Segmentation of Short Axis Cardiac MR Images using PCA with Active Appearance Model

Abstract

We describe the method for segmentation of Left Ventricle (LV) in short axis cardiac MR Images in order to visibly identify the LV, the endocardium and the epicardium. The interpretation and evaluation of images obtained from Magnetic Resonance Imaging (MRI) is not a trivial task and also it is very time- consuming if done completely manually. This is especially the case for higher dimensional image data such as volumetric and/or time-dependent data such as the Cardiac Images. The objective of this study was show clearly the LV in particular so that any deviation from the standard dimensions in terms of shape, size or texture, can be unmistakably identified. Early diagnosis of heart abnormalities can potentially decrease the risk of heart attacks as well as benefit people who have just undergone Heart Transplant Surgery. For segmentation of the Cardiac MRI, Principal Component Analysis (PCA) is used in the Active Appearance Model (AAM) building process. The extraction of statistical features with PCA can be regarded as a projection of high-dimensional data vectors into a space of much lower dimensionality. The process of building a computerized model is equated with making the computer learn possible appearances of an object. The basis for such a learning process is the training set which is prepared from the data obtained from a reputed hospital in Pune. I. Introduction According to the Organ Procurement and Transplantation Network (OPTN), out of approximately 4000 heart transplants performed every year in the United States, 543 patients die in the first year, only 78% survive 3 years after the transplant and 72% after 5 years. Most of these deaths are due to rejection of the transplanted organs. To prevent rejection, patients are given immuno-suppressants, which suppress the immune system to enable acceptance of the newly transplanted organ. However, excess of these drugs destroy the patient's immune system making them susceptible to numerous diseases. These patients are monitored closely by doctors to adjust the dosage of immuno-suppressants given to them. One method of doing this is by biopsy, which involves taking a small piece of the heart muscle and inspecting it under the microscope for damaged cells. This procedure is highly invasive and prone to sampling errors. MRI can be used to effectively monitor any functional abnormality of the heart in a non-invasive manner. Segmentation of the cardiac MRI images into epicardium and ventricles helps to detect any deformities and thus could indicate the on-setting of rejection. This segmentation can be performed manually by experts, but it is a time consuming process, and may delay the detection of rejection at a crucial stage. The manually performed segmentation also suffers the problem of inconsistency. Hence, efforts have been made to make this segmentation procedure completely automatic. (1, 2) The heart is a muscular organ of the circulatory system that constantly pumps blood throughout the body. As seen in figure 1, the heart has four separate chambers. The upper chambers are called the left and right atria, and the lower chambers are called the left and right ventricles. The septum separates the right and left sides of the heart. The endocardium is the inner surface of the myocardium (the heart muscle tissue) and the epicardium is the outer surface of the myocardium. The muscular tissues that attach to the lower portion of the interior wall of the ventricles are the papillary muscles. The periodic motion of the walls of the heart chambers during one heartbeat is referred to as one cardiac cycle. One heartbeat consists of two phases, namely systole and diastole. The rhythmic contraction of the ventricles, by which blood is driven through the aorta and pulmonary artery is the systole. Diastole is the normal rhythmically occurring relaxation and dilatation of the ventricles, during which they fill with blood. The right ventricle delivers blood to the lungs. The left ventricle delivers blood to the rest of the body. So the left ventricular wall is thicker. This fact is often used for distinguishing between the left and right ventricles. Here, we study the MRI images of the transverse section of the heart and segment them into epicardium, right ventricle, and left ventricle. (1, 2) Magnetic Resonance Imaging (MRI) is a method of creating images of the inside of opaque organs in living organisms. When the object to be imaged is placed in a powerful uniform magnetic field, the spins of the hydrogen nuclei with non-zero spin numbers, within the tissue, all align in one of two opposite directions: parallel to the magnetic field or three orthogonal image gradients are applied, to selectively image the different voxels (3-D pixels). They are the slice selection gradient, the phase encoding gradient, and the frequency