Alternating-SSFP for whole-brain functional magnetic resonance imaging [electronic resource]
Keywords: Electrical Engineering.

Functional Magnetic Resonance Imaging (fMRI) is a powerful noninvasive tool that extends MRI technology to mapping brain activity. FMRI is used to measure brain activity by detecting vascular changes associated with neuronal activation. Currently, the most widely used methods to acquire fMRI images are T2*-weighted Gradient- Echo (GRE) sequences. These methods exhibit excellent sensitivity to blood oxygenation level-dependent (BOLD) contrast. However, GRE sequences require a long echo time (TE) for good BOLD sensitivity and use long, single-shot readouts for efficiency, resulting in signal dropout and image distortion in regions near air-tissue interfaces such as the orbitofrontal cortex and inferior temporal regions. Recent studies have shown that pass-band steady-state free precession (pb-SSFP) fMRI is a promising alternative. Pb-SSFP fMRI has several advantages over conventional GRE-echo-planar imaging (EPI) acquisitions, including small-vessel BOLD sensitivity, reduced image distortion, and reduced signal dropout from susceptibility field gradients due to the short TE and rapid acquisition. However, banding artifacts remain a challenge for whole-brain imaging, as current solutions are impractical for many functional studies. Recently, an improved pb-SSFP fMRI technique called alternating-SSFP (alt- SSFP) was proposed. This technique permits whole-brain, banding-artifact-free-SSFP fMRI in a single scan. However, many challenges need to be overcome to make the method practical and robust for human fMRI studies. Therefore, a complete and practical alt-SSFP fMRI image acquisition sequence and image reconstruction method is developed for whole-brain fMRI. First, methods regarding RF catalyzation, k-space trajectory design, and parallel imaging are developed for alt-SSFP to ensure signal stability, achieve whole-brain coverage, and maintain sufficiently high temporal resolution. In addition, the alt-SSFP sequence's inherent bright fat signal combined with the echo-planar k-space trajectory causes chemical-shift artifacts. A short spatial-spectral RF pulse is designed to reduce artifacts associated with the bright fat signal and increase temporal SNR for alt-SSFP fMRI. Lastly, the alternate banding patterns of alt-SSFP are used to improve the conditioning of parallel imaging for image reconstruction in a method called Extended Parallel Imaging, which would allow greater acceleration for higher temporal and/or spatial resolution. Artifact-suppressed images from breath-hold and visual stimulus studies show that the alt-SSFP fMRI method permits whole-brain imaging with excellent blood oxygen level-dependent sensitivity and fat suppression. In addition, image reconstruction with Extended Parallel Imaging increases temporal SNR for alt-SSFP fMRI and improves activation maps in highly accelerated cases. These combined developments result in a practical pb-SSFP fMRI method capable of functional imaging in regions currently inaccessible to conventional fMRI acquisition methods. This could potentially become a powerful tool for better understanding how different parts of the brain are interconnected, and for studying the brain in its entirety.