## 3 Technology
### Current Research Challenges
#### 3.7.5 Impact of the Overall Detector Response Shape  
In a recent study performed by some of us [102], the effect of instrumental, optical and geometrical parameters on TG diffuse optics measurements within heterogeneous media have been estimated by performing comprehensive simulations. They take into account the strong interplay between all the different parameters in order to both provide a reference for scientist to devise the best measurement conditions depending on the problem under investigation and to identify the most critical challenges to be addressed by researchers working in the optimization of the hardware performance of TG system. The most critical parameters limiting the performance of a TD-DO system based on TG detection were found to be the memory effect and the detector active area. Figure 10 shows the simulated contrast and contrast-to-noise ratio (CNR, i.e., an index of the statistical significance of the contrast within the noise fluctuations [60], in formula: \(CNR=\frac{N_{0}-N}{\sigma(N_{0})}\) where \(\sigma(N_{0})\) is the standard deviation of a series of repeated acquisition for the unperturbed case) produced by localized absorption perturbation within highly scattering media using a realistic IRF shape (measurements conditions can be found in Ref. [102]). A more comprehensive figure, which includes also possible different perturbations and different optical properties of the bulk medium, can be found in Ref. [113]. In panels at different rows the perturbation is set at different depths within inside the medium probed in a reflectance geometry (from 10 mm to 30 mm). Panels in different columns shows the performance of systems characterized by different amplitudes of the memory tail (attaching four or six decades below the IRF peak, or not present in the last column--ideal case). Different colors of curves represent different \(\rho\), from 2 to 30 mm. In particular, the blue curve represents the small \(\rho\)(2 mm) configuration. While classical large \(\rho\)are not affected by the memory tail amplitude, in the 2 mm case both contrast and CNR strongly depend on the memory tail amplitude. Using a 2 mm \(\rho\)in presence of a high memory tail the perturbation is undetectable at the largest depths (contrast \(<1\%\), CNR \(<1\) at a 30 mm depth), while in absence of memory effect a 2 mm \(\rho\)gives the best results in terms of both contrast and CNR (contrast \(>20\%\), CNR \(>2.5\) at a 30 mm depth), as it was theorized in Ref. [38]. It is worth noting that here a realistic IRF shape was considered, featuring \(\sim\)150 psjitter FWHM and a diffusion tail time constant of 80 ps starting one decade below the IRF peak. Similar considerations can be drawn for the detector active area, whose effect can be found in Ref. [114]. The memory effect reduction and the increase of the detector collection area represent the main technological challenges to be addressed in the future to achieve the best performance in TG-DO systems, maximizing in this way the penetration depth inside scattering media, unleashing the full power of the technique when probing muscles and brain, but even reaching organs in the human body whose access is presently forbidden, such as heart or cerebellum, opening new applications in biomedical field and others [39].  
Figure 10: Contrast (**top panel group**) and contrast-to-noise ratio (CNR) (**bottom panel group**) produced by a localized absorption perturbation within a scattering medium (see Ref. [102] for additional details) as a function of the gate-ON delay of the SPAD. In each panel group, the perturbation is set at different depths along the rows, while along the columns different intensities of the memory effect are simulated. Different colors represent different source-detector distances, listed in the legend in mm.