Fusion, Co-Registration, Technical Issues & Limitations Flashcards
What is the need and rationale for fusion, co-registration and technical issues?
Research:
- understanding functional pathways in a subjects population
What are the clinical applications for the need and rationale?
- EEG: location of focus epilepticus
- fMRI: vicinity of eloquent areas to lesions
- DTI: vicinity of tracts to lesion
What are the problem for the need and rationale?
Poor anatomical definition in DTI and functional imaging (fMRI, EEG, PET)
What is the solution to the problem for the need and rationale?
Co-registration to images with high anatomical definition and high resolution
What is 3D Rigid-body transformations?
3 translations: in X, Y & Z directions
3 rotations: about X, Y & Z
The order of the operations matters
What is affine transformation?
3 translations: in X, Y & Z directions
3 rotations: about X, Y & Z axes
Zoom
Shear
What are the different methods for the fusion methods?
- Manual
- Interactive
- Semi-automatic
- Automatic
Manual method
Provide tools to align the images manually
Interactive method
Perform certain key operations automatically: user guides the registeration
Semi-automatic method
Perform more of the registration steps automatically; user verifies correctness of refisteration
Automatic method
Do not allow any user interaction; perform all registeration steps automatically
What is semi-automatic methods?
- Coarse image registeration done manually
- More precise image registeration done automatically
- User verified registeration and is able to finely modify it if necessary
What is automatic methods?
Image registration is done by optimisation
Optimisation involves finding some best parameters according to an “objective function” which is either minimised or maximised
The “objective function” is often related to a probability based on some model
What are the 2 different types of objective functions?
- Intra-model
2. Inter-model
What are examples of intra-model?
- Mean squared difference (minimise)
- Normalised cross correlation (maximise)
- Entropy of difference (minimise)
What are examples of inter-modal?
- Mutual information (maximise)
- Normalised mutual information (maximise)
- Entropy correlation coefficient (maximise)
- AIR cost function (minimise)
What is intra-modal?
Within subject; within modality
What is mean-squared difference (minimise)?
Simple relationship between intensities in one image, versus those in the other
- assumes normally distributed differences
What can re-sampling introduce?
Interpolation errors
What can cause aliasing artefacts?
Gaps between slides
What is not accounted for by rigid body model?
Rapid movements
Why may image artefacts not move according to a rigid body model?
- Image distortion (possible correction; unwrap, field maps)
- Image dropout
- Nyquist ghost
What is co-registeration?
Match images from same subject but different modalities
What does spatial normalisation minimise?
Mean squared difference from template image(s)
What is the consequence of no regularisation?
The non-linear spatial normalisation can introduce unneccessary warps
What is DARTEL?
Non-linear and regularised registration method
More accurate than normalised
Used for VBM
What are the limitations or re-aligning images?
Possible residual errors (interpolation, aliasing artefacts, distortion, dropouts)
What are the limitations for co-register?
Possible residual errors, especially if signal drop-out or distortions present
What are the limitations for normalisation?
- Introduction of unnecessary warps
2. Presence of lesions
What doesn’t the activation maps have?
Anatomical information
Not possible to check alignment
Person performing the analysis need to make appropriate checks
What is visualisation?
Check co-registeration of functional, structural MR and DTI
What are the basic concepts behind image co-registeration?
- Objective function created using data from two images
2. Optimisation (minimisation or maximisation) of the objective function
