1. A method of processing an image, the method comprising: receiving an image captured by irradiating radiation to a chest; extracting an image of a trachea region based on pixel values in a predetermined region of the image, and adjusting pixel values corresponding to the trachea region in the image; generating an image of the trachea region based on the adjusted pixel values; and extracting an image of a lung region from the generated image of the trachea region.