1. A method of detecting a boundary of a target object by using an ultrasound diagnostic image, the method comprising the steps of: (a) detecting an edge of the ultrasound diagnostic image to form an edge detection boundary candidate model; (b) performing a simplification operation upon the edge detection boundary candidate model to form a simplification boundary candidate model; (c) performing a sessionization operation upon the simplification boundary candidate model to form a sessionization boundary candidate model; and (d) detecting a boundary of the target object of the ultrasound diagnostic image based on the edge detection boundary candidate model, the simplification boundary candidate model and the sessionization boundary candidate model.