1. A method for extracting an arterial input function, comprising steps: Step (a): contacting a target tissue with a contrast agent to obtain a plurality of images; Step (b): using said plurality of images to work out a time intensity curve of said contrast agent in each voxel; Step (c): using said time intensity curve to calculate a purity of said time intensity curve of each voxel; and Step (d): selecting said time intensity curve of a voxel having the highest purity in step (c) as an arterial input function, wherein calculating said purity (ρ) of said time intensity curve of each voxel by ρ =  x ⁡ ( i )  2  x ⁡ ( i )  1 , wherein ∥•∥ 1 is the 1-norm operator, ∥•∥ 2 is the 2-norm operator, and x(i) is the voxel.