## 2 Experiments and Methods  
### 3D Imaging System with Cylindrical Motion of Linear Arrays  
We developed an original breast ultrasound imaging system with the cylindrical motion of four linear arrays. The setup was mainly used to diagnose breast lesions. Figure 1 shows the schematic diagram of the experimental setup and the connection of main components. Ultrasound imaging with the cylindrical motion of linear arrays is as shown in Figure 2. The transducer configuration scheme is shown in Figure 2a. The 1 \(\times\) 128 PMUT was placed 90\({}^{\circ}\) cross vertically on the circle with the length of the arrays parallel to the axis of the circle, which was roughly parallel to the central axis of the breast. Figure 2b shows the dimensions (7 cm \(\times\) 3 cm \(\times\) 13.5 cm) of the linear array transducer and the active area (1.8 cm \(\times\) 12.8 cm) of the transducer for imaging. The sensors were customized for the breast detection. The transducers surrounded the breast, which allowed for non-invasive detection. The detection was carried out in the water tank and the working temperature was constant at 32 \({}^{\circ}\)C. By controlling the electric rotary table controller through the PC workstation, the electric rotary table drives the PMUT to perform the cylindrical scanning. The cylindrical scanning diagram is shown in Figure 2c. The four PMUT linear arrays were arranged in a cross shape. The distance of the opposite PMUT was 180 mm and the array element center to center spacing was 1 mm. Hence, the maximum test aperture was 18 cm \(\times\) 13 cm. Set 1 (Adjacent PMUT) and Set 2 (Another adjacent PMUT) operate alternately. A 64 channel ultrasonic transmitting/receiving acquisition circuit was employed to control the linear array. The data acquisition circuits had a sampling frequency of 40 MHz. The breast volume data were obtained by performing the cylindrical scanning with a constant interval. For four linear array transducers, a full circle requires rotating the sensors 90 degrees. The step angle was set to 2\({}^{\circ}\), therefore, it took 1.2 s for each rotation and 3 s for the vertical slice data collection. Hence, the whole scan time was about 3 minutes using a HP workstation (16G). The smaller the interval angle, the longer the scan time. Besides, the rotation precision was 0.05\({}^{\circ}\), which allows for obtaining 7200 pulse echo signals. The optimized segmented dynamic focusing technology was used to improve the emission energy of the A-scan line, thereby improving the imaging quality. The high density was 256 scan lines and the low density was 128 scan lines, respectively; hence, the maximum data were 7200 \(\times\) 256 A-scan lines. Multi-perspective ultrasound imaging of the breast could be obtained through the data reconstruction where the data could be reconstructed to obtain 7200 vertical slices, 256 horizontal slices and a three-dimensional imaging. By adopting four rotatable linear arrays to perform cylindrical scanning, it could make the sensor's fabrication process easier and gain better consistency and reliability than a cylindrical array [30]. The 1 \(\times\) 128 PMUT linear array was characterized using a precision impedance analyzer (Agilent E4990A) as shown in Figure 3. The ultrasound transducer had a static capacitance of 664 p, an impedance of 64 \(\Omega\) and the operating frequency of 3.5 MHz [30]. Eight elements were randomly selected to test the transmitting sensitivity Sv and receiving sensitivity M as shown in Figure 3a. The average value of Sv is -220.5875 dB and the average value of M is 166.775 dB. The consistency is \(\pm\)1 [EMAIL_ADDRESS] MHz. The bandwidth experiment was conducted with the ultrasound transducer working  
Figure 1: Schematic diagram of the experimental setup. Figure 2: Ultrasound imaging with cylindrical motion of linear arrays. (**a**) Transducer configuration scheme. (**b**) The dimensions of the linear array transducer. (**c**) Cylindrical scanning diagram. as a transmitter and a standard transducer working as a receiver.