Modeling, Analysis, and Visualization of Anisotropy by Thomas Schultz Evren Özarslan & Ingrid Hotz

Author:Thomas Schultz, Evren Özarslan & Ingrid Hotz
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

3 Measurements of Diffusion with Diffusion-Weighted MRI

The estimation of diffusion anisotropy can be thought, in first approximation, as the assessment of the amount of preference that the diffusion process has for a specific spatial direction, compared to the others, in terms of diffusivity. Therefore, this assessment requires sensing the diffusion signal along multiple spatial directions, regardless of the representation adopted to describe the signal itself. In MRI, this is typically done by acquiring a collection of images of the target object, e.g. the brain. Each image is acquired when the experimental conditions within the magnet’s bore determine a specific diffusion-weighting along the selected spatial direction: this is a Diffusion-Weighted Image (DWI). The diffusion-weighting is globally encoded by the b-value [46], measured in s∕mm2, a quantity that is the reciprocal of the diffusivity, D (mm2∕s). The intensity of the diffusion-weighting, i.e. the b-value, is determined by the acquisition setup.

The most common type of acquisition is the Pulsed Gradient Spin-Echo sequence (PGSE) [60], where a DWI is obtained by applying two diffusion gradients with intensity G = ∥G∥ (T∕m) and duration δ (s) to the tissue, separated by the separation time Δ (s). We illustrate this sequence in Fig. 2. The resulting signal is ‘weighted’, along the applied gradient direction, with b-value [60]

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