# Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging
## 4 Discussion  
The basic assumption for the proposed method is that the target lesion is stationary. In practice, however, tissue motion in response to heart beat, respiration, and patient motion happens. This causes an increase in residual HIFU interference because two HIFU interference signals acquired in between the time interval of \(T_{RPI}\) are not equal to each other and the HIFU inference is not completely removed. From the experimental results, it can be concluded that the discrepancy between the two HIFU interference signals depends on a target moving a distance in the axial direction, thus increasing the amount of remaining HIFU interference. Note that in this study only the axial motion of the target was considered because the fundamental and odd harmonic elimination efficiency of the pulse inversion method is more influenced by the axial motion of the target than the lateral motion [27]. The efficiency of the HIFU interference signals and the harmonic elimination is closely related to the phase aberration between the two sets of echoes obtained between the time intervals of \(T_{PRI}\). The phase aberration occurs much less when a target moves in the lateral direction than the axial direction [28]. Although target motion causes HIFU interference to remain in ultrasound monitoring signals, the moving distance up to 10 \(\upmu\)m is not expected to have a significant effect on the quality of ultrasound monitoring images based on the experimental results; this can also be seen in the ultrasound monitoring images of the tissue mimicking phantom (see Figure 10). Note that this phantom did not contain the graphite rod because the remaining HIFU interferences occur in the scattering region as shown in Figure 8. When the phantom was not moved, the ultrasound image obtained after simultaneous delivery of ultrasound and HIFU (Figure 10b) was similar to the image obtained under the condition of no HIFU (Figure 10a). Additionally, we could observe the speckle pattern in the phantom in the ultrasound image related to a target moving distance of 10 \(\upmu\)m (Figure 10c) although a small amount of the remaining HIFU interference appeared in the image. This indicates that the residual HIFU interference  
Figure 9: Boxplots of (**a**) correlation coefficients between the reference scanline and other scanline signals and (**b**) energy of each scanline calculated by squaring and summing the scanline signals as a function of the target moving distance. Note that the scanline signals were acquired using the oscilloscope. had a magnitude smaller than the speckles. In contrast, it was difficult to recognize the speckle pattern in the ultrasound image when the target was moved by 50 \(\upmu\)m (Figure 10d). The fastest moving organ in the body is the heart and the maximum moving velocity is reported as 25 cm/s [29]. This velocity corresponds to a target moving distance of 32.5 \(\upmu\)m under the condition where the frame rate of ultrasound imaging is 30 frames/s in the case of pulse inversion and the number of scanlines for an image is 128 (i.e., 0.25 \(\times\)\(T_{PRI}\)). Note that two transmissions of a scanline should be conducted for the proposed method and thus T\({}_{\text{PRI}}\) is calculated as 130 \(\upmu\)s (i.e., 1/(2 \(\times\) 30 \(\times\) 128)). In this case, unremoved HIFU interference may lead to degradation of ultrasound image quality. However, the heart is not the target organ for current HIFU surgery. On the other hand, the maximum moving velocity of the liver due to respiratory organ motion is 10 mm/s [27] that corresponds to a target moving distance of 1.3 \(\upmu\)m (i.e., 0.01 \(\times\)\(T_{PRI}\)). If the frame rate is reduced to 15 frames/s for pulse inversion imaging of a deep-lying target, this target moving distance is changed to only 2.6 \(\upmu\)m because \(T_{PRI}\) is doubled. Therefore, it can be concluded that the organs in the abdomen such as the liver are considered to be stationary and the basic assumption for the proposed method is valid.