# RF Exposure Assessment for Various Poses of Patient Assistant in Open MRI Environment
## 3 Results and Discussion  
The conditions and amount of RF exposure are dependent on the desired image contrast, RF coils used, and target body parts. In an MRI system, the image contrast can be adjusted by applying different RF powers (resulting different flip angle) to the  
Figure 5: Assessment scenarios of RF energy exposure: The patient assistant (**a**) stands next to the patient (scenario 1), (**b**) sits by the patient (scenario 2), (**c**) holds the patient’s hand (scenario 3), (**d**) holds the patient’s hand and chest (head coil case, scenario 4), and (**e**) holds the patient’s hand and head (body coil case, scenario 5). subject [27]. The RF exposure levels are also dependent on the loaded body parts because more loading in a RF coil requires additional RF power to flip the spins at a specific angle for the desired MRI scans [28]. In this study, 1 W normalized RF power was applied to all SAR computations for each scenario, which is a commonly applied method for the numerical assessment of RF energy deposition in an MRI system [29; 30; 31]. This helps with analytical comparisons of SAR computations under different MRI scan protocols requiring different levels of RF power. Meanwhile, there are a few factors that can affect the RF exposure of the patient assistant during MRI scans: (1) the distance of the patient assistant from the RF coils or patients, (2) the poses of patient assistant, (3) the dielectric tissue properties of the patient assistant and patients, and (4) the different conditions of the electrical current flow within the patient assistant, which is induced by the EMF of the RF coils. The maximum values (so-called 'hot-spots') of 1 g peak averaged SARs in the Billie and Thelonious heads were 1.67 and 1.62 W/kg, respectively, when loaded in the head RF coil. The maximum SAR values were observed at the nose. For the body RF coil, the maximum values of 1 g peak averaged SARs in Billie and Thelonious were 0.87 and 1.16 W/kg respectively, found at the center of the back. The existence and poses of the patient assistant did not notably affect the SAR levels of the patient because the patient assistant was located outside of the RF coils. The difference in the SARs of the patient's head/body was below 1.5% when the patient assistant was involved, as summarized in Tables 2-5. In addition to scenarios 1-5, note that S\({}_{\text{ab}}\) was defined in Tables 2-5 for the absence of a patient assistant. Figure 6a-d shows the 1 g averaged SAR distribution in the patient assistant when the patient ([PERSON]) was loaded in the head RF coil, whereas Figure 6e shows the 1 g SAR distribution when the body RF coil was used. The patient did not notably affect the hot-spot locations of the patient assistant in this study. The hot-spots in each scenario were observed at different locations on the patient assistant. The levels of the hot-spots also depended on the patient assistant's location and pose. For scenario 1, the hot spot was found at the abdomen, which is close to the head/body RF coils. For a similar reason, the hot-spot in scenario 2 was found at the thigh. The levels of scenarios 1 or 2 were relatively lower than those of scenarios 3-5 mainly because of (1) the physical distance from the RF coils to the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Scenario** & \multicolumn{2}{c}{**Patient ([PERSON])**} & \multicolumn{2}{c}{**Patient assistant ([PERSON])**} \\ \hline  & 1 g & 10 g & 1 g & 10 g \\ \hline S\({}_{\text{ab}}\) & 1.6670 & 1.0411 & - & - \\
1 & 1.6960 & 1.0596 & 0.00038 & 0.00019 \\
2 & 1.6888 & 1.0551 & 0.00016 & 0.00008 \\
3 & 1.6614 & 1.0380 & 0.00353 & 0.00162 \\
4 & 1.6583 & 1.0359 & 0.19928 & 0.09240 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Peak spatially averaged SAR (W/kg) in [PERSON] (patient) and [PERSON] (patient assistant) according to head RF coil (input power = 1 W).