## 2 Materials and Methods  
_Calculation of Effective Dose (E\({}_{d}\))_  
This study, conducted at King Fahad University Hospital, aimed to estimate and compare different methods used to calculate the effective doses: the computational method based on CT dose index (CTDIvol), the dose-length product (DLP) using Monte Carlo simulation CT-Expo 2.6v, and the well-known standard method based on organ dose estimates which uses tissue weighting factors specified by ICRP [13, 14, 15]. The effective doses (E\({}_{\text{d}}\)) of the most common routine radiological investigations were evaluated for 165 CT studies. A total of 112 patients were male (68%), and 53 were female (32%). In this work, the effective dose (E\({}_{\text{d}}\)) was calculated for CT scan studies as follows:  
\[\text{Effective dose (E${}_{\text{d}}$)}=\text{DLP}\times\text{k} \tag{1}\]  
where DLP is the parameter displayed on the screen after the scan and k is the coefficient factor reported by European Commission NRPB-W67 (2005) with reference to regional anatomy (Table 2). This is referenced as E\({}_{\text{DLP}}\), which refers to the effective dose calculated with reference to the dose length product values displayed on the screen. For example, in Table 3, the first row is as follows:  
\[\text{E${}_{\text{DLP}}$}=\text{DLP}\times\text{K}=634\times 0.0021=1.3314 \tag{2}\]  
The effective doses represented by E\({}_{60}\) and E\({}_{103}\) were calculated using organ dose-based estimates (tissues weighting factor) from ICRP 60, 103. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & **Body Organs** & **Gonads** & **Lung** & **Stomach** & **Breast** & **Liver** & **Thyroid** & **Bladder** & **Brain** & **Reminder** \\ \hline \multirow{4}{*}{
\begin{tabular}{c} **N** \\ **N**  
Monte Carlo simulation is considered to be the best complete computational method for the estimation of organ and tissue radiation. Additionally, it accounts for many scanner specifications such as beam collimation, filtration, scanner geometry, and tube potential. The effective doses were estimated by Monte Carlo simulation and referenced as E\({}_{860}\) and E\({}_{8103}\) for comparison. In this work, the CT-Expose light model simulation was used, in which the calculation can be performed independently of the type of scanner in use and only requires the corresponding CTDIvol and DLP values to be set as inputs for the simulation (Table 4). The values of CTDIvol and DLP were used as inputs for the simulation. Currently, all newly developed CT scanners have protocol information and parameters (CTDIvol, DLP) that provide an effective dose calculation displayed on the screen [16]. In computed tomography, CTDI is the measure of the radiation output obtained from the absorbed dose at a line parallel to the rotation of the \(z\)-axis, when the X-ray tube has a single rotation. In ICRU report no. 87, CTDI is defined by [17]:  
\[\text{CTDI}_{\infty}=\frac{1}{nT}\ \int_{-\infty}^{+\infty}A(z)dz \tag{3}\]  
\(A(z)\) is air kerma, \(z\) and \(n\) are tomographic sections, and \(T\) is the width. The absorbed dose accumulated in a range of minus (\(\pm 50\) mm) of the beam center, giving the scale of \(\text{CTDI}_{100}\):  
\[\text{CTDI}_{100}=\frac{1}{nT}\ \int_{-50}^{+50}A(z)dz \tag{4}\]  
\(\text{CTDI}_{100}\) is measured by an ionizing chamber, and the weighted CTDI (CTDIw) is given as:  
\[\text{CTDIW}=\frac{1}{3}\text{.CTDIC}+\frac{2}{3}\text{.CTDIIP} \tag{5}\]  
The change in the radiation dose due to the pitch factor (\(P\)) is known as volume (CTDIvol):  
\[\text{CTDIvol}=\frac{\text{CTDIW}}{P} \tag{6}\]  
The dose-length product is introduced to provide the scanning range and is directly estimated as:  
\[\text{Dose length product}=\text{CTDIvol}\times\text{scan length} \tag{7}\]  
DLP accounts for all the energy absorbed in mGy.cm [18]. Table 1 shows the tissue weighting factors for ICRP 60, published in 1991, and different tissue weighting factors for primary organs updated by ICRP 103, published in 2007.