Ultra-fast fMRI of human visual cortex using echo-shifted magnetic resonance inverse imaging

Functional MRI (fMRI) with blood-oxygen level dependency (BOLD) contrast [1, 2] measures the hemodynamic response secondary to neural activity [3]. Typically fMRI uses the echo-planar imaging (EPI) [4] to acquire images covering the whole brain with a TR of a few seconds. Using the magnetic resonance inverse image (InI) technique, the scan time per brain volume can be reduced to 100 ms [5]. To detect the fine temporal property of hemodynamic response, we aim to increase the scanning rate to 40 Hz per volume. However, the optimal BOLD contrast is achieved at TE = 30 ms at 3T. Such an echo time limits the shortest TR using steady-state incoherent sequences. This challenge can be mitigated by echo-shifted pulse sequences, such as PRESTO [6]. Here we propose the echo-shifted InI (esInI) to achieve TR = 25 ms with whole-brain coverage with TE = 34 ms to achieve an unprecedented temporal resolution while maintaining the optimal functional contrast. We used an event-related design to probe the spatial and temporal properties of task-related hemodynamic responses in the human visual cortex. The accelerated esInI scans generated two dimensional projection images, which were subsequently reconstructed for volumetric images by the minimum-norm estimation (MNE) approach [7]. We found that esInI localized the visual cortex, and the time courses in functional areas were consistent with the waveforms measured by EPI.