## Results and Discussion
### Phantom Study  
Figure 5a,b shows the Doppler spectrograms obtained from the commercial rack-based ultrasound machine (i.e., Accuvix V10) and the developed wireless carotid wearable ultrasound Doppler system prototype, respectively. From visual assessment, the developed prototype successfully visualizes the pulsatile flow waveform similarly to the commercial rack-based ultrasound machine. The systolic and diastolic phases are clearly differentiated, and comparable spectral broadening is observed in the two spectrograms. Moreover, two systems show parabolic velocity profiles, as shown in the right side of the Figure 5a,b, at the peak systolic phase indicated by white dotted lines. Compared to the  
Figure 4: (**a**) Prototype of the wireless carotid wearable ultrasound Doppler system and (**b**) the pre-processing module and the power module.  
commercial rack-based ultrasound machine, the developed prototype presents more noise spikes and lower frequency resolution (i.e., a blocky representation of the velocity).  
The \(PSV\) and \(RI\) values from the commercial machine and the developed prototype were measured, and the results are summarized in Table 1. As listed in Table 1, the \(PSV\) and \(RI\) values for the commercial machine and the prototype were \(131.49\pm 3.97\) and \(0.75\pm 0.02\) vs. \(131.89\pm 2.06\) and \(0.74\pm 0.02\), respectively. There is no statistically significant difference in the measured \(PSV\) and \(RI\) values between the commercial rack-based machine and the wearable Doppler device prototype (\(p=0.66\) and \(p=0.70\), respectively). In addition, the velocity component at a specific time was also similarly measured in the same manner as shown in Figure 5.  
Boxplots of the measured \(PSV\) and \(RI\) values are also shown in Figure 6. As indicated in Table 1, the commercial rack-based machine and the wearable Doppler device prototype show comparable results.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Accuvix V10 (\(n=10\))** & **Wearable Doppler** & **\(p\)-Value** \\ \hline \(PSV\) [cm/s] & \(131.49\pm 3.97\) & \(131\pm 2.06\) & \(0.64\) \\ \(RI\) & \(0.75\pm 0.02\) & \(0.74\pm 0.02\) & \(0.70\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: The measured peak systolic velocity (\(PSV\)) and resistive index (\(RI\)) values (mean \(\pm\) std) from the commercial rack-based Accuvix V10 machine and the developed wireless carotid wearable ultrasound Doppler system prototype.  
Figure 5: Doppler spectrogram obtained from (**a**) the commercial rack-based ultrasound machine (i.e., Accuvix V10, Samsung Medison Co., Seongnam, South Korea) and (**b**) the developed wireless carotid wearable ultrasound Doppler system prototype (PRF = 4 KHz). The velocity profile at a peak systolic phase, indicated by a white dotted line, is given in the right side of each Doppler spectrogram.  
In the developed system, the system PRF setting determines the measurable maximum speed of blood flow. For example, with the default setting (i.e., PRF = 4 kHz), the developed prototype is capable of measuring up to 150.0 cm/s in common carotid arteries, as shown in Figure 5b. With PRF = 6 kHz, the measurable speed is extended up to 225.0 cm/s without aliasing. Figure 7 shows the obtained result when using PRF = 6 kHz from the string phantom for an acquisition time of 10 s.