How can one compute the global threshold of a ROI polygon in an image using Otsu's method in Matlab?

Suppose I have obtained a ROI using roipoly in an image. Is there a way that one can compute a global threshold using Otsu's method for that area only. If one uses graythresh(I) function, all pixels including those outside the ROI are taken into account in the computation of the threshold. Is there a way that can only make use of the pixels belonging to the ROI.