Evaluation des Potentials der Dosisreduktion in der Computertomographie der Nase und Nasennebenhöhlen unter Berücksichtigung der klinisch notwendigen Bildqualität

Chronic rhinosinusitis is a common disease. Having used all conservative procedures, an operation using functional endoscopic sinus surgery is the therapy of choice. Anatomical structures showing normvariants are important due to the fact that they are at risk of getting destroyed during the operation. Therefore, a Computertomography scan is required before each operation. Radiation dose is a key factor in CT, which should not be underestimated. Thus, in this study, we examined the question of how radiation dose in the CT of the nose and the paranasal sinuses can be reduced while still maintaining a clinically good and, satisfactory picture quality. The aim of this study was to find an optimal range of technical parameters that would allow, from a clinical-surgical point of view, a consistently good picture quality, while also saving the maximum radiation dose possible for the patient. Using three human head phantoms, 178 – 183 CT-images per head were made with varying voltage, current (= all together 46 different CTDI-values) and reconstruction algorithms, keeping constant section thickness and pitch. The blinding element of the study was achieved by anonymising and randomising the data records. In total, 14 anatomical structures in each picture were evaluated with grades 1 = very good to see, 2 = good to see, 3 = bad to see and, 4 = not to see. These structures were the following: the bony margin of maxillary sinus, the bony margin of the infraorbital nerve, the os turbinale of the inferior nasal concha, the uncinate process, the lamina papyracea on a level with uncinate process and on a level with anterior ethmoidal artery, the anterior ethmoidal artery, the lateral lamina of the olfactory fossa, the cribriform plate, the bony canal of the optic nerve, the canal of the Vidian nerve, the foramen rotundum, the bony canal of the internal carotid artery and the posterior wall of frontal sinus. In order to achieve a satisfactory image quality, the mean value including the standard deviation was set constantly at ≤ 2,0 in grading. As a result, a strong dosage-dependency of the presentability of anatomical structures could be shown. The higher the dosage was, the better the evaluation. But there were differences. Stable structures, such as the frontal sinus, the maxillary sinus or the carotid artery, were more dosage-independent whereas fine, thin structures, such as the os turbinale, the lamina papyracea or the canal of the Vidian nerve, were strongly dosage-dependent. Concerning the reconstruction algorithms, our results showed that the lower the kernel, the better the picture quality and grade in the low-dose-area were. Kernel H47f/s and H50f/s were the best adjustments, in which good images were generated at a CTDI-value of 2.0 – 2.5 mGy. Independent of the kernel, a CTDI-area of 3.5 – 4.5 mGy could be established as the optimal range, in which a clinically satisfactory, good image quality exists even for fine anatomical structures, while ensuring a low radiation dose for the patient. For clinical practical work these results consist of a dose reduction of at least 50 % compared to the current diagnostic reference value of 9 mGy. These findings are consistent with fundamental studies carried out by other research groups. The dosage range of the kernel proposed in this paper should now be further evaluated in the clinic and verified through follow-up studies. Starting points for further research are the evaluation of using smooth kernel as well as working with a higher layer of thickness in the CT of the nose and the paranasal sinuses.