Hi,

these are warnings concern DTI data coming from a Siemens Scanner. Vb13 and vb15 are MRI scanner software generations (vb15 being the actual I think).

1. Polarity means that you probably have to invert x,y or z (you will know after tensor reconstruction, when the tensors are not correct)

2. if your scanner still runs vb13 you need to be aware of possible bugs...

If you have any doubt you can directly contact MRIConvert:

https://lists.uoregon.edu/mailman/listinfo/mriconvert

best, M

First, you should check if you are running a (Siemens) MR scanner with software version VB13 (from around 2006) or VB15 (from around 2008) - or if you are processing older data acquired on a scanner with either VB13 or VB15. If not (most systems with VB software today have software versions VB17 or VB 19), the warnings shouldn't bother you.

The first warning ("Polarity of the bvecs may be wrong") should be visible in the resulting diffusion tensor data: The main diffusion direction would be oriented in a non-physiological direction (e.g., diffusion in the corpus callosum would not follow the normal course of nerve fibers).

The second warning has not so clear wording ("bvecs are sometimes wrong"); if this refers to the polarity as well, then the same as written above applies. If this refers to the magnitude of the b-value, then the trace of the tensor (mean diffusion) may just be slightly wrong, which is more difficult to detect. It might make sense to compare your calculated diffusion tensors (or derived quantities) with those calculated directly on the Siemens scanner (or on a Siemens post-processing workstation), since the latter should (hopefully) compensate any problems with the DICOM meta data.