[Insight-users] MI Registration example translation parameters

[David Wikler]

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