Since 1997, a group of scientists in a wide range of areas at CRS4, the Centre for Advanced Studies, Research and Development in Sardinia, is involved in ViVa, a three-year project, funded by the European Commission in the Eureka programme. The Virtual Vascular Project partners aim at the design of advanced tools for the emodynamicist and cardiovascular surgeon to study and interpret the ever increasing number of data which is produced by non-invasive imaging equipment. In particular, the team is working on a system which will be able to reconstruct the geometry of patients' arteries as well as to simulate blood flow in them. Initially, the applications will be used for clinical research and training purposes but in a later stage, the partners want to explore the system's potential for surgical planning.
The CRS4 centre constitutes an interdisciplinary institute for the development of innovative simulation techniques in order to solve large scale computational problems by means of high performance computing. The ViVa project creates a synergy between biomedical researchers; specialists in fluid dynamics and combustion; visualization and virtual reality experts; as well as waves, materials and structures' scientists. ViVa indeed is based on an integrated set of tools, each of which is dedicated to a specific aspect of the complex data processing and simulation procedure which includes image processing and segmentation; real time 3D volume visualization; 3D geometry reconstruction and 3D mesh generation; and blood flow simulation and visualization.
In the cardiovascular system, morphology and functionality are very closely related. If flow conditions alter because of arterial wall changes due to the thickening of the blood or atherosclerotic plaques, they are likely to have a severe impact on the development of arterial diseases. A detailed insight of the influence of wall modifications on flow patterns can help the surgeon to perform reconstructing interventions and revascularization. Furthermore, the ability of simulating vascular operations on an accurate virtual model of the vessel tree will allow to anticipate the effects on the blood flow of changes in the vessel geometry due to the insertion of prostheses, such as a stent, or to the addition of new vessel junctions, like a bypass.
The "virtual" vessel models can equally be used as the basis for sophisticated training systems. In this regard, the ViVa project team plans the simulation of angioplastic procedures. Angioplasty consists of a range of surgical interventions to cure vascular pathologies, like vessel narrowing or stenosis, from within the vessel lumen. The procedures include the insertion of a catheter in the patient's artery; the accurate positioning of the catheter by navigating the arterial tree; and the inflation of a balloon in the stenotic region as to enlarge the vessel lumen. The pre-operative simulation with 3D virtual models of the vascular tree augmented by a computational model of the interaction between the catheter and the vessel walls allows the doctor to interact with the system by means of a specialized force-feedback hardware and software. Thus, the surgeon is able to feel the response of the vessel on the advancing catheter.
The ViVa team is building a system which extracts geometrical information from Computed Tomography (CT) and Magnetic Resonance (MR) scans. The researchers use several segmentation algorithms, controlled by specifically designed user interfaces. These are written as Java clients in a distributed implementation, including dedicated server processes for image retrieval and processing. The 2D contours obtained from the segmentation step are then connected to reconstruct a 3D model of the arteries. The physician can use this to analyse the vessels' geometrical structure and to build computational meshes for the flow simulation step. A geometrical modelling tool will allow the interactive manipulation of 3D vessel geometry and procedures, such as the reconnection of vessels in a simulated by-pass operation.
At present, the partners are creating an environment to enable the numerical simulation of fluid flow, specifically tailored to research in emodynamics. As such, a flow solver, a specialized grid generation as well as flow visualization packages are being developed to be integrated into the geometrical modelling tool. The main aim is to obtain a numerical model, which is a mathematical formulation of the problem suitable for numerical calculations. In particular, the numerical model is obtained with a mathematical continuous model of blood flow, given by equations linking the different physical quantities, like velocity and pressure, on the basis of physical laws. The equations are the classical Navier-Stokes equations of fluid dynamics for incompressible flow.
Currently, the team's effort is aimed at the systematic study, based on vessel geometries obtained from autopsy specimens, of how geometrical features of the vessels effect the large scale structures of the flow and the stability of the latter against small perturbations in the geometrical models, due to errors in the geometry reconstruction procedures. Also, a series of prototypes will be developed in order to investigate the feasibility of specific technical solutions, applied to the field of emodynamics. This involves active contributions from medical centres and from other research institutions, such as the Politecnico di Milano. For more details about the Virtual Vascular Project, you can visit the ViVa home page.