A computed tomography image processing apparatus for processing a CT image of an object, the CT image processing apparatus comprising: a data obtainer configured to detect X-rays having different energy spectra transmitted through the object and obtain raw data in each of energy ranges of the X-rays, through the detected X-rays; a processor configured to generate a CT image by using the raw data, set a region of interest on the CT image based on a user input, generate a plurality of virtual monochromatic images (VMls, 210) respectively corresponding to a plurality of energy levels by reconstructing the CT image, measure a contrast-to-noise ratio of the ROI; a memory; and a display, characterized in that: the memory stores information about a relationship between energy level and the CNR according to a type and a concentration of a contrast agent in a form of a look-up table, and the apparatus is further configured to: obtain information about the type of the contrast agent injected into the object in the ROI, the processor is further configured to: measure the concentration of the contrast agent injected into the object in the ROI, determine the VMI of the energy level at which the CNR is at a maximum according to the concentration of the contrast agent measured in the ROI and the type of the contrast agent, with reference to the LUT stored in the memory, and the apparatus is further configured to to display the determined VMI. a data obtainer configured to detect X-rays having different energy spectra transmitted through the object and obtain raw data in each of energy ranges of the X-rays, through the detected X-rays; a processor configured to generate a CT image by using the raw data, set a region of interest on the CT image based on a user input, generate a plurality of virtual monochromatic images (VMls, 210) respectively corresponding to a plurality of energy levels by reconstructing the CT image, measure a contrast-to-noise ratio of the ROI; a memory; and a display, characterized in that: the memory stores information about a relationship between energy level and the CNR according to a type and a concentration of a contrast agent in a form of a look-up table, and the apparatus is further configured to: obtain information about the type of the contrast agent injected into the object in the ROI, the processor is further configured to: measure the concentration of the contrast agent injected into the object in the ROI, determine the VMI of the energy level at which the CNR is at a maximum according to the concentration of the contrast agent measured in the ROI and the type of the contrast agent, with reference to the LUT stored in the memory, and the apparatus is further configured to to display the determined VMI. the memory stores information about a relationship between energy level and the CNR according to a type and a concentration of a contrast agent in a form of a look-up table, and the apparatus is further configured to: obtain information about the type of the contrast agent injected into the object in the ROI, the processor is further configured to: measure the concentration of the contrast agent injected into the object in the ROI, determine the VMI of the energy level at which the CNR is at a maximum according to the concentration of the contrast agent measured in the ROI and the type of the contrast agent, with reference to the LUT stored in the memory, and the apparatus is further configured to to display the determined VMI.