Influence of biological subject, shielding cage, and resonance frequency on radio wave propagation in a birdcage coil

Abstract

Radio wave propagation of three types of birdcage transmit/receive radiofrequency (RF) coils (lowpass filter, highpass filter, and bandpass filter configurations) was analysed for mouse body magnetic resonance imaging at main magnetic fields of 1.5, 3.0, 4.7, 7.0, 9.4, and 11.7 T (Larmor frequencies of 63.87, 127.74, 200, 300, 400, and 500 MHz) in terms of magnetic (|B 1 |) field sensitivity and homogeneity. The observed radio wave propagation in the central axial |B 1 | field, as influenced by the biological subject, was calculated numerically in the finite-difference time-domain method and compared with assessment criteria using the mean value and standard deviation. The results for different Larmor frequencies RF shielding cage configurations, and biological subject loading versus unloading were compared and are discussed in detail.

A Preliminary Study for Reference RF Coil at 11.7 T MRI: Based on Electromagnetic Field Simulation of Hybrid-BC RF Coil According to Diameter and Length at 3.0, 7.0 and 11.7 T

“…The current intensity and phase of the RF can be defined at each current drive point replacing the capacitor, and the EM-field can be obtained at a specific resonance frequency. This simulation was performed under the assumption that the current was ideally drive by the micro-stripe line of the BC RF coil [5,9,19,28,32,36,44]. The results of the FDTD complex data matrix were computed using MATLAB (version 2020a, Mathworks, Inc., Natick, MA, USA) for the positioning of the B 1 -, B 1 + -, and B 1 − -components within their phase map and for the comparison of the electric field (E-field) and SAR components under various B 0 -field strengths.…”

“…However, LP-BC RF coils are characterized by performance limitations in terms of their signal sensitivity in ultrahigh-field (UHF) MRI systems above 3.0 T. In fact, LP-BC RF coils provide excellent B 1 -field sensitivity in low-field MRI systems below 3.0 T, and their B 1 -field sensitivity decreases significantly as magnetic field strength increases above 3.0 T compared to other types of BC RF coils [5,6]. Despite the low signal sensitivity problem of LP-BC RF coils, MRI manufacturers perform the aforementioned evaluation on 3.0 and 7.0 T MRI systems using LP-BC RF coils as reference coils.…”

“…An LP-BC RF coil can reduce the unit cost and can improve its quality due to the advantages of using a small quantity of required capacitors and fewer capacitance values [16,17]. The high-pass-type BC (HP-BC) and hybrid-type BC (Hybrid-BC) RF coil configurations exhibit higher |B 1 |-field sensitivity than the LP-BC RF coil at various magnetic field (B 0 -field) strengths [5,18]. In fact, the HP-BC RF coil exhibited better |B 1 |-field sensitivity than the other configurations, whereas the |B 1 |-field sensitivity of the LP-BC RF coil decreased dramatically with the increase in the B 0 -field strength.…”

“…In fact, the HP-BC RF coil exhibited better |B 1 |-field sensitivity than the other configurations, whereas the |B 1 |-field sensitivity of the LP-BC RF coil decreased dramatically with the increase in the B 0 -field strength. On the other hand, the Hybrid-BC RF coil provides the highest B 1 -field sensitivity and homogeneity when used at UHF MRI systems over 7.0 T [5]. In addition, the diameter (D) and length (L) of the BC RF coils determine B 1 -field sensitivity and uniformity; thus, these coils are designed to operate in close proximity to the image acquisition target.…”

A Comparative Study of Birdcage RF Coil Configurations for Ultra-High Field Magnetic Resonance Imaging

“…We have previously described a set of electromagnetic field (EM-field) simulations that combine various methods to improve B + 1 -field efficiency and B − 1 -field sensitivity and have proposed the use of an optimal combination of a band-pass type bird cage coil (BP-BC RF coil) with a multi-channel wireless RF element (MCWE) [43]. A BP-BC RF coil was the optimal choice because it provided the greatest uniformity and signal sensitivity among the three types of BC RF coils (Low-pass, High-pass, and Band-pass) tested in UHF MRI [44][45][46].…”