Hi Tugan,
Thank you very much for the information. Now I can rest on my back:)
Wayne
On 05/05/09 10:43 AM, "Tugan" <[log in to unmask]> wrote:
> We are also using Philips 3T Achieva and my understanding is just like what
> you described. I am using no-overplus-high and all gradient tables turn out
> the same. I have confirmed this with Philips' clinical science people, too.
> Just like the imaging gradients, the DTI gradients are also rotated to
> accommodate the oblique acquisition. So, the same gradient table is applied
> for all subjects in the imaging plane. That makes sense because you
> generally choose a set of anatomical landmarks to align your DTI acquisition
> so that you standardize acquisition orientation and minimize inter-subject
> variance. Applying the same gradient tables in the imaging plane will also
> help minimize the inter-subject variance.
>
> The matlab software from KKI also produces the same tables for all subjects.
>
> But I recently started using yes-overplus-high for a different study and the
> tables turned out to be different for each subject. I wonder why they did
> not use the same approach.
>
> The gradient tables in the par file are in magnet coordinates, so they have
> to be rotated based on the angulation and offsets described in the par file.
>
> Tugan
>
>
>
> On Tue, 5 May 2009 09:21:44 -0700, Wayne Su <[log in to unmask]> wrote:
>
>> I am also confusing on this issue. According to Jonathan Farrell's website (
>> http://godzilla.kennedykrieger.org/~jfarrell/software_web.htm), the DTI
>> gradient tables are same for all subjects regardless of the slice
>> angulations if using "No Overplus" option. He also said " The V4.1 par files
>> list the diffusion weighting directions. HOWEVER, these directions are not
>> in pixel space. Specifically, in the case of gradient overplus = YES
>> tables, the directions need to be rotated and corrected for slice
>> angulation." My scans are using "No Overplus" option, If I am going to use
>> the gradient table listed in the PAR file, should I correct it for the FOV
>> or not?
>>
>> So for the Philips 3T scanner, if using "No overplus" option, the gradient
>> tables are same for all subjects because they have been corrected already on
>> the scanner for the FOV. For my case, the bvec files used in FDT are same.
>> Am I right?
>>
>> I haven't reached Jonathan Farrell. I would appreciate if anyone can confirm
>> on this issue.
>>
>> Regards,
>>
>> Wayne
>>
>>
>> On 05/05/09 7:03 AM, "Matt Glasser" <[log in to unmask]> wrote:
>>
>>> Well, regardless of what option you select in the scanner, the gradients
>>> outputted by DICOM to NIFTI conversion programs like MRIConvert or MRICron's
>>> converter may vary from subject to subject if you have rotated the FOV. The
>>> gradients applied by the scanner are the same; they are just corrected for
>>> these rotations.
>>>
>>> Peace,
>>>
>>> Matt.
>>>
>>> -----Original Message-----
>>> From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
>>> Of Yuzheng HU
>>> Sent: Monday, May 04, 2009 11:36 PM
>>> To: [log in to unmask]
>>> Subject: Re: [FSL] gradient table for Philips 3T scanner
>>>
>>> Hi,
>>> Thanks for answer.
>>>
>>> I was told that '... if all subjects use same protocal, the gradients sould
>>> be
>>> same. however,for Philips, if you select "Overplus" option during scanning
>>> the
>>> gradients will be changed dynamically accordin to the tilt-angles...'
>>> The gradients I have might be in such cases.
>>>
>>> Best!
>>>
>>>
>>> Yuzheng
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