How to obtain statistical parameters of images by comparing with ground truth image

i stored my images in im1,im2,im3,...., i want to find statistical parameters by comparing with ground truth image. i want to use loop for this. i am getting error when i call image in the loop using im' num2str(j) '. Here i have included my code

grn_i=im2bw(I1,0.5);
im1=im2bw(I2,0.5);
im2=im2bw(I3,0.5);
im3=im2bw(I4,0.5);
im4=im2bw(I5,0.5);
im5=im2bw(I6,0.5);
%m=imread(['folder_path\im' num2str(i) '.jpg']). 
for j=1:5
    [Accuracy, Sensitivity, Specitivity] = EvaluateImageSegmentationScores(grn_i, im' num2str(j) ');
    [Jaccard,Dice,rfp,rfn]=sevaluate(grn_i, im'num2str(j)');
end

i am getting error in the loop

  [Accuracy, Sensitivity, Specitivity] = EvaluateImageSegmentationScores(grn_i, im' num2str(j) ');