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.

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.

In this paper, we will provide a methodology for head phantom development based on in vivo imaging data attained utilizing MRI. The anthropomorphic phantom can be designed to mimic human anatomy. In medical physics, phantoms are physical and/or numerical tools created to represent specified characteristics of human anatomy [1-5]. Phantoms are a less expensive approach for testing and validation of a variety of applications, such as communication or medical diagnostic imaging tools [1,5-7]. Phantoms offer the capability to perform safety testing of diagnostic imaging tools and potentially minimize harmful exposure to a human. A variety of simple commercial [8] and geometric [3,5] phantoms have been used in various medical applications. The development of anthropomorphic phantoms dates back to the 1960s [9,10]. Additive manufacturing, referred to as 3D printing, has become an increasingly useful tool to develop these complex and anatomically accurate phantoms [11]. In a recent review [12], *

Resumo -Este trabalho apresenta uma proposta de antena monopolo planar operando na faixa para sistemas de comunicação de banda ultralarga (UWB) de 3.1 a 10.6 GHz. Inicialmente, é analisada uma antena monopolo planar constituída por um condutor retangular de largura w = 12 mm e altura h = 20 mm sobre um plano de terra, a qual tem largura de banda de 3,5 GHz. Aqui, foram plotadas: a curva de perda de retorno e o diagrama de irradiação, os quais estão de acordo com os resultados da literatura. Foi projetada uma outra antena monopolo planar com altura h = 14 mm e largura w = 8 mm, a qual possui largura de banda igual a 6,2GHz. Em seguida, foram utilizados três elementos parasitas em conjunto com a antena anterior, sendo obtida uma antena de largura de banda de 6,5 GHz dentro da faixa de 3.1 a 10.6GHz. As freqüências de corte superior e inferior, utilizadas para calcular a largura de banda, foram obtidas em -10 dB. Todas as análises foram realizadas a partir de simulações utilizando o método FDTD.

Recently cleared by the FDA, 7 Tesla (7T) 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 7T; to date, this device was used in more than 1,300 patient/volunteer neuro 7T MRI scans. Electromagnetic simulations were performed for each of the coil’s antennas 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, and successfully produced homogeneous magnetic field distributions at low levels of power deposition in the tissues. The simulations were then successfully validated in-vivo using the Tic Tac Toe RF head coil system on a 7T MRI scanner.