Increased brain coverage and efficiency when measuring current-induced magnetic fields by use of simultaneous multi-slice echo-planar MRI

Purpose Magnetic resonance current density imaging (MRCDI) can non-invasively validate electric field simulations in volume conductor head models. Weak electric currents are injected using scalp electrodes while measuring the MR phase perturbations caused by the tiny magnetic fields (1-2 nT) induced by the current flow in tissue. MRCDI generally has a low signal-to-noise ratio, making it susceptible to technical imperfections and physiological noise. Here, we tested and optimized simultaneous multi-slice (SMS) EPI for time-efficient and robust brain MRCDI.

MRCDI data was acquired in a phantom and five human brains using SMS-EPI optimized for measuring current-induced phase perturbations. Multiband factors and interslice gaps were systematically varied and the resulting image quality assessed. In particular, the impact of interslice signal leakage on the measured phase was tested.

Current-free acquisitions showed the expected noise amplification with decreasing interslice distances. However, physiological noise generally dominated the human data, making the overall noise levels identical to single-slice EPI for interslice gaps of at least 12 mm and multiband factors between 3 and 5. Upon application of electric currents, the phantom data revealed subtle artifacts for multiband factors 5 and 6, even for large gaps. Nevertheless, artifacts were absent in the human brain for multiband factors up to 5, where the performance of SMS-EPI approached that of single-slice measurements for sufficient interslice distances.

Optimized SMS-EPI with multiband factors up to 5 and minimum interslice gaps of 12 mm performs on par with single-slice EPI, making it attractive for increasing brain coverage in MRCDI. Competing Interest Statement The authors have declared no competing interest. Funding Statement This study was supported by the Lundbeck Foundation (https://lundbeckfonden.com/, grant R313-2019-622 to AT, grant R324-2019-1784 to LGH) and the German Research Foundation (https://www.dfg.de/, DFG grants TH 1330/6-1 and TH 1330/7-1, part of Research Unit FOR 5429 'MeMoSLAP'). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study complied with the Helsinki Declaration on human experimentation and was approved by the Ethics Committee of the Capital Region of Denmark (De Videnskabsetisk Medicinske Komiteer, approval number 2206924). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability Data of the human subjects cannot be shared publicly because of privacy restrictions. The phantom data is fully available on OSF (DOI 10.17605/OSF.IO/DCWUN, https://osf.io/dcwun/).