## 1 Introduction  
Sinusitis is a frequent disorder and one of the most common conditions treated by primary care physicians [1]. Each year in the United States, sinusitis affects one in seven adults, and is diagnosed in 31 million patients [2]. The direct costs of sinusitis, including medications, outpatient and emergency department visits, ancillary tests and procedures, are estimated to be $3 billion per year in the United States. Sinusitis is the fifth most common diagnosis for which antibiotics are prescribed [2, 3] and can occur as acute or chronic infection. Multi-slice computed tomography (MST) is frequently used for imaging of the parasinus region [4, 5]. It provides essential information for planning the surgical approach, image-guided navigation, and robotic surgery [6, 7]. According to the current guidelines, it is mandatory before functional endoscopic sinus surgery (FESS) to visualize individual anatomic variants and the extension of the disease [8]. The computed tomography (CT) scan of the paranasal sinuses have superseded the conventional standard radiography as it offers more precise anatomic information to the surgeon on the complex anatomy of the sinus cavities and their drainage pathways, in particularly the ostiomeatal complex [9; 10]. To adhere to the ALARA principle (\"as low as reasonably achievable\") different approaches to lower radiation exposure in parasinus CT have been proposed like low kV scanning and spectral shaping [11; 12]. Alternatively, cone beam CT (CBCT) has been shown to be an efficient alternative to conventional CT scans to identify sinusitis disease and serves as a guide for surgical intervention [13]. With its flat-panel detector technology it is characterized by high spatial resolution to provide excellent detail of the bony anatomy with the drawback of limited soft tissue information [14]. Because of the relatively lower costs in comparison with modern helical CT scanners, CBCTs with small detectors, also named as digital volume tomography (DVT), are increasingly used for imaging of the paranasal sinuses. Originally developed for dentistry and orthodontic diagnostics, DVTs with increasing field of view (FoV) sizes due to bigger flat detectors and further advances in technology are becoming increasingly popular for ENT specialists [15; 16]. They are even able to perform tomography independently using DVT for diagnostic and preoperative imaging [17] assuming they have the appropriate technical qualifications in radiation protection. For this reason, an X-ray machine that has a 3D CBCT option would be interesting for X-ray practices, in order to be able to offer CBCTs in addition to classic X-rays with a small fleet of equipment.  
A new type of X-ray machine developed primarily for digital X-ray and fluoroscopic examinations is additionally equipped with flat-panel detector technology for CBCT [18]. With this multipurpose device, 3D imaging of the paranasal sinuses can be performed. Different pre-filtration options like copper and tin, metal artefact reduction algorithms (as metal implants can also affect image quality in CBCT as in MSCT) and different kV levels are available. As imaging of the sinuses is among the most common scans requested in the field of otorhinolaryngology, this additional option would be a considerable advantage in terms of cost efficiency as fluoroscopic examinations are becoming increasingly rare.  
The aim of this study was to define and evaluate dose-optimized protocols for the diagnosis of sinusitis and preoperative sinus surgery planning using different kV levels, pre-filtration options and metal artefact reduction algorithms.