## 4 Discussion  
In this study, we investigated the impact of two types of orthogonal codes on phased array based MLT imaging schemes in order to see whether they could be of (added) value to suppress the associated crosstalk artifacts. The performance of these coded imaging schemes was investigated without and with TT apodization and benchmarked against non-coded MLT schemes with TT apodization in both simulation and experiments. The results show that without apodization, for a 2 MLT scheme, the crosstalk was completely removed using orthogonal Golay codes with a 55 dB dynamic range, whereas it was slightly suppressed with the orthogonal chirps (Figures 6 and 9). Indeed, this implied extremely low cross-correlation between the used orthogonal Golay pair. Compared to a non-coded non-apodized four or 6-MLT scheme, a coded non-apodized four or 6-MLT scheme showed less crosstalk artifacts and the appearance of its artifacts largely replicated that of a non-coded non-apodized two or 3-MLT scheme, respectively (Figure 7). This was to be expected given that only two sets of orthogonal codes were applied on neighboring MLT beams in an interleaved manner. Hence, to suppress the remaining crosstalk, TT apodization was applied (Figure 8). While almost no crosstalk artifacts could have been seen on the TT-apodized coded images (Figure 8), crosstalk quantification showed that the chirp-coded schemes had slightly higher crosstalk level than that of the Golay-coded ones (Figure 9). This was in agreement with the fact that the cross-correlation of the two \"orthogonal\" chirps remained high due to the overlapping sweeping frequency bands. The information in two different directions can thus not be completely separated. Moreover, in the near field, the PSF of the chirp-coded 6 MLT scheme distorted [e.g., Figure 8f the scatterer at a depth of 30 mm]. This might be due to the fact that the inter-beam space was very small for the 6 MLT scheme when it was close to the probe, since the duration of the chirps were relatively long, the level of interference between simultaneously transmitted waveforms could be too high to be well separated in the near field. Nonetheless, with respect to crosstalk artifacts, experimental images of the apodized coded MLT schemes showed similar quality compared to that the non-coded schemes (Figures 8 and 11-13). It is important to note that the effective gain in frame rate of the Golay-coded schemes was compromised by a factor of two due to the transmission of complementary codes (Table 1). Concerning spatial resolution, the coded schemes had slightly worse axial resolution whereas the lateral resolution was almost not affected (Figure 10); the latter, as is known, is affected by TT apodization. Moreover, as expected, the coded schemes had significantly improved the penetration, SNR, and contrast, especially at larger depths (Figures 11 and 12, Table 1). This was also expected given that more energy was transmitted into the tissue with the coded schemes given the increased time-bandwidth product [28]. It should be noticed that the transmit power of different imaging schemes was not normalized in order to make the different imaging schemes operating at their individual optimal conditions. Furthermore, the different blind regions of the experimental images are due to different signal lengths; indeed, longer excitations keep the transmitters switched on for longer times, during which the receivers are disabled. Regarding crosstalk reduction, the tested Golay and chirp codes did not show much added value, and indeed, the apodization is still mandatory; the number of MLT beams is usually larger than the number of the orthogonal codes, hence the cross-talk cannot be perfectly suppressed; the frame rate might be decreased by a factor of two when complementary codes are needed. On the other hand, coded signals are beneficial in terms of SNR and penetration depth that are limited by MLT transmission schemes.