Four-Dimensional Flow Magnetic Resonance Imaging for Cardiovascular Imaging: from Basic Concept to Clinical Application

[4D FLOW Magnetic Resonance Imaging in the Study of Blood Flow in Patients With Aortic Coarctation in the Long-Term After Surgery]

Aim      Comprehensive evaluation of blood flow in the thoracic aorta using a software for 4D processing of magnetic resonance (MR) images of the heart and blood vessels (4D Flow) in patients with aortic coarctation in the late postoperative period.Materials and methods The MR study of the heart was performed for 10 patients (7 boys and 3 girls) aged 8 to 13 years (median, 9.5 [8.3; 10.8] years) who underwent resection with end-to-end anastomosis for aortic coarctation at age of 2 weeks to 10 months. MR tomography was performed on a 1.5 T MR scanner using a multichannel surface coil for scanning, electrocardiographic synchronization, and a specialized package of pulse sequences for scanning of the heart. Blood flow was evaluated with a 4D data handling software for processing of MR images of heart and blood vessels (4D Flow). The following blood flow parameters were analyzed: blood flow volume per second, peak blood flow velocity, peak and minimum blood flow area at the levels of ascending aorta, arch, isthmus, and descending aorta, and pressure gradient at the level of maximum narrowing of the aorta. 3D-MR images were used for evaluation of aortic geometry. Blood flow formation, distribution, and trajectories were analyzed by maps of vectors, particle trace, and stream lines. Statistical analysis was performed with a Statistica (v. 6.0 StatSoft Inc.) package.Results Accelerated flow in the region of residual aortic stenosis in systole was observed in all patients; 4 patients had an additional vortex flow below the aortic stenosis and a spiral flow in the descending aorta. The pressure gradient on the aortic isthmus was directly correlated with the left ventricular myocardial mass index (r=0.65; р=0.04) and indexes of blood flow in the ascending and descending aorta (р=0.03; р=0.026). No significant correlation was found for blood flow indexes and geometry of the aortic arch (H / L). Delayed contrast enhancement MR imaging did not detect any fibrotic changes in the myocardium in only one patient. The fibrosis severity inversely correlated with the right ventricular ejection fraction (r=0.65; р=0.04) and directly correlated with the pressure gradient at the aortic isthmus (r=0.63; p=0.05).Conclusion      The 4D MR image processing software for the heart and blood vessels allows studying the blood flow in detail under natural conditions, provides potential advantages in comprehensive evaluation of patients with aortic coarctation during a dynamic follow-up. For a definitive conclusion about the relationship between the altered blood flow in the thoracic aorta and markers of residual, post-correction pathology, larger studies are required as well as long-term follow-up of patients with documented pathological patterns of blood flow (changes in blood flow velocity and volume throughout the entire thoracic aorta in combination with disorders in the normal flow geometry during the cardiac cycle).

Hydrodynamic Noise of Pulsating Jets through Bileaflet Mechanical Mitral Valve

Experimental research results of hydrodynamic noise of pulsating flow through a bileaflet mechanical mitral valve are presented. The pulsating flow of pure water corresponds to the diastolic mode of the cardiac rhythm heart. The valve was located between the model of the left atrium and the model of the left ventricle of the heart. A coordinate device, on which a block of miniature sensors of absolute pressure and pressure fluctuations was installed, was located inside the model of the left ventricle. It is found that the hydrodynamic noise of the pulsating side jet of the semiclosed valve is higher than for the open valve. The pressure fluctuation levels gradually decrease with the removal from the mitral valve. It is established that at the second harmonic of the pulsating flow frequency, the spectral levels of the hydrodynamic noise of the semiclosed bileaflet mechanical mitral valve are almost 5 times higher than the open valve. With the removal from the mitral valve, spectral levels of hydrodynamic noise are decreased, especially strongly at the frequency of the pulsating water flow and its higher harmonics.

Four-dimensional-flow Magnetic Resonance Imaging of the Aortic Valve and Thoracic Aorta

Blood flow through the heart and great vessels is sensitive to time and multiple velocity directions. The assessment of its three-dimensional nature has been limited. Recent advances in magnetic resonance imaging (MRI) allow the comprehensive visualization and quantification of in vivo flow dynamics using four-dimensional (4D)-flow MRI. In addition, the technique provides the opportunity to obtain advanced hemodynamic measures. This article introduces 4D-flow MRI as it is currently used for blood flow visualization and quantification of cardiac hemodynamic parameters. It discusses its advantages relative to other flow MRI techniques and describes its potential clinical applications.