Producing a homogenous radiofrequency field within the patient at high field is challenging since the electromagnetic wavelength inside the body is significantly less than the body dimensions. An associated problem is the spatially inhomogeneous electric field that can produce localized thermal hot-spots. The use of transmit array coils, in which the magnitude and phase of the inputs to each element of the array are individually controlled, can significantly improve the RF field uniformity within a patient. Despite the challenges, there are also exciting new designs for RF coils and possibilities to control the RF fields which can only be put into practice at high fields. This chapter explains the basic principles of radiofrequency transmission and reception with specific focus on the challenges of UHF-MR and gives an overview of the state-of-the-art in this rapidly changing field.
KeywordsBlood Oxygen Level Dependent Body Coil Patch Antenna Lower Field Strength Proton Resonance Frequency
The MATLAB code used to generate values of dielectric constant versus frequency (http://pennstatehershey.org/web/nmrlab/resources/software/rftools) were provided by Chris Collins and Sukhoon Oh from the Penn State Center for NMR Research. Coil photographs were generously provided by Mark Ladd from the Erwin L. Hahn Institute, University of Essen-Duisberg and Chris Collins. Figures were also provided by Figures were kindly provided by Thomas Vaughan, Pierre-Francois van der Moortele, and Gregor Adriany from the Center for Magnetic Resonance Research, University of Minnesota.
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