## 5 Discussion
Although, parasternal view acquisitions are mostly used to asses aortic and pulmonary valves, they are also used for circumferential strain evaluation which is not possible to be appraised with the current approach. Due to mainly hardware limitations, in vitro and in vivo experiments of a MLT implementation could not be studied and therefore the capability of MLT-MLA systems to precisely record the motion of the heart could not be corroborated. On the one hand, transmitting into several directions simultaneously requires the application of non-identical electric excitation pulses (EEPs) to all elements in the phased array transducer. These pulses are defined as the sum of the EEPs that would be applied on the individual elements when emitting the transmit beams separately. As this sum will result in the superposition of the EEPs, to date, MLT implementations were done on systems having arbitrary waveform generators (AWG). This is particularly relevant for 3D imaging as the superposition of EEPs can become significant (e.g., up to 16 for a 16 MLT imaging sequence). However, as most of the available scanners are equipped with a tri-state pulse generator, the implementation of an MLT sequence on such scanner is not straightforward. One way of overcoming this limitation is by clipping the superposed MLT sequences to the maximum of the available pulser. It has been found in preliminary tests that the clipped EEPs are capable of well-resembling the ideal MLT sequence, despite the drop of the transmit energy which might reduce the capability of the acoustic wave to propagate in depth and might decrease the acoustic pressure. On the other hand, matrix array transducers (essential to 3D imaging) have between 2000 and 4000 individual elements which makes wiring costly and impractical. Several methods have been proposed to limit cabling between the US probe and the back-end of the console. Among these methods is subaperture beamforming (SAP) which is most commonly implemented in clinical scanners. SAP divides the transducer aperture in subsets of elements that are beamformed inside the probe and the resulting signal is transferred through a single wire. In this way, the number of wires required to connect the transducer and the back-end of the console are reduced to the number of \"sub-apertures\". Therefore, beamforming is typically done in two phases where part of the beamforming is done in the transducer itself while the pre-beam formed signals are combined in the front-end in order to generate the individual image lines (i.e., two-stage beamforming [27]). Recently, it has been shown that severe imaging artifacts are introduced while using this technique for DWI [28]. This is due to delay errors which cause grating lobes and reduced penetration. Thus, due to the complexity of MLT sequences, an adaptation for clinical transducers using SAP must be implemented, which represents a challenge that needs to be investigated. Moreover, current state-of-the-art commercial systems are fully equipped to beamform MLA beams in real time. Therefore, the translation of the proposed method into commercial systems remains challenging, since this would not only require changing imaging methodology but also redesigning hardware.