[Insight-users] Re: mutual information again

[Daniel Rueckert]

Hi,

it is common for mutual information to sometimes not work very well if
there is not much overlap between the images. To solve this problem a
variant of mutual information has been proposed by Colin Studholme,
called normalized mutual information:

"An overlap invariant entropy measure of 3D medical image alignment",
Studholme C, Hill DJG, Hawkes DJ, Pattern Recognition, 32, 71-86 (1999)

Normalized mutual information is calculated as (H(x) + H(y)) / H(x, y)
rather than the standard mutual information (H(x) + H(y) - H(x,y))...

Maybe this helps,

Daniel

-- 
Daniel Rueckert, Ph.D.
Department of Computing
Imperial College London
180 Queens' Gate                         Tel: +44 20 7594 8333
London SW7 2AZ                           Fax: +44 20 7581 8024
mailto:D.Rueckert@imperial.ac.uk         http://www.doc.ic.ac.uk/~dr

> Dear itk-users,
> 
> First of all, thanks for your useful answers to my questions so far.
> Not all problems are solved, yet, so here a question on the use of 
> mutual information using itk, again!
> 
> To get "good" results, I am still working on a solution for a particular
> problem. Whenever the two image volumes that I want to register with
> mutual information, do not have "enough" overlap, an exception is thrown
> when I start the registration (Exception message:
> 
>>itk::ERROR: MutualInformationImageToImageMetric(003C6D20): All the
>>sampled point mapped to outside of the moving image ).
> 
> Somehow I understand this problem, but what to do when I have to images
> that are already aligned up to some level, but where the volumes are
> not overlapping sufficiently (eg when I have two cubes with the 
> center-of-mass points on top of each other but where one is rotated 
> wrt to other, along some axis, by eg TENS of degrees; A FEW degrees does
> not give a problem!). A more practical example is shown in the attached
> jpg-file where you see a MRI (grey) and CT (colour) slice of medical image
> volumes that I want to register. The MRI is rotated (with about 14
> degrees) wrt the CT, so that I do not have a good overlap between the two
> images. Moreover, the CT and MRI are already aligned reasonably. Note: The
> reason for this rotation is that the MRI has a rotation in the
> index2physicaltransform and the CT has not!
> 
> Personally, I see a solution by making a mask where I set all grey values
> of pixels that do not overlap to zero and then, when using mutual
> information, NOT using those pixels with zero grey value in the itk mutual
> information program. But how to do this? Is it somehow possible to exclude
> pixels in the registration calculation using
> itkMutualInformationImageToImageMetric. Hopefully someone has an answer to
> this question.
> 
> Note: To my opinion it would be very useful to be able to exclude some
> pixels in the mutual information registration: then, not only areas where
> no overlap is present can be excluded, but also parts where there is a lot
> of noise, or parts that you do not want to use due to deformations (of
> organs or tissue) that are present.
> 
> Thanks in advance,
> Regards,
> Jorn.