Validation and reproducibility of aortic pulse wave velocity as assessed with velocity‐encoded MRI

Purpose

To validate magnetic resonance imaging (MRI) assessment of aortic pulse wave velocity (PWV_MRI) with PWV determined from invasive intra‐aortic pressure measurements (PWV_INV) and to test the reproducibility of the measurement by MRI.

Materials and Methods

PWV_MRI was compared with PWV_INV in 18 nonconsecutive patients scheduled for catheterization for suspected coronary artery disease. Reproducibility of PWV_MRI was tested in 10 healthy volunteers who underwent repeated measurement of PWV_MRI at a single occasion. Velocity‐encoded MRI was performed on all participants to assess PWV_MRI in the total aorta (Ao_total), the proximal aorta (Ao_prox), and the distal aorta (Ao_dist).

Results

The results are expressed as mean ± SD, Pearson correlation coefficient (PCC), and intraclass correlation (ICC). Good agreement between PWV_MRI and PWV_INV was found for Ao_total (6.5 ± 1.1 m/s vs. 6.1 ± 0.8 m/s; PCC = 0.53), Ao_prox (6.5 ± 1.3 m/s vs. 6.2 ± 1.1 m/s; PCC = 0.69), and for Ao_dist (6.9 ± 1.1 m/s vs. 6.1 ± 1.0 m/s; PCC = 0.71). Reproducibility of PWV_MRI was high for Ao_total (4.3 ± 0.5 m/s vs. 4.6 ± 0.7 m/s; ICC = 0.90, P < 0.01), Ao_prox (4.3 ± 0.9 m/s vs. 4.7 ± 1.0 m/s; ICC = 0.87, P < 0.01), and Ao_dist (4.3 ± 0.6 m/s vs. 4.4 ± 0.8 m/s; ICC = 0.92, P < 0.01).

Conclusion

MRI assessment of aortic pulse wave velocity shows good agreement with invasive pressure measurements and can be determined with high reproducibility. J. Magn. Reson. Imaging 2009;30:521–526. © 2009 Wiley‐Liss, Inc.     

Aortic flow patterns in patients with Marfan syndrome assessed by flow-sensitive four-dimensional MRI

Purpose:

To apply time‐resolved three‐dimensional (3D) phase contrast MRI with three‐directional velocity encoding (flow‐sensitive 4D MRI) for the characterization of flow pattern changes in patients with Marfan syndrome (MFS) compared with normal controls.

Materials and Methods:

Flow‐sensitive 4D MRI of the thoracic aorta (temporal resolution ～45 ms, spatial resolution ～2.4 × 2.1 × 2.8 mm³) was performed in 24 MFS patients and 10 volunteers. Aortic flow patterns were visualized by 3D particle traces and streamlines. Global (affecting the complete lumen) and local (parts of the vessel lumen) helix and vortex flow in the ascending aorta (AAo), aortic arch, and descending aorta (DAo) were graded in 3 categories (blinded reading, two observers): none = 0, moderate = 1, pronounced = 2.

Results:

Flow grading revealed similar global helix and vortex flow in the AAo and arch for MFS patients and controls. Local helix flow in the AAo was significantly (P = 0.011) increased in patients and was associated with aortic sinus dilatation. The incidence of global helix and vortex flow in the DAo was increased in patients (77% and 50% of subjects) compared with controls (none and 10%).

Conclusion:

The 4D flow analysis revealed marked differences of the aortic flow patterns between Marfan patients and controls: Local helix flow in the patients' AAo may be associated with the increased incidence of aortic root dilatation. The flow alterations in the proximal DAo could explain the occurrence of Type‐B dissection originating from this site. J. Magn. Reson. Imaging 2012;35:594‐600. © 2011 Wiley Periodicals, Inc.     

Improved aortic pulse wave velocity assessment from multislice two-directional in-plane velocity-encoded magnetic resonance imaging

Purpose

To evaluate the accuracy and reproducibility of aortic pulse wave velocity (PWV) assessment by in‐plane velocity‐encoded magnetic resonance imaging (MRI).

Materials and Methods

In 14 patients selected for cardiac catheterization on suspicion of coronary artery disease and 15 healthy volunteers, PWV was assessed with multislice two‐directional in‐plane velocity‐encoded MRI (PWV_i.p.) and compared with conventionally assessed PWV from multisite one‐directional through‐plane velocity‐encoded MRI (PWV_t.p.). In patients, PWV was also obtained from intraarterially acquired pressure–time curves (PWV_pressure), which is considered the gold standard reference method. In volunteers, PWV_i.p. and PWV_t.p. were obtained in duplicate in the same examination to test reproducibility.

Results

In patients, PWV_i.p. showed stronger correlation and similar variation with PWV_pressure than PWV_t.p. (Pearson correlation r = 0.75 vs. r = 0.58, and coefficient of variation [COV] = 10% vs. COV = 12%, respectively). In volunteers, repeated PWV_i.p. assessment showed stronger correlation and less variation than repeated PWV_t.p. (proximal aorta: r = 0.97 and COV = 10% vs. r = 0.69 and COV = 17%; distal aorta: r = 0.94 and COV = 12% vs. r = 0.90 and COV = 16%; total aorta: r = 0.97 and COV = 7% vs. r = 0.90 and COV = 13%).

Conclusion

PWV_i.p. is an improvement over conventional PWV_t.p. by showing higher agreement as compared to the gold standard (PWV_pressure) and higher reproducibility for repeated MRI assessment. J. Magn. Reson. Imaging 2010;32:1086–1094. © 2010 Wiley‐Liss, Inc.     

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 R² = 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. J. Magn. Reson. Imaging 2012;35:1162‐1168. © 2012 Wiley Periodicals, Inc.     

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. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Aortic wall shear stress in Marfan syndrome

The aim of this study was to quantify changes in thoracic aortic wall shear stress (WSS) in asymptomatic patients with Marfan syndrome (MFS) compared with healthy controls. WSS in the thoracic aorta was quantified based on time‐resolved 3D phase contrast MRI with three‐directional velocity encoding (4D flow MRI, temporal resolution ～44 ms, spatial resolution ～2.5 mm) in 24 patients with confirmed MFS (age = 18 ± 12 years) and in 12 older healthy volunteers (age = 25 ± 3 years). Diameters of the thoracic aorta normalized to body surface area were similar for both groups. Peak systolic velocity, absolute WSS, time‐averaged WSS, circumferential WSS, peak systolic WSS, and WSS eccentricity were calculated in eight analysis planes distributed along the thoracic aorta. Plane‐wise comparison revealed significant differences between MFS patients and volunteers in the proximal ascending aorta for peak systolic velocities (1.11 ± 0.23 m/s vs. 1.34 ± 0.18 m/s, P = 0.004) and circumferential WSS (0.14 ± 0.03 N/m² vs. 0.11 ± 0.02 N/m², P = 0.007). WSS eccentricity was altered in most of the ascending aorta and proximal arch (P = 0.009–0.020). MFS patients demonstrated segmental differences in peak systolic WSS with a significantly higher WSS at the inner curvature in the proximal ascending aorta and at the anterior part in the more distal ascending aorta (P < 0.01). These findings indicate differences in WSS associated with MFS despite similar aortic dimensions compared to controls. Magn Reson Med, 70:1137–1144, 2013. © 2012 Wiley Periodicals, Inc.     

Nontriggered MRI quantification of aortic pulse‐wave velocity

Pulse‐wave velocity is an index of arterial stiffness, which is a strong indicator of cardiovascular risk. We present a high‐speed technique that generates time‐resolved complex difference signal intensity simultaneously in the ascending and descending aorta from velocity‐encoded projections without gating, allowing quantification of pulse‐wave velocity. The velocity‐time curve was approximated with a time‐resolved complex difference signal intensity to estimate the propagation time of the pulse wave in the aortic arch. The path length of the pulse wave is measured from an oblique sagittal image in a plane encompassing thoracic ascending and descending aorta, and pulse‐wave velocity is computed from the ratio between the path length and pulse‐wave propagation time. The method was implemented at 1.5 T and 3 T, and pulse‐wave velocity was quantified in healthy subjects (ages 20–70 years, N = 23) without symptoms or prior history of cardiovascular events. In addition, the method was compared against retrospectively EKG‐gated PC‐MRI. The overall results were found to be in good agreement with literature data showing age‐related increase in aortic stiffness. The RMS differences between the projection and gated PC‐MRI methods were less than 4%. Key benefits of the proposed method are simplicity in both data acquisition and processing requiring only computation of the complex difference between the velocity‐encoded projections rather than absolute velocity. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Four‐dimensional velocity‐encoded magnetic resonance imaging improves blood flow quantification in patients with complex accelerated flow

Purpose:

To evaluate the use of four‐dimensional (4D) velocity‐encoded magnetic resonance imaging (VEC MRI) for blood flow quantification in patients with semilunar valve stenosis and complex accelerated flow.

Materials and Methods:

Peak velocities (Vmax) and stroke volumes (SV) were quantified by 2D and 4D VEC MRI in volunteers (n = 7) and patients with semilunar valve stenosis (n = 18). Measurements were performed above the aortic and pulmonary valve with both techniques and, additionally, at multiple predefined planes in the ascending aorta and in the pulmonary trunk within the 4D dataset. In patients, 4D VEC MRI streamline analysis identified flow patterns and regions of highest flow velocity (4D_{max‐targeted}) for further measurements and Vmax was also measured by Doppler‐echocardiography.

Results:

In patients, 4D VEC MRI showed higher Vmax than 2D VEC MRI (2.7 ± 0.6 m/s vs. 2.4 ± 0.5 m/s, P < 0.03) and was more comparable to Doppler‐echocardiography (2.8 ± 0.7 m/s). 4D_{max‐targeted} revealed highest Vmax values (3.1 ± 0.6 m/s). SV measurements showed significant differences between different anatomical levels in the ascending aorta in patients with complex accelerated flow, whereas differences in volunteers with laminar flow patterns were negligible (P = 0.004).

Conclusion:

4D VEC MRI improves MRI‐derived blood flow quantification in patients with semilunar valve stenosis and complex accelerated flow. J. Magn. Reson. Imaging 2013;37:208–216. © 2012 Wiley Periodicals, Inc.     

Age-related and regional changes of aortic stiffness in the Marfan syndrome: assessment with velocity-encoded MRI

Purpose:

To study age‐related change in aortic stiffness in patients with Marfan syndrome (MFS) versus healthy volunteers using velocity‐encoded (VE) MRI.

Materials and Methods:

Twenty‐five MFS patients (age range, 18–63 years; mean age 36 ± 14 years, 13 men) and 25 age‐/gender‐matched healthy volunteers were examined with VE‐MRI. Aortic stiffness was expressed by pulse wave velocity (PWV), assessed in the proximal and distal part of the aorta and in the total aorta. PWV was compared between patients and volunteers and age‐relation was determined by linear regression.

Results:

PWV was significantly higher in all parts of the aorta in patients when compared with healthy volunteers (proximal aorta 5.7 ± 1.5 m/s versus 4.8 ± 0.9 m/s, distal aorta 6.4 ± 2.4 m/s versus 5.0 ± 1.2 m/s and total aorta 5.9 ± 1.7 m/s versus 4.9 ± 1.1 m/s, all P < 0.004). PWV correlated significantly with age (Pearson R between 0.45 and 0.94). Only in the proximal aorta, the increase in PWV with age was significantly higher in patients (7 ± 2 cm/s increase with age) than in volunteers (3 ± 1 cm/s increase, P = 0.03); in the distal or total aorta, the increase in PWV with age was not different between patients and volunteers.

Conclusion:

Velocity‐encoded MRI detects more pronounced age‐related aortic stiffening in the proximal aorta in MFS patients versus healthy volunteers, suggesting more severe wall disease in MFS. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Right coronary artery flow velocity and volume assessment with spiral K‐space sampled breathhold velocity‐encoded MRI at 3 tesla: Accuracy and reproducibility

Purpose:

To evaluate accuracy and reproducibility of flow velocity and volume measurements in a phantom and in human coronary arteries using breathhold velocity‐encoded (VE) MRI with spiral k‐space sampling at 3 Tesla.

Materials and Methods:

Flow velocity assessment was performed using VE MRI with spiral k‐space sampling. Accuracy of VE MRI was tested in vitro at five constant flow rates. Reproducibility was investigated in 19 healthy subjects (mean age 25.4 ± 1.2 years, 11 men) by repeated acquisition in the right coronary artery (RCA).

Results:

MRI‐measured flow rates correlated strongly with volumetric collection (Pearson correlation r = 0.99; P < 0.01). Due to limited sample resolution, VE MRI overestimated the flow rate by 47% on average when nonconstricted region‐of‐interest segmentation was used. Using constricted region‐of‐interest segmentation with lumen size equal to ground‐truth luminal size, less than 13% error in flow rate was found. In vivo RCA flow velocity assessment was successful in 82% of the applied studies. High interscan, intra‐ and inter‐observer agreement was found for almost all indices describing coronary flow velocity. Reproducibility for repeated acquisitions varied by less than 16% for peak velocity values and by less than 24% for flow volumes.

Conclusion:

3T breathhold VE MRI with spiral k‐space sampling enables accurate and reproducible assessment of RCA flow velocity. J. Magn. Reson. Imaging 2010;31:1215–1223. © 2010 Wiley‐Liss, Inc.     

Rapid measurement of aortic wave velocity: in vivo evaluation.

A 1D MR sequence has been developed for determining aortic flow wave velocity (WV), a metric of arterial compliance, within a single cardiac cycle. Studies were carried out on the thoracic aortas of 10 normal volunteers. Correlative WV data were also acquired from each subject using a conventional phase-velocity 2D mapping technique. Aortic WV in this cohort was found to range from 411 to 714 cm/s and was highly correlated (R = 0.95) between the two methods. Peak blood velocity was also measured using both methods and found to agree closely. The reproducibility of WV measurements using the rapid 1D method averaged 7.6%, which is comparable or better than that achieved using existing noninvasive techniques. Magn Reson Med 46:95-102, 2001.     

Quantification of the pulse wave velocity of the descending aorta using axial velocity profiles from phase-contrast magnetic resonance imaging.

The pulse wave velocity (PWV) of aortic blood flow is considered a surrogate for aortic compliance. A new method using phase-contrast (PC)-MRI is presented whereby the spatial and temporal profiles of axial velocity along the descending aorta can be analyzed. Seventeen young healthy volunteers (the YH group), six older healthy volunteers (the OH group), and six patients with coronary artery disease (the CAD group) were studied. PC-MRI covering the whole descending aorta was acquired, with velocity gradients encoding the in-plane velocity. From the corrected axial flow velocity profiles, PWV was determined from the slope of an intersecting line between the presystolic and early systolic phases. Furthermore, the aortic elastic modulus (Ep) was derived from the ratio of the brachial pulse pressure to the strain of the aortic diameter. The PWV increased from YH to OH to CAD (541 +/- 94, 808 +/- 184, 1121 +/- 218 cm/s, respectively; P = 0.015 between YH and OH; P = 0.023 between OH and CAD). There was a high correlation between PWV and Ep (r = 0.861, P < 0.001). Multivariate analysis showed that age and CAD were independent risk factors for an increase in the PWV. Compared to existing methods, our method requires fewer assumptions and provides a more intuitive and objective way to estimate the PWV.     

Feasibility of aortic pulse pressure and pressure wave velocity MRI measurement in young adults

PURPOSE: To investigate the feasibility of assessing, noninvasively, aortic pulse pressure (APP) and pulse wave velocity (PWV) in the ascending aorta of young adults by means of velocity-encoded magnetic resonance (MR) imaging. MATERIALS AND METHODS: In a series of 11 healthy volunteers, velocity-encoded MR imaging provided pairs of magnitude and phase-contrast images. Blood flow velocity and aortic cross-sectional area (CSA) were determined with a 30-msec temporal resolution. A model analysis revealed that variation in aortic CSA and in maximal blood flow velocity throughout systole could be used to estimate APP and, hence, to derive PWV by means of two different methods. RESULTS: Mean +/- SD values of the APP for the series were 54.2 +/- 16.4 mmHg (range 32.2-84.1 mmHg). The ascending aortic PWV mean +/- SD values were 5.03 +/- 1.10 m/second and 5.37 +/- 1.23 m/second according to the two methods, and both estimates were not significantly different (95% confidence level). CONCLUSION: These results are in agreement with previously published data, suggesting that APP and PWV can be determined, noninvasively, in young adults using MRI.     

Compliance and pulse wave velocity assessed by MRI detect early aortic impairment in young patients with mutation of the smooth muscle myosin heavy chain

Purpose

To evaluate aortic elasticity with MRI on young asymptomatic individuals with mutation of the smooth muscle myosin heavy chain in whom aortic enlargement is not present.

Materials and Methods

Aortic compliance, aortic distensibility, and pulse wave velocity (PWV) were semiautomatically measured from MRI in 8 asymptomatic subjects having a mutation of the MYH11 gene (M+) and 21 nonmutated relatives (M?) of similar age, sex, and blood pressure characteristics.

Results

Despite a similar aortic diameter in both groups, the aortic compliance and distensibility were significantly lower in M+ subjects compared with M? (0.84 ± 0.33 versus 2.03 ± 0.54 mm²/mmHg, 1.18 ± 0.62 10^?3 versus 5.11 ± 1.58 10^?3 mmHg^?1, respectively), and PWV was significantly higher (5.35 ± 1.53 versus 3.60 ± 0.64 m.s^?1). A threshold aortic compliance value of 1.3 mm²/mmHg separated the two groups. The receiver operating characteristics curve analysis indicated an optimal threshold of 2.9 10^?3 mmHg^?1 for aortic distensibility (sensitivity: 87.5%, specificity: 90%), and of 4.4 m.s^?1 for PWV (sensitivity: 75%, specificity: 100%).

Conclusion

Young asymptomatic adults with MYH11 mutation have an aortic compliance impairment which is not detectable by the sole measurement of the aortic size. Aortic compliance measurement might be part of routine examination in patients suspected of inherited aortic disease even with a normal aortic diameter. J. Magn. Reson. Imaging 2008;28:1180–1187. © 2008 Wiley‐Liss, Inc.

     

Effect of acquisition parameters on the accuracy of velocity encoded cine magnetic resonance imaging blood flow measurements.

PURPOSE: To investigate the effect of acquisition parameters on the accuracy of 2D velocity encoded cine magnetic resonance imaging (VEC MRI) flow measurements. MATERIALS AND METHODS: Using a pulsatile flow phantom, through-plane flow measurements were performed on a flexible vessel made of polyvinyl alcohol cryogel (PVA), a material that mimics the MR signal and biomechanical properties of aortic tissue. RESULTS: Repeated VEC MRI flow measurements (N = 20) under baseline conditions yielded an error of 0.8 +/- 1.5%. Slice thickness, angle between flow and velocity encoding directions, spatial resolution, velocity encoding range, and radio frequency (RF) flip angles were varied over a clinically relevant range. Spatial resolution had the greatest impact on accuracy, with a 9% overestimation of flow at 16 pixels per vessel cross-section. CONCLUSION: VEC MRI proved to be an accurate and reproducible technique for pulsatile flow measurements over the range of acquisition parameters examined as long as sufficient spatial resolution was prescribed.     

A new method for the determination of aortic pulse wave velocity using cross-correlation on 2D PCMR velocity data

PURPOSE: To evaluate the reproducibility of a new multisite axial pulse wave velocity (PWV) measurement technique that makes use of 2D PCMR data and cross-correlation analysis. MATERIALS AND METHODS: PWV was estimated with MRI in 13 healthy volunteers by a transit-time technique (TT), a multisite technique utilizing 1D PCMR data in the descending aorta (FOOT), and a new multisite axial technique that uses 2D PCMR data over the ascending, transverse, and descending sections of the aorta (2D-XC). RESULTS: No significant difference was observed between PWV measurements values measured by the three techniques. However, 2D-XC displayed significantly better intertest reproducibility than either the TT or FOOT methodologies. Average percent difference between scans: TT: 15.8% +/- 13.4%, FOOT: 21.3% +/- 16.9%, 2D-XC: 7.72% +/- 4.73%. P = 0.02 for both 2D-XC/TT comparison and 2D-XC/FOOT comparison. CONCLUSION: 2D-XC is a more reproducible method than either the established TT or FOOT methods to estimate the aortic PWV.