# Radius of Neighborhood Correlation
**URL:** https://discourse.itk.org/t/radius-of-neighborhood-correlation/503
**Category:** Beginner Questions
**Created:** [December 12, 2017, 3:44pm UTC](https://discourse.itk.org/t/radius-of-neighborhood-correlation/503 "2017-12-12T15:44:57Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Ray](https://discourse.itk.org/u/Ray)
#### Post date: [December 12, 2017, 3:44pm UTC](https://discourse.itk.org/t/radius-of-neighborhood-correlation/503/1 "2017-12-12T15:44:57Z")
For the metric “ANTSNeighborhoodCorrelationImageToImageMetricv4”, we can set up the radius of the neighborhood by SetRadius(). I wonder if the radius is defined on physical space or image space? i.e. the unit is mm or voxel?
Thanks!
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### Author:  [@matt.mccormick](https://discourse.itk.org/u/matt.mccormick)
#### Post date: [December 12, 2017, 4:37pm UTC](https://discourse.itk.org/t/radius-of-neighborhood-correlation/503/2 "2017-12-12T16:37:41Z")
Hi Ray,
In ITK, `Radius` is a common term used to refer the size of local neighborhoods, and it is given in voxel units. It can be anisotropic, and the size in a given direction is `2 * radius[i] + 1`. The `+ 1` is for counting the center voxel.
Hope this helps,
Matt
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### Author:  [@Ray](https://discourse.itk.org/u/Ray)
#### Post date: [December 12, 2017, 5:50pm UTC](https://discourse.itk.org/t/radius-of-neighborhood-correlation/503/3 "2017-12-12T17:50:01Z")
Thank you Matt!
What if we do multi-resolution registration? Say the shrinking factor is 2x1, and the radius is set to be 4. The voxel spacing is 1mmx1mmx1mm. At the second level, the radius is still 4 VOXELs, which means the actual radius at that level is 8 MMs?
-Ray
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### Author:  [@matt.mccormick](https://discourse.itk.org/u/matt.mccormick)
#### Post date: [December 12, 2017, 6:01pm UTC](https://discourse.itk.org/t/radius-of-neighborhood-correlation/503/4 "2017-12-12T18:01:43Z")
👍 correct.