Radiofrequency field gradient encoding methods and applications for magnetic resonance imaging

This article reviews the research area of encoding magnetic resonance imaging (MRI) with radiofrequency (RF) transmit field gradients. RF gradients are generally investigated as an alternative to the static magnetic field gradients more conventionally employed for imaging. Since first being demonstrated in the 1970s, the number of approaches explored for RF encoding has increased, as have potential applications. Methods such as rotating frame imaging, FREE, TRASE, Bloch-Siegert shift encoding, nonlinear RF encoding, and SENF have been developed and show promise for providing certain advantages over B₀-encoded MRI. Magnetic resonance spectroscopy (MRS) applications also have a long history and certain advantages when RF gradients are used for localization. Despite the interest in developing these methods, challenges remain with the design of associated hardware, integration of encoding, and translation to in-vivo imaging. Here, we provide a survey of the various methods of RF encoding for imaging. We conclude with future directions for RF encoding in its technical development, research utility, and clinical application.