# Numerical Study on the Application of Near-Infrared Temperature Distribution Measurement of HIFU
## 4 Discussion  
In the existing transient temperature measurement methods, thermocouples are often used. The error introduced by metal thermocouples in biological tissues under ultrasound is related to the VHA in measurement [20]. VHA occur at the thermocouple tissue interface, caused by the difference in the densities between the thermocouple and the surrounding tissue [21]. In 2008,Morris et al. showed that temperature measurements using metal wire thermocouples can have an error of up to 80% [22]. Different methods have been proposed to minimize VHA to obtain the actual temperature in response to this issue. Research has shown that the contribution of viscous heating is significant at the onset of ultrasound action and decreases over time [20]. This method, also known as the \"wait and then measure\" method, involves waiting for a certain amount of time to measure the temperature in order to minimize the impact of viscous heating. A drawback of this method is the inconsistent waiting times reported in different studies, attributed to variations in the tissue characteristics, acoustic parameters, and thermocouple specifications. For instance, waiting times such as 0.5 s after the start of ultrasound or 200 milliseconds [23], 2 s [24], and 5 s [25] after ultrasound cessation have been reported. Another method is simulation, used to estimate the temperature increase caused by viscous heating and correct the temperature reading [22]. No matter which method is used, there are inconveniences, which also highlight the limitations of thermocouples. Based on the simulation results in this study, it is observed that the temperature rise due to non-linear effects surpasses that caused by shear waves and thermal viscosity. Importantly, under high-order harmonic excitation, non-linear effects significantly impact the temperature. Hence, accounting for additional temperatures induced by non-linear effects during the analysis is imperative. It is worth noting that although the thermal viscosity effect was considered in this study, the influence of non-linear effects was not considered. Therefore, we still need to understand the influence of non-linear effects on thermal viscosity. However, it is worth noting that there is also a non-linear effect within the thermocouple, which will have a pronounced impact on the thermal viscosity. In summary, compared to thermocouples, the thermal viscosity effect of infrared glass has a relatively small error. It means that infrared glass can effectively overcome the limitations of thermocouples and provide more accurate temperature distribution values, reflecting the temperature diffusion. Figure 12: Temperature rise curves corresponding to different incident angles when considering the thermal viscous boundary layer. In clinical practice, the presence of bone structure can be analogized to that of infrared glass, which will potentially affect the ultrasound field distribution, in turn generating an additional temperature rise during HIFU tumor treatment surgery. By conducting simulations on infrared glass, we can gain a deeper understanding of the characteristics of bones. However, there are noteworthy distinctions between infrared glass and bones. Infrared glass exhibits good thermal sensitivity, yet its attenuation coefficient is small, preventing any significant additional temperature rise. Conversely, bones possess a relatively large structural attenuation coefficient, causing more substantial temperature increases during irradiation, leading to higher heat absorption. Therefore, when conducting HIFU practice, it is necessary to avoid bone parts in order to avoid potential damage risks. Through in-depth research on the relationship between infrared glass and bones, we can provide the necessary guidance and inspiration for clinical practice. These results have significant implications for future scientific research and medical technology innovation.