Introduction

Vascular diseases are one of the major sources of deaths in the U.S. A report [1] states that "Aneurysm rupture is not a rare occurrence as evidenced by the fact that this event currently ranks thirteenth on the list for leading causes of death in the U.S.A." Also, this report [1] says that "Chronic venous insufficiency is a common problem in the U.S., affecting approximately 5% of the general population. It is estimated that half a million patients suffer from ulceration of the lower extremity as a result of longstanding venous disease." This report raises the importance of research needed in the area of angiography, the branch of medicine which deals with veins and arteries. In the field of cerebrovascular diseases [1], "Ischemic and hemorrhagic stroke account for one of the principal causes of death and disability in older aged population. In fact, stroke currently ranks third on the list of leading causes of death in the United States (see also trial reports from North America (NASCET) [2] and Europe (ECST) [3], and Wiebers et al. [4]). Each year approximately 500,000 people suffer a new or recurrent stroke in the U.S., and approximately 150,000 die as a result of this process. In addition, more than one third of stroke survivors are left with permanent physical disability which currently accounts for the leading cause of nursing home admissions in the U.S."

The branch of medicine that deals with studies of the arteries and veins or vasculature is angiography. The different ways angiography can be performed are: bi-plane x-ray/digital subtraction angiography (DSA), MR angiography (MRA), CT angiography (CTA), and ultrasound angiography. From the clinical point of view, DSA1 is considered the most reliable and accurate method for vascular imaging, a subset of x-rays. On the contrary, this method lacks three-dimensional information,2 which is easily available via MR and CT techniques. The MR and CT techniques lack the ability to locate tiny vessels and the morphological estimation of stenoses and aneurysms. One of the main reasons MR and CT techniques are not as successful in detection as DSA is due to the presence of nonvascular structures and background noise.

This chapter focuses on white-blood angiography (WBA) and black-blood angiography (BBA) using MR, and more importantly, on the prefiltering of the MR data sets for removal of nonvascular tissues and background noise suppression. The main motivation for performing vascular filtering is for three-dimensional segmentation, which in turn helps in the following areas:

1. Neurosurgical planning, interventional procedures, and treatments

2. Time saving for performing three-dimensional segmentation

3. Distinguishing between the veins and arteries

4. Relative placement of the anatomical structures with respect to vasculature

5. Blood flow process and hemodynamics

6. Stenosis and aneurysm assessment/quantification

7. Disease monitoring/remission, and

8. Quantification of vascular structures

Having discussed some of the sources of motivations for performing vascular image processing (VIP), we now present the major difficulties in performing filtering of the vasculature in MR data sets. The following include the main reasons that contribute to the complexity in vasculature filtering from MRA data sets (see [18-20]):

1. Vessel shape complexity and variability. The vessels are curvey, twisted (tortuous), and sometimes occluded,3 or there could be superposition of structures/vessels corresponding to the vasculature in various areas of the body, such as in the head, heart, and neck.

2. Vessel density and diameter of small vessels. The complexity of the filtering increases with density and reduction of the diameter of vessels (see Figure 2.1).

1 Subtraction of the x-ray images without contrast material from x-ray angiograms.

2One can still achieve the three-dimensional information using the stereo reconstruction from x-ray angiograms with multiple viewpoints.

3Overlap of vessels with one another.

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