Experimental setup for bulk susceptibility effect-minimized, multi-orientation MRI of ex vivo tissue samples.

Purpose: Many conventional ex vivo magnetic resonance imaging (MRI) setups utilize cylindrical or other nonspherical tissue containers which can cause static field (B ) inhomogeneity affecting the accuracy of the measurements in an orientation-dependent manner. In this work we demonstrate an experimental method to obtain MRI of ex vivo tissue samples held in a spherical container in order to minimize bulk susceptibility-induced B inhomogeneity in arbitrary orientations.

Methods: B inhomogeneity caused by tissue-air susceptibility mismatch can be theoretically eliminated if the surface of susceptibility discontinuity is spherical. This situation can be approximated by putting a tissue sample in a spherical shell filled with materials with tissue-like magnetic susceptibility. We achieved this on an intact monkey brain by (a) holding the brain with a three-dimensional (3D)-printed holder with tissue-like (within 0.5 ppm) susceptibility, and (b) enclosing the brain and the holder in an acrylic spherical shell filled with diamagnetic liquid. Furthermore, the sphere and the radio-frequency coil for MRI were mounted on a 3D-printed frame designed to reduce B inhomogeneity contributions. The sphere could be rotated freely without disturbing the RF coil to facilitate multi-orientation imaging. We verified our setup by mapping B in the monkey brain at 13 different orientations in a human 7T scanner, and measuring orientation-dependent relaxation rates in the white and gray matters of the brain. The results were then compared with a setup where the brain was held inside a cylindrical container.

Results: In all orientations, the B standard deviation in the brain in the spherical setup (converted to Larmor frequency offset) was less than about 10 Hz. This corresponds to two-sigma deviation of B of <0.07 ppm. The B gradient was <9 Hz/mm in 95 % of the brain voxels in all orientations. In high-resolution imaging with e.g. voxel size <0.4 mm, this corresponds to voxel line broadening of <4 Hz (0.013 ppm). in the corpus callosum showed distinctly different orientation dependence compared to the gray matter. The B uniformity and reliability were much reduced in the cylindrical container setup.

Conclusions: We have demonstrated an experimental method to effectively minimize bulk susceptibility-induced B perturbation in multi-orientation ex vivo MRI. The method promises to benefit a range of tissue orientation-dependent MR property studies.