1. A method of operating a diagnostic ultrasound system comprising: measuring a cross-section region of a subject by a probe; determining displacement of tissue in the cross-section; determining an elastic information of the tissue in the cross-section region on the basis of the displacement; generating an elastic image of the cross-section region on the basis of the elastic information and displaying the elastic image on a display device; determining a change of compression state over time by the result of compression for the subject; and displaying the change of the compression state as a compression state information together with the elastic image on the display device.