Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Introduction
There are multiple aspects of imaging in the context of coronary artery disease. On the one hand, imaging is used to identify the presence of coronary artery stenoses, through two possible approaches. One approach is to visualize ischemia as the consequence of hemodynamically relevant coronary artery lesions. In clinical practice, this is most frequently done by stress echocardiography, stress cardiac magnetic resonance, or nuclear medicine techniques ( functional imaging ). The alternative approach is to directly visualize the coronary arteries and identify atherosclerotic lesions. Given the small dimensions and fast motion of the coronary vessels, this is technically challenging and requires a combination of high spatial resolution, high temporal resolution, and the ability to capture the entire complex course of the coronary artery tree. On the other hand, next to the mere identification of coronary artery disease, imaging fulfills other needs regarding management of patients, such as the assessment of left ventricular function or myocardial injury and viability.
Computed tomography (CT) and cardiac magnetic resonance (CMR) play an increasingly important role in the evaluation of patients with known or suspected coronary artery disease. The main application of CT in the context of chronic coronary artery disease is coronary CT angiography, that is, direct visualization of the coronary artery lumen to rule in or rule out coronary artery stenoses. Bypass grafts and stents can also be assessed but are significantly more challenging to evaluate than native coronary vessels. To some extent, CT can be used to characterize nonobstructive coronary atherosclerotic plaque. This may have applications in the context of risk stratification, but it is not yet a method with firmly established clinical applications. Other areas in which CT is used include the support of coronary interventions (in particular for chronic total coronary artery occlusions) and the identification of ischemia through myocardial perfusion imaging or simulation of the fractional flow reserve (FFR).
CMR is not used for visualization of the coronary arteries to the same extent as CT; rather, it is focused on imaging the myocardium. Late gadolinium enhancement imaging is a reliable, high-resolution technique to visualize and quantify myocardial scar and differentiate it from viable myocardial tissue, whereas stress CMR, typically after adenosine or dobutamine infusion, is an accurate method to identify myocardial ischemia.
Both methods complement each other regarding the assessment of patients with known or suspected chronic coronary artery disease. They have widespread clinical application and are firmly established in professional guidelines. Nevertheless, technical challenges exist that may impair image quality or lead to misinterpretation. Meticulous care in patient preparation and image acquisition, as well as sufficient expertise in interpretation, is therefore essential to maximize benefit to the patient.
Cardiac Computed Tomography
Imaging Protocols
Cardiac computed tomography is most frequently used to visualize the coronary artery lumen. The method is referred to as coronary CT angiography or coronary CTA . To achieve sufficient spatial and temporal resolution, high-end CT equipment and adequate imaging protocols must be used. Currently, 64-slice CT is considered the state of the art for coronary artery imaging. Newer technology, such as dual source CT or volume scanners that have wide detectors with 256 or 320 detector rows, provides further improved and more robust image quality.
Typical datasets for coronary artery visualization by CT consist of approximately 200–300 transaxial slices with a thickness of 0.5 mm to 0.75 mm ( Fig. 13.1 ). Data interpretation is based on interactive manipulation of these datasets using an image processing workstation, enhanced by post-processing tools such as maximum intensity projections and multiplanar reconstructions. Three-dimensional renderings, although impressive for visualization of the heart and coronary arteries, are not accurate for stenosis detection and play no role in data interpretation. Whereas many workstations provide prerendered reconstructions that are intended to show the coronary arteries over their entire course, readers should not rely on such automated post-processing tools alone. In fact, official recommendations mandate that the reader manipulate the original data and not rely on prerendered reconstructions of any kind.
There are some conditions for patients to be suitable for coronary CT angiography ( Box 13.1 ). Importantly, they include the ability to understand and follow breathhold commands, because even slight respiratory motion during data acquisition will cause substantial artifact. A regular and, preferably, low heart rate substantially improves image quality and reliability (optimally below 60 beats/min, even though this is not as strictly required for dual-source CT). To achieve a low heart rate, patients usually receive premedication with short-acting β-blockers, and nitrates are given to achieve coronary dilatation. For vascular enhancement during the scan, contrast agent is injected intravenously. Depending on scanner type and acquisition protocol, approximately 40 mL to 100 mL of iodinated contrast agent is used. Data acquisition can follow various principles, and the mode of data acquisition has profound implications regarding radiation exposure. Retrospectively electrocardiogram (ECG)-gated acquisition in helical mode (also called spiral mode ) provides for high and robust image quality and maximum flexibility to choose the cardiac phase during which images are reconstructed, including the ability to reconstruct functional datsasets throughout the entire cardiac cycle in order to assess wall motion (which, however, is not frequently necessary or clinically desired). Prospectively ECG-triggered axial acquisition is associated with substantially lower radiation exposure. Image quality is high, especially in patients with stable and low heart rates. Less flexibility to reconstruct data at different time instants in the cardiac cycle, as well as greater susceptibility to artifacts caused by arrhythmia, can be downsides of this acquisition mode but rarely affect individuals if they are well prepared. Overall, prospectively ECG-triggered axial acquisition is the preferred image acquisition mode in many experienced centers. Finally, prospectively ECG-triggered high-pitch helical or spiral acquisition, often referred to as flash acquisition, is an imaging mode that combines aspects of the former two techniques but can only be used on single source or dual-source CT systems with very wide detectors and only in patients with low and truly regular heart rates. It allows coverage of the volume of the heart within a very short time and maximizes efficacy of radiation use, so that it is associated with very low radiation exposure ( Fig. 13.2 ).
BOX 13.1
Patient Characteristics for Optimal Image Quality in Cardiac CT and Coronary CT Angiography
- –
Ability to follow breathhold commands and perform a breathhold of approximately 10 seconds
- –
Regular heart rate (sinus rhythm) < 65 beats/min, optimally < 60 beats/min
- –
Lack of severe obesity
- –
Ability to establish a sufficiently large peripheral venous access (cubital vein preferred)
- –
Absence of contraindications to radiation exposure and iodinated contrast media
The radiation exposure of coronary CTA (and cardiac CT in general) varies widely. When CT of the heart was first developed, use of radiation was not efficient and effective doses up to 25 mSv were not uncommon for standard acquisition protocols. With the use of improved data acquisition protocols, complemented by image reconstruction techniques that compensate for image noise, radiation exposure in the context of coronary CT angiography has been substantially reduced and typical values for effective radiation dose of contemporary CT protocols range between 1 and 5 mSv. In very strictly selected patient cohorts, it has even been reported that doses below 0.5 mSv and even below 0.1 mSv are possible, but image quality at this extreme end of the spectrum is not robust enough for routine clinical practice. Without going to the extreme and by using measures that are widely available, do not require special training, and are straightforward to implement, Chinnaiyan et al. reported a mean effective dose of 6.4 mSv across 15 centers routinely performing coronary CTA. In a 2014 multicenter trial, the average effective dose for coronary CT angiography was 3.2 mSv.
Accuracy of Coronary CT Angiography
Coronary CT angiography has high accuracy for the detection of coronary artery stenoses ( Figs. 13.3 and 13.4 ). Three multicenter trials assessed the accuracy of coronary CT angiography for the identification of coronary artery stenosis in comparison with invasive coronary angiography. Two trials performed in patients with suspected coronary artery disease using 64-slice CT have demonstrated sensitivities of 95% to 99% and specificities of 64% to 83%, as well as negative predictive values of 97% to 99% for the identification of individuals with at least one coronary artery stenosis. The positive predictive values were 64% and 86% in these two trials, which is due to a tendency to overestimate stenosis degree in coronary CTA, as well as the fact that image artifacts often result in false-positive interpretations. In a third multicenter study of 291 patients with 56% prevalence of coronary artery stenoses, as well as 20% of patients with previous myocardial infarction and 10% with prior revascularization, specificity was high (90%) and the resulting positive predictive value was 91%. However, this came at the cost of decreased sensitivity (85%) and negative predictive value (83%).
A 2016 meta-analysis summarized 30 clinical trials that evaluated the accuracy of coronary CTA performed with systems composed of 64 slices or greater in comparison with invasive angiography. A total of 3722 patients were included. The authors determined that, on average, 6.6% of studies were unevaluable. They also reported a pooled sensitivity of 95.6% and a specificity of 81.5% for systems with at least 64 detector rows. Of particular importance, the negative likelihood ratio was 0.022, rendering coronary artery stenoses extremely unlikely if coronary CTA is normal.
Accuracy values are not uniform across all patients. High heart rates, obesity, and extensive calcification negatively influence accuracy. Degraded images will lead to false-positive rather than false-negative findings. Specificity and positive predictive value will therefore be most affected. Along with patient factors that influence image quality, the accuracy of coronary CTA depends on pretest likelihood of disease. In an analysis of 254 patients referred to invasive angiography and also studied by CT, it was demonstrated that coronary CTA performs best in patients with a low to intermediate clinical likelihood of coronary artery stenoses (negative predictive value: 100% in both groups), while accuracy is substantially lower in high-risk patients ( Table 13.1 ).
TABLE 13.1
Diagnostic Performance of 64-slice CT Depending on the Clinical Pretest Likelihood of Coronary Artery Disease in 254 Patients
Meijboom WB, van Mieghem CA, Mollet NR, et al. 64-slice computed tomography coronary angiography in patients with high, intermediate, or low pretest probability of significant coronary artery disease. J Am Coll Cardiol . 2007;50:1469–1475.
| Pretest probability ∗ |
n | Sensitivity | Specificity | Positive pred. value | Negative pred. value |
|---|---|---|---|---|---|
| High | 105 | 98% | 74% | 93% | 89% |
| Intermediate | 83 | 100% | 84% | 80% | 100% |
| Low | 66 | 100% | 93% | 75% | 100% |
Overall, the ability of coronary CTA to reliably rule out the presence of coronary artery stenoses and the fact that it performs best in situations of low to intermediate likelihood of disease indicate that coronary CTA is a clinically useful tool in symptomatic patients who do not have a high pretest likelihood of coronary artery disease but require further work-up to rule out significant coronary stenoses. A negative coronary CTA scan, if of high quality, will obviate the need for further testing. Indeed, several observational trials and registry reports with up to 35,000 patients clearly demonstrated that symptomatic patients, when coronary CTA is negative, have an extremely favorable clinical outcome even without further additional testing.
Randomized Clinical Outcome Trials Evaluating Coronary CTA
Two pivotal randomized clinical trials emphasize the fact that coronary CTA is a clinically useful tool that may be used for management decisions in patients with suspected chronic coronary artery disease. In the multicenter Prospective Multicenter Imaging Study for Evaluation of Chest Pain (PROMISE) trial, published in 2015, 10,003 patients with suspected coronary artery disease were randomized to either ischemia testing or coronary CTA as the initial test. After 2 years, outcome in terms of major cardiovascular adverse events or complications associated with testing was equal between the two groups. The rate of invasive coronary angiograms (12.2% vs 8.1%) and the rate of revascularizations (6.2% vs 3.2%, p < 0.001) were significantly higher if coronary CTA was used as the initial test. On the other hand, catheterization showing no obstructive lesions occurred significantly less frequently if coronary CTA had been used as the initial test (3.4% vs 4.3% of the population, p < 0.02). In summary, the trial demonstrated that there is no clinical risk to using coronary CTA as an anatomic test, as opposed to functional imaging, as a first diagnostic method in patients with suspected coronary artery disease.
The Scottish Computed Tomography of the Heart (SCOT-HEART) multicenter trial randomized 4146 patients with stable chest pain to receiving only functional testing or functional testing plus coronary CTA in the setting of suspected coronary artery disease. The additional information from coronary CTA to standard care changed planned management (15% vs 1%, p < 0.001) and treatment (23% vs 5%, p < 0.001) but did not affect 6-week symptom status ( p = 0.22) or the frequency of initial admissions ( p = 0.21) or subsequent hospital admissions for chest pain (11.9% vs 12.7%, p = 0.40) as compared to standard care alone. However, after 1.7 years of follow-up, the trial demonstrated that there was a trend toward lower event rates of fatal and nonfatal myocardial infarction by 38% ( p = 0.05) if ischemia testing was complemented by coronary CTA.
These two pivotal randomized imaging clinical outcome trials (PROMISE and SCOT-HEART) and several smaller trials demonstrated coronary CTA to have a proven role in management of patients with suspected chronic coronary artery disease. An overview of the PROMISE and SCOT-HEART trials is presented in Table 13.2 .
TABLE 13.2
Overview of the Design and the Main Findings of the Multicenter Randomized Trials PROMISE and the SCOT-HEART
Data from Douglas PS, Hoffmann U, Patel MR, et al. Outcomes of anatomical versus functional testing for coronary artery disease. Engl J Med. 2015;372(14):1291–1230; SCOT-HEART Investigators. CT coronary angiography in patients with suspected angina due to coronary heart disease (SCOT-HEART): an open-label, parallel-group, multicentre trial. Lancet. 2015;385(9985):2383–2391.
| Promise | Scot-Heart | |
|---|---|---|
| Patients | n = 10,003 patients | n = 4146 patients |
| Inclusion criteria |
|
|
| Methods | Functional stress testing ∗ versus coronary CTA | Usual care (ECG stress testing) versus usual care plus coronary CTA |
| Study endpoints | Death, nonfatal MI Hospitalization for unstable angina, Major procedural complications † |
Certainty of diagnosis Angina due to CAD |
| Management |
|
Increased preventive prescription |
| Outcome | No difference |
|
CAD, Coronary artery disease; CTA, computed tomography angiography; ICA, invasive coronary angiography; MI, myocardial infarction.
∗ Exercise treadmill, nuclear stress, or stress echocardiography.
† Major procedural complications: anaphylaxis, stroke, major bleeding, renal failure.
Imaging of Patients With Bypass Grafts and Stent
The follow-up of patients after previous revascularization is a frequent question in clinical cardiology. It needs to be taken into account that coronary CTA has relevant limitations in patients with previous coronary revascularization. Assessment of coronary artery stents ( Fig. 13.5 ) is often unreliable because the dense metal of the stents can cause artifacts that render the stent lumen unevaluable or create false-positive findings of stenosis. The ability to assess stents concerning in-stent restenosis depends on many factors. They include stent type and diameter, as well as the overall image quality. The analysis of large stents (eg, stents implanted in the left main coronary artery) may be possible by CT in most cases. In general, however, there is uncertainty about the accuracy of coronary CTA to detect and rule out in-stent stenosis. A meta-analysis reported that 20% of stents were unevaluable by CT, and sensitivity for stenosis detection was only 82% in evaluable stents. With the exception of large stents (≥ 3.0-mm diameter) in locations very amenable to CT imaging (eg, left main coronary artery), and if invasive coronary angiography is to be avoided, imaging of patients with previously implanted stents by coronary CTA should therefore not be routinely considered. Bioresorbable vascular scaffolds, on the other hand, are typically made of material that does not have the high attenuation of metal in CT imaging. No systematic evaluations have been performed, but imaging of the coronary lumen should not be impaired by these devices. CT may therefore be a useful method for the follow-up after percutaneous coronary intervention (PCI) performed with bioresorbable scaffolds ( Fig. 13.6 ).
Regarding the follow-up after bypass surgery, the accuracy of coronary CTA for the detection of bypass graft stenosis and occlusion is very high ( Fig. 13.7 ). However, assessing the native coronary arteries in patients after bypass surgery is typically difficult. The native vessels frequently have a small diameter and substantial calcification ( Fig. 13.8 ). Consequently, accuracy for detecting and ruling out stenoses in nongrafted and run-off vessels is relatively low, false-positive findings are frequent, and unevaluable segments impair the clinical utility of the test.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here