 Coronary
CT angiography (CTA) is a relatively new test that has been
made possible by the rapid advancement of CT technology in
recent years. As discussed above, current CT scanners acquire
images very quickly, allowing the scanner to “freeze”
cardiac motion. Until the development of very fast modern
CT scanners, coronary artery imaging with CT was not possible
due to the fact that, because the heart is beating during
the scan, the coronary arteries are moving during the study.
Rapidly moving structures are very challenging to image effectively,
particularly when the target structures are small. Modern
CT scanners have the ability to scan rapidly using very thin
slices, which maximizes the ability to see small detail. This
capability has made it possible, for the first time, to directly
visualize the internal portion of the coronary artery –
the lumen (Figure 4). Modern CT scanners can,  therefore,
create images that resemble those obtained with coronary artery
catheterization (see below). However, unlike coronary artery
catheterization, coronary CTA can also visualize the wall
of the coronary artery, and the wall of the coronary artery
is where atherosclerosis begins. So, it is now possible for
physicians to directly visualize both the coronary artery
lumen and wall for atherosclerotic plaque in a non-invasive
fashion – this capability is truly revolutionary and
is changing the way physicians think about CAD. Coronary CTA
allows physicians to create whole-heart images (Figure 5)
and assess cardiac function by watching the heart contract
in real time.
What is the benefit of
coronary CTA over other techniques used to assess for CAD?
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Early CAD detection. Coronary artery
CTA has the ability to detect CAD at a far earlier stage
than the perfusion techniques discussed above. Recall
that, for a perfusion study (such as stress echocardiography,
myocardial perfusion SPECT) to become abnormal, the coronary
artery lumen must be narrowed by about 60-70%. This means
that CAD is fairly advanced by the time it can be detected
by these techniques. In contrast, coronary artery CTA
can detect CAD when the coronary artery lumen is narrowed
as little as 10-20% - the hope here is that detection
at this stage would allow more aggressive treatment to
halt the progression of CAD.
-
Detection of type of CAD. Recall from
the discussion above that several types of coronary atherosclerotic
plaques exist: calcified “hard” plaques, softer
fibrous or lipid (fatty)-rich plaques, and mixed plaques.
Perfusion techniques cannot distinguish among these types
of plaques, nor can treadmill stress testing.  However,
it is clear that soft plaque - particularly those with
abundant lipid or fat - are the plaques that are most
prone to rupture and cause acute coronary syndromes. Detection
of such plaques may be very important for directing therapy,
and this is a very active area of research. As an example,
consider the example of a patient with a calcium score
of zero. From the foregoing, we know that such a patient
has a very low likelihood of having a coronary event.
However, the limitations of calcium scoring include the
inability to visualize non-calcified plaque and the fact
that a calcium score provides a measure of relative risk
for coronary events compared to a reference population,
but does not give specific anatomic information about
the individual patient. Such a patient is presented in
Figure 6. This patient had 2 major risk factors
for CAD, but a calcium score of zero. Coronary CTA shows
a single, noncalcified plaque in one of the major coronary
arteries - the first portion of the left anterior descending
coronary artery. Such a plaque, if it were to rupture,
places a large amount of heart muscle at risk for damage.
These plaques are potentially deadly and have been referred
to as ‘widow-makers’ for this reason. This
plaque could not be seen with coronary artery calcium
scanning and would not be detected with any perfusion
or treadmill tests.
-
Detection of causes of chest pain or
angina-like symptoms unrelated to CAD. Coronary artery
CTA directly visualizes the heart, the tissues surrounding
the heart, between the two lungs (called the mediastinum),
the lungs themselves, the thin membrane lining the surfaces
of the lungs (called the pleura), and the structures of
the chest wall. Patients with chest pain or symptoms that
resemble angina may be caused by derangements of any one
of these anatomic regions. Furthermore, coronary CTA can
detect potentially aggressive processes in these regions
before symptoms develop.
Who should have a coronary artery CTA study?
This question is currently a very active area of research.
Coronary CTA has been suggested by some to be a useful test
for patients at low to intermediate risk for CAD who present
with chest pain. A number of such patients currently undergo
nuclear perfusion scintigraphy, and some of these patients
will have abnormal or equivocal results. The latter group
of patients may then undergo coronary artery catheterization
(see below), only to be found to have no significant CAD.
Such patients could be evaluated with coronary artery CTA
- negative results exclude CAD with high confidence, and
also have the capability to detect other causes of chest
pain, such as lung disease. In contrast, patients at high
risk for CAD who present with chest pain, or patients who
are presenting with ACS are not appropriate referrals for
coronary CTA. These patients should be evaluated by a cardiologist
and usually require urgent coronary artery catheterization
because the likelihood of obstructive CAD is high, and CAD
can be treated during coronary artery catheterization procedures.
Other investigators have suggested that coronary
artery CTA may be a good test for risk stratification for
outpatients at low or intermediate risk for CAD. Negative
coronary artery CTA results in such patients would conclusively
exclude CAD. Some believe that positive results, particularly
if predominantly soft plaque is identified, would be useful
for justifying aggressive therapy.
Coronary CTA is also a very useful test for
evaluating patients with inconclusive or unclear tests for
CAD, such as treadmill tests or perfusion studies.
Coronary artery CTA also has value in assessing
patients for congenital anomalies of the coronary arteries.
Such patients are born with coronary arteries that have
an unusual course or position that renders the patient at
risk for heart-related chest pain or coronary events. Coronary
artery CTA is the test of choice for such patients. Coronary
artery CTA may also be useful for assessing patients with
new-onset heart failure for CAD as the cause of the heart
failure. Coronary artery CTA may also be employed to assess
for cardiac valve and muscle function and to assess for
masses in the heart or space surrounding the heart. Finally,
coronary artery CTA may be used to assess patency of bypass
grafts following coronary artery bypass grafting surgery.
Does the foregoing mean that coronary artery
CTA is a good test for everyone? Absolutely not! Coronary
artery CTA involves the injection of intravenous contrast
(sometimes also referred to as “dye”) to opacify
the arteries so that they may be visualized. The contrast
used for coronary artery CTA, which is the same intravenous
contrast used for other CT examinations and catheter coronary
angiography, contains iodine and occasionally may be associated
with allergic reactions and has the potential to worsen
pre-existing disorders of kidney function. In a very small
number of patients, such allergic reactions may be quite
severe, even deadly. In would be inappropriate to expose
a patient to such risks, no matter how small, if the information
gleaned from the study would not be useful. Coronary artery
CTA, like other CT examinations, uses x-ray radiation to
create images of the human heart. X-ray radiation has the
ability to harm human tissue, and may have the potential
to cause cancers. This risk of cancer induction by x-ray
radiation may be small, but cannot be assumed to be zero.
Therefore, it is important that the information that may
be gained by coronary artery CTA is worth the potential
risks of a serious intravenous contrast reaction or radiation-induced
malignancy.
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