[Mristudio-users] Why deleting b0 from one of the runs increases FA a lot.

[Dorian P.]

Hi Dr. Mori

That was my strategy. I  combined the two runs, b0+32DWI+32DWI. This was
done of course after registration, so that I could use the new gradient
table. Images were saved in raw format, and loaded back with the new
gradient table.

I tried both using all images with gradient 100, 100, 100 for the bad b0 or
completely deleting the bad b0 (and the corresponding gradient line).
Either way FA goes up a lot for the same measured tract.

Here are some more tests on left arcuate from a good subject with 2 DTI
runs. Values are FA:
2 DTI runs (normal) = 0.489
2 DTI runs (exclude 1 b0) = 0.55
1 DTI run (normal) = 0.499

It seems that excluding one b0 is much worse than excluding the whole DTI
run. In other words, adding DWI images from a sequence that doesn't have
its own b0 may invalidate the findings. If seems that a set of DWI images
without a b0 acquired during the same sequence is not useful to to include
in DTI calculation.

Does this make sense?

Dorian



2013/9/27 Susumu Mori <smoriw at gmail.com>

> Do you mean, you used different gradient tables for different runs?
> I wonder if the gradient table and images are not matched for a part of
> data.
> Suppose you have 1 b0 + 12 DWIs x 2 runs and you lost one b0 in the second
> run.
> The only way to analyze this data, I can think of, is, combine the two
> runs as 1 b0 + 12 DWIs + 12 DWIs and crate one 25-element gradient table.
>
> In general, if you have less data, you have lower SNR and FA goes up but I
> doubt if the loss of one b0 could cause such a large FA increase.
>
>
> On Fri, Sep 27, 2013 at 10:38 AM, Dorian P. <alb.net at gmail.com> wrote:
>
>> Dear developers and Dr. Mori,
>>
>> I am comparing FA for some tracts at two time points. At each time point
>> I acquire 2-3 DTI runs to increase SNR. However, the scanner has
>> malfunctioned for one of the runs in one subject. As a result the B0 image
>> is not saved, but all other DWIs are there. So, I set the collapsed B0 to
>> gradient 100, 100, 100 and align all DWIs to the only B0 available. For
>> some results, the resulting FA is increased a lot.
>>
>> I tried the same procedure with another subject, when I set one of the B0
>> to gradient 100, 100, 100, the resulting FA of the tract goes up from 0.51
>> to 0.62.
>>
>> Is there any explanation why this is happening, and whether I should
>> delete completely the collapsed B0 instead of setting it to 100, 100, 100?
>>
>> Thank you.
>> Dorian
>> TJU
>>
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