Fast fMRI and Its Application to Events with Short Duration (2014)

Introduction: Simultaneous Multislice imaging using multiband (MB) RF pulse with slice-selective gradient can be used to reduce the acquisition time of whole-brain echo planar imaging (EPI) [1]. This technique has the potential to improve the temporal resolution of EPI considerably [2]. Here, we evaluated the statistical power of event-related fMRI (visual stimulation using standard checkerboards) at various MB factors to determine the optimal imaging parameters to detect brief, single events. As we plan to employ this technique for evaluation of single interictal spikes in epileptic patients, we generated our design (temporal occurrence of events) based on the spike discharge pattern of an epileptic patient (so called pseudo spike paradigm). Methods: Five healthy volunteers [mean age 24.4, SD 2.0 years) participated in this fMRI study. Images were acquired with an isotropic resolution of 3 mm (40 slices) using a 3T MRI system (Magnetom Trio, Siemens, Germany) equipped with a 32-channel head coil. In the pseudo spike paradigm (duration: 10 minutes), 23 checkerboard stimuli per MB factor were presented for 500 ms (see Table 1). Participants were instructed to respond to the stimuli by a button press with their right thumb. In addition to the pseudo spike paradigm, a block paradigm (duration: 3 minutes) was additionally employed at a MB factor of 4 (TR=660 ms). The block paradigm was used as an independent fMRI localizer to define regions of interest (ROI) for the event-related responses. Preprocessing and statistical analysis relied on SPM8. Only events with a response time &lt; 1.5 seconds were included in the analysis. The percentage of detected events in the ROIs during the pseudospike design were determined at uncorrected voxel height thresholds of p &lt; 0.001, p &lt; 0.01, and p &lt; 0.05. Results: Based on the activation in the block-design fMRI localizer (p &lt; 0.05, FWE corrected), five spherical ROIs (radius: 6 mm) were defined: left visual area (left_vis, 20;-86;-6), left motor area (left_mot,-44;-14; 48), right motor area (right_mot, 42;-8; 48), right visual area (right_vis,-20;-86;-6) and supplemental motor area (SMA, 0;-6; 62). Fig. 1 depicts the activation map obtained during the block paradigm for a typical subject. Averaged T-values within the ROIs are shown in Fig. 2. As expected, the highest T-values were found within the visual cortex. There was an increase of T-values for all selected ROIs as a function of TR. Correspondingly, the percentage of single event analysis (Fig. 3) also increases as a function of TR. Conclusions: In summary, multislice imaging using accelerated MB EPI is an important MRI technique capable of providing high temporal resolution which can be used to improve detection of single events with a short duration. Although the image quality decreases at higher MB factors we gain in terms of statistical significance. Up to 70 of the brief stimuli elicited a significant hemodynamic response within the ROIs suggesting that this technique can be useful to effectively capture the response of short random events such as epileptic spike discharges.