[Mrtrix-discussion] Variability in diffusion acquisition gain affecting FA measures

[Amanda Ng]

Hi all,

I was hoping someone might be able to explain an 'anomaly' I've come across.

We have some 64 direction diffusion scans taken on our Siemens Skyra
(10 subjects, 2 time points each). We've used the same protocol for
all the scans. Looking at the DWI files (ie b0 volume + 64 directional
volumes), most of the b0 volumes have a mean intensity of ~160-210 and
the directional volumes have mean intensities of ~18. There are three
scans that have mean B0 intensities of ~70 and mean directional volume
intensities of ~6. This drop in intensity is affecting the FA measures
(we assume this is because of the lower SNR). I've ruled out software
and hardware upgrades as a cause. I've been told there were no changes
to the protocol, and I can't find anything in the DICOM headers
indicating a change in protocol.

Would anyone know why this is occurring and how to avoid it? Has
anyone come across it before?

Thanks in advance!
Amanda Ng