## 4 Discussion
In addition, when evaluating the FWHM, the configuration with 2 lines ON/2 lines OFF and 1 line ON/1 line OFF seems to be the closer to the 1024 elements, but the measured values for all these configurations remain quite close. Moreover, a lower transmit energy compromises the image quality by diminishing the CR and CNR, from 7.71 dB (CR) and 2.99 dB (CNR), when using full active array, to 2.02 dB (CR) and \(-\)7.31 dB (CNR), respectively, for the configuration with 256 active elements. The results obtained experimentally are in good agreement with preliminary numerical simulations, and the image degradation is explained by the different values of the pitch in the \(y\) direction. By using the presented 4-D US platform, the power Doppler technique was exploited to detect moving particles. Here, only the full configuration array was used to prove the feasibility of the method, the experiments with lower number of elements being beyond the scope of this article. This method is particularly useful when examining superficial structures. The capability of obtaining entire volumes at a high frame-rate (used PRF acquisitions on blood vessel mimicking phantom was 2000 Hz) opens a wide field of vascular and cardiac applications. The feasibility of the ultrafast imaging is very important in the context of cardiac imaging. Here, the difficulty is the heart beating. Therefore, the high frame-rate allows performing the acquisition in less time in order to not be disrupted by heart movement [23]. The combination of hardware and software-based technologies provides full flexibility of the developed platform, its reconfurability being one of its major advantages. This makes it suitable for different US applications requiring 1024 channels or less in regular or optimized sparse arrays. As the US scanners are completely open systems, our platform can be programmed to support different strategies which typically cannot be implemented in conventional data flow architectures. Different methods can be investigated: novel transmission and/or reception methods and sequences, apodization windowing [35], non-conventional beamforming techniques, excitation amplitude and aperture control, etc., but also custom-data processing. Such a platform will indeed be crucial to objectivize the real adjunct and efficiency of advanced reconstruction strategy, aiming at providing similar image quality as from highly undersampled acquisition since they can be compared against ground truth available for a full configuration. Indeed, over-performing methods are often published without a true reference implementation of the acquisition, and data are generally virtually undersampled, lacking realism. The connectors of the US probe were chosen in order to allow adapting the impedance while for all 4 systems we have the possibility to have the HIFU (High Intensity Focused Ultrasound) option. Different studies have already illustrated the possibility to use HIFU for pre-clinical and clinical cardiac applications such as: treatment of arrhythmias [36; 37], atrial septostomy [38] or atrio-ventricular nodal-ablation in dogs [39]. Therefore, the developed 4-D US platform can be considered as dual-mode, imaging and therapy. This aspect can be exploited, even using low intensity therapies (the US probe presented in this manuscript is not developed for HIFU mode), for diagnostic applications in the heart, for example. Here, the major interest is represented by the use of the same system for imaging and therapy. One of the technical limitations of this platform when using the described US probe involves the physical addressing of the elements. When using 512 active elements in configurations \"middle ON\" and \"4 lines OFF/4 lines ON\", we still need 4 synchronized US systems even if not all the channels are used. This issue could be improved by using US probes with a different addressing connectivity.