How can keep all objects who are in the specified area from center of 3D image?

I want to keep all objects who are in the specified area from center of image in 3D. For example in below image, how can keep all objects in red circle and remove all of the object out of the red circle? This circle has specified radius from center of image . I want to do that in 3D. Can any one help me?

Answer 1

[Image Analyst]

Use the FAQ: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

Create a spherical version of the circle mask, then multiply voxel by voxel:

out = array3D .* sphericalMaskImage;