# Shear Wave Elastography Implementation on a Portable Research Ultrasound System: Initial Results
## 2 Materials and Methods
### Push Beam Generation  
The us4R-lite research platform with the TXPB-256 extension module, as described above, was used in this work to generate various push beams alongside a 128-element L7-4 linear array probe (Philips Healthcare, Amsterdam, The Netherlands) with a center frequency of 5 MHz and 0.3 mm of pitch. Three different push beams generation methods were used, all of which are presented schematically in Figure 4.  
The details of each method were as follows.  
* FCUSE: The first method followed the principle that was proposed by [PERSON] et al. [21]. The transducer aperture was divided into three groups of 42 elements, each simultaneously producing a focused beam at a depth of 30 mm (\(F/\#=2.4\)). The push length was 800 us.
* SSI: The second method followed the principle that was proposed by [PERSON] et al. in [4]. Three acquisitions were used in total, each producing three laterally aligned beams that were focused at successive depths, which were generated sequentially by increasing the aperture size along with focusing the depth to maintain a constant f-number. The consecutive beams used 24, 50 and 66 elements and were focused at depths of 15 mm, 30 mm and 40 mm, respectively (constant \(F/\#=2\)). The single push length was 80 us.
* FCUSE-SSI: In the third method, we proposed a combination of the two previous methods. Two focused beams were generated simultaneously, as in FCUSE, and then the beams were focused at consecutive depths and generated sequentially, as in SSI. It was equivalent to the generation of one FCUSE beam after another, increasing the focal point of the beam in each iteration. The consecutive beams used 24, 50 and 64 elements and were focused at depths of 15 mm, 30 mm and 40 mm and resulted in \(F/\#\) values of 2, 2 and 2.1, respectively. The single push length was 120 us.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Feature** & **Value** \\ \hline TX channels & 256 \\ HV transmit voltage & Configurable, up to 180 Vpp \\ Output current per channel & 2 A or 4 A \\ Transmit frequency range & 0.1–50 MHz 1  \\ Transmit aperture & Arbitrary \\ Transmit pulse capability & Arbitrary 3-level square wave pattern for each channel \\ Transmit apodization & Yes 2 \\ Transmit delay & 0–327 μs \\ Timing resolution & 5 ns \\ Theoretical maximum PRF & 100 kHz \\ \hline \hline \end{tabular}
\end{table}
Table 1: The transmit capabilities of the us4R-lite research platform with the (TXPB-256) extension module.  
Figure 4: The push beam patterns that were used in this study: **(a)** FCUSE; **(b)** SSI; **(c)** FCUSE-SSI.  
A push pulse frequency of 4.4 MHz was used for all experiments. For a fair comparison of each method, the total transmitted energy for all schemes was kept at the same level. The energy transmitted from a single transducer element was obtained as in [22]:  
\[E=\frac{V_{rms}^{2}}{Z}\cdot T \tag{3}\]  
where \(V_{rms}\) is the root mean square of the excitation voltage waveform, \(Z\) is the transducer element impedance and \(T\) is the push pulse duration. The total energy is equal to the sum of the transmitted energy from all of the individual elements in all acquisitions. Because the same probe was used at the same center frequency of the push pulse in all cases, \(Z\) was assumed to be constant, as well as \(V_{rms}\). Thus, only the total number of active elements and the total push duration could be used to compare the transmitted energies.