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Concepts Magn Reson Part B Magn Reson Eng. 2016 Feb;46(1):25-40. Epub 2016 Feb 4.

Dependence of B1+ and B1- Field Patterns of Surface Coils on the Electrical Properties of the Sample and the MR Operating Frequency.

Author information

1
Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201.
2
Department of Radiology, Center for Advanced Imaging Innovation and Research (CAIR) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016; NYU WIRELESS, Polytechnic Institute of New York University, Brooklyn, NY 11201.
3
Department of Radiology, Center for Advanced Imaging Innovation and Research (CAI R) and Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY 10016.

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 [Formula: see text] and receive [Formula: see text] 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 [Formula: see text] and [Formula: see text]. 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.

KEYWORDS:

B1 field patterns; B1 twisting; MRI; dyadic Green’s functions; electrical properties; electromagnetic field simulations; high-field MRI; interference patterns; magnetic resonance imaging

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