I've recently started a mini project on typical doses that patients receive when having particular CT examinations and there is just something that I don't really understand that I was hoping someone here might be able to help out with. Now when a doctor takes some CT images the CT machine will give him dose information. That is, it will provide him with a CTDI (computed tomography dose index) value and a DLP (dose length product) value. From all the material I've read so far on this the CTDI value is something that is determined using phantoms (head or body) with pencil ionisation chambers. So how does the CT machine estimate CTDI and DLP for patients? It's just confusing because I thought to get a CTDI value you had to perform certain measurements with a head or body phantom and I'm pretty sure the doctors wouldn't do that before processing the patient. Any help would be appreciated.