Lilian Kornet

Assessment of the spatial homogeneity of artery dimension parameters with high frame rate 2-D B-mode

To elicit vessel wall inhomogeneities in diameter and distension along an arterial segment, a 2-D vessel wall-tracking system based on fast B-mode has been developed. The frame rate of a 7.5-MHz linear-array transducer (length 36 mm) is enhanced by increasing the pulse-repetition frequency to 10 kHz, decreasing the number of echo lines per frame from 128 to 64, or increasing the interspacing between echo lines with a factor of two or four. Dedicated software has been developed to extract for each echo-line the end-diastolic diameter from the B-mode image and the 2-D distension waveform from the underlying radiofrequency (RF) information. The method is validated in tubes with various focal lesion sizes. Straight segments of presumably homogeneous common carotid arteries have also been tested. The temporal and spatial SD of diameter or distension reveals inhomogeneities in time or space (i.e., inhomogeneities in artery characteristics). The method can be implemented in echo systems supporting high frame rates and real-time processing of radiofrequency data.

Differences in Near-Wall Shear Rate in the Carotid Artery Within Subjects Are Associated With Different Intima-Media Thicknesses

In the common carotid artery, reflections originating from the periphery and the flow divider may affect the shape of the flow velocity profile and, hence, near-wall shear rate (WSR) differently just before the bifurcation (location B) than 20 to 30 mm farther upstream (location A). Recent developments in ultrasound technology allow the assessment of WSR and intima-media thickness (IMT) at the same site in the carotid artery in vivo. We therefore determined WSR at locations A and B and investigated whether the differences between both sites, if any, were associated with different IMTs and different mechanical properties of the arterial wall. The effect of age on the possible differences was assessed as well. The study was performed on presumably healthy volunteers (n=53). In all individuals, IMT was larger at location B than at location A. The relative difference in IMT between both locations was not affected by age. No significant differences in diameter and distension were found between locations. Near peak systolic and near mean WSR at the posterior wall (PWSRp and MWSRp, respectively) were significantly lower at location B than at location A. The relative differences in PWSRp and MWSRp between both locations within subjects were independent of age. The velocity profiles were more blunted at location A than at location B. PWSRp and MWSRp significantly decreased and IMT significantly increased with age at both locations. IMT was negatively correlated with PWSRP and MWSRP at location B, but this correlation was not significant at location A. In summary, in the common carotid artery, the lower WSR near the bifurcation, as compared with 20 to 30 mm upstream, is associated with a larger IMT than at the more proximal site. The relative difference between both locations within subjects is independent of age.

Assessment of Local Pulse Wave Velocity in Arteries Using 2D Distension Waveforms

The reciprocal of the arterial pulse wave velocity contains crucial information about the mechanical characteristics of the arterial wall but is difficult to assess noninvasively in vivo. In this paper, a new method to assess local pulse wave velocity (PWV) is presented. To this end, multiple adjacent distension waveforms are determined simultaneously along a short arterial segment, using a single 2D-vessel wall tracking system with a high frame rate (651 Hz). Each B-mode image consists of 16 echo lines spanning a total width of 15.86 mm. Dedicated software has been developed to extract the end-diastolic diameter from the B-mode image and the distension waveforms from the underlying radiofrequency (rf) information for each echo-line. The PWV is obtained by determining the ratio of the temporal and spatial gradient of adjacent distension velocity waveforms. The proposed method is verified in a phantom and in the common carotid artery (CCA) of humans. Phantom experiments show a high concordance between the PWV obtained from 2D distension velocity waveforms (4.21 ± 0.02 m/s) and the PWV determined using two pressure catheters (4.26 ± 0.02 m/s). Assuming linear spatial gradients, the PWV can also be obtained in vivo for CCA and averages to 5.5 ± 1.5 m/s (intersubject variation, n = 23), which compares well to values found in literature. Furthermore, intrasubject PWV compares well with those calculated using the Bramwell-Hill equation. It can be concluded that the PWV can be obtained from the spatial and temporal gradient if the spatial gradient is linear over the observed length of the artery, i.e. the artery should be homogenous in diameter and distension and the influence of reflections must be small.

In the Femoral Artery Bifurcation, Differences in Mean Wall Shear Stress Within Subjects Are Associated With Different Intima-Media Thicknesses

In elastic arteries, intima-media thickening is more pronounced in areas with low than with high mean and peak wall shear stress. These findings in elastic arteries are not necessarily representative of the situation in muscular arteries. The former arteries have to store volume energy, whereas the latter are mainly conductive vessels. It was the aim of the present study to investigate noninvasively whether differences in wall shear stress within a muscular artery bifurcation, if any, were associated with different intima-media thicknesses (IMTs). The effect of age on the possible differences was assessed as well. We determined IMT and mean, peak systolic, and the maximum cyclic change in shear stress near the posterior wall in the common (FC) and the superficial (FS) femoral artery 20 to 30 mm from the flow divider in 54 presumed healthy subjects between 21 and 74 years of age. Results were considered in terms of intrasubject differences. Before the study, the reliability of the ultrasonic system to assess wall shear rate and IMT was determined in terms of intrasubject variability. IMT at the posterior wall was significantly larger in the FC than in the FS, probably owing to the significantly lower mean wall shear stress at this site in the FC. The relative differences in IMT and mean wall shear stress between FC and FS were independent of age. The difference in wall shear stress between both arteries can likely be explained by a different influence of reflections. In both the FC and FS, mean, peak systolic, and maximum cyclic change in shear stress near the posterior wall did not change significantly with age, whereas IMT did increase significantly with age.

Carotid diameter variations as a non-invasive tool to examine cardiac baroreceptor sensitivity.

Objective and design Recently we developed a non-invasive ultrasound technique, enabling the determination of end-diastolic diameter and distension (increase in diameter during cardiac cycle) over a long time period. Using this technique we test the hypothesis that low-frequency variations derived from non-invasively determined common carotid artery diameter signals are superior to those derived from non-invasively determined arterial pressure to predict heart rate variability, a method used to assess baroreceptor sensitivity. Moreover, we investigate whether the reduced baroreflex sensitivity in the elderly persists after eliminating the influence of the stiffness of the vessel wall and can be attributed to an impairment of the neural baroreflex pathways. Results The main finding of the present study is that variability in distension rate, i.e. increase in diameter during the cardiac cycle per systolic time interval, of the common carotid artery is a considerably more accurate predictor of R–R interval variability than variability in systolic arterial finger pressure. Moreover, distension rate variability is a more accurate predictor of R–R interval variability than distension and diameter variability. The reduced baroreflex sensitivity in the elderly persists in spite of the elimination of the influence of the stiffness of the vessel wall. Conclusions We conclude that: (1) in the evaluation of baroreceptor sensitivity the assessment of variations in parameters derived from carotid arterial diameter is superior to the assessment of parameters derived from peripheral arterial finger pressure; and (2) conduction by the neural baroreflex pathways deteriorates with age.

Mean Wall Shear Stress in the Femoral Arterial Bifurcation Is Low and Independent of Age at Rest

In elastic arteries, mean wall shear stress appears to be close to 1.5 Pa, the value predicted by the theory of minimal energy loss. This finding in elastic arteries does not necessarily represent the situation in muscular arteries. Elastic arteries have to store potential energy, while muscular arteries have mainly a conductive function. Therefore, we determined wall shear stress and its age dependency in the common and superficial femoral arteries, 2–3 cm from the flow divider in 54 presumed healthy volunteers between 21 and 74 years of age, using a non-invasive ultrasound system. Prior to the study, the reliability of this system was determined in terms of intrasubject variation. Mean wall shear stress was significantly lower in the common femoral artery (0.35 ± 0.18 Pa) than in the superficial femoral artery (0.49 ± 0.15 Pa). In all age categories, peak systolic wall shear stress and the maximal cyclic change in wall shear stress were not significantly different in the common and the superficial femoral arteries. Peak systolic wall shear stress in the common and the superficial femoral arteries was not significantly different from the value previously determined in the common carotid artery, but mean wall shear stress was lower in the common and superficial femoral arteries than in the common carotid artery by a factor of 2–4. In both the common and the superficial femoral arteries, mean, peak systolic and maximal cyclic change in wall shear stress did not change significantly with age, nor did diameter. We conclude that, as compared to elastic arteries, mean wall shear stress is low in the conductive arteries of a resting leg, due to backflow during the first part of the diastolic phase of the cardiac cycle and the absence of flow during the rest of the diastolic phase. Mean wall shear stress is lower in the common than in the superficial femoral artery due to additional reflections from the deep femoral artery.

Element of Physiology and Mechanics of Human Arteries

Some elements concerning the physiology and the corresponding mechanical properties of human arteries are introduced. The non-invasive ultrasound techniques developed to assess strain, distensibility and compliance and IMT, velocity and wall shear rate/stress distribution in arteries are introduced. Artery wall properties, and intima-media thickness (IMT) change with age and disease are discussed with the relation with artery wall function and structure and role in the genesis of atherosclerosis. In hypertension and with aging elastic arteries become stiffer (loss of distensibility and compliance), the degree of stiffening also varies along artery bifurcations and differs in elastic and muscular arteries. These variations give differences in intima-media thickness and sites of preference for atherosclerosis. Alterations in structure and composition of the arterial wall responsible for the loss of distensibility and compliance and the increase in intima-media thickness (IMT) in disease and aging. Reasons and effects are discussed.