Difference between revisions of "Projects:SlicerFAQ"

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*How do I fix an wrong orientation of a loaded image volume?  / My image appears upside down / facing the wrong way / I have incorrect/missing axis orientation
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= Image Loading & Viewing =
[[Projects:SlicerFAQ:ImageOrientationFix]]
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*[[Projects:SlicerFAQ:ImageOrientationFix|How do I fix a wrong image orientation?  / My image appears upside down / facing the wrong way / I have incorrect/missing axis orientation]]
**Slicer presents and interacts with images in '''physical'' space, which differs from the way the image is stored by a separate ''transform'' that defines how large the voxels are and how the image is oriented in space, e.g. which side is left or right.  This information is stored in the image header, and different image file formats have different ways of storing this information. If Slicer supports the image format, it should read the information in the header and display the image correctly. If the image appears upside down or with distorted aspect ratio etc, then the image header information is either missing or incorrect. You can correct the voxel dimensions and the image origin in the Info tab of the ''Volumes'' module, and you can reorient images via the ''Transforms'' module. Reorientation however will work only if the incorrect orientation involves rotation or translation. If axes are flipped (e.g. if left & right are flipped because the x-axis direction is defined incorrectly), then there is no tool in Slicer to alter that. This is by design; Slicer does not want to offer tools to (inadvertedly) manipulate image information that should be static and could lead to misrepresented image data. However fixing incorrect orientations such as flipped axes is not that hard and can be done with the help of Slicer:
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*what do the coordinate labels R,A,S and (negative numbers) mean?
:1. load the image into slicer (Load Volume, Add Data,Load Scene..)
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*[[Projects:SlicerFAQ:VoxelSizeFix|How do I fix incorrect voxel size / aspect ratio of a loaded image volume?]]
:2. save the image back out as ''NRRD (.nhdr)'' format.
 
:3. open the ''.nhdr'' with a text editor of your choice. You should see a line that looks like this:
 
  space: left-posterior-superior
 
  sizes: 448 448 128
 
  space directions: (0.5,0,0) (0,0.5,0) (0,0,0.8)
 
:4. the three brackets ( ) represent the coordinate axes as defined in the ''space'' line above, i.e. the first one is left-right, the second anterior-posterior, and the last inferior-superior. To flip an axis place a minus sign in front of the respective number, which is the voxel dimension. E.g. to flip left-right, change the line to
 
  space directions: ('''-'''0.5,0,0) (0,0.5,0) (0,0,0.8)
 
:5. alternatively if the entire orientation is wrong, i.e. coronal slices appear in the axial view etc., you may easier just change the ''space'' field to the proper orientation. Note that Slicer uses ''RAS'' space by default, i.e. first (x) axis = left-'''r'''ight, second (y) axis = posterior-'''a'''nterior, third (z) axis = inferior-'''s'''uperior
 
:6. save & close the edited ''.nhdr'' file and reload the image in slicer to see if the orientation is now correct.
 
  
 
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= Registration =
*what do the coordinate labels R,A,S and (negative numbers) mean?
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*[[Projects:SlicerFAQ:ImageCenter|two images are very far apart / do not overlap]]
*Loaded images are very far apart
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*[[Projects:SlicerFAQ:DWI_Registration|How do I register a DWI image dataset to a structural reference scan?  (''Cookbook'')]]
*How do I fix incorrect voxel size / aspect ratio of a loaded image volume?
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*[[Projects:SlicerFAQ:RegistrationAdjust|Can I manually adjust or correct a registration?]]
*How do I register a DWI image dataset to a structural reference scan? -> (''Cookbook'')
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*[[Projects:SlicerFAQ:RegistrationMethods|What's the difference between the various registration methods listed in Slicer?]]
*Can I manually adjust a registration?  
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*[[Projects:SlicerFAQ:RegistrationMasking|What's the purpose of masking / VOI in registration?]]
*What's the difference between the various registration methods listed in Slicer?
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*[[Projects:SlicerFAQ:RegistrationError|Registration failed with an error. What should I try next?]]
*What's the purpose of masking / VOI in registration?
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*[[Projects:SlicerFAQ:RegistrationRerun|Registration result is wrong or worse than before?]]
*Registration failed with an error. What should I try next?
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*[[Projects:SlicerFAQ:SamplePoints|How many sample points should I choose for my registration?]]
*Registration result is wrong or worse than before?
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*[[Projects:SlicerFAQ:RegistrationContrast|I want to register two images with different intensity/contrast.]]
*I want to register two images with different intensity/contrast
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*How important is bias field correction / intensity inhomogeneity correction?
*how important is bias field correction / intensity inhomogeneity correction?
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*Have the Slicer registration methods been validated?
*what is the purpose of masking?
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*I want to register many image pairs of the same type. Can I use Slicer registration in batch mode?
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*How can I save the parameter settings I have selected for later use or sharing?
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*Registration is too slow. How can I speed up my registration?
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*Registration results are inconsistent and don't work on some image pairs. Are there ways to make registration more robust?
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*One of my images has a clipped field of view. Can I still use automated registration?
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*I ran a registration but cannot see the result. How do I visualize the result transform?
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*What's the difference between Rigid and Affine registration?
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*What's the difference between Affine and BSpline registration?
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*The nonrigid (BSpline) registration transform does not seem to be nonrigid or does not show up correctly.
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*Can I combine multiple registrations?
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*Can I combine image and surface registration?
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*What's the difference between BRAINSfit and BRAINSDemonWarp?
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*Is the BRAINSfit registration for brain images only?
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*What's the difference between ''Fast Rigid'' and ''Linear Registration''?
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*Which registration methods offer non-rigid transforms?
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*Which registration methods offer masking?

Latest revision as of 14:19, 17 November 2010

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Image Loading & Viewing

Registration