## 5 Discussion
Given the characteristics of the typical conical volumes to be encountered in a clinical setting, the 8 MLT, 9 MLT and 10 MLT systems seemed to be able to generate high frame rate images while preserving an appropriate image quality as these implementations reached our predefined threshold of \(-30\) dB cross-talk. Even though this threshold was at first chosen from retrospectively analyzing our MLT findings in 2D [22], our recent 3D in vitro experiments corroborated that this threshold is indeed a reliable proxy for good image quality [13]. In fact, very recently [PERSON] et al. [23] demonstrated both in vitro and in vivo that a 12 MLT system can generate 2D images of clinically diagnostic value while this system had inter-beam cross-talk levels well above our pre-defined \(-30\) dB threshold [11]. As such, our \(-30\) dB threshold was likely conservative implying that also the 12 MLT system tested in the context of anatomical imaging remains an attractive option that should be evaluated experimentally. Overall, the combination of MLT and the definition of an anatomic FOV would thus imply a gain in frame rate up to 24 (considering 12 MLT an option). As it has previously been proposed that at least 4 MLA (two in azimuth and two in elevation) can be reconstructed per focused beam for a typical 2D cardiac matrix array transducer [14], this implies that the proposed sequences could effectively gain a factor up to 96 in frame rate at full line density. Similarly, if an MLT-4 MLA sequence is implemented an actual frame rate gain factor of 64, 72 and 80 could be obtained when using 8 MLT, 9 MLT and 10 MLT beams respectively. Finally, in order to further improve the temporal resolution of the data set, line density and/or interpolating the received scanline data could be considered. As such, if required for the desired cardiac application, frame rates in the order of hundreds or even thousands of Hertz could be achieved if a sparse anatomical scan sequence is implemented. For example, according to the current guideline for cardiac chamber quantification [1], for the assessment of regional left ventricular (LV) function (e.g., wall motion or regional strain), the ventricle is divided into segments. One of the segmentation models commonly used divides the apex, the basal and midventricular levels, in six segments each. Resulting in an 18-segment model. As such, as little as six beams would be required to scan the left ventricle with a clinically relevant spatial sampling (i.e., one sample per LV segment). Thus, using MLT beamforming, a single transmit event could evaluate all LV segments defined this way at a volume rate of 5 kHz (i.e., equal to the Pulse Repetition Frequency (PRF) typically used for cardiac imaging). However, six beams around the left ventricular myocardium may be too few to correctly resolve cardiac kinematics. As a consequence, a trade-off between the frame rate and the spatial sampling of the volume has to be made. For instance, if the 12 MLT-4 MLA sequence is applied to scan the FOV, 48 image lines could be reconstructed per transmit event. If five transmit events are then performed, 240 samples around the circumference could be acquired at a frame rate of 1 kHz. Depending on the application, the transmit events could be performed radially or circumferentially,while properly staggering the scanning sequence should ensure that the inter-beam distance remains constant. Although such sequence would result in inhomogeneous spatial sampling (i.e., transmit beams are further apart than MLA beams), the MLA beams could be used to assess local kinematics (e.g., 3D motion) while the MLT beams allow for sampling different segments of the LV simultaneously. Only MLT systems with 8 to 12 parallel beams were evaluated in this study, as this was the range of number of beams that gave a significant increase in frame rate while having a moderate cross-talk energy level.