CARDIAC COMPUTED TOMOGRAPHY
For decades, investigators sought to develop new technologies that would allow rapid noninvasive imaging of the heart. One such technology that has evolved rapidly in the past several decades has been CCT. CCT now permits visualization of the coronary arteries and lumen, as well as providing assessment of cardiac function, valvular structures, prosthetic materials, the pericardium, left atrial anatomy, congenital heart disease, pulmonary arterial and venous anatomies, and diseases of the aorta.
Technology of
Cardiac Computed Tomography
Imaging the heart and coronary arteries with CT is an extremely challenging
undertaking for several reasons, and requires more sophisticated hardware and
software analysis tools than are required for imaging other body regions. Major
difficulties arise because coronary arteries are relatively small moving
structures with branches of interest in the range of 2 to 4 mm in diameter. The
coronary arteries show rapid cyclic motion throughout the cardiac
cycle—essentially moving in three dimensions with each heartbeat. Furthermore,
when the subject breathes, the heart and vessels move within the chest due to
diaphragmatic motion. Several major advances in recent years have dramatically
improved the resolution of CCT images. Improvement in scanner technology in the past 5 years includes increased
Z-axis coverage, now up to 16 cm with some vendors, which means that the entire
heart can be imaged in a single gantry rotation. In addition, gantry rotation
speeds have also continued to improve; they currently approach 0.2 seconds per
360-degree rotation, which results in improved temporal resolution. ECG gating
has also improved. These improvements have significantly decreased overall
patient radiation exposure, which is now substantially less than traditional
SPECT and is often less than 2 mSv. As the United States continues to deal with
the obesity epidemic, vendors have started manufacturing CT scanners with
increasingly larger bore diameters and
table weight limits to better accommodate these patients.
Over its relatively short history, several different CT technologies have been used for cardiac imaging. Electron beam CT (EBCT), which was initially introduced in the mid-1970s, uses an electron source reflected onto a stationary tungsten target to generate x-rays, which allows for rapid scan times. EBCT is well suited for cardiac imaging because of its high temporal resolution (50–100 ms), with an estimated slice thick- ness of 1.5 to 3 mm and the ability to scan the heart in a single breath hold. The primary use of this technique was for coronary arterial vessel wall calcium volume and density, which generated a patient-specific score. Coronary calcium scores are independent of other traditional cardiac risk factors in the prediction of cardiac events, and as such, can be considered an excellent biomarker for the presence of coronary artery disease (CAD) and the risk of future cardiac events.
EBCT has since been largely supplanted by multidetector CT (MDCT) technology, which involves a mechanically rotated x-ray source within a cylindrical gantry with a collimated detector located 180 degrees opposite that permits the simultaneous acquisition of more data (“slices”). Collimator rows that measure 0.625 mm provide for markedly increased spatial resolution and for complete acquisition of data during one breath hold. However, what MDCT offers is improved spatial resolution to make coronary CT angiography (CTA) feasible.
Coronary CTA was initially performed using MDCT machines capable of
obtaining only four to eight slices per scan. As technology has advanced,
256-slice (and higher) scanners are now available that allow acquisition of
higher resolution images without the requirement for long breath holds or
extremely slow heart rates. It is currently recommended that CTA be performed
using a minimum 64-slice scanner. Newer technology allows up to 320 to 512
anatomic slices to be simultaneously acquired. With a minimal slice thickness
of 0.6 mm, an entire heart can be imaged in a single heartbeat. Even with newer
generation scanners, temporal resolution at its best now approaches 140 ms, and
cannot reach what can be obtained routinely in a cardiac catheterization
laboratory, which is closer to 33 ms. To overcome the necessity of a slow heart
rate, one vendor has placed two x-ray sources in the scanner (termed
dual-source imaging) at 90-degree angles to one another. This technology offers
an improved temporal resolution even with heart rates approaching ≥100
beats/min.
Data Acquisition Techniques
In the past, for coronary CTA using a single x-ray source scanner, it was
typically necessary to obtain images with heart rates at <65 beats/ min. An
oral or intravenous β-blocker usually was given to patients to slow the heart
rate. Newer generation scanners have somewhat lessened these rigid low heart
rate requirements, although the adage still holds when imaging the heart that
slower tends to be better. Coronary CTA requires intravenous administration of
a contrast agent to opacify the lumen of the coronary arteries. The intravenous
contrast agents used for CTA carry the same dose-dependent risks in patients
with renal dysfunction as contrast agents used for cardiac catheterization, as
well as the risk of an allergic reaction to iodine. Respiratory motion is
minimized by patient breath holds up to 10 seconds, depending on scanner
generation and patient body size. Data acquisition varies somewhat based on
scanner type. The most common data acquisition protocol uses a spiral mode involving continuous data acquisition during
constant rotation of the x-ray tube
while the patient is simultaneously and continually advanced on the table
through the x-ray gantry. To minimize radiation exposure, data acquisitions can
be performed in sequential mode (step and shoot). This involves acquisition of
single transaxial slices sequentially as a patient is advanced stepwise through
the scanner. Excessive cardiac motion can lead to blurring of the contours of
the coronary vessels. For this reason, a regular heart rate is necessary for
optimal imaging of the coronary arteries. Relative contraindications to
performing CTA include the presence of frequent ectopic beats or atrial
fibrillation. Coordinating data acquisition and analysis to the cardiac cycle
involves either prospective triggering or retrospective gating. In prospective
triggering, data are acquired in late diastole, based on simultaneous ECG
recordings. In retrospective gating, data are collected during the entire
cardiac cycle. Postprocessing then allows only data from specific periods of
the cardiac cycle to be used for image reconstruction.
Coronary artery
calcium (CAC) has emerged as a robust marker of subclinical atherosclerosis.
CAC scoring uses no contrast and readily detects calcium because of its high
x-ray attenuation coefficient (or CT number) measured in Hounsfield units (HUs)
(Fig. 11.1). The Agatston scoring system assigns a
calcium score based on the maximal CT number and the area of calcium deposits.
Recently, analysis of several large clinical data sets confirmed that the CAC
score is a robust predictor of coronary events, particularly in the asymptomatic
patient population, independent of traditional risk factors. In at least one
study, the calcium score was more predictive than C-reactive protein and
standard risk factors for predicting CAD events.
 |
FIG
11.1 Coronary calcium scoring. LAD,
Left anterior descending; LCX, left circumflex; LM, left main; RCA, right coronary artery. |
The coronary calcium score is derived by identifying coronary arterial tree segments that have attenuation characteristics (HU) greater than a certain value (100–130 depending on software and patient size) that correlate with the attenuation due to calcium. These calcified lesions are scored by size and density, with a weighting factor for increasing density. Discrete lesions are scored separately, and the density of calcium within each lesion is graded from 1 to 4 according to the HU. The sums of all the lesions are totaled to arrive at a single coronary calcium score. In general, the higher the score, the greater the amount of calcified plaque within the arterial tree. There is a positive correlation of cardiac events with this score. However, it is important to remember that exclusively noncalcified coronary artery plaques have been reported in up to 4% of asymptomatic patients.
The Multiethnic Study of Atherosclerosis (MESA) Group published a series
of articles that suggests that the calcium score is an independent risk factor
for cardiac events. Has the
capacity to allow comparison of the calcium score of an individual patient
against their large database. This score takes into account age, sex, and race,
and generates a percentile compared with the database studies. The presence
of a high calcium score may prompt clinicians to use more aggressive therapy as
if they were reclassified into a higher risk group or to convince patients who
are reluctant to take drugs (e.g., statins) to take their disease more
seriously. Recent studies have shown that a zero CAC score demonstrated an
annual event rate in asymptomatic subjects of only 0.11% (10-year risk of only
1.1%). Among asymptomatic patients, the incidence of abnormal nuclear stress
testing is 1.3%, 11.3%, and 35.2% for calcium scores <100, 101 to 400, and
>400, respectively. Studies that have looked at serial calcium scanning have
noted that calcified plaque progression is significantly and independently
associated with a worse overall prognosis.
Coronary Artery Imaging (Cardiac CT Angiography)
Chest pain is a common clinical problem and one of the most common
complaints of individuals presenting for urgent medical evaluation. One of the
most important, life-threatening causes of chest pain is CAD. Although cardiac
catheterization is the best method to assess for the presence of
hemodynamically significant obstructive CAD, it is impractical as a screening
test. It is invasive and costly, can be especially dangerous in some patients,
and when used broadly as a screening tool, it is performed on a substantial
number of patients who have no significant CAD and/or whose chest pain is
unrelated to cardiac causes. Approximately 10% to 25% of the patients who are
referred for invasive coronary angiography are found to have normal coronary
arteries or nonobstructive CAD. Furthermore, several meta-analyses have
demonstrated poorer prognosis and increased numbers of hard cardiac endpoints
with nonobstructive CAD compared with normal coronary arteries.
CT angiography (CTA) uses intravenous contrast to differentiate the
vessel lumen from the vessel wall. In 2010, the American College of Cardiology
and many other societies with interests in cardiac imaging put together
recommendations of appropriateness criteria for the use of cardiac CTA (CCTA) that include appropriate (Box 11.1) and inappropriate uses of this technology. The most common appropriate use is diagnostic study of patients
presenting with chest pain who do not have significant ECG changes or elevated
cardiac biomarkers, but who do have an intermediate probability of CAD (Fig. 11.2). At experienced centers with careful data
acquisition, sensitivities range from 83% to 99% and specificities range from
93% to 98%, with remark- ably high estimated negative predictive values
(95%–100%), indicating that CCTA may
be used to reliably rule out the presence of significant flow-limiting coronary atherosclerotic disease. It should be
pointed out that CCTA would be inappropriate for patients at high risk for or
with other indications of cardiac ischemia, such as elevated biomarkers or
significant ECG changes. Those patients should be referred immediately for
invasive imaging. Currently, there is no indication for performing CCTA in
asymptomatic patients. The appropriateness criteria definitively recommend
against the use of CCTA in the asymptomatic population until further evidence
suggests that it would positively affect outcomes.
 |
FIG 11.2 Three-dimensional cardiac computed tomography volume rendering showing patent bypass grafts. |
Bypass graft imaging is more easily accomplished than coronary artery imaging because of the larger size of bypass grafts (particularly saphenous vein grafts) and less rapid movement of bypass grafts compared with native coronary arteries. The patency or occlusion of grafts can be determined by the presence or absence of distal target vessel contrast enhancement (Fig. 11.3). Imaging internal mammary grafts are often more difficult because of artifacts caused by metallic clips near the grafts. Imaging of coronary artery stents is challenging because of artifacts caused by metal that can obscure visualization of the coronary artery lumen. Studies that evaluated CCTA to assess in-stent restenosis have been somewhat disappointing, yielding sensitivities of 54% to 83%. Stents <3.0 mm in diameter are much more likely to be difficult to evaluate. An additional important application of CCTA is in patients with congenital abnormalities of their coronary arteries, including anomalous coronary artery origins and the presence of intramyocardial bridges (coronary arteries that, for a portion of their course, are not epicardial but rather covered by a layer of myocardial tissue).
 |
FIG 11.3 Conventional diagnostic coronary
angiogram and coronary CT angiogram.
The past several years have seen several new developments in CCTA technology that have focused not just on the coronary artery anatomy, but have also evaluated functional data simultaneously on the presence or absence of myocardial ischemia. There have been several publications in the past few years that have focused on the use of CCT for myocardial perfusion imaging, transluminal attenuation gradients, and corrected coronary opacification indexes and fractional flow reserve calculated from resting CCTA data.
Through appropriate timing of intravenous chamber contrast enhancement,
extensive cardiac morphological and functional information can be obtained by
CCT. Myocardial mass and ventricular function can be estimated with a high
level of accuracy. CCT can also provide a detailed morphological picture of the
left atrium and left atrial appendage anatomy, which is information that can be
useful before planned catheter
ablation for atrial fibrillation or device implantation in the left atrial
appendage. Three-dimensional anatomic data obtained by CCT can be fused with
electrical mapping data acquired in the electrophysiology laboratory, which greatly
facilitates the procedure.
The past several years has seen an explosive growth in the use of
transcatheter therapies for the treatment of valvular and structural heart
disease. Advancement in transcatheter aortic valve replacement (Fig. 11.4) technology has relied heavily on appropriate
aortic annular sizing data obtained
with CCT. Studies have demonstrated that accurate
three-dimensional aortic annular sizing assessment with CCT results in reduced paravalvular
regurgitation, which has been associated with increased mortality following
these procedures. In 2012, the Society of Cardiovascular Computed Tomography
released guidelines for the use of CCT before TAVR. Furthermore, several new
therapies on the horizon for mitral and tricuspid valve repair will also rely
heavily on preprocedural CCT for accurate sizing and morphological assessment.
FIG 11.4 Cardiac CT images pretranscatheter and
posttranscatheter aortic valve replacement (TAVR). 3D,
Three-dimensional.
Congenital Heart Disease
Assessment of complex congenital heart disease, including anomalous
coronary artery circulation, great vessels, cardiac chambers, and valves are
all appropriate indications for CCT. Specific indications include shunt
assessment, aortic geometry in coarctation or Marfan syndrome, partial or total
anomalous pulmonary venous return, and pulmonary artery visualization in
patients with cyanotic heart disease.
Evaluation of Intracardiac and Extracardiac Structures
In patients with technically limited images on echocardiography and who
are unable to undergo MRI, CCT can be used to evaluate for cardiac mass (i.e.,
tumor, thrombus, or potentially vegetation). Pericardial diseases can also be
evaluated using CCT by looking specifically for pericardial cysts, pericardial
calcification that may be suggestive of constrictive pericarditis, or
complications of cardiac surgery. Thickening of the pericardium (normal
thickness is <4 mm) can be suggestive of an inflammatory process.
Evaluation of Thoracic Aorta and Pulmonary Artery Disease In
patients with suspected pulmonary embolism, CCT has both high sensitivity and
specificity (>90%) for the diagnosis of proximal pulmonary embolism. Emboli
can be visualized in the main pulmonary artery
and as far distally as the segmental pulmonary artery branches. Evaluation of
the pulmonary venous anatomy is useful before (and after) atrial fibrillation
ablation to assess for pulmonary vein stenosis or pulmonary venous anomalies. CCT assessment of the aorta
typically requires contrast enhancement.
Three-dimensional reconstruction can be
useful diagnostically and also before planned endovascular repair. Typical indications for
CCT in assessment of the aorta include aneurysm, dissection, and intramural
hematoma.
CCT involves exposure to radiation and the potential for radiation-
related risk (particularly related to the risk of cancer induction). Radiation
exposure (effective dose) is quantified in millisieverts. Patient radiation
doses are dependent upon tube current (milliamperes) and tube voltage
(kiloelectron volts), as well as duration of radiation exposure and patient body
size. Current generation CT scanners are now able to perform
electrocardiographically gated cardiac imaging with 1 to 2 mSv. For comparison
purposes, typical gated cardiac SPECT carries a similar radiation dose
(effective dose: 10–15 mSv), whereas conventional coronary angiography carries
a lower radiation dose (effective dose: 6 mSv) compared with CCT.
ECG-correlated tube current modulation (reduction of tube current in systole)
with retrospectively gated studies can reduce radiation exposure by 30% to 50%.
Retrospectively gated studies have more recently been replaced by prospectively
triggered studies that limit radiation exposure to a single portion of the
cardiac cycle, further significantly decreasing overall patient radiation
exposure. In addition, improvements in iterative reconstruction techniques have
also been developed that also further reduce patient radiation exposure. Although the
risk from radiation is relatively low, it does mean that CCT is not well suited
for use as a screening test on a regular or repeated basis.
In addition, allergic contrast reactions have been reported in 0.2% to
0.7% of patients who receive nonionic contrast materials. In the absence of
preexisting renal disease, the risk of renal dysfunction due to contrast
administration is low.
Future Directions
It is estimated that nearly 60 million CT scans were performed in the
United States in 2001, with growth estimated at 9% per year in the coming
decade. Current CCT use has not constituted a broad replacement for
conventional coronary angiography, but in appropriately selected patients, it
may serve as a useful alternative. Dual-source CCT has improved temporal
resolution, and 320-detector row coronary CTA now allows imaging of the entire
heart in a single heartbeat. Combination cardiac PET/CT promises to provide
additional information regarding cardiac morphology, perfusion, and metabolism.
At present, CCT is not covered by many insurance carriers. Based on the results
of ongoing clinical studies demonstration of both efficacy and
cost-effectiveness of CCT as a diagnostic modality there may well be expanded
coverage of CCT by insurers.
