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the lamina bones (just behind the spinal cord). Thick ligaments also connect the
bones of the lumbar spine to the sacrum (the bone below L5) and pelvis.
Muscles in the lower back are arranged in layers. The super
cial layer, the
closest to the skin, is covered by a thick tissue called Fascia. The middle layer,
called the Erector Spinae, has strap-shaped muscles that run up and down over the
lower ribs, chest, and low back. They join in the lumbar spine to form a thick
tendon that binds the bones of the low back, pelvis, and sacrum. The deepest layer
of muscles attaches along the back surface of the spine bones, connecting the low
back, pelvis, and sacrum. These deepest muscles coordinate their actions with the
muscles of the abdomen to help hold the spine steady during activity [
95
].
Spinal segments is a notion that includes two vertebrae separated by an inter-
vertebral disc, the nerves that leave the spinal column at each vertebra, and the
small facet joints that link each level of the spinal column. The intervertebral disc
separates the two vertebral bodies of the spinal segment. The disc normally works
like a shock absorber. It protects the spine against the daily pull of gravity. It also
protects the spine during heavy activities that put strong force on the spine, such as
jumping, running, and lifting. The spinal segment is connected by two facet joints
described earlier. When the facet joints of the lumbar spine move together, they
bend and turn the low back [
95
].
3 Popular Lumbar Imaging Modalities
Most image based research literature focuses on X-ray radiography, Dual-energy X-
ray Absorptiometry (DEXA or DXA), CT, and MRI. X-ray radiography and DEXA
are cheaper and widely popular modalities as an initial diagnostic tool. Hence, the
availability of the data provided researchers with great opportunities to investigate
labeling, localization, and even diagnosis problems.
On the other hand, MRI (Fig.
8
) and CT (Fig.
9
) are more expensive and less
available for researchers. Hence, fewer researchers obtained access to such data and
were able to investigate localization, segmentation, and diagnosis problems on the
various anatomical structures. Few efforts utilized other modalities such as ultra-
sound, especially, for fetal spine detection and abnormality detection.
Both X-ray and DEXA (aka DXA) radiography consist only from one 2D slice
that shows the area of interest. On the other hand, CT scans show a full 3D volume
for the area of interest. Clinical CT spans the whole area in slice-by-slice fashion
that can be directly used to produce a full 3D volume. Usually CT consists of a set
of axial slices with speci
c thickness depending on the available technology.
Moreover, clinical MRI consists of few protocols that vary depending on the
available technology. The current standard in MRI in North America, for low back,
is the 3 T MRI. Most of the current MRI radiology centers produce: (1) T1-
Weighted sagittal (T1 W-sagittal), (2) T2-Weighted sagittal (T2 W-sagittal), (3) T2-
Weighted axial (T2 W-axial) for a set of selected discs, (4) T2-Weighted axial
(T2 W-axial) and (5) Myelo MR images (Fig.
8
). While the sagittal views span the
