The simple equation goes something like
S/S0=exp(-bD)
-> D=log(S0/S)/b
so if you know what your mean overestimation of S0 is, you should be
able to figure out what your overestimation of D will be.
Cheers
T
On 4 Feb 2008, at 10:27, Wen, Wei wrote:
> Thanks Tim,
>
> The b0 in the 4D DTI is located at the 2nd last in the scan,
> therefore what
> you say makes sense. What is the possible magnitude of the MD
> overestimation?
>
> Wei Wen
>
> -----Original Message-----
> From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On
> Behalf
> Of Tim Behrens
> Sent: Monday, 4 February 2008 7:38 PM
> To: [log in to unmask]
> Subject: Re: [FSL] Multiple b0s: different intensities
>
> The reason for this will be that the b0 scans in your DTI sequence
> will be prepared by previous relaxations (the previous in the EPI
> timeseries, or dummy scans if the b0 is the first) - your standalone
> b0 scans have no preparation - if you want to make them comparable
> you need to prepare them with dummies.
>
> They should have negligible effect on your FA maps, but they will
> lead you to overestimate MD.
>
>
> Cheers
>
> T
>
>
>
> On 4 Feb 2008, at 06:54, Wen, Wei wrote:
>
>> -
>>
>> Hi FSLers,
>>
>> We acquired separate b0s to go with our DTI (because we use Philips
>> scanner and it doesn't allow us to acquire multiple b0s in the 4D
>> DTI scan). However, we found our separately acquired b0 images have
>> higher intensities (about 1/3 higher )than the b0 in the 4D DTI
>> image, even though we used the same TR/TE etc. It might be a
>> scaling thing I think.
>>
>> I was then wondering what/how this different intensity of the
>> separate b0 would impact on the computations. What we intend to do
>> is: a) merge separately acquired b0s into the 4D image and then b)
>> eddy correction etc c) usual dtifit ...
>>
>> Hopefully you can shed some insight to this question/problem.
>>
>> Many thanks and regards,
>>
>> Wei Wen
>>
>
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