Extended focal-film distance technique: an analysis of the factors in dose reduction for the AP knee radiograph

Abstract

Purpose : While an increase in focal film distance (FFD) is an effective method of reducing the entrance skin dose received by a patient, the literature is not agreed on the principal factors involved. Past work has suggested that the reasons for dose reductions may include geometric factors, air attenuation, reciprocity law failure, extra focal radiation, changes in the scatter to primary radiation ratio (s/p) and back-scatter factor variations. This diversity of opinion can lead to confusion when the technique is applied to other areas of skeletal radiography, and hence is investigated in this paper.Method : The effect of increasing FFD from 100cm to 176cm was investigated for a non-Bucky knee radiograph. A 16.2cm thick anthropomorphic knee phantom was radiographed at 60kVp with the mAs varied to give a constant density on the radiograph. Steel ball bearings were taped to the phantom surface to calculate s/p on the resulting radiographs. Dose measurements were made both with and without the phantom present allowing the backscatter factor to be calculated for both varying FFD and collimator settings. Air attenuation measurements were calculated using a computer programme.Results : Results showed that the skin dose reduction was 13.4% at 176cm FFD with 0.8% due to extra focal radiation variations. This compares to 14.8% (theoretically calculated due to geometric considerations alone) with 3.5% calculated to be due to air attenuation. No variations in s/p and backscatter factor were measured.Conclusions : Geometric factors are the main factors in dose reduction with increasing FFD. Caution is urged when extending this technique to include a grid, as grid-focus decentring would be expected to lessen dose reduction.