High-field MRI of brain cortical substructure based on signal phase

TLDR

MRI detection of brain anatomy is limited by contrast‑to‑noise ratio, which depends on the contrast mechanism and spatial resolution. The study demonstrates that exploiting MRI signal phase at high field can greatly improve contrast‑to‑noise in high‑resolution brain imaging. The authors used 7.0‑T gradient‑echo MRI with a multichannel detector to acquire 0.24 × 0.24 × 1.0 mm³ voxels. High‑resolution phase images at 7 T revealed strong contrast between and within gray and white matter, with CNR up to 20:1—an almost 10‑fold improvement over conventional MRI and ≈100‑fold when accounting for resolution gains—attributable to susceptibility differences from iron, suggesting that phase contrast can delineate fiber bundles and cortical layers.

Abstract

The ability to detect brain anatomy and pathophysiology with MRI is limited by the contrast-to-noise ratio (CNR), which depends on the contrast mechanism used and the spatial resolution. In this work, we show that in MRI of the human brain, large improvements in contrast to noise in high-resolution images are possible by exploiting the MRI signal phase at high magnetic field strength. Using gradient-echo MRI at 7.0 tesla and a multichannel detector, a nominal voxel size of 0.24 × 0.24 × 1.0 mm 3 (58 nl) was achieved. At this resolution, a strong phase contrast was observed both between as well as within gray matter (GM) and white matter (WM). In gradient-echo phase images obtained on normal volunteers at this high resolution, the CNR between GM and WM ranged from 3:1 to 20:1 over the cortex. This CNR is an almost 10-fold improvement over conventional MRI techniques that do not use image phase, and it is an ≈100-fold improvement when including the gains in resolution from high-field and multichannel detection. Within WM, phase contrast appeared to be associated with the major fiber bundles, whereas contrast within GM was suggestive of the underlying layer structure. The observed phase contrast is attributed to local variations in magnetic susceptibility, which, at least in part, appeared to originate from iron stores. The ability to detect cortical substructure from MRI phase contrast at high field is expected to greatly enhance the study of human brain anatomy in vivo .