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

[Susumu Mori]

how about MD?
Maybe intensities of two scans were different.


On Fri, Sep 27, 2013 at 2:56 PM, Dorian P. <alb.net at gmail.com> wrote:

> 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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>>
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