Anisotropy Induced by Macroscopic Boundaries: Surface Normal Mapping Using DWI

INTRODUCTION Diffusion imaging has traditionally been used to infer information regarding tissue microstructure when the characteristic dimensions of the underlying geometry are smaller than the voxel size. However, the diffusion signal is sensitive to structures whose sizes are comparable to or greater than the voxels of the image. In this work, using a simple system of infinite parallel planes, we demonstrate that when the voxel is situated close to the boundaries, diffusion signal may be anisotropic even if the voxel itself contains no structure. This information can be used to extract the direction perpendicular to the bounding surfaces. The expected behavior is experimentally verified on a sample of a hollow cylinder where water molecules are trapped between two nearly concentric cylinders. The proposed method can be utilized to infer information from various tissues with macroscopic boundaries such as the cerebral cortex and the colon wall.