Open AccessIn-vivo and numerical analysis of the eigenmodes produced by a multi-level Tic-Tac-Toe head transmit array for 7 Tesla MRI
Author(s) -
Tales Santini,
Yang Zhao,
Sossena Wood,
Narayanan Krishnamurthy,
Junghwan Kim,
Nadim Farhat,
Salem Alkhateeb,
Tiago Martins,
Min Seok Koo,
Tiejun Zhao,
Howard Aizenstein,
Tamer S. Ibrahim
Publication year - 2018
Publication title -
plos one
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.99
H-Index - 332
ISSN - 1932-6203
DOI - 10.1371/journal.pone.0206127
Subject(s) - physics , specific absorption rate , excitation , imaging phantom , superposition principle , quadrature (astronomy) , finite difference time domain method , nuclear magnetic resonance , human head , radio frequency , excited state , computational physics , optics , acoustics , atomic physics , computer science , telecommunications , antenna (radio) , absorption (acoustics) , quantum mechanics
Radio-frequency (RF) field inhomogeneities and higher levels of specific absorption rate (SAR) still present great challenges in ultrahigh-field (UHF) MRI. In this study, an in-depth analysis of the eigenmodes of a 20-channel transmit Tic-Tac-Toe (TTT) RF array for 7T neuro MRI is presented. The eigenmodes were calculated for five different Z levels (along the static magnetic field direction) of the coil. Four eigenmodes were obtained for each Z level (composed of 4 excitation ports), and they were named based on the characteristics of their field distributions: quadrature, opposite-phase, anti-quadrature, and zero-phase. Corresponding finite-difference time-domain (FDTD) simulations were performed and experimental B 1 + field maps were acquired using a homogeneous spherical phantom and human head (in-vivo). The quadrature mode is the most efficient and it excites the central brain regions; the opposite-phase mode excites the brain peripheral regions; anti-quadrature mode excites the head periphery; and the zero-phase mode excites cerebellum and temporal lobes. Using this RF array, up to five eigenmodes (from five different Z levels) can be simultaneously excited. The superposition of these modes has the potential to produce homogeneous excitation with full brain coverage and low levels of SAR at 7T MRI.