Nathalie Bijnens

Toward Noninvasive Blood Pressure Assessment in Arteries by Using Ultrasound

A new method has been developed to measure local pressure waveforms in large arteries by using ultrasound. The method is based on a simultaneous estimation of distension waveforms and velocity profiles from a single noninvasive perpendicular ultrasound B-mode measurement. Velocity vectors were measured by applying a cross-correlation based technique to ultrasound radio-frequency (RF) data. From the ratio between changes in flow and changes in cross-sectional area of the vessel, the local pulse wave velocity (PWV) was estimated. This PWV value was used to convert the distension waveforms into pressure waveforms. The method was validated in a phantom set-up. Physiologically relevant pulsating flows were considered, employing a fluid which mimics both the acoustic and rheologic properties of blood. A linear array probe attached to a commercially available ultrasound scanner was positioned parallel to the vessel wall. Since no steering was used, the beam was perpendicular to the flow. The noninvasively estimated pressure waveforms showed a good agreement with the reference pressure waveforms. Pressure values were predicted with a precision of 0.2 kPa (1.5 mm Hg). An accurate beat to beat pressure estimation could be obtained, indicating that a noninvasive pressure assessment in large arteries by means of ultrasound is feasible.

Periprocedural variations of platelet reactivity during elective percutaneous coronary intervention

Summary.  Background: Percutaneous coronary intervention (PCI) modulates platelet reactivity (PR). Objectives: To assess: (i) the impact of coronary interventions on periprocedural variations (Δ) of PR; (ii) whether ΔPR correlates with periprocedural myocardial infarction (PMI); and (iii) the mechanisms of these variations in vitro. Methods and results: We enrolled 65 patients on aspirin (80–100 mg day−1) and clopidogrel (600 mg, 12 h before PCI): 15 with coronary angiography (CA group), 40 with PCI (PCI group), and 10 with rotational atherectomy plus PCI (RA group). PR was assessed by ADP, high‐sensitivity ADP and thrombin receptor activator peptide 6 tests prior to, immediately after and 24 h after the procedure. E‐selectin and ICAM‐1 were assessed prior to and immediately after the procedure. In vitro, PR was measured during pulsatile blood flow at baseline, after balloon inflation and after stent implantation in six porcine carotid arteries and five plastic tubes. PR declined in the CA group, but significantly increased in the PCI and RA groups immediately postprocedure, and decreased to baseline at 24 h. ΔPR increased across the three groups (P < 0.0001). In the PCI group, ΔPR was directly related to total inflation time (r = 0.435, P = 0.005) and total stent length (r = 0.586, P < 0.001). The change in E‐selectin significantly and inversely correlated with ΔPR (P < 0.001). No correlation was found with sICAM‐1. PR increased significantly more in patients with PMI than in patients without PMI (P = 0.013). In vitro, platelet activation was observed in the presence of carotid arteries but not in the presence of plastic tubes. Conclusions: Despite dual antiplatelet therapy, PCI affected platelet function proportionally to procedural complexity and the extent of vascular damage.

2-D left ventricular flow estimation by combining speckle tracking with Navier-Stokes-based regularization: an in silico, in vitro and in vivo study.

Despite the availability of multiple ultrasound approaches to left ventricular (LV) flow characterization in two dimensions, this technique remains in its childhood and further developments seem warranted. This article describes a new methodology for tracking the 2-D LV flow field based on ultrasound data. Hereto, a standard speckle tracking algorithm was modified by using a dynamic kernel embedding Navier-Stokes-based regularization in an iterative manner. The performance of the proposed approach was first quantified in synthetic ultrasound data based on a computational fluid dynamics model of LV flow. Next, an experimental flow phantom setup mimicking the normal human heart was used for experimental validation by employing simultaneous optical particle image velocimetry as a standard reference technique. Finally, the applicability of the approach was tested in a clinical setting. On the basis of the simulated data, pointwise evaluation of the estimated velocity vectors correlated well (mean r = 0.84) with the computational fluid dynamics measurement. During the filling period of the left ventricle, the properties of the main vortex obtained from the proposed method were also measured, and their correlations with the reference measurement were also calculated (radius, r = 0.96; circulation, r = 0.85; weighted center, r = 0.81). In vitro results at 60 bpm during one cardiac cycle confirmed that the algorithm properly measures typical characteristics of the vortex (radius, r = 0.60; circulation, r = 0.81; weighted center, r = 0.92). Preliminary qualitative results on clinical data revealed physiologic flow fields.

2D intracardiac flow estimation by combining speckle tracking with navier-stokes based regularization: a study with dynamic kernels

Echocardiographic transducers record two-dimensional (2D) datasets in a sector reference after which a scan-conversion is applied to obtain the images in Cartesian coordinates. To assess left ventricular(LV) flow dynamics by a low dose contrast injection, we recently developed a 2D tracking methodology by combining speckle tracking (ST) with Navier-Stokes based regularization and it has been tested in synthetic ultrasound datasets prior to the scan conversion. However, in clinical settings the estimation becomes challenging due to the inhomogeneous image patterns which are inherently introduced by scan-conversion and are more likely to be locally strengthened by non homogeneous bubble seeding and high velocity gradient. To better deal with that, the aim of this study was hereby to modify the previous method by using a dynamic tracking kernel size. Its performance was first quantified in synthetic scan-converted ultrasound data based on a computational fluid dynamics model of LV flow. The applicability of the approach was tested in an experimental phantom setup with pulsed flow that mimics the normal human heart and simultaneously allows for optical particle image velocimetry as a standard reference technique. Both qualitative and quantitative comparison of the estimated flow fields and reference measurements showed that the modified methodology can correctly characterize the flow field properties and is promising to offer new insights into the flow dynamics inside the left ventricle.

In-Vivo Real-Time Contrast-Free Ultrasonic Blood Flow Velocity Profile Measurement

Recently, Ultrasonic Perpendicular Velocimetry (UPV) based algorithms, as opposed to commonly used Doppler technique (Figure 1), were applied to Radio Frequency (RF)-data acquired in an in-vitro setup [1,3]. Thus, the estimation of velocity components perpendicularly to the ultrasound beam and the simultaneous and accurate assessment of wall position and axial velocity profiles were made feasible. By integrating the measured velocity profile an accurate flow estimation was made possible. Furthermore, the ratio between the changes in flow Q(t) and the changes in cross-sectional area of the vessel A(t) was found to offer an accurate estimation of the local Pulse Wave Velocity (PWV). By combining the PWV with the diameter waveform, accurate local pressure estimation was obtained indicating that a non-invasive pressure assessment by means of ultrasound is feasible [3]. However, the abovementioned method is time consuming due to the data size and the post-processing procedure required. Additionally, the Fast Fourier Transform (FFT) on Butterworth Band Pass Filters (BPF) for vessel’s wall removal requires contrast agents dispersion in the fluid for the application of UPV. A real-time approach, of the previously described techniques, was applied [2] in-vitro using a Blood Mimicking Fluid (BMF), as contrast agent, resembling the rheological (shear thinning) and acoustical (backscattering) properties of blood and ex-vivo using BMF or contrast-free real blood implementing Wavelet Transform (WT) filtering. The use of a Graphics Processing Unit (GPU) [4], succeeded in considerable acceleration and WT [5] filtering on the rough RF-data, in improvement of the discrimination between reflections from the vessel wall and scattering from small particles. In this research the method is extended to include in-vivo measurements.Copyright

The Use of Wavelets for Wall Removal and Scatter Enhancement in Ultrasound Blood Velocity Profile Measurements

Ultrasound waves, transmitted by a transducer into a body, are reflected and scattered by the materials they encounter in the body. In case of blood flow measurements in an artery, the received signal will contain the information not only from the moving red blood cells, but the reflections from the vessel wall of other soft tissue structures as well. The discrimination between ultrasound signals originating from scattering of red blood cells and reflection of tissue is one of the major problems for blood velocity assessment. Traditionally, in Doppler processing, where the highest blood velocities in the middle of the vessel are estimated, this discrimination is obtained via a high pass filter with a static cut-off frequency related to the maximum frequency content of the reflections. This is illustrated in Fig.1 (top). As illustrated in Fig. 1 (bottom), problems occur for velocity estimation of slowly moving blood cells close to the vessel wall and in case of perpendicular insonification [1]. In these cases, there is no frequency shift in the signal received from scattering on blood cells. Furthermore, the intensity of the reflections from the vessel wall is highest in case of perpendicular insonification. Filtering in these cases is very challenging since it allows the assessment of blood velocity profiles without contrast agents.Copyright

Towards Non-Invasive Pressure Assessment

In clinical practice, ultrasound is frequently applied to non-invasively assess blood velocity, blood volume flow and blood vessel wall properties such as vessel wall thickness and vessel diameter waveforms. To convert these properties into relevant biomechanical properties that are related to cardiovascular disease (CVD), such as elastic modulus and compliance of the vessel wall, local pressure has to be assessed simultaneously with vessel wall thickness and vessel diameter waveforms. Additionally, accurate estimates of vascular impedance (transfer function between pressure and blood flow) can be a valuable tool for the estimation of the condition of the vessel, e.g., to diagnose stenosis. Studies of arterial impedance in humans, however, are hampered by the lack of reliable non-invasive techniques to simultaneously record pressure and flow locally as a function of time. Local pressure assessment together with flow has great potential for improving the ability to diagnose and monitor CVD.Copyright