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In-Depth Information
signi
cant inter-observer variability. At the same time, segmentation of interver-
tebral discs is a very important part of lumbar CAD systems in order to diagnose
and quantify abnormalities such as herniation, desiccation and degeneration.
Requirements for CAD systems of the lumbar region are unique since we need to
segment the Dural Sac and localize, label and segment the lumbar intervertebral
discs before we can initiate the diagnosis. Figure
3
shows an illustration of auto-
mated segmentation [
34
] of the discs, vertebrae and the Dural Sac of a clinical MRI
using two methods, the
first using a probability map and HOG features, while the
second method uses neighborhood label information as well, in a Gibbs Sampling
approach.
2 The Vertebral Column
This section is dedicated to present the anatomy of the vertebral column in general
with focus on the lumbar area. It also provides the standardized nomenclature of the
various abnormalities in the vertebral column as endorsed by the North American
Spine Society (NASS), the American Society of Spine Radiology (ASSR), and the
American Society of Neuroradiology (ASNR) [
24
].
The vertebral column, also known as the backbone or the spinal column, is
typically made up of (33) individual bones called vertebra (plural: vertebrae) that
interlocks with each other. These vertebrae are classi
five areas from top to
bottom: Cervical (7), Thoracic (12), Lumbar (5), Sacral (5), and Coccyx (4).
Among these (33) vertebrae, only the top (24) are movable due to which clinicians
often state that the vertebral column consists only of (26) vertebrae counting the
Sacral vertebrae as one and the Coccyx as one. In each of these four regions, the
vertebrae have unique features that allows certain functionality [
88
].
There are
ed into
five distinct regions in the vertebral column. The top most region is the
Cervical region which consists of seven vertebrae anatomically named from top to
bottom as C1 (also called Atlas), C2 (also called Axis), C3, C4, C5, C6, and C7.
The main function of this region is to support the weight of the head [normally
weighs about 10 pounds (4.5 kg)]. The cervical has the most range of motion due to
the
first two specialized vertebrae that connect to the skull.
The Thoracic region comes next and consists of twelve vertebrae named from
top to bottom as T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, and T12. The major
role of the thoracic spine is to protect the organs that lie in the chest by supporting
the rib cage. The motion is limited due to the nature of the chest.
The Lumbar region comes next and has the largest vertebrae. This region is
responsible for the whole
flexibility of the back as well as bearing the weight of the
body. Five vertebrae exist in this area that are named (from top to bottom) as L1,
L2, L3, L4, and L5. The spinal cord stops, typically at the L1-L2 area where the
nerves hang down inside the Pachymeninx (Dural Mater) which is the tough and
in
fl
fl
exible outermost of the three layers of the meninges surrounding the brain and
spinal cord. The Sacral and the Coccyx regions have less functionality and they are
