[Digital radiography of the thorax with selenium detector. Clinical results and optimization potential of the technique].
INTRODUCTION: We compared the image quality of the chest radiograph obtained with a digital selenium detector and with a conventional system and investigated the possible improvements in the digital technique resulting from kilovoltage (kV) lowering, antiscattering grid addition and image format reduction. MATERIAL AND METHODS: 150 subjects in the first series were submitted to posteroanterior chest radiography with both the selenium and the conventional systems. Image quality was compared by giving a score to the depiction of anatominal and pathologic findings. Thirty-two and 31 subjects from two other series were submitted to two digital chest radiographs each: the former at high (150) and low (90) kV, and the latter at 150 kV with the antiscattering grid and at 90 kV without the grid. Comparisons were made by choosing the better of the two images of each subjects. A score was given to the depiction quality of several difficult-to-detect findings in full-size and small-size format digital images obtained in another series of 27 subjects. RESULTS: As for anatomical detailing, digital selenium images were of much better quality than conventional images: the mean scores given by 3 observers to digital images (5.32; 5.55; 6.68) are higher than those given to the corresponding conventional images (4.49; 5.02; 5.81) and the difference is statistically significant (p < 0.001 in all cases). The advantage of digital over conventional images is also significant with reference to diagnostic confidence in the identification of pathologic findings (p < 0.001; p < 0.005; p < 0.01), but to a lessere extent (mean scores: 3.98; 4.22; 3.60 for the digital system, versus 3.43; 3.69; 3.38 for the conventional system). The digital images acquired at lower kV (90 kV) were much more frequently chosen by the two observes (87.5% and 96.8% of cases) than the images acquired at 150 kV; the entry dose at lower energies (91 muGy using an anthropomorphic phantom) is not significantly higher than the dose given at 150 kV (85 muGy). No significant difference was found in the two observers' choice between the digital images taken at 90 kV without antiscattering grid and those taken at 150 kV with the grid, the former being preferred in 38.7% and 58% of cases. The level of diagnostic confidence in the detailing of difficult-to-detect findings was slightly higher in full-size digital images (mean scores: 5.33 and 6.77) than in small-size ones (4.88 and 5.96). DISCUSSION AND CONCLUSIONS: Digital selenium images always exibit better quality than conventional images: the difference is very marked relative to anatomical detailing and not so striking, though still significant, in showing pathologic findings. Digital selenium image quality can be improved relative to the manufacturer's guidelines (150 kV exposure with no grid), by lowering the kV and adding the antiscattering grid, without increasing patient exposure too much. Digital image format reduction allows cost containment without affecting diagnostic reliability.