Dear Leo,
This is a fairly standard question and the answer is that
there is no good answer! The metrics you suggest in 1
and 2 are flawed, as you pointed out. Using the same
metric that the registration uses won't tell you much, as
it is explicitly optimised by the registration method.
However, you can get cost function values from FLIRT
by using the schedule file: $FSLDIR/etc/flirtsch/measurecost1.sch
This returns the cost for the given -init matrix (this matrix
should be the output of the registration step you've run
previously). It is difficult to interpret the cost function values
when comparing different images, as there may be differences
in the image intensities that modify the overall cost without
necessarily affecting the accuracy of the registration.
The best solution to assess the quality of the registration is
to inspect the results by eye, or use manually placed landmarks
and quantify the displacement errors of the landmarks after
registration. There are quite a few papers dealing with the
assessment of registration quality (often for comparing methods)
which you can read more about if you are interested.
All the best,
Mark
On 30 Apr 2011, at 00:24, Yiou Li wrote:
> Dear FSLers,
>
> I want to assess the quality of the FLIRT registration of different T1
> images to MNI152_T1 template. Could you please advise what is a good
> numeric metric for the assessment?
>
> I am thinking of the following ones:
>
> 1. Sum of mean square error between the registered and the reference
> image (this might not be a good one because I found the intensity
> scales of the two images are quite different).
>
> 2. Determinant of the transformation matrix (I know this has been used
> to evaluate local deformations, but large deformation could result in
> good registration quality, so this metric is not relevant?)
>
> 3. The final cost function value obtained by FLIRT (corrate, mutual
> information, etc.) However, flirt doesn't output this information, is
> there easy way to get this value?
>
> Any comment on this will be appreciated.
>
> Leo
>
> PS. Congrats to new royal couple ^_^
>
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