Image Registration

Question 1

Maual & Automatic wasy to register data?

Name another way

Answer 1

Manual:

• Landmarks, extrinsic markers, geometrical landmarks

Automatic

• point based, surface based, intensity based
• Or you can do it with Hybrid scanners*

Question 2

Describe Procrastes method

Answer 2

• 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

Question 3

4 components of image registration

Answer 3

1. Pre-process
2. comparison - use a cost function to assess if the images match, maximise or minimise
3. Transformation/mapping
4. Optimisation - adjust the cost function to edge towards the correct solution

Question 4

Pre-processing

What might you need to do?

Answer 4

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

Question 5

How do you compare images with a cost function?

2 methods

Answer 5

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

Question 6

3 types of transformation

Answer 6

• 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

Question 7

Optimisation process

How do you do it, why?

Answer 7

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

Question 8

4 reasons why you would want to register images

Answer 8

1. Same patient, same scan - correct for serial changes or movement
2. same patient, different scan - functional and structural info
3. Patient to normal database - highlight abnormalities
4. Population intercomparisons

Question 9

Some disadvantages for registration

Answer 9

• 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