1. A method of displaying a diagnostic image, the method comprising: setting an examination target of a subject; dividing the examination target into a plurality of examination regions; detecting whether a signal region indicated by a signal transmitted from a probe collides with each of the plurality of examination regions; calculating a collision frequency of each of the plurality of examination regions based on a result of the detecting; and displaying a diagnostic image that indicates a sectional view of the subject and includes information indicating the collision frequency of each of the plurality of examination regions included in the sectional view based on the calculation result wherein the collision frequency is calculated by increasing a value of the collision frequency whenever a collision occurs, and wherein the collision indicates that predetermined portions of the signal region and each of the plurality of examination regions contact each other for a predetermined period of time.