1. A method of updating a pixel map, the method comprising: generating an X-ray transmission image by detecting an X-ray penetrating an object; determining whether a difference between a value associated with each one of the pixels forming the X-ray transmission image and a value associated with a nearby pixel adjacent to the each one of the pixels is equal to or greater than a predetermined reference value by comparing the value of the each one of the pixels with the value of the nearby pixel; determining a pixel having a value representing a difference equal to or greater than the reference value as a candidate for a bad pixel among the pixels if the difference is equal to or greater than the reference value; displaying a message which requests a selection of whether to determine the candidate for the bad pixel is the bad pixel; and updating the pixel map by showing the determined bad pixel on the pixel map if the candidate for the bad pixel is determined as the bad pixel.