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 a degree of the collision frequency of each of the plurality of examination regions included in the sectional view, based on the calculation result.