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.