For practical reasons, we scan only one piece at a time as dozens of technical parameters have to be fine-tuned for each piece. 

Parameters are adjusted for the diameter and length of the piece, its material, complexity and the reasons for the CT scan. 

So the first rule is 'one piece at a time', and the second is 'all or nothing' because the whole piece must be CT scanned from top to toe, or from nose to tail, for its structure and anomalies to be analysed as precisely as possible.