Biomedical Engineering Reference
In-Depth Information
Two-dimensional digital substraction angiography (DSA) is considered the
gold standard technique for the detection and quantification of brain aneurysms.
However, other less invasive acquisition techniques like Computed Tomogra-
phy Angiography (CTA) or Magnetic Resonance Angiography (MRA) are also
used as complementary methods for these aims [9, 10]. In order to estimate
the dimensions of an aneurysm from 3D CTA or MRA, it is customary to per-
form lineal measurements on Maximum Intensity Projections (MIP) of the
original volumetric scan. The MIP provides a 2D image of the 3D data in an
angle considered optimal by the neuroradiologist. The optimal viewpoint is
chosen so that the magnitudes of interest are maximum. Manual measure-
ments are then carried out on the basis of this 2D image using electronic
callipers.
The selection of the optimal view angle, introduces a high degree of subjec-
tivity to the quantification of the aneurysm. Window levelling is often used to
enhance the arterial structures of interest thus increasing the subjectivity in the
quantification of the aneurysm morphology. In some images, the presence of
nearby bony structures imposes restrictions to the selection of the viewpoint.
This forces, in some cases, the selection of suboptimal views to perform the
measurements. In MIP images, it is often difficult to determine depth relation-
ships between the aneurysm and the surrounding and sometimes overlapping
vessels. Therefore, the use of computerized 3D segmentation techniques is cru-
cial for accurate quantification of the aneurysm dimensions as well as for a
correct interpretation of the 3D morphology.
5.1.3
Implicit Deformable Models
Many of the recent approaches used to segment vascular structures from medi-
cal images use deformable models. The use of implicit deformable models com-
bined with level set methods in arterial structures has become very popular
over the last few years. The ability to capture the topology of complex struc-
tures makes it a very suitable technique for the extraction of the shape of arte-
rial structures. Loncaric
et al
. [11] use classic Geodesic Active Contours (GAC)
model combined with level set methods in the segmentation of aortic abdomi-
nal aneurysms (AAA). However, classic GAC methods are not able to deal with
narrow and twisted vessels. To this end, some improvements to the traditional
method have been proposed in the literature.
