Abstract
Ultrafast functional magnetic resonance imaging (fMRI) can measure blood oxygen level dependent (BOLD) signals with high sensitivity and specificity. Here we propose a novel method: simultaneous multi-slice inverse imaging (SMS-InI) — a combination of simultaneous multi-slice excitation, simultaneous echo refocusing (SER), blipped controlled aliasing in parallel imaging echo-planar imaging (EPI), and regularized image reconstruction. Using a 32-channel head coil array on a 3 T scanner, SMS-InI achieves nominal isotropic 5-mm spatial resolution and 10 Hz sampling rate at the whole-brain level. Compared with traditional inverse imaging, we found that SMS-InI has higher spatial resolution with lower signal leakage and higher time-domain signal-to-noise ratio with the optimized regularization parameter in the reconstruction. SMS-InI achieved higher effective resolution and higher detection power in detecting visual cortex activity than InI. SMS-InI also detected subcortical fMRI signals with the similar sensitivity and localization accuracy like EPI. The spatiotemporal resolution of SMS-InI was used to reveal that presenting visual stimuli with 0.2 s latency between left and right visual hemifield led to 0.2 s relative hemodynamic response latency between the left and right visual cortices. Together, these results indicate that SMS-InI is a useful tool in measuring cortical and subcortical hemodynamic responses with high spatiotemporal resolution.
Highlights
There is accumulating evidence suggesting the advantage of collecting blood oxygen level dependent (BOLD) signals with sub-second sampling rate
This study proposed the simultaneous multi-slice inverse imaging (SMS-inverse imaging (InI)) method by optimizing the combination of simultaneous echo refocusing (SER), blipped-CAIPI-echo-planar imaging (EPI), and highly parallel detection to achieve 100-ms whole-brain sampling of hemodynamic responses with 5-mm isotropic resolution
We explicitly investigated the trade-off between tSNR and signal leakage associated with the regularization parameter in blipped-CAIPI reconstruction (Fig. 4)
Summary
There is accumulating evidence suggesting the advantage of collecting BOLD signals with sub-second sampling rate. Parallel-accelerated simultaneous multi-slice (SMS) imaging[17] using blipped controlled aliasing (CAIPI) EPI has been introduced[18] This method was further integrated with simultaneous echo refocusing (SER) to achieve a 2.5 Hz volumetric sampling rate in fMRI experiments[7,19]. We proposed an optimized simultaneous multi-slice inverse imaging (SMS-InI), which integrates the state-of-the-art blipped-CAIPI-EPI and SER in image acquisition and regularized reconstruction in order to achieve high spatiotemporal resolution with high computational efficiency. With the optimized volume prescription, slice arrangement, and choice of regularization, we were able to achieve isotropic 5-mm spatial resolution, 100-ms volumetric sampling interval, and high tSNR at 3 T MRI using a 32-channel head coil array. The performance of SMS-InI was empirically quantified and further compared with the traditional InI and EPI in the human visual and sensorimotor systems
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