1. A system of medical imaging, comprising: a marking module for processing data, wherein the marking module is stored in a memory, marking feature regions from a plurality of pre-scanned images; a classifying module for processing data, wherein the classifying module is stored in the memory, classifying the feature regions by using a classification algorithm model wherein the classification algorithm model comprises a K-means classification model or a Vector Space Model (VSM), and the classifying module uses feature parameters of the marked feature regions as input values of the K-means classification model or the VSM to acquire output values of the K-means classification model or the VSM, and classifies the feature regions based on the output values of the K-means classification model or the VSM and wherein the feature parameters of the feature regions comprise the following parameters: a relationship of CT values of pixels in the feature regions as a function of the scanning time points of the plurality of pre-scanned images, the number of pixels in the feature regions, 1st-Nth moments of the pixel values in the feature regions, and centroid of the pixels in the feature regions, where N is an integer more than 1; a region-of-interest determining module for processing data, wherein the region-of-interest determining module is stored in the memory, selecting a feature region, among the classified feature regions, of a same type as a specific diagnostic tissue as a region of interest; a curve acquiring module for processing data, wherein the curve acquiring module is stored in the memory, acquiring a curve of Computed Tomography (CT) values of the region of interest as a function of time, based on a relationship between the CT values of the region of interest in the plurality of pre-scanned images and scanning time points of the corresponding pre-scanned images; and a delay calculating module for processing data, wherein the delay calculating module is stored in the memory: detecting a peak value of the curve; calculating a scan delay time based on a time point corresponding to the peak value with regard to an origin of a time axis; and determining, a transformed scan delay time based on at east one of: a composition, and a dosage of a contrast media.