Monitoring blood flow velocity with aflexible Doppler ultrasound device3 November 2021, by Thamarasee Jeewandara

Schematic diagram of the principle and design of theDoppler ultrasonic device. (A) Schematic of the Dopplerultrasonic device. The device continuously transmitsultrasound waves and receives echoes from a movingscatterer (such as red blood cells). (B) Schematics (left)and exploded view (right) of the device structure. ACF,Anisotropic Conductive Film. (C) Detection concept.Each received echo is Doppler-shifted relative to thefrequency transmitted by the transducer, which is relatedto the velocity of the scatterer. A mixture of Dopplerfrequency shifts that changes from moment to momentand from place to place within the vessel lumen isproduced by many moving scatterers in blood flow. Afterdemodulation and power spectral density (PSD)analysis, the Doppler spectrum that contains theabsolute frequency shift or velocity is obtained. (D)Optical image of the cross section of the semifinisheddevice before top electrode bonding. The device withtransducer arrays, bottom electrodes, and substrate wasplaced on a glass plate. (E) Optical image of the devicewhen bent around a curved surface. Photo credit: FengleWang, Tsinghua University. Credit: Science Advances,10.1126/sciadv.abi9283

Researchers aim to implement a clinical strategy to

monitor vascular conditions such as restenosis andthrombosis after vascular reconstructionprocedures. Conventional ultrasound probes arerigid devices for patients with post-operativelyfragile skin. In a new report now published on Science Advances, Fengle Wang and researchteam in engineering, mechanics and medicine inBeijing, China, introduced a flexible Dopplerultrasound device to continuously monitor theabsolute velocity of blood flow in deeply embeddedarteries using the Doppler effect. The skinconforming device was thin and light weight. Usingthe dual-beam Doppler (DBUD) method, Wang etal. obtained accurate measurements of the flowvelocity. The wearable Doppler device can enhancethe quality of care for patients after reconstructionsurgery.

The cardiovascular system

Clinicians assess variations of the cardiovascularsystem by examining variations in blood flow intime and space. Blood flow in arteries can vary in apulsatile manner due to intermittent heart pumpingwhere the flow velocity near the blood vessel wall islower than at the center of a vessel. Vesselcharacteristics including their dimensions andcompliance have an impact on the hemodynamicsoccurring within them. Flow velocity parameterscan also determine thrombus, artery stenosis,hardening occlusion and other diseases.Continuously monitoring the blood flow of patientsis therefore necessary in clinical practice andhealthcare. In present healthcare systems, ultrasound equipment including the handheldultrasound device are incorporated to measureblood flow. In this work, Wang et al. presented acontinuous-wave Doppler ultrasound device for real-time, continuous monitoring of absolute blood flow.The array design and dual-beam ultrasoundDoppler method allowed them to obtain absolutevelocity and eliminate the need for calibration.Wang et al. demonstrated the device on ultrasoundphantoms and on human arteries to display its

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excellent capacity to monitor blood flow velocity.

Device characterization and performance. (A) Schematicof oblique incidence of the ultrasound beam through theEcoflex substrate and skin. The ultrasound beam willrefract on the interface because of acoustic impedancemismatch. (B) Relationship between the intensity ratio ofthe transmitted wave to the incident wave and theinclination angle of the transducer ?. (C) Simulated beampattern of a piezoelectric transducer with a 20°inclination angle. (D and E) Impulse response (A) andfrequency response (B) of the transducer before andafter being bent 500 times. (F) Comparison of the amountof pressure applied to the skin during measurements.The dark, dark gray, and light gray lines represent threemeasurements of variations in the pressure with timeproduced by the commercial probe. The red linerepresents the results of our Doppler device. Credit:Science Advances, 10.1126/sciadv.abi9283

Working principle and device design

Ultrasonic measurements of blood flow velocitywere based on the Doppler effect with piezoelectrictransducers that transmit ultrasound waves with afrequency into the skin. When moving scattererssuch as red blood cells produce an echo, the

received frequency deviated from the transmittedfrequency to form a Doppler shift. This usuallydepends on the speed of sound, flow velocity andthe known Doppler angle between the axis of theultrasound beam and the direction of flow lookingtowards the transducer. The Doppler signal ofblood flow indicated a mixture of many single-frequency signals of scatterers with differentvelocities in the blood flow, with a correspondingamplitude, frequency and phase. The team used acontinuous wave Doppler in the device where onetransducer continuously transmitted an ultrasoundbeam, and an adjacent transducer received thescattered echoes from the blood. The deviceoffered simple signal processing and easyfunctionality, compared to pulse-wave dopplerdevices. To design the flexible device, Wang et al.addressed a few key limitations by using an angledtransducer array to form opaque ultrasound beamsto determine relative motion between theultrasound beam and scatterers. They thendetermined the Doppler angle, by using a dual-beam ultrasound Doppler method (known as DBUDmethod) to determine the inclination angle. Thetransducer showed excellent beam directivity. Theteam determined the pulse response and frequencyresponse of the transducer and noted a stablecentral frequency approximating 5 MHz.

In vitro device validation. (A) Structure specification ofultrasound phantom. (B) Doppler spectrum of blood-mimicking flow with PFVs varying from 60 to 20 cm/s byusing 17° transducers. The time is represented on the

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horizontal axis of the Doppler spectrum at the base of thedisplay. The frequency bin is shown on the vertical axisof the spectrum, which can be converted into a velocityvalue using the Doppler equation. Both the forward andreverse Doppler signals are displayed simultaneously onthe same spectrum, with flow toward the transducerdisplayed on the positive half-axis and flow away fromthe transducer displayed on the negative half-axis. Thedistribution of frequency bins within the sample volume isillustrated by the brightness of the spectral display. Thebrightness of a pixel is proportional to the number ofscatterers causing that frequency shift at that specificpoint in time. (C) Measurements by 17°, 20°, and 23°transducers at three PFVs of 20, 40, and 60 cm/s. (D)Schematic of the DBUD method. (E) Calculations of theDoppler angle ? by using the pairwise results of theDoppler peak shifts produced by the 17° (1), 20° (2),and 23° (3) transducers. Error bars represent ±SD (N =5). (F) Measured PFV versus true PFV curves. Error barsrepresent ±SD (N = 5). (G) Schematic of the flow througha constriction followed by a rapid expansion downstream,showing the regions of turbulence. (H) Spectra of flow inthe normal segment of the tube (left) and flow around thenarrowing site (right). The velocity increases as the bloodflows through the narrowing site (from left to right)followed by an area of flow reversal beyond thenarrowing. Credit: Science Advances,10.1126/sciadv.abi9283

Monitoring of blood flow and artery wall motion in cuffexperiments. (A) Measurement setup, with our deviceand an inflatable cuff wrapped around the upper arm. (B)Schematic of the measurement. The cuff is initiallyinflated above 140 mmHg to occlude arterial flow. Then,the artery reopens, and blood flow resumes at somepoint as the cuff pressure declines. (C and D) Dopplerspectra and cuff pressure variations (white dashed line)of the right brachial artery (C) and left brachial artery (D).The graphs in the right panels of (B) and (C) show themotion of the artery wall. The small red circles representthe systolic arterial pressure. Credit: Science Advances,

10.1126/sciadv.abi9283

Characterizing the device and understandingthe DBUD method

The team next tested the device on a standardultrasound phantom designed to mimic tissuematerial and blood fluid that simulated the acousticcharacteristics of real human tissues and fluid.Wang et al. measured the Doppler spectrum of theblood-mimicking flow under different velocities andadjusted the peak flow velocity accordingly. Withincreasing inclination angle and velocity, the peakfrequency shift also increased. To accuratelymeasure the blood flow velocity using the Dopplereffect, the researchers had to know the anglebetween the direction of flow and the ultrasoundbeam. The angle between the direction of thevessel and the skin surface is, however, typicallyuncertain; therefore, to overcome this, Wang et al.used piezoelectric transducer arrays with differentinclination angles to qualitatively measure the flowvelocity. They derived the principle of this methodas DBUD (dual-beam ultrasound Doppler method)and determined a unique solution of Dopplerangles, flow velocity and the inclination angle of thevessel. The team then calculated the Doppler angleand measured the absolute velocity withoutcalibration.

Simulated experiments

Atherosclerosis can cause artery narrowing(stenosis), which leads to increased resistance toblood flow and artery occlusion. Blood flow througharteries is generally laminar, but it can rapidlyincrease when encountering a stenosis to causeturbulence beyond the narrowing. By developing anin vitro narrowing simulator, Wang et al. simulatednarrowing to capture blood flow characteristicsunder different conditions. The team nextconducted spectrum analysis to obtain quantitativeinformation for blood vessels. When compared withconstant flow, blood flow was pulsatile andproduced a jumble of Doppler frequency shifts.Wang et al. obtained an accurate measurement ofthe carotid blood flow velocity and compared it withthe result of a commercial ultrasound machine. The

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carotid artery is associated with cerebral blood flowand has a typical spectrum of five feature points.Wang et al. validated the outcome relative tocommercial ultrasound machines and displayed themeasurements in the form of Doppler audio in theaudible range as well. The device continuouslyrepresented changes in vascular resistance toprove its role in capturing the characteristics ofblood flow.

Blood flow velocity monitoring. (A) Typical carotid bloodflow spectra during a cardiac cycle (left) and severalcycles (right). Feature points are marked in the leftimage. Insets: An image showing the device mounted onthe neck. (B) Spectrum of carotid blood flow from acommercial ultrasound machine. (C) Calculated keyblood flow parameters compared with those obtained bythe commercial instrument. Error bars represent ±SD (N= 5). (D) Doppler audio waveform of measured carotidblood flow. (E) Continuous measurement of radial arteryblood flow after exercise. The flow waveform graduallytransitions from a low-pulsatility monophasic pattern [lowresistance (LR)] to a high-pulsatility pattern [highresistance (HR)]. All the spectra of (A) to (F) use thesame color bar as in (A). (F) Measurements of peripheralarteries (bottom row) and validation (top row) by acommercial instrument. Columns (left to right): brachialartery, radial artery, and dorsalis pedis artery. The smallred circles represent the PSV (solid line) and maximumreverse flow velocity (dotted line). The numbers 1, 2, and3 represent the three phases of blood flow. Credit:Science Advances, 10.1126/sciadv.abi9283

Outlook

In this way, Fengle Wang and colleaguesdeveloped a flexible, and lightweight Dopplerultrasound device to non-invasively monitor real-time blood flow velocity within human arteries. Thedevice was simple without complex imagingcomponents when compared to the clinicalultrasound machine. The team employed atechnique to provide absolute velocities of allmoving scatterers in the sample range, based onthe dual-beam Doppler (DBUD) method. Theresearchers propose the use of the device tomonitor blood flow velocity after vascularreconstruction procedures.

More information: Fengle Wang et al, FlexibleDoppler ultrasound device for the monitoring ofblood flow velocity, Science Advances (2021). DOI:10.1126/sciadv.abi9283

Clementine M. Boutry et al, Biodegradable andflexible arterial-pulse sensor for the wirelessmonitoring of blood flow, Nature BiomedicalEngineering (2019). DOI:10.1038/s41551-018-0336-5

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APA citation: Monitoring blood flow velocity with a flexible Doppler ultrasound device (2021, November3) retrieved 24 April 2022 from https://medicalxpress.com/news/2021-11-blood-velocity-flexible-doppler-ultrasound.html

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