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Subject:

Re: Question about rmdiff

From:

Yiou Li <[log in to unmask]>

Reply-To:

FSL - FMRIB's Software Library <[log in to unmask]>

Date:

Tue, 31 Jan 2012 16:18:42 -0800

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (69 lines)

Hi Mark,

Thanks for the link. According to the formula (36),

M  = T2*inv(T1) - I = xmat_1_to_2 - I   when I put T1 = identity and
T2 = xmat_1_to_2
M = inv(xmat_1_to_2) - I                     when I put T2 = identity
and T1 = xmat_1_to_2;

By this, M seems not invariant to the order of T1 and T2?

Best,
Leo


On Tue, Jan 31, 2012 at 3:00 PM, Mark Jenkinson <[log in to unmask]> wrote:
> Hi,
>
> No, that's not the implementation that we use.
> See: http://www.fmrib.ox.ac.uk/analysis/techrep/tr99mj1/tr99mj1/node5.html
>
> All the best,
>        Mark
>
>
> On 31 Jan 2012, at 22:56, Yiou Li wrote:
>
>> Hi Mark,
>>
>> Thanks for the prompt feedback! As I understand, the rmsdiff measure
>> is invariant to the order of transform matrices just as |X1 - X2|^2 is
>> symmetric to both variables?
>>
>> Thanks!
>> Leo
>>
>> On Tue, Jan 31, 2012 at 2:44 PM, Mark Jenkinson <[log in to unmask]> wrote:
>>> Dear Leo,
>>>
>>> There is no right answer.
>>> The two different ways just average over a slightly different
>>> spherical ROIs.  However, the difference will be pretty minor
>>> and I can't think of an example where these small differences
>>> would matter.
>>>
>>> All the best,
>>>        Mark
>>>
>>>
>>>
>>> On 31 Jan 2012, at 22:08, Yiou Li wrote:
>>>
>>>> Hi Mark,
>>>>
>>>> I want to use rmsdiff to measure the difference between image X1 and
>>>> X2 in terms of affine linear transformation, to do that, I ran flirt
>>>> to register X1 to X2, taking X2 as reference image and get the
>>>> transformation xmat_1_to_2, now I got two slightly different number by
>>>> running:
>>>>
>>>> 1. rmsdiff xmat_identity xmat_1_to_2 X2
>>>> 2. rmsdiff xmat_1_to_2 xmat_identity X2
>>>>
>>>> Can you please advise which one is the correct order in this case?
>>>>
>>>> Thanks in advance!
>>>> Leo
>>>>
>>

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