Appending the sheets in excel file

hi all, i am having some 24 image files. i want to read each file and perform some operations on the image file to get an output of 13x1 matrix, and want to store it in sheet1 in an excel file. after that i want to perform the same operation on 2nd image and want to store output in sheet2 of the same excel file. i am providing the code below

for i=1:1:24 

     fname='figure'; 

      ext='.png'; 

      fnamer=sprintf('%s %d%s',fname,i,ext); 

      I=imread(fnamer); 

      a=rgb2gray(I);

end

% i want to perform some mathematical operations on matrix 'a' , which gives me an output of 13x1 matrix and i want to save this in sheet1 in a excel file. when i value increments to 2 again it will give me a 13x1 matrix as output and i want to save this in sheet2 of the same excel file and as the loop progress i want it to save in the appending sheets. some one please help me, thanks in advance.