PCA on differences

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Michela Bisighini

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Feb 14, 2021, 10:16:14 AMFeb 14
to scalismo
Hi Scalismo comunity. I have two question about PCA with Scalismo.
There is a way in ScalismoLab to create a model using PCA with only two shapes (one reference and one target)?
Moreover, there is a way in Scalismo to build a model using PCA by considering the differences in shape and not the anatomy of shapes? I'm interesting in evaluate the differences before and after a surgical treatment on one patient.
Thanks very much

Marcel Luethi

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Feb 16, 2021, 7:34:04 AMFeb 16
to Michela Bisighini, scalismo
Hi Michela,

Sorry for the delayed reply. If you only have 2 meshes, you don't need a PCA. For a fixed correspondence, there is exactly one deformation field, which relates the two meshes.
The PCA would in this case just have 1 component, which corresponds exactly to this deformation field.

To visualize the changes between pre and postop, I would advice you to visualize different strength of this deformation, I.e. if u is your deformation field and Gamma_R your reference,
you could plot Gamma_R + 0.5 u, Gamma_R + 1 u, Gamma_R + 2 u, etc. Extending the deformation beyond the target gives you a "caricature" of the deformation field and makes it easier to spot what is changing.

Best regards,
Marcel

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Michela Bisighini

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Feb 18, 2021, 2:42:54 AMFeb 18
to scalismo
Thank you very much Marcel. Now, I am able to create a deformation field between two shapes of one patient. Now, I would like to build a PCA model based on n° deformation fields that I created before. There is the possibility to build a PCA model not of shapes but based on deformation fields?
Best regards,
Michela
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