[Physics homogenization and the choice of screen-film combination in conventional thoracic radiography]

The homogenization of conventional chest radiographs is obtained with mechanical filtration of the X-ray beam and it allows opacity compensation of the different anatomical structures under examination; consequently, more pieces of diagnostic information on the mediastinum can be obtained. The authors used a mechanical device for equalization (Tau-Gil, Gilardoni SpA) with five different X-ray film/rare-earth screen combinations (3M Trimax) to investigate the following problems: a) choice of the best film-screen combination; b) results of the dosimetric comparison of filtered and non-filtered examinations; c) qualitative evaluation of equalization in terms of diagnostic information. One thousand analogic chest studies, 500 with and 500 without hard-filtering equalization, were performed with high-tension technique and statistically evaluated with a dedicated program. The authors concluded that: 1) the best qualitative results are obtained with GTU film/T6 screen combination, which seems to make the most satisfying compromise between sensitivity and spatial resolution and allows low X-ray exposure to be combined with good diagnostic results; 2) dosimetric tests clearly show that in hard-filtered studies the average incident dose to the patient is 12% lower than that given with no filter. Particularly, skin and lung doses decrease, while mediastinum dose doubles to allow the simultaneous visualization of mediastinum and parenchyma; 3) a substantial difference exists between the frequencies of excellent, good and bad results in the comparative series (with and without equalization) for the evaluation of central chest areas, especially for mediastinum, retrocardiac region and carena. The overall rate of poor results was reduced by equalization, ranging 11.6% to 2.8% (mean: 6.3%) in equalized studies, and topping 64.2% (average: 35.5%) in non-equalized studies. Therefore, in the authors' experience, equalization with this type of mechanical device improves the depiction of the mediastinal region, with no significant imaging loss in lung parenchyma demonstration. Moreover, hard-filtering decreases average incident dose to the chest and skin-lung dose, increasing mediastinum dose to allow its correct demonstration.