Fixing Aliasing

If a low VENC is selected, and the fluid flows faster than the VENC, then the image will 'alias,' and the true velocity will not be measured. Aliasing typically happens in the center of a vessel, where flow is typically the fastest. If aliasing happens, then it can sometimes be fixed during analysis by adjusting the encoded velocity.

The following workflow assumes the image is displaying the intensity of the phase image in units of cm/sec. If it is displaying the intensity in other units, correct that before proceeding.

Steps for Fixing Aliasing with ImageJ

  1. Open Image J

  2. Click File → Open

  3. Use the scrolling tool that looks like a hand to drag your image to your desired location

  4. Select the freehand ROI tool to place your ROI around the area you want to un-alias

  5. You will need the following information to unalias correctly:

    • Mean: the mean intensity (in units of cm/sec) in the phase image near the vessel. This is used to compute what voxels to unalias.
    • Standard Deviation: the standard deviation of intensity in the phase image near the vessel. This is used to compute what voxels to unalias.
    • VENC: the encoding velocity used to acquire the images. This is used to compute the correct velocity in the voxels that are unaliased.

      Tip: if you need the mean or standard deviation, then you can use "ROI" and "Measure" to compute these values.

      Tip: if you do not have the VENC, then you can often get this information from the DICOM header.

  6. Select: Process→ Math Macro

  7. In the command box change the code as follows and select OK

If (v\\<-( (mean) - (standard deviation) * 2) ) v= (2 * (VENC) ) -- v

Note: replace (mean), (standard deviation), and (VENC) with the values captured earlier, but leave 'v' as a variable.

  1. Select \'No\' when the dialog box asks if you want to process all 3 images

  2. To save your un-aliased image as a tiff format file. Select: File→Save as→Tiff