Physical orientation in the magnetic field affects diffusion measures: a hardware phantom study

Introduction It was shown recently in an isotropic water phantom that ADC values are not equal in each measured diffusion direction due to gradient mis-calibrations [1]. MR resonance frequency is also dependent on the orientation of anisotropic structures in the magnetic field [2], which possibly contributes to deviations in the diffusion measurement. We investigated the effect of orientation of an anisotropic diffusion phantom on the non-DW (b0) signal, FA and the first eigen value (λ1). Our results show considerable variations in b0, FA and λ1 in the anisotropic phantom at different orientations in the magnetic field. Methods Acquisition A commercially available spherical diffusion phantom (Brain Innovation, NL)[3] with 3 tubular anisotropic phantoms in each principal direction was used. Fig 1 shows the opened physical phantom and a rendering of the phantoms x: red, y: green, z: blue. The sphere, marked with an angular grid was rotated in 3 planes in the scanner (x-z, y-z and x-y plane) in steps of ~45 degrees. At each orientation, a DW-MRI scan was made: Siemens Allegra 3T birdcage coil, FOV 192x192mm, 70 slices, 2x2x2 mm voxel size. b-value 1000 s/mm, 27 diffusion directions and 1 b0 image. A GRE phase and magnitude image were also obtained at the same resolution. TE(1)=10.0ms, TE(2)=12.46ms. Data Analysis Data analysis was done in FSL (FMRIB, UK). FA and the first eigenvector λ1 were calculated in each phantom orientation. The b0 image from each orientation was co-registered with the reference orientation (X, Y and Z phantoms aligned with x, y, z scanner axis) and the FA and λ1 images were transformed into the same principal orientation. The mean, re-oriented FA map was skeletonized to obtain ROIs containing voxels from the tubular phantoms for each principal direction. Results Fig 2 shows the b0 images of the phantom rotated in the x-z plane, with the angle of the z-phantom wrt the scanner’s zaxis. The phantom was rotated in a similar fashion in the other two planes.