1. A method of detecting a boundary of a target object from an ultrasound diagnostic image having a plurality of pixels using an image processor in an ultrasound diagnostic system, the method comprising: (a) filtering, via the image processor, the ultrasound diagnostic image to calculate a vertical characteristic value and a horizontal characteristic value of each of the plurality of pixels, detect at least one vertical edge of the target object from the ultrasound diagnostic image based on the vertical characteristic value, detect at least one horizontal edge of the target object from the ultrasound diagnostic image based on the horizontal characteristic value, and form an edge detection boundary candidate model based on the at least one vertical edge and the at least one horizontal edge; (b) performing, via the image processor, a simplification operation upon the edge detection boundary candidate model to form a simplification boundary candidate model; (c) performing, via the image processor, a sessionization operation upon the simplification boundary candidate model to form a sessionization boundary candidate model; and (d) detecting, via the image processor, 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, wherein step (b) includes, (b1) performing a morphological operation to morphologically transform the edge detection boundary candidate model, and step (b1) includes, (b11) performing an erosion operation upon the edge detection boundary candidate model to connect an unconnected portion of the edge and remove a small noise, (b12) performing a reduction operation upon the edge detection boundary candidate model to connect the unconnected portion of the edge by reducing the size of the edge detection boundary candidate model, (b13) performing a dilation operation upon the edge detection boundary candidate model to expand outermost pixels, and (b14) performing the erosion operation upon the edge detection boundary candidate model to which the dilation operation was performed.