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:

    Spinner to set the search range for 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.

  • Check box to smooth the B0 field inhomogeneity estimate. 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 from the drop-down menu: Menu to set the degree of 0 field inhomogeneity smoothness. 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.

  • Check box to to process slice-by-slice. 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.