image processing - How does `radon` in Matlab works? -

i need find thickness of segmented object in binary image. trying use 'radon' transform find intensity values value "1" (white color) along specified orientation. in way can find thickness @ specified locations want. before using function radon want understand how works. not able understand how radon works. example took 1 simple image , coded follows.

i = zeros(100,100); i(1:25,1:25) = 1; figure;imshow(i) [r,xp] = radon(i, 90); figure;plot(r) 

i plotted r , checked theta = 90,0. plot shows impulse wave both , there shift theta = 0. dimensions of r size(r) = 145. how r takes 145, when size of image 100? how theta = 0, r gives intensity values? can give me clear understanding on radon?