Technical Validation of ARTSENS-An Image Free Device for Evaluation of Vascular Stiffness

Advancing the Local Pulse Wave Velocity Measurement-Wave Confluence Decomposition Using a Double Gaussian Propagation Model

Background Pulse wave velocity (PWV) is a marker of arterial stiffness and local measurements could facilitate its widescale clinical use. However, confluence of incident and early reflected waves leads to biased spatiotemporal PWV estimates. Objective We introduce the Double Gaussian Propagation Model (DGPM) to measure local PWV in consideration of wave confluence (PWV[Formula: see text]) and compare it against conventional spatiotemporal PWV (PWV[Formula: see text]), with Bramwell-Hill PWV (PWV[Formula: see text]) and blood pressure (BP) as reference measures. Methods Ten subjects ranging from normotension to hypertension were repeatedly measured at rest and with induced PWV changes. Carotid distension waveforms over a 19 mm wide segment were acquired from ultrasonography, simultaneously with noninvasive continuous BP. Per cardiac cycle, the 8-parameter DGPM (amplitude, centroid, width, and velocity, respectively of forward and backward propagating wave) was fitted to the distension waveforms' systolic foot and dicrotic notch complexes. Corresponding PWV[Formula: see text] was computed from linear fittings of respective feature timings and distances. Regression analyses were conducted with PWV[Formula: see text] and PWV[Formula: see text] as predictors, and various PWV and BP measures as response variables. Results Whereas PWV[Formula: see text] correlations were insignificant, PWV[Formula: see text] estimated the reference PWV[Formula: see text] with a significant reduction in errors (P < 0.001), explained up to 65% PWV[Formula: see text] variability at rest, demonstrated higher intra-method consistency and correlated significantly with all BP measures (P < 0.001). Conclusion The proposed DGPM measures local carotid PWV in consideration of wave confluence, showing significant correlations with Bramwell-Hill PWV and BP at two distinct waveform complexes. Thereby PWV[Formula: see text] outperforms the conventional PWV[Formula: see text] in all investigated respects, potentially enabling PWV assessment in routine clinical practice.

Characterizing the Effect of Hold-Down Pressure for Local and Regional Stiffness Markers

Vascular aging occurs due to pathologies and old age. Early detection of stiffness markers and arterial geometry parameters in the carotid artery serves as an essential indicator for the progression of vascular aging. The assessment of stiffness markers is typically conducted using ultrasound and tonometer-based devices. However, both types of devices present challenges, requiring skilled operators and exhibiting variability in results due to different hold-down pressures when different operators are involved. In this study, we aim to quantify the impact of hold-down pressure on diameter-based measurements and evaluate its effects on local and regional stiffness markers. To carry out this investigation, we utilized ARTSENS Plus device along with a multi-modal probe comprising an ultrasound transducer and a tonometer. A-mode ultrasound scanning was performed on the left common carotid artery of each participant, tonometer indicated the applied hold-down pressure on the participant's skin. Four trials were conducted at hold-down pressure levels of 50, 100, 150, and 200 mmHg, and RF echo frames were recorded. ARTSENS Plus signal processing algorithms were applied to obtain the recorded frames' carotid diameter and pressure, central pressure, and local and regional stiffness. The beat-to-beat repeatability of diameter values was examined, and the coefficient of variation was calculated to assess the consistency of the measurements. The system's signal-to-noise ratio exceeded 25 dB. The results section delves into the impact of hold-down pressure on diameter, carotid pressure, and stiffness markers, providing insights into the variables influencing the reliability of the measurements in this cardiovascular assessment. From all the results and observations optimal hold-down pressure can be slightly higher than diastolic pressure.

Arterial Distension Monitoring Scheme Using FPGA-Based Inference Machine in Ultrasound Scanner Circuit System

This paper presents an arterial distension monitoring scheme using a field-programmable gate array (FPGA)-based inference machine in an ultrasound scanner circuit system. An arterial distension monitoring requires a precise positioning of an ultrasound probe on an artery as a prerequisite. The proposed arterial distension monitoring scheme is based on a finite state machine that incorporates sequential support vector machines (SVMs) to assist in both coarse and fine adjustments of probe position. The SVMs sequentially perform recognitions of ultrasonic A-mode echo pattern for a human carotid artery. By employing sequential SVMs in combination with convolution and average pooling, the number of features for the inference machine is significantly reduced, resulting in less utilization of hardware resources in FPGA. The proposed arterial distension monitoring scheme was implemented in an FPGA (Artix7) with a resource utilization percentage less than 9.3%. To demonstrate the proposed scheme, we implemented a customized ultrasound scanner consisting of a single-element transducer, an FPGA, and analog interface circuits with discrete chips. In measurements, we set virtual coordinates on a human neck for 9 human subjects. The achieved accuracy of probe positioning inference is 88%, and the Pearson coefficient (r) of arterial distension estimation is 0.838.

Baroreflex sensitivity is impaired in survivors of mild COVID-19 at 3-6 months of clinical recovery; association with carotid artery stiffness

The association between the stiffening of barosensitive regions of central arteries and the derangements in baroreflex functions remains unexplored in COVID-19 survivors. Fifty-seven survivors of mild COVID-19 (defined as presence of upper respiratory tract symptoms and/or fever without shortness of breath or hypoxia; SpO2 > 93%), with an age range of 22-66 years (27 females) participated at 3-6 months of recovering from the acute phase of RT-PCR positive COVID-19. Healthy volunteers whose baroreflex sensitivity (BRS) and arterial stiffness data were acquired prior to the onset of the pandemic constituted the control group. BRS was found to be significantly lower in the COVID survivor group for the systolic blood pressure-based sequences (BRSSBP ) [9.78 (7.16-17.74) ms/mmHg vs 16.5 (11.25-23.78) ms/mmHg; p = 0.0253]. The COVID survivor group showed significantly higher carotid β stiffness index [7.16 (5.75-8.18) vs 5.64 (4.34-6.96); (p = 0.0004)], and pulse wave velocity β (PWVβ ) [5.67 (4.96-6.32) m/s vs 5.12 (4.37-5.41) m/s; p = 0.0002]. BRS quantified by both the sequence and spectral methods showed an inverse correlation with PWVβ in the male survivors. Impairment of BRS in the male survivors of mild COVID-19 at 3-6 months of clinical recovery shows association with carotid artery stiffness.

Computation of Vascular Parameters: Implementing Methodology and Performance Analysis

This paper presents the feasibility of automated and accurate in vivo measurements of vascular parameters using an ultrasound sensor. The continuous and non-invasive monitoring of certain parameters, such as pulse wave velocity (PWV), blood pressure (BP), arterial compliance (AC), and stiffness index (SI), is crucial for assessing cardiovascular disorders during surgeries and follow-up procedures. Traditional methods, including cuff-based or invasive catheter techniques, serve as the gold standard for measuring BP, which is then manually used to calculate AC and SI through imaging algorithms. In this context, the Continuous and Non-Invasive Vascular Stiffness and Arterial Compliance Screener (CaNVAS) is developed to provide continuous and non-invasive measurements of these parameters using an ultrasound sensor. By driving 5 MHz (ranging from 2.2 to 10 MHz) acoustic waves through the arterial walls, capturing the reflected echoes, and employing pre-processing techniques, the frequency shift is utilized to calculate PWV. It is observed that PWV measured by CaNVAS correlates exponentially with BP values obtained from the sphygmomanometer (BPMR-120), enabling the computation of instantaneous BP values. The proposed device is validated through measurements conducted on 250 subjects under pre- and post-exercise conditions, demonstrating an accuracy of 95% and an average coefficient of variation of 12.5%. This validates the reliability and precision of CaNVAS in assessing vascular parameters.

Long-term cardiovascular health status and physical functioning of nonhospitalized patients with COVID-19 compared with non-COVID-19 controls

Coronavirus disease 2019 (COVID-19) is reported to have long-term effects on cardiovascular health and physical functioning, even in the nonhospitalized population. The physiological mechanisms underlying these long-term consequences are however less well described. We compared cardiovascular risk factors, arterial stiffness, and physical functioning in nonhospitalized patients with COVID-19, at a median of 6 mo postinfection, versus age- and sex-matched controls. Cardiovascular risk was assessed using blood pressure and biomarker concentrations (amino-terminal pro-B-type-natriuretic-peptide, high-sensitive cardiac troponin I, C-reactive protein), and arterial stiffness was assessed using carotid-femoral pulse wave velocity. Physical functioning was evaluated using accelerometry, handgrip strength, gait speed and questionnaires on fatigue, perceived general health status, and health-related quality of life (hrQoL). We included 101 former patients with COVID-19 (aged 59 [interquartile range, 55-65] yr, 58% male) and 101 controls. At 175 [126-235] days postinfection, 32% of the COVID-19 group reported residual symptoms, notably fatigue, and 7% required post-COVID-19 care. We found no differences in blood pressure, biomarker concentrations, or arterial stiffness between both groups. Former patients with COVID-19 showed a higher handgrip strength (43 [33-52] vs. 38 [30-48] kg, P = 0.004) and less sleeping time (8.8 [7.7-9.4] vs. 9.8 [8.9-10.3] h/day, P < 0.001) and reported fatigue more often than controls. Accelerometry-based habitual physical activity levels, gait speed, perception of general health status, and hrQoL were not different between groups. In conclusion, one in three nonhospitalized patients with COVID-19 reports residual symptoms at a median of 6 mo postinfection, but we were unable to relate these symptoms to increases in cardiovascular risk factors, arterial stiffness, or physical dysfunction.NEW & NOTEWORTHY We examined cardiovascular and physical functioning outcomes in nonhospitalized patients with COVID-19, at a median of 6 mo postinfection. When compared with matched controls, minor differences in physical functioning were found, but objective measures of cardiovascular risk and arterial stiffness did not differ between groups. However, one in three former patients with COVID-19 reported residual symptoms, notably fatigue. Follow-up studies should investigate the origins of residual symptoms and their long-term consequences in former, nonhospitalized patients with COVID-19.

High Frame-Rate A-Mode Ultrasound System for Jugular Venous Pulse Tracking: A Feasibility Study

Jugular venous pulse (JVP) helps in the early detection of central venous pressure abnormalities and various cardiovascular diseases. Studies have been reported indicating that contour features of the JVP waveform provide crucial information regarding cardiac function. Although current ultrasound systems reliably provide the diameter measurements, they are limited by low frame rates resulting in poor resolution JVP cycles that are inadequate to yield distinguishable critical points. In this work, we propose an image-free high frame rate system for the assessment of JVP signals. The proposed A-mode ultrasound system acquires high fidelity JVP pulses with a temporal resolution of 4 ms and amplitude resolution of 10 µm. The functionality verification of the proposed system was performed by comparing it against a clinical-grade B-mode imaging system. A study was conducted on a cohort of 25 subjects in the 20-30 age group. While the system provided diameter measurements comparable to that of the imaging ones (r > 0.98, p < 0.05), it also yielded high-resolution JVP exhibiting the presence of all fiduciary points. This was a leveraging feature as opposed to the imaging system that possessed limited temporal and amplitude resolution. Clinical Relevance- The proposed system is a potential ultrasound means for measuring the diameter values from JV at the same time yielding the JVP critical points necessary for clinical analysis.

High-frame-rate A-mode ultrasound for calibration-free cuffless carotid pressure: feasibility study using lower body negative pressure intervention

PURPOSE

Existing technologies to measure central blood pressure (CBP) intrinsically depend on peripheral pressure or calibration models derived from it. Pharmacological or physiological interventions yielding different central and peripheral responses compromise the accuracy of such methods. We present a high-frame-rate ultrasound technology for cuffless and calibration-free evaluation of BP from the carotid artery. The system uses a pair of single-element ultrasound transducers to capture the arterial diameter and local pulse wave velocity (PWV) for the evaluation of beat-by-beat BP employing a novel biomechanical model.

MATERIALS AND METHODS

System's functionality assessment was conducted on eight male subjects (26 ± 4 years, normotensive and no history of cardiovascular risks) by perturbing pressure via short-term moderate lower body negative pressure (LBNP) intervention (-40 mmHg for 1 min). The ability of the system to capture dynamic responses of carotid pressure to LBNP was investigated and compared against the responses of peripheral pressure measured using a continuous BP monitor.

RESULTS

While the carotid pressure manifested trends similar to finger measurements during LBNP, the system also captured the differential carotid-to-peripheral pressure response, which corroborates the literature. The carotid diastolic and mean pressures agreed with the finger pressures (limits-of-agreement within ±7 mmHg) and exhibited acceptable uncertainty (mean absolute errors were 2.4 ± 3.5 and 2.6 ± 4.0 mmHg, respectively). Concurrent to the literature, the carotid systolic and pulse pressures (PPs) were significantly lower than those of the finger pressures by 11.1 ± 9.4 and 11.3 ± 8.2 mmHg, respectively (p < .0001).

CONCLUSIONS

The study demonstrated the method's potential for providing cuffless and calibration-free pressure measurements while reliably capturing the physiological aspects, such as PP amplification and dynamic pressure responses to intervention.

Phantom Assessment of an Image-free Ultrasound Technology for Online Local Pulse Wave Velocity Measurement

Cardiovascular community has started clinically adopting the assessment of local stiffness, contrary to the traditionally measured carotid-femoral pulse wave velocity (PWV). Though they offer higher reliability, ultrasound methods require advanced hardware and processing methods to perform real-time measurement of local PWV. This work presents a system and method to perform online PWV measurement in an automated manner. It is a fast image-free ultrasound technology that meets the methodological requirements necessary to measure small orders of local pulse transit, from which PWV is measured. The measurement accuracy and repeatability were assessed via phantom experiments, where the measured transit time-based PWV (PWVTT) was compared against the theoretically calculated PWV from Bramwell-Hill equation (PWVBH). The beat-to-beat variability in the measured PWVTT was within 3%. PWVTT values strongly correlated (r=0.98) with PWVBH, yielding a negligible bias of -0.01 m/s, mean error of 3%, and RMSE of 0.27 m/s. These pilot study results demonstrated the presented system's reliability in yielding online local PWV measurements.

Gaussian-Mixture Modelling of A-Mode Radiofrequency Scans for the Measurement of Arterial Wall Thickness

Measurement of arterial wall thickness is an integral component of vascular properties and health assessment. State-of-the-art automated or semi-automated techniques are majorly applicable to B-mode images and are not available for entry-level in-expensive devices. Considering this, we have earlier developed and validated an image-free (A-mode) ultrasound device, ARTSENS^® for the evaluation of vascular properties. In this work, we present a novel gaussian-mixture modeling-based method to measure arterial wall thickness from A-mode frames, which is readily deployable to the existing technology. The method's performance was assessed based on systematic simulations and controlled phantom experiments. Simulations revealed that the method could be confidently applied to A-mode frames with above-moderate SNR (>15 dB). When applied to A-mode frames acquired from the flow-phantom setup (SNR > 25 dB), the mean error was limited to (2 ± 1%), and RMSE was 19 μm, on comparison with B-mode measurements. The measured and reference wall thickness strongly agreed with each other (r = 0.88, insignificant mean bias = 7 μm, p = 0.16). The proposed method was capable of performing real-time measurements.

High-Framerate A-Mode Ultrasound for Vascular Structural Assessments: In-Vivo Validation in a Porcine Model

Capturing vascular dynamics using ultrasound at a high framerate provided a unique way to track time-dependent and transient physiologic events non-invasively. In this work, we present an A-model high-framerate (500 frames per second) image-free ultrasound system for monitoring vascular structural and material properties. It was developed based on our clinically validated ARTSENS^® technology. Following in-vitro verification on arterial flow phantoms, its measurement accuracy and high-framerate data acquisition and processing were verified in-vivo on 2 anesthetized Sus scrofa swine. Measurements of the carotid artery (the luminal diameter, distension, and wall thickness) obtained using the high-framerate system were comparable to those provided by a clinical-grade reference ultrasound imaging device (absolute error < 4%, < 6.3%, and < 6.6%, respectively). Notably, the morphology of the arterial distension waveforms obtained at high-framerate depicted vital physiological fiduciary points compared to the low-framerate reference waveform. The compression-decompression pattern of the arterial wall was also captured with the high-framerate system, which is challenging with low-framerate ultrasound. Potential applications of these high temporal structural waveforms have also been discussed.

Pulse Arrival Time Segmentation Into Cardiac and Vascular Intervals - Implications for Pulse Wave Velocity and Blood Pressure Estimation

OBJECTIVE

This study demonstrates a novel method for pulse arrival time (PAT) segmentation into cardiac isovolumic contraction (IVC) and vascular pulse transit time to approximate central pulse wave velocity (PWV).

METHODS

10 subjects (38 ± 10 years, 121 ± 12 mmHg SBP) ranging from normotension to hypertension were repeatedly measured at rest and with induced changes in blood pressure (BP), and thus PWV. ECG was recorded simultaneously with ultrasound-based carotid distension waveforms, a photoplethysmography-based peripheral waveform, noninvasive continuous and intermittent cuff BP. Central PAT was segmented into cardiac and vascular time intervals using a fiducial point in the carotid distension waveform that reflects the IVC onset. Central and peripheral PWVs were computed from (segmented) intervals and estimated arterial path lengths. Correlations with Bramwell-Hill PWV, systolic and diastolic BP (SBP/DBP) were analyzed by linear regression.

RESULTS

Central PWV explained more than twice the variability (R²) in Bramwell-Hill PWV compared to peripheral PWV (0.56 vs. 0.27). SBP estimated from central PWV undercuts the IEEE mean absolute deviation threshold of 5 mmHg, significantly lower than peripheral PWV or PAT (4.2 vs. 7.1 vs. 10.1 mmHg).

CONCLUSION

Cardiac IVC onset signaled in carotid distension waveforms enables PAT segmentation to obtain unbiased vascular pulse transit time. Corresponding PWV estimates provide the basis for single-site assessment of central arterial stiffness, confirmed by significant correlations with Bramwell-Hill PWV and SBP.

SIGNIFICANCE

In a small-scale cohort, we present proof-of-concept for a novel method to estimate central PWV and BP, bearing potential to improve the practicality of cardiovascular risk assessment in clinical routines.

Assessment of Carotid Arterial Stiffness in Community Settings With ARTSENS®

OBJECTIVE

We investigate the field feasibility of carotid stiffness measurement using ARTSENS® Touch and report the first community-level data from India.

METHOD

In an analytical cross-sectional survey among 1074 adults, we measured specific stiffness index ([Formula: see text]), pressure-strain elastic modulus ([Formula: see text]), arterial compliance (AC), and one-point pulse wave velocity (PWV[Formula: see text]) from the left common carotid artery. Data for established risk factors (waist circumference, blood pressure, plasma glucose, triglycerides, and HDL-C) were also collected. The association of carotid stiffness with age, gender, hypertension/diabetes, smoking, and clustering of risk factors was studied.

RESULTS

Measurements were repeatable with a relative difference (RD) between consecutive readings of < 5% for blood pressure and < 15% for [Formula: see text]% of arterial diameter values. The average RDs for [Formula: see text], [Formula: see text], AC, and PWV[Formula: see text], were 20.51%, 22.31%, 25.10%, and 14.13%, respectively. Typical range for stiffness indices among females and males were [Formula: see text]: 8.12 ± 3.59 vs 6.51 ± 2.78, [Formula: see text]: 113.24 ± 56.12 kPa vs 92.33 ± 40.65 kPa, PWV[Formula: see text]: 6.32 ± 1.38 ms-1 vs 5.81 ± 1.16 ms-1, and AC: 0.54 ± 0.36 mm2 kPa-1 vs 0.72 ± 0.38 mm2 kPa-1. Mean [Formula: see text], [Formula: see text], and PWV[Formula: see text] increased (and mean AC decreased) across decades of age; the trend persisted even after excluding hypertensives and subjects with diabetes. The odds ratio of presence of multiple risk factors for [Formula: see text] kPa and/or PWV[Formula: see text] ms-1 was ≥ 2.12 or above in males. In females, it was just above 2.00 for [Formula: see text] kPa and/or PWV[Formula: see text] ms-1 and increased to ≥ 3.33 for [Formula: see text] kPa and ≥ 3.25 for PWV[Formula: see text] ms-1.

CONCLUSION

The study demonstrated the feasibility of carotid stiffness measurement in a community setting. A positive association between the risk factors and carotid artery stiffness provides evidence for the device's use in resource-constrained settings. Clinical Impact: The device paves the way for epidemiological and clinical studies that are essential for establishing population-level nomograms for wide-spread use of carotid stiffness in clinical practice and field screening of 'at-risk' subjects.

Current understanding of intimal hyperplasia and effect of compliance in synthetic small diameter vascular grafts

Despite much effort, synthetic small diameter vascular grafts still face limited success due to vascular wall thickening known as intimal hyperplasia (IH). Compliance mismatch between graft and native vessels has been proposed to be one of a key mechanical factors of synthetic vascular grafts that could contribute to the formation of IH. While many methods have been developed to determine compliance both in vivo and in vitro, the effects of compliance mismatch still remain uncertain. This review aims to explain the biomechanical factors that are responsible for the formation and development of IH and their relationship with compliance mismatch. Furthermore, this review will address the current methods used to measure compliance both in vitro and in vivo. Lastly, current limitations in understanding the connection between the compliance of vascular grafts and the role it plays in the development and progression of IH will be discussed.

High-Throughput Vascular Screening by ARTSENS Pen During a Medical Camp for Early-Stage Detection of Chronic Kidney Disease

Intervention in the early stages of cardiovascular and kidney diseases is proven to be more effective in preventing disease progression. Large artery stiffness measurement can be a potential early predictor of future risks. The purpose of the study reported in this work was to demonstrate the feasibility of our ARTSENS^® Pen device as a high-throughput vascular screening tool for risk assessment. The study was performed during a medical camp conducted for awareness and early-stage detection of kidney diseases. Screening procedures included biosample tests and blood pressure measurements. Alongside, various clinically relevant measures of the arterial stiffness were evaluated using the ARTSENS^® Pen, by measuring vessel wall dynamics via our proprietary image-free ultrasound algorithms. Stiffness measurement from the left common carotid artery on 85 participants could be completed within 4 hours, employing two units of ARTSENS^® Pen; this also includes time taken for all the procedures enlisted in the study protocol. The associations of carotid stiffness indices with age-, gender-, and risk factor-dependent variations were established.

Ultrasonic Characteristics of Cardiovascular Changes in Children with Hutchinson-Gilford Progeria Syndrome: A Comparative Study with Normal Children and Aging People

Background

The cardiovascular characteristics of children with Hutchinson-Gilford progeria syndrome (HGPS) remain unclear. The present study is aimed at evaluating the cardiovascular changes with ultrasound examination in children with HGPS and compared these with those in normal children and older people.

Methods

Seven HGPS children, 21 age-matched healthy children, and 14 older healthy volunteers were evaluated by three-dimensional echocardiography (including strain analysis) and carotid elasticity examination with the echo-tracking technique.

Results

Children with HGPS had higher left ventricular ejection fraction (LVEF) and global longitudinal strain, when compared to older healthy volunteers (P < 0.05). However, these parameters were not significantly different, when compared to those in healthy children. Furthermore, children with HGPS had lower average peak times in the left ventricle, when compared with the other two groups. For the structure of the carotid artery detected by ultrasound, the abnormality rates were similar between children with HGPS and older healthy volunteers (83.3% vs. 71.4%). The elastic parameters, elastic modulus, stiffness parameter, and pulsed wave transmittal velocity of children with HGPS were lower, when compared to those in older healthy volunteers (P < 0.05), while they were higher with arterial compliance (P > 0.05). Furthermore, no significant difference existed among the vascular elastic parameters between HGPS and normal children.

Conclusion

HGPS children had impaired left ventricular (LV) synchrony, when compared to normal children, although the difference in LVEF was not statistically significant. Furthermore, the structural abnormality of the carotid artery in HGPS children was similar to that in older people, although the index of elasticity appears to be more favorable. These results suggest that the cardiovascular system in HGPS children differs from natural aging.

ARTSENS® Pen-portable easy-to-use device for carotid stiffness measurement: technology validation and clinical-utility assessment

OBJECTIVE

The conventional medical imaging modalities used for arterial stiffness measurement are non-scalable and unviable for field-level vascular screening. The need for an affordable, easy-to-operate automated non-invasive technologies remains unmet. To address this need, we present a portable image-free ultrasound device-ARTSENS^® Pen, that uses a single-element ultrasound transducer for carotid stiffness evaluation.

APPROACH

The performance of the device was clinically validated on a cohort of 523 subjects. A clinical-grade B-mode ultrasound imaging system (ALOKA eTracking) was used as the reference. Carotid stiffness measurements were taken using the ARTSENS^® Pen in sitting posture emulating field scenarios.

MAIN RESULTS

A statistically significant correlation (r > 0.80, p < 0.0001) with a non-significant bias was observed between the measurements obtained from the two devices. The ARTSENS^® Pen device could perform highly repeatable measurements (with variation smaller than 10%) on a relatively larger percentage of the population when compared to the ALOKA system. The study results also revealed the sensitivity of ARTSENS^® Pen to detect changes in arterial stiffness with age.

SIGNIFICANCE

The easy-to-use technology and the automated algorithms of the ARTSENS^® Pen make it suitable for cardiovascular risk assessment in resource-constrained settings.

Continuous Assessment of Carotid Diameter using an Accelerometer Patch Probe for Ambulatory Arterial Stiffness Monitoring

We present a system with an accelerometer patch probe design for non-invasive evaluation of carotid arterial stiffness. The proposed system could continuously measure the acceleration signal derived due to the propagation of blood pulse wave through the left carotid artery, double integrating and scaling it to estimate the accelerometer-derived carotid wall displacement. This functional principle was proved by comparing the accelerometer-derived carotid wall displacement with the carotid distension signal from the reference system ARTSENS® (ARTerial Stiffness Evaluation for Noninvasive Screening device) for all the recruited human subjects. Assuming the relationship to be linear, a one-time subject-specific calibration was performed with the simultaneously acquired reference distension signal and the accelerometer-derived carotid displacement signals on its anachrotic limbs data points (at systolic phases) for each subject. This calibration equation was tested with latterly acquired accelerometer signals and results in the measurement of accelerometer-derived carotid distension and lumen-diameter values. The ability of the accelerometer system to measure real-time carotid distension and lumen diameter in a repeatable beat-by-beat manner for arterial stiffness index evaluation was validated in-vivo. The accuracy of the obtained results was studied with our clinically validated reference system. The experimental validation study results exhibit the feasibility of using the developed accelerometer system for continuous carotid distension and lumen diameter measurements, whereby the estimation of carotid arterial stiffness.

Image-Free Technique for Flow Mediated Dilation Using ARTSENS® Pen

Flow mediated dilation (FMD) is a clinically accepted non-invasive tool for assessing endothelial dysfunction. FMD is conventionally performed with B-mode ultrasound systems that involve recording of the image sequences as DICOM files or video-graphic files and processing them offline. Sometimes the examinations may have to be rejected due to poor or unstable image sequences resulting non-reliable diameter estimates. We had earlier developed and extensively validated an image-free ultrasound technology, ARTSENS^®, for the measurement of carotid artery wall dynamics and arterial stiffness metrics. In this work, we evaluate the feasibility of using the technology for continuous real-time diameter measurement of the brachial artery and thereby FMD. To investigate the performance of the ARTSENS^® device an in-vivo study was conducted on 5 subjects as pilot. As a reference the measurements were also performed by a B-mode imaging system with a help of a commercially available clinically validated offline FMD analysis tool. The brachial artery diameter and FMD measurements performed by the ARTSENS^® device were consistent with the earlier reported literature. The beat-to-beat repeatability of the baseline diameter measurements was acceptable with a CoV <; 4% for all the subjects. The diameter measurements performed by the two devices exhibited a significant correlation (r-square = 0.81, p <; 0.05). The RMSE for the diameter and FMD% measurements was 0.32 mm and 0.63% respectively, illustrating the measurement accuracy. The study demonstrated that the ARTSENS^® can be reliably employed for performing FMD measurements and assessing endothelial dysfunction. This would help realize a field deployable solution for real-time automated FMD measurement and consequently for the acceleration of large population studies in this research area.

Multimodal Image-Free Ultrasound Technique for Evaluation of Arterial Viscoelastic Properties: A Feasibility Study

In this article, we have presented a multimodal system and a novel probe design that was built around an image-free ultrasound technology, ARTSENS®, for measurement of arterial viscoelastic properties. ARTSENS® was extensively validated over the years, for performing measurements of arterial wall dynamics and stiffness with an accuracy that meets clinical standards. Concerning this work, several enhancements were incorporated to this basic technology that allowed high frame rate A-scan imaging (1 kHz) and integration of a pressure measuring module for automated measurements of the viscoelastic parameter (elastic index, viscous index and wall buffering function). The functionality of the developed multimodal system and probe were investigated by conducting an in-vivo on 8 young subjects (both normotensive and hypertensive were included). The beat-to-beat measurements of the viscoelastic parameters exhibited acceptable repeatability with a variability <; 6.5%. It was observed that the group average for viscosity index and the wall buffering function were higher for hypertensive subjects as compared to normotensive subjects. The study observations were consistent with the reported literature. The proposed system addresses several issues associated with the traditional image-based systems and offers huge advantage of field amenability thus making it favorable for large population screening and studies.

Local Pulse Wave Velocity: Theory, Methods, Advancements, and Clinical Applications

Local pulse wave velocity (PWV) is evolving as one of the important determinants of arterial hemodynamics, localized vessel stiffening associated with several pathologies, and a host of other cardiovascular events. Although PWV was introduced over a century ago, only in recent decades, due to various technological advancements, has emphasis been directed toward its measurement from a single arterial section or from piecewise segments of a target arterial section. This emerging worldwide trend in the exploration of instrumental solutions for local PWV measurement has produced several invasive and noninvasive methods. As of yet, however, a univocal opinion on the ideal measurement method has not emerged. Neither have there been extensive comparative studies on the accuracy of the available methods. Recognizing this reality, makes apparent the need to establish guideline-recommended standards for the measurement methods and reference values, without which clinical application cannot be pursued. This paper enumerates all major local PWV measurement methods while pinpointing their salient methodological considerations and emphasizing the necessity of global standardization. Further, a summary of the advancements in measuring modalities and clinical applications is provided. Additionally, a detailed discussion on the minimally explored concept of incremental local PWV is presented along with suggestions of future research questions.

Local Evaluation of Variation in Pulse Wave Velocity over the Cardiac Cycle using Single-Element Ultrasound Transducer

A method and system for single-site measurement of local pulse wave velocity (PWV) and its variation over the cardiac cycle are presented. The proposed system employs a single-element ultrasound transducer and associated custom technology to record arterial diameter and wall thickness waveforms in real-time. Simultaneously acquired blood pressure, diameter and wall thickness parameters were used to evaluate diastolic local PWV (CD) and systolic local PWV (Cs) from an arterial site of interest. The developed prototype system was validated on a cohort of 15 subjects (age $=43\pm 12$ years) that includes normotensives and hypertensives. Cs and CD measurements were obtained from the left carotid artery. A significant difference between carotid Cs and CD $(\Delta \mathrm{C})$ was observed in all recruited subjects (group average $\Delta \mathrm{C} = 0.92\pm 0.76\mathrm{m}/\mathrm{s})$, illustrating the arterial pressure dependency of local PWV. The absolute values of Cs and CD were within a range of 3.39 m/s to 7.5 m/s and 3.12 m/s to 5.82 m/s respectively. Normotensive versus hypertensive group-wise analysis was performed to investigate the degree of variation in the carotid local PWV over a cardiac cycle among different BP categories. Study results demonstrated that the proposed approach has a potential to provide valuable surrogate markers for cardiovascular risk assessment.

Brachial artery stiffness estimation using ARTSENS

Central and peripheral arteries stiffening prominently affect hemodynamics thus increasing the risk of coronary heart disease, chronic kidney disease and end stage renal disease. There are several commercially available non-invasive measurement technologies for the evaluation of stiffness that are expensive, demand dedicated expertise and fall short for mass screening. Considering this, we have developed ARTSENS^®, a highly compact and portable image-free ultrasound device for evaluation of arterial stiffness. The capability of the device to perform accurate measurements of carotid artery stiffness has been validated through extensive in-vivo studies. In this paper we demonstrate the feasibility of using ARTSENS^® for measuring brachial artery stiffness. An inter-operator repeatability study was done based on in-vivo experiments on 9 young healthy subjects. The study included measurement of distension, end diastolic lumen diameter, arterial compliance and stiffness index performed both on carotid artery and brachial artery by two operators successively. The degree of agreement between the measurements made by operators has been investigated based on Bland-Altman plots and paired t-test. The measurements were populated within the limits of agreement. No statistically significant difference (p-values from paired t-test for end-diastolic diameter, distension, stiffness index, arterial compliance were 0.36, 0.24, 0.47 and 0.11 respectively) was seen for the brachial artery measurements performed by the two operators. The correlation between the measurement made by the operators was highly significant (r=0.86, p-value=0.003).

Arterial compliance probe for cuffless evaluation of carotid pulse pressure

OBJECTIVE

Assessment of local arterial properties has become increasingly important in cardiovascular research as well as in clinical domains. Vascular wall stiffness indices are related to local pulse pressure (ΔP) level, mechanical and geometrical characteristics of the arterial vessel. Non-invasive evaluation of local ΔP from the central arteries (aorta and carotid) is not straightforward in a non-specialist clinical setting. In this work, we present a method and system for real-time and beat-by-beat evaluation of local ΔP from superficial arteries-a non-invasive, cuffless and calibration-free technique.

METHODS

The proposed technique uses a bi-modal arterial compliance probe which consisted of two identical magnetic plethysmograph (MPG) sensors located at 23 mm distance apart and a single-element ultrasound transducer. Simultaneously measured local pulse wave velocity (PWV) and arterial dimensions were used in a mathematical model for calibration-free evaluation of local ΔP. The proposed approach was initially verified using an arterial flow phantom, with invasive pressure catheter as the reference device. The developed porotype device was validated on 22 normotensive human subjects (age = 24.5 ± 4 years). Two independent measurements of local ΔP from the carotid artery were made during physically relaxed and post-exercise condition.

RESULTS

Phantom-based verification study yielded a correlation coefficient (r) of 0.93 (p < 0.001) for estimated ΔP versus reference brachial ΔP, with a non-significant bias and standard deviation of error equal to 1.11 mmHg and ±1.97 mmHg respectively. The ability of the developed system to acquire high-fidelity waveforms (dual MPG signals and ultrasound echoes from proximal and distal arterial walls) from the carotid artery was demonstrated by the in-vivo validation study. The group average beat-to-beat variation in measured carotid local PWV, arterial diameter parameters-distension and end-diastolic diameter, and local ΔP were 4.2%, 2.6%, 3.3%, and 10.2% respectively in physically relaxed condition. Consistent with the physiological phenomenon, local ΔP measured from the carotid artery of young populations was, on an average, 22 mmHg lower than the reference ΔP obtained from the brachial artery. Like the reference brachial blood pressure (BP) monitor, the developed prototype device reliably captured variations in carotid local ΔP induced by an external intervention.

CONCLUSION

This technique could provide a direct measurement of local PWV, arterial dimensions, and a calibration-free estimate of beat-by-beat local ΔP. It can be potentially extended for calibration-free cuffless BP measurement and non-invasive characterization of central arteries with locally estimated biomechanical properties.

An In-Vivo Study on Intra-Day Variations in Vascular Stiffness using ARTSENS Pen

This work presents investigations on intraday variations in arterial stiffness. For this purpose, an in-vivo study was conducted on five subjects over a duration of five consecutive days. Five stiffness index ($\beta $) measurements were obtained per day for each individual. Our clinically validated ARTSENS device was used to perform fully automated reliable stiffness measurements on the carotid artery. For each measurement, two trials were performed and averaged. These trials were observed to be repeatable with the coefficient of variation $< 0.72$%. For each day, one measurement that was performed immediately after the lunch was subject to intervention due to the consumed food, which significantly $( \mathrm {p}< 0.001)$ deviated from the mean baseline $\beta $ of the day. Such significant deviations were not observed for the rest measurements that were performed in the absence of an intervention. Two subjects who consumed caffeinated beverages during the lunch exhibited an increment in $\beta $ measurement (taken immediately after lunch) as compared to mean baseline $\beta $ of the day. Further, there was no significant (p $=$ 0.97) difference between the mean baseline $\beta $ measured over a day and the mean baseline $\beta $ measured over the entire course of the study. Results obtained from the present study demonstrated that the arterial stiffness does not vary significantly over a short period but varies progressively. However, significant temporary variations in stiffness could be observed due to dietary interventions.

ARTSENS® orientation navigation system: A study towards faster arterial stiffness measurements

ARTSENS® (ARTerial Stiffness Evaluation for Non-invasive Screening) is a technology for image-free evaluation of carotid artery stiffness with intelligent real time ultrasound signal processing algorithms for arterial wall identification and tracking. However, the influence of the ultrasound probe orientation on the clarity of the artery wall echoes makes measurement a challenge for inexperienced operators. We present an orientation navigation system (ONS), which performs real time tracking of the probe orientation and gives feedback to the user to help take faster readings with more ease for novice ARTSENS® operators. An controlled usability study on seven operators demonstrated that the ONS could reduce the average measurement time in ARTSENS® by nearly 50%. The usability of the system was rated positive with a score of 75.4/100 and a grading of 8.3/10 by the operators. The ONS system makes ARTSENS® easier to operate thus enables it to be used for quick and reliable measurements in vascular screening scenarios.

ARTSENS® Mobile: a portable image-free platform for automated evaluation of vascular stiffness

Vascular stiffness is a key physiological marker significant in screening and early diagnosis of vascular disease. However, state of art techniques for vascular stiffness evaluation are not suited for field deployment. We present ARTSENS® Mobile, a portable, image-free ultrasound technology platform for automated evaluation of carotid artery stiffness, amenable for field deployment. The technology utilises a single element ultrasound transducer operated in pulse echo modality to investigate arterial wall dynamics, assisted by intelligent algorithms for wall identification and motion tracking to completely automate the measurement. The ARTSENS Mobile is built around the NI sbRIO 9651 embedded SOM which runs all algorithms for pulsed transducer excitation, high speed synchronised ultrasound echo capture, real-time signal processing and automated measurement. The system is designed to communicate wirelessly with any smart tablet for data display and wire-free control. The accuracy of automated arterial dimension measurements made by ARTSENS Mobile was verified by phantom studies using a reference ultrasound imaging system. The ability of the system to reliably perform automated measurements was verified by a systematic in-vivo study on 21 volunteers. The intra-operator and inter-operator variability of stiffness index β, was found to be good with a coefficient of variation (CoV) less than 12% and 16 % respectively. The ARTSENS® Mobile can provide accurate and repeatable measures of arterial stiffness in an easy manner and has strong potential in large scale vascular screening.