## 3 Results and Discussion
For example, a shear wave elastography technique based on plane-wave transmission has already been commercialized and it is widely used in a clinical setting (e.g., liver fibrosis evaluation) [11; 23]. In shear wave elastography, the position and size of the ROI map visualizing tissue elasticity are changed according to the position and size of the interest area (i.e., lesion) [24; 25]. In that case, e.g., the number of plane-waves and the degree of steering angles, can be adjusted for providing comparable image quality and field-of-view [9]. Therefore, transmit delays need to be recalculated while the transmission parameters are adjusted, and the proposed method can improve the computational efficiency. Moreover, it is useful for supporting emerging applications where high image quality is necessary by increasing the number of plane-wave transmission. To demonstrate the feasibility of the proposed method in terms of image quality enhancement, a tissue-mimicking phantom (040 GSE, CIRS Inc., Norfolk, VA, USA) was scanned using a C5-2 curved array transducer and a programmable ultrasound research platform (Vantage 128, Verasonics Inc., Redmond, WA, USA) with 9 or 65 plane-wave insonifications. In addition, in vivo abdominal data were obtained from a healthy volunteer under Institutional Review Board (IRB) approval. The contrast ratio (CR) and contrast-to-noise ratio (CNR) [26] were also measured in the phantom study as the following:  
\[\text{CR}=20\log_{10}\left(\frac{\mu_{cyst}}{\mu_{bkgd}}\right) \tag{4}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Variables** & \multicolumn{2}{c}{**Conventional**} & **Proposed** \\ \hline \(N_{de}\) & \(N_{PW}\) & \(k_{m}\) & Computation Time (\(\upmu\)s) & Computation Time (\(\upmu\)s) \\ \hline \multirow{4}{*}{128} & 9 & 64 & 19.6 \(\pm\) 0.7 & 8.4 \(\pm\) 0.7 \\  & 128 & 19.4 \(\pm\) 0.7 & 8.9 \(\pm\) 0.8 \\  & 65 & 64 & 78.6 \(\pm\) 0.5 & 8.4 \(\pm\) 0.7 \\  & 128 & 78.8 \(\pm\) 0.8 & 8.4 \(\pm\) 0.2 \\ \hline \multirow{4}{*}{192} & 9 & 96 & 32.6 \(\pm\) 0.6 & 8.5 \(\pm\) 0.2 \\  & 192 & 32.8 \(\pm\) 0.7 & 9.1 \(\pm\) 0.2 \\ \cline{1-1}  & 65 & 96 & 148.2 \(\pm\) 2.4 & 8.8 \(\pm\) 0.8 \\ \cline{1-1}  & 192 & 149.0 \(\pm\) 0.7 & 9.3 \(\pm\) 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Total computation times of Equation (1) for the conventional method and proposed method. \[\text{CNR}=\frac{\left|\mu_{cyst}-\mu_{bkgd}\right|}{\sqrt{\sigma_{cyst}^{2}+ \sigma_{bkgd}^{2}}} \tag{5}\]  
where \(\mu_{cyst}\) and \(\mu_{bkgd}\) are the mean intensities of the image in the cyst and the background regions, respectively, and \(\sigma_{cyst}\) and \(\sigma_{bkgd}\) are the corresponding standard deviations of the intensities in the regions. Figure 7 represents the B-mode images of the tissue mimicking phantom by using nine or 65 plane-waves. As shown in Figure 7, the image based on 65 plane-waves has clearer cyst regions (i.e., yellow dotted circle) than the image based on nine plane-waves. Quantitatively, the measured CRs are \(-11.3\) dB and \(-22.5\) dB for the 9-plane-wave image and the 65-plane-wave image, respectively. Additionally, the CNRs of the 9-plane-wave image and the 65-plane-wave image are 1.9 and 2.7, respectively. These results show the 65-plane-wave image has much higher contrast resolution. Moreover, Figure 8a shows the result of B-mode image for the in vivo liver data acquired using only nine plane-waves, which suffers from very low contrast resolution. On the other hand, Figure 8b, which shows an image acquired using 65 plane-waves, can provide a clear depiction of a hepatic vessel (yellow arrows) due to increased contrast. Figure 7: B-mode images of a tissue-mimicking phantom acquired using (**a**) nine plane-waves and (**b**) 65 plane-waves. The yellow dotted and red circles in each image indicates the cyst and background regions, respectively, which are used for calculating the contrast ratio (CR) and contrast-to-noise ratio (CNR).