Image field size and beam collimation in fluoroscopic equipment

There are no internationally established limiting values for image field size and beam collimation in fluoroscopy. In Britain it is accepted that the ratio of the image field size diameter to the nominal field size diameter should have a minimum limiting value of 0.85, and it is proposed that this be adopted. It is also proposed that beam collimation be compared with image field size rather than nominal field size, and that the ratio of the radiation field size diameter to image field size diameter should have a maximum limiting value of 1.1. Tolerances inherent in both these measurements are discussed. The frequency with which image field size and beam collimation measurements outside these limiting values were identified on a large number of fluoroscopic units is presented. Image field size measurements support the prudence of choice of the limiting value although beam collimation measurements do not. The results are discussed and a further source of patient dose reduction by the use of octagonal diaphragms identified. (author)