A well trained radiographer would not include the skull and shoulder joints on a chest xray. Lack of collimation increases the radiation dose enormously. The problem is in trying to find T7 and using that as a centring point. No one knows where it is, and no one judges the success of a chest xray by finding T7 in the centre. Instead, collimate to the size of the cassette, then use the illuminated field as your only positioning guide.