1. A system of medical imaging, comprising: a marking module for marking one or more feature regions from a plurality of pre-scanned images; a classifying module for classifying the feature regions by using a classification algorithm model; a region-of-interest determining module for selecting a feature region of the same type as a specific diagnostic tissue as a region of interest; a curve acquiring module for acquiring a curve of CT values of the region of interest in function of time, based on a relationship between the CT values of the region of interest in the pre-scanned images and scanning time points of the corresponding images; and a delay calculating module for detecting a peak value of the curve and calculating a scan delay time based on a time point corresponding to the peak value.