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Abstract
Purpose: Many conventional ex vivo magnetic resonance imaging (MRI) setups utilize cylindrical or other nonspherical tissue containers which can cause static field (B0 ) 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 B0 inhomogeneity in arbitrary orientations.

Methods: B0 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 B0 inhomogeneity contributions. The sphere could be rotated freely without disturbing the RF coil to facilitate multi-orientation imaging. We verified our setup by mapping B0 in the monkey brain at 13 different orientations in a human 7T scanner, and measuring orientation-dependent R∗2R2∗ 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 B0 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 B0 of <0.07 ppm. The B0 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). R∗2R2∗ in the corpus callosum showed distinctly different orientation dependence compared to the gray matter. The B0 uniformity and R∗2R2∗ reliability were much reduced in the cylindrical container setup.

Conclusions: We have demonstrated an experimental method to effectively minimize bulk susceptibility-induced B0 perturbation in multi-orientation ex vivo MRI. The method promises to benefit a range of tissue orientation-dependent MR property studies.

Affiliations

So-Hee Lee  1   2 , Min-Jun Han  1   2 , Joonyeol Lee  1   2 , Seung-Kyun Lee  1   2   3   4
1 Center for Neuroscience Imaging Research (CNIR), Institute for Basic Science (IBS), Suwon, South Korea.
2 Department of Biomedical Engineering, Sungkyunkwan University, Suwon, South Korea.
3 Biomedical Institute for Convergence, Sungkyunkwan University, Suwon, South Korea.
4 Department of Physics, Sungkyunkwan University, Suwon, South Korea.