Time-resolved three-dimensional magnetic resonance velocity mapping of aortic flow in healthy volunteers and patients after valve-sparing aortic root replacement

In vivo visualization and analysis of 3-D hemodynamics in cerebral aneurysms with flow-sensitized 4-D MR imaging at 3 T

INTRODUCTION

Blood-flow patterns and wall shear stress (WSS) are considered to play a major role in the development and rupture of cerebral aneurysms. These hemodynamic aspects have been extensively studied in vitro using geometric realistic aneurysm models. The purpose of this study was to evaluate the feasibility of in vivo flow-sensitized 4-D MR imaging for analysis of intraaneurysmal hemodynamics.

METHODS

Five cerebral aneurysms were examined using ECG-gated, flow-sensitized 4-D MR imaging at 3 T in three patients. Postprocessing included quantification of flow velocities, visualization of time-resolved 2-D vector graphs and 3-D particle traces, vortical flow analysis, and estimation of WSS. Flow patterns were analyzed in relation to aneurysm geometry and aspect ratio.

RESULTS

Magnitude, spatial and temporal evolution of vortical flow differed markedly among the aneurysms. Particularly unstable vortical flow was demonstrated in a wide-necked parophthalmic ICA aneurysm (high aspect ratio). Relatively stable vortical flow was observed in aneurysms with a lower aspect ratio. Except for a wide-necked cavernous ICA aneurysm (low aspect ratio), WSS was reduced in all aneurysms and showed a high spatial variation.

CONCLUSION

In vivo flow-sensitized 4-D MR imaging can be applied to analyze complex patterns of intraaneurysmal flow. Flow patterns, distribution of flow velocities, and WSS seem to be determined by the vascular geometry of the aneurysm. Temporal and spatial averaging effects are drawbacks of the MR-based analysis of flow patterns as well as the estimation of WSS, particularly in small aneurysms. Further studies are needed to establish a direct link between definitive flow patterns and different aneurysm geometries.

Three-dimensional magnetic resonance flow analysis in a ventricular assist device

OBJECTIVE

The assessment of flow characteristics inside ventricular assist devices by magnetic resonance imaging techniques may provide insight into the mechanisms underlying the high rate of thromboembolic events after implantation of a ventricular assist device. Furthermore, these investigations may form the basis to optimize the device's design and its need for anticoagulation. The purpose of this study was to integrate a clinical routine ventricular assist device into a flow circuit with realistic geometric and pulsatile inflow conditions. Combination with flow-sensitive magnetic resonance imaging at 3 T permitted the detailed analysis of local and global 3-dimensional flow dynamics in a realistic environment.

METHODS

A commercially available ventricular assist device was integrated into a magnetic resonance-compatible flow circuit. Flow-sensitive 3-dimensional magnetic resonance imaging was performed to measure time-resolved 3-directional flow velocities in the entire device. Advanced computer-aided 3-dimensional flow visualization methods were used to derive a comprehensive picture of flow dynamics within the ventricular assist device system.

RESULTS

On the basis of the ventricular assist device model system, magnetic resonance imaging, and flow visualization, the first 4-dimensional functional magnetic resonance imaging analysis of flow characteristics inside an operating clinical routine ventricular assist device chamber system is reported. Detailed visualization of flow patterns and local changes in flow characteristics were successfully performed and revealed locally accelerated, vortical, and helical flow regions inside the geometry of the device.

CONCLUSIONS

Complex flow patterns such as vortex formation and locally accelerated flow demonstrate the potential of the presented method to further deepen the understanding of complex and regionally different flow characteristics inside ventricular assist devices.

[MR-based tridirectional flow imaging. Acquisition and 3D analysis of flows in the thoracic aorta]

Tridirectional MR flow imaging is a novel method that extends the well-established technique of phase-contrast flow measurement by vectorial velocity encoding, i.e., by encoding in all three spatial directions. Modern sequence protocols allow the acquisition of velocity vector fields with high spatial resolutions of 1-3 mm and temporal resolutions of 20-50 ms over the heart cycle. Using navigating techniques, data on the entire thoracic aorta can be acquired within about 20 min in free breathing. The subsequent computer-based data processing includes automatic correction of aliasing effects, eddy currents, gradient field inhomogeneities, and Maxwell terms. The data can be visualized in three dimensions using vector arrows, streamlines, or particle traces. The parallel visualization of morphological slices and of the surface of the vascular lumen in 3D enhances spatial and anatomical orientation. Furthermore, quantitative values such as blood flow velocity and volume, vorticity, and vessel wall shear stress can be determined. Modern software systems support the integrated flow-based analysis of typical aortic pathologies such as aneurysms and aortic insufficiency. To what extent this additional information will help us in making better therapeutic decisions needs to be studied in clinical trials.

Anatomical and Functional Evaluation of Pulmonary Veins in Children by Magnetic Resonance Imaging

Pulmonary vein pathologies often present a diagnostic challenge. Among the different imaging modalities used for the evaluation of pulmonary veins, magnetic resonance is the most comprehensive in assessing anatomy and pathophysiology at the same time. Bright blood cine sequences and contrast-enhanced magnetic resonance angiography outline the course and connections of the pulmonary veins. Phase-contrast velocity mapping measures flow patterns, velocities, and volumes throughout the pulmonary circulation. This paper reviews contemporary utilization of magnetic resonance in the evaluation of pulmonary venous abnormalities in children, based on our experience over the last 5 years and on that of other investigators. We summarize how magnetic resonance imaging enhances our understanding of pulmonary vein physiology and how it can influence the diagnostic approach to children and adults with a pulmonary venous pathology, and we discuss its limitations.

Impact of patient-prosthesis mismatch and aortic valve design on coronary flow reserve after aortic valve replacement

OBJECTIVES

This prospective-randomized study investigated the effect of aortic valve design and patient-prosthesis mismatch (PPM) on coronary flow reserve (CFR) after mechanical or biological aortic valve replacement (AVR) in patients with aortic stenosis (AS).

BACKGROUND

Coronary flow reserve may be an important parameter of long-term survival after AVR in patients with AS. Reduced CFR may contribute to more cardiovascular events and greater rates of mortality.

METHODS

A total of 48 patients undergoing AVR underwent magnetic resonance imaging for the measurement of coronary flow preoperatively, 5 days postoperatively, and at 6-month follow-up with measurement of CFR. Patients scheduled for mechanical AVR were randomized to a tilting disc or bileaflet prosthesis (n = 12 in each group). For biological AVR, patients were scheduled to receive a stented (n = 12) or stentless (n = 12) valve. Patients also underwent echocardiography with measurement of transvalvular pressure gradients and left ventricular mass regression.

RESULTS

Postoperatively, coronary flow increased significantly in all groups (p < 0.001). Only stentless valves demonstrated a normal CFR (3.4 +/- 0.3 vs. 2.3 +/- 0.1 for stented biological valves, 2.1 +/- 0.2 for tilting disc, and 2.2 +/- 0.3 for bileaflet mechanical valves). Patient-prosthesis mismatch with an indexed effective orifice area <0.85 cm2/m2 led to decreased rates of CFR in the tilting disc, stentless, and stented groups. Pressure gradients were 14 +/- 3 mm Hg for tilting disc, 12 +/- 4 mm Hg for bileaflet, 19 +/- 6 mm Hg for stented, and 10 +/- 4 mm Hg for stentless valves.

CONCLUSIONS

Normalization of CFR after AVR in patients with AS was observed only for stentless valves. Coronary flow reserve might explain the excellent long-term results for stentless valves. (Impact of Patient-Prosthesis Mismatch on Coronary Flow Reserve; http://www.clinicaltrials.gov/ct/show/NCT00310947?order=1; NCT00310947).

Emerging vascular applications of magnetic resonance imaging: a picture is worth more than a thousand words

Vascular applications of magnetic resonance (MR) imaging are reviewed, with emphasis on algorithms that use nonpictorial information contained in the MR data set. Current clinical vascular practice generally limits use of MR angiography and three-dimensional vessel images to qualitative pictorial rendering without routinely using the available quantitative information contained within the MR data. This review is dedicated to recent advances that include characterization of vessel histology, assessment of carotid plaque vulnerability, characterization of blood flow dynamics, quantitative analysis of disease severity, and prediction of vascular intervention outcome. Examples from histologic preparation, in vitro and in vivo experiments, are discussed, with an emphasis on potential clinical applications and advances in acquisition technology.

Computational simulations of the aortic valve validated by imaging data: evaluation of valve-sparing techniques☆

The goal of this study has been to develop a numerical model of the aortic valve, to validate it with in-vivo data and to computationally evaluate the effect of two types of aortic valve-sparing reconstructions on valve dynamics and hemodynamics. A model of the native aortic valve and two models of the valve after surgical reconstruction (reimplantation with a straight conduit and remodeling with a shaped conduit) were created. These models were transferred to a finite element analysis software where the interaction between valve structures and blood was taken into account in a dynamic manner. Leaflet and blood dynamics, as well as tissue compliance and stresses were evaluated. Leaflet dynamics and blood velocities were also assessed by magnetic resonance imaging in 15 healthy volunteers. Computational results in the native valve model correlated closely with the in-vivo imaging data. The creation of neo-sinuses was shown to restore leaflet opening and closing dynamics. Loss of compliance at the commissures led to altered stress distribution patterns. Preservation of sinus geometry was an important factor in end systolic vortex formation. This is the first study to have incorporated the effect of blood flow in the numerical evaluation of aortic reconstructions using a computational model validated by in-vivo data. Differences in valve dynamics after surgical reconstruction reported in this computational study match trends previously reported in other in-vivo studies. Numerical models such as this one can serve as increasingly sophisticated tools in the study of aortic valve pathologies and in the optimization of new surgical reconstruction techniques.

Stentless bioprostheses improve postoperative coronary flow more than stented prostheses after valve replacement for aortic stenosis

OBJECTIVE

In some randomized studies, stentless aortic valves have demonstrated hemodynamic advantages in comparison with stented prostheses. The effect of more physiologic flow dynamics on coronary artery flow has not been investigated yet. This study compares coronary perfusion after aortic valve replacement with stented or stentless porcine bioprostheses in a prospective randomized study.

METHODS

A total of 24 patients (73 +/- 6 years) referred for treatment of aortic stenosis were randomized to aortic valve replacement with stented (Medtronic Mosaic; (Medtronic Inc, Minneapolis, Minn) or stentless (Medtronic Freestyle; Medtronic Inc) prostheses. Coronary flow was measured by means of magnetic resonance imaging preoperatively, 5 days after the operation, and 6 months postoperatively, then with evaluation of coronary flow reserve. Echocardiography was performed to quantify transvalvular gradients and left ventricular mass regression.

RESULTS

Coronary flow increased in both groups significantly (P < .001) after aortic valve replacement. This increase was higher in the stentless group compared with that seen in the stented group (343 +/- 137 vs 221 +/- 66 mL/min). Also, coronary flow reserve was higher for stentless valves (3.4 +/- 0.3 for stentless valves and 2.3 +/- 0.1 for stented valves). Mean pressure gradients for Freestyle valves were lower (10 +/- 4 and 8 +/- 3 mm Hg, respectively, vs 19 +/- 6 postoperatively and 15 +/- 4 mm Hg at follow-up for Mosaic valves, P < .05). Left ventricular mass regression was similar in both groups.

CONCLUSIONS

Normalization of coronary artery flow after aortic valve replacement for aortic stenosis was more pronounced for stentless valves compared with stented valves. The fact that the stentless design also demonstrated a superior hemodynamic performance with lower pressure gradients might be explained by the design being closer to physiologic anatomy and thus the presence of lower turbulence levels in the sinuses of Valsalva.