Quantification of Anatomical and Fluid-Dynamic Anomalies in Fontan Patients Based on Magnetic Resonance Imaging

ІНФОРМАЦІЙНІ ТЕХНОЛОГІЇ, СИСТЕМНИЙ АНАЛІЗ ТА КЕРУВАННЯ 79 UDC 519.63: 536.25: 532.5 DOI: 10.20535/1810-0546.2017.1.94670 G. Signorini1, S. Tirelli1, F. Piatti1, F. Pluchinotta2, S. Siryk3, E. Votta1*, M. Lombardi2, A. Redaelli1 1 2 Politecnico di Milano, Milan, Italy IRCCS “S. Donato” Hospital, San Donato Milanese, Milan, Italy 3 Igor Sikorsky Kyiv Polytechnic Institute, Kyiv, Ukraine QUANTIFICATION OF ANATOMICAL AND FLUID-DYNAMIC ANOMALIES IN FONTAN PATIENTS BASED ON MAGNETIC RESONANCE IMAGING Background. Univentricular diseases are lethal congenital diseases affecting about 2 % of newborns in the western world. Due to these pathologies, only one ventricle pumps blood into the circulatory bed, and arterial and venous blood are mixed, preventing from properly providing tissues and organs with oxygen. These pathologies are currently treated through the so-called Fontan procedure, which is a multi-step and complex surgical approach. The Fontan procedure aims at obtaining the anatomical separation between the systemic and pulmonary circulations, and hence between oxygenated and non-oxygenated blood. However, the only ventricle present in the heart remains the only pumping organ, and blood flow in the pulmonary circulation is merely passive. Also, and importantly, the postsurgical anatomy of the junction between systemic veins and pulmonary arteries is markedly non-physiological. As such, it is associated with altered blood fluid dynamics, undesired energy losses, and, ultimately, sub-optimal quality of life and short life expectancy. Objective. On this basis, clinicians need tools to 1) quantify the post-surgical anatomical and fluid-dynamic alterations, 2) correlate these anatomies to the patients’ prognosis, and 3) identify criteria to improve Fontan surgery. Methods. In order to support the pursue of these goals, we developed a computational tool for the processing of 4D flow data, i.e., phase contrast magnetic resonance images yielding information on the velocity of tissues within a 3D domain. The tool allows for reconstructing the 3D geometry of the surgically treated anatomical district and, through a semi-automated user-interface, extracting relevant geometrical scores, as well as quantifying flow rates in the different vessels, energy losses, and wall shear stresses. A numerical method based on the finite element approach was implemented to estimate relative pressures. Results. The developed tool was preliminarily applied to the analysis of the datasets of six pediatric patients. The analysis of data obtained by two independent users highlighted a good repeatability of geometrical reconstructions, and hence of the quantification of geometrical scores. The method for the quantification of relative pressures was preliminarily tested in a simplified model of the thoracic aorta, with encouraging results. Conclusions. The developed computational tool, which, to the best of our knowledge, is completely novel, helps clinicians to quantify the post-surgical anatomical and fluid-dynamic alterations. Ongoing activities include its application to the real datasets, and the extension of the analysis to a wider cohort of patients, so to check for correlations between the quantitative geometrical and fluid-dynamic indexes with the patients’ prognosis. Such possible correlations could help identifying criteria to improve Fontan surgery. Keywords: magnetic resonance imaging; 4D flow; fluid dynamics; Fontan procedure. Introduction Univentricular diseases are lethal congenital heart diseases that affect every year about 2 % of newborns in the United States [1], and require surgical treatment. Early surgical approaches consisted in palliative treatments (i.e, BT shunt and BDG shunt) conceived to save the patient’s life in the short term. Advances in cardiac surgery have led to the approach that is the current state of the art, i.e., the Fontan procedure (Fig. 1). Through a multi-step surgery, the Fontan procedure aims to a long term solution by reconfiguring the native structures involved in the pathology. The inferior vena cava (IVC), i.e., the vein returning venous blood from the lower systemic cir* culation is connected to the pulmonary arteries (LPA and RPA, respectively) either through the right atrium (atrium-pulmonary connection or APC) or through a graft bypassing the right atrium (total cavopulmonary connection or TCPC) and through the superior vena cava (SVC). In this way, the blood pumped by the left ventricle feeds the peripheral organs and is directly redirected to the lungs without the action of a proper right ventricle, and successively returns into the left atrium through the pulmonary veins. Unfortunately, this configuration is far from being physiological [2]: the hydraulic impedance downstream of the left ventricle is much higher than the physiological one, and, due to the abnormal anatomy of the reconstructed APC/TCPC, local fluid dyna- corresponding author: The authors gratefully acknowledge the support of the project AMMODIT funded within EU H2020-MSCA-RICE  80 Наукові вісті НТУУ "КПІ" Normal heart 2017 / 1 Hypoplastic left heart syndrome Patent ductus arteriosus Mitral valve (closed) Mitral valve Aortic Left valve ventricle a Aortic Left valve ventricle b c d Fig. 1. Physiological anatomy of the heart (a); heart anatomy in case of univentricular disease (b); heart anatomy following APC (c); heart anatomy following TCPC (d) mics is altered and characterized by large eddies and stagnation regions that may lead to thromboembolitic events. Also, sub-optimal distribution of blood flow rates between the left and right pulmonary arteries can be obtained, leading to atrophic remodeling of the branch receiving poor flow rate and hence to an increase in its hydraulic impedance [3]. Based on these evidences, the Fontan procedure should be performed aiming at optimizing the balance between flow rates into the pulmonary arteries, as well as to minimize geometrical distortions that could lead to flow disturbances. Also, flow rate redistribution and energy loss at the APC/TCPC, which is a surrogate measure of flow disturbances, could be used as prognostic indexes to judge the postsurgery evolution of the patient. In this work we hypothesized that blood fluiddynamics with the APC or TCPC of Fontan patients could be quantified based on 4D flow imaging, i.e., phase-contrast magnetic resonance imaging sequences that yield the 3D velocity components that characterize biological tissues within a volume of interest [4]. In particular, we developed in house software to exploit such information to quantify the 3D geometry of the surgically treated anatomical district and to computing the flow rates in the pulmonary arteries, local viscous energy losses, wall shear stresses, and pressure drops. Problem Statement Our main objective is to develop numerical approaches and corresponding computational tools for the processing of 4D flow data aimed to reconstruc (...truncated)