In-vivo and numerical analysis of the eigenmodes produced by a multi-level Tic-Tac-Toe head transmit array for 7 Tesla MRI

Abstract

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 B1+ 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.

Computational and experimental evaluation of the Tic-Tac-Toe RF coil for 7 Tesla MRI

“…The overall results show that transmit B 1 + field homogeneity measured by CV as well as max/min (maximum B 1 + over minimum B 1 + in the ROI) are substantially improved with the TTT coil when compared to the TEM coil. The TTT coil is well suited for exciting cerebellum and brain stem [32] at 7T. That being said, the TEM coil design with its arrangement of multiple transmission lines aims at providing increased excitation in the middle of the brain [31] as opposed to extended coverage in the peripheries.…”

“…In this work, we study RF characteristics due to different human heads and compare different coils that are used at 7T. Specifically, the transverse electromagnetic resonator (TEM) coil [31] and 5-sided Tic-Tac-Toe (TTT) coil [32, 33] are evaluated via RF simulations using different anatomically detailed human head models and measurements utilizing a network analyzer and in-vivo 7T B 1 + mapping on different human heads. We chose the 16-element TEM and TTT designs due to the availability of both coils as well as FDTD coil models in our facility and because they are based on distributed-circuit approach and have shown to produce the homogenous B 1 + field distributions at 7T [34, 35].…”

Improved 7 Tesla Transmit Field Homogeneity with Reduced Electromagnetic Power Deposition Using Coupled Tic Tac Toe Antennas

“…Based on the eigenmodes of the Tx coil design 11 , the 16-channel Tx coil is composed of 4 excitation levels spatially positioned in the Z direction ( Figure 1a); each level is composed of 4 Tx coil channels. In this work, Level 1 was chosen to be excited with half of the total supplied RF power because of the higher power efficiency associated with this level and its capacity to produce center bright, as previously demonstrated in reference 11 . Levels 2 and 3 play an essential role in exciting the lower brain regions, including the hippocampus and the temporal lobes 11 .…”

Neuroimaging of Small Vessel Disease in Late-Life Depression

“…The tic-tac-toe (TTT) RF system for 7 T head MRI is composed by a 16-channel transmit array (4 TTT elements located around the head) and a 32-channel receive insert, for optimal receiving performance. The TTT transmit array has been shown to produce a homogenous spin excitation at 7 T frequencies [42] and superior subject insensitiveness (low variations of the RF performance with different head shapes) [43]. It also presents lower global and local (SAR) when compared with other transmit coil designs [42, 43], increasing patient safety and minimizing the risks of temperature increase and tissue damage.…”

“…High resolution in vivo gradient echo (GRE) whole-brain acquisitions at 7 T, showing details of the brain structure and a comparison with histology. The images were acquired with the tic-tac-toe RF system [42, 43] and with the following GRE sequence parameters: TR = 1960, TE = 14, in-plane resolution 0.21 × 0.21 mm 2 , slice thickness of 1.5 mm, and 64 slices (114.9 mm) in total (20% gap between slices). In ( a ), selected slices showing the whole-brain acquisition; in ( b ), zoomed image showing the medullary draining veins throughout the deep white matter acquired without acceleration (18 min acquisitions); in ( c ), similar zoomed images acquired with acceleration factor (GRAPPA) 2 (9 min acquisition); in ( d ), the zoomed image in the cortical region, showing cortical microvessels (black arrows), without acceleration in the acquisition; in ( e ), similar slice showing the cortical regions and cortical microvessels (black arrows), acquired with GRAPPA 2; in ( f ), figure adapted from Ref.…”