How to loop on a data cube of an image

Hi, I'm trying to calculate gradient at each pixel in a data cube of an image an i'm getting this error "index out of bounds because size(X)=[280,307,191]."

Any help would be much appreciated Thank you

[Walter Roberson]

duplicates http://www.mathworks.co.uk/matlabcentral/answers/70424-how-to-loop-on-a-data-cube-of-an-image