Primary beam exposure outside the fluoroscopic field of view

Fluoroscopic units can irradiate an area outside the displayed field of view (FOV) which provides no additional information to the fluoroscopist and may significantly increase the integral dose to the patient. This unnecessary exposure can be reduced using region of interest (ROI) filtration techniques to attenuate the x rays peripheral to the ROI as well as outside the displayed FOV [Granger et al., Med. Phys. 23, 1059 (1996)]. A survey was conducted to quantitatively determine the deviation between the total exposed area and the displayed FOV on 18 fluoroscopic x-ray units which have different shaped collimation. A film was taken in fluoroscopic mode and the area of the FOV was determined from the image of markers placed at the edge of the displayed FOV. For each unit, a measurement was made for each magnification mode at various source to image distances. The measured difference between the total exposed area and the area of the displayed FOV ranged between 5% and 32% for units meeting federal compliance standards and between 22% and 48% for units which were found to be out of compliance. The results of the survey and the application of ROI imaging techniques as a possible method to reduce dose due to inexact fluoroscopic collimation are presented.