Continuous Simultaneous Recording of Brachial Artery Distension and Wall Shear Rate: A New Boost for Flow-Mediated Vasodilation

Dynamic Coronary Blood Flow Velocity and Wall Shear Stress Estimation Using Ultrasound in an Ex Vivo Porcine Heart

PURPOSE

Wall shear stress (WSS) is a critically important physical factor contributing to atherosclerosis. Mapping the spatial distribution of local, oscillatory WSS can identify important mechanisms underlying the progression of coronary artery disease.

METHODS

In this study, blood flow velocity and time-varying WSS were estimated in the left anterior descending (LAD) coronary artery of an ex vivo beating porcine heart using ultrasound with an 18 MHz linear array transducer aligned with the LAD in a forward-viewing orientation. A pulsatile heart loop with physiologically-accurate flow was created using a pulsatile pump. The coronary artery wall motion was compensated using a local block matching technique. Next, 2D and 3D velocity magnitude and WSS maps in the LAD coronary artery were estimated at different time points in the cardiac cycle using an ultrafast Doppler approach. The blood flow velocity estimated using the presented approach was compared with a commercially-available, calibrated single element blood flow velocity measurement system.

RESULTS

The resulting root mean square error (RMSE) of 2D velocity magnitude acquired from a high frequency, linear array transducer was less than 8% of the maximum velocity estimated by the commercial system.

CONCLUSION

When implemented in a forward-viewing intravascular ultrasound device, the presented approach will enable dynamic estimation of WSS, an indicator of plaque vulnerability in coronary arteries.

Forward-viewing estimation of 3D blood flow velocity fields by intravascular ultrasound: Influence of the catheter on velocity estimation in stenoses

Coronary artery disease is the most common type of cardiovascular disease, affecting > 18 million adults, and is responsible for > 365 k deaths per year in the U.S. alone. Wall shear stress (WSS) is an emerging indicator of likelihood of plaque rupture in coronary artery disease, however, non-invasive estimation of 3-D blood flow velocity and WSS is challenging due to the requirement for high spatial resolution at deep penetration depths in the presence of significant cardiac motion. Thus we propose minimally-invasive imaging with a catheter-based, 3-D intravascular forward-viewing ultrasound (FV US) transducer and present experiments to quantify the effect of the catheter on flow disturbance in stenotic vessel phantoms with realistic velocities and luminal diameters for both peripheral (6.33 mm) and coronary (4.74 mm) arteries. An external linear array ultrasound transducer was used to quantify 2-D velocity fields in vessel phantoms under various conditions of catheter geometry, luminal diameter, and position of the catheter relative to the stenosis at a frame rate of 5000 frames per second via a particle imaging velocimetry (PIV) approach. While a solid catheter introduced an underestimation of velocity measurement by > 20% relative to the case without a catheter, the hollow catheter introduced < 10% velocity overestimation, indicating that a hollow catheter design allowing internal blood flow reduces hemodynamic disturbance. In addition, for both peripheral and coronary arteries, the hollow catheter introduced < 3% deviation in flow velocity at the minimum luminal area compared to the control case. Finally, an initial comparison was made between velocity measurements acquired using a low frequency, catheter-based, 3-D intravascular FV US transducer and external linear array measurements, with relative error < 12% throughout the region of interest for a flow rate of 150 mL/min. While further system development is required, results suggest intravascular ultrasound characterization of blood flow velocity fields in stenotic vessels could be feasible with appropriate catheter design.

Wall shear stress mapping for human femoral artery based on ultrafast ultrasound vector Doppler estimations

PURPOSE

Wall shear stress (WSS), a type of friction exerted on the artery wall by flowing blood, is considered a crucial factor in atherosclerotic plaque development. Currently, achieving a reliable WSS mapping of an artery noninvasively by using existing imaging modalities is still challenging. In this study, a WSS mapping based on vector Doppler flow velocity estimation was proposed to measure the dynamic WSS on the human femoral artery.

METHODS

Because ultrafast ultrasound imaging was used here, flow-enhanced imaging was also performed to observe the moving blood flow condition. The performance of WSS mapping was verified using both straight (8 mm in diameter) and stenosis (70% of stenosis) phantoms under a pulsatile flow condition. A human study was conducted from five healthy volunteers.

RESULTS

Experimental results demonstrated that the WSS estimation was close to the standard value that was obtained from maximum velocity estimation in straight phantom experiments. In a stenosis phantom experiment, a low WSS region was observed at a site downstream of an obstruction, which is a high-risk area for plaque formation. Dynamic WSS mapping was accomplished in measurement in the femoral artery bifurcation. In measurements, the time-averaged WSS of the common femoral artery, superficial femoral artery, and deep femoral artery was 0.52± 0.19, 0.44 ± 0.21, and 0.29 ± 0.16 Pa, respectively, for the anterior wall and 0.29 ± 0.11, 0.54 ± 0.24, and 0.23 ± 0.10 Pa, respectively, for the posterior wall.

CONCLUSIONS

All results indicated that WSS mapping has the potential to be a useful tool for vessel duplex scanning in the future.

Arterial wall shear rate response to reactive hyperaemia is markedly different between young and older humans

KEY POINTS

The vasodilatory response to reactive hyperaemia is impaired with advancing age, but it is unclear whether this is because of an altered wall shear rate (WSR) stimulus or an altered flow-mediated dilatation (FMD) response. Using new technology that allows detailed WSR measurement, we assessed the WSR-FMD response in healthy older people. Our data show that older people have a markedly altered and diminished WSR response to reactive hyperaemia compared to young people, but reduced WSR alone does not fully explain reduced FMD. In young people, WSR appears to be coupled to FMD but, by age ∼65 years, the arterial vasodilatory response has begun to uncouple from the WSR stimulus. These findings point to the importance and utility of comprehensively characterizing the WSR-FMD response when using reactive hyperaemia to assess vascular function, as well as giving new insight into the age-related alteration in vascular function.

ABSTRACT

The vasodilatory response to reactive hyperaemia is impaired with age, but it is unknown whether this is because of an altered wall shear rate (WSR) stimulus or an altered flow-mediated dilatation (FMD) response to the WSR stimulus. Inherent difficulties in measuring blood flow velocity close to the arterial wall have prevented detailed assessment of the WSR-FMD response. Using an enhanced multigate spectral Doppler ultrasound system (ultrasound advanced open platform), we aimed to produce new data on the WSR-FMD relationship in healthy older adults. Sixty healthy people, comprising 28 young (27.5 ± 5.5 years) and 32 older (64.9 ± 3.7 years) individuals, underwent FMD assessment. Raw data were post-processed using custom-designed software to obtain WSR and diameter parameters. The data revealed that older people have a much altered and diminished WSR response to reactive hyperaemia compared to younger people [e.g. WSR peak: 622 (571-673) vs. 443 (396-491) 1/s in young and older respectively; P < 0.05]. However, reduced WSR alone does not appear to fully explain the reduced FMD response in older people because associations between WSR and FMD were few and weak. This was in contrast to young adults, where associations were strong. We conclude that WSR during FMD is much altered and diminished in older people, and there appears to be an 'uncoupling' of WSR from FMD in older people that may reflect a loss of precision in the reactive hyperaemia stimulus-response relationship. These findings also point to the importance and utility of comprehensively characterizing the WSR-FMD response when using reactive hyperaemia to assess vascular function.