# CT Resampling and Registration order

**URL:** <https://discourse.slicer.org/t/ct-resampling-and-registration-order/4840>\
**Category:** Support\
**Tags:** registration\
**Created:** [November 21, 2018, 7:09pm UTC](https://discourse.slicer.org/t/ct-resampling-and-registration-order/4840 "2018-11-21T19:09:31Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![ZA\_S](https://avatars.discourse-cdn.com/v4/letter/z/ea5d25/32.png) [@ZA\_S](https://discourse.slicer.org/u/ZA_S)\
**Post date:** [November 21, 2018, 7:09pm UTC](https://discourse.slicer.org/t/ct-resampling-and-registration-order/4840/1 "2018-11-21T19:09:31Z")

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Hello guys,

I have non-contrast 3D CT images. I would like to resample them to 1mm and perform registration. In what order it should be; resampling then registration, or firstly registration then resampling?

Thanks

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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:** [November 21, 2018, 7:16pm UTC](https://discourse.slicer.org/t/ct-resampling-and-registration-order/4840/2 "2018-11-21T19:16:15Z")

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I don’t think it will affect the registration if you resample the images before. During registration, linear interpolation is used for point sampling. If you use a different interpolator (bspline, sinc, …) for the 1mm resampling then the sampled values will be slightly different, but it is unlikely to affect the registration result.

Of course, once registration is completed, apply the computed transform and resample to 1mm in one step (to minimize sampling artifacts). You can use Crop volume module for this.

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**Author:** ![ZA\_S](https://avatars.discourse-cdn.com/v4/letter/z/ea5d25/32.png) [@ZA\_S](https://discourse.slicer.org/u/ZA_S)\
**Post date:** [November 23, 2018, 11:19am UTC](https://discourse.slicer.org/t/ct-resampling-and-registration-order/4840/3 "2018-11-23T11:19:29Z")

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Thank you @lassoan. I am very appreciate for your help.