Projects:RegistrationLibrary:RegLib C31

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v3.6.1 Slicer Registration Library Case xx: TBI

Modules

• Slicer 3.6.1 recommended modules: BrainsFit, Expert Automated Registration

Objective / Background

We seek to align two exams (acute baseline and follow-up) as well as all series within exams into a common space for direct evaluation of regional change.

Input Data

• inter-exam
  • reference/fixed : T1_e1 baseline exam of acute TBI , 512 x 512 x 160 ; 0.5 x 0.5 x 1 mm voxel size, axial acquisition, RAS orientation.
  • moving:T1_e2 follow-up exam of acute TBI , 512 x 512 x 160 ; 0.5 x 0.5 x 1 mm voxel size, axial acquisition, RAS orientation.
• intra-exam

Registration Challenges

• we have multiple nested transforms: each exam is co-registered within itself, and then the exams are aligned to eachother
• strong pathology (hemorrhage?) present at different amounts in the two exams

Key Strategies

• we first register the scans within each exam to the T1
• second we register the follow-up T1 scan to the baseline T1
• we then nest the first alignment within the second

Procedure

• Load & Center

1. Load T1 MPRAGE datasets via Load Volume...
2. volumes are note well centered, i.e. their physical origin defined in the image header differs; we therefore first center both images:
   1. Go to Volumes module, open Info tab
   2. From Active Volume menu, select each image in turn, then click the Center Image button
   3. note how Image origin changes for T1_e1 from 121,-97,-97 to 128,-128,-80 and for T1_e2 from 129,-106,-66 to 128,-128,-80
   4. now that both images have same center their initial misalignment can be seen by placing T1_e1 in the background and T1_e in the foreground and using the toggle-switch

• Align Exam 2 to Exam 1: T1 1st pass: unmasked

1. Open Registration / BRAINSFit module
   1. To set all parameters from presets, from the ParameterSet menu, select "Xf1_e1_FLAIR-T1_unmasked", else choose settings below:
   2. Fixed Image: e1_T1, moving image: e1_FLAIR
   3. Registration Phases: select "Initialize with CenterOfHeadAlign", Include Rigid, "IncludeScaleVersor3D" and Include Affine
   4. Output Settings: under SlicerLinear Transform, select "Create New Linear Transform, then select Rename" and rename it to Xf1_e1_FLAIR-T1
   5. Registration Parameters: increase the Number of Samples field to 200,000
   6. Leave all other settings at defaults & Click: Apply. Registration should complete within ~ 30 seconds
   7. Go back to the Data module: you should see the e1_FLAIR image moved under the newly created transform
   8. Select "E1_T1" as background and e1_FLAIR as new foreground, toggle to see alignment
   9. you will notice a small but distinct residual translational misalignment, apparent on axial and sagittal slices. This likely arises from dominant edges of the skull. We rerun a 2nd pass with masks

• Align Exam 1: FLAIR 2nd pass: masked

1. we obtain a mask for the moving image (e1_FLAIR) by sending the mask for the fixed image (e1_ICC) through the inverse of the above transform
   1. From File menu, select Add Data & reload a second copy of the above transform Xf1_e1_FLAIR-T1 and the labelmap "e1_ICC.nrrd"
   2. rename Xform to " Xf1i_e1_T1-FLAIR and the mask to "e1_FLAIR_mask"
   3. go to the Transforms module, select the Xf1i_e1_T1-FLAIR transform and then click on the Invert button
   4. go to the Data module and drag the node for "e1_FLAIR_mask" inside the transform node Xf1i_e1_T1-FLAIR
   5. right click on the Xf1i_e1_T1-FLAIR node and select Harden Transform. We now have an approximate mask for the moving FLAIR image.
2. Open Registration / BRAINSFit module
   1. To set all parameters from presets, from the ParameterSet menu, select "Xf1c_e1_FLAIR-T1_masked", else choose settings below:
   2. Fixed Image: e1_T1, moving image: e1_FLAIR
   3. Registration Phases: select Include Rigid, "IncludeScaleVersor3D" and Include Affine. Make sure no initialization phases are selected.
   4. Output Settings: under SlicerLinear Transform, select "Create New Linear Transform, then select Rename" and rename it to Xf1c_e1_FLAIR-T1_masked
   5. Registration Parameters: increase the Number of Samples field to 200,000
   6. Control of Mask Processing tab: check "ROI" box,
      1. Input Fixed Mask, select "e1_ICC",
      2. Input Moving Mask, select "e1_FLAIR_mask"
   7. Leave all other settings at defaults & Click: Apply. Registration should complete within ~ 40 seconds
   8. You should see the earlier residual misalignment mostly gone.

• Align Exam 1: T2

1. The T2 image is not far in pose from the T1 and also does not have the same skull-contrast issue as the FLAIR, we register directly to the T1 w/o masking
2. Open Registration / BRAINSFit module
   1. To set all parameters from presets, from the ParameterSet menu, select "Xf2_e1_T2-T1_unmasked", else choose settings below:
   2. Fixed Image: e1_T1, moving image: e1_T2
   3. Registration Phases: select Include Rigid, "IncludeScaleVersor3D" and Include Affine.
   4. Output Settings: under SlicerLinear Transform, select "Create New Linear Transform, then select Rename" and rename it to Xf2_e1_T2-T1
   5. Registration Parameters: increase the Number of Samples field to 200,000
   6. Leave all other settings at defaults & Click: Apply. Registration should complete within ~ 30 seconds

• Align Exam 2:
  • repeat the above steps for exam 2, i.e. align e2_FLAIR and e2_T2 with e2_T1
  • the above issue with the FLAIR-T1 residual does not arise for the follow-up exam, hence direct registration (unmasked) is possible
  • Registration presets: BRAINSFit see Xf3_ and Xf4_ for FLAIR and T2, respectively

the MRML node tree shows the hierarchy of multiple transfoms.

• Align Exam 2-1:

1. finally we align the T1 references of exam 1 and 2, i.e. we align e2_T1 with e1_T1:
2. Registration presets: BRAINSFit see Xf5_
3. parameters: defaults except increase sample rate to 200,000.
4. here we have genuine masks for both images: Control of Mask Processing Tab:
   1. Control of Mask Processing tab: check "ROI" box,
   2. Input Fixed Mask, select "e1_ICC",
   3. Input Moving Mask, select "e2_ICC"
   4. Leave all other settings at defaults & Click: Apply. Registration should complete within ~ 20 seconds

• Combine Transforms:

1. to align all images in a common space, we combine a hierarchy of transforms, i.e. the e2_FLAIR undergoes both the Xf3_e2_FLAIR-T1 and the Xf4_e2-e1: See image on the right for what the final hierarchy should look like.
2. right-click on the nested transforms and select Harden Transforms from the pulldown menu. Rename the hardened transforms to indicate the direct alignment with e1_T1.

• Resample

1. resample all images with their respective transforms, using the Filtering -> Resample Scalar/Vector/DWI Volume module.
2. within the module, select "ws" as interpolation type

• Nonrigid extension

1. there is some residual distortion in the alignment of e2_T1 to e1_T1. Optionally one can address this via another separate nonrigid BSpline registration. However combining the Affine with the BSpline is not straightforward (as of v.3.6.1), to do this follow this outline:
   1. resample e2_T1 according to Xf5 : e2_T1_Xf5
   2. Perform a BSpline only alignment : Xf6_e2-e1_BSpline that aligns e2_T1_Xf5 with e1_T1. Grid size 3-5.
   3. Save resulting Xform
   4. Open Xform file and other saved affine transforms in Text Editor
   5. Paste the affine portion at the bottom of the BSpline and save under new name.

Registration Results

unregistered exam 1 & 2
Exam 2 aligned to Exam 1 (affine only)
Exam 2 aligned to Exam 1 (affine+BSpline)
BSpline deformation only of Exam 2