Purpose: To systematically investigate the scan-rescan reproducibility and observer variability of flow-sensitive four-dimensional (4D) MRI in the aorta for the assessment of blood flow and global and segmental wall shear stress.Materials and Methods: ECG and respiration-synchronized flow-sensitive 4D MRI data (spatio-temporal resolution ¼ 1.7 Â 2.0 Â 2.2 mm 3 /40.8 ms) were acquired in 12 healthy volunteers. To analyze scan-rescan variability, flow-sensitive 4D MRI was repeated in 10 volunteers during a second visit. Data analysis included calculation of time-resolved and total flow, peak systolic velocity, and regional and global wall shear stress (WSS) in up to 24 analysis planes distributed along the aorta.Results: Scan-rescan, inter-observer, and intra-observer agreement was excellent for the calculation of total flow and peak systolic velocity (mean differences <5% of the average flow parameter). Global WSS demonstrated moderate agreement and increased variability regarding wall shear stress (scan-rescan, inter-observer, and intra-observer agreement; mean differences <10% of the average WSS parameters). The segmental distribution of wall shear stress in the thoracic aorta could reliably be reproduced (r > 0.87; P < 0.001) for different observers and examinations.Conclusion: Flow-sensitive 4D MRI-based analysis of aortic blood flow can be performed with good reproducibility. Robustness of global and regional WSS quantification was limited, but spatio-temporal WSS distributions could reliably be replicated.

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 5 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 6 0.24 m/ sec) compared to age-matched controls (6.40 6 0.32 m/sec). Both were significantly (P < 0.001) increased compared to younger volunteers (4.39 6 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 fourdimensional 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 63:1575-1582, 2010. V C 2010 Wiley-Liss, Inc. Key words: pulse wave velocity; compliance; phase contrast; atherosclerosis; aorta Increased pulse wave velocity (PWV) as a measure of aortic stiffness is an important marker for both agerelated changes in aortic compliance and the presence of atherosclerosis (1,2) and is an independent predictor of cardiovascular mortality and stroke (3,4). Reliable measurement of PWV is of particular interest for monitoring the progression or regression of vessel compliance during therapy (5,6).Pressure catheter measurements can be considered the gold standard for the assessment of the development of aortic pressure wave, but their use is limited due to the invasiveness of the procedure (7). To provide a noninvasive estimate of PWV, phase contrast (PC) MRI has been applied in a number of in vivo studies determining blood flow based on flow waveform measurements in single two-dimensional (2D) planes transecting the aorta (8-10). For PWV estimation, typically transit-time methods are employed estimating temporal differences of specific features of blood flow waveforms, e.g., time from foot to foot or peak to peak, between two locations of the vessel with known distance (11-24). However, the precision of this method highly depends on the exact calculation of flow difference and distance between only two measuring points (16,25,26).Methodological improvements include a more continuous evaluation along a vessel center line and cross-correlation (XCor) analysis for the estimation of waveform delays, which improved the accuracy of PWV...

Purpose Changes in aortic geometry or presence of aortic valve disease can result in substantially altered aortic hemodynamics. Dilatation of the ascending aorta or aortic valve abnormalities can result in an increase in helical flow. Methods 4D flow MRI was used to test the feasibility of quantitative helicity analysis using equidistantly distributed 2D planes along the entire aorta. The evaluation of the method included three parts: 1) the quantification of helicity in 12 healthy subjects, 2) an evaluation of observer variability and test-retest reliability, and 3) the quantification of helical flow in 16 patients with congenitally altered bicuspid aortic valves. Results Helicity quantification in healthy subjects revealed consistent directions of flow rotation along the entire aorta with high clockwise helicity in the aortic arch and an opposite rotation sense in the ascending and descending aorta. The results demonstrated good scan-rescan and inter- and intra-observer agreement of the helicity parameters. Helicity quantification in patients revealed a significant increase of absolute peak relative helicity during systole and a considerably greater heterogeneous distribution of mean helicity in the aorta. Conclusion The method has the potential to serve as a reference distribution for comparisons of helical flow between healthy subjects and patients or between different patient groups.

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 6 3 years) and 86 acute stroke patients (68 6 9 years) with aortic atherosclerosis. Retrospectively positioned 28 6 4 analysis planes along the entire aorta (inter-slice-distance ¼ 10 mm) and frame wise lumen segmentation yielded flow-time-curves for each plane. Global aortic PWV was calculated from timeshifts 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 6 2.9 versus 3.8 6 0.8 m/s; P ¼ 0.02). Among 17 patient characteristics considered, statistical analysis revealed significant (P < 0.05) but low correlation of PWV with age (r ¼ 0.25), aortic valve insufficiency (r ¼ 0.29), and pulse pressure (r ¼ 0.28). Multivariate modeling indicated that aortic valve insufficiency and elevated pulse pressure were significantly associated with higher PWV (adjusted R 2 ¼ 0.13). INCREASED PULSE WAVE velocity (PWV) is a measure of aortic stiffness and closely associated with atherosclerosis. A recent meta-analysis revealed that increased PWV and thus reduced aortic compliance is a strong predictor of future cardiovascular events and all-cause mortality. Moreover, the authors identified that an increase in aortic PWV by 1 m/s corresponded to an age-, sex-, and risk factor-adjusted risk increase of ca. 15% in total cardiovascular events (1). Reliable measurement of PWV is of particular interest for monitoring vessel compliance during therapy (2,3). Carotid-femoral PWV using tonometry is the current reference standard to measure aortic compliance (4). This method, however, does not focus on compliance of the thoracic aorta. Instead, wall characteristics of both the abdominal aorta and the carotid and femoral arteries are also included. Time-resolved (CINE) Phase contrast (PC) MRI provides a noninvasive estimate of PWV based on flow waveform measurements in analysis planes and allows focusing on the region of interest in stroke patients, i.e., the thoracic aorta (5). Transit-time (TT) methods are typically used estimating temporal differences of specific flow waveforms features, e.g., timing differences of the foot of the waveform between two locations with known distance (6-14). Alternatively, PWV was calculated using targeted excitation of the vessel of interest in Conclusion

Flow-sensitive 4D MRI at 3 T provided improved image quality without additional artifacts related to higher fields. Imaging at 1.5 T MRI, which is more widely available, was also feasible and provided information on aortic 3D hemodynamics of moderate quality with identical performance regarding quantitative analysis.

Purpose: To compare 3D magnetic resonance imaging (3D MRI) with transesophageal echocardiography (TEE) for the detection of complex aortic plaques (!4 mm thick, ulcerated, or containing mobile thrombi). Materials and Methods:In all, 99 consecutive patients with acute cryptogenic stroke and !3 mm thick aortic plaques in TEE were prospectively included. 3D MRI comprised T1-weighted bright blood MRI with complete aortic coverage (spatial resolution 1 mm 3 ). Wall thickness and occurrence of complex plaques in ascending aorta (AAo), aortic arch (AA), and descending aorta (DAo) and image quality for each segment was rated for MRI and TEE.Results: MRI detected more complex plaques than TEE (MRI vs. TEE): AAo 13 vs. 7; AA 37 vs. 11; differences were smaller in the DAo: 101 vs. 70. Image quality was higher for MRI in AAo and AA (P < 0.001) and superior for TEE in DAo (P < 0.001). MRI revealed additional complex plaques in the proximal aorta in 19 of 58 patients (32.8%) categorized as cryptogenic after complete routine diagnostics including TEE. Conclusion:Due to improved visualization of the aorta 3D MRI allows to detect more complex plaques than TEE. This renders 3D MRI particularly valuable for patients with cryptogenic stroke and for trials evaluating optimal treatment in aortic atherosclerosis.

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