## 4 Discussion  
This study investigated the feasibility of inducing rotational sensations through acoustic streaming in the SCC using LIFU. Through a 3D finite element model, we evaluated the effects of LIFU on endolymph dynamics within the SCC and assessed its thermal safety. The findings demonstrate the potential of LIFU as a vestibular stimulation method, with its capability to control rotational sensations by adjusting the focal position of the ultrasound. First, the analysis of the acoustic field and energy dissipation showed that higher ultrasound frequencies lead to a more focused intensity distribution, resulting in narrower FWHM. As the frequency increased from 400 kHz to 800 kHz, the focal point moved closer to the designed focal distance of 42 mm, and the intensity and energy dissipation density increased. This result confirms that higher frequencies can more precisely target the SCC, which is critical for generating strong acoustic streaming. However, since the frequency of transcranial ultrasound cannot be increased arbitrarily [33; 34; 35], optimizing transducer parameters is essential for achieving effective rotational stimulation. Moreover, while the maximum intensities for the three frequency results exceeded the typical LIFU range, they were based on continuous wave conditions with a duty cycle of 1. In practical applications, pulsed ultrasound can be used to adjust the duty cycle, effectively reducing the spatial  
Figure 11: Cupula displacement occurs when the focal position of the ultrasound transducer is shifted along the x-axis from x = \(-\)3 mm to x = 3 mm. (**a**–**c**) When the focus is positioned on the left side of the SCC center (x < 0), the dissipated energy generates endolymph flow in the clockwise direction. (**d**–**f**) Conversely, when the focus is positioned on the right side (x > 0), it generates endolymph flow in the counterclockwise direction. peak temporal average intensity. This suggests that the findings of this study can be safely applied in clinical settings by appropriately scaling the intensity. The study also analyzed the acoustic streaming and cupula displacement induced by energy dissipation. The results showed that the strength of acoustic streaming within the endolymph increased with frequency, leading to larger cupula displacements. At 800 kHz, the cupula displacement reached a maximum of 1.46 \(\upmu\)m, exceeding the minimum displacement of 1.3 \(\upmu\)m observed in human simulation reported in Table 2[52]. Although this displacement is smaller than those observed in animal studies [48; 49], it suggests that at least 10\({}^{\circ}\)/s\({}^{2}\) of rotational sensation could be induced under this setup. This indicates that LIFU can generate meaningful fluid motion within the SCC, mimicking natural vestibular stimulation. The strong correlation between frequency and displacement suggests that LIFU offers a platform for controlling the intensity of sensory input based on its application. In the thermal safety evaluation, the maximum temperature rise during LIFU exposure was 0.81 \({}^{\circ}\)C, which is below the FDA safety limit of 1 \({}^{\circ}\)C [21; 57; 58]. Despite the relatively long 1-second exposure, the temperature increase remained within the safety threshold. Thanks to the higher specific heat and lower attenuation coefficient of the endolymph [40; 41], the temperature rise was reduced compared to when only the temporal bone was present. This demonstrates that the thermal effects of LIFU can be effectively managed, suggesting the potential for future applications with further research. While the possibility of additional cooling effects from fluid flow was noted, further studies are required to verify this. Lastly, the variation in the focal position demonstrated that both the direction and magnitude of cupula displacement can be controlled by adjusting the ultrasound focal position.