B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneity correction of volumetric brain NOEMTR via high permittivity dielectric padding at 7 T

Abstract

Purpose: Nuclear Overhauser effect magnetization transfer ratio (NOE_MTR ) is a technique used to investigate brain lipids and macromolecules in greater detail than other techniques and benefits from increased contrast at 7 T. However, this contrast can become degraded because of $B_1^{+}$ inhomogeneities present at ultra-high field strengths. High-permittivity dielectric pads (DP) have been used to correct for these inhomogeneities via displacement currents generating secondary magnetic fields. The purpose of this work is to demonstrate that dielectric pads can be used to mitigate $B_1^{+}$ inhomogeneities and improve NOE_MTR contrast in the temporal lobes at 7 T.

Methods: Partial 3D NOE_MTR contrast images and whole brain $B_1^{+}$ field maps were acquired on a 7 T MRI across six healthy subjects. Calcium titanate DP, having a relative permittivity of 110, was placed next to the subject's head near the temporal lobes. Pad corrected NOE_MTR images had a separate postprocessing linear correction applied.

Results: DP provided supplemental $B_1^{+}$ to the temporal lobes while also reducing the $B_1^{+}$ magnitude across the posterior and superior regions of the brain. This resulted in a statistically significant increase in NOE_MTR contrast in substructures of the temporal lobes both with and without linear correction. The padding also produced a convergence in NOE_MTR contrast toward approximately equal mean values.

Conclusion: NOE_MTR images showed significant improvement in temporal lobe contrast when DP were used, which resulted from an increase in $B_1^{+}$ homogeneity across the entire brain slab. DP-derived improvements in NOE_MTR are expected to increase the robustness of the brain substructural measures both in healthy and pathological conditions.

Keywords: B 1 + $$ {\mathrm{B}}_1^{+} $$ inhomogeneity correction; NOEMTR; dielectric padding; ultra-high field.