## 4 Discussion  
This study illustrates the development and feasibility of a real-time 4-D US imaging platform in full array mode by synchronizing emission and reception of up to 1024 elements independently. The initial experiments conducted in this study demonstrate the flexibility of the modular US platform, having the possibility to reconfigure it according to the number of available Vantage systems (between 1 and 4). This allows us to use full array, dense/halves/quarters of the array or to downsample (regular or sparse) the full array, as needed. Different imaging strategies were investigated using this US platform, including plane, diverging or focused waves and also high frame-rate imaging techniques. Moreover, a Doppler detection strategy was implemented as a first application by using volumetric fast imaging. Preliminary numerical simulations indicated the image quality for some cases with full and with regular downsampled array when using the 32 \(\times\) 32 US matrix. This allowed us to understand how the image quality will change for different configurations and to determine the best physical addressing of the US probe final design. The experimental data obtained when using the same configurations of the transducer showed the feasibility of real-time 4D US acquisitions in simultaneous mode in different systems. As expected, the best configuration in terms of image quality was obtained experimentally when the full matrix array was active while using 4 Vantage systems. The worst configuration included only 256 elements, with only one US system was required. At the same time, this is the only configuration  
Figure 8: Examples of Power Doppler illustrations of two US acquisitions on a blood vessel mimicking phantom. The US probe is parallel (\(\theta\) = 0) to the phantom-tube for the first example (first row) while the probe was tilted at an angle of 12\({}^{\circ}\) from the phantom-tube. Axial (**a**), lateral (**b**) and 3-D view rendering (**c**) is illustrated here. A dynamic of 50 dB has been used for the B-mode grayscale image. allowing processing of the data and displaying the image in real time, this issue being very important for positioning and guidance. Future work will be focused on developing strategies for increasing the image quality when using one single system, in order to be able to provide images of clinical use, especially for moving organs. The implementation in synchronous mode will follow an increase in the number of active elements, but the data must be reprocessed offline. Knowing that 3-D volume images are acquired, the data offer great utility for offline analyses even after the examination is done, reducing repeat examination and making it more cost-effective for the patient. The integration of the four 256-channels systems into a single 1024-channels system is theoretically possible, but requires further technical developments of the constructor. Such an evolution could facilitate the transition to clinical applications, opening the possibility to collect and process the data acquired by all 1024 channels in near real-time, improving the image quality. Overall, when inspecting the (\(x\), \(z\)) plane positioned in the center of the transducer array, no major differences were detected due to the same number of active elements (32) in the \(x\) direction with all the tested configurations. When using a larger pitch (in \(y\) direction) for a reduced number of elements, more important secondary lobes are visible in the (\(y\), \(z\)) plane and therefore, we have lower image contrast. Depending on the pitch value, the secondary lobes appear more or less visible, illustrating the degradation of the images. The same result is obtained when using PW and DW. Among the compared configurations with 512 elements, in the near-field, no visual difference is observed when using the same number of elements, while in the far-field and on the lateral parts of the images, the contours are seriously affected when using only the central part of the US probe (see Figures 5 and 7).