## 4 Results  
The experimental platform was set up as shown in Figure 11a. It consisted of a workstation, the breast model (Figure 11b), transducer configuration (Figure 11c), ultrasound transmitting/receiving circuits, water tank, and a rotary table controller [22]. A key component of this system was the transducer configuration, which consisted of four vertical ultrasonic transducers in a cross shape surrounding the breast model to achieve non-invasive detection. By controlling the electric rotary table controller through a PC workstation, the electric rotary table induces the PMUT to perform cylindrical scanning. A 64-channel ultrasonic transmitting/receiving acquisition circuit with a sampling frequency of 40 MHz was employed to control the linear array. The breast volume data were obtained by performing cylindrical scanning with a constant interval. Besides, the rotation interval angle is adjustable. A 1 \(\times\) 128 PMUT linear array was utilized, with a center frequency of 3.5 MHz, consistency of \(\pm\)1 dB at 3.5 MHz, and a bandwidth of 86.7%. The breast model was customized (Shenzhen Well Come Technology co. LTD, China), and its overall dimensions were \(15.5\times 13.5\times 8\) cm. The breast phantom was made from a proprietary gel with a physical consistency similar to that of human tissue. The gel was surrounded by an elastic skin-like membrane and is suitable for ultrasound imaging research. The size of the embedded mass in the center was 5 cm, made of urethane rubber and colored for easy visualization. The attenuation coefficient was 0.6 dB/(MHz-cm), and the speed of sound used was 1520 m/s. The operating working range was from 2 to 4 MHz. The experiments were carried out in the water tank with a constant temperature of 32 \({}^{\circ}\)C.  
The ultrasound tomography test method and reconstruction slice scheme are shown in Figure 12. The four PMUT linear arrays were placed 90\({}^{\circ}\) cross vertically at 0\({}^{\circ}\), 90\({}^{\circ}\), 180\({}^{\circ}\), and 270\({}^{\circ}\). An electric rotary table controller was regulated by the workstation to drive the ultrasonic transducer to perform cylindrical scanning with a constant interval. Then, 128 horizontal slices were achieved by rotating the Nth element through a full circle. The circular scanning test diagram of one slice is shown in Figure 12a. In the experiment, the rotation interval was set to 2\({}^{\circ}\). Each slice consisted of 180 A-scan lines (RF line). A single line of the tomographic image was created from RF data received by a single element, and the image was reconstructed using the proposed tomography algorithm. A breast ultrasound horizontal slice  
Figure 11: The experimental platform with cylindrical motion of piezoelectric micromachined ultrasonic transducer (PMUT) linear array. (**a**) Experimental setup. (**b**) Breast model. (**c**) Transducer configuration. The breast horizontal ultrasound tomography images without TGC and with adaptive TGC were realized as shown in Figure 13, respectively. In the center of the breast model was a mass of approximately 5 cm. The incomplete contour was detected, and the experimental results were consistent with theoretical values. On the other hand, the border of the phantom was not complete. The main reason was the asymmetric shape of the breast model. The tilted side (right side) was not easily detected because the incident angle was relatively large. When dynamic focusing technology is used, the whole border of the phantom will be clearly visible. A slice without TGC compensation is shown in Figure 13a, and a slice with adaptive TGC is shown in Figure 13b. By contrast analysis, the improved algorithm with adaptive TGC detected the internal mass clearly; thus, the compensation for ultrasound attenuation was effective.