Accuracy and repeatability of fourier velocity encoded M‐mode and two‐dimensional cine phase contrast for pulse wave velocity measurement in the descending aorta

Purpose:

To assess the accuracy and repeatability of Fourier velocity encoded (FVE) M‐mode and two‐dimensional (2D) phase contrast with through‐plane velocity encoding (2D‐PC) for pulse wave velocity (PWV) evaluation in the descending aorta using five different analysis techniques.

Materials and Methods:

Accuracy experiments were conducted on a tubular human‐tissue‐mimicking phantom integrated into a flow simulator. The theoretical PWV value was derived from the Moens‐Korteweg equation after measurement of the tube elastic modulus by uniaxial tensile testing (PWV = 6.6 ± 0.7 m/s). Repeatability was assessed on 20 healthy volunteers undergoing three consecutive MR examinations.

Results:

FVE M‐mode PWV was more repeatable than 2D‐PC PWV independently of the analysis technique used. The early systolic fit (ESF) method, followed by the maximum of the first derivative (1st der.) method, was the most accurate (PWV = 6.8 ± 0.4 m/s and PWV = 7.0 ± 0.6 m/s, respectively) and repeatable (inter‐scan within‐subject variation δ = 0.096 and δ = 0.107, respectively) for FVE M‐mode. For 2D‐PC, the 1st der. method performed best in terms of accuracy (PWV = 6.8 ± 1.1 m/s), whereas the ESF algorithm was the most repeatable (δ = 0.386).

Conclusion:

FVE M‐mode allows rapid, accurate and repeatable central PWV evaluation when the ESF algorithm is used. 2D‐PC requires long scan times and can provide accurate although much less repeatable PWV measurements when the 1st der. method is used. J. Magn. Reson. Imaging 2010;31:1185–1194. © 2010 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.     

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.     

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.     

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.     

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.     

High-resolution slice-selective Fourier velocity encoding in congenital heart disease using spiral SENSE with velocity unwrap

Quantification of peak velocity is important in the assessment of stenotic flow jets in patients with congenital heart disease. Phase-contrast magnetic resonance underestimates peak velocities. Hence, clinically Doppler ultrasound is used as the reference standard for assessing stenoses. It is possible to accurately measure peak velocity in MR using Fourier velocity encoding (FVE). In this study, a fast, high-resolution slice-selective FVE sequence was developed with the use of spiral trajectories, parallel imaging, and partial Fourier in the velocity dimension and a novel velocity-unwrap technique. The resulting sequence was acquired within a short breath-hold (more than 15 heartbeats) making this FVE technique clinically achievable. Peak velocities were compared from Doppler ultrasound, phase-contrast magnetic resonance, and FVE. Experiments were carried out in vitro and in vivo in 25 patients with congenital heart disease with stenoses. It was shown that in vitro and in vivo phase-contrast magnetic resonance tended to underestimate peak velocity when compared with Doppler ultrasound, whereas FVE agreed well with Doppler ultrasound.     

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.     

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.     

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.     

Reducing artifacts in one‐dimensional Fourier velocity encoding for fast and pulsatile flow

When evaluating the severity of valvular stenosis, the peak velocity of the blood flow is routinely used to estimate the transvalvular pressure gradient. One‐dimensional Fourier velocity encoding effectively detects the peak velocity with an ungated time series of spatially resolved velocity spectra in real time. However, measurement accuracy can be degraded by the pulsatile and turbulent nature of stenotic flow and the existence of spatially varying off‐resonance. In this work, we investigate the feasibility of improving the peak velocity detection capability of one‐dimensional Fourier velocity encoding for stenotic flow using a novel echo‐shifted interleaved readout combined with a variable‐density circular k‐space trajectory. The shorter echo and readout times of the echo‐shifted interleaved acquisitions are designed to reduce sensitivity to off‐resonance. Preliminary results from limited phantom and in vivo results also indicate that some artifacts from pulsatile flow appear to be suppressed when using this trajectory compared to conventional single‐shot readouts, suggesting that peak velocity detection may be improved. The efficiency of the new trajectory improves the temporal and spatial resolutions. To realize the proposed readout, a novel multipoint‐traversing algorithm is introduced for flexible and automated gradient‐waveform design. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Improved aortic stiffness assessment in the elderly using a one-dimensional fluid displacement MR method

Purpose

To assess the in vitro accuracy of two rapid projective MR wave velocity measurement sequences, and their relative performance for assessing aortic stiffness in adults of all ages.

Materials and Methods

In vitro testing was performed using latex tube phantoms with precisely‐known flow wave velocities, both in the presence and absence of simulated static tissue. A total of 104 adults representing a large age range (21–83 years) underwent aortic wave velocity (AWV) measurements using multiple trials of each method in a single MR session. The relative agreement between the two AWV results in each tertile of subject age and the coefficient of variation of the AWV data were assessed.

Results

In vitro wave velocities did not differ significantly from the known values for either MR method, with or without simulated static tissue. In vivo, the mean AWVs for the young and middle‐aged cohorts did not differ significantly between the two MR methods. However, in the elderly group, the two methods did not agree, and one sequence was found to be superior in this age cohort.

Conclusion

In elderly individuals, a one‐dimensional MR method for evaluating aortic stiffness based on aortic blood displacement yields a smaller coefficient of variation and superior overall performance than a similar method based on aortic blood velocity. The two methods perform equivalently in young and middle‐aged subjects. J. Magn. Reson. Imaging 2006. © 2006 Wiley‐Liss, 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.     

多方法检测肱-踝动脉脉搏波传导速度的比较研究

目的:比较分析同一组受试者运用多种方法检测获得的肱-踝动脉段脉搏波传导速度(brachial ankle pulse wave velocity,baPWV),探讨将超声技术引入脉搏波传导速度检测的可行性及准确性。方法:纳入30例正常人,分别行日本Colin动脉硬化仪检测、联合多普勒超声及脉搏波传感器同步检测、仅多普勒超声检测baPWV,并将结果进行比较分析。结果:动脉硬化仪检测得到的baPWV为(1 118.20±176.42)cm/s;将脉搏波传感器放置于肱动脉、多普勒探头放置于胫后动脉同步检测的baPWV为(1 135.74±246.26)cm/s;仅用多普勒超声分2次分别将探头放置于肱动脉及胫后动脉时测得的baPWV为(1 048.96±117.24)cm/s。多普勒超声及脉搏波传感器联合应用与动脉硬化仪检测得到的baPWV呈显著正相关(r=0.549,P0.01)。多普勒超声与动脉硬化仪检测得到的baPWV无相关性(r=0.277,P0.05)。结论:联合运用超声多普勒与脉搏波传感器为PWV的检测提供了新途径,测量数据准确度较高;仅运用多普勒超声分2次检测PWV的方法误差较大。