Fitting a Masked Image. How to skip zeros?

Hi,

    I've used the imtool to define a region of interest on a series of image files.  I then use these image files to perform a pixel-wise fit.  The masked images contain a large number of zero entries (outside the ROI), and only a few non-zero entries.  Is there a way to identify and skip these zero entries in the fit loop to speed up the process?

Thanks,

Avi