Electrodynamics and radiofrequency antenna concepts for human magnetic resonance at 23.5 T (1 GHz) and beyond

Abstract

Materials and MethodsElectromagnetic field simulations (EMF) are performed in phantoms with average tissue simulants for dipole antennae using discrete frequencies (300MHz (7.0T) to 3GHz (70.0T)).To advance to a human setup EMF simulations are conducted in anatomical human voxel models of the human head using a 20-element dipole array operating at 1 GHz. ResultsOur results demonstrate that transmission fields suitable for 1 H MR of the human brain can be achieved at 1GHz. An increase in transmit channel density around the human head helps to enhance B1 + in the center of the brain. The calculated relative increase in specific absorption rate (SAR) at 23.5T vs. 7.0T was below 1.4 (in-phase phase setting) and 2.7 (circular polarized phase setting) for the dipole antennae array. ConclusionThe benefits of multi-channel dipole antennae at higher frequencies render MR at 23.5Tfeasible from an electrodynamic standpoint. This very preliminary finding opens the door on further explorations that might be catalyzed into a 20 Tesla class human MR system.2

An analytic expression for the ultimate intrinsic SNR in a uniform sphere

“…In fact, recent work has shown that because in a spherical geometry all points on the surface are equidistant from a central spin, the optimal receive currents that maximize performance at the center are closed and create loop‐like patterns at all field strengths. Nevertheless, electric dipoles, which also have a divergence‐free component, can still capture a substantial portion of the available central SNR at ultra‐high field, if the current‐bearing surface is not a complete sphere . At ultra‐high field, electric dipole arrays can even exceed loops' performance in maximizing central SNR in the case of body‐size objects, for which the receive elements are normally arranged on a cylindrical surface …”

In vivo human head MRI at 10.5T: A radiofrequency safety study and preliminary imaging results

“…As a result, destructive interference of the magnetic fields causes signal inhomogeneity and degradation in image quality. On the other hand, both local and global specific absorption rate (SAR) increase with B 0 . Local SAR turns out to be the most critical radiofrequency (RF) safety concern as its limit is typically reached earlier than global SAR based on international guidelines .…”

“…On the other hand, both local and global specific absorption rate (SAR) increase with B 0 . 2,[9][10][11] Local SAR turns out to be the most critical radiofrequency (RF) safety concern as its limit is typically reached earlier than global SAR 11,12 based on international guidelines. 13 Multichannel excitation solutions have been proposed in the literature to deal with these issues using hardware and software solutions.…”

From ultrahigh to extreme field magnetic resonance: where physics, biology and medicine meet

“…The issue provides an overview of the state of the art and discusses the clinical relevance of what we have already observed and can clearly foresee. Articles are devoted to development of novel methodology [12][13][14][15][16], safety topics [17][18][19] early multicenter trials [20], frontier human studies [21][22][23][24][25][26][27][28], breakthrough clinical applications [29][30][31][32][33][34][35][36] and future directions of UHF-MR [37,38]. At the moment some of these new concepts and clinical applications are merely of proof-of-principle nature and visions, but they are compelling enough to drive the field forward.…”

“…This envisions human MR at 20 T [37,38], and it is an important leap of the imagination because it aims to fill a crucial "resolution gap" in our understanding of human biology [39,40]. While discoveries are pouring in on the molecular and cellular level every day, it is The development of ultrahigh-field magnetic resonance (UHF-MR) is moving forward at an amazing speed that is breaking through technical barriers almost as fast as they appear.…”