## 4 Discussion  
In the last years, 90Y PET imaging has posed a unique challenge, primarily due to the extremely low positron yield of 32 \(\times\) 10\({}^{-6}\) per 90Y disintegration [9]. Many authors have shown the feasibility of 90Y PET for imaging microspheres after radioembolization procedures [10, 15, 18, 26, 27]. In this sense, the main goal of 90Y PET is the estimation of the dose delivered to the tumor and normal tissues. Therefore, the accuracy of the PET image in terms of the quantitative pixel value in Bq/mL is of the highest relevance, especially for the possibility to establish a dose-response correlation also at local level [15, 16, 28]. In this study, different set-ups of reconstruction for 90Y PET-images were investigated, and different types of analyses and metrics were used. Starting from the choice of matrix size, the effects of different matrix sizes were investigated without the bias generated by using a Gaussian filter and for a fixed number of iterations and subsets. Since there is independence for RCs and SUV from the matrix reconstruction dimension, it is reasonable to choose a matrix size of about 256 \(\times\) 256, which provides a pixel value of 3.18 \(\times\) 3.18 mm, considering that the resolution for the 90Y PET is approximately 3 mm [12]. Moving to the choice of the number of EIs, linear regression fits of RCs as functions of the number of EIs show independence of RCs from the number of EIs. For the 21 EIs, a convergence is achieved, and an increase in EIs only results in higher image noise. For this reason, the minimum number of EIs (i.e. 21) which are available for a Siemens mCT Biograph, is fixed. Lastly, the study of Gaussian filters was realized using dosimetric approaches. The evaluation of RMSE in iDVH proposed by [PERSON] et al. [21] and the evaluation of FWHM in dDVH proposed in this work as a complementary metric allows for the evaluation of the accuracy of dose  
Figure 6: Normalized RMSE for the liver (black) and for the IEC NEMA spheres as a function of the Gaussian filter. Figure 7: dDVHs for liver compartment (**A**) and 37 mm sphere (**B**) for different Gaussian filters. quantification. This analysis shows that there is an inverse trend for smaller volumes and larger ones. For larger volumes, RMSE and mean absorbed doses show independence from the choice of filter (slope m values of 0.0151 and 0.083 respectively), and FWHM decreases with increasing filter values, passing from 41.3 with no filter to 30.8 with a 12 mm filter. On the other hand, for smaller volumes, there is an inverse behavior; RMSE increases with increasing filter thickness. Moreover, for all P2's spheres, the Gaussian-like shape distribution of dDVH is lost, likely due to partial volume effects, poor counting statistics and limited spatial resolution. In this condition, it is not possible to use the FWHM metric, and more studies should be conducted in this way to establish a cut-off value that represents the real limit of this new metric, especially when the volumes of interest are close to scanner spatial resolution. Evaluations of the possibility to introduce a new metric should also be considered. It should be noticed that this alternative metric can be used only on phantom studies with a uniform and known activity to evaluate the best scanner's reconstruction parameters. In conclusion, a 6 mm Gaussian filter, corresponding to twice the resolution, can provide an adequate level of accuracy both for small and large volumes, even if adjustments should be performed when the lesion size is smaller than the scanner resolution. Generally, the Gaussian filter can be a free parameter that should be modeled depending on the clinical volume of interest. Moreover, in this aspect, more studies should be conducted to establish an objective guideline to follow in all situations. These future studies should be based firstly on phantoms and should subsequently be validated on patients. Surely, the higher effective sensitivity of new digital PET/CT scanners could particularly improve 90Y-PET-CT imaging.