# Contrast-Enhanced Ultrasound Imaging Based on Bubble Region Detection
## 3 Experiment and Results  
### Phantom Results  
Two types of phantom experiments were designed. In the first, there were two wall-less pipes with different thicknesses in the phantom experiment (0.3 mm and 0.5 mm). In the other experiment,  
Figure 5: The fitted eigenvalue curve.  
Figure 6: The fitted maximum ESBCF index curve.  
we placed a piece of fresh pork over the phantom, which only had one single pipe (about 0.5 mm). The detailed parameters for the phantom experiment are illustrated in Table 2.  
Figure 7 provides the first phantom result (transmit frequency: 3 MHz, transmit voltage: 10 V). Figure 7a-c show the imaging using traditional DAS, MV, and ESBMV beamformer without BWT, respectively. Figure 7d is the image using BWT based on ESBMV beamformer and Figure 7e is the image with bubble region detection by setting the maximum eigenvalue and ESBCF index threshold based on Figure 7d. Some impurities in the phantom are removed and the two tubes filled with microbubbles are well-preserved using the proposed method.  
Figure 8 provides the pork experiment results. As shown in Figure 8, DAS suffers from severe artefact interference, althouth the artefact interference was weakened with MV. The image CR can be further improved with ESBMV, although an obvious black region distortion appears inside the tube. The brightness of the UCA inside the tube became more uniform and UCA information loss could be solved with the help of BWT, although the interference of tissues still exists. In comparison, for bubble region detection, the region except for the microbubble was able to be fully eliminated.  
\begin{table}
\begin{tabular}{c c} \hline
**Experiment Parameters** & **Value** \\ \hline Transducer element number & 128 \\ Transducer element kerf & 0.05 mm \\ Transducer element width & 0.27 mm \\ Transducer element pitch & 0.3 mm \\ Transducer spacing between elements & 0. 295 mm \\ Transmit frequency & 3 MHz, 4 MHz, 5 MHz, 6.25 MHz \\ Transmit voltage & 1.6 V, 2.5 V, 5 V, 7.5 V, 10 V, 12.5 V, 15 V, 17.5 V, 20 V \\ Transmit pulse & Sine wave with two cycles \\ \hline \end{tabular}
\end{table}
Table 2: Parameters for the experiment.  
Figure 7: _Cont._  
Figure 8: The image results of phantom experiment (with pork): (**a**) DAS; (**b**) MV; (**c**) ESBMV; (**d**) BWT; (**e**) bubble region detection.  
Figure 7: The image results of the phantom experiment (without pork): (**a**) DAS; (**b**) minimum variance (MV); (**c**) Eigenspace-based minimum variance (ESBMV); (**d**) BWT; (**e**) bubble region detection (dynamic range is 60 dB).  
CTR was calculated to describe the performance of different beamformers:  
\[CTR=20\log\frac{I_{UCA}}{I_{tissue}} \tag{14}\]  
where \(I_{UCA}\) is the average intensity of the UCA region and \(I_{tissue}\) is that of the tissue region. The two regions were enclosed in a rectangular area (the upper white one is the tissue region and the lower red one is the UCA region).  
Figure 9 shows the CTR curve between different beamformers under different transmit conditions. The performance of bubble region detection was remarkable. Figure 10 explains that, with bubble region detection, the CTR has an enhancement of 7.5 dB on average compared with ESBMV, and an enhancement of 4.2 dB compared with BWT.  
Figure 10: Increase in CTR with bubble region detection: (**a**) Increase in CTR between region detection and ESBMV; and (**b**) increase in CTR between region detection and BWT.  
Figure 9: The image contrast-to-tissue ratio (CTR) curve between different beamformers: (**a**) 3 MHz; (**b**) 4 MHz; (**c**) 5 MHz; and (**d**) 6.25 MHz.  
In the bubble region detection, the choice of eigenvalue and ESBCF index are two critical factors. Taking the pork experiment as an experiment, the pork and artifact disturbance remain under a low threshold while part of the UCA information is also removed with a high threshold, as shown in Figure 11.