Quantification of CINE phase contrast (PC)-MRI data is a challenging task because of the limited spatiotemporal resolution and signal-to-noise ratio (SNR). The method presented in this work combines B-spline interpolation and Green's theorem to provide optimized quantification of blood flow and vessel wall parameters. The B-spline model provided optimal derivatives of the measured three-directional blood velocities onto the vessel contour, as required for vectorial wall shear stress (WSS) computation. Eight planes distributed along the entire thoracic aorta were evaluated in a 19-volunteer study using both high-spatiotemporal-resolution planar two-dimensional (2D)-CINE-PC ( approximately 1.4 x 1.4 mm(2)/24.4 ms) and lower-resolution 3D-CINE-PC ( approximately 2.8 x 1.6 x 3 mm(3)/48.6 ms) with three-directional velocity encoding. Synthetic data, error propagation, and interindividual, intermodality, and interobserver variability were used to evaluate the reliability and reproducibility of the method. While the impact of MR measurement noise was only minor, the limited resolution of PC-MRI introduced systematic WSS underestimations. In vivo data demonstrated close agreement for flow and WSS between 2D- and 3D-CINE-PC as well as observers, and confirmed the reliability of the method. WSS analysis along the aorta revealed the presence of a circumferential WSS component accounting for 10-20%. Initial results in a patient with atherosclerosis suggest the potential of the method for understanding the formation and progression of cardiovascular diseases.

Correspondence to Alex J. Barker, PhD, Department of Radiology, Northwestern University, 737 N Michigan Ave, Suite 1600, Chicago, IL 60611. E-mail alex.barker@northwestern.edu Background-Hemodynamics may play a role contributing to the progression of bicuspid aortic valve (BAV) aortopathy.This study measured the impact of BAV on the distribution of regional aortic wall shear stress (WSS) compared with control cohorts. Methods and Results-Local WSS distribution was measured in the thoracic aorta of 60 subjects using 4-dimensional (4D) flow-sensitive magnetic resonance imaging. WSS analysis included 15 BAV patients: 12 with fusion of the right-left coronary cusp (6 stenotic) and 3 with fusion of the right and noncoronary cusp. The right-left BAV cohort was compared with healthy subjects (n=15), age-appropriate subjects (n=15), and age-/aorta size-controlled subjects (n=15). Compared with the age-appropriate and age-/aorta size-matched controls, WSS patterns in the right-left BAV ascending aorta were significantly elevated, independent of stenosis severity (peak WSS=0.9±0.3 N/m 2 compared with 0.4±0.3 N/m 2 in age-/aorta size-controlled subjects; P<0.001). Time-resolved (cine) 2D images of the bicuspid valves were coregistered with 4D flow data, directly linking cusp fusion pattern to a distinct ascending aortic flow jet pattern. The observation of right-anterior ascending aorta wall/jet impingement in right-left BAV patients corresponded to regions with statistically elevated WSS. Alternative jetting patterns were observed in the right and noncoronary cusp fusion patients. Conclusions-The results of this study demonstrate that bicuspid valves induced significantly altered ascending aorta hemodynamics compared with age-and size-matched controls with tricuspid valves. Specifically, the expression of increased and asymmetric WSS at the aorta wall was related to ascending aortic flow jet patterns, which were influenced by the BAV fusion pattern. (Circ Cardiovasc Imaging. 2012;5:457-466.)

Purpose:To assess the distribution and regional differences of flow and vessel wall parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) in the entire thoracic aorta. Materials and Methods:Thirty-one healthy volunteers (mean age ϭ 23.7 Ϯ 3.3 years) were examined by flow-sensitive four-dimensional (4D)-MRI at 3T. For eight retrospectively positioned 2D analysis planes distributed along the thoracic aorta, flow parameters and vectorial WSS and OSI were assessed in 12 segments along the vascular circumference. Conclusion:The normal distribution of vectorial WSS and OSI in the entire thoracic aorta derived from flow-sensitive 4D-MRI data provides a reference constituting an important perquisite for the examination of patients with aortic disease. Marked regional differences in absolute WSS and OSI may help explaining why atherosclerotic lesions predominantly develop and progress at specific locations in the aorta. COMPLEX VASCULAR GEOMETRY AND PULSATILE FLOW in the human arterial system lead to regionally different flow characteristics and thus spatial and temporal changes in shear forces acting on the vessel wall. These forces can be characterized by wall shear stress (WSS) or oscillatory shear index (OSI) that play an important role in flow-mediated atherogenesis and arterial remodeling (1-3). While WSS values reported in the literature typically reflect the time-averaged shear forces acting on the vessel wall, OSI describes the existence and magnitude of WSS changes over the cardiac cycle. Recent reports stressed the importance of WSS and OSI with respect to the formation and stability of atherosclerotic plaques (4). A number of studies have shown that low WSS and high OSI represent sensitive markers for formation of plaques in the aorta, carotid, or coronary arteries (5,6). Particularly, the assessment of both WSS and OSI can help to determine the complexity of the lesions. A recent study with animal models and deliberately altered flow characteristics in the carotid arteries demonstrated the close correlation of low WSS with the development of vulnerable high-risk plaques whereas high OSI induce stable lesions (4). In addition, the effects of selected pathologies on regionally-varying WSS and OSI values have been reported (7,8).Among other methods, MRI is a feasible and extensively validated technique to derive quantitative flow information from arterial vessels (9 -12). Due to its intrinsic sensitivity to flow and the possibility to acquire true time-resolved three-dimensional (3D) data, in vivo analyses of blood-flow and derived vessel wall parameters are promising. However, earlier reports on MRbased analysis of aortic hemodynamics were either based on incomplete vascular coverage and separately acquired 2D slices (13-17), a combination of MR mea-

Time-resolved phase contrast (PC) MRI with velocity encoding in three directions (flow-sensitive four-dimensional MRI) can be employed to assess three-dimensional blood flow in the entire aortic lumen within a single measurement. These data can be used not only for the visualization of blood flow but also to derive additional information on vascular geometry with three-dimensional PC MR angiography (MRA). As PC-MRA is sensitive to available signal-to-noise ratio, standard and novel blood pool contrast agents may help to enhance PC-MRA image quality. In a group of 30 healthy volunteers, the influence of different contrast agents on vascular signalto-noise ratio, PC-MRA quality, and subsequent three-dimensional stream-line visualization in the thoracic aorta was determined. Flow-sensitive four-dimensional MRI data acquired with contrast agent provided significantly improved signal-to-noise ratio in magnitude data and noise reduction in velocity data compared to measurements without contrast media. The agreement of three-dimensional PC-MRA with reference standard contrast-enhanced MRA was good for both contrast agents, with improved PC-MRA performance for blood pool contrast agent, particularly for the smaller supraaortic branches. While most clinical applications of MRA rely on the application of Gadolinium (Gd) contrast agent (CA), three-dimensional (3D) phase contrast (PC)-MRA based on velocity-encoded 3D MRI with encoding in three directions has proven to be a useful alternative (8-11). PC-MRA can provide detailed information on vascular geometry and may offer additional information on flow direction. However, most PC-MRA implementations used nongated data acquisition, which can result in artifacts for pulsatile blood flow. Further drawbacks of the PC-MRA method are long scan times and lack of respiration control, which limited most applications of 3D PC-MRA to static regions with low pulsatile flow such as the cranial vessels (12,13).Recently, improved time-resolved (CINE) 3D PC MRI techniques using electrocardiography (ECG) gating and advanced navigator respiration control (flow-sensitive four-dimensional [4D] MRI) have been successfully applied for the analysis of pulsatile 3D blood flow in the aorta (14-24). Such techniques offer the opportunity for the detailed analysis of pulsatile 3D blood flow but require scan times up to 20 min. We previously reported an approach to derive 3D angiographic information (3D PC-MRA) from flow-sensitive 4D MRI (24,25). We showed that it was possible to exploit the information in the acquired flow-sensitive 4D data to derive angiographic information without performing additional MRA measurements. Although the derived 3D PC-MRA does not provide the same detailed depiction of anatomy and morphology compared to CE-MRA, it can improve considerably the presentation of the results by combining 3D visualization of anatomy and flow for large vascular geometries such as the thoracic aorta.Based on this strategy, an improved data processing workflow including noise masking was impleme...

In this work, we present a systematic phantom comparison and clinical application of noninvasive pressure difference mapping in the human aorta based on time-resolved 3D phase contrast data. Relative pressure differences were calculated based on integration and iterative refinement of pressure gradients derived from MR-based three-directional velocity vector fields (flow-sensitive 4D MRI with spatial/temporal resolution~2.1 mm 3 /40 ms) using the Navier-Stokes equation. After in vitro study using a stenosis phantom, timeresolved 3D pressure gradients were systematically evaluated in the thoracic aorta in a group of 12 healthy subjects and 6 patients after repair for aortic coarctation. Results from the phantom study showed good agreement with expected values and standard methods (Bernoulli). Data of healthy subjects showed good intersubject consistency and good agreement with the literature. In patients, pressure waveforms showed elevated peak values. Pressure gradients across the stenosis were compared with reference measurements from Doppler ultrasound. The MRI findings demonstrated a significant correlation (r 5 0.96, P < 0.05) but moderate underestimation (14.7% 6 15.5%) compared with ultrasound when the maximum pressure difference for all possible paths connecting proximal and distal locations of the stenosis were used. This study demonstrates the potential of the applied approach to derive additional quantitative information such as pressure gradients from time-resolved 3D phase contrast MRI. Magn Reson Med 66:1079-1088,

To determine three-dimensional (3D) blood flow patterns in the carotid bifurcation, 10 healthy volunteers and nine patients with internal carotid artery (ICA) stenosis ≥50% were examined by flow-sensitive 4D MRI at 3T. Absolute and mean blood velocities, pulsatility index (PI), and resistance index (RI) were measured in the common carotid arteries (CCAs) by duplex sonography (DS) and compared with flow-sensitive 4D MRI. Furthermore, 3D MRI blood flow patterns in the carotid bifurcation of volunteers and patients before and after recanalization were graded by two independent readers. Blood flow velocities measured by MRI were 31-39% lower than in DS. However, PI and RI differed by only 13-16%. Rating of 3D flow characteristics in the ICA revealed consistent patterns for filling and helical flow in volunteers. In patients with ICA stenosis, 3D blood flow visualization was successfully employed to detect markedly al- The assessment of the severity and progression of internal carotid artery (ICA) stenosis is of clinical interest since high-grade stenoses constitute a major source of ischemic stroke. While the influence of cardiovascular risk factors on the common carotid artery (CCA) is expressed by a proportional increase of intima-media thickness and decrease of vessel distensibility (1,2), the development of atherosclerosis in the naturally bulbar ICA is related to anatomical and local hemodynamic conditions such as flow deceleration and reduced and oscillating wall shear stress (WSS), as shown in vitro, in animal models, and in two-dimensional (2D) MRI studies of healthy volunteers (3-9).However, current clinical diagnostic tools are limited since they provide either functional (2D duplex sonography [DS]) or morphological (digital subtraction, CT, or MR angiography [MRA]) data (10 -12). At present, a combined assessment and analysis of both anatomy and function in 3D is not available. While the evaluation of carotid plaque composition by MRI is well established (12), associated individual 3D blood flow patterns that influence plaque type, as recently shown by Cheng et al. (7), have not yet been studied in humans by MRI in vivo. Furthermore, little is known about the underlying flow characteristics, such as helical flow in the bulb of ICA and its relationship to the development of carotid artery stenosis.In this context, time-resolved phase-contrast (PC) MRI with three-directional velocity encoding (flow-sensitive 4D MRI) provides full hemodynamic information on 3D blood flow for both left and right carotid bifurcations. Previously reported results demonstrated the potential of this technique for the assessment of normal and altered blood flow in the heart and the aorta (13-19). In a number of studies, advanced 3D visualization of blood flow based on vector fields, 3D streamlines, and time-resolved 3D particle traces was successfully employed to detect and illustrate complex in vivo 3D blood flow patterns (20 -26).Moreover, application of this MRI technique to intracranial and peripheral vessels yielded objective...

• Secondary blood flow patterns (helices, vortices) are commonly observed in the aorta • Secondary flow patterns predominantly depend on patient age and aortic diameter • Geometric factors show a lesser impact on blood flow patterns than age and diameter • Future analyses of flow patterns should incorporate age- and diameter dependencies.

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.