An improved method for detection of carotid walls in ARTSENS

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.

A Machine Learning Pipeline for Measurement of Arterial Stiffness in A-Mode Ultrasound

Arterial stiffness (AS) of the carotid artery is an early marker of stratifying cardiovascular disease risk. This article aims to improve the performance of ARTSENS, a noninvasive A-mode ultrasound-based device for measuring AS. The primary objective of ARTSENS is to enable the measurement of elastic modulus using A-Mode ultrasound and blood pressure. As this device is image-free, there is a need to automate: 1) carotid detection; 2) wall localization; and 3) inner lumen diameter measurement. This has been performed using conventional signal processing methods in some of the earlier works in this domain. In this article, deep neural network (DNN) models are employed to perform the above three tasks. The DNNs were trained over data acquired from 82 subjects at two different medical centers. Ground-truth labeling was performed by a trained operator using corresponding measurements from the state-of-the-art Aloka e-Tracking system. All three DNN models had significantly lower errors compared to earlier signal processing methods and could perform their measurements using a single A-Mode frame. Using the DNNs, two different machine learning pipelines have been proposed here to measure the elastic modulus; the best among them could achieve an error of 9.3% with the Pearson correlation coefficient of 0.94 ( ). The models were tested on Raspberry Pi and Jetson Nano single board computers to demonstrate real-time processing on low computational resources.

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.

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.

An Imageless Ultrasound Device to Measure Local and Regional Arterial Stiffness

Arterial stiffness (AS) has been shown to be an important marker for risk assessment of cardiovascular events. Local arterial stiffness (LAS) is conventionally measured by evaluating arterial distensibility at particular arterial sites through ultrasound imaging systems. Regional arterial stiffness (RAS) is generally obtained by evaluating carotid to femoral pulse wave velocity (cfPWV) through tonometric devices. RAS has a better prognostic value than LAS and cfPWV is considered as the gold standard of AS. Over the past few years our group has been developing ARTerial Stiffness Evaluation for Non-Invasive Screening (ARTSENS), an inexpensive and portable device to measure the LAS. It uses a single element ultrasound transducer to obtain A-Mode frames from the desired artery and is fully automated to enable a non-expert to perform measurements. In this work, we report an extension of ARTSENS to enable measurement of cfPWV that now makes it the only fully automatic device that can measure both LAS and RAS. In this paper, we provide a general review of the ARTSENS and compare it with other state-of-the-art AS measurement systems. cfPWV measurement using ARTSENS was cross-validated against SphygmoCor by successive measurements with both devices on 41 human subjects and excellent agreement between both devices was demonstrated (Coefficient of determination and, limits of agreement m/s). The inter-device correlation between ARTSENS and SphygmoCor was found to be better than other similar studies reported in the literature.

Automatic Measurement of End-Diastolic Arterial Lumen Diameter in ARTSENS

Over past few years, we are developing a system for facilitating large scale screening of patients for cardiovascular risk—arterial stiffness evaluation for noninvasive screening (ARTSENS). ARTSENS is an image-free device that uses a single element ultrasound transducer to obtain noninvasive measurements of arterial stiffness (AS) in a fully automated manner. AS is directly proportional to end-diastolic lumen diameter (Dd). Multilayered structure of the arterial walls and indistinct characteristics of intima-lumen interface (ILI) makes it quite difficult to accurately estimate Dd in A-mode radio-frequency (RF) frames obtained from ARTSENS. In this paper, we propose a few methods based on fitting simple mathematical models to the echoes from arterial walls, followed by a novel method to fuse the information from curve fitting error and distension curve to arrive at an accurate measure of Dd. To bring down the curve fitting time and facilitate processing on low-end processors, a novel approach using the autocorrelation of echoes from opposite walls of the artery has been discussed. The methods were analyzed for their comparative accuracy against reference Dd obtained from 85 human volunteers using Hitachi-Aloka eTRACKING system. Dd from all reported methods show strong and statistically significant positive correlation with eTRACKING and mean error of less than 7% could be achieved. As expected, Dd from all methods show significant positive correlation with age.