## 4 Conclusions  
In this article, the influence of several parameters, such as frequency, lesion depth, and number of acquisitions, was investigated for the localization of lung nodule by microwave imaging. This was studied through finite difference time domain (FDTD) simulations where a simple monostatic antenna was placed in contact with the lung tissue. A deflated lung was assumed as a medium to mimic the case during a thoracoscopic surgery and a pulmonary nodule was approximated by a sphere with different dielectric properties.  
Several antenna grids were investigated spanning from \(3\times 3\) to \(5\times 5\). In all the simulations, the nodule was positioned between 1 and 6 cm below the lung surface and the frequency of the microwave pulse was varied between 0.5 and 5 GHz.  
The results show that the best localizations were those performed for deep pulmonary nodules, between 5 and 6 cm of depth, and at the lowest frequencies, below 1 GHz.  
At lower depths, some artifacts appeared in the confocal images hindering the localization and at the highest frequencies, the attenuation given by the lung tissue was too strong to show any meaningful target in the confocal images.  
However, despite the strong attenuation of the lung tissue and the low dielectric contrast between the tissue and the nodule, on theoretical ground, deep pulmonary nodules could be successfully localized through microwave imaging, even though a very simple antenna model without any directionality was used.  
The best performances were found for 0.6 GHz for a \(5\times 5\) antenna grid and showed that the nodule localization was possible up to 10 cm of depth.  
This work gives some first guidelines for the usage of microwave imaging directly in the proximity of the lung which could be implemented in a thoracoscopic surgery as a simple and non-invasive method.  
Further refinements are necessary to really establish the limit of this technique. In fact, higher working frequencies are preferred because of their improved spatial resolution and a more sophisticated antenna and a less homogeneous and ideal lung model should be used to realistically mimic the surgery condition. Moreover, different strategies can be employed to identify also those nodules that lay in shallow positions, as for example to change the distance between the antennas and the lung tissue, or to use spacing materials with predefined dielectric properties. Furthermore, measurements done at different frequencies could be used to improve accuracy. Finally, testing with tissue samples is foreseen in order to confirm and refine the numerical results.  
The following are available online at [[https://www.mdpi.com/article/10_3390/app11094282/s1](https://www.mdpi.com/article/10_3390/app11094282/s1)]([https://www.mdpi.com/article/10_3390/app11094282/s1](https://www.mdpi.com/article/10_3390/app11094282/s1)).  
investigation, A.B.; writing--original draft preparation, A.B.; writing--review and editing, P.P.P. and K.M.; funding acquisition, K.M. All authors have read and agreed to the published version of the manuscript.  
Figure 9: Ratio \(R\) at 0.6 GHz for \(5\times 5\) antenna distribution at different target depths.  
**Funding:** This research was partially funded by BMBF grant number FKZ: 13 FH5l05 IA COHMED-DigiMed-OP and grant AIRLobe funded by \"Innovative Projects\" MWK-BW.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**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.