1. A method for locating boundaries of metal inserts in an array of attenuation data of a set of CT X-ray attenuation data comprising a plurality of arrays of attenuation data, wherein each array of attenuation data represents attenuation data acquired for a plurality of substantially contiguous paths through a slice of a subject, comprising: (a) evaluating the derivative of the attenuation data with respect to a position coordinate of the array of attenuation data; (b) setting a derivative threshold; (c) identifying positive and negative boundary peaks as positive and negative peaks in the derivative respectively that have an absolute value greater than the derivative threshold; (d) determining the positions of the positive and negative boundary peaks; and (e) determining the position of boundaries of metal inserts in the array of attenuation data to be positions of positive and negative boundary peaks.