Solved by KFrank in post #4 
              
              
                Hi gkrisp! 

It would almost certainly be worse, if I understand correctly that you 
are working with 3d images. 
The point is that your images have substantive spatial structure. 
That is, just as a pixel with x = 17 is right next to x = 18, but far 
away from x  = 148, a slice in your 3d “z-stack”…