Evaluating the impact of fast-fMRI on dynamic functional connectivity in an event-based paradigm.

Ashish Kaul Sahib,Serge Vulliemoz,Adham Elshahabi,Niels K Focke,Silke Klamer,Thomas Ethofer,Michael Erb,Pascal Martin,Klaus Scheffler,Justus Marquetand

doi:10.1371/journal.pone.0190480

Abstract

The human brain is known to contain several functional networks that interact dynamically. Therefore, it is desirable to analyze the temporal features of these networks by dynamic functional connectivity (dFC). A sliding window approach was used in an event-related fMRI (visual stimulation using checkerboards) to assess the impact of repetition time (TR) and window size on the temporal features of BOLD dFC. In addition, we also examined the spatial distribution of dFC and tested the feasibility of this approach for the analysis of interictal epileptiforme discharges. 15 healthy controls (visual stimulation paradigm) and three patients with epilepsy (EEG-fMRI) were measured with EPI-fMRI. We calculated the functional connectivity degree (FCD) by determining the total number of connections of a given voxel above a predefined threshold based on Pearson correlation. FCD could capture hemodynamic changes relative to stimulus onset in controls. A significant effect of TR and window size was observed on FCD estimates. At a conventional TR of 2.6 s, FCD values were marginal compared to FCD values using sub-seconds TRs achievable with multiband (MB) fMRI. Concerning window sizes, a specific maximum of FCD values (inverted u-shape behavior) was found for each TR, indicating a limit to the possible gain in FCD for increasing window size. In patients, a dynamic FCD change was found relative to the onset of epileptiform EEG patterns, which was compatible with their clinical semiology. Our findings indicate that dynamic FCD transients are better detectable with sub-second TR than conventional TR. This approach was capable of capturing neuronal connectivity across various regions of the brain, indicating a potential to study the temporal characteristics of interictal epileptiform discharges and seizures in epilepsy patients or other brain diseases with brief events.

Highlights

Identifying and understanding brain connectivity from non-invasive and in-vivo functional magnetic resonance imaging is desirable both for physiological and pathological processes
For short TRs at a window size of 7.8 s and 13.2 s, functional connectivity degree (FCD) in the region of interest (ROI) peaked around 5–6 s after stimulus onset, which follows the behavior of the event-related BOLD timecourse
We examined the impact of TR and window size on dynamic functional connectivity degree in a sliding window approach in event-based paradigms

Summary

Introduction

Identifying and understanding brain connectivity from non-invasive and in-vivo functional magnetic resonance imaging (fMRI) is desirable both for physiological and pathological processes. Previous investigations on functional connectivity (FC), especially in resting state studies, have relied largely on static approaches, i.e. correlation of signals calculated for the entire scan duration [1]. These approaches represent the functional relationship between regions without reflecting the dynamics of functional connectivity over time. DFC analysis of brief events could be biologically very relevant One such condition is epilepsy that is characterized by spontaneous interictal epileptiform discharges (IED, i.e. spikes or sharp waves). Enabling dFC analysis of IED in individual patients could, be clinically very relevant

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Conclusion