New MRI scan technology
There is a lot of difference between MRI scanners.
First, there is a large difference in image quality
between MRI scanners.
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The strength of the magnet is very important, and
high-field scanners (1.5 Tesla magnets) are generally
regarded as the best magnet strength.
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Low-field scanners (0.25 to 0.5 Tesla magnets)
may generate enough power in some cases, but where
a lot of detail is needed (e.g. the foramen where
the nerve exits the spine) they may not be adequate.
Another disadvantage of a low-field scanner is that
it often takes longer to do the scan.
From the standpoint of making a surgical decision,
one important technical point is how the images are
angled. If the images do not cut parallel to the disc
space at each spinal level in question, the scan will
be of limited usefulness in generating a surgical decision.
This is a technical issue and all scanners are capable
of doing this, but some technicians do not know how
important this is to the surgeon. It takes a little
more work on the technician’s part but it is worth
doing.
The older MRI scanners are basically a tunnel, and
for a complete scan a patient must lie still for about
40 to 60 minutes. However, due to varying needs of patients,
a few different types of MRI scanners are now available.
Some patients who may have difficulty with a traditional
MRI scan include:
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Claustrophobia. A small percentage
of patients are claustrophobic and cannot tolerate
the confined space within the magnet. The vast majority
of these individuals will be sufficiently relaxed
with sedatives taken orally. All of the highest
resolution MRI scanners are completed tunnels and
tend to accentuate the claustrophobic reaction.
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Size. The MRI scanner is a confined
space, and some patients may be too large to fit
in the MRI scanner tunnel. Some MRI scanners also
have weight limits.
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Pain. If a patient is in a lot
of pain, lying still for a prolonged period may
not be well tolerated.
Because the traditional tunnel scanners may not work
well for some people, Open MRI scanners
have been developed. While some patients may find the
more open MRI scanners easier to tolerate, the machines
use a lower-field magnet and do not generate as good
an image. With a lower quality image (the images are
not as clear or detailed), it is often difficult to
make a definitive anatomic diagnosis.
Recently, short bore magnets have
been developed that combine the accuracy of a tunnel
scanner and the comfort of an open MRI scan. Although
they are not completely open, they are much less constrictive
because of the short bore magnet (shorter tunnels),
but can produce a high- field.
Most modern, high-field magnets are now designed with
these shorter tunnels (Short Bore architecture)
and, theoretically, maintain the highest image quality
while causing less claustrophobia and being more easily
tolerated by patients who are in a lot of pain. The
high-field units (1.5 Tesla or greater) tend to be faster,
which means less time in the tube, less patient discomfort.
Summary
MRI imaging has revolutionized the diagnosis and treatment
of spine-related pain syndromes. MRI technology is constantly
improving, although at great financial costs to the
end consumers (doctors and their patients). The newest
and usually most expensive equipment tends to be available
in the largest hospitals and imaging centers that specialize
in spine care and other complicated medical disorders.
In many communities there is no choice in the scanner
to be used, but where there is a choice it is advisable
to get the best quality scan the first time - especially
if a patient is considering surgery. MRI scans have
taken a considerable amount of guesswork out of spine
surgery as they can give very accurate anatomic detail
that can be used to plan a surgical approach.
By:
Philip R. Shalen, MD
December 20, 2002
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