Dependence of and field patterns of surface coils on the electrical properties of the sample and the MR operating frequency

Abstract

In high field MRI, the spatial distribution of the radiofrequency magnetic (B1) field is usually affected by the presence of the sample. For hardware design and to aid interpretation of experimental results, it is important both to anticipate and to accurately simulate the behavior of these fields. Fields generated by a radiofrequency surface coil were simulated using dyadic Green’s functions, or experimentally measured over a range of frequencies inside an object whose electrical properties were varied to illustrate a variety of transmit false(B1+false) and receive false(B1−false) field patterns. In this work, we examine how changes in polarization of the field and interference of propagating waves in an object can affect the B1 spatial distribution. Results are explained conceptually using Maxwell’s equations and intuitive illustrations. We demonstrate that the electrical conductivity alters the spatial distribution of distinct polarized components of the field, causing “twisted” transmit and receive field patterns, and asymmetries between false|B1+false| and false|B1−false|. Additionally, interference patterns due to wavelength effects are observed at high field in samples with high relative permittivity and near-zero conductivity, but are not present in lossy samples due to the attenuation of propagating EM fields. This work provides a conceptual framework for understanding B1 spatial distributions for surface coils and can provide guidance for RF engineers.

Synthesized tissue‐equivalent dielectric phantoms using salt and polyvinylpyrrolidone solutions

“…Figure illustrates how the object's dielectric properties influence the field pattern at 64 MHz. This effect is even more pronounced as the operating frequency increases, in which features such as the so‐called dielectric brightening and B 1 twisting are clearly visible, indicating the presence of interference patterns and conduction currents in the sample .…”

“…To motivate the use of tissue‐equivalent phantoms in MRI, we performed full‐wave simulations using an analytic framework based on dyadic Green's functions with objects of various dielectric properties. In particular, we simulated the field of a surface loop coil in the central transverse plane of a head‐sized cylindrical phantom.…”

Quadrature operation of segmented dielectric resonators facilitated with metallic connectors

“…Figure c shows the frequency dependence of the B y mode versus the copper connector length. Finally, the effect of the phantom's conductivity on the was examined and is shown in http://onlinelibrary.wiley.com/store/10.1002/mrm.26301/asset/supinfo/mrm26301-sup-0001-suppinfo.pdf?v=1&s=697352a1a323cae037b1da7b64abfeb284bd332d, which shows an asymmetry of the that increases with higher conductivity, in line with work on different coil geometries .…”

Use of dielectric padding to eliminate low convective field artifact in cr‐MREPT conductivity images

“…Regarding the mechanism of influence of a pad on the magnetic field in the object, it is known that dielectric and conductive pads behave differently . It has been discussed and demonstrated that according to the modified Ampere’s law, displacement currents in a pad, which are induced by the “primary” B 1 field of the RF coil, act as a secondary source and result in enhancement of the B 1 field in regions of the object proximal to the pad.…”

“…where is the conductivity, is relative permittivity, is free‐space permittivity, is the Larmor frequency, and is the transmit magnetic field . Equation will be referred to as the standard MREPT (std‐MREPT) method for the reconstruction of EPs.…”