Single-Shot 2D Radial Echo Planar Imaging for Functional MRI

To develop a novel single-shot radial echo planar imaging (ss-rEPI) technique for rapid, distortion-free brain imaging in functional MRI experiments.

Radial multi-gradient echo (radial mGRE) data were acquired on a 3T clinical scanner using a 2D ss-rEPI readout with small golden-angle rotations between echoes. Images were reconstructed using an iterative conjugate-gradient method incorporating coil sensitivities, B₀ field inhomogeneities, transverse relaxation, and field-drift correction to account for signal inconsistencies in the extended mGRE readout. Additional k-space-weighted image contrast (KWIC) filtering prior to reconstruction helped reduce model mismatches at low spatial frequencies. Single-shot rEPI image quality, contrast, and distortion were assessed against multi-shot radial mGRE reference data. Retrospective adjustment of the KWIC filter and target TE in the reconstruction allowed the generation of multiple T₂*weighted and phase contrast images from a single ss-rEPI scan, enabling quantitative T₂* mapping and QSM. Visual BOLD fMRI experiments were conducted and evaluated against Cartesian EPI measurements.

Twenty-four 3 mm thick slices of distortion-free, multi-contrast brain images were obtained at 2 × 2 and 1.5 mm² (ramp-sampled) in-plane resolution with an acquisition time of under 1.7 s. In multi-session fMRI experiments on two subjects, ss-rEPI demonstrated BOLD activation in the visual cortex comparable to standard EPI while also enabling functional T₂* measurements.

Single-shot rEPI enables rapid, distortion-free 2D multi-contrast brain imaging, offering a promising alternative to Cartesian EPI. Accurate ∆B₀ modeling is critical for ss-rEPI performance. Advanced reconstruction techniques and self-calibration methods could further enhance its speed, performance, and applicability across diverse MRI techniques.