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Coronary computed tomography angiography: a new wave of
cardiac imaging
K. Soon and C. Wong
Centre for Cardiovascular Therapeutics, Western Hospital, Footscray and Epworth Hospital, Richmond, Melbourne, Victoria, Australia
Key words
coronary, CT, angiography, artery, chest pain,
radiation.
Correspondence
Kean Soon, Heartcare, 54 Eleanor Street,
Footscray, Vic. 3011, Australia.
Email: [email protected]
Received 24 April 2012; accepted 11 May
2012.
Abstract
Coronary computed tomography angiography (CTA) is becoming a popular noninvasive imaging modality for coronary arteries. It has high sensitivity and specificity in
the evaluation of coronary artery disease (CAD). Due to its high negative predictive
value, the main clinical role of coronary CTA is in the exclusion of significant CAD in the
low to intermediate pretest probability patients. In the emergency department, coronary
CTA helps to fast-track the triage of acute chest pain patients. One of the major coronary
CTA’s limitations is its false positive findings in the presence of heavily calcified lesions.
With the advance of CT technology, the radiation dose of coronary CTA has been
reduced significantly. At present, coronary CTA is still not indicated for screening
asymptomatic patients.
doi:10.1111/j.1445-5994.2012.02901.x
Introduction
CT technology and coronary CTA
In many parts of the world, coronary computed tomography angiography (CTA) is fast becoming a popular
cardiac imaging modality. In Australia, the acceptance
of coronary CTA has increased since it became a Medicare rebate item from 1 July 2011. This was largely due
to the effort of Conjoint Committee for Recognition of
Training in CT coronary angiography consisting of representatives from Cardiac Society Australia and New
Zealand, Royal Australian and New Zealand College of
Radiologists, and Australian and New Zealand Association of Physicians in Nuclear Medicine. Nonetheless,
many doctors are still uncertain about the indications
and unaware of the limitations of coronary CTA. Some
are concerned about its radiation dose in comparison
with other imaging modalities. This review article aims
to clarify all these issues.
Despite the advance of CT technology, coronary CTA is
still quite challenging to perform in comparison with CTA
of the peripheral vascular system. Coronary arteries are
small and moving with each heartbeat. In order to image
coronary arteries well, the CT scanner used in coronary
CTA must have high spatial and temporal resolutions.
The scan also needs to be gated to the cardiac cycle so that
the axial images obtained at a particular point in time of
the cardiac cycle can be retrieved and reconstructed in
phase (Fig. 1). There are two modes of electrocardiography (ECG) gating: retrospective and prospective gating
modes. A retrospective gated scan acquires images
throughout the whole cardiac cycle, whereas a prospective gated scan only acquires images during a particular
time in cardiac cycle, such as during the diastolic phase
when the heart is least mobile.
22
© 2012 The Authors
Internal Medicine Journal © 2012 Royal Australasian College of Physicians
Coronary CTA angiography
Figure 1 Coronary computed tomography
angiography can be performed with (a) retrospective ECG gating or (b) prospective ECG
gating. ECG, electrocardiography.
Nowadays, coronary CTA is predominantly performed
with a multi-detector row CT (MDCT) for its wide
z-axis coverage, high spatial resolution (0.4–0.6 mm)
and temporal resolution (330 ms per rotation). Electron
beam CT has been phased out due to its suboptimal
spatial resolution in comparison with MDCT. Most
MDCT scanners used to perform coronary CTA have a
minimum of 64 rows of detectors, with a spatial resolution of 0.4–0.6 mm.1 It normally takes about 5–8
heartbeats to scan from the cranial to caudal aspects
of the heart. The ECG-gated scan images are then
restacked to form the linear images of coronary arteries
(Fig. 2). A wider z-axis coverage with 126, 256 and
even 320 row MDCT has the advantage of scanning the
whole heart in a shorter time period, hence there is
lesser demand on breath-holding duration and lesser
subjection to heart rate variability, such as atrial or ventricular ectopics. The wide-array 320 row MDCT allows
Figure 2 Curved multiplanar reconstruction
(a), maximal intensity projection (b) of coronary computed tomography angiography
correlate well with the finding of invasive
coronary angiography (c) showing a severe
stenosis in the mid-left anterior descending
artery.
© 2012 The Authors
Internal Medicine Journal © 2012 Royal Australasian College of Physicians
23
Soon & Wong
Figure 3 Misregistration artefacts due to
heart rate variability (a) with a heart rate of
71 beats per minute during CT scan in comparison with (b) without artefacts with a
heart rate of 50 beats per minute.
the whole heart to be scanned in a single or two heartbeats;2 it essentially eliminates the issue of misregistration artefacts as seen on MDCT with lesser row number
of detectors in the event of ectopic heartbeat or heart
rate variation (Fig. 3).
The temporal resolution of the MDCT is limited by the
rotational speed of X-rays gantry. Current MDCT has a
rotational speed of 330 ms per round. Further increment
of rotational speed of X-rays gantry is limited by the
gravitational force it generates. By virtue of half-scan
reconstruction, that is the X-ray image of one side of the
vessel is similar to the X-ray image of the half of the
vessel, a virtual temporal resolution of 155 ms can be
achieved. Although the heart beats faster than the rotational speed of MDCT during the systolic phase, it has a
relatively static period during the diastolic phase. Hence,
the diastole is the opportunistic window period for scanning the heart and its coronary arteries with less motion
artefacts. The duration of diastolic phase can be prolonged with the use of beta-blockers or other atrioventricular (AV) nodal blocking agents, such as diltiazem or
verapamil. Thetarget heart rate normally aimed for in
coronary CTA is 60 b.p.m. or below.3
Clinical indications for coronary CTA
The clinical indications for coronary CTA should be based
on the diagnostic accuracy of coronary CTA. Most published data suggest that coronary CTA has high sensitivity
and specificity, with high negative predictive value but
moderately high positive predictive value.1 In segmentbased analysis, Schroeder et al.’s meta-analysis showed
that coronary CTA had sensitivity and negative predictive
value of 89% and 98% respectively. In patient-based
24
analysis, patient with at least one coronary segment
>50% stenosis, the sensitivity and negative predictive
value of coronary CTA were reported as 98% and 95%
respectively. Its high sensitivity and negative predictive
value make coronary CTA a very useful tool to exclude
significant coronary artery disease (CAD), in most cases
defined as at least a coronary stenosis >50%. Its positive
predictive value is only moderately good in segmentbased analysis (78%) due to the presence of false positive
findings. False positive findings of coronary CTA are predominantly related to overestimation of lesion severity
in the presence of calcified plaques.4–6 Calcified lesions
appear worse on coronary CTA because of calcium
blooming and beam hardening effects (Fig. 4). There is
not much we can do to eliminate the calcium-related
artefacts other than using a higher tube voltage and
trying to slow down heartbeats even further during coronary CTA scanning. Motion artefacts can also be the
source of false positive (Fig. 3).
Current Society of Cardiovascular Computed
Tomography/American Heart Association/American
College of Cardiology (SCCT/AHA/ACC) guidelines
suggest that in the detection of CAD in symptomatic
patients without known heart disease, coronary CTA is
appropriate for intermediate pretest probability patients.
It is also appropriate in the low pretest probability
patients who cannot perform exercise stress test or with
equivocal functional tests.7,8 A negative coronary CTA
provides us a high level of confidence in excluding
obstructive CAD and relieve the patients from other
investigations. However, it should not be used in the high
pretest or high risk profile patients as it is more likely
than not going to show the presence of calcified and
non-calcified plaques. In this patient group, it is not
© 2012 The Authors
Internal Medicine Journal © 2012 Royal Australasian College of Physicians
Coronary CTA angiography
Figure 4 Heavy calcification in the left
anterior descending on coronary computed
tomography angiography (a) results in a
false positive finding in comparison with
invasive coronary angiography (b).
appropriate to use coronary CTA to decide precisely how
narrow the coronary lumen is in view of its limited
spatial and temporal resolutions. Such patients would be
better off having their coronary arteries assessed with the
invasive coronary angiogram or functional tests.
In the setting of acute chest pain suspicious of acute
coronary syndrome (ACS), both low and intermediate
pretest probability patients are appropriate for coronary
CTA. Coronary CTA can exclude significant obstructive
CAD more rapidly than current acute chest pain management pathway which relies on serial cardiac biomarkers and ECGs.9–11 The cost of care using coronary CTA in
the chest pain triage is lower than the conventional chest
pain management without the use of coronary CTA (38%
lower).11 In addition, a triple rule out of coronary
obstruction, aortic dissection and pulmonary embolism
can be performed within a single scan of cardiac CT. In
patients with positive ECG changes or highly elevated
serial troponin assays, they would be better off evaluated
with the invasive coronary angiography (ICA) and not
coronary CTA.
Post revascularisation, coronary CTA is fairly useful in
evaluating the patency of coronary bypass grafts.7,8 Due
to its relatively static nature, coronary bypass graft is
easier to scan, and the image quality is often excellent
(Fig. 5). The diagnostic accuracy of coronary CTA for
bypass graft is very high, with sensitivity, specificity, and
negative and positive predictive values all close to
100%.12,13 However, coronary CTA is less useful in the
evaluation of coronary stents due to stent blooming artefacts. Coronary CTA of stents suffers from a significant
high number of false positive findings.14 Currently, only
stents of ⱖ3 mm in diameter are suitable for coronary
CTA, such as left main artery stent or proximal coronary
Figure 5 Curved MPR of a patent LIMA
graft to the LAD of patient with recurrent
left shoulder pain post CABG (a). The 3D
picture shows a patent LIMA graft and also
a severe stenosis in the left subclavian
artery proximal to the origin of the LIMA
graft as the cause of left shoulder pain (b).
CABG, coronary artery bypass graft; LAD,
left anterior descending artery; LIMA, left
internal mammary artery; MPR, multiplanar
reconstruction.
© 2012 The Authors
Internal Medicine Journal © 2012 Royal Australasian College of Physicians
25
Soon & Wong
Figure 6 Curved MPR of the left main artery
with a patent 4.0-mm stent and the left anterior descending artery with a patent 3.0-mm
stent (a). Curved MPR of the LCx with a
2.5-mm stent. It is difficult to visualise the
stent lumen due to stent blooming and misregistration artefacts (b). LCx, left circumflex
artery; MPR, multiplanar reconstruction.
segment stent (Fig. 6). In both coronary artery bypass
graft and stented patients, the degree of CAD in the
native vessels cannot be accurately assessed with coronary CTA. The referrers need to understand that coronary
CTA may not provide them the answer to the cause of
chest pain post revascularisation per se.
In recent years, coronary CTA is also increasingly used
in the evaluation of patients with suspected dilated cardiomyopathy. A negative coronary CTA further confirms
that the cause of dilated cardiomyopathy is not due to
obstructive epicardial CAD.7,8
By its tomographic nature, coronary CTA is quite a
unique imaging modality in comparison with the conventional ICA. Coronary CTA provides an excellent crosssectional view of all cardiac structures and coronary
arteries. Calcified and non-calcified plaques are appreciated much better on coronary CTA than the ICA. It also
shows a superb 3D relationship between coronary arteries
and other cardiac structure. Hence, coronary CTA is very
useful in the characterisation of coronary anomaly, such
as anomalous coronary artery. Coronary CTA helps
delineate the proximal course of anomalous coronary
artery to see if it runs between aorta and pulmonary
artery. An interarterial anomalous coronary artery can be
associated with sudden cardiac death as compared with
retroaortic or prepulmonic anomalous coronary artery
(Fig. 7).15
Other clinical scenario potentially
indicated for coronary CTA
Not infrequently, we come across a situation where the
outcome of a functional test does not match the patient’s
clinical presentation. For examples, patients with a good
history of ischaemic sounding chest pain produce a negative functional test, or patients with an atypical chest pain
end up with a positive functional test. In both situations,
further testing with coronary CTA may help clarify if
patients require an ICA.
In the setting of acute chest pain, troponin assays are
invariably used in the emergency department to investigate the cause of chest pain. Not infrequently, we come
Figure 7 Anomalous left circumflex artery
(arrow) with a benign retroaortic course (a)
and anomalous right coronary artery (double
arrows) with a malignant interarterial
course (b).
26
© 2012 The Authors
Internal Medicine Journal © 2012 Royal Australasian College of Physicians
Coronary CTA angiography
across patients with slightly elevated troponin for
whatever reasons. If such a small elevation of troponin is
not accompanied by ischaemic changes on ECG or not
compatible with the clinical presentation, further investigation with coronary CTA may help speed up the triaging process of acute chest pain. Coronary CTA in this
setting not only helps rule out significant CAD, it may
also help rule out aortic dissection or pulmonary embolism as the cause of troponin elevation.
Screening
Despite its popularity in certain parts of the world, at
present there is not enough evidence to support the use of
coronary CTA to screen asymptomatic patients for significant CAD. A large study performed in Korea, screening
1000 middle-aged asymptomatic patients, yielded a low
incidence of significant CAD (5%).16 In those with significant CAD detected on coronary CTA, currently there is no
data to support any form of treatment for subclinical CAD.
More often than not, coronary CTA in asymptomatic
patients may show mild atherosclerosis in the coronary
arteries (which naturally comes with age), and subsequently may pose more uncertainties and unnecessary
anxiety to the patients. As it currently stands, coronary
CTA is not a screening tool for CAD.7,8
Medicare rebate for coronary CTA
Medicare Australia stipulates that coronary CTA must be
performed on a minimum 64-slice CT (or equivalent) at
an experienced centre serviced by recognised coronary
CTA reporters accredited by the conjoint committee
(Australia-New Zealand Conjoint Committee for the Recognition of Training in CT Coronary Angiography). Coronary CTA referral can only be made by a specialist or
physician. Consistent with the guidelines published by
SCCT/AHA, Medicare Australia stipulates that coronary
CTA is only indicated in symptomatic patient suspicious
of coronary ischaemia but with a low to intermediate risk
profile and would have been considered for ICA. The
other indication of coronary CTA is exclusion or characterisation of anomalous coronary artery or coronary
fistula. In young patients going for non-coronary cardiac
surgery, coronary CTA is also considered an appropriate
indication for Medicare rebate. The Medicare rebate
item number for coronary CTA is 57360 (http://
www9.health.gov.au/mbs/fullDisplay.cfm?type=item&q=
57360&qt=item&criteria=57360).
Limitations of coronary CTA
Despite its attractiveness as a form of non-invasive
imaging modality, coronary CTA has several limitations.
© 2012 The Authors
Internal Medicine Journal © 2012 Royal Australasian College of Physicians
Due to its significant dependence on the regularity of
heart rhythm, coronary CTA in patients with markedly
irregular heart rhythm, such as atrial fibrillation or frequent premature complexes, may not produce a good
image quality. It is more certainly so if coronary CTA is
performed using MDCT scanners that require a few rotations to ‘stitch’ up cardiac images as compared with a
single rotation acquisition with a wide-array 320 MDCT
or the latest Siemens flash scanner with a fast table speed.
Coronary CTA performed at a high heart rate may also
yield suboptimal image quality as motion artefacts can
cause poor visualisation of coronary lumen, and hence
inaccurate assessment of luminal stenosis. A genius way
to improve the temporal resolution of MDCT has been
the creation of dual source CT (DSCT) scanner. The availability of second X-ray source at 90 degrees to the other
X-ray source means the x-gantry needs to spin only a
quarter of a cycle to obtain a particular segment of the
coronary artery. Virtually, the temporal resolution of the
DSCT reaches 83 ms.17 Such a high temporal resolution
allows coronary CTA to be performed at higher heart
rates than 60 b.p.m. and preclude the need for routine
beta-blocker. Nonetheless, most DSCT operators still find
that the use of beta-blockers helps obtain the best image
quality and reduce the radiation dose of DSCT.18
As mentioned earlier, heavy calcification of coronary
vessels is a common source of false positive findings.
Coronary CTA is not that helpful in patients who are
likely to have extensive calcification, such as elderly
patients or patients with chronic renal failure. Cast aside
the issue of calcification, coronary CTA itself often overestimates the severity of lesion in comparison to ICA. The
spatial resolution of coronary CTA is in the order of about
0.4–0.6 mm as compared with 0.1–0.2 mm with conventional ICA.
As coronary CTA involves the use of iodine-based contrast, patients with renal impairment and contrast allergy
are not suitable to have coronary CTA.
Radiation dose
A few years back, coronary CTA was a contentious
imaging modality due to its radiation dose. In the early
day, with 16-slice CT, the radiation dose was estimated to
be about 15–20mSv. As the CT technology improves,
coupled with the industrial efforts in reducing the radiation dose of coronary CTA, nowadays coronary CTA can be
performed at a much lower radiation dose. The use of
prospective ECG-gated scanning mode helps reduce the
radiation dose from 15–20mSv to 3–5mSV, a sheer reduction of about 70–80%!19,20 Coronary CTA performed with
a lower tube voltage from 120 kVp to 100 kVp in appropriate patients can also reduce the dose by 40–50%.21
27
Soon & Wong
Even in retrospective gated scanning mode due to clinical
needs, ECG-gated tube current modulation helps reduce
the dose by about 25–40%.21,22 Lately, with the introduction of Siemens flash DSCT scanner, coronary CTA can be
performed with a radiation of about 1mSv in one heartbeat using a high table speed.23–25 For comparison, conventional ICA yields a radiation dose of about 5–8mSv,
while nuclear perfusion study yields a radiation dose of
10–15mSv in total for both rest and stress components.
Our background cosmic radiation gives us 2.0–2.5mSv per
year living in Australia. In short, the effective dose of
coronary CTA has come down quite substantially over the
last 5 years.
reserve (FFR-CT). It is hard to comprehend exactly how
FFR-CT is derived, but it is predominantly based on the
fluid haemodynamics mathematical computation. The
preliminary result of FFR-CT looks promising. In a study
looking at 159 coronary vessels, FFR-CT had a good correlation with the referenced invasive FFR with a receiveroperator characteristics curve of 0.90. FFR-CT helped
reduce the false positive findings of coronary CTA by
70% and improved the diagnostic accuracy of coronary
CTA by 25%.29 It provides the much needed information
about coronary flow, which is not currently obtainable
on routine coronary CTA.
Functional information of coronary CTA
Conclusions
CT myocardial perfusion study
Coronary CTA has emerged as a very useful non-invasive
imaging modality for the evaluation of CAD. Its main
clinical role is in ruling out significant CAD in the low to
intermediate pretest probability patients and in the
setting of acute chest pain. Heavy calcification remains a
major limitation of coronary CTA. With improved CT
technology, the radiation dose of coronary CTA has
decreased substantially. With the availability of functional information from cardiac CT stress perfusion and
CT-FFR, coronary CTA poses to provide a one-stop investigation for anatomical and functional information of
CAD.
Coronary CTA provides excellent anatomical information, such as luminal stenosis, plaque characteristics and
coronary anatomy. However, it lacks functional information, such as coronary flow and physiology. Stress cardiac
CT is now possible albeit at a research level. Stress cardiac
CT can be performed in conjunction with coronary CTA
using adenosine induced myocardial stress. So far, cardiac
CT stress perfusion studies yield comparable results with
nuclear myocardial perfusion imaging. Hence, it has the
potential to become a one-stop investigation to obtain
anatomical and functional information of CAD, and to
reduce the incidence of false positive findings of coronary
CTA.26–28
Non-invasive CT fractional flow reserve
A new horizon in the field of coronary CTA is the introduction of non-invasive CT coronary fractional flow
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