Improved 7 Tesla transmit field homogeneity with reduced electromagnetic power deposition using coupled Tic Tac Toe antennas

Abstract

Recently cleared by the FDA, 7 Tesla (7 T) MRI is a rapidly growing technology that can provide higher resolution and enhanced contrast in human MRI images. However, the increased operational frequency (~ 297 MHz) hinders its full potential since it causes inhomogeneities in the images and increases the power deposition in the tissues. This work describes the optimization of an innovative radiofrequency (RF) head coil coupled design, named Tic Tac Toe, currently used in large scale human MRI scanning at 7 T; to date, this device was used in more than 1,300 neuro 7 T MRI scans. Electromagnetic simulations of the coil were performed using the finite-difference time-domain method. Numerical optimizations were used to combine the calculated electromagnetic fields produced by these antennas, based on the superposition principle, resulting in homogeneous magnetic field distributions at low levels of power deposition in the tissues. The simulations were validated in-vivo using the Tic Tac Toe RF head coil system on a 7 T MRI scanner.

An open-source MRI compatible frame for multimodal presurgical mapping in macaque and capuchin monkeys

“… A) The frame next to a human head model (addapted from model published in Christ et al, 2009). B) Frame fitted into 2 nd generation Tic-tac-toe radiofrequency coil for the Siemens 7T scanner (Sajewski et al, 2023; Santini et al, 2021). …”

“…In vivo T1-weighted MRIs were acquired for presurgical planning at the University of Pittsburgh on a human, whole body 7T MRI scanner (400 μm isotropic, TR/TE: 6000/3.7 ms, FOV: 148×148×96 mm) and using the 2 nd generation of the Tic-Tac-Toe radiofrequency (RF) system, with 60 transmit channels and 32 receive channels. This RF system family provides images with excellent homogeneity, coverage, and signal to noise ratio (Sajewski et al, 2023; Santini et al, 2021). Animals (MK-EF, MK-SC, MK-SZ, MK-OP, MK-HS, MK-JC, MK-JN, MK-BB) were anesthetized and intubated (induced with Ketamine, 10 mg/kg, maintained with isoflurane, 2%) with all vital signs continuously monitored.…”

“…This allows the frame to fit in most clinical MRI scanners either with a commercial or custom designed radio frequency coil (see Fig. 2B for the custom high-sensitivity conformal coil used here, Krishnamurthy et al, 2019;Sajewski et al, 2023;Santini et al, 2021Santini et al, , 2018. The CAD designs have been made freely and publicly available with the hope of prompting open collaboration and supporting the 3R's in NHP research (Russell and Burch, 1959).…”

Characterization of oscillations in the brain and cerebrospinal fluid using ultra-high field magnetic resonance imaging

“…The sequence is capable of achieving fast acquisitions, high contrast to the CSF flow, and high sensitivity to BOLD signal, thus making it a good candidate for studies of sleep and neurodegenerative and psychological disorders. The imaging was done with an in-house developed and fabricated 16-channel Tic-Tac-Toe transmit array with a 32-channel receive head coil (Krishnamurthy et al, 2019; Santini et al, 2018; Santini, Wood, et al, 2021) that is load insensitive (Ibrahim et al, 2008; Kim et al, 2016; Santini, Wood, et al, 2021) and capable of whole brain homogenous imaging at 7T (Ibrahim et al, 2013). By using this coil design, we were able to acquire signal from the entire brain with minimal regions of significant excitation losses using the single transmit mode of the 7T scanner.…”

“…The enhanced SNR can be used either to increase the resolution of the images or to decrease the scanning time (with the use of higher acceleration factors). Other advantages of 7 Tesla (T) field strength are the higher sensitivity to blood-oxygen-level-dependent (BOLD) signal and better vasculature conspicuity (Moser et al, 2012; Santini, Wood, et al, 2021). The high signal-to-noise ratio (SNR) and fast acquisitions of 7T MRI scanners allow studies to perform analysis of blood and cerebrospinal fluid (CSF) flow within the brain (Scouten & Constable, 2008).…”