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

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 Ϸ100-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.anatomy ͉ contrast mechanism ͉ cortex

Cortical phase changes in Alzheimer's disease at 7T MRI: A novel imaging marker

“…Based on the visual observation on unwrapped phase images of a higher contrast between gray and white matter in AD patients than in control subjects, resulting from a higher signal intensity within the gray matter, the phase values of the cortical gray matter were determined in ROIs in four different areas of the brain-temporoparietal left, temporoparietal right, frontal, and parietal-which were positioned according a detailed human brain atlas. Because of the laminar variation in the cortical areas [29], histograms perpendicular to the cortex within these regions were created to measure peak gray matter phase values, over at least 10 cortical regions per slice and per region. To correct for local macroscopic magnetic field inhomogeneities subcortical white matter phase values were measured and used as an internal reference value.…”

“…An alternative method to measure these susceptibility changes in the brain is to measure the relative phase in regions of interest (ROIs), because it has been shown that this is a reliable indicator of the iron content in the brain [25][26][27][28]. Although previous studies have demonstrated the potential of this approach in highfield MRI [29,30], no clinical studies have been performed yet on AD patients in vivo for the detection of AD pathology.…”

In vivo assessment of myelination by phase imaging at high magnetic field

“…On the other hand, MR phase imaging, acquired with a standard gradient recalled echo, has been shown to emphasize contrast between gray and white matter (Duyn et al, 2007) when compared to conventional magnitude imaging. Phase contrast was shown to be affected by tissue lipid and iron content (Duyn et al, 2007;Fukunaga et al, 2010) and the local microstructure (He and Yablonskiy, 2009).…”

“…Phase contrast was shown to be affected by tissue lipid and iron content (Duyn et al, 2007;Fukunaga et al, 2010) and the local microstructure (He and Yablonskiy, 2009). The effect of cerebral blood volume has been demonstrated to have a minor, if any, impact on the phase contrast observed between white and gray matter (Lee et al, 2010a;Marques et al, 2009).…”

Measurement of brain iron distribution in Hallevorden‐Spatz syndrome

“…Analysis of MR image phase variations may be confounded by numerous background factors such as global magnetic field inhomogeneities, nonuniform radiofrequency (RF) penetration, magnetic field gradients from air-tissue interfaces, eddy currents, etc. These artifacts lead to phase rolling and phase 2p wrapping and must be eliminated or at least minimized before true iron-related effects on phase can be assessed (27).…”