**5. Conclusions**

This study has demonstrated the use of the tools Gmsh and GetDP, known together as ONELAB, as a single simulation platform for modeling both optical, as well as RF-induced, thermoacoustic computed tomography, i.e. PACT and RACT, respectively. To achieve PACT and RACT results, the propagation of optical, radio frequency, and acoustic waves were effectively modeled using solutions of the optical diffusion equation, Maxwell's equations, and time-domain wave equations. We validated our PACT and RACT algorithms using two types of tissue mimicking phantoms: a homogeneous phantom consisting of two absorbing targets and a breast phantom consisting of a tumor, with pre-defined optical and RF properties. Our results demonstrated that the optical and RF absorption properties of the respective tissue phantoms were accurately reconstructed using the proposed dual-modality computed tomography simulations in ONELAB. The use of the ONELAB software package allows for accurate multimodal thermoacoustic modeling, in order to develop and validate more robust algorithms for functional imaging of the human body.

**Author Contributions:** C.F. designed the simulations, analyzed the results, and wrote the paper. S.R.K. assisted in both the analysis of the results and writing of the paper.

**Funding:** The authors would like to thank the NIH for funding this work under NIH-NIBIB 4R00EB017729 (Sri-Rajasekhar Kothapalli) and PennState Hershey Center Institute Cancer Startup Funds (Sri-Rajasekhar Kothapalli).

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