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Optical density variations in CT films and their effect on image quality

¹ Medical Physics Unit, 'Konstantopoulio - Agia Olga' Hospital, 3-5 Agias Olgas, Nea Ionia, 142 33, Athens, ² Field Service Engineering Department, General Electric Medical Systems, 156 Cyprou Avenue and 91 Konstantinoupoleos Str, Argyroupolis, 164 51, Athens, ³ Computed Tomography Department, 'Konstantopoulio - Agia Olga' Hospital, 3-5 Agias Olgas, Nea Ionia, 142 33, Athens, ⁴ Nuclear Medicine Department, 'Hygeia' Hospital, 4 Erythrou Stavrou, 151 23, Maroussi, Athens, Greece

It was recently reported that optical density (OD) variations were observed in CT films printed with a laser camera, depending on the printing format and the frame position within the film. The purpose of the present study was to investigate if these variations are common to both laser and dry-film printers and if the different OD settings along with day-to-day and frame-to-frame variations may affect the image quality. Eight laser and five dry-film printers installed at 12 different CT facilities were tested. For each one, the SMPTE test pattern was printed on all frames of a film using the same printing format. The ODs of the 0%, 10%, 40% and 70% patches of the 11-step greyscale of the SMPTE patterns were measured with a densitometer in all frames, while all films were examined on a viewing box to assess subjectively the image quality by visual inspection of the test pattern. A wide range of OD settings and variations were recorded. Frame-to-frame variations in the same film of up to 0.19, 0.15 and 0.21 OD, were observed for contrast index (CI, the OD difference of patches 10% and 70%), speed index (SI, the OD of patch 40%) and maximum OD (OD_max, the OD of patch 0%), respectively. The variations were not always of the same magnitude, nor always followed the same pattern, even for printers of the same model. Considering all films and frames, the CI ranged from 1.26 to 1.74, the SI from 0.68 to 1.43 and the OD_max from 2.5 to 3.11 OD, well beyond the proposed settings and tolerances of 1.55±0.15, 1.15±0.1 and 2.45±0.1 given in the literature for CI, SI and OD_max, respectively. Despite these large differences, the various problems that were identified in image quality from the visual inspection of the films could not be directly attributed to OD settings, as films with similar CI, SI and OD_max presented quite different image quality levels. Therefore, for routine quality control, thorough visual inspection of the SMPTE test pattern provides all the necessary information about the imaging chain status.