Frequently asked questions for the PSMD toolbox.
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What MRI sequences do I need to calculate PSMD?
Only a diffusion tensor imaging (DTI) sequence is required.
Are there special requirements for the DTI sequence?
We recommend the following parameters:
- Single-shell DTI
- B-value 1000 s/mm2 (between 700 and 1200 should also work).
- At least 20 directions, ideally 30 or more
- 2 mm isotropic resolution recommended, but will also work with thicker slices
- Coverage of the entire hemispheres. Cropped brains (usually on top) can be problematic and may even introduce a bias into your analysis (see "known problems" below).
- PSMD seems to work best with data from Siemens or GE scanners. Data from Philipps scanners showed problems in some studies, especially if there was a software update (see below).
Are there any known problems that might affect PSMD?
Scanner software upgrades can be problematic. We have seen this especially with Philips scanners. Even minor software upgrades can have a strong effect on the MD histogram and therefore MD. We recommend to check for different software version (even minor differences) in your data, especially if you have Philips data.
Another issue is when the two hemispheres are not covered completely by the field-of-view. If e.g. the top is cropped, this will alter the histogram shape. If only large brains are cropped in your study, you might introduce a systematic bias.
Acute infarcts will also affect the PSMD calculation. We are currently working on a new version of PSMD, which will exclude the area with acute infarcts.