RF penetration in ultra high field MRI: Challenges in visualizing details within the center of the human brain

Purpose: The purpose of this work is to discuss radio frequency (RF) penetr ation and its relevance to imaging the human head and to acquire images con taining intricate structures located at the center of the brain with ultra high field MRI (UHFMRI). Method: A simple plane wave analysis of RF penetration was performed based on Maxwell equations as a function of frequency up to 900 MHz. Gradient-rec alled images were acquired at 8 T (340 MHz) using an RF resonator operating in quadrature. Typical acquisition parameters were as follows: TR = 750 ms , TE = 17 ms, slice thickness = 2 mm, FOV = 20 x 20 cm, matrix = 1,024 x 1, 024. The specific absorption rate was well below 1 W/kg. Results: A simple analytical treatment, for a plane wave up to 900 MHz, rev eals a lack of decreasing penetration depth with frequency beyond 200 MHz. Gradient-recalled echo images acquired from the human head displayed good c ontrast, homogeneity, and resolution. Importantly, excellent structural det ail was observed on the resulting MR images, demonstrating that RF penetrat ion is not a problem at 8 T. Images reveal excellent detail including the r ed nucleus, anterior commissure, fornix, mamillary body, pineal gland, and ependymal lining of the fourth ventricle. Conclusion: Structures located at the center of the human brain can be clea rly visualized at 8 T with no detectable loss in signal intensity arising f rom RF penetration. The ability to examine these structures with UHFMRI wil l provide a powerful new modality for diagnostic radiology.