A method for determining a beam width of a computed tomography scanner includes calculating a dose length product DLP and dividing it by an accumulated radiation dose D(0). In some embodiments, DLP is calculated from measurements obtained using a pencil ionization chamber and a radiopaque mask that is slid over the chamber. In other embodiments, DLP is calculated from measurements obtained using a radiation dosimeter that is moved long a longitudinal axis of the scanner at a known velocity v.
