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.     

A one-dimensional velocity technique for NMR measurement of aortic distensibility

A technique is presented for rapidly and noninvasively determining aortic distensibility, by NMR measurement of wave velocity in the aorta. A two–dimensional NMR selective-excitation pulse is used to repeatedly excite a cylinder of magnetization in the aorta, with magnetization read out along the cylinder axis each time. A toggled bipolar flow-encoding pulse is applied prior to readout, to produce a one-dimensional phase-contrast flow image. Cardiac gating and data interleaving are employed to improve the effective time resolution to 2 ms. Wave velocities are determined from the slope of the leading edge of flow measured on the resulting M-mode velocity image. The technique is sensitive over a range of distensibilities from 10^?8 to 10^?3 m s²/kg. The average value in the descending thoracic aorta in seven normal subjects was found to be 4.8 × 10^?5 m s²/kg, with a significant inverse correlation with age.     

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.     

Rapid measurement of pulse wave velocity via multisite flow displacement.

A MR method is presented for measuring pulse wave velocity (PWV) and its application to assessing stiffness in the human thoracic aorta. This one-dimensional (1D) flow displacement method applies a single RF comb excitation to the vessel, followed by an oscillating frequency encoding gradient, each oscillation providing a 1D projection of the vessel, enabling one to track fluid motion. The currently implemented sequence excites nine slices within a 20-cm length of vessel and has a temporal resolution of 2.03 msec and a total acquisition time of 140 msec. Offline-reconstructed position-versus-time plots show curvilinear flow displacement trajectories corresponding to fluid motion at each of the excitation positions. The PWV can be reliably calculated by curve-fitting these trajectories to a model. In vitro studies using compliant tubes demonstrate no significant difference between results obtained using this method and those directly obtained using pressure transducers. Compared to another MR method previously developed in our laboratory, the proposed method displays improved temporal resolution and enhanced ability to extract PWV from vessels exhibiting low peak flow velocity. Preliminary data suggest that this method is feasible for in vivo application and may provide a more accurate estimation of aortic wave velocity among subjects exhibiting low peak flow velocity, such as the elderly or those with impaired cardiac function.     

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.     

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. J. Magn. Reson. Imaging 2011;33:1321–1329. © 2011 Wiley‐Liss, Inc.     

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.

     

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.     

Quantification of aortic compliance in mice using radial phase contrast MRI

Purpose

To investigate the feasibility of radial phase contrast MR imaging to measure in vivo pulse wave velocity (PWV) and wall shear stress (WSS) in small animals on a 7 Tesla scanner.

Materials and Methods

The aortic compliance of 9‐month‐old ApoE deficient (ApoE‐KO) mice (n = 10) on a normal diet was studied in comparison to that of wild‐type (WT) mice (n = 10). An undersampled, asymmetric echo radial phase contrast MR technique was developed to measure through plane blood flow velocity at axial slices along the descending aorta. The PWV and the time averaged WSS was calculated from the spatiotemporal flow data. The reproducibility of PWV and WSS was evaluated by taking multiple measures on a separate cohort of WT (n = 4) mice.

Results

The mean percentage standard deviation among repeated measures was 10.1% for PWV and 24.8% for WSS. The PWV of ApoE‐KO mice (5.84 ± 2.15 m/s) was significantly higher (p = 0.02) than that of WT (3.55 ± 0.97 m/s), whereas WSS was lower in ApoE‐KO mice (1.44 ± 0.31Pa) compared with WT (1.55 ± 0.36Pa).

Conclusion

This study demonstrates that in vivo PWV derived from radial phase contrast MR imaging can be potentially used as a surrogate marker for impaired vascular function in mice. J. Magn. Reson. Imaging 2009;30:286–291. © 2009 Wiley‐Liss, Inc.     

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.     

Age‐related changes of regional pulse wave velocity in the descending aorta using Fourier velocity encoded M‐mode

Aortic pulse wave velocity (PWV) is an independent determinant of cardiovascular risk. Although aortic stiffening with age is well documented, the interaction between aging and regional aortic PWV is still a debated question. We measured global and regional PWV in the descending aorta of 56 healthy subjects aged 25–76 years using a one‐dimensional, interleaved, Fourier velocity encoded pulse sequence with cylindrical excitation. Repeatability across two magnetic resonance examinations (n = 19) and accuracy against intravascular pressure measurements (n = 4) were assessed. The global PWV was found to increase nonlinearly with age. The thoracic aorta was found to stiffen the most with age (PWV [thoracic, 20–40 years] = 4.7 ± 1.1 m/s; PWV [thoracic, 60–80 years] = 7.9 ± 1.5 m/s), followed by the mid‐ (PWV [mid‐abdominal, 20–40 years] = 4.9 ± 1.3 m/s; PWV [mid‐abdominal, 60–80 years] = 7.4 ± 1.9 m/s) and distal abdominal aorta (PWV [distal abdominal, 20–40 years] = 4.8 ± 1.4 m/s; PWV [distal abdominal, 60–80 years] = 5.7 ± 1.4 m/s). Good agreement was found between repeated magnetic resonance measurements and between magnetic resonance PWVs and the gold‐standard. Fourier velocity encoded M‐mode allowed to measure global and regional PWV in the descending aorta. There was a preferential stiffening of the thoracic aorta with age, which may be due to progressive fragmentation of elastin fibers in this region. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

主动脉缩窄术后磁共振脉搏波传导速度法评估主动脉顺应性

目的 使用磁共振脉搏波传导速度法(PWV)评估主动脉缩窄(CoA)术后主动脉顺应性.方法 回顾性分析22例CoA术后患儿心脏磁共振,使用MRI相位对比序列测量并计算主动脉PWV,同时期行超声心动图测量双侧心室功能及心肌厚度.另选取年龄、性别匹配的22例健康儿童组成对照组测量主动脉PWV.结果 研究组平均PWV明显高于对照组[(4.42±3.02)m/s vs(2.73±0.76)m/s,P=0.02].在研究组中,吻合口中度狭窄患儿PWV值最高,受试者工作特征(ROC)曲线分析发现,取PWV为3.37 m/s时,区分轻、中度吻合口狭窄的敏感性、特异性最高.左侧心室舒张末期容积、收缩末期容积、射血分数及左室壁厚度与PWV无明显相关性.结论 CoA术后患儿PWV升高,中度吻合口狭窄患儿存在较显著主动脉重构.     

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.     

Pulse-wave velocity measured in one heartbeat using MR tagging.

A noninvasive method for measuring the aortic pulse-wave velocity (PWV) in a single heartbeat is introduced. The method sinusoidally tags a column of blood within the vessel, and rapidly acquires a series of 1D projections of the tags as they move (in practice, 64 projections at 4-ms intervals). From these projections, the relative motion of blood at different positions along the vessel is measured. The PWV is obtained by fitting a mathematical model of blood flow to the tag trajectories. Tests of this method in a pulsatile flow phantom are presented using latex and polyurethane tubes. The PWV measured in these tubes was (mean +/- standard deviation) 4.4 +/- 0.5 m/s and 2.3 +/- 0.2 m/s, respectively. The distensibility of each tube was calculated from the PWV (latex = (7 +/- 2) 10(-3) mm Hg(-1), poly. = (25 +/- 4) 10(-3)mmHg(-1)) and found to agree within error with distensibility measurements based on the change of tube area with pressure (latex = (6.3 +/- 0.3) 10(-3)mmHg(-1), poly. = (27 +/- 1) 10(-3) mmHg(-1)). To test its feasibility, the PWV measurement was applied to four normal volunteers. The measured PWV values were 3.9 +/- 0.8 m/s, 3.6 +/- 0.9 m/s, 3.9 +/- 0.5 m/s, and 5.3 +/- 0.8 m/s. By acquiring an independent PWV measurement each heartbeat, errors introduced by arrhythmia and trigger variability appear to be avoided with this method.     

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 pulse wave velocity in main pulmonary artery with phase contrast MRI: preliminary investigation

PURPOSE: To assess the feasibility and reproducibility of a noninvasive MRI method to measure pulse wave velocity (PWV) in the main pulmonary artery (MPA). MATERIALS AND METHODS: A total of 17 subjects without history of pulmonary diseases (38.2 +/- 18.4 years) participated in this study. Series of MR velocity maps of the MPA were acquired at 2 cm above the pulmonary valves using a two-dimensional phase-contrast sequence. Effective temporal resolution was 11 msec after interleaving two dynamic series with different values of electrocardiograph (ECG) trigger delay. PWV was derived as the rate of MPA flow variations per unit change in MPA cross-sectional area, during early systole. Seven healthy subjects underwent three repetitive examinations to investigate intrascan and interscan reproducibility. RESULTS: Flow vs. area was highly linear in the MPA during early systole, with Pearson's coefficients ranging from 0.982 to 0.999, rendering derivation of PWV with little difficulty. Average value of PWV in MPA was 1.96 +/- 0.27 m/second, in good agreement with literature values measured using invasive means. The percentage intra- and interscan differences were 5.46% and -10.86%, respectively. CONCLUSION: Phase-contrast MRI to noninvasively measure PWV in the MPA is feasible with good reproducibility.