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Last update; 19.06.2009 |
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Hybrid Imaging: coronary or carotid? |
 Myocardial
Perfusion SPECT showing a small inferobasal infarction and a
relatively large ischemia of the inferior wall. This patient has a
significant narrowing of the right coronary artery and a high risk
for a re-infarction of the inferior wall. |
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 MSCT
of the right coronary artery showing a significant narrowing of
the proximal und midportion (> 50%), therefore defining a high
risk situation based on a luminogram. The accuracy of the
luminogram for ischemia in stenoses with 50-75% narrowing is about
50%: many stenosis of about 50% do NOT define a high risk
condition. |
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 Coronary
calcifications, as evidenced in this example within the left
anterior descending artery increase the risk for acute myocardial
infarction: the more calcium, the higher the risk. However, based
on pretest probability, patients may not have coronary calcium but
a significant narrowing of the coronary system due to SOFT plaques. |
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 Carotid
atherosclerosis, which may be quantified using the total plaque
area (TPA, see
www.tpainfo.ch) is a very good
substitute for the risk of having a coronary stenosis > 50% or
having a high risk for myocardial infarction. |
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What's the
problem? |
Hybrid imaging is defined by the combination of perfusion and
plaque imaging of the heart. The rationale is to better risk
stratify subjects with respect to the presence of a significant
lumen narrowing in a subject with a normal Stress SPECT study.
Currently, there exist no guidelines for this type of
combination.
Usually, a SPECT study is performed with physicial or
pharmacologic testing and a coronary angiography is added using
MSCT with contrast and cardiac gating. Therefore, anatomical data
on the possible presence of a coronary stenosis and funcional data
on the risk that a stenosis causes ischemia, are collected.
This kind of imaging goes along with a radiation burden of 10
mSv (at best if Technetium and scan-and-scan is used) to 40 mSv
(at worst, if dual isotope plus restrospective oversampling of the
heart with CT is used). |
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Is there an
alternative ?
Yes! |
Better risk stratification is certainly needed, but not with MSCT!
In direct comparison (Arterioscler Thromb Vasc Biol.
2006;26:656-662), carotid imaging was superior to rule out a
significant stenosis defined by MSCT 50% or more in 36 men and
8 women with 18 subjects having significant CAD. Further, carotid
imaging, especially total plaque area (TPA) has an excellent
record for the prediction of myocardial infarction and vascular
death (AMI and STROKE) both in men and women. Therefore, using
carotid imaging, we obtain a more global view of vascular risk,
than what is possible with coronary imaging. |
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Conclusions |
In a subject with a normal SPECT study (or a normal exercise
stress ECHO) and absence of carotid plaques defined by TPA, the
addition of a MSCT coronary imaging study is absolutely
useless because the risk, that this subject will develop a
myocardial infarction is minimal. This can be deduced
mathematically. If you have a normal SPECT study, your risk for
AMI is 7/1000/year. If overmore, your TPA = 0, posttest risk is
3/1000/year.
Beside a certain amount of additional radiation burden when
MSCT hybrid imaging is used, we should not forget the additional
costs. TPA imaging would cost about 75 Swiss francs, coronary
imaging with MSCT about 1500 Swiss francs, contrast media included.
The promotion of MSCT hybrid imaging in clinical practice is at
the current point of evidence and knowledge not recommendable. |
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Further
information |
www.tpainfo.ch
http://scopri.ch/posttestcalculators1.html
http://www.nuk.usz.ch/nuk.aspx
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