Maual & Automatic wasy to register data?
Name another way
Manual:
- Landmarks, extrinsic markers, geometrical landmarks
Automatic
- point based, surface based, intensity based
- Or you can do it with Hybrid scanners*
Describe Procrastes method
- Identify landmark points (fiducial markers)
- In source image Si
- and in target image
- Determine what you need to get from si to ti
- Apply to all points si
4 components of image registration
- Pre-process
- comparison - use a cost function to assess if the images match, maximise or minimise
- Transformation/mapping
- Optimisation - adjust the cost function to edge towards the correct solution
Pre-processing
What might you need to do?
The pixels are the a-priori information. You may need to use information in the image such as zoom, or manual re-alignment.
You may need to segment, or remove features e.g. metal fillings, areas of high uptake that are obstructing theimage
You might want to smooth it to reduce noise
How do you compare images with a cost function?
2 methods
Can do this with sum of squares for serial images or cross correlation for images of same modality but different person.
Low entropy = aligned, high = misaligned.
Mutual information = H (I) + H (J) - H (I, J).
- H (I) = entropy in image I
- H (J) = entropy in image J
- H (I, J) = joint entropy in both images
3 types of transformation
- Rigid - multiply with transformation matrices, translation and 3 rotation directions
- Affine - add scaling and shear in x,y,z planes
- Deformable/elastic - Spline thing. Constrained local deformations
Optimisation process
How do you do it, why?
Very time consuming to work out for every single parameter change. So you do an iterative search for maxima or minima, e.g. down-hill simplex
4 reasons why you would want to register images
- Same patient, same scan - correct for serial changes or movement
- same patient, different scan - functional and structural info
- Patient to normal database - highlight abnormalities
- Population intercomparisons
Some disadvantages for registration
- Scans on different says - abdominal organs can change position
- Poaition of diaphragm
- Normalisation
- Variable slice thicknesses
- Metal = hot spot on CT, don’t want to mistake that for a region of high uptale on PET scan
