Pencil excitation with interleaved fourier velocity encoding: NMR measurement of aortic distensibility

Accelerated dual-venc 4D flow MRI for neurovascular applications

PURPOSE

To improve velocity-to-noise ratio (VNR) and dynamic velocity range of 4D flow magnetic resonance imaging (MRI) by using dual-velocity encoding (dual-venc) with k-t generalized autocalibrating partially parallel acquisition (GRAPPA) acceleration.

MATERIALS AND METHODS

A dual-venc 4D flow MRI sequence with k-t GRAPPA acceleration was developed using a shared reference scan followed by three-directional low- and high-venc scans (repetition time / echo time / flip angle = 6.1 msec / 3.4 msec / 15°, temporal/spatial resolution = 43.0 msec/1.2 × 1.2 × 1.2 mm³ ). The high-venc data were used to correct for aliasing in the low-venc data, resulting in a single dataset with the favorable VNR of the low-venc but without velocity aliasing. The sequence was validated with a 3T MRI scanner in phantom experiments and applied in 16 volunteers to investigate its feasibility for assessing intracranial hemodynamics (net flow and peak velocity) at the major intracranial vessels. In addition, image quality and image noise were assessed in the in vivo acquisitions.

RESULTS

All 4D flow MRI scans were acquired successfully with an acquisition time of 20 ± 4 minutes. The shared reference scan reduced the total acquisition time by 12.5% compared to two separate scans. Phantom experiments showed 51.4% reduced noise for dual-venc compared to high-venc and an excellent agreement of velocities (ρ = 0.8, P < 0.001). The volunteer data showed decreased noise in dual-venc data (54.6% lower) compared to high-venc, and improved image quality, as graded by two observers: fewer artifacts (P < 0.0001), improved vessel conspicuity (P < 0.0001), and reduced noise (P < 0.0001).

CONCLUSION

Dual-venc 4D flow MRI exhibits the superior VNR of the low-venc acquisition and reliably incorporates low- and high-velocity fields simultaneously. In vitro and in vivo data demonstrate improved flow visualization, image quality, and image noise.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:102-114.

Cardiovascular magnetic resonance phase contrast imaging

Cardiovascular magnetic resonance (CMR) phase contrast imaging has undergone a wide range of changes with the development and availability of improved calibration procedures, visualization tools, and analysis methods. This article provides a comprehensive review of the current state-of-the-art in CMR phase contrast imaging methodology, clinical applications including summaries of past clinical performance, and emerging research and clinical applications that utilize today's latest technology.

Review of MRI-based measurements of pulse wave velocity: a biomarker of arterial stiffness

Atherosclerosis is the leading cause of cardiovascular disease (CVD) in the Western world. In the early development of atherosclerosis, vessel walls remodel outwardly such that the vessel luminal diameter is minimally affected by early plaque development. Only in the late stages of the disease does the vessel lumen begin to narrow-leading to stenoses. As a result, angiographic techniques are not useful for diagnosing early atherosclerosis. Given the absence of stenoses in the early stages of atherosclerosis, CVD remains subclinical for decades. Thus, methods of diagnosing atherosclerosis early in the disease process are needed so that affected patients can receive the necessary interventions to prevent further disease progression. Pulse wave velocity (PWV) is a biomarker directly related to vessel stiffness that has the potential to provide information on early atherosclerotic disease burden. A number of clinical methods are available for evaluating global PWV, including applanation tonometry and ultrasound. However, these methods only provide a gross global measurement of PWV-from the carotid to femoral arteries-and may mitigate regional stiffness within the vasculature. Additionally, the distance measurements used in the PWV calculation with these methods can be highly inaccurate. Faster and more robust magnetic resonance imaging (MRI) sequences have facilitated increased interest in MRI-based PWV measurements. This review provides an overview of the state-of-the-art in MRI-based PWV measurements. In addition, both gold standard and clinical standard methods of computing PWV are discussed.

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.

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.

Non-triggered quantification of central and peripheral pulse-wave velocity

PURPOSE

Stiffening of the arteries results in increased pulse-wave velocity (PWV), the propagation velocity of the blood. Elevated aortic PWV has been shown to correlate with aging and atherosclerotic alterations. We extended a previous non-triggered projection-based cardiovascular MR method and demonstrate its feasibility by mapping the PWV of the aortic arch, thoraco-abdominal aorta and iliofemoral arteries in a cohort of healthy adults.

MATERIALS AND METHODS

The proposed method "simultaneously" excites and collects a series of velocity-encoded projections at two arterial segments to estimate the wave-front velocity, which inherently probes the high-frequency component of the dynamic vessel wall modulus in response to oscillatory pressure waves. The regional PWVs were quantified in a small pilot study in healthy subjects (N = 10, age range 23 to 68 yrs) at 3T.

RESULTS

The projection-based method successfully time-resolved regional PWVs for 8-10 cardiac cycles without gating and demonstrated the feasibility of monitoring beat-to-beat changes in PWV resulting from heart rate irregularities. For dual-slice excitation the aliasing was negligible and did not interfere with PWV quantification. The aortic arch and thoracoabdominal aorta PWV were positively correlated with age (p < 0.05), consistent with previous reports. On the other hand, the PWV of the iliofemoral arteries showed decreasing trend with age, which has been associated with the weakening of muscular arteries, a natural aging process.

CONCLUSION

The PWV map of the arterial tree from ascending aorta to femoral arteries may provide additional insight into pathophysiology of vascular aging and atherosclerosis.

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.

Self-gated Fourier velocity encoding

Self-gating is investigated to improve the velocity resolution of real-time Fourier velocity encoding measurements in the absence of a reliable electrocardiogram waveform (e.g., fetal magnetic resonance or severe arrhythmia). Real-time flow data are acquired using interleaved k-space trajectories which share a common path near the origin of k-space. These common data provide a rapid self-gating signal that can be used to combine the interleaved data. The combined interleaves cover a greater area of k-space than a single real-time acquisition, thereby providing higher velocity resolution for a given aliasing velocity and temporal resolution. For example, this approach provided velocity spectra with a temporal resolution of 19 ms and velocity resolution of 22 cm/s over an 818 cm/s field-of-view. The method was validated experimentally using a computer-controlled pulsatile flow apparatus and applied in vivo to measure aortic-valve flow in a healthy volunteer.

Degree of fusiform dilatation of the proximal descending aorta in type B acute aortic dissection can predict late aortic events

OBJECTIVE

Predicting the risk factors for late aortic events in patients with type B acute aortic dissection without complications may help to determine a therapeutic strategy for this disorder. We investigated whether late aortic events in type B acute aortic dissection can be predicted accurately by an index that expresses the degree of fusiform dilatation of the proximal descending aorta during the acute phase; this index can be calculated as follows: (maximum diameter of the proximal descending aorta)/(diameter of the distal aortic arch + diameter of the descending aorta at the pulmonary artery level).

METHODS

Patients with type B acute aortic dissection without complications (n = 141) were retrospectively analyzed to determine the predictors of late aortic events; these include aortic dilatation, rupture, refractory pain, organ ischemia, rapid aortic enlargement, and rapid enlargement of ulcer-like projections.

RESULTS

The fusiform index in patients with late aortic events (0.59) was higher than that in patients without late aortic events (0.53, P < .01). Patients with a higher fusiform index exhibited aortic dilatation earlier than those with a lower fusiform index. By multivariate analysis, we conclude that the predominant independent predictors of late aortic events were a maximum aortic diameter of 40 mm or more, a patent false lumen, and a fusiform index of 0.64 or more (hazard ratios, 3.18, 2.64, and 2.73, respectively). The values of actuarial freedom from aortic events for patients with all 3 predictors at 1, 5, and 10 years were 22%, 17%, and 8%, respectively, whereas the values in those without these predictors were 97%, 94%, and 90%, respectively.

CONCLUSIONS

The degree of fusiform dilatation of the proximal descending aorta, a patent false lumen, and a large aortic diameter can be predominant predictors of late aortic events in patients with type B acute aortic dissection. Patients with these predictors should be recommended to undergo early interventions (surgery or stent-graft implantation) or at least be closely followed up during the chronic phase before such events develop.

Restricted field of view magnetic resonance imaging of a dynamic time series

A restricted field of view (rFOV) approach for imaging a dynamic time series of volumes of limited spatial extent within a larger subject is described. The shorter readout with rFOV-MRI can be exploited to either limit image artifacts or increase spatial resolution. To accomplish rFOV imaging of a multislice volume for a dynamic series, an outer volume suppression (OVS) preparation that saturates signal external to a cylinder through the subject is followed by slice-selective excitation and a spiral readout. The pass- and stopband efficiencies of the OVS in an agar gel phantom were 97% (+/-1.5%) and 3% (+/-1%), respectively. Profiles of the temporal signal-to-noise ratio (SNR) were measured in a phantom and an adult brain. The rFOV sequence reduced distortions from off-resonance signal and T2*-induced blurring compared to a conventional sequence. Sequence utility is demonstrated for high-resolution rFOV functional MRI (fMRI) in the visual cortex. The rFOV sequence may prove to be useful for other multislice dynamic and high-resolution imaging applications.

Assessing normal pulse wave velocity in the proximal pulmonary arteries using transit time: A feasibility, repeatability, and observer reproducibility study by cardiovascular magnetic resonance

PURPOSE

To calculate pulse wave velocity (PWV) in the proximal pulmonary arteries (PAs) by cardiovascular magnetic resonance (CMR) using the transit-time method, and address respiratory variation, repeatability, and observer reproducibility.

MATERIALS AND METHODS

A 1.9-msec interleaved phase velocity sequence was repeated three times consecutively in 10 normal subjects. Pulse wave (PW) arrival times (ATs) were determined for the main and branch PAs. The PWV was calculated by dividing the path length traveled by the difference in ATs. Respiratory variation was considered by comparing acquisitions with and without respiratory gating.

RESULTS

For navigated data the mean PWVs for the left PA (LPA) and right PA (RPA) were 2.09 +/- 0.64 m/second and 2.33 +/- 0.44 m/second, respectively. For non-navigated data the mean PWVs for the LPA and RPA were 2.14 +/- 0.41 m/second and 2.31 +/- 0.49 m/second, respectively. No statistically significant difference was found between respiratory non-navigated data and navigated data. Repeated on-table measurements were consistent (LPA non-navigated P = 0.95, RPA non-navigated P = 0.91, LPA navigated P = 0.96, RPA navigated P = 0.51). The coefficients of variation (CVs) were 12.2% and 12.5% for intra- and interobserver assessments, respectively.

CONCLUSION

One can measure PWV in the proximal PAs using transit-time in a reproducible manner without respiratory gating.

Quantification of aortic elasticity: development and experimental validation of a method using computed tomography

Aortic distensibility depending on aortic cross-sectional area changes is an important parameter for the grading of vascular diseases. This study measured aortic area changes by multidetector computed tomography. An image reconstruction algorithm was developed to assess aorta diameter and area as a function of the cardiac cycle with sufficient time resolution along the entire length of the aorta by four-detector row computed tomography. The algorithm was tested on porcine aortic specimens and compared with an optical reference method. The error of the relative vessel area change comparing the two methods was found to be about 3%. Initial tests on patient datasets indicate that clinical application is feasible. The proposed method has the advantage that it can easily be integrated into a modified routine CT angiography study and allows the measurement of aortic cross-sectional area changes.

Variable-density one-shot fourier velocity encoding

In areas of highly pulsatile and turbulent flow, real-time imaging with high temporal, spatial, and velocity resolution is essential. The use of 1D Fourier velocity encoding (FVE) was previously demonstrated for velocity measurement in real time, with fewer effects resulting from off-resonance. The application of variable-density sampling is proposed to improve velocity measurement without a significant increase in readout time or the addition of aliasing artifacts. Two sequence comparisons are presented to improve velocity resolution or increase the velocity field of view (FOV) to unambiguously measure velocities up to 5 m/s without aliasing. The results from a tube flow phantom, a stenosis phantom, and healthy volunteers are presented, along with a comparison of measurements using Doppler ultrasound (US). The studies confirm that variable-density acquisition of kz-kv space improves the velocity resolution and FOV of such data, with the greatest impact on the improvement of FOV to include velocities in stenotic ranges.

Semiquantitative fast flow velocity measurements using catheter coils with a limited sensitivity profile

Flow measurements can be used to quantify blood flow during MR-guided intravascular interventional procedures. In this study, a fast flow measurement technique is proposed that quantifies flow velocities in the vicinity of a small RF coil attached to an intravascular catheter. Since the small RF coil receives signal from only a limited volume around the catheter, a spatially nonselective signal reception is employed. To enhance signal from flowing blood, and suppress unwanted signal contributions from static material, a slice-selective RF excitation is used. At a velocity sensitivity of 150 cm/s, a temporal resolution of 2 x TR = 10.2 ms can be achieved. The flow measurement is combined with an automatic slice positioning to facilitate measurements during interventional procedures. The influence of the catheter position in the blood vessel on the velocity measurement was analyzed in simulations. For blood vessels with laminar flow, the simulation showed a systematic deviation between catheter measurement and true flow between -15% and 80%. In four animal experiments, the catheter velocity measurement was compared with results from a conventional ECG-triggered 2D phase-contrast (PC) technique. The shapes of the velocity time curves in the abdominal aorta were nearly identical to the conventional measurements. A relative scaling factor of 0.69-1.19 was found between the catheter velocity measurement and the reference measurement, which could be partly explained by the simulation results.

Computer-assisted evaluation of aortic stiffness using data acquired via magnetic resonance

Aortic stiffness is frequently assessed through pulse wave velocity (PWV) measurements. Based on data acquired by magnetic resonance (MR) using a one-dimensional time-of-flight technique, a new computational tool has been developed to rapidly construct flow velocity images and automatically calculate PWV. Comparison between PWV results obtained from this and a manual analysis demonstrates good agreement (correlation coefficient of 0.9951), while the new method improves the time efficiency by more than 20 times. The new method can also significantly improve flow signal quality and yield more credible results when strong interfering background signals are present.

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.

Determination of aortic compliance from magnetic resonance images using an automatic active contour model

The possibility of monitoring changes in aortic elasticity in humans has important applications for clinical trials because it estimates the efficacy of plaque-reducing therapies. The elasticity is usually quantified by compliance measurements. Therefore, the relative temporal change in the vessel cross-sectional area throughout the cardiac cycle has to be determined. In this work we determined and compared the compliance between three magnetic resonance (MR) methods (FLASH, TrueFISP and pulse-wave). Since manual outlining of the aortic wall area is a very time-consuming process and depends on an operator's variability, an algorithm for the automatic segmentation of the artery wall from MR images through the entire heart cycle is presented. The reliable detection of the artery cross-sectional area over the whole heart cycle was possible with a relative error of about 1%. Optimizing the temporal resolution to 60 ms we obtained a relative error in compliance of about 7% from TrueFISP (1.0 x 1.0 x 10 mm3, signal-to-noise ratio (SNR) > 12) and FLASH (0.7 x 0.7 x 10 mm3, SNR > 12) measurements in volunteers. Pulse-wave measurements yielded an error of more than 9%. In a study of ten volunteers, a compliance between C = 3 x 10(-5) Pa(-1) and C = 8 x 10(-5) Pa(-1) was determined, depending on age. The results of the TrueFISP and the pulse-wave measurements agreed very well with one another (confidence interval of 1 x 10(-5) Pa(-1)) while the results of the FLASH method more clearly deviated from the TrueFISP and pulse-wave (confidence interval of more than 2 x 10(-5) Pa(-1)).

Selective arterial spin labeling (SASL): perfusion territory mapping of selected feeding arteries tagged using two-dimensional radiofrequency pulses

To date, most perfusion magnetic resonance imaging (MRI) methods using arterial spin labeling (ASL) have employed slab-selective inversion pulses or continuous labeling within a plane in order to obtain maps derived from all major blood vessels entering the brain. However, there is great potential for gaining additional information on the territories perfused by the major vessels if individual feeding arteries could be tagged. This study demonstrates noninvasive arterial perfusion territory maps obtained using two-dimensional (2D) selective inversion pulses. This method is designated "selective ASL" (SASL). The SASL method was used to tag the major arteries below the circle of Willis. A combination of 2D selective tagging and multislice readout allows perfusion territories to be clearly visualized, with likely applications to cerebrovascular disease and stroke.

Peak velocity determination using fast Fourier velocity encoding with minimal spatial encoding

For quantitative peak velocity determination, a technique was developed that uses Fourier velocity encoding (FVE) for the fast acquisition of images of velocity with no spatial encoding other than slice selection. The technique produces images of velocity versus temporal frequency. In applications where the quantity of interest is the peak velocity and in-plane spatial localization is not required, high SNR images are produced with reduced sensitivity to errors due to slice thickness and motion. The technique was validated using steady and pulsatile flow in a straight tube, and compared to both phase contrast measurements and numerical models using steady flow in a 50% and a 75% cosinusoidal stenosis phantom. Results show that for slices as large as 2 cm and/or undergoing periodic motion, FVE can accurately measure the peak velocity in cases where a distribution of velocities exist.

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.

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

A magnetic resonance imaging projective velocity encoding sequence was used to determine the pulse-wave velocity in an artery model. To this end, a well-defined flow phantom simulating flow propagation in large arteries was used. In order to validate the measurement method in the presence of large reflected waves, these were deliberately created in the phantom. The projective sequence was applied to two measurement sites and the wave velocity was determined from the spatial and temporal separations of the foot of the velocity waveform. A theoretical model describing reflection and attenuation phenomena was compared with experimental velocity waveforms. The model showed that reflections and attenuation can explain the important changes in velocity waveforms. The model also confirmed that in the presence of reflecting waves, the foot of the waveform can be used as a characteristic point for measurements through changes in the waveform.

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.

Biophysical properties of the normal-sized aorta in patients with Marfan syndrome: evaluation with MR flow mapping

PURPOSE

To investigate the feasibility of magnetic resonance (MR) flow mapping in the assessment of aortic biophysical properties in patients with Marfan syndrome and to detect differences in biophysical properties in the normal-sized aorta distal to the aortic root between these patients and matched control subjects.

MATERIALS AND METHODS

Seventy-eight patients with Marfan syndrome with aortic root dilatation and 23 matched control subjects underwent MR flow mapping in four locations in the normal-sized aorta (1, ascending aorta; 2, thoracic descending aorta; 3, descending aorta at the level of the diaphragm; and 4, abdominal descending aorta). Distensibility at each location and flow wave velocity between locations were calculated.

RESULTS

Compared with the control subjects, patients with Marfan syndrome had decreased aortic distensibility at three of the four locations (levels 1, 2, and 4; P <.05) and increased flow wave velocity between all locations (P <.05) in the aorta. In patients with Marfan syndrome, flow wave velocity was also significantly increased along the entire aortic tract beyond the aortic root (from level 1 to level 4).

CONCLUSION

MR imaging reveals abnormal biophysical properties of the normal-sized aorta in patients with Marfan syndrome. Monitoring of these properties is relevant for evaluating disease progression and treatment options.

Rapid aortic wave velocity measurement with MR imaging

The utility of a one-dimensional magnetic resonance (MR) sequence to rapidly and accurately measure wave velocity in vivo was evaluated. Studies were conducted in the thoracic aortas of 20 healthy subjects of varying age, and the MR method was validated in a compliant tube model. Aortic wave velocity ranged from 3.8 to 9.7 m/sec and demonstrated a positive correlation with subjects' age. Peak blood velocity ranged from 47 to 125 cm/sec and exhibited a strong negative correlation with subjects' age.

Fast measurements of the motion and velocity spectrum of blood using MR tagging

A new method is presented for tracking the motion of blood and determining its velocity spectrum from magnetic resonance data collected within a single heartbeat. The method begins by tagging a column of blood in a vessel by combining a 1D SPAMM excitation with a 2D cylindrical excitation. A series of 1D projections of the tagging pattern is acquired from a train of gradient echoes. The influence of specific excitation profiles and velocity profiles on the motion of the tags is explored for steady flow. It is shown mathematically, and confirmed with phantom experiments, that the velocity of a tag equals the mean velocity of the excited fluid when the velocity spectrum is symmetric about its mean velocity. The velocity spectrum is derived by analyzing the interference between tags moving at different velocities. This appears to be the first use of magnitude tagging to obtain velocity spectra. Representative measurements in a human aorta are presented to assess feasibility in vivo. Magn Reson Med 45:461-469, 2001.

In vivo measurement of pulsewave velocity in small vessels using intravascular MR

A one-dimensional intravascular MR (IVMR) technique for the measurement of pulsewave velocity in a single cardiac cycle is presented. The technique was used to measure pulsewave velocity in vivo in the intact rabbit model, where its sensitivity to different hemodynamic states was demonstrated using a pharmacological intervention with phenylephrine and nitroprusside. IVMR measurements of pulsewave velocity were found to increase with mean arterial pressure, as expected. Further, IVMR-based pulsewave velocity estimates were in agreement with those measured by pressure catheters and direct distensibility measurement. Because of their rapidity and highly localized nature, these measurements of vessel elasticity may complement the high-resolution vascular imaging information gained in an IVMR examination. This could allow assessment of atherosclerotic plaques and facilitate immediate treatment decisions. Magn Reson Med 45:53-60, 2001.

Estimation of aortic compliance using magnetic resonance pulse wave velocity measurement

A method for compliance estimation employing magnetic resonance pulse wave velocity measurement is presented. Time-resolved flow waves are recorded at several positions along the vessel using a phase contrast sequence, and pulse wave velocity is calculated from the delay of the wave onsets. Using retrospective cardiac gating in combination with an optically decoupled electrocardiogram acquisition, a high temporal resolution of 3 ms can be achieved. A phantom set-up for the simulation of pulsatile flow in a compliant vessel is described. In the phantom, relative errors of pulse wave velocity estimation were found to be about 15%, whereas in a volunteer, larger errors were found that might be caused by vessel branches. Results of pulse wave velocity estimation agree with direct aortic distension measurements which rely on a peripheral estimate of aortic pressure and are therefore less accurate. Studies in 12 volunteers show values of pulse wave velocity consistent with the literature; in particular the well-known increase in pulse wave velocity with age was observed. Preliminary results show that the method can be applied to aortic aneurysms.

Flow quantification using low-spatial-resolution and low-velocity-resolution velocity images

In this report, a flow-quantification method using Fourier velocity encoding (FVE) with limited spatial and velocity resolution is presented. The total flow rate in a vessel corresponds to the first moment of the velocity histogram of spins in the vessel, whereas the spin density of flowing spins is the normalization constant. Because the measured histogram using FVE is distorted by RF saturation effects, the RF saturation effects are first estimated and then accurately compensated by acquiring five velocity-encoded images. The spatial resolution in each image can be relatively low because all stationary spins vanish in the resultant flow map. In a phantom study, the errors in measured flow rates were within +/-10% even when the pixel size was greater than the vessel size. This method was also successfully applied to measure flow in the femoral artery. In general, this method constitutes a basis for analyzing multiple velocity-encoded images and is particularly useful for quantifying slow flow or flow in small vessels. Magn Reson Med 42:682-690, 1999.

Separation of haemodynamic flow waves measured by MR into forward and backward propagating components

Physiological information on the action of the heart and on the reflection sites in the arterial system can be derived respectively from the forward and the backward propagating pressure or flow wave components. Earlier work on the separation of these components was exclusively based on invasive measurements of pressure or flow. In this study magnetic resonance (MR), which is a non-invasive imaging technique, was used to measure the blood flow waveform simultaneously at multiple positions along a vessel. Linear one dimensional transmission-line theory was used to separate the flow waves into forward and backward propagating components. First results, obtained from the thoracic aorta of five healthy male volunteers, consistently showed a negative reflection with a delay of about 100 ms between the foot of the forward and the foot of the backward propagating flow wave. Our model, consisting of a single vessel segment with constant diameter and wall properties, was validated by the excellent agreement between the vessel area as calculated from the flow data using the law of mass conservation and as directly measured with a different independent MR technique.

Accuracy of arterial pulse-wave velocity measurement using MR

The performance of a one-dimensional MR technique for the estimation of pulse-wave velocity in the aorta was evaluated. An expression for the error in this estimate was formulated and verified both by simulation and by experiment. On the basis of this formulation, guidelines for increasing the efficiency of the acquisition were established. The technique was further validated by comparison with pulse-wave velocity measurements made with a pressure catheter. All data were acquired from a latex tube driven by a pulsatile flow system. MR measurements of pulse-wave velocity in the tube were found to be very reproducible in the presence of white noise. Measurements by other techniques were in good agreement, falling within 2 SD of the mean. Because of its sensitivity and spatial resolution, this technique shows promise for making spatially resolved estimates of vessel distensibility. This would allow assessment of diseases, such as atherosclerosis, that cause local changes in the material properties of the vasculature.

Cine MR Fourier velocimetry of blood flow through cardiac valves: comparison with Doppler echocardiography

Noninvasive measurement of blood flow velocity through the cardiac valves has important clinical applications. A wide variety of MR methods are available for flow measurement. The aim of this study was to investigate the ability of cine MR Fourier velocimetry to measure flow through healthy cardiac valves and to compare MR and Doppler peak velocity measurements. Ten healthy volunteers (age mean +/- SD, 24 +/- 4 years) without history of valvular disease were studied. Four of the subjects were females. In each subject, aortic, pulmonary, mitral, and tricuspid valves were evaluated with MR and Doppler imaging. A whole-body mobile MR machine was used, operating at .5-T with actively shielded magnetic field gradient coils on all three axes capable of 20 mT/m at a slew rate of 60 mT/ m/msec. The heart rate during MR and Doppler studies was not significantly different. The mean difference between the two studies was 2 beats/min, with a 95% confidence interval of -22 beats/min, +25 beats/ min. Peak systolic flow velocity in the aortic and pulmonary valves and peak diastolic flow velocity in the mitral and tricuspid valves measured with MRI and Doppler echocardiography correlated well. The mean difference between the two measurements (MR-Doppler) was 63 mm/sec, with a 95% confidence interval of -180 mm/sec, +310 mm/sec. The agreement between two observers interpreting the same MR velocity maps was close. The mean difference between their two measurements was 23 mm/sec, with a 95% confidence interval of -20 mm/sec, +60 mm/sec. There was no significant difference between MR and Doppler imaging or between the two MR observers. MR Fourier velocimetry has the necessary ease, reliability, and speed to measure blood flow through the cardiac valves, although measurement of late diastolic flow in the atrioventricular valves is limited. Measurement of peak blood velocity through the cardiac valves by this method showed satisfactory agreement with Doppler, but its clinical application for assessing diseased cardiac valves must be established.

In Vitro Validation of Rapid MR Measurement of Wave Velocity

A one-dimensional time-of-flight MR sequence, having a total acquisition time of approximately 60 ms, has been employed to determine flow-wave propagation velocities for pulsatile flow in compliant latex tubes. The results were compared with those of two independent methods and were found to be in good agreement. An extension of the same MR method was used to test the validity of the "water-hammer" relationship as a means to assess pulse pressure. Very good agreement was found with direct manometric determinations of pulse pressure.