S1 through S5
S1 through S5 is a reference to the five (S1, S2, S3, S4,
S5) vertebrae (bones that form an opening in which the
spinal cord passes) that are near an area of the body
known as the sacrum. The sacrum is a large bone in the
lower part of the spine that is shaped like a triangle. The
letter "S" stands for "sacrum." So S1 refers to the 1st sacral
bone, S2 to the 2nd sacral bone, S3 to the 3rd sacral bone,
and so on, until S5, which is the last sacral bone. S1 is at
the top and S5 is at the bottom, with the numbers in
between going down the body in numerical order. For
example, S3 is below S2, S4 is below S3, and so on.

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The S1-S5 region.
These numeric levels are often mentioned on imaging studies of the spine such as
computerzed axial tomography (CT) scans or magnetic resonance imaging (MRI). CT
scanning is an advanced imaging technique that uses x-rays and computer technology to
produces clear and detailed picture of the body area being examined. MRI scans
produce extremely detailed pictures of the inside of the body by using very powerful
magnets and computer technology. Other designations for other levels of the vertebrae
include a) C1 through C7, where C stands for the cervical (neck) area; b) T1 through
T12, where T stands for the thoracic (chest) area, and c) L1 through L5, where L stands
for the lumbar (lower back) area.

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