**5. Conclusions**

Preliminary trials were conducted in 20 conditionally healthy volunteers and 15 patients with suspected maxillary sinus inflammation. The influence of anatomical and gender features of the study area on the diagnostic results (differences in skin structure, skull bone thickness, and sinus size) was revealed. The sensitivity of the prototype device to detect pathological changes was confirmed by the results of MRI studies.

The simulation results show the regularity of changes in the light scattering and parameters of the probing and measuring parts of the experimental setup. The mathematical model developed via Monte Carlo simulation made it possible to take into account the anatomical and gender features of the study area, as well as the absorption and scattering of optical radiation.

The prototype of the device was upgraded to obtain similar scattering patterns of light for different patients and to ensure their comparison. To adjust the output power of the probing applicator, a device for controlling the LED applicator brightness was designed, and additional software was developed.

The obtained results can be used to create modern diagnostic devices for the diagnosis of maxillary sinus pathology based on visualization and analysis of the low-intensity radiation scattering pattern. The application of the developed digital diaphanoscopy technology will make it possible to conduct timely, reliable, and painless diagnostics of maxillary sinus pathology, assess the dynamics of changes in the pathological processes within the framework of the therapy, and analyze its effectiveness. It is important to note that, due to the portability and simplicity of its instrument implementation, the technology can be used as a screening method for assessing the condition of the maxillary sinuses both in hospital and medical institutions and remotely in the absence of otolaryngologists and diagnosticians.

**Author Contributions:** Original draft preparation and, measurements, E.O.B. and I.N.N.; data acquisition setup, methodology, and numerical simulation, V.V.D.; numerical simulation, R.Y.G.; experiments at the Diagnostic Medical Center "MediSсan" (Orel, Russia), E.O.B.; funding acquisition, supervision, and project administration, A.V.D.; and original development of the project idea and supervision, O.A.B. and V.G.A. All authors edited the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The reported study was funded by RFBR according to the research project No. 20-32-90147 (original draft preparation, measurements, numerical simulation). The work was also supported by the gran<sup>t</sup> of the President of the Russian Federation for state support of young Russian scientists No. MK-2634.2019.8 (development of experimental setup and data acquisition). V.D. kindly acknowledges personal support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie gran<sup>t</sup> agreemen<sup>t</sup> No. 839888.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki by the World Medical Association and approved by the Ethics committee of the Orel State University (record of meeting No. 15 of 21 February 2019).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data that support the findings of this study are available upon reasonable request request from the corresponding author.

**Acknowledgments:** Thanks for assistance in project realization are given to Olaf Minet (Charité Universitätsmedizin Berlin, Berlin, Germany) and Urszula Zabarylo (Charité—Universitätsklinikum Berlin, Berlin, Germany). Special thanks are extended to the head doctor of Diagnostic Medical Center "MediSсan" (Orel, Russia) Boris Shuraev for assistance in conducting experiments and conceptualization.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
