# Dosimetric Evaluation in Micro-CT Studies Used in Preclinical Molecular Imaging
## 4 Discussion  
Several investigators have reported different experimental procedures using ionization chambers, Gafchromic films, implanted thermoluminescence dosimeters (TLDs), and Monte-Carlo-based simulations and analytical calculations to determine the received dose in rodents during preclinical studies that use microCT imaging [29; 30; 31; 32]. However, there are no accepted methods, guidelines, or regulations defining a unified parameter for microCT dose. Some researchers have adapted the computed tomography dose index (CTDI) used in the clinic as a metric to quantify the radiation output from a microCT examination [29; 31]. We chose the imparted dose at the surface (IDS) as the dosimetric parameter to estimate the dose received by the animals used in microCT studies. This decision considers the limitations in the availability of the pencil ionization chambers and phantoms needed to \begin{table}
\begin{tabular}{c c c} \hline
**mA** & **Modality** & **IDS\({}_{\text{ms}}\) \(\pm\) \(\mathbf{\delta_{\text{IDS}}}\) (mGy)** \\ \hline
0.2 & _Standard_ & \(55\) \(\pm\) \(8\) \\
0.2 & _Good_ & \(74\) \(\pm\) \(4\) \\
0.2 & _Best_ & \(109\) \(\pm\) \(17\) \\
0.4 & _Standard_ & \(89\) \(\pm\) \(5\) \\
0.4 & \(HR\) & \(298\) \(\pm\) \(9\) \\ \hline \end{tabular} Values are corrected by the corresponding energy dependence correction factor.  
\end{table} 
Table 7: Imparted Dose at the Surface of the mice (IDS\({}_{\text{ms}}\)) at the 45 kV potential of the X-ray tube. Figure 6: (**A**) Comparison of the imparted dose at surface (IDS) within the different microCT acquisition modalities. No statistical differences were observed between IDS\({}_{\text{0}}\) vs. IDS\({}_{\text{TL}}\) or IDS\({}_{\text{ms}}\) vs. IDS\({}_{\text{T}}\). (**B**) Comparison of the imparted dose at surface (IDS) within the _Standard_ modality. measure the CTDI in microCT acquisitions, but mainly because of the belief that that CTDI estimates the absorbed dose in patients, as opposed to quantifying the radiation output of CT systems [33]. The analytical model presented here was implemented specifically for the Albira system. However, if the spectra of other X-ray tubes are known (or can be estimated) together with the characteristics of the tomographic acquisition geometry, a similar model can be easily implemented for the imparted dose assessment of other microCT systems. The IDS results estimated by the analytical model and measured with the TLDs implanted in the animals were in good agreement, indicating the validity and utility of the analytical calculations. By contrast, results from TLDs at the lucite block were not representative of the dose delivered at the surface of the animals submitted for tomographic study. It was assumed that this experimental arrangement did not satisfy the animal irradiation conditions, particularly the shape and dimensions of the animals and, likely, the measurements reported by the manufacturer do not satisfy this condition either. Previous works have reported that the Apogee's X-ray tube output per mAs has a linear behavior in terms of voltage, so for a fixed voltage, its effective energy (\(E_{\textit{ff}}\)) is independent of the filament current and exposure time. We also observed no differences in \(E_{\textit{ff}}\) at 0.2 and 0.4 mA at either 35 or 45 kV. Differences between our measurements and the \(E_{\textit{ff}}\) calculated or measured by other researchers [1,7,9] are around 10%, depending on the analytical calculation or the experimental setting parameters. In any case, our \(E_{\textit{ff}}\) average values of 14.5 keV for 35 kV and 16.6 keV for 45 kV are representative of Albira's microCT. Knowing the radiation dose received by the animals that might result in biological effects is critical to designing longitudinal studies (of the same animal over time). Albira's manufacturer has reported a total dose of 13.7 \(\mathrm{G}\mathrm{G}\mathrm{y}\) in mice after a representative microPET/SPECT/CT study [12].