# Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging
## 2 Materials and Methods
### Experimental Arrangement
Another A-mode signal was acquired after moving the target up by 5 \(\upmu\)m and subsequently transmitting both HIFU and ultrasound with a phase of 180\({}^{\circ}\) at the same time; 20 A-mode signals were acquired while moving the target in 5 \(\upmu\)m increments. When the reference and these A-mode signals were used to obtain each scanline, the assumption was not valid  
Figure 3: Experimental arrangement for performance evaluation of the implemented HIFU interference elimination method. and residual HIFU interference existed in the scanline signal. For quantitative analysis of residual HIFU interference as a function of target motion distance, correlation coefficients between the reference scanline and other scanline signals were calculated. Note that the reference scanline signal is defined as a signal obtained using the reference and the A-mode signal acquired in the case of no target motion. Additionally, the energy of each scanline was calculated by squaring and summing the scanline signals that were acquired using the oscilloscope. It was expected that the energy would increase as residual HIFU interference increases. The experiment was repeated 5 times to calculate the mean and standard deviation of both correlation coefficient and scanline energy. For this experiment of investigating the target motion effect, a single-element ultrasound transducer (NDT, Olympus Corp., Waltham, MA, USA) with a center frequency of 3.5 MHz and a \(-\)6 dB bandwidth of 2.5 MHz was used instead of the convex array. This transducer was completely inserted in the central opening of the HIFU transducer to acquire ultrasound A-mode signals. By doing so, the strong echoes from the rear surface of the HIFU transducer and the side of its hole, that occurred when the convex array was used, could be avoided. In addition, the single-element transducer facilitated placing the graphite rod in the on-axis beam profile regardless of moving the target, as compared to the convex array. A two-channel function generator (AFG3252, Tektronix Inc., Beaverton, OR, USA) was used to generate a 3.5-MHz, 5-cycle rectangular-shaped electrical pulse for the monitoring signal (i.e., a pulse length of about 1.42 us) and a 1.1-MHz, 120-cycle sinusoid signal for HIFU (i.e., a pulse length of about 109 us). These signals were amplified to 28.12 V\({}_{\text{peak-peak}}\) for the monitoring signal and 70.45 V\({}_{\text{peak-peak}}\) for HIFU by using two RF power amplifiers (75A250A, Amplifier Research Corp., Souderton, PA, USA; A150, ENI Corp., Rochester, NY, USA). The monitoring signal reflected from the graphite rod was received by a pulse-receive system (UT340, UTEX Scientific Instruments Inc., Mississaug, ON, Canada) in the pitch-catch mode with an amplified gain of 20 dB. An oscilloscope (DPO7054, Tektronix Inc., Beaverton, OR, USA) was used to record the received monitoring signal. A customized expander was used to protect the pulse-receive system from the high voltage output of the 55-dB power amplifier. The DC cancellation was conducted using a 257-tap band-pass filter to remove out-of-band noise of the scanline signals [21]; its cutoff frequencies were 2.5 and 4 MHz. All signal processing was conducted using MATLAB (Math-Works Inc., Natick, MA, USA).