B1+ inhomogeneity correction of volumetric brain NOEMTR via high permittivity dielectric padding at 7 T
Paul S. Jacobs, Blake Benyard, Quy Cao, Anshuman Swain, Neil Wilson, Ravi Prakash Reddy Nanga, M. Dylan Tisdall, John Detre, Mark A. Elliott, Mohammad Haris, Ravinder Reddy
Abstract
Purpose
Nuclear Overhauser effect magnetization transfer ratio (NOEMTR) 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 B1+ 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 B1+ inhomogeneities and improve NOEMTR contrast in the temporal lobes at 7 T.
Methods
Partial 3D NOEMTR contrast images and whole brain B1+ 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 NOEMTR images had a separate postprocessing linear correction applied.
Results
DP provided supplemental B1+ to the temporal lobes while also reducing the B1+ magnitude across the posterior and superior regions of the brain. This resulted in a statistically significant increase in NOEMTR contrast in substructures of the temporal lobes both with and without linear correction. The padding also produced a convergence in NOEMTR contrast toward approximately equal mean values.
Conclusion
NOEMTR images showed significant improvement in temporal lobe contrast when DP were used, which resulted from an increase in B1+ homogeneity across the entire brain slab. DP-derived improvements in NOEMTR are expected to increase the robustness of the brain substructural measures both in healthy and pathological conditions.