The deformation image contains all the necessary information about how
your registration algorithm has aligned the fixed and moving images. The
definition of a deformation vector field can vary, therefore it is
important to note that for the EMPIRE10 challenge the values in the
deformation fields must be displacement distances in mm. For those
unfamiliar with deformation field data, the data will be interpreted as
follows:
Suppose we consider a voxel location (xVoxFixed, yVoxFixed,
zVoxFixed) in the fixed image, and we wish to know which
location in the moving image this corresponds to (according to a
particular registration algorithm). This will be determined using the
following steps:
- First, we get the deformation information relating to this location
from the three deformation images:
defXmm = defX(xVoxFixed, yVoxFixed, zVoxFixed)
and similarly for defYmm and defZmm. Each of defXmm, defYmm and defZmm
is a floating point distance, specified in millimetres.
- We need to consider that voxel spacing may be different in the fixed
and moving images, so from now on we will work in physical space (rather
than in voxel space). We use the fixed image voxel spacing to convert
the voxel location (xVoxFixed, yVoxFixed, zVoxFixed)
into a physical location (xPhysFixed, yPhysFixed,
zPhysFixed). (Note that the image origin is at (0,0,0) in all
our images so it is not required when converting to physical
space.)
- The moving image location which we require is obtained (in physical
space) by:
xPhysMoving = xPhysFixed + defXmm
and similarly for yPhysMoving and zPhysMoving.
- Finally, the moving image location in voxel coordinates
(xVoxMoving, yVoxMoving, zVoxMoving) may be obtained
from the physical location (xPhysMoving, yPhysMoving,
zPhysMoving) using the moving image voxel spacing
information.
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