CEST WASSR Analysis

Estimating B0 Inhomogeneity with WASSR

CEST is sensitive to B₀ inhomogeneity, and it may be important to correct this. This module uses the WASSR method of direct water saturation imaging to estimate the absolute water frequency in each voxel, allowing proper centring of Z-spectra on a voxel-by-voxel basis. To perform WASSR analysis, select the WASSR tab:

Tab to perform WASSR analysis

Set up the input section, then set:

The search range for B₀ inhomogeneity. Using the spinner, set the expected range of B₀ inhomogeneity:

You should try to roughly estimate the actual range of B₀ inhomogeneity, since this will result in more precise estimation, with less tendency to get the estimate badly wrong. However, you should not set the search range less than the true range of B₀ inhomogeneity.

. If not selected, the B₀ inhomogeneity is estimated in each pixel independent of all the other pixels. If selected, continuity of B₀ inhomogeneity is maintained between adjacent pixels by applying a smoothness criterion. The degree of smoothness is set by selecting it fron the drop-down menu: . You will need to experiment to find the appropriate degree of smoothness, since it will depend on the signal-to-noise ratio in the input images.

. De-select to maintain B₀ inhomogeneity smoothness between slices in 3-D datasets. This setting has no effect for single slice images, or when B₀ smoothing is not applied.

When set up, press Button to perform the analysis

to perform the WASSR analysis to estimate the B₀ inhomogeneity. When complete, a new image is created which shows a map of the B₀ inhomogeneity, with units of parts-per-million. The image name will be taken from the input image (or first input image, in the case of multiple input images), with the suffix "_B0" appended.

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