Understanding the orientation dependent T2* contrast of the cingulum in ultra high fields

Introduction: It was shown recently that high resolution T2*-weighted images, acquired at ultra high field, display T2* heterogeneity in white matter [1], and an unexpected signal dependence in the cingulum bundles on head orientation to the main magnetic field [2]. The orientation dependence was not been observed in ex-vivo samples at lower field strength [3]. Because the effect of susceptibility variations increases with field strength, the orientation dependence might thus only be observable at higher fields [2], and/or in-vivo [4]. The signal change with orientation is still poorly explained. Here we used, in addition, phase images arising from high resolution spoiled gradient echo sequences, because such images are more sensitive to susceptibility effects [5]. We also obtained TSE images to determine whether the orientation dependence of the signal appears in that sequence as well. We also compared the measurements with simulations of simple model using the forward field calculation [6].