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.

We present a new method based on B-spline snakes (active contours) for measuring high-accuracy contact angles. In this approach, we avoid making physical assumptions by defining the contour of the drop as a versatile B-spline curve. When useful, we extend this curve by mirror symmetry so that we can take advantage of the reflection of the drop onto the substrate to detect the position of the contact points. To keep a wide range of applicability, we refrain from discretizing the contour of the drop, and we choose to optimize an advanced image-energy term to drive the evolution of the curve. This term has directional gradient and region-based components; additionally, another term-an internal energy-is responsible for the snake elasticity and constrains the parameterization of the spline. While preserving precision at the contact points, we limit the computational complexity by constraining a non-uniform repartition of the control points. The elasticity property of the snake links the local nature of the contact angle to the global contour of the drop. A global knowledge of the drop contour allows us to use the reflection of the drop on the substrate to automatically and precisely detect a line of contact points (vertical position and tilt). We apply cubic-spline interpolation over the image of the drop; then, the evolution procedure takes part in this continuous domain to avoid the inaccuracies introduced by pixelization and discretization.We have programmed our method as a Java software and we make it freely available [A.F. Stalder, DropSnake, Biomedical Imaging Group, EPFL, [ON LINE] visited 2005. http://bigwww.epfl.ch/demo/dropanalysis]. Our experiments result in good accuracy thanks to our high-quality image-interpolation model, while they show applicability to a variety of images thanks to our advanced image-energy term.

a b s t r a c tA new method based on the Young-Laplace equation for measuring contact angles and surface tensions is presented. In this approach, a first-order perturbation technique helps to analytically solve the Young-Laplace equation according to photographic images of axisymmetric sessile drops. When appropriate, the calculated drop contour is extended by mirror symmetry so that reflection of the drop into substrate allows the detection of position of the contact points. To keep a wide range of applicability, a discretisation of the drop's profile is not realised; instead, an optimisation of an advanced image-energy term fits an approximation of the Young-Laplace equation to drop boundaries. In addition, cubic B-spline interpolation is applied to the image of the drop to reach subpixel resolution. To demonstrate the method's accuracy, simulated drops as well as images of liquid coal ash slags were analysed. Thanks to the high-quality image interpolation model and the image-energy term, the experiments demonstrated robust measurements over a wide variety of image types and qualities. The method was implemented in Java and is freely available [A.F. Stalder, LBADSA, Biomedical Imaging Group, EPFL,

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...

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...

Our purpose was to correlate atherogenic low wall shear stress (WSS) and high oscillatory shear index (OSI) with the localization of aortic plaques. Flow-sensitive four-dimensional MRI was used to acquire three-dimensional blood flow in the aorta of 62 patients with proven aortic atherosclerosis and 31 healthy volunteers. Multiplanar data analysis of WSS magnitude and OSI in 12 wall segments was performed in analysis planes distributed along the aorta. Disturbed WSS and OSI were defined as areas exposed to low WSS magnitude and high OSI beyond individual 15% thresholds. Planewise analysis revealed a good correlation (r 5 0.85) of individual low WSS magnitude but not of high OSI with plaque distribution. Although plaques occurred only rarely in the ascending aorta, the incidence of low WSS magnitude and high OSI was similar to findings in other aortic segments where plaques occurred more frequently. Case-by-case comparisons of plaque location and critical wall parameters revealed a shift of atherogenic WSS magnitude (78% of all cases) and OSI (91%) to wall segments adjacent to the atheroma. Our results indicate that the predictive value of WSS for plaque existence depends on the aortic segment and that locations of critical wall parameters move to neighboring segments of regions affected by atherosclerosis. Magn Reson Med 63:1529-1536,

Purpose To develop a highly accelerated phase contrast cardiac-gated volume flow measurement (4D flow) MR imaging technique based on spiral sampling and dynamic compressed sensing, and to compare with established phase contrast imaging techniques for the quantification of blood flow in abdominal vessels. Methods In this prospective IRB approved study, 10 subjects (9 males, mean age 51 y) including 7 patients with liver disease were enrolled. Two 4D flow acquisitions were performed, one using Cartesian sampling with respiratory tracking, the other using spiral sampling and acquired in a breath hold. Cartesian 2D cine phase contrast was also acquired in the portal vein. Two independent observers assessed vessel conspicuity on phase contrast 3D angiogram. Quantitative flow parameters were measured by two independent observers in major abdominal vessels. Inter-technique concordance was quantified using Bland-Altman analysis and Pearson correlation. Results There was no significant difference in vessel conspicuity between 4D flow acquisitions (p >0.069, for both observers), while more artifacts were observed with spiral 4D flow (p <0.016). Quantitative measurements in abdominal vessels showed strong correlation between spiral and Cartesian 4D flow techniques (for total flow r = 0.96, p <0.001). For portal venous flow, spiral 4D flow was in better agreement with 2D cine phase contrast (−8.8/9.3 mL/s) than was Cartesian 4D flow (−10.6/14.6 mL/s). Conclusion Combining highly efficient spiral sampling with dynamic compressed sensing results in major acceleration for 4D flow MR imaging, which allows comprehensive assessment of abdominal vessel hemodynamics in a breath hold.