Choice of phantom material and test protocols to determine radiation exposure rates for fluoroscopy

The image intensifier entrance exposure rate (IIEER)) and entrance skin exposure rate (ESER) are two characteristics that can be used to determine whether a fluoroscope is adjusted to operate at an optimal dose rate level. This article presents solutions to some of the practical problems that are encountered when measuring these parameters. Because the energy response of the ion chamber used to measure exposure rates is different from that of the image intensifier, a fluoroscope with a perfect automatic exposure rate control will have an IIEER that depends on the phantom composition, phantom thickness, and x-ray tube potential. The authors developed a simple mathematic model to investigate the relationship of ion chamber response to image intensifier response and to determine the effects of phantom composition and thickness. Results from this model were compared with measurements made with two C-arm fluoroscopes. Variations of 40% or more in the IIEER and ESER can be ascribed to these phenomena.