Assessing the Z-spectrum Asymmetry

This module analyses the asymmetry of the Z-spectrum with respect to the water frequency. The asymmetry can be assessed at a particular frequency offset, or within a range of frequency offsets. To perform asymmetry analysis, select the Asymmetry tab:

Set up the input section, then set:

• The frequency offset, in ppm relative to water, at which to assess the asymmetry. This can be either:
• A single frequency offset: .

• A range of offset frequencies. Select the check box, and then enter the range of frequencies: . The asymmetry is evaluated by averaging over the frequency range set, which can help to improve robustness to measurement noise.

• S0 image. The asymmetry value is normalised by dividing the asymmetry of the Z-spectrum by the image intensity with no saturation pulses applied. Supply an image acquired using the same pulse sequences as for Z-spectrum but without saturation pulses here:

  

  This image must have the same dimensions as the Z-spectrum images, and with a number of slices matching that set in the input images section.

• B0 map. This is a map of B0 inhomogeneity produced from the WASSR analysis to correct for field deviations.

  

  This image must have the same dimensions as the Z-spectrum images, and with a number of slices matching that set in the input images section.

When set up, press to assess the asymmetry. When complete, a new image is created which shows a map of the normalised asymmetry. The image name will be taken from the input image (or first input image, in the case of multiple input images), with a suffix related to the asymmetry frequency offset (e.g. "_4.2" or "_4.0-4.4" in the examples above).