In magnetic resonance imaging (MRI), when a radiofrequency (RF) wave propagates through the air-body boundary into the human body, one would expect to see the differences outside and inside human body. This chapter discusses RF wave reflection and refraction at several media boundaries conforming to Snell's law. Based on these observations, the wave propagation at the surface of human body is predicted, and several dielectric methods are proposed to improve the RF field magnitude within the region-of-interest (ROI). It introduces a numerical method, finite-difference-time domain (FDTD), and used to evaluate the RF fields inside the human body and in a specific target organ, with and without the aid of dielectric methods. The chapter uses dielectric pads to obtain optimal RF transmission within the ROI, and to use thin dielectric boards to reduce energy flowing out of the ROI, which are evaluated with XFDTD, a software based on the FDTD method.