Hi,
That's correct - it maps the spherical ROI to slightly
different locations effectively. As I said before, it is
hard to imagine when this would be important though.
If you are desperate for an invariant formulation then
you would need to implement the equations in the
technical report but use M=T2-T1 rather than M=T2*inv(T1)-I.
This is also mentioned in that technical report. However,
as I've said before, I can't really see a case where this would
be that important.
All the best,
Mark
On 1 Feb 2012, at 00:18, Yiou Li wrote:
> 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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