## 4 Conclusions  
In our work on breast imaging and lesion characterization, we wanted to assess the performances of a Time Domain Diffuse Optical Tomography (TD-DOT) system working in reflectance geometry, formed by eight couples of sources and detectors placed in a rectangular pattern, with a particular focus on the quantification of the system figures of merit. Thus, we developed a general-purpose simulation platform to carry out a quantitative performance assessment of clinical TD-DOT systems. The platform is able to study up to five parameters at the same time and to extract the corresponding figures of merit. It also provides an immediate and concise visualization tool for an analysis and comparison of results. We used the platform to provide a thorough characterization and validation of our TD-DOT system using realistic levels of signal and noise and studying important parameters, such us inclusion position, hardware gating of the detector, regularization parameter (\(\tau\)) and the total photon count rate. The main figures of merit used are the localization error of the reconstructed inclusion with respect to the simulated one and the relative volumetric error on the absorption coefficient. We found that the hardware gating of the detector significantly improves absorption quantification, enabling one to probe the tissue down to 2 cm-depth with a volumetric error as low as 10%. At the same time a software gating of the reconstructed photon distribution of time of flights improves the penetration depth if the time window is shorter than 500 ps. Figure 9: Cross sections at different depths of the reconstruction. Localization is good in all of the coordinates and the quantification error is low (5%). Better inclusion localization in the \(x\)-\(y\) plane (error \(<1\) mm) was reached under most of the experimented conditions when \(\tau\geq 10^{-3}\) and down to 25 mm-depth in the source-detector rectangular region. The localization error is low along the \(z\) axis for \(\tau=10^{-3}\) (error \(<1\) mm down to 25 mm-depth) but increases with higher values of \(\tau\), reaching a \(-4\) mm value when \(\tau=1\). Thus, the inclusion is reconstructed at a shallower depth and there is a strong underestimation of the absorption coefficient. Finally, once we fixed the other parameters, a photon count rate \(\geq 10^{6}\) cps leads to a wide volume (under the source-detector pattern and down to a 2 cm-depth) in which the relative volumetric error is low (\(<\)10%), indicating a high accuracy of the proposed TD-DOT system and fostering its use for breast imaging. To further improve these results multiple options are being studied: The use of a priori information coming from US images should further improve the localization and penetration depth, and a finite element fitting procedure should improve the quantification of the absorption coefficient as inversion algorithms are avoided for the reconstruction in favor of a more robust data fitting procedure, using an analytical solution of the problem. These approaches could also benefit from the implementation of a discretization error model, thus further improving its quantification capability [37]. The use of a multi-wavelengths approach is being considered to improve quantification. A spectral fitting procedure would provide the concentrations of the main components of the breast which results in a more direct method for breast and lesion type identification. Conceptualization, A.P. and A.F. ; methodology, S.A. and F.M. ; software, A.P., E.F. and A.F. ; formal analysis, A.P., A.F. and E.F.; writing--original draft preparation, E.F., P.T., A.F., A.P. ; writing--review and editing, P.T., A.F., A.P., S.A. and F.M. ; visualization, E.F.; supervision, P.T. ; funding acquisition, P.T. The research leading to these results has received partial funding from the European Union's Horizon 2020 research and innovation program under project SOLUS: \"Smart Optical Laser and Ultrasound Diagnostics of Breast Cancer\" (www.solus-project.eu, grant agreement No. 731877).