In vitro validation of MR measurements of arterial pulse-wave velocity in the presence of reflected waves

Quantification of aortic pulse wave velocity from a population based cohort: a fully automatic method

BACKGROUND

Aortic pulse wave velocity (PWV) is an indicator of aortic stiffness and is used as a predictor of adverse cardiovascular events. PWV can be non-invasively assessed using magnetic resonance imaging (MRI). PWV computation requires two components, the length of the aortic arch and the time taken for the systolic pressure wave to travel through the aortic arch. The aortic length is calculated using a multi-slice 3D scan and the transit time is computed using a 2D velocity encoded MRI (VE) scan. In this study we present and evaluate an automatic method to quantify the aortic pulse wave velocity using a large population-based cohort.

METHODS

For this study 212 subjects were retrospectively selected from a large multi-center heart-brain connection cohort. For each subject a multi-slice 3D scan of the aorta was acquired in an oblique-sagittal plane and a 2D VE scan acquired in a transverse plane cutting through the proximal ascending and descending aorta. PWV was calculated in three stages: (i) a multi-atlas-based segmentation method was developed to segment the aortic arch from the multi-slice 3D scan and subsequently estimate the length of the proximal aorta, (ii) an algorithm that delineates the proximal ascending and descending aorta from the time-resolved 2D VE scan and subsequently obtains the velocity-time flow curves was also developed, and (iii) automatic methods that can compute the transit time from the velocity-time flow curves were implemented and investigated. Finally the PWV was obtained by combining the aortic length and the transit time.

RESULTS

Quantitative evaluation with respect to the length of the aortic arch as well as the computed PWV were performend by comparing the results of the novel automatic method to those obtained manually. The mean absolute difference in aortic length obtained automatically as compared to those obtained manually was 3.3 ± 2.8 mm (p < 0.05), the manual inter-observer variability on a subset of 45 scans was 3.4 ± 3.4 mm (p = 0.49). Bland-Altman analysis between the automataic method and the manual methods showed a bias of 0.0 (-5.0,5.0) m/s for the foot-to-foot approach, -0.1 (-1.2, 1.1) and -0.2 (-2.6, 2.1) m/s for the half-max and the cross-correlation methods, respectively.

CONCLUSION

We proposed and evaluated a fully automatic method to calculate the PWV on a large set of multi-center MRI scans. It was observed that the overall results obtained had very good agreement with manual analysis. Our proposed automatic method would be very beneficial for large population based studies, where manual analysis requires a lot of manpower.

Abnormal aortic stiffness in patients with bicuspid aortic valve: phenotypic variation determined by magnetic resonance imaging

The aim of this study was to assess aortic stiffness in patients with bicuspid aortic valve (BAV), and to determine if differences exist among the BAV phenotypes. Stiffness was measured by pulse wave velocity (PWV) determined using velocity-encoded magnetic resonance imaging (VENC-MRI). VENC-MRI was performed in 100 BAV patients and 45 normal controls. PWV was determined between the mid ascending and mid descending aorta. The BAV phenotypes were characterized using steady-state free precession (SSFP) images acquired across the face of the aortic valve, and classified as follows: right-left cusp (R-L) fusion, right and non-coronary cusp (R-NC) fusion, and left and non-coronary cusp (L-NC) fusion. The following BAV phenotypes were identified: 76 R-L, 23 R-NC, and 1 L-NC fusion. BAV patients demonstrated significantly greater PWV compared to normal controls, after adjusting for age (9.16 vs. 3.83 m/s; p < 0.0001). Furthermore, PWV was significantly greater in patients with R-NC fusion than those with R-L fusion phenotype (12.27 vs. 7.97 m/s; p < 0.001). There was significantly increased PWV from VENC-MRI in BAV patients compared to normal controls. Thisis the first to demonstrate the association of different BAV phenotypes and aortic stiffness. VENC-MRI PWV assessment potentially represents a novel parameter for enhanced surveillance and may alter surgical triage of aorta in this high risk group.

Aortic Arch Stiffness Is Associated With Incipient Brain Injury in Patients With Hypertension

BACKGROUND

It has been shown that microstructural brain tissue damage can be detected in hypertension patients, while the underlying mechanisms are not fully understood. We aim to explore the association between diffusion tensor imaging (DTI) measures of brain injury and aortic arch pulse wave velocity (PWV) in hypertensive patients without clinically manifest cerebrovascular disease.

METHODS

Sixty-six hypertension patients (30 men, mean age 46±14 years) were prospectively included. Aortic arch PWV was assessed using velocity-encoded magnetic resonance imaging (VE-MRI). Brain tissue integrity was assessed by using DTI. Multivariable linear regression analysis was performed to assess the association between aortic arch PWV and fractional anisotropy (FA), axial diffusivity (AxD), and radial diffusivity (RD).

RESULTS

Increased aortic arch PWV was associated with decreased white matter FA (β = -0.30, P = 0.018), increased gray matter AxD (β = 0.28, P = 0.016), and increased gray and white matter RD (β = 0.30, P = 0.008 and β = 0.35, P = 0.003, respectively). These effects were independent of age, sex, body mass index, smoking, and white matter hyperintensity (WMH) volume.

CONCLUSIONS

Aortic arch stiffness relates to incipient brain injury before overt brain abnormalities may become apparent in patients with hypertension.

Robust segmentation methods with an application to aortic pulse wave velocity calculation

Aortic stiffness has proven to be an important diagnostic and prognostic factor of many cardiovascular diseases, as well as an estimate of overall cardiovascular health. Pulse wave velocity (PWV) represents a good measure of the aortic stiffness, while the aortic distensibility is used as an aortic elasticity index. Obtaining the PWV and the aortic distensibility from magnetic resonance imaging (MRI) data requires diverse segmentation tasks, namely the extraction of the aortic center line and the segmentation of aortic regions, combined with signal processing methods for the analysis of the pulse wave. In our study non-contrasted MRI images of abdomen were used in healthy volunteers (22 data sets) for the sake of non-invasive analysis and contrasted magnetic resonance (MR) images were used for the aortic examination of Marfan syndrome patients (8 data sets). In this research we present a novel robust segmentation technique for the PWV and aortic distensibility calculation as a complete image processing toolbox. We introduce a novel graph-based method for the centerline extraction of a thoraco-abdominal aorta for the length calculation from 3-D MRI data, robust to artifacts and noise. Moreover, we design a new projection-based segmentation method for transverse aortic region delineation in cardiac magnetic resonance (CMR) images which is robust to high presence of artifacts. Finally, we propose a novel method for analysis of velocity curves in order to obtain pulse wave propagation times. In order to validate the proposed method we compare the obtained results with manually determined aortic centerlines and a region segmentation by an expert, while the results of the PWV measurement were compared to a validated software (LUMC, Leiden, the Netherlands). The obtained results show high correctness and effectiveness of our method for the aortic PWV and distensibility calculation.

Robust estimation of pulse wave transit time using group delay

PURPOSE

To evaluate the efficiency of a novel transit time (Δt) estimation method from cardiovascular magnetic resonance flow curves.

MATERIALS AND METHODS

Flow curves were estimated from phase contrast images of 30 patients. Our method (TT-GD: transit time group delay) operates in the frequency domain and models the ascending aortic waveform as an input passing through a discrete-component "filter," producing the observed descending aortic waveform. The GD of the filter represents the average time delay (Δt) across individual frequency bands of the input. This method was compared with two previously described time-domain methods: TT-point using the half-maximum of the curves and TT-wave using cross-correlation. High temporal resolution flow images were studied at multiple downsampling rates to study the impact of differences in temporal resolution.

RESULTS

Mean Δts obtained with the three methods were comparable. The TT-GD method was the most robust to reduced temporal resolution. While the TT-GD and the TT-wave produced comparable results for velocity and flow waveforms, the TT-point resulted in significant shorter Δts when calculated from velocity waveforms (difference: 1.8±2.7 msec; coefficient of variability: 8.7%). The TT-GD method was the most reproducible, with an intraobserver variability of 3.4% and an interobserver variability of 3.7%.

CONCLUSION

Compared to the traditional TT-point and TT-wave methods, the TT-GD approach was more robust to the choice of temporal resolution, waveform type, and observer.

Estimation of aortic pressure waveforms from 4D phase-contrast MRI

Several approaches for the non-invasive MRI-based measurement of the aortic pressure waveform over the heart cycle have been proposed in the last years. These methods are normally based on time-resolved, two-dimensional phase-contrast sequences with uni-directionally encoded velocities (2D PC-MRI). In contrast, three-dimensional acquisitions with tridirectional velocity encoding (4D PC-MRI) have been shown to be a suitable data source for detailed investigations of blood flow and spatial blood pressure maps. In order to avoid additional MR acquisitions, it would be advantageous if the aortic pressure waveform could also be computed from this particular form of MRI. Therefore, we propose an approach for the computation of the aortic pressure waveform which can be completely performed using 4D PC-MRI. After the application of a segmentation algorithm, the approach automatically computes the aortic pressure waveform without any manual steps. We show that our method agrees well with catheter measurements in an experimental phantom setup and produces physiologically realistic results in three healthy volunteers.

Aortic pulse wave velocity measurements with undersampled 4D flow-sensitive MRI: comparison with 2D and algorithm determination

PURPOSE

To compare pulse wave velocity (PWV) measurements obtained from radially undersampled 4D phase-contrast magnetic resonance imaging (PC-MRI) with 2D PC measurements and to evaluate four PWV algorithms.

MATERIALS AND METHODS

PWV was computed from radially undersampled 3D, 3-directionally velocity-encoded PC-MRI (4D) acquisitions performed on a 3T MR scanner in 18 volunteers. High temporal resolution 2D PC scans serving as a reference standard were available in 14 volunteers. Four PWV algorithms were tested: time-to-upstroke (TTU), time-to-peak (TTP), time-to-foot (TTF), and cross-correlation (XCorr). Bland-Altman analysis was used to determine inter- and intraobserver reproducibility and to compare differences between algorithms. Differences in age and PWV measurements were analyzed with Student's t-tests. The variability of age-corrected data was assessed with a Brown-Forsythe analysis of variance (ANOVA) test.

RESULTS

2D (4.6-5.3 m/s) and 4D (3.8-4.8 m/s) PWV results were in agreement with previously reported values in healthy subjects. Of the four PWV algorithms, the TTU, TTF, and XCorr algorithms gave similar and reliable results. Average biases of +0.30 m/s and -0.01 m/s were determined for intra- and interobserver variability, respectively. The Brown-Forsythe test revealed that no differences in variability could be found between 2D and 4D PWV measurements.

CONCLUSION

4D PC-MRI with radial undersampling provides reliable and reproducible measurements of PWV. TTU, TTF, and XCorr were the preferred PWV algorithms.

MRI-assessed regional pulse wave velocity for predicting absence of regional aorta luminal growth in marfan syndrome

BACKGROUND

In patients with Marfan syndrome (MFS), increased aortic wall stiffening may lead to progressive aortic dilatation. Aortic Pulse Wave Velocity (PWV), a marker of wall stiffness can be assessed regionally, using in-plane multi-directional velocity-encoded MRI. This study examined the diagnostic accuracy of regional PWV for prediction of regional aortic luminal growth during 2-year follow-up in MFS patients.

METHODS

In twenty-one MFS patients (mean age 36 ± 15 years, 11 male) regional PWV and aortic luminal areas were assessed by 1.5 T MRI. At 2-year follow-up, the incidence of luminal growth, defined as mean luminal diameter increase >2mm was determined for five aortic segments (S1, ascending aorta; S2, aortic arch; S3, thoracic descending aorta, S4, supra-renal and S5, infra-renal abdominal aorta). Regional PWV at baseline was considered increased when exceeding age-related normal PWV (healthy volunteers (n=26; mean age 30 ± 10 years, 15 male)) by two standard-errors. Sensitivity and specificity of regional PWV-testing for prediction of regional luminal growth were determined.

RESULTS

Regional PWV at baseline was increased in 17 out of 102 segments (17%). Significant luminal growth at follow-up was reported in 14 segments (14%). The specificity of regional PWV-testing was ≥ 78% for all aortic segments, sensitivity was ≤ 33%.

CONCLUSIONS

Regional PWV was significantly increased in MFS patients as compared to healthy volunteers within similar age range, in all aortic segments except the ascending aorta. Furthermore, regional PWV-assessment has moderate to high specificity for predicting absence of regional aortic luminal growth for all aortic segments in MFS patients.

Analysis of pulse wave velocity in the thoracic aorta by flow-sensitive four-dimensional MRI: reproducibility and correlation with characteristics in patients with aortic atherosclerosis

PURPOSE

To measure aortic pulse wave velocity (PWV) using flow-sensitive four-dimensional (4D) MRI and to evaluate test-retest reliability, inter- and intra-observer variability in volunteers and correlation with characteristics in patients with aortic atherosclerosis.

MATERIALS AND METHODS

Flow-sensitive 4D MRI was performed in 12 volunteers (24 ± 3 years) and 86 acute stroke patients (68 ± 9 years) with aortic atherosclerosis. Retrospectively positioned 28 ± 4 analysis planes along the entire aorta (inter-slice-distance = 10 mm) and frame wise lumen segmentation yielded flow-time-curves for each plane. Global aortic PWV was calculated from time-shifts and distances between the upslope portions of all available flow-time curves.

RESULTS

Inter- and intra-observer variability of PWV measurements in volunteers (7% and 8%) was low while test-retest reliability (22%) was moderate. PWV in patients was significantly higher compared with volunteers (5.8 ± 2.9 versus 3.8 ± 0.8 m/s; P = 0.02). Among 17 patient characteristics considered, statistical analysis revealed significant (P < 0.05) but low correlation of PWV with age (r = 0.25), aortic valve insufficiency (r = 0.29), and pulse pressure (r = 0.28). Multivariate modeling indicated that aortic valve insufficiency and elevated pulse pressure were significantly associated with higher PWV (adjusted R(2) = 0.13).

CONCLUSION

Flow-sensitive 4D MRI allows for estimating aortic PWV with low observer dependence and moderate test-retest reliability. PWV in patients correlated with age, aortic valve insufficiency, and pulse pressure.

Bramwell-Hill modeling for local aortic pulse wave velocity estimation: a validation study with velocity-encoded cardiovascular magnetic resonance and invasive pressure assessment

BACKGROUND

The Bramwell-Hill model describes the relation between vascular wall stiffness expressed in aortic distensibility and the pulse wave velocity (PWV), which is the propagation speed of the systolic pressure wave through the aorta. The main objective of this study was to test the validity of this model locally in the aorta by using PWV-assessments based on in-plane velocity-encoded cardiovascular magnetic resonance (CMR), with invasive pressure measurements serving as the gold standard.

METHODS

Seventeen patients (14 male, 3 female, mean age ± standard deviation = 57 ± 9 years) awaiting cardiac catheterization were prospectively included. During catheterization, intra-arterial pressure measurements were obtained in the aorta at multiple locations 5.8 cm apart. PWV was determined regionally over the aortic arch and locally in the proximal descending aorta. Subsequently, patients underwent a CMR examination to measure aortic PWV and aortic distention. Distensibility was determined locally from the aortic distension at the proximal descending aorta and the pulse pressure measured invasively during catheterization and non-invasively from brachial cuff-assessment. PWV was determined regionally in the aortic arch using through-plane and in-plane velocity-encoded CMR, and locally at the proximal descending aorta using in-plane velocity-encoded CMR. Validity of the Bramwell-Hill model was tested by evaluating associations between distensibility and PWV. Also, theoretical PWV was calculated from distensibility measurements and compared with pressure-assessed PWV.

RESULTS

In-plane velocity-encoded CMR provides stronger correlation (p = 0.02) between CMR and pressure-assessed PWV than through-plane velocity-encoded CMR (r = 0.69 versus r = 0.26), with a non-significant mean error of 0.2 ± 1.6 m/s for in-plane versus a significant (p = 0.006) error of 1.3 ± 1.7 m/s for through-plane velocity-encoded CMR. The Bramwell-Hill model shows a significantly (p = 0.01) stronger association between distensibility and PWV for local assessment (r = 0.8) than for regional assessment (r = 0.7), both for CMR and for pressure-assessed PWV. Theoretical PWV is strongly correlated (r = 0.8) with pressure-assessed PWV, with a statistically significant (p = 0.04) mean underestimation of 0.6 ± 1.1 m/s. This theoretical PWV-estimation is more accurate when invasively-assessed pulse pressure is used instead of brachial cuff-assessment (p = 0.03).

CONCLUSIONS

CMR with in-plane velocity-encoding is the optimal approach for studying Bramwell-Hill associations between local PWV and aortic distensibility. This approach enables non-invasive estimation of local pulse pressure and distensibility.

Measurement of aortic arch pulse wave velocity in cardiovascular MR: comparison of transit time estimators and description of a new approach

PURPOSE

To investigate the efficiency of a new method (TT-Upslope) for transit time (Δt) estimation from cardiovascular MR (CMR) velocity curves.

MATERIALS AND METHODS

Fifty healthy volunteers (40 ± 15 years) underwent applanation tonometry to estimate carotid-femoral pulse wave velocity (cf-PWV) and carotid pressure measurements, and CMR to estimate aortic arch-PWV and ascending aorta distensibility (AAD). The Δt was calculated with TT-Upslope by minimizing the area delimited by two sigmoid curves fitted to the systolic upslope of the ascending (AAC) and descending (DAC) aorta velocity curves, and compared with previously described methods: TT-Point using the half maximum of AAC and DAC, TT-Foot using AAC and DAC feet, and TT-Wave by minimizing the area between AAC and DAC curves using cross correlation.

RESULTS

All the Δt methods provided a high reproducibility of arch-PWV. However, TT-Upslope and TT-Wave resulted in better correlations with aging (r = 0.83/r = 0.83 versus r = 0.47/r = 0.72), cf-PWV (r = 0.69/r = 0.70 versus r = 0.34/r = 0.59), and AAD (r = 0.81/r = 0.71 versus r = 0.61/r = 0.60). Furthermore, TT-Upslope resulted in stronger relationship between arch-PWV and AAD according to a theoretical model and provided better characterization of older subjects compared with TT-Wave.

CONCLUSION

Arch-PWV estimated with CMR using the TT-Upslope method was found to be reproducible and accurate, providing strong correlations with age and aortic stiffness indices.

Using nanosecond pulse near-field sensing technology for non-contact cardiovascular parameters measurement

Clinically arterial stiffness has shown that it is the most important cause of cardiovascular complications and also an independent risk factor to several cardiovascular diseases. In routine, there are many preferable non-invasive methods, including pressure-sensitive transducers, applanation tonometry, Doppler ultrasound and MRI, to get insight of cardiovascular condition. However, the operation of traditional monitors is relied on professionals' experience, and also the sensing probes needed to exert pressure to the user directly. The measurement procedure is short-term and easy to cause discomfort. To improve the issues of these measuring techniques, the non-contact and non-invasive measuring method will become an important innovation. In this paper, the novel nanosecond pulse near-field sensing (NPNS) based screening technology, which includes radio frequency (RF) pulse transmission and a flat antenna connected to transceiver of miniature radar, is proposed to monitor cardiovascular activity. A dedicated analysis software is also provided to calculate cardiovascular parameters, including PWV, average systolic time, reflection index (RI), heart and heart rate variability (HRV), for clinical applications. To evaluate the performance, the proposed method was applied on aortic pulse measurement at the body site of chest. As a result, it shows 0.92 correlations with the measurement result from commercial product, and performs the capability of continuously long-term monitoring in real-time.

Estimation of global aortic pulse wave velocity by flow-sensitive 4D MRI

The aim of this study was to determine the value of flow-sensitive four-dimensional MRI for the assessment of pulse wave velocity as a measure of vessel compliance in the thoracic aorta. Findings in 12 young healthy volunteers were compared with those in 25 stroke patients with aortic atherosclerosis and an age-matched normal control group (n = 9). Results from pulse wave velocity calculations incorporated velocity data from the entire aorta and were compared to those of standard methods based on flow waveforms at only two specific anatomic landmarks. Global aortic pulse wave velocity was higher in patients with atherosclerosis (7.03 +/- 0.24 m/sec) compared to age-matched controls (6.40 +/- 0.32 m/sec). Both were significantly (P < 0.001) increased compared to younger volunteers (4.39 +/- 0.32 m/sec). Global aortic pulse wave velocity in young volunteers was in good agreement with previously reported MRI studies and catheter measurements. Estimation of measurement inaccuracies and error propagation analysis demonstrated only minor uncertainties in measured flow waveforms and moderate relative errors below 16% for aortic compliance in all 46 subjects. These results demonstrate the feasibility of pulse wave velocity calculation based on four-dimensional MRI data by exploiting its full volumetric coverage, which may also be an advantage over standard two-dimensional techniques in the often-distorted route of the aorta in patients with atherosclerosis.

Arterial stiffness in the young: assessment, determinants, and implications

Arterial stiffness describes the rigidity of the arterial wall. Its significance owes to its relationship with the pulsatile afterload presented to the left ventricle and its implications on ventricular-arterial coupling. In adults, the contention that arterial stiffness as a marker and risk factor for cardiovascular morbidity and mortality is gaining support. Noninvasive methods have increasingly been adopted in both the research and clinical arena to determine local, segmental, and systemic arterial stiffness in the young. With adoption of these noninvasive techniques for use in children and adolescents, the phenomenon and significance of arterial stiffening in the young is beginning to be unveiled. The list of childhood factors and conditions found to be associated with arterial stiffening has expanded rapidly over the last decade; these include traditional cardiovascular risk factors, prenatal growth restriction, vasculitides, vasculopathies associated with various syndromes, congenital heart disease, and several systemic diseases. The findings of arterial stiffening have functional implications on energetic efficiency, structure, and function of the left ventricle. Early identification of arterial dysfunction in childhood may provide a window for early intervention, although longitudinal studies are required to determine whether improvement of arterial function in normal and at-risk paediatric populations will be translated into clinical benefits.

Aortic stiffness is associated with cardiac function and cerebral small vessel disease in patients with type 1 diabetes mellitus: assessment by magnetic resonance imaging

OBJECTIVE

To evaluate, with the use of magnetic resonance imaging (MRI), whether aortic pulse wave velocity (PWV) is associated with cardiac left ventricular (LV) function and mass as well as with cerebral small vessel disease in patients with type 1 diabetes mellitus (DM).

MATERIALS AND METHODS

We included 86 consecutive type 1 DM patients (49 male, mean age 46.9 +/- 11.7 years) in a prospective, cross-sectional study. Exclusion criteria included aortic/heart disease and general MRI contra-indications. MRI of the aorta, heart and brain was performed for assessment of aortic PWV, as a marker of aortic stiffness, systolic LV function and mass, as well as for the presence of cerebral white matter hyperintensities (WMHs), microbleeds and lacunar infarcts. Multivariate linear or logistic regression was performed to analyse the association between aortic PWV and outcome parameters, with covariates defined as age, gender, mean arterial pressure, heart rate, BMI, smoking, DM duration and hypertension.

RESULTS

Mean aortic PWV was 7.1 +/- 2.5 m/s. Aortic PWV was independently associated with LV ejection fraction (ss = -0.406, P = 0.006), LV stroke volume (ss = -0.407, P = 0.001), LV cardiac output (ss = -0.458, P = 0.001), and with cerebral WMHs (P < 0.05). There were no independent associations between aortic stiffness and LV mass, cerebral microbleeds or lacunar infarcts.

CONCLUSION

Aortic stiffness is independently associated with systolic LV function and cerebral WMHs in patients with type 1 DM.

Reduced Aortic Elasticity and Dilatation Are Associated With Aortic Regurgitation and Left Ventricular Hypertrophy in Nonstenotic Bicuspid Aortic Valve Patients

OBJECTIVES

This study sought to assess elasticity and dimensions of the aorta and their impact on aortic valve competence and left ventricular (LV) function in patients with a nonstenotic bicuspid aortic valve (BAV).

BACKGROUND

Intrinsic pathology of the aortic wall is a possible explanation for reduced aortic elasticity and aortic dilatation in patients with BAVs, even in the absence of a stenotic aortic valve. The relationship between aortic wall elasticity, aortic dimensions, aortic valve competence, and LV function in patients with BAVs has not previously been studied with magnetic resonance imaging.

METHODS

Magnetic resonance imaging was performed in 20 patients with nonstenotic BAVs (mean +/- SD, age 27 +/- 11 years) and 20 matched control patients.

RESULTS

The BAV patients showed reduced aortic elasticity as indicated by increased pulse wave velocity in the aortic arch and descending aorta (5.6 +/- 1.3 m/s vs. 4.5 +/- 1.1 m/s, p = 0.01; and 5.2 +/- 1.8 m/s vs. 4.3 +/- 0.9 m/s, p = 0.03, respectively), and reduced aortic root distensibility (3.1 +/- 1.2 x 10(-3) mm Hg(-1) vs. 5.6 +/- 3.2 x 10(-3) mm Hg(-1), p < 0.01). In addition, BAV patients showed aortic root dilatation as compared with control patients (mean difference 3.6 to 4.2 mm, p < or = 0.04 at all 4 predefined levels). Minor degrees of aortic regurgitation (AR) were present in 11 patients (AR fraction 6 +/- 8% vs. 1 +/- 1%, p < 0.01). The LV ejection fraction was normal (55 +/- 8% vs. 56 +/- 6%, p = 0.61), whereas LV mass was significantly increased in patients (54 +/- 12 g/m2 vs. 46 +/- 12 g/m2, p = 0.04). Dilatation at the level of the aortic annulus (r = 0.45, p = 0.044) and reduced aortic root distensibility (r = 0.37, p = 0.041) correlated with AR fraction. Increased pulse wave velocity in the aortic arch correlated with increased LV mass (r = 0.42, p = 0.041).

CONCLUSIONS

Reduced aortic elasticity and aortic root dilatation were frequently present in patients with nonstenotic BAVs. In addition, reduced aortic wall elasticity was associated with severity of AR and LV hypertrophy.

Determination of vessel cross section for flow rate quantification

This study was motivated by the interest of measuring different cardiac parameters for which changes in the flow rate during a cardiac cycle needs to be determined at different positions along a vessel segment. These measurements result in a great number of images for which automatic contour detection is very helpful. A model-based algorithm for intraluminal contour detection has been developed in order to allow an accurate quantitative image analysis. The algorithm permits to select contours automatically on all the frames and slices of an imaging study. Images obtained on a flow phantom simulating the effects of blood circulation in large arteries have been used to validate the method. They were acquired with a specially designed interleaved multi slice and phase sequence, using a standard whole-body 2 Tesla NMR scanner. A potential in vivo application of the algorithm has been demonstrated on abdominal aorta images.