# Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging
## 2 Materials and Methods
### Experimental Arrangement  
An experimental arrangement was set to verify and evaluate the proposed method as shown in Figure 3. For the simultaneous delivery of HIFU and ultrasound for imaging, the ultrasound imaging  
Figure 2: Functional block diagram for the real-time HIFU interference elimination method implemented in a commercial ultrasound scanner. Rx stands for ultrasound reception. system in which the proposed method had been implemented was modified to generate a trigger signal whenever ultrasound was transmitted for imaging. This trigger signal was sent to a function generator (AFG3252, Tektronix Inc., Beaverton, OR, USA) that was responsible for generating a 1.1 MHz sinusoid wave with a pulse length of 136 \(\upmu\)s. \(T_{PRI}\) was set to be 500 \(\upmu\)s, which means that the pulse repetition time of HIFU was also 500 \(\upmu\)s and the duty cycle was 27.2%. Since the monitoring ultrasound images consisted of 128 scanlines, the frame rate was 7.8 Hz. The 1.1 MHz sinusoid signal was amplified by 49 dB with a RF power amplifier (75A250A, Amplifier Research Corp., Souderton, PA, USA) and passed to a ring-shaped HIFU transducer (H102, Sonic Concepts, Bothell, WA, USA) through an impedance matching network; the electrical signal with a voltage of 70 V\({}_{\text{peak-peak}}\) was used to generate HIFU. A convex array (C5-2, Verasonics Inc., Redmond, WA, USA) used for ultrasound imaging was placed in the central opening of the HIFU transducer. As a target, an excised bovine liver was placed on a 55 mm high BSA (bovine serum albumin) phantom immersed in a deionized-water-filled container. The distance between the convex array and the bottom of the phantom was 130 mm. Note that the fabrication process of the BSA phantom can be found in [20]. The HIFU and imaging transducers were also immersed in the deionized-water-filled container to induce coagulation in the bovine liver while acquiring ultrasound monitoring images. Note that the synchronized transmission of HIFU and imaging ultrasound was controlled by the imaging system as described above. Since the imaging system has a pulse inversion function with a minimal jitter between two imaging ultrasounds with a 180\({}^{\circ}\) phase difference, it is possible to avoid the problem of image quality deterioration due to asynchronous transmission. If two HIFU pulses with phase 0\({}^{\circ}\) and 180\({}^{\circ}\) are transmitted as in the previously proposed method, a HIFU system should have a jitter removal circuit. Otherwise, white straight lines may appear in ultrasound images [16]. Another experimental setup was constructed to ascertain the effect of target motion on the performance of the proposed method for HIFU interference elimination as shown in Figure 4. When the ultrasound image of a moving target is acquired, the target position changes every pulse repetition interval, i.e., \(T_{PRI}\) for imaging. Therefore, residual HIFU interference signal increases when a target moves quickly at a given \(T_{PRI}\). This is also true when \(T_{PRI}\) is increased at a given target motion speed. To mimic this situation, a 0.9 mm diameter graphite rod serving as a target was inserted in the BSA phantom. The graphite-contained phantom was placed in the deionized-water-filled container on a motorized stage (SGSP160-YAW, SIGMAKOKI Co. Ltd., Tokyo, Japan). In the beginning, the graphite rod was located at the focal length of the HIFU transducer, i.e., 62.6 mm. A pulse-echo signal \(r_{+}(t)\) (i.e., ultrasound A-mode signal) was acquired after transmitting ultrasound with a phase of 0\({}^{\circ}\) while delivering HIFU, which is called a reference in this paper. In the next step, both HIFU and ultrasound with a phase of 180\({}^{\circ}\) were simultaneously delivered to the target located at the same position to obtain \(r_{-}(t-T_{PRI})\). Since the target position did not change, the assumption for Equation (5) is valid: \(h^{\prime}(t)=h^{\prime}(t-T_{PRI})\) and \(u^{\prime}(t)=u^{\prime}(t-T_{PRI})\).