A novel, hands-free ultrasound patch for continuous monitoring of quantitative Doppler in the carotid artery

Figures

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  Figure 1. Metrics derived from the spectral Doppler signal for a single cardiac cycle. The velocity time integral (VTI) is the area under the maximum velocity envelope traced from the Doppler spectral waveform for a single cardiac cycle (highlighted in orange) bounded by the cycle time. VTI is also known as the stroke distance and is measured in centimeters.

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  Figure 2. Using Doppler ultrasound to measure blood flow velocity in the carotid artery. (a) Using conventional PW Doppler, the handheld transducer is placed on the neck, and the Doppler sample window (dark red) is positioned in the centre of the artery. The Doppler angle cursor (black dashed line, see inset) is then aligned with the direction of blood flow to obtain an accurate velocity measurement. (b) With a broad-beam CW Doppler patch, the device is placed on the neck with the wide beam intersecting the cross-section of the carotid at approximately 60°. Doppler signals are simultaneously acquired from the entire beam area. (c) An adhesive strip secures the CW Doppler patch on the neck to maintain its position and angle over the course of repeated measurements.

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  Figure 3. The CW Doppler patch. (a) System diagram showing the transmit and receive electronics paths. (b) Pressure map of the wide transmitted ultrasound beam measured in the lateral plane with a hydrophone, normalized to the peak pressure and log-scaled; with an outline of the approximate cross section of a 7-mm-diameter carotid artery superimposed on the beam (black dashed line). (c) Transmit beam pressure profile, log-scaled, at distance of 1 cm from the transducer showing the uniformity of the pressure amplitude (± 0.7 dB) over the active region. (d) Exploded view of CW Doppler patch components, including (from bottom) transducer module mounted in angled wedge, water-resistant gasket, PCB for Doppler processing and wireless communication, rechargeable battery, case with indicator LEDs, and silicone sleeve. Image by Kyle Fredricks/Flosonics Medical. (e) Wireless configuration of device, on a 2″ grid for scale, with translucent adhesive strips on each side and clear tabs, visible around the patch, to remove liner of adhesive.

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  Figure 4. In vitro validation results of CW patch velocity measurement, showing good tracking of absolute and relative velocities. (a) Absolute velocity measured by CW patch on a string phantom. A 1° angle correction improves the agreement between measured and actual velocities. (b) Comparison between measured relative change in velocity and actual relative change on a string phantom. (c) Comparison of absolute measured maximum velocity and nominal maximum velocity on a flow phantom. A 2° angle correction improves the agreement. (d) Good agreement between measured and nominal maximum velocity in a flow phantom.

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  Figure 5. (a) Continuous carotid Doppler waveform monitoring with automatically traced maximum velocity envelope (white line). (b) Beat-to-beat quantification of carotid velocity time integral (VTI).

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  Figure 6. Time course of velocity time integral (VTI) following a passive leg raise (PLR) intervention to modulate cardiac preload in a healthy volunteer. The leg raise was initiated at t = 0 s (vertical dashed-line). VTI is calculated from Doppler waveforms acquired simultaneously from the carotid using the CW Doppler Patch (blue, open circles) and the descending aorta using PW Doppler (orange, crosses) and normalized to the mean VTI between − 10 and 0 s of baseline. Solid lines are moving average of 12 beats.