[Elastix] RAS or LPS coordiante system?

Bilal Tahir b.tahir at sheffield.ac.uk
Thu Apr 18 02:52:12 CEST 2013


Dear All,

I recently moved from VV to Slicer for defining expert landmarks on fixed
and moving images prior to registration. I then use transformix to deform
the fixed points so I can compare them with original moving points via the
target registration error.

One thing I noticed is that Slicer defines the points (via fudicial markers
in the annotations module) in the RAS coordinate system as default. Putting
these values into transformix gives me completely wrong results. Is Elastix
based on the LPS coordinate system as default or another one? Below is part
of  my header data for the fixed image:

ObjectType = Image
NDims = 3
BinaryData = True
BinaryDataByteOrderMSB = False
CompressedData = False
TransformMatrix = 1 0 0 0 1 0 0 0 1
Offset = -215.592 -417.592 -323.5
CenterOfRotation = 0 0 0
AnatomicalOrientation = RAI
ElementSpacing = 0.816406 0.816406 1.78125
DimSize = 512 512 160
ElementType = MET_SHORT
ElementDataFile = fixed.raw

Here is the header data for the moving image:

ObjectType = Image
NDims = 3
BinaryData = True
BinaryDataByteOrderMSB = False
CompressedData = False
TransformMatrix = 1 0 0 0 1.52656e-015 -1 0 1 5.55112e-017
Offset = -212.209 -155 200.502
CenterOfRotation = 0 0 0
AnatomicalOrientation = RSA
ElementSpacing = 1.6797 1.6797 10
DimSize = 256 256 24
ElementType = MET_SHORT
ElementDataFile = moving.raw

I look forward to hearing your replies.

Cheers,

Bilal Tahir,
James Morrison Researcher in Radiotherapy Imaging
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