of engineering challenges that need to be overcome. Firstly, electrode heating becomes more likely owing to both the reduced wavelength of the RF compared to wire lengths [2] and the increased RF power required to achieve the same pulse sequence at 7T compared to 3T [3].
Work is required to develop a new EEG configuration that is compatible with the existing 7T RF coils, minimises EEG artefacts and maximises safety by reducing induced voltages. This will be achieved by altering wiring configurations and wire impedance for example by distributing impedance to minimise effective wire lengths.
EEG quality is degraded by an increased sensitivity to both pulsatile and bulk head motion. We will build on a recently awarded project grant (GOSHCC) that will develop individualised head cushions for paediatric 7T MRI to minimise motion by including EEG wiring and electrodes into the customised cushion design. The safety of these new configurations requires a combination of simulation and experimental evaluation both of which have been performed by the supervisory team [4-6]. This is required because existing MR-compatible 64-channel scalp EEG systems are not certified for 7T MRI. To date preliminary data has been obtained at 7T by the primary supervisor, however this was achieved using a sub-optimal RF coil limiting the fMRI quality and clinical applicability.
There is a general research ethics approval submission in place for the 7 Tesla at St Thomas’ Hospital and will be extended to including any in-house built hardware. We will follow the 7T MRI safety SOP (first supervisor is involved in the local committee and it is now in preparation) before using the EEG device on humans. Before using it on the human, its function and safety will be validated by computer simulations and simultaneous MRI-EEG measurements on MRI test objects with temperature probe sensors. Therefore most of the technical development can be achieved without human participants. The supervisors have extensive experience of testing and imaging using in-house hardware such as RF coils and imaging patients with EEG systems[4,5]. We include a placement at Nottingham Sir Peter Mansfield Imaging Centre where EEG-fMRI has been performed at 7T.
References:
[1] Centeno M., Tierney T.M., Perani S., Shamshiri E.A., StPier K., Wilkinson C., Konn D., Vulliemoz S., Grouiller F., Lemieux L., Pressler R.M. Combined EEG-fMRI and ESI improves localisation of paediatric focal epilepsy. Ann. Neurol. 2017; 82 (2), 278–287.
[2] Ipek Ö., Hawsawi H., Papadaki A., Jorge J., Carmichael D., Gruetter R., Lemieux L. Computational EM field simulations and empirical measurements for the assessment of RF-induced heating in the vicinity of human electrocorticography and depth brain electrodes during MRI. In preparation.
[3] Le T.P., Ipek Ö., Jorge J., Gruetter R. Improved transmit field homogeneity in simultaneous EEG-fMRI at 7T by increasing EEG wire resistance. Proc. Intl. Mag. Reson. Med. 2018; 26, 4314.
[4] Jorge J., Grouiller F., Ipek Ö., Stoermer R., Michel C.M., Figueiredo P., van der Zwaag W., Gruetter R. Simultaneous EEG-fMRI at ultra-high field: Artifact prevention and safety assessment. Neuroimage 2015; 105, 132-144.
[5] Safety of localizing epilepsy monitoring intracranial electroencephalography electrodes suing MRI: radiofrequency-induced heating. Carmichael D.W., Thornton J.S., Rodionov R., Thornton R., McEvoy A., Allen P.J., Lemieux L. Safety of localizing epilepsy monitoring intracranial electroencephalography electrodes suing MRI: radiofrequency-induced heating. J. Magn Reson Imaging 2008; 28(5), 1233-1244.
[6] Hawsawi H.B., Carmichael D.W., Lemieux L. Safety of simultaneous scalp or intracranial EEG during MRI: A review. Frontiers in Physics, 2017; 5(42).
Top row (left to right): Simulated EEG cap (the arrows pointing to wire and insulation, electrode, gel, 5k Ω current limiting resistor) and Commercial EEG cap. Bottom row: Simulated EEG/MRI setup.