## 3 Results  
The simulated PSFs of different MLT schemes without and with coded excitation are presented in Figures 6-8. Without apodization on transmit and receive, no crosstalk was seen in the image of the Golay-coded 2-MLT scheme (Figure 6c) within a 55 dB-dynamic range, whereas a slight crosstalk suppression was observed in the image of the chirp-coded 2-MLT scheme (Figure 6d, highest crosstalk at about \(-\)46 dB) compared to that of the non-coded 2-MLT scheme (Figure 6b, highest crosstalk at about \(-\)40 dB). For the non-apodized coded 4-MLT and 6-MLT schemes, obvious crosstalk artifacts was observed (Figure 7c-f), though less artifacts were found on the Golay-coded images than on the chirp-coded images in particular in the near field. With TT apodization, most of the crosstalk was suppressed within the 55 dB-dynamic range (Figure 8), despite the fact that the image of the point scatterer at the depth of 30 mm was severely distorted for the TT-apodized chirp-coded 6 MLT scheme. Quantitative crosstalk evaluation is shown in Figure 9. It was found that, without apodization and for the same number of MLT beams, Golay-coded MLT schemes presented the lowest crosstalk level. In particular, zero level crosstalk was detected for Golay-coded 2-MLT schemes. Moreover, with apodization, the crosstalk level of both Golay and chirp-coded 4 MLT and 6 MLT schemes was significantly reduced compared to that of the non-apodized ones. In general, despite of the apodized chirp-coded 6 MLT schemes, all apodized coded MLT schemes showed lower crosstalk compared to the corresponding non-apodized non-coded MLT schemes. In addition, for the mean radial beam widths, all imaging schemes showed similar values (around 1 mm), although Golay-coded schemes had slightly larger values (about 1.07 mm) (Figure 10, top plot). With respect to lateral beam widths, the non-apodized coded schemes showed similar widths, which was about 2.30 mm, whereas the values of apodized schemes were around 2.89 mm (Figure 10, bottom plot). Based on the simulation results, TT-apodized 4 MLT and 6 MLT schemes were tested in in-vitro experiments. Results are presented in Figures 11 and 12 where in Figure 12 the cystic regions were zoomed for better visualization. The images of the tissue phantom presented in Figure 11 showed that the crosstalk artifacts were hardly observed for all MLT schemes. However, the coded images showed better contrast for both the middle and bottom cystic regions (Figure 12). Moreover, coded images presented less electronic background noise compared to the non-coded ones (Figure 11, indicated by the arrows). Quantitatively, seven regions of interest (ROI) were defined on those images to compute the SNR, CNR, and CR. As indicated in Figure 11a, three ROIs were defined in the three cystic regions by adjusting the radius of the cysts, whereas three ROIs were accordingly defined in the hyperechoic background next to individual cysts. The CNR and CR of each cystic region were thus computed using Equations (2) and (3). Moreover, the last ROI was defined in the noise region (Figure 11, N\({}_{\text{ROI}}\)), where the SNR was computed based on signal in N\({}_{\text{ROI}}\) and B\({}_{\text{ROI}}\) using Equation (4). The results are presented in Table 1. It can be seen that in terms of CNR, all MLT imaging schemes showed similar values for the top and middle cysts, whereas for the bottom cyst, the coded 4-MLT and 6-MLT schemes showed higher values than that of the non-coded 4-MLT and 6-MLT schemes, respectively. In terms of CR, except for the top cyst, coded schemes showed about 2.1-5.5 dB higher CR compared to that of the non-coded ones; the chirp-coded MLT schemes presented the highest CR values. Similarly, with respect to SNR, the coded schemes had about 2.6-5.4 dB higher SNR than that of the non-coded ones; still, the Golay-coded performed the best and presented the highest SNR. Figure 6: Point-spread-function (PSF) of different MLT schemes without apodization on transmit and receive.