Conversion between image pixel and the millimeter

I get a image with matlab, how can I know the conversion between the image pixel and the millimeter. Does the different resolution of the image lead to the different conversion between image pixel and the millimeter?

Thanks very much!

Answer 1

[Walter Roberson]

Please read my Answer in http://www.mathworks.com/matlabcentral/answers/18380-getting-x-and-y-for-3d

I know the question is not exactly the same, but the give my answer a fly-over anyhow.