1. A method of detecting arterial input function (AIF) in dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) scans of a tissue or an organ in a subject, comprising: (a) designing a mask to a major region of a scan imaging; (b) calculating a concentration of intravascular contrast agent Cp (t) by Cp ⁡ ( t ) = k ⁢ ⁢ Δ ⁢ ⁢ R 2 * = - 1 TE ⁢ ln ⁢ S ⁡ ( t ) S ⁡ ( 0 ) , wherein t represents time; ΔR 2 * represents transverse relaxation at time t, S (t) is a signal intensity at time t, S (0) is a pre-injection baseline signal intensity, k is a scaling factor, wherein S (0) is the mean intensity of the time slot that the contrast agent arrives at the tissue; (c) calculating the area under the concentration curve; (d) discarding P aus percent of the curves with the smallest area, and P aub percent of the remaining curves with the biggest area; (e) determining peak values of every pixel in every scan imaging and excluding P pks percent of a pixel which peak value is smaller than a big peak value of all pixels in the scan; (f) calculating a dissimilarity value between an AIF and a processed concentration time curve by 
 dis ij =√{square root over (( x i −x j )( x i −x j )′)}{square root over (( x i −x j )( x i −x j )′)}, and selecting P sim percent of small dissimilarity values, wherein x i represents the processed concentration time curve and x j represents the AIF curve; (g) clustering concentration time curves using a fast-AP clustering algorithm, in which concentration time curves of remaining pixels are simultaneously considered as input data in the cluster analysis; (h) selecting additional scans to be processed by above steps (a) to (g); (i) processing the additional scans by above steps (a) to (g); and (j) calculating a final estimate of AIF, which is a mean curve with the lowest first moment based on the selection of the scans.