Electromagnetic perspective on the operation of RF coils at 1.5–11.7 Tesla

Abstract

In this work experimental and numerical studies of the MR signal were performed at frequencies ranging from 64 MHz to 485 MHz, utilizing three different MRI coils: a single-strut transverse electromagnetic (TEM)-based coil, a TEM resonator, and a high-pass birdcage coil. The experimental analyses were conducted using 1.5 and 8 Tesla whole-body systems and volume RF head coils. The simulation data were obtained utilizing an in-house-developed finite difference time domain (FDTD) model. Pertinent data from the numerical and experimental setups were compared, and a remarkable agreement between the two methods was found that clearly demonstrates the effectiveness of the FDTD method when it is applied rigorously. The numerical and experimental studies demonstrate the complexity of the electromagnetic (EM) fields and their role in the MR signal. These studies also reveal unique similarities and differences between the transmit and receive field distributions at various field strengths. Finally, for ultra high-field operations, it was demonstrated mathematically, numerically, and experimentally that highly asymmetric inhomogeneous images can be acquired even for linear excitation, symmetrical load geometries, and symmetrical load positioning within the coil. Experiments with human whole-body imaging at Ն3 Tesla and head imaging at Ն7 Tesla have shown the complexity of the electromagnetic fields induced within the associated radiofrequency (RF) coils (1-5). With increasing recognition of the role played by tissue-coil interactions, it became clear that numerical techniques based on full-wave methods can provide essential information for understanding the behavior of RF coils when they are loaded with biological structures. Computational tools based on full-wave methods can be very effective for feasibility studies of RF coils, which are extremely difficult to carry out in experimental settings. As such, for whole-body human applications at Ն3 Tesla, and head applications at Ն7 Tesla, computational electromagnetics can be effectively utilized to design and evaluate RF coils before they are constructed and tested experimentally.At high magnetic fields (Ն3 Tesla), the performance of the RF coil becomes increasingly dependent on its interactions with the human head/body, because the human head/body size is comparable to the operating wavelength. These interactions can be accurately explored through full-wave electromagnetic modeling. Until the current decade, full-wave numerical EM methods were seldom used to model the fields produced by RF coils used with MRI systems. There has not been much need for such an approach because most MRI magnets for humans, even the research-dedicated systems, had magnetic field strengths of 1.5 Tesla and below. Thus, circuit approximations remained appropriate for designing and predicting the performance of the associated RF coils. However, with the growth in the number of high-field human magnets, including 4 Tesla (6 -9), 7 Tesla (10), and 8 Tesla (11) systems, full-wave electromagnetic mod...

In‐depth study of the electromagnetics of ultrahigh‐field MRI

“…For instance, the finite difference time domain (FDTD) method (21) has shown excellent correlation with experimental data at 7 T for surface coils (22), providing in-depth understanding of the operation of these coils at such field strength. With respect to volume coils, excellent correlation showing the interactions of coil drive ports and load has been demonstrated between data obtained experimentally at 8 T and data obtained numerically using FDTD simulations in which the load and the coil were treated as a single system (5). These studies have led to unique observations of electromagnetic field behavior of loaded UHF MRI volume coils (5,10), including the verification of the reciprocity theorem in predicting the NMR signal (23,24).…”

“…This approach permits the electromagnetic interactions between the drive port(s) and the load to be accurately considered in all the calculations through the rigorous modeling of the coil (4,5,10,12,13,28,29). In terms of the resonator model, a stair-step approximation was used to model the shield, the top and bottom rings of the coil and the coaxial elements.…”

“…Advancing the UHF MRI RF technology to achieve the RF coils needed requires a clear understanding of their operation when they are loaded (4,5). At UHF MRI frequencies, the dimensions of both head and body coils, as well as the dimensions of the load, are significant fractions of the operating wavelength (exceeding multiple wavelengths for applications at !7 T).…”

“…At UHF MRI frequencies, the dimensions of both head and body coils, as well as the dimensions of the load, are significant fractions of the operating wavelength (exceeding multiple wavelengths for applications at !7 T). As such, a combination of experimental approaches with full wave modeling (2,(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) is required in order to accurately analyze the behavior of UHF MRI RF coils when they are loaded. For instance, the finite difference time domain (FDTD) method (21) has shown excellent correlation with experimental data at 7 T for surface coils (22), providing in-depth understanding of the operation of these coils at such field strength.…”

“…When empty, the TEM resonator (26) produces modes that are transverse electromagnetic (TEM) to the resonator axis. Theoretically, however, these modes, including the normal mode of operation (referred to as the 'homogeneous' mode) are not TEM at any field strength owing to the load (dielectric or lossy) and to the perturbations in the coil fields as a result of the feed and termination loads (5,28). While not theoretically TEM, these modes can be referred to as quasi-TEM with low field/small electrical loads since the load has minimal effect on the volume coil performance and the TEM characteristics are mostly retained.…”