Cardiac MRI based numerical modeling of left ventricular fluid dynamics with mitral valve incorporated
Introduction
Despite numerous studies of blood flow in the left ventricle (LV) (Pedrizzetti and Domenichini, 2014), the association between the LV flow structure and the cardiac remodeling is still not well understood. Various approaches have been applied to visualize the flow structure in the LV such as echocardiography and cardiac magnetic resonance (CMR) (Charonko et al., 2013, Garcia et al., 2010, Rodriguez Muñoz et al., 2013). Besides in-vivo studies, extensive in-vitro studies have been conducted to measure the intraventricular flow using a heart simulator (Falahatpisheh and Kheradvar, 2012, Kheradvar and Gharib, 2007). Owing to the difficulties in mimicking a native heart ventricle, the primary purpose of in-vitro studies is to understand the fluid dynamics in an idealized heart ventricle. Computational fluid dynamics (CFD) in conjunction with CMR is emerging as an alternative tool to further facilitate the visualization of patient-specific LV flow, as it is capable of providing a more detailed flow structure with much higher resolutions (Chnafa et al., 2014, Mangual et al., 2013, Saber et al., 2003, Schenkel et al., 2009, Su et al., 2014a).
As an asymmetric bileaflet mitral valve (MV) is located at the inflow tract of the LV, it has been shown that incorporation of the MV into the numerical model will give more realistic flow field predictions (Domenichini and Pedrizzetti, 2014, Seo et al., 2014, Votta et al., 2013). However, the MV was only modeled in very limited numerical simulations, due to the difficulties in reconstructing and modeling the dynamic MV. In the studies focusing on the mitral leaflets, the MV was scanned using real-time 3D transesophageal echocardiography (TEE) (Chandran and Kim, 2014). To obtain better accuracy such as papillary muscles and chordae, CMR was also adopted to scan a number of evenly rotated long-axis planes (Dimasi et al., 2012, Stevanella et al., 2011). It is worth noting that this method was not routinely used in clinical CMR imaging. A few studies used high spatial resolution of Computed Tomography (CT) images to reconstruct the MV and LV to study the intraventricular flow (Chnafa et al., 2014, Chnafa et al., 2012). Its usage is limited by the complications of ionizing radiation and relatively low temporal resolution. Routine clinical CMR scanning (i.e. two-chamber, three-chamber, four-chamber and short-axis planes) is able to provide adequate information to reconstruct a patient-specific LV. Nevertheless, it is difficult to segment mitral leaflet in all views, and a generic one defined by mathematical equations is a feasible approach (Domenichini and Pedrizzetti, 2014).
In the literature, most LV numerical simulations focused on understanding the LV fluid dynamics in normal subjects, while limited studies modeled the patient-specific LVs with heart failure, dilated cardiomyopathy, and hypertrophic cardiomyopathy (Khalafvand et al., 2014, Mangual et al., 2013, Su et al., 2014a). These studies have demonstrated that the structure of vortices is different in normal cases compared to patient cases. To the best of our knowledge, no numerical study of LV flow has been conducted in pulmonary arterial hypertension (PAH) patients. In addition, these patient-specific studies ignored the influence of MV, which impaired the reliability of the numerical results to a certain extent. In this study, CMR images acquired from a normal subject and a PAH patient were selected for MV and LV reconstructions and the following numerical simulations. Geometrical parameters (e.g. ventricular volume and vortex formation time (VFT)) and the detailed fluid structures in these two models were analyzed to observe the impact of PAH. In addition, the study outlined a framework for more realistic LV flow based CMR images.
Section snippets
Data acquisition
One healthy volunteer and one PAH patient were recruited in this study, and CMR scans were carried out on a Philips 3.0 T system (Ingenia, Philips Healthcare, Netherlands) with a dStream Torso coil (maximum number of channels 32). BTFE end-expiratory breath hold cine images were acquired in multi-planar views (namely two-chamber, three-chamber, four-chamber, short-axis views as indicated in Fig. 1). Electrocardiographically (ECG) gated consecutive cine short-axis images were acquired from apex
Global parameters of numerical models
Based on CMR images, the LV geometries in both cases were reconstructed as demonstrated in Fig. 4A. The normal LV was essentially elliptical and its cross section on the short-axis plane was circular. However, the PAH in the right heart considerably flattened the septum and remodeled the LV into D-shape in the short-axis view. The saddle-shaped mitral annulus tilted towards the septum, and the presence of asymmetric bileaflet mitral valve rectified the opening direction toward the apex
Discussion
Most of the numerical simulations focused on the normal subjects, and only a few types of myocardial diseases were investigated in the literature. To the best of our knowledge, this study was the first attempt to model LV in a PAH patient using patient-specific CMR images. In addition, the MV was incorporated in the numerical model, which was mostly neglected in other studies. Although Dimasi et al. (2012) reconstructed the LV, MV and aorta based on CMR images, they only investigated the flow
Conclusion
This study was the first attempt to simulate the flow in LV with MV incorporated based on CMR, which is more widely used for patients with myocardial diseases than CT. Based on the geometries and opening area of mitral valve, the resultant VFTs in the normal subject and the PAH patient varies significant, which is consistent with the observations in the literature. Therefore, VFT is a promising parameter to differentiate normal and diseased LVs. The detailed vortex structures showed that the MV
Conflict of interest statement
The authors declare that they have no conflict of interest in regards to this study.
Acknowledgment
The authors would like to acknowledge Goh Cardiovascular Research Award (Duke-NUSGCR/2013/0009) for funding this research.
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