Analyse des Einflusses der Dosisreduktion auf die Bildqualität in der Darstellung der Nase und Nasennebenhöhlen in der täglichen Routine

During the last decade Digital Volume Tomography has become increasingly important. Along with CT it is the most relevant imaging technique concerning preoperative and follow-up diagnostics evaluation. It is used to provide information about the anatomy of the nose, the paranasal sinuses, the anterior and lateral skull base, the temporal bone and the ossicles and thereby to minimize intraoperative risks (Stuck et al. 2012, Bremke et al. 2009a, Kontorinis et al. 2011, Kontorinis et al. 2012). DVT, which has been constantly developed since 1998, needs in comparison to the technique of CT just one rotation around the region of interest for imaging (Schwenzer and Ehrenfeld 2010) and it enables a good spatial representation with a layer thickness of up to 0,125 mm (CV Dalchow et al. 2006). Out of specific cylindrical volumes it is possible to generate section images of the coronar-, sagittal- und axial-sections and also it is possible to visualize smallest anatomical structures (Pasler 2008). Regarding to the guiding principles of the radiation protection and to the official German X-Ray Regulation every single examination has to be performed with the smallest amount of radiation dosage and simultaneously with the best image quality (ALARA principle) (Strahlenschutz 2008). In order to comply with these principles, previous studies with DVT were implemented on a phantom scull as well as on three deep frozen human complete heads. An optimal range of the dosage for evaluation of the paranasal sinuses of 2.0 to 3.0 mGy was found and for the petrous bone from 3.0 to 4.0 mGy (Bitterwolf et al. 2013). Further examination showed that a dose reduction of 50 % is possible by changing the rotation angle from 360° to 180° maintaining the quality of imaging. Based on these findings standard settings were implemented at „4 mA; 84 kV; 180°; ! CTDI 2.4 mGy“ for daily clinical practice (Güldner et al. 2012a). Since ethical requirements and rules of radiation protection do not allow a series of comparative examinations on living human beings, the ideal relation between minimal dosage and best imaging quality has not yet been confirmed. Caused by that issue this study was carried out. There for the quality of 165 sectional images - using the previous standard DVT settings (average values: 5m A; 88 kV; 360°; ! CTDI 6.6 mGy) were taken, evaluated and compared with the quality of 151 sectional images - using the new defined standard DVT settings (average values: 4 mA; 85 kV; 180°; ! CTDI 2.9 mGy). The results should provide a scientific basis for dosage optimized DVT protocols applicable at the everyday practice. The image quality of both groups was analysed by the evaluation of 17 important surgical anatomic structures (16 of them were two paired structures). According to the recognizable visibility, a special score was assigned, which was based on a German school performance grading system: 1 - Excellent, 2 - well, 3 - poor, 4 - not evaluable. The summation of the grades can lead to a possible score of 33 for best image quality (grade 1) or a score of 132 for the worst quality (grade 4). To find out a possible influence of pathologies of the nose and paranasal sinuses to the image quality, the images were analysed using the Lund-Mackay score. This classification followed the extents of radiological shade level. The results of this examination showed a significant difference (p 0.001) of the image quality of anatomical structures between the two groups. Contrary to the expected results, the investigation showed even a better image quality by using the new defined settings of the DVT. This could be caused by the influence of more artefacts when using higher dosages. All anatomical structures in group 1 as well in group 2 were always excellent or well visible. In addition a positive correlation (r = 0.382) between pathologies of the paranasal sinuses and image quality could be shown. The evaluation of the results establishes a sufficient scientific proven basis for using dosage-optimized protocols of DVT in daily practice. That is why the newly defined settings of DVT are still recommendable and should be used in daily practice in accordance to the ALARA principle. In addition to these results there are no reference values for the applied radiation dosage by using DVT (normally given by the „Bundesamt für Strahlenschutz” once a year). Further research is required, to generate these reference settings and to optimize them in order to get the radiation exposure as low as possible while the picture quality will stay sufficient.