**5. Conclusions**

The numerical approach proposed in this paper provides a complete and effective way of designing axicon lenses for many high-resolution applications, such as mapping or detection. This is also suitable for focused ultrasound through human skull bone. In view of providing better lateral resolution with lower sidelobes, the use of design programs for this task is not as straightforward. The choice of a good starting point is an important factor for successful optimization. It is easier to obtain a starting design using relatively simple formulas and then use it in a lens design program for future analysis and optimization. We believe that this will be an effective way of designing axicon lenses, for example, to build focused windows to the brain for clinically viable transparent cranial implant for chronic ultrasonic therapy and stimulation of the brain.

**Author Contributions:** Conceptualization, F.A., S.E.L., and S.N.G.; data curation, F.A. and S.N.G.; formal analysis, F.A.; methodology, F.A. and S.N.G.; project administration, S.E.L.; software, F.A. and S.N.G.; supervision, S.E.L.; validation, F.A.; writing—original draft, F.A.; writing—review and editing, S.E.L. and S.N.G.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
