# Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging
## 3 Results
### Effect of Target Motion on the Performance of the Proposed Method  
The A-mode signals were obtained by the proposed pulse-inversion method while moving the target in 5 \(\upmu\)m increments to ascertain the effect of target motion on the performance of the proposed HIFU interference elimination method. For the purpose of comparison, the A-mode signal and its frequency spectrum were also acquired without HIFU delivery as shown in Figure 7a,b. The three signal peaks appeared in the A-mode signal: the first peak signal generated at the interface between the water and phantom surface, the second at the graphite rod, and the third at the bottom of the phantom. When HIFU and ultrasound were simultaneously transmitted under the condition where the target was not moved, only a very small amount of the HIFU interference was observed in the A-mode signal (see Figure 7c) because the proposed method effectively eliminated the HIFU interference. From Figure 7d and Table 1, it was learned that the HIFU interference signal appeared mainly in the out-of-band of the spectrum of the monitoring ultrasound signal in this case; the fundamental, second harmonic, and fourth harmonic components of the HIFU interference signal located at 1.1, 2.2, and 4.4 MHz primarily affected the increase in the spectral magnitude. The magnitude at its third harmonic frequency (i.e., 3.3 MHz) similar to the center frequency of the monitoring ultrasound transducer increased by only 1.12 dB. However, the HIFU interference signal increased as the distance of the target motion increased. When the target was moved by 10, 30, 50 \(\upmu\)m after transmitting and receiving ultrasound with a phase of 0\({}^{\circ}\), the considerable amount of the HIFU interference remained in the A-mode signals as shown in Figure 8; the fundamental, second harmonic, third harmonic, and fourth harmonic signals of the HIFU interference predominantly appeared in the frequency spectra. From Figures 7 and 8, it was seen that the HIFU echoes received by the transducer were generated from the scatterers in the phantom as well as the graphite rod. The HIFU interference signals appeared after the first peak signal corresponding to the interface between the water and phantom when the interference signals were not effectively removed. However, the amplitude of the HIFU interferences from the region between the phantom surface and the graphite rod was relatively low because the density of the scatterers was possibly low in this region, which frequently occurs in custom-made phantoms. Figure 6: Photograph of the lesion formation induced in the excised bovine liver, which corresponds to the dotted white circle shown in Figure 5d. Figure 8: A-mode signal and its frequency spectrum obtained after transmitting both HIFU and the ultrasound imaging signal in the case of a moving distance of 10 \(\upmu\)m (**a**,**b**), 30 \(\upmu\)m (**c**,**d**), and 50 \(\upmu\)m (**e**,**f**). Figure 7: A-mode signal and its frequency spectrum obtained after transmitting the ultrasound imaging signal only (**a**,**b**) and those obtained after transmitting both HIFU and the ultrasound imaging signal without the target motion (**c**,**d**). For quantitative analysis, correlation coefficients between the reference scanline and other scanline signals were calculated as a function of the target moving distance. As shown in Figure 9a, the correlation coefficient was gradually decreased as the target moving distance increased; the decrease rate was 1.9%, 4.8%, and 7.1% in the cases of a moving distance of 5, 10, and 15 \(\upmu\)m. This value was 19.8%, 39%, and 66.9% at a moving distance of 30, 50, and 100 \(\upmu\)m (see Table 2). This result indicates that an increase in target motion leads to reducing the efficiency of the proposed method in HIFU interference elimination and this adverse effect becomes more dominant after a moving distance of 30 \(\upmu\)m. The remaining HIFU interference acts as a noise, thus decreasing a signal-to-noise ratio of monitoring ultrasound signal.