1. An X-ray CT apparatus including an X-ray source for irradiating X-rays onto an object, a multi-slice detector for detecting the irradiated X-rays through an object, a reconstruction unit for reconstructing images based on the projection data collected through the multi-slice detector, and a display unit for displaying reconstructed projection data as a tomography image, the X-ray CT apparatus comprising: a first scan unit configured to perform a helical scan over a wide area of a whole diagnosis portion of the object; a wide range projection data collecting unit configured to collect projection data of the wide range through the helical scan; an ROI designating unit configured to designate a region of interest (ROI) in a reconstructed wide range tomography image displayed on the display; a narrow range projection data collecting unit configured to collect narrow range projection data of the ROI through a volume scan by controlling a collimator to provide a single X-ray radiation that covers the designated ROI; a wide range projection data generating unit configured to generate regenerated wide range projection data from the reconstructed wide range tomography image; and a compounding unit configured to compound the regenerated wide range projection data with the narrow range projection data for display on the display unit.