DOI: 10.1148/rg.264055709
RadioGraphics 2006;26:963-978
© RSNA, 2006
Cardiac CT in Emergency Department Patients with Acute Chest Pain1
Udo Hoffmann, MD,
Antonio J. Pena, MD,
Ricardo C. Cury, MD,
Suhny Abbara, MD,
Maros Ferencik, MD, PhD,
Fabian Moselewski, MD,
Uwe Siebert, MD, PhD,
Thomas J. Brady, MD and
John T. Nagurney, MD, MPH
1 From the Departments of Radiology (U.H., A.J.P., R.C.C., S.A., M.F., F.M., U.S., T.J.B.) and Emergency Medicine (J.T.N.), Massachusetts General Hospital, 165 Charles River Plaza, Suite 400, Boston, MA 02114; and Harvard School of Public Health, Boston, Mass (U.H.). Received April 29, 2005; revision requested June 9 and received August 31; accepted October 13. All authors have no financial relationships to disclose.
Address correspondence to U.H. (e-mail: uhoffman{at}partners.org).
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Abstract
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Current strategies for the triage of patients who have chest pain but normal initial cardiac enzyme levels and nondiagnostic electrocardiograms do not permit efficient risk stratification. The potentially fatal consequences and high malpractice costs of missed acute coronary syndromes lead every year to the unnecessary hospital admission of about 2.8 million patients who present with acute chest pain in emergency departments in the United States. Most of these patients are at very low risk for an acute coronary syndrome. However, the standard clinical work-up does not provide information about the presence and extent of coronary artery disease. In most patients (80%–94%) with an acute coronary syndrome, a significant coronary artery stenosis can be detected with selective coronary angiography. High levels of diagnostic accuracy also have been established for the detection of significant coronary artery stenosis with the use of 16- and 64-section multidetector computed tomography (CT) in patients with stable angina. Preliminary data indicate that multidetector CT also can help quantify and characterize coronary atherosclerotic plaque and that the CT findings are in good agreement with those at intravascular ultrasonography. Although multidetector CT provides accurate information about the presence of coronary artery disease, large blinded observational studies are warranted to identify CT characteristics with high accuracy for diagnosis of acute coronary syndromes. Such information would enable the conduct of randomized controlled trials to determine whether the detection of coronary stenosis and plaque with multidetector CT improves triage and reduces the costs or increases the cost-effectiveness of management of acute chest pain.
© RSNA, 2006
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Introduction
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More than 5 million patients with acute chest pain present to emergency departments in the United States each year (1). Early triage of these patients is important both for prognosis and treatment but remains difficult. Patients who are at highest risk for adverse outcomes derive the greatest benefit from glycoprotein IIb and IIIa inhibitor therapy and early revascularization (2). By contrast, patients at low risk may be discharged without a long-term effect on their risk of death or myocardial infarction (3).
The term acute coronary syndrome describes clinical manifestations of acute myocardial ischemia induced by coronary artery disease. The American Heart Association (AHA) differentiates among acute coronary syndromes that involve myocardial infarction with acute ST segment elevation, myocardial infarction without ST segment elevation, and unstable angina pectoris (4–6). Unfortunately, triage decisions that are based on estimates of acute coronary syndrome risk levels derived from various clinical predictors are often ineffective, especially for patients who have chest pain but normal initial cardiac enzyme levels and normal or nondiagnostic electrocardiograms. The predictive value of single variables such as patient age, sex, presence of risk factors, and biochemical markers for adverse outcomes is limited (4,6–10).
Moreover, the rate of missed diagnosis of acute coronary syndromes, which remains unacceptably high (2%–4%), is associated with a twofold increase in mortality (11–15). This factor contributes to a low threshold for hospital admission of patients with chest pain by emergency department physicians. As a result of the limited ability to make correct triage decisions for patients with acute chest pain, the possibly fatal consequences of a missed acute coronary syndrome, and resultant liability issues (20% of emergency department malpractice dollar losses [11]), more than 2 million patients with acute chest pain are unnecessarily admitted to the hospital (12,14). Because approximately 60% of patients who are eligible for early discharge are instead admitted to the hospital (16), the numbers of potentially unnecessary hospital days per 100 patients are high, ranging from 65 in New Zealand to 839 in Germany (15).
Overall, current management methods do not permit effective emergency department triage of patients with acute chest pain in whom initial troponin levels are not elevated and changes are not evident at electrocardiography (ECG). There is no diagnostic tool available that can provide morphologic information about the presence and severity of coronary artery disease. As a result, the confirmation or exclusion of an acute coronary syndrome, particularly in patients with unstable angina pectoris, requires extensive testing, and this necessity leads to unnecessary hospital admission or, possibly, a delay in necessary treatment (17,18). A quick and noninvasive examination with multidetector CT for the presence and extent of coronary artery disease could substantially improve the clinical care of patients with acute chest pain who present to emergency departments.
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Multidetector CT Technique and Protocols
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With very short examination times of approximately 5 minutes and robust image quality, multidetector cardiac CT constitutes a highly attractive approach for initial work-up in the emergency department setting. In addition, most CT systems now provide easy handling of the large resultant image data sets, including reconstruction of data from different phases of the cardiac cycle as well as further postprocessing. Thus, a diagnostic image evaluation can usually be completed within a few minutes after scanning. However, proper patient preparation and selection are required to guarantee diagnostic image quality.
Patient Selection and Preparation
In a candidate for CT coronary angiography, a history of severe allergic reaction to an iodinated contrast material or of impaired renal function (creatinine level of > 1.5 mg/dL) is considered a contraindication. To ensure diagnostic image quality, patients should have a normal sinus rhythm, and a targeted heart rate of less than 65 beats per minute should be achieved during image acquisition (19,20). The patient’s heart rate should be measured during a breath-holding test to determine whether the administration of a ß-blocker is necessary. The heart rate often decreases by 5–10 beats per minute during the first few seconds of a postinspiration breath hold (A.J.P., unpublished data, June 22, 2005). At our institution, patients receive an intravenous short-acting ß-blocker (eg, 5–20 mg of metoprolol) immediately before the CT examination unless a contraindication such as congestive heart failure, asthma, bradycardia, or atrioventricular block is present. In our experience, approximately 30% of patients with chest pain receive a ß-blocker as part of standard clinical care before undergoing CT.
Standard CT Protocol
The standard CT protocol used at our institution for the evaluation of patients with acute chest pain is similar to previously described protocols for CT coronary angiography. It consists of three steps.
Localization.—
The position of the heart is identified by obtaining a projectional anteroposterior topographic scan of the chest. Typically, the imaging volume should extend from 1–2 cm below the carina to the bottom of the heart.
Determination of Contrast Agent Transit Time.—
A total of 15 mL of the contrast agent, immediately followed by 40 mL of saline (optional), is injected at a flow rate of 5 mL/sec. Scanning is initiated 10 seconds after the start of the contrast medium injection. Axial images are acquired at the level of the aortic root (10-mm collimation) at intervals of 2 seconds and are instantly displayed. Scanning is terminated when sufficient contrast enhancement of the aortic root is detected. The time interval from initiation of the contrast medium injection to the peak opacification of the ascending aorta represents the contrast agent transit time.
Data Acquisition.—
Images are acquired in helical mode during injection of 60–100 mL of the contrast agent, with the exact amount depending on the duration of scanning, followed by 40 mL of saline solution (optional), at a rate of 4–5 mL/ sec. The start of the image acquisition is delayed in accordance with the previously determined contrast agent transit time.
Image Reconstruction
Images are reconstructed by using retrospective electrocardiographic gating and the half-scan reconstruction technique. At heart rates of more than 65 beats per minute, multisegment reconstruction based on information obtained from different detectors during as many as four consecutive heart cycles is recommended (21). However, this algorithm requires a stable heart rate. In most cases, beat-to-beat variation in subsequent heart cycles may cause spatial blurring and impaired image quality. Optimal image quality usually can be achieved in diastole (starting at approximately 65% of the R-R cycle); however, in some cases, the right coronary artery is better visualized around 30% or 80% of the R-R cycle. Images are typically reconstructed with 1-mm section thickness and a 0.5-mm overlap at 16-section multidetector CT and with a 0.75-mm section thickness and 0.4-mm overlap at 64-section multidetector CT. In order to reduce noise, images may sometimes be reconstructed with a larger section thickness. Nearly isotropic resolution (voxel size, 0.4 x 0.4 x 0.6 mm) permits reformatting of images in any arbitrary plane without a significant loss of image information.
Postprocessing
The CT data set of overlapping two-dimensional axial images can be used to detect coronary stenosis.
The raw data may be postprocessed to enable image display and interpretation in various orientations and formats. In this section, typical postprocessing techniques are described.
Multiplanar Reformation.—
This technique is most often used to generate cross-sectional images of coronary arteries or other typical views, such as short-axis or long-axis views. The image plane can be chosen arbitrarily.
Curved Multiplanar Reformation.—
This postprocessing algorithm allows the depiction of the entire course of a coronary artery on a single image. The image plane is adjusted to follow the centerline of the vessel. The resultant display is most useful for depicting the lumen of a coronary artery from its ostium to its distal end.
Maximum Intensity Projection.—
Images can be displayed singly (as individual sections) or stacked (as a summary of several adjacent sections). Stacked display results in loss of detail but improved image contrast and decreased image noise. Maximum intensity projection (MIP) allows visualization of longer lengths of the coronary lumen and has proved more accurate for the detection of significant coronary artery lesions than has multiplanar reformation or three-dimensional (3D) volume rendering (22).
Volume Rendering.—
Volume rendering involves reconstruction of the entire volume of image data and display of the data from a selected viewer orientation. The contributions of each voxel along a line from the viewer’s eye through the data set are summed, pixel by pixel, to obtain a single composite image. This technique is most useful for detecting coronary artery anomalies, and it may be useful also for surgical planning (22,23).
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Radiation Exposure at CT Coronary Angiography
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With use of an ECG-controlled dose modulation technique to reduce the tube current during systole, the effective radiation dose at cardiac CT is 6.7–7.6 mSv in men and 8.1–9.2 mSv in women (24). With the use of 64-section scanners, the radiation dose at cardiac CT is 6.9–11.1 mSv (25). Without tube current modulation, the radiation dose is estimated to be approximately 16–20 mSv. By comparison, a mean effective radiation dose of approximately 5 mSv is incurred at selective coronary angiography.
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Coronary Anatomy
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A thorough understanding of the coronary artery anatomy is a prerequisite for the correct diagnostic interpretation of CT coronary angiograms. By general consensus, the coronary artery tree is divided into 17 segments, according to the AHA system of classification (26,27). The segmental anatomy is shown in Figures 1–4.
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Figure 1a. (a) Axial CT image (0.75-mm section thickness) shows the left main (LM) coronary artery at the level of the ostium. The left main artery arises from the left Valsalva sinus, courses posterior to the right ventricular outflow tract (RVOT), and bifurcates into the left anterior descending (LAD) and the left circumflex (LCX) branches. In about 15% of patients, a separate intermediate branch, or ramus intermedius (RI), also arises from the left main coronary artery. (b) Axial CT image (0.75-mm section thickness) at the midventricular level shows a middle segment of the right coronary artery (RCA) and distal segments of the left anterior descending and left circumflex branches. The latter is seen in the left atrioventricular groove, in close proximity to the great cardiac vein (GCV). (c) Axial MIP image (5-mm section thickness) at the level of the bottom of the heart shows a distal segment of the right coronary artery at the origins of the posterior descending artery (PDA) and the posterior left ventricular (PLV) artery. The posterior descending artery is seen in the posterior longitudinal sulcus, in close proximity to the middle cardiac vein (MCV). A distal segment of the left anterior descending artery also is visible. This case demonstrates right coronary artery dominance in blood supply to the ventricles, a common finding (85%–90% of patients).
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Figure 1b. (a) Axial CT image (0.75-mm section thickness) shows the left main (LM) coronary artery at the level of the ostium. The left main artery arises from the left Valsalva sinus, courses posterior to the right ventricular outflow tract (RVOT), and bifurcates into the left anterior descending (LAD) and the left circumflex (LCX) branches. In about 15% of patients, a separate intermediate branch, or ramus intermedius (RI), also arises from the left main coronary artery. (b) Axial CT image (0.75-mm section thickness) at the midventricular level shows a middle segment of the right coronary artery (RCA) and distal segments of the left anterior descending and left circumflex branches. The latter is seen in the left atrioventricular groove, in close proximity to the great cardiac vein (GCV). (c) Axial MIP image (5-mm section thickness) at the level of the bottom of the heart shows a distal segment of the right coronary artery at the origins of the posterior descending artery (PDA) and the posterior left ventricular (PLV) artery. The posterior descending artery is seen in the posterior longitudinal sulcus, in close proximity to the middle cardiac vein (MCV). A distal segment of the left anterior descending artery also is visible. This case demonstrates right coronary artery dominance in blood supply to the ventricles, a common finding (85%–90% of patients).
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Figure 1c. (a) Axial CT image (0.75-mm section thickness) shows the left main (LM) coronary artery at the level of the ostium. The left main artery arises from the left Valsalva sinus, courses posterior to the right ventricular outflow tract (RVOT), and bifurcates into the left anterior descending (LAD) and the left circumflex (LCX) branches. In about 15% of patients, a separate intermediate branch, or ramus intermedius (RI), also arises from the left main coronary artery. (b) Axial CT image (0.75-mm section thickness) at the midventricular level shows a middle segment of the right coronary artery (RCA) and distal segments of the left anterior descending and left circumflex branches. The latter is seen in the left atrioventricular groove, in close proximity to the great cardiac vein (GCV). (c) Axial MIP image (5-mm section thickness) at the level of the bottom of the heart shows a distal segment of the right coronary artery at the origins of the posterior descending artery (PDA) and the posterior left ventricular (PLV) artery. The posterior descending artery is seen in the posterior longitudinal sulcus, in close proximity to the middle cardiac vein (MCV). A distal segment of the left anterior descending artery also is visible. This case demonstrates right coronary artery dominance in blood supply to the ventricles, a common finding (85%–90% of patients).
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Figure 2a. Right coronary artery anatomy. (a) Schema shows the right coronary artery segments according to the modified 17-segment coronary artery classification system (26). 1 = proximal segment of the main artery, 2 = middle segment of the main artery, 3 = distal segment of the main artery, 4 = posterior descending branch, 16 = posterior left ventricular branch, CB = conal branch, SN = sinonodal branch. (b–d) Multidetector CT images show the right coronary artery (RCA) anatomy with MIP (5-mm section thickness) along the vessel centerline (b), 3D volume rendering from a posterior oblique perspective with partial deletion of the right atrium (RA) and left atrium (LA) (c), and curved multiplanar reformation (d). AM = acute marginal branch, LV = left ventricle, PLV = posterior left ventricular branch, RV = right ventricle. (e) Conventional selective angiogram shows the artery in a right anterior oblique cranial projection.
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Figure 2b. Right coronary artery anatomy. (a) Schema shows the right coronary artery segments according to the modified 17-segment coronary artery classification system (26). 1 = proximal segment of the main artery, 2 = middle segment of the main artery, 3 = distal segment of the main artery, 4 = posterior descending branch, 16 = posterior left ventricular branch, CB = conal branch, SN = sinonodal branch. (b–d) Multidetector CT images show the right coronary artery (RCA) anatomy with MIP (5-mm section thickness) along the vessel centerline (b), 3D volume rendering from a posterior oblique perspective with partial deletion of the right atrium (RA) and left atrium (LA) (c), and curved multiplanar reformation (d). AM = acute marginal branch, LV = left ventricle, PLV = posterior left ventricular branch, RV = right ventricle. (e) Conventional selective angiogram shows the artery in a right anterior oblique cranial projection.
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Figure 2c. Right coronary artery anatomy. (a) Schema shows the right coronary artery segments according to the modified 17-segment coronary artery classification system (26). 1 = proximal segment of the main artery, 2 = middle segment of the main artery, 3 = distal segment of the main artery, 4 = posterior descending branch, 16 = posterior left ventricular branch, CB = conal branch, SN = sinonodal branch. (b–d) Multidetector CT images show the right coronary artery (RCA) anatomy with MIP (5-mm section thickness) along the vessel centerline (b), 3D volume rendering from a posterior oblique perspective with partial deletion of the right atrium (RA) and left atrium (LA) (c), and curved multiplanar reformation (d). AM = acute marginal branch, LV = left ventricle, PLV = posterior left ventricular branch, RV = right ventricle. (e) Conventional selective angiogram shows the artery in a right anterior oblique cranial projection.
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Figure 2d. Right coronary artery anatomy. (a) Schema shows the right coronary artery segments according to the modified 17-segment coronary artery classification system (26). 1 = proximal segment of the main artery, 2 = middle segment of the main artery, 3 = distal segment of the main artery, 4 = posterior descending branch, 16 = posterior left ventricular branch, CB = conal branch, SN = sinonodal branch. (b–d) Multidetector CT images show the right coronary artery (RCA) anatomy with MIP (5-mm section thickness) along the vessel centerline (b), 3D volume rendering from a posterior oblique perspective with partial deletion of the right atrium (RA) and left atrium (LA) (c), and curved multiplanar reformation (d). AM = acute marginal branch, LV = left ventricle, PLV = posterior left ventricular branch, RV = right ventricle. (e) Conventional selective angiogram shows the artery in a right anterior oblique cranial projection.
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Figure 2e. Right coronary artery anatomy. (a) Schema shows the right coronary artery segments according to the modified 17-segment coronary artery classification system (26). 1 = proximal segment of the main artery, 2 = middle segment of the main artery, 3 = distal segment of the main artery, 4 = posterior descending branch, 16 = posterior left ventricular branch, CB = conal branch, SN = sinonodal branch. (b–d) Multidetector CT images show the right coronary artery (RCA) anatomy with MIP (5-mm section thickness) along the vessel centerline (b), 3D volume rendering from a posterior oblique perspective with partial deletion of the right atrium (RA) and left atrium (LA) (c), and curved multiplanar reformation (d). AM = acute marginal branch, LV = left ventricle, PLV = posterior left ventricular branch, RV = right ventricle. (e) Conventional selective angiogram shows the artery in a right anterior oblique cranial projection.
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Figure 3a. Left coronary artery anatomy. (a) Schema shows the left coronary artery segments according to the modified 17-segment coronary artery classification system (26). 5 = main artery, 6 = proximal segment of the left anterior descending (LAD) branch, 7 = middle segment of the LAD branch, 8 = distal segment of the LAD branch, 9 = first diagonal branch, 10 = second diagonal branch, 11 = proximal segment of the left circumflex (LCX) artery, 12 = first obtuse marginal branch of the LCX artery, 13 = middle segment of the LCX artery, 14 = second obtuse marginal branch of the LCX artery, 15 = distal segment of the LCX artery, 17 = intermediate branch. (b–d) Multidetector CT images show the left main (LM) artery and LAD branch with MIP (5-mm section thickness) along the centerline (b), 3D volume rendering (anterior view) (c), and curved multiplanar reformation (d). The arrowhead in c indicates the intermediate branch (segment 17). LA = left atrium, LV = left ventricle, SP = septal perforator branch. (e) Conventional selective angiogram shows the left main artery and LAD branch in a left anteroposterior cranial projection.
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Figure 3b. Left coronary artery anatomy. (a) Schema shows the left coronary artery segments according to the modified 17-segment coronary artery classification system (26). 5 = main artery, 6 = proximal segment of the left anterior descending (LAD) branch, 7 = middle segment of the LAD branch, 8 = distal segment of the LAD branch, 9 = first diagonal branch, 10 = second diagonal branch, 11 = proximal segment of the left circumflex (LCX) artery, 12 = first obtuse marginal branch of the LCX artery, 13 = middle segment of the LCX artery, 14 = second obtuse marginal branch of the LCX artery, 15 = distal segment of the LCX artery, 17 = intermediate branch. (b–d) Multidetector CT images show the left main (LM) artery and LAD branch with MIP (5-mm section thickness) along the centerline (b), 3D volume rendering (anterior view) (c), and curved multiplanar reformation (d). The arrowhead in c indicates the intermediate branch (segment 17). LA = left atrium, LV = left ventricle, SP = septal perforator branch. (e) Conventional selective angiogram shows the left main artery and LAD branch in a left anteroposterior cranial projection.
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Figure 3c. Left coronary artery anatomy. (a) Schema shows the left coronary artery segments according to the modified 17-segment coronary artery classification system (26). 5 = main artery, 6 = proximal segment of the left anterior descending (LAD) branch, 7 = middle segment of the LAD branch, 8 = distal segment of the LAD branch, 9 = first diagonal branch, 10 = second diagonal branch, 11 = proximal segment of the left circumflex (LCX) artery, 12 = first obtuse marginal branch of the LCX artery, 13 = middle segment of the LCX artery, 14 = second obtuse marginal branch of the LCX artery, 15 = distal segment of the LCX artery, 17 = intermediate branch. (b–d) Multidetector CT images show the left main (LM) artery and LAD branch with MIP (5-mm section thickness) along the centerline (b), 3D volume rendering (anterior view) (c), and curved multiplanar reformation (d). The arrowhead in c indicates the intermediate branch (segment 17). LA = left atrium, LV = left ventricle, SP = septal perforator branch. (e) Conventional selective angiogram shows the left main artery and LAD branch in a left anteroposterior cranial projection.
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Figure 3d. Left coronary artery anatomy. (a) Schema shows the left coronary artery segments according to the modified 17-segment coronary artery classification system (26). 5 = main artery, 6 = proximal segment of the left anterior descending (LAD) branch, 7 = middle segment of the LAD branch, 8 = distal segment of the LAD branch, 9 = first diagonal branch, 10 = second diagonal branch, 11 = proximal segment of the left circumflex (LCX) artery, 12 = first obtuse marginal branch of the LCX artery, 13 = middle segment of the LCX artery, 14 = second obtuse marginal branch of the LCX artery, 15 = distal segment of the LCX artery, 17 = intermediate branch. (b–d) Multidetector CT images show the left main (LM) artery and LAD branch with MIP (5-mm section thickness) along the centerline (b), 3D volume rendering (anterior view) (c), and curved multiplanar reformation (d). The arrowhead in c indicates the intermediate branch (segment 17). LA = left atrium, LV = left ventricle, SP = septal perforator branch. (e) Conventional selective angiogram shows the left main artery and LAD branch in a left anteroposterior cranial projection.
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Figure 3e. Left coronary artery anatomy. (a) Schema shows the left coronary artery segments according to the modified 17-segment coronary artery classification system (26). 5 = main artery, 6 = proximal segment of the left anterior descending (LAD) branch, 7 = middle segment of the LAD branch, 8 = distal segment of the LAD branch, 9 = first diagonal branch, 10 = second diagonal branch, 11 = proximal segment of the left circumflex (LCX) artery, 12 = first obtuse marginal branch of the LCX artery, 13 = middle segment of the LCX artery, 14 = second obtuse marginal branch of the LCX artery, 15 = distal segment of the LCX artery, 17 = intermediate branch. (b–d) Multidetector CT images show the left main (LM) artery and LAD branch with MIP (5-mm section thickness) along the centerline (b), 3D volume rendering (anterior view) (c), and curved multiplanar reformation (d). The arrowhead in c indicates the intermediate branch (segment 17). LA = left atrium, LV = left ventricle, SP = septal perforator branch. (e) Conventional selective angiogram shows the left main artery and LAD branch in a left anteroposterior cranial projection.
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Figure 4a. Left circumflex coronary artery segments according to the modified 17-segment classification system (26). (a–c) Multidetector CT images show the left circumflex (LCX) artery with MIP (5-mm section thickness) along the centerline (a), 3D volume rendering with an anterior view (b), and curved multiplanar reformation (c). A = ascending aorta, LA = left atrium, RI = ramus intermedius. 5 = left main artery, 11 = proximal segment of the left circumflex artery, 12 = first obtuse marginal branch of the left circumflex artery, 13 = middle segment of the left circumflex artery. (d) Conventional selective angiogram shows the vessel in a right anterior oblique caudal projection. 14 = second obtuse marginal branch of the left circumflex artery, 15 = distal segment of the left circumflex artery.
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Figure 4b. Left circumflex coronary artery segments according to the modified 17-segment classification system (26). (a–c) Multidetector CT images show the left circumflex (LCX) artery with MIP (5-mm section thickness) along the centerline (a), 3D volume rendering with an anterior view (b), and curved multiplanar reformation (c). A = ascending aorta, LA = left atrium, RI = ramus intermedius. 5 = left main artery, 11 = proximal segment of the left circumflex artery, 12 = first obtuse marginal branch of the left circumflex artery, 13 = middle segment of the left circumflex artery. (d) Conventional selective angiogram shows the vessel in a right anterior oblique caudal projection. 14 = second obtuse marginal branch of the left circumflex artery, 15 = distal segment of the left circumflex artery.
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Figure 4c. Left circumflex coronary artery segments according to the modified 17-segment classification system (26). (a–c) Multidetector CT images show the left circumflex (LCX) artery with MIP (5-mm section thickness) along the centerline (a), 3D volume rendering with an anterior view (b), and curved multiplanar reformation (c). A = ascending aorta, LA = left atrium, RI = ramus intermedius. 5 = left main artery, 11 = proximal segment of the left circumflex artery, 12 = first obtuse marginal branch of the left circumflex artery, 13 = middle segment of the left circumflex artery. (d) Conventional selective angiogram shows the vessel in a right anterior oblique caudal projection. 14 = second obtuse marginal branch of the left circumflex artery, 15 = distal segment of the left circumflex artery.
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Figure 4d. Left circumflex coronary artery segments according to the modified 17-segment classification system (26). (a–c) Multidetector CT images show the left circumflex (LCX) artery with MIP (5-mm section thickness) along the centerline (a), 3D volume rendering with an anterior view (b), and curved multiplanar reformation (c). A = ascending aorta, LA = left atrium, RI = ramus intermedius. 5 = left main artery, 11 = proximal segment of the left circumflex artery, 12 = first obtuse marginal branch of the left circumflex artery, 13 = middle segment of the left circumflex artery. (d) Conventional selective angiogram shows the vessel in a right anterior oblique caudal projection. 14 = second obtuse marginal branch of the left circumflex artery, 15 = distal segment of the left circumflex artery.
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There are a few common pitfalls in the evaluation of the coronary anatomy as depicted on CT angiograms. The shape of the left main coronary artery often appears distorted because the axial image does not represent a true cross section of this segment and the Valsalva sinus is depicted in an oblique plane of section. For this reason, the left anterior descending branch commonly appears to extend upward, above the level of the left main coronary ostium. Coronary veins may overlap and be confused with coronary arteries. For example, the great cardiac vein frequently overlaps the proximal part of the left anterior descending branch, the intermediate branch, and the proximal and middle parts of the left circumflex branch.
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Relevant Findings at Cardiac CT
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Information about the presence and extent of coronary artery disease—information unavailable with the standard clinical evaluation but easily obtainable with a fast and noninvasive CT examination—could substantially improve the clinical care and management of patients with acute chest pain. Data from clinical trials in patients with acute coronary syndromes indicate that the detection of a significant stenosis may help improve risk stratification. Although a significant coronary stenosis cannot necessarily be equated with an acute coronary syndrome, in most patients with unstable angina or non–ST-segment-elevation myocardial infarction (80%–94%) a significant coronary stenosis has been detected at coronary angiography performed during the index hospitalization (27–29). However, the results of observational studies demonstrate that a myocardial infarction also may occur in patients with coronary luminal narrowing of less than 50% (30,31). In addition, hemodynamically nonsignificant lesions with a thin fibrous cap and a large underlying lipid core may be more likely than other lesions to rupture (32,33). The results of several studies with intravascular ultrasonography (US) demonstrated that many coronary atherosclerotic lesions that cause acute events have a distinct morphology that includes a thrombus, a small residual vessel lumen, a greater plaque burden, and more pronounced positive remodeling (34–36). Thus, the detection and characterization of coronary atherosclerotic plaque may aid in the identification of patients at risk for an acute coronary syndrome.
The primary use for coronary multidetector CT may be in patients with an intermediate likelihood of experiencing an acute coronary syndrome. CT might well help decrease the number of unnecessary hospital admissions in this cohort by helping improve early triage and cost-effectiveness. However, CT may have little role in the management of patients with a very low likelihood of acute coronary syndrome, who are discharged directly from the emergency department.
To our knowledge, no systematic analysis of contrast-enhanced coronary CT angiographic patterns of stenosis and plaque in patients with acute chest pain has yet been performed in which the CT findings were compared between patients who experienced myocardial infarction and those who did not. In the next section, we review the currently available data about coronary artery calcification in patients with acute chest pain and the feasibility of contrast-enhanced CT for the detection and characterization of coronary artery stenosis and plaque.
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Diagnostic Value of Coronary Calcification Findings
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Investigators in a few studies have evaluated the utility of electron-beam CT–based detection of coronary artery calcification for predicting the likelihood of acute coronary syndromes in patients with acute chest pain (Table) (37–39). The results of these studies demonstrate a high negative predictive value of the absence of coronary calcifications for acute coronary syndrome. In addition, the characteristics of coronary calcification in patients with stable angina were found to differ from those in patients with unstable angina (35,40–43). However, the diagnostic value of a finding of coronary calcification is controversial. Calcification was present in only approximately 50% of culprit lesions in the coronary arteries of people who experienced sudden death from cardiac causes (44). In a study by Greenland et al (45), 14% of events (myocardial infarction and death) were observed in patients in whom no evidence of coronary calcification was found at CT. Thus, the absence of coronary calcification may not imply the absence of coronary atherosclerotic plaque, especially in young patients. Electron-beam CT therefore may be of limited value for the triage of patients with acute chest pain.
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Detection of Significant Coronary Artery Stenosis
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The results of studies with electron-beam CT scanners and early multidetector CT scanners equipped with four detector rows and a temporal resolution of 250–330 msec demonstrated moderate sensitivity and excellent specificity both for electron-beam CT (82% ± 6.4 and 87% ± 0.6) and for multidetector CT (81% ± 7.2 and 91% ± 0.9) compared with selective coronary angiography. However, the translation of those study results into the clinical routine has been limited because roughly one-third of all coronary artery segments could not be evaluated because of stair-step and motion artifacts and the presence of calcification and were excluded from analysis. The diagnostic accuracy significantly decreased if these coronary segments were included in the statistical calculations for both electron-beam CT and multidetector CT (sensitivity, 68% ± 4.3 and 63% ± 2.9, respectively; specificity, 68% ± 0.2 and 71% ± 0.2, respectively) (46–51). The results of studies with 16- and 64-section CT scanners are promising and indicate that improved image quality achieved through better spatial and temporal resolution leads to a significant decrease in the number of coronary vessel segments with luminal diameters of more than 1.5 mm that cannot be evaluated (7%–12% of coronary segments). Subsequently, sensitivity and specificity increased substantially when all coronary artery segments were considered (84% and 85%, respectively) (52,53).
The high negative predictive value suggests that 64-section multidetector CT can help reliably rule out the presence of hemodynamically significant coronary artery disease (54, 55).
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Detection and Characterization of Noncalcified Plaque
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There is growing evidence that the presence, amount, and composition of noncalcified coronary atherosclerotic plaque and the degree of coronary remodeling can be assessed with multidetector CT with a level of accuracy comparable to that achievable with intravascular US (Figs 5, 6). Sixteen-section multidetector CT allows the identification of calcified plaque, noncalcified plaque, and mixed (calcified and noncalcified) plaque with high sensitivity (92%) and specificity (88%) (56,57) and may enable the further stratification of noncalcified plaques as either fibrous or lipid rich (39,56). Furthermore, the characterization of other features associated with plaque vulnerability, such as positive coronary remodeling (growth of atherosclerotic plaque into the vessel wall rather than the vessel lumen), also is feasible and may be important for the short- and long-term risk stratification of patients with acute chest pain (58).
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Figure 5a. Significant stenosis of the left anterior descending artery in a 67-year-old patient with unstable angina and multiple risk factors (history of premature coronary artery disease, hypercholesterolemia, hypertension) but negative results at testing for biochemical markers and no acute ECG changes. (a) Axial thin-section (5-mm) MIP image from multidetector CT shows a significant luminal narrowing (arrowhead) in the middle segment of the artery. (b) Selective coronary angiogram demonstrates an eccentric high-grade (94%) stenosis (arrowhead). (c, d) A comparison of cross-sectional images obtained with multidetector CT in a proximal reference segment (c) and at the location of significant luminal narrowing (d) shows residual contrast material filling in d, with enhancement of a large eccentric lesion consisting of noncalcified plaque.
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Figure 5b. Significant stenosis of the left anterior descending artery in a 67-year-old patient with unstable angina and multiple risk factors (history of premature coronary artery disease, hypercholesterolemia, hypertension) but negative results at testing for biochemical markers and no acute ECG changes. (a) Axial thin-section (5-mm) MIP image from multidetector CT shows a significant luminal narrowing (arrowhead) in the middle segment of the artery. (b) Selective coronary angiogram demonstrates an eccentric high-grade (94%) stenosis (arrowhead). (c, d) A comparison of cross-sectional images obtained with multidetector CT in a proximal reference segment (c) and at the location of significant luminal narrowing (d) shows residual contrast material filling in d, with enhancement of a large eccentric lesion consisting of noncalcified plaque.
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Figure 5c. Significant stenosis of the left anterior descending artery in a 67-year-old patient with unstable angina and multiple risk factors (history of premature coronary artery disease, hypercholesterolemia, hypertension) but negative results at testing for biochemical markers and no acute ECG changes. (a) Axial thin-section (5-mm) MIP image from multidetector CT shows a significant luminal narrowing (arrowhead) in the middle segment of the artery. (b) Selective coronary angiogram demonstrates an eccentric high-grade (94%) stenosis (arrowhead). (c, d) A comparison of cross-sectional images obtained with multidetector CT in a proximal reference segment (c) and at the location of significant luminal narrowing (d) shows residual contrast material filling in d, with enhancement of a large eccentric lesion consisting of noncalcified plaque.
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Figure 5d. Significant stenosis of the left anterior descending artery in a 67-year-old patient with unstable angina and multiple risk factors (history of premature coronary artery disease, hypercholesterolemia, hypertension) but negative results at testing for biochemical markers and no acute ECG changes. (a) Axial thin-section (5-mm) MIP image from multidetector CT shows a significant luminal narrowing (arrowhead) in the middle segment of the artery. (b) Selective coronary angiogram demonstrates an eccentric high-grade (94%) stenosis (arrowhead). (c, d) A comparison of cross-sectional images obtained with multidetector CT in a proximal reference segment (c) and at the location of significant luminal narrowing (d) shows residual contrast material filling in d, with enhancement of a large eccentric lesion consisting of noncalcified plaque.
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Figure 6a. Multidetector CT images show acute thrombotic occlusion of the left circumflex artery in a patient with a recent myocardial infarction. Although cardiac CT is unlikely to be performed in patients with a confirmed acute myocardial infarction, it can help improve the planning of interventional strategy for some of these patients. (a) Curved multiplanar reformatted image of the left circumflex artery demonstrates a proximal high-grade stenosis (arrowhead) and a thrombotic occlusion of the middle segment (arrow). (b) Cross-sectional multiplanar reformatted image of the left circumflex artery shows the typical CT appearance of a thrombotic occlusion (arrow), with low-attenuation material obstructing the coronary lumen, and positive remodeling of the vessel segment.
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Figure 6b. Multidetector CT images show acute thrombotic occlusion of the left circumflex artery in a patient with a recent myocardial infarction. Although cardiac CT is unlikely to be performed in patients with a confirmed acute myocardial infarction, it can help improve the planning of interventional strategy for some of these patients. (a) Curved multiplanar reformatted image of the left circumflex artery demonstrates a proximal high-grade stenosis (arrowhead) and a thrombotic occlusion of the middle segment (arrow). (b) Cross-sectional multiplanar reformatted image of the left circumflex artery shows the typical CT appearance of a thrombotic occlusion (arrow), with low-attenuation material obstructing the coronary lumen, and positive remodeling of the vessel segment.
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In addition, coronary multidetector CT data sets may be used to assess global and regional left ventricular function (59–61) and to obtain information about myocardial perfusion (62–64). In patients with acute myocardial infarction, cardiac multidetector CT enables detection of the thrombi that typically occur in the presence of left ventricular aneurysms and impaired left ventricular function. It also can demonstrate the success or failure of thrombolysis (Fig 7) and the patency of coronary artery bypass grafts (Fig 8). Multidetector CT has high sensitivity and specificity for the assessment of bypass graft patency and could be used in place of invasive coronary angiography for this purpose. Three-dimensional reconstructions are particularly useful for visualization of the complex anatomy and spatial relations of bypass grafts and native coronary vessels (Fig 9).
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Figure 7a. Success of thrombolysis in a patient with acute myocardial infarction. (a) Coronary angiogram obtained before thrombolysis shows occlusion of the left anterior descending (LAD) artery (arrow) distal to the ostium of the second diagonal branch (2nd diag). 1st diag = first diagonal branch. (b) Volume-rendered image from 16-section multi-detector CT, obtained 24 hours after thrombolysis, shows patency of the distal segment of the left anterior descending artery (arrow). (c) Cross-sectional image of the left ventricle obtained at the same time as b shows a region of low attenuation (white arrows) in the anteroseptal segment of the middle and apical portions of the left ventricular myocardium, a finding indicative of a perfusion deficit due to occlusion of a distal segment of the left anterior descending artery. A large thrombus (arrowhead) is visible in the left ventricular cavity. LA = left atrium, LV = left ventricle, RV = right ventricle.
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Figure 7b. Success of thrombolysis in a patient with acute myocardial infarction. (a) Coronary angiogram obtained before thrombolysis shows occlusion of the left anterior descending (LAD) artery (arrow) distal to the ostium of the second diagonal branch (2nd diag). 1st diag = first diagonal branch. (b) Volume-rendered image from 16-section multi-detector CT, obtained 24 hours after thrombolysis, shows patency of the distal segment of the left anterior descending artery (arrow). (c) Cross-sectional image of the left ventricle obtained at the same time as b shows a region of low attenuation (white arrows) in the anteroseptal segment of the middle and apical portions of the left ventricular myocardium, a finding indicative of a perfusion deficit due to occlusion of a distal segment of the left anterior descending artery. A large thrombus (arrowhead) is visible in the left ventricular cavity. LA = left atrium, LV = left ventricle, RV = right ventricle.
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Figure 7c. Success of thrombolysis in a patient with acute myocardial infarction. (a) Coronary angiogram obtained before thrombolysis shows occlusion of the left anterior descending (LAD) artery (arrow) distal to the ostium of the second diagonal branch (2nd diag). 1st diag = first diagonal branch. (b) Volume-rendered image from 16-section multi-detector CT, obtained 24 hours after thrombolysis, shows patency of the distal segment of the left anterior descending artery (arrow). (c) Cross-sectional image of the left ventricle obtained at the same time as b shows a region of low attenuation (white arrows) in the anteroseptal segment of the middle and apical portions of the left ventricular myocardium, a finding indicative of a perfusion deficit due to occlusion of a distal segment of the left anterior descending artery. A large thrombus (arrowhead) is visible in the left ventricular cavity. LA = left atrium, LV = left ventricle, RV = right ventricle.
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Figure 8a. Multidetector CT evaluation of coronary artery bypass grafts. (a) Three-dimensional volume-rendered image demonstrates the patency of a left internal mammary artery graft (arrows) to the left anterior descending artery and a saphenous vein graft (arrowheads) to the first obtuse marginal branch. (b) Three-dimensional volume-rendered image shows a patent saphenous vein graft (arrows) to the posterior descending coronary artery.
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Figure 8b. Multidetector CT evaluation of coronary artery bypass grafts. (a) Three-dimensional volume-rendered image demonstrates the patency of a left internal mammary artery graft (arrows) to the left anterior descending artery and a saphenous vein graft (arrowheads) to the first obtuse marginal branch. (b) Three-dimensional volume-rendered image shows a patent saphenous vein graft (arrows) to the posterior descending coronary artery.
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Figure 9a. Multidetector CT evaluation for syncope and exertional chest pain. (a) Thin-slab MIP image depicts an anomalous origin of the right coronary artery (arrow) from the left Valsalva sinus as well as narrowing of the proximal segment of the right coronary artery between the main pulmonary artery (MPA) and the ascending aorta (AO). Note the normal origin of the left main coronary artery (arrowhead). (b) Three-dimensional volume-rendered image provides an overview of the coronary anatomy and helps confirm the anomalous origin of the right coronary artery from the left Valsalva sinus (arrow), adjacent to the origin of the left main coronary artery (arrowhead).
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Figure 9b. Multidetector CT evaluation for syncope and exertional chest pain. (a) Thin-slab MIP image depicts an anomalous origin of the right coronary artery (arrow) from the left Valsalva sinus as well as narrowing of the proximal segment of the right coronary artery between the main pulmonary artery (MPA) and the ascending aorta (AO). Note the normal origin of the left main coronary artery (arrowhead). (b) Three-dimensional volume-rendered image provides an overview of the coronary anatomy and helps confirm the anomalous origin of the right coronary artery from the left Valsalva sinus (arrow), adjacent to the origin of the left main coronary artery (arrowhead).
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In patients with atypical chest pain, nonobstructive calcified and noncalcified coronary atherosclerotic plaques may be detected. Such findings may indicate the absence of acute coronary syndrome and may be important for long-term risk stratification (Figs 10, 11).
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Figure 10. Multidetector CT evaluation for atypical chest pain. Thin-slab MIP image depicts two nonstenotic lesions (arrows), both consisting of noncalcified plaque, in the middle segment of the right coronary artery.
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Figure 11a. Multidetector CT evaluation for atypical chest pain. (a) Curved multiplanar reformatted image shows positive remodeling of the left main coronary artery lumen by small lesions consisting of mixed calcified and noncalcified plaque (arrow). (b) Cross-sectional image at the level of the left main coronary artery shows mixed plaque with calcified and noncalcified components (arrow). The lesions do not compromise flow through the coronary artery lumen but are evidence of positive remodeling, which cannot be detected with conventional coronary angiography.
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Figure 11b. Multidetector CT evaluation for atypical chest pain. (a) Curved multiplanar reformatted image shows positive remodeling of the left main coronary artery lumen by small lesions consisting of mixed calcified and noncalcified plaque (arrow). (b) Cross-sectional image at the level of the left main coronary artery shows mixed plaque with calcified and noncalcified components (arrow). The lesions do not compromise flow through the coronary artery lumen but are evidence of positive remodeling, which cannot be detected with conventional coronary angiography.
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Extended CT Protocol
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Expectations are high that the fast volume coverage of 64-section multidetector CT will enable combined imaging of the coronary arteries, ascending aorta, and pulmonary arteries. This would give emergency department physicians a tool to rule out a broader spectrum of potentially life-threatening causes of chest pain, including pulmonary embolism, thoracic aortic dissection, and coronary artery disease. An extended CT protocol is conceptually feasible (Fig 12), but it would involve a significant expansion of the volume coverage and field of view as well as a prolonged administration of the intravenous contrast medium for 25 seconds to ensure adequate contrast enhancement of both the pulmonary and the thoracic aortic vasculature. The actual feasibility of such a protocol would have to be determined by comparing it directly with the individual standard protocols for each indication, before any change in the clinical standard of care would be warranted. ]
Moreover, the implementation of this approach in the clinical setting depends on whether a specific population of patients with chest pain can be identified beforehand and whether this population has a sufficient prevalence of thoracic aortic dissection and pulmonary embolism to justify a probably substantial number of CT examinations.
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Figure 12a. Protocol for combined evaluation of the coronary arteries, thoracic aorta, and pulmonary arteries with a 64-section multidetector CT scanner. (a) Topographic scan of the chest shows extension of the volume coverage and field of view to depict the level just above the aortic arch (red frame) and to include the pulmonary arteries and thoracic aorta in addition to the coronary arteries (green frame). The enlarged field of view is similar to that used for a coronary bypass CT study and may require prolongation of the image acquisition time by 2–4 seconds. (b) Short-and long-axis images show adequate contrast enhancement of the right and left ventricular circulation. The time difference between maximal enhancement in the aorta and that in the main pulmonary artery indicates that a prolongation of contrast material administration to 20–25 seconds is necessary to ensure adequate enhancement of the pulmonary artery tree.
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Figure 12b. Protocol for combined evaluation of the coronary arteries, thoracic aorta, and pulmonary arteries with a 64-section multidetector CT scanner. (a) Topographic scan of the chest shows extension of the volume coverage and field of view to depict the level just above the aortic arch (red frame) and to include the pulmonary arteries and thoracic aorta in addition to the coronary arteries (green frame). The enlarged field of view is similar to that used for a coronary bypass CT study and may require prolongation of the image acquisition time by 2–4 seconds. (b) Short-and long-axis images show adequate contrast enhancement of the right and left ventricular circulation. The time difference between maximal enhancement in the aorta and that in the main pulmonary artery indicates that a prolongation of contrast material administration to 20–25 seconds is necessary to ensure adequate enhancement of the pulmonary artery tree.
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For now, the concept of an extended CT protocol for evaluation of patients with acute chest pain is still in the preliminary stages of development.
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Summary
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The current triage of emergency department patients with acute chest pain is ineffective, leading to many unnecessary hospital admissions. It is conceivable that fast and noninvasive detection of the presence and extent of coronary artery disease with multidetector CT, which provides information that cannot be obtained with the standard clinical evaluation, may help substantially improve the clinical care of such patients. In particular, patients with typical unstable angina but non-specific ECG changes (ST segment depression of less than 1 mm, T-wave inversion) and negative results at initial testing of biochemical markers for myocardial ischemia may benefit from early detection of significant coronary artery stenosis at multidetector CT and receive appropriate treatment (thrombolysis, percutaneous transluminal coronary angioplasty, or bypass surgery) more promptly.
The accurate detection of significant stenosis and the presence and characteristics of atherosclerotic plaque may be important criteria for improved risk stratification among these patients. However, an analysis of CT angiographic patterns of stenosis and plaque in patients with acute chest pain and of the relationship of these patterns to clinical outcomes in patients with versus without myocardial infarction is warranted. As the use of multidetector CT increases in the imaging of patients with acute chest pain, it should become easier to relate the anatomic findings to the presence or absence of an acute coronary syndrome. If it is determined that particular multidetector CT findings are highly predictive of an acute coronary syndrome, then large prospective randomized diagnostic trials will be necessary to assess whether coronary CT helps improve early triage of patients with acute chest pain and is cost effective when compared with standard clinical care. While 64-section multidetector CT has the potential to image the coronary arteries, aorta, and pulmonary arteries in a single scan, further studies are necessary to test the accuracy of this protocol.
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Footnotes
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Abbreviations: ECG = electrocardiography, MIP = maximum intensity projection, 3D = three-dimensional
See the commentary by White following this article.
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Abstract
Introduction
