How to vectorize a problem

Hi everyone, I'd like some tips about vectorizing a problem.

I make a calculation for image registration and the thing is that I use two for loops in order to browse the whole images and it takes much time. So I would like to know how I could vectorize it.

Here is a part of my code:

 [a,b]=size(I);
  for i=1:2*a-1

    %translation vectors
    tx=a:-1:-a+1;
    tz=b:-1:-b+1;

        %set I for each loop
        I_trans=I;
        %x translation
        I_trans=imtranslate(I_trans, [0 tx(i)-1]);

        for j=1:2*b-1

        %set I_trans_x for each loop
        I_trans_x=I_trans;

        %z translation
        I_trans_z=imtranslate(I_trans_x, [tz(j)-1 0]);

        %removing the zeros from rows and columns
        I_trans_z(~any(I_trans_z,2),:)=[];
        I_trans_z(:,~any(I_trans_z,1))=[];

        [crop_x, crop_z]=size(I_trans_z);

        if(crop_x>=i)

        c=b-crop_z+1;
        d=a-crop_x+1;
        rect_J=[c d  crop_z crop_x];
        J_crop=imcrop(J, rect_J);

        else
            rect_J=[0 0 crop_z crop_x];
            J_crop=imcrop(J, rect_J);
        end

         %GI computation
         GI=GI_1(I_trans_z,J_crop);
         GI_tab(i,j)=GI;

 %          figure
%           imshowpair(I_trans_z,J_crop,'montage');
%          title(int2str(GI));

     end      
 end

I hope someone will be able to help me.