Relationships between assumed directions of gradients used during reconstruction of synthesized ADC distributions, and those specified during data acquisition using a prolate distribution. To accentuate spatial curvature of the distributions, the -value was assumed to be . Panel (A): Reconstruction directions for oblate (2) and (3) distributions, at 0° and 45° in the reconstruction frame, map into the acquisition directions shown on the prolate distribution (1). Panel (B): Acquisition gradient directions for synthesizing oblate, , and distributions. Reconstruction directions, uniformly distributed over a sphere, are shown as points in the upper left plot. For the synthesized distributions shown in the top row, the position of dots in the bottom row indicates the applied gradient directions, with the dot shading indicating where the ADC data will be remapped in reconstruction coordinates.
Schematic of phantom, with closeups of the array configuration (ROI) showing individual capillary arrays (shaded areas).
Comparisons of theoretical ADC distributions (small points) and measured or synthesized ADC distributions (large dots). Data in panel A use the same directions during acquisition and reconstruction. Synthesized ADC distributions include oblate (column B), (column C), and (column D). Dashed lines show the orientation of the principal eigenvalues for the prolates in the two-tensor distributions.
The effect of SNR decrease on prolate (top row) and synthesized ADC distributions (middle ; bottom ). Theoretical predictions are shown in the first column, as a outline for high SNR and a smaller outline for low SNR. The measured and synthesized distributions (large dots) for the same high SNR (middle row) and low SNR (right row) values are shown for comparison. Note the “flattening” of ADC measurements close to the tip of the distribution, within the circles. The shape of the expected ADC distributions at high SNR is also shown as a cloud of small points.
Rotation of the scan plane from the orientation used to calculate appropriate diffusion gradients. The synthesized ADC distributions (large dots) compare less favorably to theoretical ADC distributions (small points), after misalignment of the scan plane approximately by 0° (left column), 5° (center column), and 10° (right column).
A comparison of DTI metrics for measured and synthesized data.
DTI metrics for synthesized distributions after phantom misalignment or faulty eigenvalue assumption.
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