The method of performing CT imaging by injecting the contrast medium into the object includes: setting a region of interest (ROI) in a scout image of the object； acquiring X-ray projection images by detecting X-rays that have passed through the ROI； measuring a difference between pixel values of at least one X-ray projection image acquired before an injection of a contrast medium into the ROI and pixel values of at least one X-ray projection image acquired after the injection of the contrast medium into the ROI； and determining whether to initiate a diagnostic CT scan by comparing the measured difference with a preset threshold.
