## 1 Introduction  
Noninvasive monitoring of blood flow in the common carotid arteries is important for evaluating not only cerebrovascular diseases [1, 2] but also cardiovascular diseases [3, 4]. Among various imaging modalities, Doppler ultrasonography is a popular tool for measuring blood flow velocity and its variability in the common carotid arteries [5, 6, 7]. In carotid Doppler ultrasonography, spectral broadening with an elevated peak systolic velocity (_PSV_) on a pulsed-wave spectral Doppler waveform is correlated with the degree of carotid stenosis [8, 9, 10, 11]. In addition, carotid Doppler ultrasonography can provide quantitative functional measurements of blood flow, e.g., the resistive index, _RI_, which is a hemodynamic parameter reflecting local wall extensibility and related vascular resistance [12, 13, 14].  
Moreover, if Doppler ultrasonography can continuously measure the blood flow of the common carotid artery, increased intracranial pressure can be assessed in patients with head trauma, stroke, brain tumor, or hydrocephalus [15; 16; 17]. Additionally, by intraoperatively monitoring emboli during endovascular or surgical procedure of aorta, heart, or carotid arteries, the risk of potential stroke and brain injury can be reduced [18; 19]. However, utilizing a conventional cart-based Doppler ultrasound system is challenging for continuous assessment of the common carotid artery due to its limited accessibility in terms of cost and size.  
Recently developed portable and handheld ultrasound imaging systems feature substantially reduced sizes and costs for bedside and intraoperative procedures, but they are still inconvenient for use in continuously monitoring the common carotid artery. To lower operator dependency, in the last two decades, various types of wearable ultrasound Doppler device prototypes have been investigated [20; 21]. For monitoring blood flow in the common carotid artery, the 'Doppler Necklace' system, in which an ultrasound array transducer was mounted to acquire blood flow signal, was proposed. However, it was not sufficient to validate the concept of the 'Doppler Necklace' since theoretical analysis and simulation results were limited [20; 21; 22; 23; 24]. Another wearable carotid ultrasound device, in which two 128-element linear array probes were mounted similarly to the original design, was also proposed. Although this carotid ultrasound device can successfully visualize common carotid arteries to measure their diameters, it cannot evaluate blood flow velocity and its variability [24]. In this paper, a new wireless carotid wearable ultrasound Doppler system, in which two single-element ultrasonic sensors are mounted in the neckband for acquiring Doppler signals from both carotid arteries, is proposed for continuously measuring blood flow velocities and functional index parameters (e.g., _RI_). In the developed wireless carotid neckband ultrasound Doppler system, the acquired Doppler signals are quantized, and pre-processing operations (i.e., demodulation and clutter filtering) are performed on a field programmable gate array chip (Sparatan-6 LX150, Xilinx Inc., San Jose, CA, USA). Then, these data are transferred to an external smartphone via Bluetooth 2.0. Post-processing (i.e., Fourier transform and image processing) is performed using an embedded application processor in the smartphone. The developed wireless carotid wearable ultrasound Doppler system prototype was evaluated with a string phantom and in vivo studies.