[Insight-users] MI Registration example translation parameters

[Luis Ibanez]

Hi David,

The origin of the physical coordinate system
for the volumes should be taken into account
during the registration process.

Your analysis looks right. It makes sense to
first discount the relative distance between
the image centers.

In general, simply creating a diagram of the
images in a common coordinate system in physical
space should clarify the discrepancy.

In any case, if the resampled images are aligned
correctly, you can be confident that the only
difficulty is the intrepretation of the transform
in a common coordinate system.

--

If the resampled images are not aligned correctly,
then we could suspect that the registration method
didn't converge as expected.

---

An easy and quick test is to tweak the origin
values of your images and verify that the
changes are reflected in the final registration.
For example, if you add (13,17,19) to x,y,z of one
of the images origin, the registration should
report a variation of (13,17,19) in the translation
offset.



   Luis


---------------------
David Wikler wrote:
> Dear Luis,
> 
> I think your comments helped me to understand
> my numbers a bit better. I focused on my numeral (3)
> 
> 
>>3. The registration result I get with datasets oriented 
>>Head to Feet along the z axis is 
>>
>>Overall transform matrix: 
>>0.999868 -0.0144369 0.00750628 
>>0.0144862 0.999874 -0.00655476 
>>-0.0074107 0.00666263 0.99995 
>>
>>Overall transform offset: 
>>36.6828 31.1387 86.6143 
> 
> 
> What I actually did was flip the images along the Z axis
> so the head goes to feet and vice-versa. I actually also 
> did flip Anterio-posterior but this is not relevant for 
> the following.
> 
> I said to you that I estimated the Z translation to 
> about 40.8 mm. Actually I can also estimate X and Y 
> translation being of the order of less than 10mm.
> 
> After reading your comments about physical space vs
> voxels space, I imagined the dicrepancy could come
> from the centers of volumes translations when going
> from voxel space to physical space.
> We could then write
> 
> Tz(slices) = slicethickness *( Tz(physical) - D)
> and
> Tz(mm) = Tz(physical) - D
> 
> where D = (PET_volume_Zcenter(physical)-MR_volume_Zcenter(physical)) /2
> which is the translation in physical space between volume centers
> (for Z coordinate)
> 
> In my case:
> 
> For Z: 86.6143 - (63*2.4- 50*1.3)/2 = 43.5 mm
> For Y: 31.1387 - (128*2.57- 256*1.05)/2 = 1.06 mm
> For X: 36.6828 - (128*2.57- 256*1.05)/2 = 6.6 mm
> 
> which is now compatible with my estimation.
> I still have got to check with other cases but
> I think it could be the solution, what is your
> impression ?
> 
> Thank a lot for your quick help.
> 
> David
> 
> David Wikler, Ir
> ULB - Erasme Hospital - PET Scan
> 808 route de Lennik - B1070 Brussels - Belgium
> Phone: 32 2 5556603 - Fax: 32 25556631
> Email: dwikler@ulb.ac.be
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