IMAGING AT HIGH MAGNETIC-FIELDS - INITIAL EXPERIENCES AT 4-T

This article reviews the preliminary experiences and the results obtai ned on the human brain at 4 T at the University of Minnesota. Anatomic al and functional images are presented. Contrary to initial expectatio ns and the early results, it is possible to obtain high-resolution ima ges of the human brain with exquisite T1 contrast, delineating structu res especially in the basal ganglia and thalamus, which were not obser ved clearly in 1.5-T images until now. These 4-T images are possible u sing a new approach that achieves maximal contrast for different T1 va lues at approximately the same repetition time and has built-in tolera nce to variations in B1 magnitude. For functional images, the high fie ld provides increased contribution from the venuoles and the capillary bed because the susceptibility-induced alterations in 1/T2 from thes e small-diameter vessels increase quadratically with the magnitude of the main field. Images obtained with short echo times at 4 T, and by i mplication at lower fields with correspondingly longer echo times, are expected to be dominated by contributions from large venous vessel or in-flow effects from the large arteries; such images are undesirable because of their poor spatial correspondence with actual sites of neur onal activity.