Research On Left Ventricle Segmentation And Dynamic Models From Cardiac MR Images

Currently, the technique of the cardiac Magnetic Resonance (MR) imaging has becomean important assistant measure in the clinical diagnosis of heart diseases. The analysis ofthe cardiac MR imaging is an important approach to measure the heart functionnon-invasively. Through the cardiac MR images, the physicians not only can observe thestructure of the heart, but also can estimate the global function and local myocardiumfunction of the ventricles. So the physicians can make the right estimation of the pathologyand physiology of the heart.In the clinical diagnosis of heart diseases, as the Left Ventricle (LV) is the pump of theblood circulation of the whole body, it leads an important role in the heart function. So theLV is the focus in the current research. From the sequence of the cardiac MR images, themotion, displacements and strain-stress of the myocardium of the LV during systole will beextracted. Then the global function and local myocardium function of the LV can bemeasured, which is significant in the clinical diagnosis. This thesis based on the cardiacMR images, will focus on the segmentation of the LV and the myocardium motion analysis.From the aspects of the 2D segmentation of the LV, 3D surface restoration, 3D motionreconstruction and strain-stress analysis, we have proposed the corresponding 2Dsegmentation models and 3D motion analysis models, building a general framework ofevaluating the 3D myocardium function of the LV from the cardiac MR images. Our workmainly includes the following parts:(1) A simulated annealing (SA) algorithm based simplified Snake model for imagesegmentation is proposed. This proposed model improves the traditional Snake model,introducing the idea of simplified Snake to make the computation easy. Also an area energyterm with variable coefficients is added to make the evolving curve not influenced by theinitial position. The SA optimization algorithm is used to solve the improved Snake model.This idea exerts the characteristics of the SA algorithm, also keeps the low computationcomplexity by applying Snake. Our model not only can deal with the weak edges andirregular edges, but also can deal with concave region and corner region.(2) Aiming to the disturbance from the tagged lines when segmenting the tagged cardiacMR images, a texture analysis method is proposed based on the detail signal energy whichapplies the signal feature provided by the tagged lines themselves. Therefore it caneffectively remove the influence from the tagged lines. In the cardiac MR images, theedges are weak and noises are a little stronger. So the geodesic active contour model isapplied to define the evolving curve, and it is solved by the level set method, thereby theinner edges of the LV are well extracted. When extracting the outer edges of the LV, adistance constraint energy term is added in order to solve the problem of the edges' breakor default. Depending on this energy term, the feature edges which cannot be directlyextracted by gradient or texture information can be well extracted.(3) By applying the tagged cardiac MR images, we build a physics-based deformablemodel with parameter functions to reconstruct the shape and 3D motion of the LV duringsystole. In the process of the surface restoration of the LV of the initial frame, we propose aretro-deducting parameter functions method to restore the mathematical surface model of the LV. And during the process of the motion tracking, we propose an effective method tocalculate the tagged forces of the model. Meanwhile these forces are optimized usingThin-Plate splines. The proposed model not only can track any material point of the LV,and also almost all the motions and deformations of the LV during systole can be reflectedon the changing curves of parameter functions. These curves are very intuitive and easy tobe understood, which will be very helpful for clinical applications.(4) In order to intuitively analyze the deformations of the LV during systole, amechanical modality analysis method of the LV is proposed. First, applying the taggedcardiac MR images data, we build a physics-based deformable model with parameterfunctions of the LV, and then based on the model built we apply the boundary data sets ofeach frame during systole to reconstruct the 3D shape of the LV. Second, we compute thetagged forces of the LV model during systole by using the tagged data sets of the adjacenttwo frames. Lastly, the tagged forces of the model will be transformed to the contractivestress component, the tangent stress component and the vertical stress component.Moreover, these three stress components will be described in color cloud pictures. So, themechanical modality analysis method of the LV proposed in this thesis will intuitively andeffectively reflect the stress distribution and deformation trends of the inner and outersurfaces of the LV during systole.(5) Based on the sequence of the tagged cardiac MR images, a Finite Element Method(FEM) based motion analysis model of the LV is proposed, reconstructing the 3Ddisplacements and strain distribution of the LV during systole. First, we proposed a kind of3D surface restoration algorithm based on Delaunay triangulation. This algorithm canrestore the surface from the sparse data points of un-uniform spatial distribution ordifferent formats. Then the FEM-based LV model is constructed, reconstructing the 3Dmotion and capturing the deformation information of the LV. The proposed surfacerestoration algorithm and the FEM-based LV model can effectively capture the shape andmotion information of the LV, which provide the useful reference data in the clinicaldiagnosis.(6) A FEM-based biomechanical model of the LV is proposed. The myocardium is thehighly optimized composite material in the biologic material. The myocardium fiberorientations of the LV vary in a continuous manner. Based on the FEM-based model of theLV, the myocardium fiber orientation is further considered, namely the material property ofthe myocardium is added to the model. Thus the biomechanical model of the LV is built.Based on this model, we analyze the strain-stress of the LV under local fiber coordinates.Also each component is shown in the color cloud picture, which is meaningful for theclinical diagnosis.