Keeping image display constant while updating spy ontop

Dear MatlabCentral

I am currently running some picture analysis, where I am iterating for many times to compute a matrix that updates every iteration. I would like to display the sparsity pattern (spy) ontop of the image (imagesc) and update this figure every run.

What i am currently doing:

for i=1:1000000
 MyMatrix=MyMatrix + Update(i) 

   imagesc('picture')
   hold on
   spy(MyMatrix,'ro')
   drawnow
  end

This is obviously very slow, as "drawnow" takes a lot of time to display the large (800x1280 pixel) image. What i would like to have is to draw the image once outside the loop and than only update the "spy" every iteration.

I searched but couldnt find many helpful answers.

Any hint would be very much appreciated regards BjoernBeyer