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cylinder (Fig.
11
). The magnetic polarity of the applied magnetic
field is alternated
at characteristic and varying frequencies and pulse settings to emphasize speci
c
properties of the tissues. A single scan utilizing characteristic phase and frequency
parameters used to emphasize speci
c tissue properties is termed a pulse sequence.
A group of these sequences, varied in terms of the tissue planes being imaged and
physical characteristics of the tissue being emphasized, forms an MRI examination.
A multitude of MRI sequences have now been devised investigate speci
c tissue
characteristics and pathologies. However, there is a fundamental set of sequences
ubiquitous in use in medical imaging, known as T1, proton density (PD), and T2,
examples of which for the spine are shown in Fig.
12
[
7
]. Each of these MRI
sequences has bene
ts of T1 include increased ana-
tomic detail relative to T2 and ability to assess tissue enhancement with the
administration of contrast. T2, on the other hand, is better for assessing edema and
has generally shorter imaging times [
8
]. PD is an intermediate sequence, which
seeks to combine T1 and T2 characteristics, with results which are intermediate
between the two. Now, on grayscale imaging, certain tissues will show up as high
signal intensity (white on the display monitor), and other as low signal intensity
ts and drawbacks. The bene
Fig. 11 MRI scanner unit. Patient is placed on a moveable table in alignment with the central axis
of the magnet gantry, which then moves into the bore of the magnet during the scanning process
