variance modification for gaussian fitting problem

i am trying to implement gaussian fitting for 2D images and am using the code from-

http://in.mathworks.com/matlabcentral/fileexchange/37087-fit-2d-gaussian-function-to-data

made by gero nootz. how can i get rid of the orientation estimation so that the estimated covariance is a scaled identity matrix.I only want to fit a circularly-symmetric Gaussian so that variances along x and y (x-width and y-width in plot) are equal.Can anyone figure this out.

Thanks in advance :)