High Field Human Imaging

This review article examines the state of knowledge regarding human imaging using MRI at high main magnetic field strengths. The article starts with a summary of the technical issues associated with magnetic field strengths in the range of 3-8 T, including magnet characteristics and the properties of radiofrequency magnetic fields, with special reference to sensitivity, power deposition, and homogeneity. The published data on tissue-water relaxation times in the brain is tabulated and the implications for contrast and pulse sequence implementation is elucidated. The behavior of the major fast imaging sequences, fast low angle shot (FLASH), rapid acquisition with relaxation enhancement (RARE), and echo planar imaging (EPI), is examined in this context. A number of anatomical images from 3 T systems are presented as examples. Particular attention is given to various forms of vascular imaging, namely, time of flight angiography, venography, and arterial spin labeling. The most complex changes in contrast with main magnetic field strength are in activation studies utilizing the blood oxygen level dependent mechanism, which are examined in detail. Improvements in spatial specificity are emphasized, particularly in conjunction with spin-echo imaging. The article concludes with a discussion of the current status and the potential impact of technical developments such as parallel imaging.