Michael Markl

Multicoil Dixon Chemical Species Separation with an Iterative Least-Squares Estimation Method

This work describes a new approach to multipoint Dixon fat–water separation that is amenable to pulse sequences that require short echo time (TE) increments, such as steady‐state free precession (SSFP) and fast spin‐echo (FSE) imaging. Using an iterative linear least‐squares method that decomposes water and fat images from source images acquired at short TE increments, images with a high signal‐to‐noise ratio (SNR) and uniform separation of water and fat are obtained. This algorithm extends to multicoil reconstruction with minimal additional complexity. Examples of single‐ and multicoil fat–water decompositions are shown from source images acquired at both 1.5T and 3.0T. Examples in the knee, ankle, pelvis, abdomen, and heart are shown, using FSE, SSFP, and spoiled gradient‐echo (SPGR) pulse sequences. The algorithm was applied to systems with multiple chemical species, and an example of water–fat–silicone separation is shown. An analysis of the noise performance of this method is described, and methods to improve noise performance through multicoil acquisition and field map smoothing are discussed. Magn Reson Med 51:35–45, 2004.

Time-resolved 3D MR velocity mapping at 3T: Improved navigator-gated assessment of vascular anatomy and blood flow

To evaluate an improved image acquisition and data‐processing strategy for assessing aortic vascular geometry and 3D blood flow at 3T.

Comprehensive 4D velocity mapping of the heart and great vessels by cardiovascular magnetic resonance.

BackgroundPhase contrast cardiovascular magnetic resonance (CMR) is able to measure all three directional components of the velocities of blood flow relative to the three spatial dimensions and the time course of the heart cycle. In this article, methods used for the acquisition, visualization, and quantification of such datasets are reviewed and illustrated.MethodsCurrently, the acquisition of 3D cine (4D) phase contrast velocity data, synchronized relative to both cardiac and respiratory movements takes about ten minutes or more, even when using parallel imaging and optimized pulse sequence design. The large resulting datasets need appropriate post processing for the visualization of multidirectional flow, for example as vector fields, pathlines or streamlines, or for retrospective volumetric quantification.ApplicationsMultidirectional velocity acquisitions have provided 3D visualization of large scale flow features of the healthy heart and great vessels, and have shown altered patterns of flow in abnormal chambers and vessels. Clinically relevant examples include retrograde streams in atheromatous descending aortas as potential thrombo-embolic pathways in patients with cryptogenic stroke and marked variations of flow visualized in common aortic pathologies. Compared to standard clinical tools, 4D velocity mapping offers the potential for retrospective quantification of flow and other hemodynamic parameters.ConclusionsMultidirectional, 3D cine velocity acquisitions are contributing to the understanding of normal and pathologically altered blood flow features. Although more rapid and user-friendly strategies for acquisition and analysis may be needed before 4D velocity acquisitions come to be adopted in routine clinical CMR, their capacity to measure multidirectional flows throughout a study volume has contributed novel insights into cardiovascular fluid dynamics in health and disease.

Quantitative 2D and 3D phase contrast MRI: Optimized analysis of blood flow and vessel wall parameters

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 Greens 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 (∼1.4 × 1.4 mm2/24.4 ms) and lower‐resolution 3D‐CINE‐PC (∼2.8 × 1.6 × 3 mm3/48.6 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. Magn Reson Med 60:1218–1231, 2008.

Time-resolved three-dimensional phase-contrast MRI.

To demonstrate the feasibility of a four‐dimensional phase contrast (PC) technique that permits spatial and temporal coverage of an entire three‐dimensional volume, to quantitatively validate its accuracy against an established time resolved two‐dimensional PC technique to explore advantages of the approach with regard to the four‐dimensional nature of the data.

4D flow MRI.

Traditionally, magnetic resonance imaging (MRI) of flow using phase contrast (PC) methods is accomplished using methods that resolve single‐directional flow in two spatial dimensions (2D) of an individual slice. More recently, three‐dimensional (3D) spatial encoding combined with three‐directional velocity‐encoded phase contrast MRI (here termed 4D flow MRI) has drawn increased attention. 4D flow MRI offers the ability to measure and to visualize the temporal evolution of complex blood flow patterns within an acquired 3D volume. Various methodological improvements permit the acquisition of 4D flow MRI data encompassing individual vascular structures and entire vascular territories such as the heart, the adjacent aorta, the carotid arteries, abdominal, or peripheral vessels within reasonable scan times. To subsequently analyze the flow data by quantitative means and visualization of complex, three‐directional blood flow patterns, various tools have been proposed. This review intends to introduce currently used 4D flow MRI methods, including Cartesian and radial data acquisition, approaches for accelerated data acquisition, cardiac gating, and respiration control. Based on these developments, an overview is provided over the potential this new imaging technique has in different parts of the body from the head to the peripheral arteries. J. Magn. Reson. Imaging 2012;36:1015–1036.

Bicuspid Aortic Valve Is Associated With Altered Wall Shear Stress in the Ascending Aorta

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/m2 compared with 0.4±0.3 N/m2 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.

Time-resolved 3-dimensional Velocity Mapping in the Thoracic Aorta: Visualization of 3-directional Blood Flow Patterns in Healthy Volunteers and Patients

Objective: An analysis of thoracic aortic blood flow in normal subjects and patients with aortic pathologic findings is presented. Various visualization tools were used to analyze blood flow patterns within a single 3-component velocity volumetric acquisition of the entire thoracic aorta Methods: Time-resolved, 3-dimensional phase-contrast magnetic resonance imaging (3D CINE PC MRI) was employed to obtain complete spatial and temporal coverage of the entire thoracic aorta combined with spatially registered 3-directional pulsatile blood flow velocities. Three-dimensional visualization tools, including time-resolved velocity vector fields reformatted to arbitrary 2-dimensional cut planes, 3D streamlines, and time-resolved 3D particle traces, were applied in a study with 10 normal volunteers. Results from 4 patient examinations with similar scan prescriptions to those of the volunteer scans are presented to illustrate flow features associated with common pathologic findings in the thoracic aorta. Results: Previously reported blood flow patterns in the thoracic aorta, including right-handed helical outflow, late systolic retrograde flow, and accelerated passage through the aortic valve plane, were visualized in all volunteers. The effects of thoracic aortic disease on spatial and temporal blood flow patterns are illustrated in clinical cases, including ascending aortic aneurysms, aortic regurgitation, and aortic dissection. Conclusion: Time-resolved 3D velocity mapping was successfully applied in a study of 10 healthy volunteers and 4 patients with documented aortic pathologic findings and has proven to be a reliable tool for analysis and visualization of normal characteristic as well as pathologic flow features within the entire thoracic aorta.

Comparison of flow patterns in ascending aortic aneurysms and volunteers using four‐dimensional magnetic resonance velocity mapping

To determine the difference in flow patterns between healthy volunteers and ascending aortic aneurysm patients using time‐resolved three‐dimensional (3D) phase contrast magnetic resonance velocity (4D‐flow) profiling.

Aortic Dilation in Bicuspid Aortic Valve Disease Flow Pattern Is a Major Contributor and Differs With Valve Fusion Type

Background— Ascending aortic dilation is important in bicuspid aortic valve (BAV) disease, with increased risk of aortic dissection. We used cardiovascular MR to understand the pathophysiology better by examining the links between 3-dimensional flow abnormalities, aortic function, and aortic dilation. Methods and Results— A total of 142 subjects underwent cardiovascular MR (mean age, 40 years; 95 with BAV, 47 healthy volunteers). Patients with BAV had predominantly abnormal right-handed helical flow in the ascending aorta, larger ascending aortas (18.3±3.3 versus 15.2±2.2 mm/m2; P <0.001), and higher rotational (helical) flow (31.7±15.8 versus 2.9±3.9 mm2/s; P <0.001), systolic flow angle (23.1°±12.5° versus 7.0°±4.6°; P <0.001), and systolic wall shear stress (0.85±0.28 versus 0.59±0.17 N/m2; P <0.001) compared with healthy volunteers. BAV with right-handed flow and right-non coronary cusp fusion (n=31) showed more severe flow abnormalities (rotational flow, 38.5±16.5 versus 27.8±12.4 mm2/s; P <0.001; systolic flow angle, 29.4°±10.9° versus 19.4°±11.4°; P <0.001; in-plane wall shear stress, 0.64±0.23 versus 0.47±0.22 N/m2; P <0.001) and larger aortas (19.5±3.4 versus 17.5±3.1 mm/m2; P <0.05) than right–left cusp fusion (n=55). Patients with BAV with normal flow patterns had similar aortic dimensions and wall shear stress to healthy volunteers and younger patients with BAV showed abnormal flow patterns but no aortic dilation, both further supporting the importance of flow pattern in the pathogenesis of aortic dilation. Aortic function measures (distensibility, aortic strain, and pulse wave velocity) were similar across all groups. Conclusions— Flow abnormalities may be a major contributor to aortic dilation in BAV. Fusion type affects the severity of flow abnormalities and may allow better risk prediction and selection of patients for earlier surgical intervention.Background—Ascending aortic dilation is important in bicuspid aortic valve (BAV) disease, with increased risk of aortic dissection. We used cardiovascular MR to understand the pathophysiology better by examining the links between 3-dimensional flow abnormalities, aortic function, and aortic dilation. Methods and Results—A total of 142 subjects underwent cardiovascular MR (mean age, 40 years; 95 with BAV, 47 healthy volunteers). Patients with BAV had predominantly abnormal right-handed helical flow in the ascending aorta, larger ascending aortas (18.3±3.3 versus 15.2±2.2 mm/m2; P<0.001), and higher rotational (helical) flow (31.7±15.8 versus 2.9±3.9 mm2/s; P<0.001), systolic flow angle (23.1°±12.5° versus 7.0°±4.6°; P<0.001), and systolic wall shear stress (0.85±0.28 versus 0.59±0.17 N/m2; P<0.001) compared with healthy volunteers. BAV with right-handed flow and right-non coronary cusp fusion (n=31) showed more severe flow abnormalities (rotational flow, 38.5±16.5 versus 27.8±12.4 mm2/s; P<0.001; systolic flow angle, 29.4°±10.9° versus 19.4°±11.4°; P<0.001; in-plane wall shear stress, 0.64±0.23 versus 0.47±0.22 N/m2; P<0.001) and larger aortas (19.5±3.4 versus 17.5±3.1 mm/m2; P<0.05) than right–left cusp fusion (n=55). Patients with BAV with normal flow patterns had similar aortic dimensions and wall shear stress to healthy volunteers and younger patients with BAV showed abnormal flow patterns but no aortic dilation, both further supporting the importance of flow pattern in the pathogenesis of aortic dilation. Aortic function measures (distensibility, aortic strain, and pulse wave velocity) were similar across all groups. Conclusions—Flow abnormalities may be a major contributor to aortic dilation in BAV. Fusion type affects the severity of flow abnormalities and may allow better risk prediction and selection of patients for earlier surgical intervention.