# Shear Wave Elastography Implementation on a Portable Research Ultrasound System: Initial Results
## 2 Materials and Methods  
### Ultrasound System Architecture  
The architecture of the us4R-lite (us4 us Ltd., Warsaw, Poland) research platform is depicted in Figure 2. The system architecture was optimized for the implementation of the software-defined ultrasound paradigm by featuring high-speed data acquisition and streaming. The system was built around two us4 OEM (us4 us Ltd., Warsaw, Poland) ultrasound front-end modules, each containing both transmit (TX) and receive (RX) electronics. Each module has individually controlled 128 TX and 32 RX channels, totaling 256 TX and 64 RX channels for the us4R-lite system. The transmitter part is based on 3-level transmit pulser-integrated circuits that drive individual probe elements with high-voltage (HV) bipolar square waves. The receiving path consists of an amplifying stage (a low-noise amplifier and time-gain control amplifier), followed by an analog to digital converter (ADC). The transmit and receive paths are connected in the transmit/receive (T/R) switch, which connects the transducer elements to either the transmitter or receiver subsystem via multiplexing. Both the TX and RX parts are controlled by a field-programmable gate array (FPGA) that is configured with custom firmware. The FPGA is also responsible for the execution of predefined TX/RX acquisition sequences, which need to be programmed by the software prior to the acquisition. For TX operation, the FPGA generates the transmit patterns that are to be amplified by the TX pulser. On the RX side, the FPGA captures the output data  
Figure 2: A simplified system architecture of the us4R-lite portable ultrasound research platform. Some connections are hidden for clarity. Abbreviations: T/R, transmit/receive; A/D, analog to digital; FPGA, field-programmable gate array; SDK, software development kit; HV, high voltage. streams from the ADCs, which are buffered in the off-chip DRAM. As there is no integrated hardware-based data processing, all of the acquired RF data are fed into the PC-based back-end computing. The lack of demodulation and beamforming results in a large volume of data being transferred to the PC, especially in real-time applications [19]. For this reason, the data in the us4R-lite system is streamed to the PC over a high-speed Thunderbolt-3 or PCIe 3.0 interface with an effective bandwidth of 2.5-6 GB/s. The other components that comprise the system include clock generation and synchronization circuits, a power supply (which generates all of the required voltage levels for the other subsystems) and a HV power supply (which generates high-voltage bipolar waves for the transducer excitation). All of the subsystems are connected by digital and analog backplanes. The probe adapter is an exchangeable interface that supports various ultrasound probes. Despite the promising results of the system evaluation of push pulse generation in the previous work by our team [20], multiple TX pulsers were damaged during the preliminary SWE test runs due to HV overshoots or overheating. Therefore, in order to increase the robustness of our system's transmit capabilities, we designed a dedicated 256-channel transmitter subsystem, called the transmit push beamformer board (TXPB-256). Taking the form of an extension board that replaces the analog backplane (see Figure 2), this system supersedes the transmitters and \(\mathrm{T/R}\) switches on the us4 OEM boards. The architecture of the TXPB-256 board is shown in Figure 3.  
The TXPB-256 utilizes new generation 16-channel STHV1600 transmit pulser ICs (STMicroelectronics, Geneva, Switzerland). Each of these pulsers consists of analog and digital parts. The analog part implements the high-voltage output drive and \(\mathrm{T/R}\) switch functionality. The digital part consists of a digital transmit beamformer and a set of registers to configure the transmit aperture, transmit delay profile, output waveforms and other minor settings.