Using the correct unit of diffusivity

Overview

Since DTI volumes store values of physical quantities, diffusivity in this case, it is important to specify the particular physical unit used to store these values. Check this to verify that your images are stored in the right unit and to modify it if ever it is not the case.

Solving the diffusivity issue

The unit used by DTI-TK is 10^-3mm².s^-1. With this unit, the mean diffusivity for CSF is around 3. You want to make sure your DTI volumes are stored in the correct unit. If they are stored using a different unit, you can first figure out the appropriate multiplication factor, (your unit)/(DTI-TK unit), that changes the values to the unit used by DTI-TK, then use the following command to create DTI-TK compatible DTI volumes

TVtool -in ${inputfile} -scale ${factor} -out ${outputfile}

e.g. if your DTI volumes use the unit of mm².s^-1, then the multiplication factor should be 1,000. If your unit is m².s^-1, then the factor should be 10⁹.

Please download the example data set to see what a DTI-TK compatible DTI volume looks like. Make sure the numerical values are of the same order of magnitude.

How to figure out the unit of diffusivity in a DTI volume?

If, for diffusion-weighting, b-value of 800 s.mm^-2 is used during the acquisition, and you input the number 800 as the b-value to your tensor reconstruction program, then the output DTI volume will have the unit of mm².s^-1.

Alternatively, you can compute the trace map of your DTI volumes. If you find that the trace values for CSF are close to 10^-2, then most likely it is in the unit of mm².s^-1.

• For AFNI users: 3dDWtoDT in AFNI does NOT factor b-value into the tensor estimation (Read more here). As a result, the program essentially always assume the b-value to be equal to 1. If the actual b-value is 1,000 s.mm², then the equivalent unit of diffusivity used by 3dDWItoDT is the same as DTI-TK. If the actual b-value is 800 s.mm², then the equivalent unit of diffusivity is 1/800 mm².s^-1 instead. The multiplication factor should then be 1.25.
• For Camino users: In general, it depends on how the scheme file is created. If the scheme file conforms to the default unit of Camino, the diffusivity unit should be m².s^-1. In this case, the multiplication factor should be 10^9.
• For FSL users: If the correct b-value is used for the corresponding entries in the bval file, then the diffusivity unit should be mm².s^-1. In this case, the multiplication factor should be 10^3.