Isotropic submillimeter fMRI in the human brain at 7 T: Combining reduced field‐of‐view imaging and partially parallel acquisitions

Abstract

Echo-planar imaging is the most widely used imaging sequence for functional magnetic resonance imaging (fMRI) due to its fast acquisition. However, it is prone to local distortions, image blurring, and signal voids. As these effects scale with echo train length and field strength, it is essential for high-resolution echo-planar imaging at ultrahigh field to address these problems. Partially parallel acquisition methods can be used to improve the image quality of echo-planar imaging. However, partially parallel acquisition can be affected by aliasing artifacts and noise enhancement. Another way to shorten the echo train length is to reduce the field-of-view (FOV) while maintaining the same spatial resolution. However, to achieve significant acceleration, the resulting FOV becomes very small. Another problem occurs when FOV selection is incomplete such that there is remaining signal aliased from the region outside the reduced FOV. In this article, a novel approach, a combination of reduced FOV imaging with partially parallel acquisition, is presented. This approach can address the problems described above of each individual method, enabling high-quality single-shot echo-planar imaging acquisition, with submillimeter isotropic resolution and good signal-to-noise ratio, for fMRI at ultrahigh field strength. This is demonstrated in fMRI of human brain at 7T with an isotropic resolution of 650 μm.