**4. Conclusions**

Phantoms with horizontal graphite absorbers were used to examine the efficacy of full-ring, diffuse, and point illumination PAT imaging using a ring US transducer. The full-ring illumination was

able to provide a more uniform fluence irrespective of the vertical depth of the imaged cross-section, while the point source and diffused-beam resulted in a high fluence at the point of entry, which diminished with vertical depth. In addition, the preliminary results indicate that illuminating an object 10–15 mm below the imaged cross-section of interest might be optimal for this system since it avoids large surface reflections and provides better coverage to the cross-section.

**Author Contributions:** N.A. and S.S.A. contributed to: conceptualization, methodology, formal analysis, investigation, data curation, writing—original draft preparation, writing—review and editing, and visualization. A.P. developed the software used for creating the PAT back-projection images. M.M. was involved in conceptualization, methodology, formal analysis, investigation, writing, the review and editing, supervision, and project administration.

**Funding:** This project was supported by Department of Defense (Breast Cancer Research program, Award number (W81XWH-18-1-0039). Naser Alijabbari (NA) was further supported by Office of the Vice President for Research (OVPR) offers the Faculty Competition for Postdoctoral Fellows.

**Acknowledgments:** The authors acknowledge Wayne State University for their financial support and the Barbara Ann Karmanos Cancer Institute. Also, we would like to acknowledge Neb Duric from Karmanos Cancer Institute for his guidance and assistance with using the UST system. MM was supported by Department of Defense (Breast Cancer Research program, Award number (W81XWH-18-1-0039), University Research Grant from Wayne State University Provost office, Technology Development Incubator Award through Wayne State University, and pilot research funding from Barbara Ann Karmanos Cancer Institute.

**Conflicts of Interest:** The authors have no relevant financial interests in the manuscript and no other potential conflicts of interest to disclose.
