The 64-detector computed tomography (CT) scanner is a powerful tool for the evaluation of patients with chest pain in the emergency room. Cardiac CT angiography (CTA) performed with these scanners can accurately evaluate a number of cardiac, pulmonary, and vascular disease states in an acute setting. The authors discuss the role of cardiac CTA in the evaluation of patients with potential acute cardiovascular disease and propose an integrated algorithm for the work-up and risk stratification of these patients.
is an Assistant Director, Cardiovascular CT Imaging, and
is an Associate Professor of Medicine and the Fellowship Program
Director, Division of Cardiology, the Los Angeles Biomedical
Research Institute, Harbor-UCLA Medical Center, Torrance, CA.
is an Assistant Professor, Department of Nuclear Medicine, The
University of Western Ontario, London, ON, Canada.
The diagnosis of acute coronary syndrome is missed in 2% to 4%
of cases and is associated with a twofold increase in mortality,
which results in a low threshold for hospital admission of patients
with chest pain presenting to the emergency room (ER).
More than 2 million patients with acute chest pain are
unnecessarily admitted to the hospital each year in the United
The limited ability to make the correct triage decisions can also
lead to resultant liability issues that account for 20% of ER
malpractice dollar losses.
The prognostic value of variables such as patient age, sex,
presence of risk factors, and biochemical markers for major adverse
cardiovascular outcomes is limited.
Therefore, ER triage decisions based on estimates of acute coronary
syndrome risk levels derived from various clinical predictors are
often ineffective, particularly for patients who have chest pain
but normal initial cardiac enzyme levels and normal or
More than 5 million patients with acute chest pain present to the
ER in the United States every year.
The current management methods do not permit an effective ER triage
of patients with acute chest pain in whom initial troponin levels
are not elevated and ischemic electrocardiographic (ECG) changes
are not evident.
Until the advent of cardiac computed tomography (CT), there has
been no noninvasive diagnostic tool available that provides
morphologic information about the presence and severity of coronary
artery disease (CAD). As a result, the confirmation or exclusion of
acute coronary syndrome, particularly in patients with unstable
angina pectoris, requires extensive testing, and this necessity may
lead to unnecessary hospital admission or, possibly, a delay in
Needless to say, early triage of these patients is important from a
management standpoint, as patients who are at the highest risk for
adverse outcomes derive the greatest benefit from prompt and
appropriate treatment, and those patients at low risk may be
discharged quickly and safely.
Cardiac CT with the use of both calcium scoring and/or CT
angiography (CTA), has the potential to substantially improve the
clinical care of patients with acute chest pain presenting to the
Traditional CT imaging of the chest in the acute setting has
been extremely valuable in the diagnosis of pulmonary embolism (PE)
and aortic dissection but is unable to provide accurate or detailed
information on cardiac structures because of limited temporal
resolution and motion arti-fact.
Significant advances have been made in the field of cardiac
imaging, particularly in the ability to view the coronary artery
lumen with sufficient diagnostic accuracy. This is quite an
accomplishment, as noninvasive coronary angiography has been
challenging, given rapid cardiac motion, small and tortuous
vessels, concomitant calcification, and overlying veins. These
factors have required imaging modalities used for noninvasive
coronary angiography to possess both high temporal and high spatial
Cardiac CT is now a robust technology for the noninvasive
assessment of a spectrum of cardiovascular disease processes and is
poised to become a noninvasive method to evaluate the lumen of the
coronary arteries. One of the most important advances has been
faster gantry rotation speed resulting in better temporal
resolution and better z-axis spatial resolution made possible by
thin collimations with extensive volumetric acquisition.
The new 64-detector multidetector computed tomographic (MDCT)
scanners provide fast scan times, improved cardiac gating options,
and isotropic resolution, which provides 3-dimensional (3D)
information free of superimposed tissues or interference, resulting
in uniform resolution throughout. The 64 detectors yield a 3D data
set: for example, near isotropic voxels of 0.35 × 0.35 × 0.5 mm
that could be rotated in any given plane without loss of
Calcium scoring in the ER
The first major application validated with this imaging modality
is the assessment of atherosclerotic plaque burden and CAD risk
through coronary artery calcium (CAC) scoring.
Three studies have documented that CAC is a rapid and efficient
screening tool for patients admitted to the ER with chest pain and
These studies show sensitivities of 98% to 100% for identifying
patients with acute myocardial infarction and long-term event rates
approaching zero for patients with negative tests (calcium scores
of zero). The high sensitivity and negative predictive value may
allow early discharge of those patients with nondiagnostic ECG and
negative CAC scans (scores = 0). The absence of calcium implies a
very low rate (0.6%) of annual incidence of coronary events, and
patients with a zero score and nondiagnostic ECG can be safely
discharged from the ER. The limitation of calcium scoring is that
there is no assessment of stenosis severity, and non-calcific
plaque, albeit rare in the setting of a score of zero, may be
Use of calcium scoring in risk stratification
Coronary artery calcium has been shown to be highly specific for
coronary atherosclerosis and adds incremental prognostic data to
traditional cardiac risk factors.
Coronary calcium scores (CCS) are easily obtained at the time of
the CT. The absence of coronary calcium (CCS of 0) has an extremely
high negative predictive value for the presence of obstructive CAD
in patients older than 40 years.
Conversely, the presence of CAC in a symptomatic patient cohort is
associated with adverse outcomes. There are situations, however,
where obstructive atherosclerotic disease may exist with a low CCS,
especially in younger patients with comorbidities such as diabetes.
CT angiography in these patients may reveal the presence of
noncalcified or "soft" plaques, reinforcing the need for aggressive
medical therapy in this patient subset.
64-MDCT in the ER
The advent of 64-detector MDCT (64-MDCT) offers the clinician in
the ER setting a powerful tool to evaluate acute cardiovascular
disease, evaluating both calcific and noncalcific plaque. CT
angiography using 64-MDCT can accurately evaluate a number of
cardiac, pulmonary, and vascular pathologies. In addition to
performing the so-called triple rule-out (to detect CAD, aortic
dissection, and PE), CCT can evaluate other cardiac disease states
that are relevant to the clinician who is making acute diagnoses.
These disease states include deep venous thromboses (DVT),
cardiomyopathies, and pericardial disease. This review will discuss
the role of CCT in the evaluation of patients with acute cardiac
disease and will propose an integrated algorithm for the work-up
and risk stratification of these patients.
The possibility of scanning the whole chest in one acquisition
to visualize the aorta, the coronary arteries, and the pulmonary
arteries provides a new angle in the triage of patients with acute
chest pain, since this will be a diagnostic test to evaluate for
CAD, PE, and aortic dissection. Though this might be a possibility,
there are some challenges the physician must consider. An attempt
to slow the heart rate to <60 bpm for coronary CTA might not be
physiologically suitable if the patient, in fact, has PE or
congestive heart failure. Further, although intravenous beta
blockade is generally viewed as prudent in acute myocardial
infarction, a recent large study showed an increased risk of
cardiogenic shock in this population, suggesting that the
underlying hemodynamic condition must first be stabilized.
A single scan for both pulmonary and coronary applications is
challenging. The timing of contrast must be optimized differently
for PE evaluation (right heart filling) as compared with coronary
artery and aortic assessment (left heart opacification). To
visualize both pulmonary arteries and coronary arteries
simultaneously, a long contrast infusion is necessary, allowing
simultaneous opacification of the right- and left-sided structures
of the heart. The coronary arteries will not be optimally
visualized for interpretation in CTA that is performed to evaluate
the presence of PE, as the bright contrast in the right ventricle
and superior vena cava may obscure portions of the right coronary
Two issues make it difficult for a single acquisition to
encompass the coronary and pulmonary vasculature. The extended
anatomy to be imaged is one consideration, the injection rate is
another, and the acquisition timing a third. While we normally want
the primary part of the bolus in the arterial phase in the heart
for the coronary arterial system, we want the bolus earlier for the
pulmonary arteries. Usual timing for a PE study is to inject 60 to
70 mL of contrast at 4 mL/sec and begin scanning when the contrast
reaches the pulmonary outflow tract. The contrast typically reaches
this point within 8 to 12 seconds from the start of the injection,
and ECG triggering is normally not required for this examination.
For optimal acquisition of the coronary arteries, a typical
injection is 70 to 80 mL of contrast at 5 mL/sec followed by a
60-mL saline flush. The authors use bolus timing in this setting
and begin scanning when the contrast is at equal density in the
ascending aorta and in the pulmonary outflow tract. This typically
occurs approximately 23 to 27 seconds after the start of
For a triple rule-out examination, the difficulty is timing the
contrast, taking into account the 10- to 12-second difference
between the 2 phases. To accomplish this, either flow rates are
slowed down or total contrast volume is greatly increased.
Concerns regarding radiation dose of this single acquisition
have been raised.
If CCT with retrospective triggering is used from the apex to the
base of the lung (to allow simultaneous acquisition of lung fields
and coronary arteries), the effective radiation dose will be 25 to
40 millisieverts (mSv). Finally, the decision to order such tests
must be based on a pre-test clinical probability of a particular
disease process being present and preferably not by a "shotgun
approach." Thus, there is a potential that this test will be
ordered unnecessarily in the ER for "chest pain work-up." This
leads to increased radiation exposure to the patient as well as
added costs. Despite these concerns, it should be noted that a
recent study found the feasibility of the triple rule-out protocol
by injecting 120 mL of contrast at a rate of 4 mL/sec with a
20-second breathhold to allow for overlap and contrast enhancement
of both systemic and pulmonary vasculatures.
A more prudent approach has been suggested.
This would entail performing CCT in the usual manner, with
acquisition of the heart using retrospective gating, with a
radiation dose between 12 and 15 mSv, with administration of 60 to
80 mL of contrast. Then, a CT to evaluate the pulmonary arteries
(thicker slices, not retrospectively gated) can be performed, with
approximately 40 mL of contrast and a radiation dose of 4 to 6 mSv.
This allows for a reduction of radiation dose, requires no increase
in contrast administration, and provides 2 optimal studies, instead
of 1 suboptimal study.
Acute coronary syndrome
During risk stratification of a patient who is being evaluated
for chest pain, the clinician is frequently confronted with
nondiagnostic results, including a nondiagnostic ECG and a normal
first troponin level.
Furthermore, waiting for the results of a second set of cardiac
biomarkers can often delay diagnosis for several hours. In the
Thrombolysis in Myocardial Infarction (TIMI) IIIB study,
approximately 15% of a cohort in >1300 patients had an initial
negative troponin with a later rise in cardiac biomarkers at 12
Perhaps the greatest benefit of 64-MDCT technology is the
improved imaging quality of the coronary arteries and plaque
assessment in the chest pain patient (Figures 1 through 3). In a
recent study, the specificity and sensitivity for the detection of
significant coronary stenosis (defined as >50% luminal
narrowing) for 64-detector CTA of the coronary arteries compared
with conventional coronary arteriography was 91% and 92%,
When compared with 16-MDCT, 64-MDCT offers faster scan times, fewer
image artifacts, and better evaluation of distal vessels. Optimal
image quality is obtained with slower heart rates (<65 bpm),
which often requires the use of intravenous or oral beta blockade.
The use of beta blockers, of course, may not be suitable in certain
patients. A "positive" CTA for significant coronary artery or
bypass graft stenosis would be of value to the interventional
cardiologist, who can then focus on treating the culprit vessel in
the cardiac catheterization laboratory without adding unnecessary
fluoroscopy time. This is particularly helpful in patients who have
anomalous coronary arteries and atypical anatomy of native
coronaries and bypass grafts (Figure 4). Limitations of MDCT
interpretation occur in calcified arterial segments and in segments
with stents, which may cause a multitude of artifacts.
Gallagher et al
compared the accuracy of CTA with that of stress nuclear imaging
for the detection of an acute coronary syndrome or 30day major
adverse cardiac events (MACE) in low-risk chest pain patients. This
was a prospective study of the diagnostic accuracy of myocardial
perfusion imaging and MDCT in low-risk chest pain patients. The
target condition was an acute coronary syndrome (confirmed >70%
coronary stenosis on coronary artery catheterization) or MACE
within 30 days. The patients were at low risk by the Reilly/Goldman
criteria and had negative serial ECGs and cardiac markers.
All had both rest/stress technetium-99m sestamibi myocardial
perfusion imaging and MDCT. Patients with abnormal stress nuclear
imaging results (reversible perfusion defects) or MDCT results
(stenosis >50% or calcium score >400) were considered for
cardiac catheterization, and those with discordant results had a
reevaluation (including ECG) by a cardiologist more than 30 days
later. All patients were followed for evidence of MACE within 30
days by review of hospital records and structured telephone
Primary outcomes were the accuracy of MDCT and myocardial
perfusion imaging for the detection of an acute coronary syndrome
and 30-day MACE. Of the 92 patients, 7 (8%) were excluded because
of uninterpretable MDCT scans. Of the remaining 85 study patients
(median age 49 ± 11 years, 53% men), 7 (8%) were found to have the
target condition, with all having significant coronary stenosis
(88% ± 9%) and none having myocardial infarction or MACE during 30
days. Stress nuclear myocardial perfusion imaging results were
negative in 72 (85%) patients, and MDCT results were negative in 73
(86%) patients. The sensitivity of stress nuclear imaging was 71%
(95% confidence interval [CI] 36% to 92%), and MDCT was 86% (95% CI
49% to 97%), and the specificity was 90% (95% CI 81% to 95%) and
92% (95% CI 84% to 96%), respectively. The negative predictive
value of stress nuclear imaging and MDCT was 97% (95% CI 90% to
99%) and 99% (95% CI 93% to 100%), respectively, and the positive
predictive value was 38% (95% CI 18% to 64%) and 50% (95% CI 25% to
75%), respectively. The authors concluded that the accuracy of MDCT
is at least as good as that of stress nuclear imaging for the
detection and exclusion of an acute coronary syndrome in low-risk
chest pain patients.
Acute aortic syndrome
Acute aortic syndrome (AAS) refers to the following
life-threatening aortic emergencies: penetrating atherosclerotic
ulcer, acute thoracic aortic injury, intramural hematoma,
dissection, and aneurysm leakage.
MDCT offers several advantages over other imaging modalities in the
diagnosis of acute aortic syndrome. Imaging can be obtained quickly
and non-invasively. Other imaging modalities, such as
transesophageal echocardiography, provide excellent imaging
capability but are invasive and require an experienced operator who
is readily available. This discussion will be limited to acute
Acute aortic dissection is a potentially catastrophic emergency
associated with high mortality. CT is a first-line diagnostic
modality for the diagnosis. In a recent review that evaluated the
accuracy of MDCT in acute aortic dissection, the authors reported a
sensitivity of 99% and a specificity of 100% and concluded that CT
is the test of choice, given its speed and cost-effectiveness.
The visualization of a dissection flap, of the true and false
lumen, and of the origin and distal extent of the dissection are
well characterized by CT (Figures 5 and 6).
The use of CT for the diagnosis of PE has been well established,
initially with helical CT technology.
MDCT offers additional technical refinements and highly detailed
visualization of the pulmonary vasculature.
The contemporary generation of scanners outperforms helical CT and
has the added advantages of speed, ability to image
sixth-generation vessels, and the capability to perform combined
imaging of the lower extremity venous system for DVT.
The traditional belief that invasive pulmonary angiography is the
"gold standard" for this diagnosis is under debate as a result of
this technological improvement (Figure 7).
The combination of CT venography (CTV) and pulmonary angiography
(CTVPA) was initially described in 1998 as a single comprehensive
noninvasive imaging examination for suspected thromboembolic
It allowed the identification of PE as well as DVT in the abdomen,
pelvis, thighs, and calves. Combining CTV with CT pulmonary
angiography (CTPA) not only increases the confidence in withholding
treatment when results for both the pulmonary arteries and leg
veins are negative but also increases the diagnosis of venous
thromboembolism by 25% over CTPA alone.
The venographic portion of CTVPA has now been studied by multiple
researchers and has been shown to be an accurate imaging study for
the thigh veins in comparison with lower extremity sonography. In
contrast to sonography, however, CTVPA readily and rapidly permits
evaluation of the inferior vena cava, the pelvic veins, the calf
veins, and all of the superficial venous system. Complex venous
anatomy can be easily surveyed, and an additional sonographic study
is not required. A review of 957 recent cases of suspected PE
examined with CTVPA revealed an overall 10.5% frequency of DVT,
with a nearly equal distribution of thrombosis at the common
femoral, superficial femoral, popliteal, and deep calf veins.
Although a variety of protocols for CTVPA may be implemented,
including a contiguous helical acquisition, obtaining 5- or
10-mm-thick images every 4 cm provides a high degree of accuracy
and decreases the overall radiation dose.
CT venography of the abdomen, pelvis, and lower extremities started
3 minutes after the start of contrast medium infusion for helical
pulmonary CTA routinely produces high mean levels of venous
In a prospective study, MDCT venography was compared with
Doppler sonography in the detection of DVT of the pelvis and the
thighs in patients with suspected PE. A total of 41 patients with
suspected PE were included, and CTV (collimation 4 × 2.5 mm, table
feed 12.5 mm, 120 kV, effective mAs 165) from the iliac crest to
the knees was done after CTA of the pulmonary arteries. Doppler
sonography was performed within 24 hours. Pulmonary embolism was
diagnosed in 20 patients with additional DVT in 11 patients. The
CTV had a sensitivity of 100%, a specificity of 96.6%, and positive
and negative predictive values of 91.7% and 100%, respectively. The
median cumulative effective dose for CTV was 8.26 mSv with a
gonadal dose of 3.87 mSv. Changing the CTV protocol to a
collimation of 4 × 5 mm with a 25-mm table feed reduced the dose by
approximately 11% (
<0.05) to 7.25 mSv and 3.35 mSv, respectively. The authors
concluded that CTV is a safe and quick diagnostic tool for
detecting DVT in patients with suspected PE. They also cautioned
that because of the relevant increase in radiation dose, the need
to perform the test has to be considered very carefully.
CT venography is as accurate as sonography in the diagnosis of
CT venography can further reveal thrombus in large pelvis veins and
the inferior vena cava, which is an important advantage over
sonographic screening for DVT.
The iliac veins and vena cava are sometimes the source of
significant PE emboli (in approximately 17% of patients with DVT).
A large multicenter study of PE was recently completed using
both CTV and CTA. The use of CTA alone for the detection of PE had
a sensitivity of 83% and a specificity of 96%. Adding CTV to CTA
significantly increased the sensitivity to 90%.
This study mainly used 16-detector scanners. The expectation is
that the use of 64-MDCT will improve sensitivity and provide very
high negative predictive values (>98%).
An integrated approach to the chest pain patient
The use of CCT with 64-detector technology is a useful and
powerful tool in the evaluation of the acute cardiac patient
(Figure 8). The challenge remains in integrating this modality
within the traditional work-up of cardiopulmonary disease. Many
patients can be spared further invasive and costly testing with
early risk stratification and imaging of a lower risk patient
subset. It should be noted that patients presenting with noncardiac
chest pain related to musculoskeletal or esophageal disease would
be unlikely to benefit, diagnostically speaking, from CT imaging.
Therefore, they require appropriate clinical management and would
be candidates for early discharge and outpatient follow-up.
Limitations of cardiac CT
Current limitations of CCT of the native vessels include
decreased resolution in obese patients and in those with markedly
elevated CCS, the need for iodinated contrast and radiation
exposure, and the need for reducing heart rate (typically with oral
and/or intravenous beta-blocker administration) prior to evaluation
Ar-rhythmias or heart-rate variability compromise the ability to
render 3D images from CTA data sets. An irregular heart rate is
currently considered a contra-indication to CTA, and patients with
irregular heart rates have been systematically excluded from most
studies to date.
Other factors that interfere with the diagnostic quality or ability
to perform CTA include difficulty with breath-holding, contrast
allergies, and significant renal dysfunction.
Studies in patients with heart rates >70 bpm have had
markedly reduced sensitivity and specificity because of motion
In the study by Raff et al,
baseline heart rate at the time of the scan was the primary
determinant for the accuracy of the study. When the heart rate was
<70 bpm, the sensitivity was 97%, the specificity was 95%, the
positive predictive value was 97%, and the negative predictive
value was 95%. For patients with heart rates >70 bpm, the
sensitivity declined to 88%, the specificity to 71%, the positive
predictive value to 78%, and the negative predictive value to 83%.
This study confirmed that patients with CCS >400, obesity (a
body mass index ≥30 kg/m
), and heart rates >70 bpm remain a challenge to diagnose with
CT venography is specific but has a lower sensitivity rate and a
lower positive predictive value for the diagnosis of acute lower
extremity DVT compared with ultrasound. Additionally, CTV is more
expensive than ultrasound scanning. Because of the lower
sensitivity rate, the lower positive predictive value, and the
increased cost of CTV, ultrasound remains the screening study of
choice in cases of suspected acute lower extremity DVT.
Calcium scanning by either electron-beam CT or MDCT requires
only a small radiation dose. The 0.6- to 1.2-mSv dose scan is
similar to an abdominal X-ray and is at least half the radiation
dose of invasive angiography. However, MDCT angiography imparts a
significantly higher radiation dose to the patient (approximately 8
to 13 mSv).
With dose modulation (turning the X-ray beam down during systole
when images are not as diagnostic due to motion), the dose is
reduced by 18% to 47% depending on the patient's heart rate.
This moderate radiation dose must be considered when patients are
selected for a CCT study. It must be kept in mind that the dose
given during CTA is similar to that given during technetium-99m
myocardial perfusion imaging protocols
; therefore, this should not be considered excessive for the
evaluation of CAD.
The use of cardiac CT in the emergency room, with its very high
(99%) negative predictive power for obstructive coronary artery
disease, PE, and aortic dissection, should allow for an expedited
work-up of chest pain patients. This unprecedented accuracy will be
attractive to ER physicians. However, the appropriate use of
cardiac CT in the ER must be considered in each patient. While
younger women on birth control pills with chest pain may have PE in
the diagnosis, the likelihood of coronary artery disease and aortic
dissection approaches zero. Thus, such patients should not get a
complete "triple rule-out" study, with associated higher radiation
doses and contrast requirements, but rather a targeted evaluation
of the problem. Appropriate utilization will lower healthcare costs
and reduce radiation exposure to patients. Limiting the use of this
new technology to appropriate patients may prove harder than some
CCT experts anticipate.