## 3 Results  
### Phantom Studies  
#### 3.1.1 Accuracy of SPECT and PET Derived Absorbed Dose Distributions  
The results of the accuracy of SPECT- and PET-derived ADD against the known reference obtained by the phantom experiment are presented in Table 2.  
The clinical reconstruction parameters used in the recovery of the activity distribution on PET-MS allowed for obtaining a satisfactory match of \(-10\%\) with the ground truth activity distribution in the biggest cylinder volume of 108 mL. The accuracy of the derived ADD in this cylinder was good, showing a mismatch with the ground truth of \(-4\%\). The other cylinders with smaller volumes, 8 and 15 mL, showed unsatisfactory activity recovery with a larger discrepancy of \(-57\%\) and \(-49\%\), respectively, and \(-54\%\) and \(-49\%\) regarding derived ADD. Regarding SPECT-MAA, the discrepancy was slightly inferior to the one obtained with PET-MS (Table 2).  
.1.2 Correlation and Agreement between Absorbed Dose Distributions Derived from SPECT and PET Images  
The voxelwise comparison between the SPECT-MAA- and PET-MS-derived ADD, assessed by the PCC and the ICC in _liver_cyl_ and _ptv_5_cyl_backg_, showed high correlation and agreement. The PCC was 0.87 and 0.94 in _liver_cyl_ and _ptv_5_cyl_backg_, respectively. The ICC was 0.85 and 0.93, respectively.  
The voxelized \(\gamma\)-index distributions were computed considering the ADD in both phantoms, one derived from SPECT-MAA and the other from PET-MS images, and seven DD/DTA test conditions. Table 3 shows the results of the \(\gamma\)-index PR. As expected, the highest \(\gamma\)-index PR values were obtained with the less conservative 15%/15 mm conditions.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**ROI** & **Cylinder 1** & **Cylinder 2** & **Cylinder 3** & **Cylinder 4** & **Cylinder 5** \\ \hline Actual volume (mL) & 15 & 8 & 15 & 8 & 108 \\ \hline SPECT-based volume (mL) & 23 & 9 & 24 & 10 & 117 \\ PET-based volume (mL) & 21 & 11 & 19 & 12 & 118 \\ \hline Activity \(\Rightarrow\) Mean AD & \(\text{MB}_{\text{q}}\Rightarrow\text{Gy}\) & \(\text{MB}_{\text{q}}\Rightarrow\text{Gy}\) & \(\text{MB}_{\text{q}}\Rightarrow\text{Gy}\) & \(\text{MB}_{\text{q}}\Rightarrow\text{Gy}\) & \(\text{MB}_{\text{q}}\Rightarrow\text{Gy}\) \\ \hline PET-MS ground truth (simulated) & 365 & 669 & 187 & 618 & 344 & 658 & 204 & 639 & 2084 & 775 \\ \hline Calculated & 185 & 365 & 81 & 309 & 175 & 378 & 92 & 294 & 1871 & 745 \\ Deviation (\%) & \(-49\) & \(-45\) & \(-57\) & \(-50\) & \(-49\) & \(-43\) & \(-55\) & \(-54\) & \(-10\) & \(-4\) \\ \hline SPECT-MAA ground truth (simulated) & 408 & 669 & 150 & 618 & 421 & 658 & 173 & 639 & 2031 & 775 \\ Calculated & 201 & 373 & 72 & 289 & 217 & 408 & 80 & 260 & 1808 & 721 \\ Deviation (\%) & \(-52\) & \(-44\) & \(-52\) & \(-53\) & \(-48\) & \(-38\) & \(-54\) & \(-59\) & \(-11\) & \(-7\) \\ \hline \hline \end{tabular} Note 1: SPECT-MAA and PET-MS based segmentation of ROI used 4.66 mm and 4.0 mm isotropic voxel size, respectively. Note 2: Deviation (\(\%\)) = \(100\times\) (calculated – ground truth)/ground truth.  
\end{table}
Table 2: Accuracy of SPECT-MAA and PET-MS based recovered activity and derived ADD. For comparison purposes, the two ground truth activity distributions were simulated based on the expected \({}^{90}\)Y activity concentration known by the phantom experiment preparation.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**ROI**} & \multicolumn{5}{c}{**DD (\%)/DTA (mm)**} \\ \cline{2-7}  & **5/5** & **10/5** & **5/10** & **10/10** & **15/10** & **10/15** & **15/15** \\ \hline _liver\_cyl_ & 98.3 & 98.7 & 98.7 & 98.9 & 99.2 & 99.0 & 99.3 \\ _ptv\_5\_cyl\_backg_ & 88.9 & 92.1 & 90.0 & 92.8 & 95.7 & 93.2 & 96.0 \\ \hline \hline \end{tabular}
\end{table}
Table 3: \(\gamma\)-index PR (%) in phantom ROI considering seven DD (%)/DTA (mm) test conditions.