# CT to optical image registration

**URL:** <https://discourse.slicer.org/t/ct-to-optical-image-registration/514>\
**Category:** Support\
**Created:** [June 16, 2017, 2:28am UTC](https://discourse.slicer.org/t/ct-to-optical-image-registration/514 "2017-06-16T02:28:20Z")\
**Posts on this page:** 1\
**Showing post:** 8

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [June 21, 2017, 10:02pm UTC](https://discourse.slicer.org/t/ct-to-optical-image-registration/514/8 "2017-06-21T22:02:02Z")

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> [@Niousha\_Aflatouni](#):
>
> the hardened  
> image can not be saved

Saving should be no problem, but maybe if it’s a color image there may be issues. Could you try if saving works if you convert the image to scalar (grayscale) volume (using Vector to scalar volume module)?

> [@Niousha\_Aflatouni](#):
>
> As for the intensity based registration, I’m more interested to know if  
> there is any way that the registration can be quantified (not performed)  
> based on Mutual Information

Mutual information minimizes the entropy of the joint histogram. You can find the final value in the application log (the metric value should decrease in each iteration), but it’s not a metric of registration quality.

Registration is typically assessed using spatial error metrics. If you have ground truth landmark points then compute TRE. You may also compute Hausdorff distance using Segment Comparison module (between ground truth segmentation in the fixed image and segmentation in moving image transformed to fixed image coordinate system).

The only intensity-based metric that I’m aware of that might be somewhat applicable for assessing registration (or, more accurately, the effect of remaining registration error) is the [module for comparison of dose volumes (in SlicerRT extension)](https://www.slicer.org/wiki/Documentation/Nightly/Modules/DoseComparison).

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