## 4 Discussion  
For imaging the parasinus region with a previously introduced multipurpose X-ray device with 3D cone beam option [18], various scan parameters and post-processing steps were examined to optimize image quality and radiation exposure. The best compromise between radiation exposure and image quality for the diagnosis of sinusitis was found at 121 kV/ DL 278 and for preoperative imaging at 121 kV/DL 1090. In our study, the effective dose for ruling out sinusitis was 0.077 mSv and for preoperative imaging 0.212 mSv. In modern medicine, examination protocols tailored to the specific patients' anatomy and clinical indication are mandatory. The scan range of the parasinus region includes radiosensitive organs like the eye lens. Due to the typically young age of the patients and repetitive scans, radiation exposure is a relevant topic for this cohort. Today numerous manufacturers offer a wide range of CBCT and DVT units that differ both in their technical characteristics and in terms of radiation exposure [25]. In a dental and maxillofacial study with a large cohort of 500 patients examined with CBCT mean CTDI\({}_{\text{vol}}\) was 9.11 mGy [26]. In other studies with CBCT, the radiation exposure is recorded as the effective dose. In a meta-analysis of 23 dental CBCT units the average effective dose was 0.212 mSv using large FoVs [27]. In a recently published review for radiation dose in non-dental CBCT applications the overall mean effective dose for imaging the paranasal sinuses was 119.0 \(\upmu\)Sv [28] with comparability limited due to the smaller FoVs of the devices used for the studies (i.e., 15 \(\times\) 12 cm, 13 \(\times\) 10 cm). The effective dose is in between our results for the low-dose scanning protocol (77 \(\upmu\)Sv) and the protocol recommended for preoperative planning (212 \(\upmu\)Sv). The slightly higher dose required for preoperative imaging than for  
Figure 6: Difference between 121 kV/DL 278 (**A**,**C**) and 121 kV/DL 1090 (**B**,**D**). Note the better detail visibility of the bony structures in (**B**) (orbital floor: arrow; olfactory fossa: arrowhead) or in (**D**) (carotid canal: arrow). the diagnosis of sinusitis is mainly due to the carotid canal. The carotid canal is surrounded by soft tissue and requires higher dose levels for preoperative assessment. One drawback of scanning patients on CBCT is its inferior soft tissue visualisation compared to CT [29] and magnetic resonance imaging (MRI) [30]. Even modern CBCTs lack sufficient soft tissue display, so using the multi-purpose device for soft tissue evaluation cannot be recommended. In case of unclear soft tissue lesions or if soft tissue lesions are suspected the method of choice is MRI [30]. MRI is also capable of ruling out sinusitis but fine bony structures cannot be visualized. Another disadvantage of MRI is the long examination time and the significantly higher costs. In summary, the multi-purpose X-ray machine equipped with a 3D CBCT option offers the opportunity to examine patients with dedicated protocols based on their clinical indication for the diagnosis of sinusitis and for preoperative imaging. The radiation dose for both protocols is within the range of DVT and other CBCTs. Some limitations of our study need to be addressed. First, four cadaver skulls were examined to evaluate image quality and to determine optimal scan protocols as well as post-processing steps. These dedicated protocols need to be verified in a larger cohort. Second, all four cadaver heads were examined in an upright position, which should correspond to the posture of a seated patient. Tilting the jaw to improve image quality or to lower radiation exposure in CBCT by reducing the impact of metal implant artifacts as proposed in [31] has not been attempted. Third, as cadaver heads were examined no motion artifacts occurred. The examination time for CBCT is approx. 20 s and movement artifacts could significantly influence image quality [32].