# Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging
## 3 Results  
### Real-Time HIFU Treatment Monitoring  
Ultrasound monitoring images of the induction of thermal coagulation in the excised bovine liver were obtained before and after the application of the proposed method (see video clips in the Supplementary Materials). As shown in Figure 5a, strong HIFU interference appeared in the ultrasound monitoring image when the proposed method was not used. In this case, it was difficult to  
Figure 4: Experimental arrangement to ascertain the effect of target motion on the performance of the proposed method.  
observe echogenicity change in the bovine liver. In contrast, the HIFU interference disappeared when the proposed method was applied. This made it possible to monitor the treatment area during the treatment period. In Figure 5b obtained before HIFU insonation, the solid white arrow indicates the boundary of the water and bovine liver. Note that the bright spots of which intensity was not changed with time appeared in the video clips and Figure 5 (see the dashed arrows in Figure 5b). These are the reverberation artifacts generated at the rear surface of the HIFU transducer and the side of its hole, which occurred because the central opening of the HIFU transducer was too small to allow the convex imaging probe to be inserted completely into the opening (see Figure 3). At 5 s after the HIFU insonation, the echogenicity change indicated by the dashed white arrows and the tissue boundary were clearly observed (see Figure 5c). The hyperechoic spot appearing during the HIFU exposure (indicated by the dashed circle in Figure 5d) is mainly due to microbubbles induced by the HIFU [7; 22] and partially acoustic impedance change of the region due to coagulation [23; 24]. Since some microbubbles disappear without tissue damage after cessation of the HIFU exposure [22], the size of the hyperechoic spot may not match the actual size of the coagulation lesion measured in the tissue (see Figure 6). However, the ultrasound monitoring image provided the information about the location and echogenicity change of the treatment area. As a result, the experimental results imply that the proposed method enables us to confirm the focal area of HIFU and to monitor the echogenicity changes of the area due to its capability of real-time HIFU interference elimination. Additionally, this result implies that the proposed method can help the thermometry [6] and echo decorrelation [25; 26] methods accurately visualize and measure the treatment area in real time because these methods require a high signal-to-noise ratio to achieve high accuracy. For this reason, the cessation of HIFU exposure is currently necessary at the time of measurement.  
Figure 5: (**a**) Ultrasound monitoring images of the excised bovine liver obtained before the application of the proposed method. The HIFU interference disappeared when the proposed method was applied: (**b**) 0 s, (**c**) 5 s, and (**d**) 15 s after HIFU insonation. In (**b**), the solid white arrow indicates the boundary of the water and bovine liver and the dashed white arrows indicate reverberation images at the rear surface of the HIFU transducer and the side of its hole. In (**c**), the echogenicity change is indicated by the dashed white arrows. The dotted white circle indicates the predicted treatment area in (**d**).