# Numerical Study on the Application of Near-Infrared Temperature Distribution Measurement of HIFU
## 3 Results
### Thermal Viscosity  
The thermal viscosity effect refers to the phenomenon of energy dissipation and conversion into thermal energy during the propagation of ultrasound waves. This happens due to the interaction and friction between medium molecules. This heating phenomenon is pronounced, especially at the adjacent interface between the tissue and glass. The thermal viscosity effect is a crucial factor that cannot be ignored in the HIFU-heating of biological tissues, and its impact on the temperature field must be comprehensively considered and accurately evaluated. Based on the shear wave model, a thermal viscous boundary layer is introduced at the interface between the glass and tissue, followed by simulation calculations. Considering the addition of a thermal viscous boundary layer and only accounting for the shear wave model, the ultrasound and temperature fields were compared under different input powers. The specific comparison results are shown in Figure 7.  
Figure 7 shows that after introducing the thermal viscosity effect, the thermal loss significantly increases, based on shear waves, and becomes more significant with the increase in power. Under thermal viscosity, the acoustic thermal effect is significantly enhanced, and the temperature is further increased. The thermal viscosity increases the friction between the media, and the heat generated by friction increases the temperature. However, at a power of 100 W, the temperature difference caused by the thermal viscosity effect is only 1.7 \({}^{\circ}\)C, which is not substantial compared to the significant temperature rise  
Figure 7: Comparison results of ultrasound and thermal fields between the thermal viscosity model and the shear wave model under different input powers. effect caused by shear waves. This is due to the fact that the temperature rise caused by the thermal viscosity effect is a local effect. The thermal energy in the medium is mainly concentrated in the friction and collision processes between molecules. This energy cannot effectively propagate throughout the entire medium, limiting the range and degree of thermal viscosity effect propagation within the medium. In contrast, shear waves can induce a comprehensive motion within the medium, leading to a more even heat distribution within the medium. ### Thermal Viscosity  
From the above, it can be concluded that when using infrared glass to measure the temperature rise of the tissue surface under the action of HIFU, it is influenced by both shear waves and thermal viscosity, primarily due to the increased sensitivity of oblique incidence to them. In this study, oblique incidences were used to detect and evaluate the additional thermal viscosity effect and shear wave effect of near-infrared temperature measurement. In changing the deflection angle of the transducer to achieve oblique incidence, it is vital to note that utilizing large-scale models for a comprehensive simulation of the target is not feasible. This is because of the high demand for computer resources in 3D modeling and the constraints of grid accuracy, among other factors. On this basis, a new simplification method is proposed, which uses a small-sized model to replace the original model. Figure 8 illustrates a small-scale, three-dimensional finite element model of focused ultrasound with oblique incidence. Except for the size, all of the other characteristic parameters remain constant, including the deflection angle (\(\theta\)) of the transducer. Figures 9 and 10 show the tangential temperature distribution corresponding to different incident angles considering shear waves without and with the presence of the thermal viscous boundary layer, respectively. Correspondingly, temperature rise curves of different incident angles are depicted in Figures 11 and 12.