Feasibility of head-tilted brain scan to reduce susceptibility-induced signal loss in the prefrontal cortex in gradient echo-based imaging

Abstract

Susceptibility-induced static field (B₀) inhomogeneity near the nasal cavity degrades high-field MRI image quality. Many studies have addressed this problem by hardware- or sequence-based methods to improve local B₀ shimming or minimize the impact of inhomogeneity. Here, we investigate the feasibility of the head-tilted brain scan as an easily accessible way to reduce B₀ inhomogeneity and associated gradient echo signal loss in the prefrontal cortex (PFC). We exploit the fact that the region of intense local B₀ gradient can be steered away from the PFC by head reorientation with respect to the main magnetic field. We found that the required chin-up head tilting by a substantial angle (> 30°) can be readily achieved for a group of healthy subjects when their back was raised by about 10 cm. Eleven subjects were scanned at 3T, using a standard 20 channel head-neck coil, for whole-head B₀ mapping and gradient-echo EPI-based functional MRI (fMRI) performing a reward-punishment task in normal and tilted head orientations. Additionally, multi-echo gradient echo and resting-state fMRI scans were performed on six subjects in both orientations. Head-tilted sessions, which lasted for at least 20 min, were well-tolerated by all subjects and demonstrated a marked reduction of localized signal loss in the gradient echo-based images and EPI images in the PFC compared to normal orientation scans. Imaging in tilted orientation reduced the group-averaged B₀ standard deviation and peak B₀ gradient in the orbital gyrus beyond what was possible with simulated 3rd order shimming. The behavioral performance in the head-tilted fMRI scans indicated that the subjects were able to perform a cognitive task with little difficulty, and the tilted fMRI scans successfully produced a robust whole-brain functional activation map consistent with the literature. Our study proposes that the back-raised, head-tilted imaging can benefit the shimming of the prefrontal brain regions while being compatible with moderate-length neuroimaging scans on healthy, cooperating subjects.

Keywords: B(0) homogeneity; Head tilt; Orbitofrontal; Prefrontal cortex; Shim; Susceptibility; fMRI.