Electromagnetic fields of surface coil in vivo NMR at high frequencies

Abstract

A high frequency solution of the electromagnetic field produced by a circular surface coil adjacent to a homogeneous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo 1H NMR spectroscopy experiments from 200 to 430 MHz (4.7-10 T). Above 200 MHz the S/N increases more rapidly with frequency and the SAR increases less rapidly compared with the respective S/N and SAR frequency dependence below 200 MHz. The difference in frequency dependence is due to dielectric resonances of the magnetic field inside the sphere at frequencies above 200 MHz. It is predicted that surface coil 1H NMR experiments may be performed on a head-sized sphere, having conductivity and relative dielectric constant of brain, at frequencies up to 430 MHz without exceeding 8 W/kg local SAR and 3.2 W/kg SAR. The calculations of the S/N and SAR are used to determine optimum surface coil geometries for NMR experiments. The power radiated by the surface coil in the absence of shielding and asymmetries in the received signal with respect to the plane defined by the surface coil axis and the direction of the static magnetic field are significant at high frequency. Experimental measurements of the magnetic field inside a head-sized sphere verify the presence of dielectric resonances at frequencies above 200 MHz.

Signal‐to‐noise ratio and absorbed power as functions of main magnetic field strength, and definition of “90°” RF pulse for the head in the birdcage coil

“…This demonstrates that the low-frequency approximations, which predict an increase of V 2 P abs proportional to f 2 (1), are less valid at high frequencies. This predicted trend of decreasing slope in absorbed power at high frequencies for a constant flip angle at one point is consistent with previous calculations that considered Maxwell's equations in their entirety for simple geometries with nearphysiologic parameters (20). An understanding of why this deviation from low-frequency estimates occurs can be gained by looking at the B 1 flux through a plane and considering Faraday's law.…”

4T Actively detuneable double-tuned 1H/31P head volume coil and four-channel 31P phased array for human brain spectroscopy

“…31 P spectroscopic images were acquired using a non-selective 37.5° excitation pulse and 13 × 13 × 13 spherical encoding over a FOV of 24 × 24 × 24 cm 3 . The data was acquired using 6 averages and a TR of 0.5 s for an acquisition time of 48 minutes.…”

“…Although 31 P in vivo magnetic resonance spectroscopic imaging (MRSI) is often limited by the available SNR, there are few reports of the use of 31 P phased arrays to increase sensitivity for human brain studies (2,3). Although receive-only 31 P phased arrays in combination with larger transmit-only surface coils have been reported (2,3); to our knowledge their use with homogeneous transmit volume coils has not been yet reported. To provide highresolution anatomical imaging and shimming with a homogeneous volume coil without removal of the coil or patient from the magnet, double-tuning of the transmit volume coil is required.…”

Optimizing signal‐to‐noise ratio of high‐resolution parallel single‐shot diffusion‐weighted echo‐planar imaging at ultrahigh field strengths

“…Thereby, the coil elements were distributed on a spherical shell concentrically enclosing the object and the radii were maximized so that adjacent coils touched but did not overlap. In this case, the RF electrodynamic fields of a single coil element are given in the form of a semianalytical multipole expansion (18,19). Because of the symmetry of this setup, the RF fields of the entire coil array can be determined through rotations of the single-coil solution (17).…”

“…PI performance at both field strengths is simulated considering a spherical object with the electrical properties of brain tissue uniformly surrounded by identical, circular coil elements (17). This setup has the advantage of allowing for the semianalytical expression of the fullwave radiofrequency (RF) electrodynamic fields (18,19). Amongst other parameters, the simulations require the knowledge of the T 2 relaxation times at both field strengths.…”