1. A method of correcting a target object in a computed tomographic (CT) image, the method comprising: obtaining a CT image of a patient; determining a size of the target object in the CT image; and correcting CT numbers of a portion of the CT image that corresponds to the target object, based on the determined size of the target object, wherein the correcting step includes correcting each CT number in the CT image that corresponds to the target object using the formula: 
 N= 1− c *exp(− a*D^b ), wherein a, b, and c are variables determined from non-linear regression, D is the determined size of the target object, and N is a reduction factor in CT number of the target object.