Francesca Pugliese, MD

Coronary artery calcium measurement with cardiac CT has been used for cardiovascular risk assessment since 1990.

 The American College of Cardiology (ACC) and American Heart Association (AHA) describe calcium scoring as an independent predictor of coronary events in asymptomatic subjects. 

Coronary artery calcium measurements can improve risk stratification by reclassifying asymptomatic individuals considered to be at intermediate risk of coronary artery disease (i.e., they have a 10% to 20% risk of a coronary event during the next 10 years) into higher or lower risk categories. Patient management can then be modified as necessary; for example, through encouragement to adopt a healthier lifestyle or introduction of medical therapy.

Recommendations published by the European Society of Cardiac Radiology and the North American Society for Cardiovascular Imaging note that a calcium score of zero will exclude most clinically relevant coronary artery disease in asymptomatic individuals.

 A zero score is associated with a low chance of relevant (≥50% reduction in diameter) coronary artery stenosis in symptomatic patients, such as those with chest pain. This is not always the case, however, because soft plaques could be causing the symptoms. Coronary CT angiography should not be ruled out in these patients.

A high coronary calcium score-for example, greater than 400 in a 50-yearold man-is associated with a very high (>90%) probability of relevant coronary artery stenosis. Immediate conventional coronary angiography should be considered in such cases.

Several factors limit the use of calcium scoring in clinical practice. Coronary calcium is a sensitive indicator of coronary atherosclerosis, but it is not specific for the detection of relevant coronary artery stenosis (91.8% sensitivity, 55% specificity).

 The presence of calcium in a plaque also cannot be considered as an indicator of that plaque's biological behavior: for instance, its clinical stability or vulnerability. Many studies have additionally reported wide differences among ethnic groups in terms of the prevalence of coronary artery calcium. The clinical implications of coronary calcium scores should consequently be interpreted with caution.

Studies reporting the clinical usefulness of calcium scoring are based mainly on electron-beam CT (EBCT). This modality has traditionally generated the most reproducible coronary calcium measurements. The growing availability of multislice CT in radiology departments means that this modality is increasingly replacing EBCT for the evaluation of coronary heart disease. Published reports now show that MSCT is comparable to EBCT for coronary calcium measurements.

 MSCT data acquisition techniques are not standardized, however.

The accuracy and reproducibility of calcium measurements made with MSCT need to be guaranteed. ACC/ AHA training guidelines have established that calcium scoring should be a formal part of cardiac MSCT training programs.

Care must still be taken to avoid some of the pitfalls that can alter  the accuracy of coronary calcium scoring with MSCT.

We perform MSCT coronary calcium scoring with a 64-slice scanner (Sensation 64, Siemens Medical Solutions). Details of the protocol and parameters are summarized in the table (above). The parameters meet minimum requirements for coronary artery calcium scanning proposed by the AHA (Figure 1).

Beta blockade is mandatory in patients with heart rates higher than 65 to 70 bpm to minimize the effects of cardiac motion. If the prospective gating technique (step-and-shoot) is used, data reconstruction in an early diastolic phase (40% of R-R interval) could be helpful. It can reduce interscan variability in calcium score comparing to the phase of 80% of the R-R interval, commonly used in EBCT.

Mild abnormalities to the heart rate that occur during scanning, such as premature beats, can be compensated for manually using a spiral technique with retrospective ECG gating. Retrospective ECG gating is associated with higher radiation exposure, however, and x-ray tube modulation should be used to lower this.

Analysis of multiple reconstructions (i.e., different phases of the R-R interval) can improve the accuracy of coronary calcium measurements.

 We have found that mid- to end-diastolic reconstruction (60% to 70%) is appropriate in patients with a low heart rate (=65 bpm), whereas systolic reconstruction is preferred in patients with a high heart rate (=80 bpm).

The Agatston method of calcium scoring is based on voxels containing calcium deposits that have an area of at least 1 mm2 and a density greater than 130 Hounsfield units. Alternatives include mass scoring and volume scoring. The mass score is based on the integration of pixels above a given threshold and is independent of slice thickness and spatial resolution.

 The volume score is the sum of the calcified area multiplied by the image increment.

 The Agatston method of calcium scoring is more variable than the volume method.

The Agatston score, volume score, and mass score are all calculated with standard algorithms. Calculations are dependent on the type of scanner and patient size. Standardized measurement protocols and algorithms have been proposed to ensure the accuracy of data, but implementation of these standards into clinical practice is difficult.

Motion is the major artifact in MSCT cardiac imaging. Coronary vessels can be blurred or duplicated (Figure 2). Motion artifacts can lead to calcium scores being overestimated and decrease the reproducibility of measurements. Blurring secondary to cardiac motion increases in systems with slower acquisition speeds. The AHA recommends that gantry rotation take no longer than 500 msec.

Poor spatial resolution and an insufficient contrastto- noise ratio may affect image quality to such an extent that small coronary branches are overlooked. The total calcium score may then be underestimated. Partial volume effects can lead to higher peak density values for small lesions. The AHA recommends that imaging be performed on four-slice (or higher) MSCT scanners to maintain adequate spatial and temporal resolution. The minimum slice thickness should be 2.5 to 3 mm to minimize radiation exposure in asymptomatic patients.

The field-of-view can also have an impact on the calcium score. The typical matrix size of CT images is 512 x 512, so different FOVs can result in a different pixel size. Investigators have found no statistically significant difference between a 210-mm FOV and a 260-mm FOV.

We use a small FOV (186 to 200 mm) to achieve good spatial resolution.

Image noise predominates at the diaphragmatic surface of the heart if MSCT coronary calcium scans are performed in the craniocaudal direction.

 Noise levels can sometimes exceed the default threshold for automatic calcium detection (130 HU). Automatic delineation of calcium deposits is inadequate in such cases (Figure 3). Image noise can be minimized and unnecessary x-ray exposure avoided by adjusting the x-ray tube current to the patient's size and using appropriate calibration factors.

Extra-arterial cardiac calcifications, such as calcified cardiac valves, left ventricular aneurysms and thrombi, pericardial calcifications, and calcified mediastinal lymph nodes should be excluded from the analysis (

). Radiopaque catheters, artificial valves, and pacemakers can produce streak artifacts (Figure 5). Good knowledge of coronary artery anatomy is mandatory. The lack of contrast enhancement makes it more difficult to recognize coronary vessels and other cardiac structures on coronary calcium images than on CT coronary angiography images.

Articles

Good technique preventscalcium scoring pitfalls

Acute coronary syndromes include ST-elevation myocardial infarction (STEMI) and unstable coronary artery disease. The latter includes non-ST-elevation myocardial infarction (NSTEMI) and unstable angina. 

Persistent ST-segment elevation generally reflects transmural ischemia by total coronary occlusion and characterizes evolving myocardial infarction (STEMI). This occurs in a relatively small population of patients with acute coronary syndrome. In these settings, the therapeutic objective is either primary angioplasty or fibrinolytic treatment to achieve rapid and complete recanalization.

Patients with NSTEMI or unstable angina present with chest pain associated with ST-depression, T-wave inversion, flat T waves, or nonspecific ECG changes; a normal ECG at presentation can also be the case. NSTEMI is distinguished from unstable angina by an elevation in serum biochemical markers of myocardial necrosis (troponin T or I). However, the elevation of troponin levels may also be caused by either transient occlusion or microembolization secondary to spontaneous thrombolysis (small infarcts), and hence in the absence of an epicardial coronary artery occlusion amenable to percutaneous revascularization.

Patients with unstable acute coronary syndrome are at risk for myocardial infarction, MI recurrence, or death. Diagnosis and risk stratification are pursued together: During the process of establishing the acute coronary syndrome diagnosis, risk is repeatedly assessed and used as guidance for choosing the best therapeutic strategy. According to the European Guidelines,2 risk stratification of patients with unstable angina/NSTEMI involves integrating relevant information from their histories; e.g., chest pain pattern, physical examination, ECG, and biochemical cardiac markers. 

Several clinical decision rules are available for risk stratification: the thrombosis in myocardial infarction (TIMI) score,

 the global registry of acute coronary events (GRACE) score, the fast revascularization in instability in coronary disease (FRISC) score,

 and the platelet glycoprotein IIb/IIIa in unstable angina receptor suppression using Integrilin (PURSUIT) score.

 According to these algorithms, patients can be assigned to low-, intermediate-, or high-risk categories. The presence of thrombus and extent of underlying coronary artery disease (CAD), in particular troponin elevation and signs and symptoms of ongoing myocardial ischemia, can modify a patient's risk category.

Optimal early management of patients with non-ST-elevation acute coronary syndrome is still being debated. Several large randomized trials

 compared a routine invasive strategy (i.e., early angiography possibly followed by revascularization) with a more conservative strategy (i.e., angiography and revascularization only if medical therapy failed or residual ischemia was documented).

 The early invasive strategy was found to be superior to the conservative strategy in reducing major cardiovascular events as well as severe angina and rehospitalization. In particular, RITA-3 (randomized intervention trial of unstable angina)

 was the first study reporting improved survival after early invasive intervention. 

 of 1200 patients with non-ST-elevation acute coronary syndrome demonstrated that if patients were optimally treated before, during, and after revascularization, early cardiac catheterization was not better than a more selective invasive strategy in terms of mortality and recurrence of acute myocardial infarction. These investigators reported that the restricted use of conventional angiography only in patients experiencing refractory angina or clinically significant ischemia on predischarge exercise tests saved 47% of catheterization procedures without any detrimental effect on long-term follow-up.

 A large number of these procedures were merely diagnostic and not followed by revascularization. Results were confirmed at four-year follow-up,

suggesting that only very high risk patients should undergo early invasive management. Medical therapy might be tailored to the patient's risk profile.

 compared multislice CT coronary angiography and conventional angiography for the detection of >50% coronary obstructions in 104 patients with NSTEMI. At a patient level, both sensitivity and negative predictive value of MSCT were 100%, with no difference between low-risk and high-risk patients. 

Dorgelo et al16 showed that MSCT could effectively be substituted for diagnostic conventional angiography in approximately one-third of patients with unstable CAD. It could be argued that a major limitation of MSCT is its inability to provide information on the hemodynamic significance of moderate (50% to 70%) lesions, which might lead to unnecessary catheterizations. However, these data showed that MSCT can rule out the diagnosis of acute coronary syndrome and has the potential to reduce hospital admissions.

Although only a minority of patients classified as low risk actually suffer from myocardial ischemia,

 it has been reported that up to 8% of patients who actually have acute coronary syndrome are inappropriately discharged home.

 These patients have increased morbidity and mortality. Because the consequences of missing the diagnosis are so severe, it has been standard practice to evaluate all patients with suspected acute coronary syndrome (including low-risk patients) with serial ECG and cardiac enzymes, followed by a rest and/or stress-imaging study. Such an approach is time-consuming and costly. 

Goldstein et al20 randomized 197 low-risk chest pain patients to either ECG-gated MSCT coronary angiography or standard diagnostic evaluation. In the MSCT group, 67% of patients had minimal CAD and were discharged immediately; 8% had at least one stenosis >70% and were referred for catheterization. The MSCT scan alone could immediately identify or exclude coronary disease as the cause of chest pain in 75% of patients. The remaining patients had intermediate lesions on MSCT or nondiagnostic scans, thus underwent stress testing. Stress testing was negative in 20% of patients, who were discharged directly from the emergency department during the index visit; importantly, none of these patients suffered from major acute cardiac events over the six-month follow-up period. In patients who required further testing, the MSCT strategy ultimately arrived at a correct diagnosis in 95% of cases.

 prospectively studied 58 patients with chest pain of uncertain origin. These authors reported 100% sensitivity, 92% specificity, and 100% negative predictive value for MSCT compared with standard diagnostic tests (i.e., new biomarker elevation, abnormal myocardial perfusion scintigraphy, and/or invasive angiography) in the diagnosis of acute coronary syndrome. At 15-month follow-up, sensitivity of MSCT for predicting major cardiovascular events was 92%, specificity 76%, and negative predictive value 97%.21 

The potential to visualize the coronary artery wall and several characteristics of atherosclerotic plaques noninvasively, rather than with such invasive techniques as intravascular ultrasound (IVUS), has raised interest in plaque imaging to define patient risk.

Noninvasive identification of the vulnerable plaque (i.e., the plaque that will rupture and cause an acute event) is probably an elusive goal with current imaging technology. Nevertheless, MSCT can detect obstructive/nonobstructive lesions, evaluate location/distribution of CAD, and estimate left ventricular function. MSCT might allow a patient to be reclassified from the low-risk category to a higher risk category when three-vessel disease, involvement of the left main stem, or impaired LV function is seen. 

 that mixed plaques were independent predictors of acute events in patients with CAD. Mixed plaques may contain a necrotic component (necrotic core) and be associated with higher risk of rupture than densely calcified plaques. The detection and differentiation of noncalcified plaque components from calcified plaque components is feasible with MSCT; the distinction between necrotic areas and fibrous areas, conversely, is more challenging. Nevertheless, some studies have explored the ability of MSCT coronary angiography to differentiate large necrotic cores from fibrous lesions by measuring the CT attenuation values within the plaques.

 Although these studies report significant variation of attenuation values within plaques of the same type, plaques with a necrotic core demonstrated by IVUS consistently showed lower CT attenuation values than fibrous plaques (Figure 1).

Bugiardini et al26 evaluated a large cohort of non-ST-elevation acute coronary syndrome patients drawn from several international multicenter trials.

 This study showed that most of the patients classified as low risk had nonobstructive CAD or normal coronary arteries. Importantly, the presence of nonobstructive CAD detected by conventional angiography was associated with a higher risk of events (one-year follow-up) than that associated with normal coronary angiograms. 

 performed MSCT in 103 chest pain patients presenting at the emergency department with no ischemic ECG changes and negative initial biomarkers. This study found that the absence of coronary artery plaque or obstructions at MSCT had a high negative predictive value for the subsequent diagnosis of acute coronary syndrome. Moreover, the presence of any plaque or coronary obstruction provided information incremental to conventional risk factors in predicting the risk of acute coronary syndrome.

 reported on the prognostic value of MSCT coronary angiography in symptomatic patients with known or suspected CAD and a high prevalence of conventional risk factors. At 16 months, a 100% event-free survival was noted in patients with normal MSCT coronary angiography. Patients with coronary atherosclerosis identified on MSCT had a worse prognosis. Although the risk of events was the highest in patients with obstructive CAD, the risk associated with nonobstructive CAD (Figure 2) was higher than that associated with a normal MSCT study. 

The study by Bugiardini shows that low-risk patients might be a heterogeneous group and risk stratification should be improved in these patients.

All three studies support the concept that detecting the presence of CAD may be used for improving risk definition within the low-risk patient category.

More hard data are needed to define the clinical and prognostic value of specific MSCT findings in patients with acute coronary syndrome. Large trials will have to prove the clinical meaning of mixed/noncalcified plaques, plaque location, and plaque distribution. In spite of tremendous technical developments, the resolution of MSCT is still far from what is needed to visualize individual structures of the atheroma such as the fibrous cap, which is regarded as the pathological marker of vulnerable plaques. The reproducibility and inter-reader agreement in MSCT coronary plaque imaging will also have to be tested in the real world.

Nevertheless, the available data consistently support the excellent negative predictive value of a normal MSCT in patients with suspected acute coronary syndrome. Using these data may consistently reduce the number of unnecessary hospital admissions. The finding that not only obstructive lesions but also nonobstructive CAD and mixed plaques are associated with a worse prognosis,

 is in keeping with the notion that many patients are entirely asymptomatic up to their first acute event and that plaque composition, in addition to stenosis severity, contributes to the occurrence of events.

Although the routine use of MSCT in patients with suspected acute coronary syndrome is not supported yet by practice guidelines, the concepts discussed here lay the foundation for use of MSCT in these patients. MSCT might find a role in the diagnostic algorithm of low-risk patients with non-ST-elevation acute coronary syndromes to rule out obstructive CAD.

DR. PUGLIESE is a radiologist, and PROF. KRESTIN is the radiology chair, both at Erasmus MC University Medical Centre Rotterdam, the Netherlands.

References
1. Bertrand ME, Simoons ML, Fox KA, et al. Management of acute coronary syndromes in patients presenting without persistent ST-segment elevation. Eur Heart J 2002;23(23):1809-1840.
2. Bassand JP, Hamm CW, Ardissino D, et al. Guidelines for the diagnosis and treatment of non-ST-segment elevation acute coronary syndromes: The Task Force for the Diagnosis and Treatment of Non-ST-Segment Elevation Acute Coronary Syndromes of the European Society of Cardiology. Eur Heart J 2007;28(13):1598-1660.
3. Antman EM, Cohen M, Bernink PJ, et al. The TIMI risk score for unstable angina/non-ST elevation MI: A method for prognostication and therapeutic decision making. JAMA 2000;284(7):835-842.
4. Eagle KA, Lim MJ, Dabbous OH, et al. A validated prediction model for all forms of acute coronary syndrome: estimating the risk of 6-month postdischarge death in an international registry. JAMA 2004;291(22):2727-2733.
5. Lagerqvist B, Diderholm E, Lindahl B, et al. FRISC score for selection of patients for an early invasive treatment stra¬¬tegy in unstable coronary artery disease. Heart 2005;91(8):1047-1052.
6. Boersma E, Pieper KS, Steyerberg EW, et al. Predictors of outcome in patients with acute coronary syndromes without persistent ST-segment elevation. Results from an international trial of 9461 patients. The PURSUIT Investigators. Circulation 2000;101(22):2557-2567.
7. Invasive compared with non-invasive treatment in unstable coronary-artery disease: FRISC II prospective randomised multicentre study. FRagmin and Fast Revascularisation during InStability in Coronary artery disease Investigators. Lancet 1999;354(9180):708-715.
8. Cannon CP, Weintraub WS, Demopoulos LA, et al. Comparison of early invasive and conservative strategies in patients with unstable coronary syndromes treated with the glycoprotein IIb/IIIa inhibitor tirofiban. NEJM 2001;344(25):1879-1887.
9. Fox KA, Poole-Wilson PA, Henderson RA, et al. Interventional versus conservative treatment for patients with unstable angina or non-ST-elevation myocardial infarction: the British Heart Foundation RITA 3 randomised trial. Randomized Intervention Trial of unstable Angina. Lancet 2002;360(9335):743-751.
10. Mehta SR, Cannon CP, Fox KA, et al. Routine vs selective invasive strategies in patients with acute coronary syndromes: a collaborative meta-analysis of randomized trials. JAMA 2005;293(23):2908-2917.
11. de Winter RJ, Windhausen F, Cornel JH, et al. Early invasive versus selectively invasive management for acute coronary syndromes. NEJM 2005;353(11):1095-1104.
12. Hirsch A, Windhausen F, Tijssen JG, et al. Long-term outcome after an early invasive versus selective invasive treatment strategy in patients with non-ST-elevation acute coronary syndrome and elevated cardiac troponin T (the ICTUS trial): a follow-up study. Lancet 2007;369(9564):827-835.
13. Calvin JE, Klein LW, VandenBerg EJ, et al. Validated risk stratification model accurately predicts low risk in patients with unstable angina. J Am Coll Cardiol 2000;36(6):1803-1808.
14. Cohen M, Demers C, Gurfinkel EP, et al. A comparison of low-molecular-weight heparin with unfractionated heparin for unstable coronary artery disease. Efficacy and Safety of Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events Study Group. NEJM 1997;337(7):447-452.
15. Meijboom WB, Mollet NR, van Mieghem CA, et al. 64-slice computed tomography coronary angiography in patients with non-ST elevation acute coronary syndrome. Heart 2007 (doi: 10.1136/hrt.2006.112771. Available online at 

http://heart.bmj.com/cgi/content/abstract/hrt.2006.112771v1

).
16. Dorgelo J, Willems TP, Geluk CA, et al. Multidetector computed tomography-guided treatment strategy in patients with non-ST elevation acute coronary syndromes: a pilot study. Eur Radiol 2005;15(4):708-713.
17. Reilly BM, Evans AT, Schaider JJ, et al. Impact of a clinical decision rule on hospital triage of patients with suspected acute cardiac ischemia in the emergency department. JAMA 2002;288(3):342-350.
18. McCarthy BD, Beshansky JR, D'Agostino RB, Selker HP. Missed diagnoses of acute myocardial infarction in the emergency department: results from a multicenter study. Ann Emerg Med 1993;22(3):579-582.
19. Pope JH, Aufderheide TP, Ruthazer R, et al. Missed diagnoses of acute cardiac ischemia in the emergency department. NEJM 2000;342(16):1163-1170.
20. Goldstein JA, Gallagher MJ, O'Neill WW, et al. A randomized controlled trial of multi-slice coronary computed tomography for evaluation of acute chest pain. J Am Coll Cardiol 2007;49(8):863-871.
21. Rubinshtein R, Halon DA, Gaspar T, et al. Usefulness of 64-slice cardiac computed tomographic angiography for diagnosing acute coronary syndromes and predicting clinical outcome in emergency department patients with chest pain of uncertain origin. Circulation 2007;115(13):1762-1768.
22. Pundziute G, Schuijf JD, Jukema JW, et al. Prognostic value of multislice computed tomography coronary angiography in patients with known or suspected coronary artery disease. J Am Coll Cardiol 2007;49(1):62-70.
23. Schroeder S, Kopp AF, Baumbach A, et al. Noninvasive detection and evaluation of atherosclerotic coronary plaques with multislice computed tomography. J Am Coll Cardiol 2001;37(5):1430-1435.
24. Schroeder S, Kuettner A, Leitritz M, et al. Reliability of differentiating human coronary plaque morphology using contrast-enhanced multislice spiral computed tomography: a comparison with histology. JCAT 2004;28(4):449-454.
25. Leber AW, Knez A, Becker A, et al. Accuracy of multidetector spiral computed tomography in identifying and differentiating the composition of coronary atherosclerotic plaques: a comparative study with intracoronary ultrasound. J Am Coll Cardiol 2004;43(7):1241-1247.
26. Bugiardini R, Manfrini O, De Ferrari GM. Unanswered questions for management of acute coronary syndrome: risk stratification of patients with minimal disease or normal findings on coronary angiography. Arch Intern Med 2006;166(13):1391-1395.
27. Antman EM, McCabe CH, Gurfinkel EP, et al. Enoxaparin prevents death and cardiac ischemic events in unstable angina/non-Q-wave myocardial infarction. Results of the thrombolysis in myocardial infarction (TIMI) 11B trial. Circulation 1999;100(15):1593-1601.
28. Cannon CP, Braunwald E, McCabe CH, et al. Intensive versus moderate lipid lowering with statins after acute coronary syndromes. NEJM 2004;350(15):1495-1504.
29. Cannon CP, McCabe CH, Wilcox RG, et al. Oral glycoprotein IIb/IIIa inhibition with orbofiban in patients with unstable coronary syndromes (OPUS-TIMI 16) trial. Circulation 2000;102(2):149-156.
30. Lindahl B, Venge P, Wallentin L. Relation between troponin T and the risk of subsequent cardiac events in unstable coronary artery disease. The FRISC study group. Circulation 1996;93(9):1651-1657.
31. Morrow DA, Cannon CP, Rifai N, et al. Ability of minor elevations of troponins I and T to predict benefit from an early invasive strategy in patients with unstable angina and non-ST elevation myocardial infarction: results from a randomized trial. JAMA 2001;286(19):2405-2412.
32. Schwartz GG, Olsson AG, Ezekowitz MD, et al. Effects of atorvastatin on early recurrent ischemic events in acute coronary syndromes: the MIRACL study: a randomized controlled trial. JAMA 2001;285(13):1711-1718.
33. Yusuf S, Zhao F, Mehta SR, et al. Effects of clopidogrel in addition to aspirin in patients with acute coronary syndromes without ST-segment elevation. NEJM 2001;345(7):494-502.
34. Hoffmann U, Nagurney JT, Moselewski F, et al. Coronary multidetector computed tomography in the assessment of patients with acute chest pain. Circulation 2006;114(21):2251-2260.

CTA seeks niche triage role in unstable angina patients

Coronary CT angiography has been found to be a powerful tool to evaluate patients suspected of coronary artery disease who are at low to intermediate risk. The task now for many researchers is to further refine specific patient populations for whom coronary CTA can be beneficial. One such patient niche being explored is the group with unstable angina and/or non-ST-segment elevation myocardial infarction.

Persistent ST-segment elevation generally reflects transmural ischemia by total coronary occlusion and characterizes evolving myocardial infarction (STEMI). This occurs in a relatively small population of patients with acute coronary syndrome. In these settings, the therapeutic objective is either primary angioplasty or fibrinolytic treatment to achieve rapid and complete recanalization (Figure 1).1

Patients with non-ST-segment elevation myocardial infarction (NSTEMI) or unstable angina present with chest pain associated with ST-depression, T-wave inversion, flat T waves, or nonspecific ECG changes; a normal ECG at presentation can also be the case.

NSTEMI is distinguished from unstable angina by an elevation in serum biochemical markers of myocardial necrosis (troponin T or I). However, the elevation of troponin levels may also be caused by either transient occlusion or microembolization secondary to spontaneous thrombolysis (small infarcts), and hence in the absence of an epicardial coronary artery occlusion, it may be amenable to percutaneous revascularization.

Patients with unstable acute coronary syndrome are at risk for myocardial infarction, MI recurrence, or death. Diagnosis and risk stratification are pursued together: During the process of establishing the acute coronary syndrome diagnosis, risk is repeatedly assessed and used as guidance for choosing the best therapeutic strategy. According to the European Guidelines,2 risk stratification of patients with unstable angina/NSTEMI involves integrating relevant information from their histories; e.g., chest pain pattern, physical examination, ECG, and biochemical cardiac markers.

Several clinical decision rules are available for risk stratification.3-6 According to these algorithms, patients can be assigned to low-, intermediate-, or high-risk categories. The presence of thrombus and extent of underlying coronary artery disease (CAD), in particular troponin elevation and signs and symptoms of ongoing myocardial ischemia, can modify a patient's risk category.

Optimal early management of patients with non-ST-elevation acute coronary syndrome is still being debated. Several large randomized trials7-9 and a meta-analysis compared a routine invasive strategy (i.e., early angiography possibly followed by revascularization) with a more conservative strategy (i.e., angiography and revascularization only if medical therapy failed or residual ischemia was documented).10 The early invasive strategy was found to be superior to the conservative strategy in reducing major cardiovascular events as well as severe angina and rehospitalization. RITA-3 (randomized intervention trial of unstable angina)9 was the first study reporting improved survival after early invasive intervention.

Another study11 of 1200 patients with non-ST-elevation acute coronary syndrome demonstrated that if patients were optimally treated before, during, and after revascularization, early cardiac catheterization was not better than a more selective invasive strategy in terms of mortality and recurrence of acute myocardial infarction.

These investigators reported that the restricted use of conventional angiography only in patients experiencing refractory angina or clinically significant ischemia on predischarge exercise tests saved 47% of catheterization procedures without any detrimental effect on long-term follow-up.11 Indeed, a large number of these procedures were merely diagnostic and not followed by revascularization. Results were confirmed at four-year follow-up,12 suggesting that only very high risk patients should undergo early invasive management. Medical therapy might also be tailored to the patient's risk profile.3,6,13,14

Meijboom et al15 compared MSCT coronary angiography and conventional angiography for the detection of > 50% coronary obstructions in 104 patients with NSTEMI. At a patient level, both sensitivity and negative predictive value of MSCT were 100%, with no difference between low-risk and high-risk patients. Another study, by Dorgelo et al,16 showed that MSCT could effectively be substituted for diagnostic conventional angiography in approximately one-third of patients with unstable CAD.

It could be argued that a major limitation of MSCT is its inability to provide information on the hemodynamic significance of moderate (50% to 70%) lesions, which might lead to unnecessary catheterizations. However, these data showed that MSCT can rule out the diagnosis of acute coronary syndrome and has the potential to reduce hospital admissions.

Although only a minority of patients classified as low risk actually suffer from myocardial ischemia,17 it has been reported that up to 8% of patients who actually have acute coronary syndrome are inappropriately discharged home.18,19 These patients have increased morbidity and mortality. Because the consequences of missing the diagnosis are so severe, it has been standard practice to evaluate all patients with suspected acute coronary syndrome (including low-risk patients) with serial ECG and cardiac enzymes, followed by a rest and/or stress-imaging study. Such an approach is time-consuming and costly.

Goldstein et al20 randomized 197 low-risk chest pain patients to either ECG-gated MSCT coronary angiography or standard diagnostic evaluation. In the MSCT group, 67% of patients had minimal CAD and were discharged immediately; 8% had at least one stenosis > 70% and were referred for catheterization.

The MSCT scan alone could immediately identify or exclude coronary disease as the cause of chest pain in 75% of patients. The remaining patients had intermediate lesions on MSCT or nondiagnostic scans and thus underwent stress testing. Stress testing was negative in 20% of patients, who were discharged directly from the emergency department during the index visit; importantly, none of these patients suffered from major acute cardiac events over the six-month follow-up period. In patients who required further testing, the MSCT strategy ultimately arrived at a correct diagnosis in 95% of cases.

Rubinshtein et al21 prospectively studied 58 patients with chest pain of uncertain origin. These authors reported 100% sensitivity, 92% specificity, and 100% negative predictive value for MSCT compared with standard diagnostic tests (i.e., new biomarker elevation, abnormal myocardial perfusion scintigraphy, and/or invasive angiography) in the diagnosis of acute coronary syndrome. At 15-month follow-up, sensitivity of MSCT for predicting major cardiovascular events was 92%, specificity 76%, and negative predictive value 97%.21

The potential to visualize the coronary artery wall and several characteristics of atherosclerotic plaques noninvasively, rather than with such invasive techniques as intravascular ultrasound, has raised interest in plaque imaging to define patient risk.

Noninvasive identification of the vulnerable plaque; (i.e., plaque that will rupture and cause an acute event) is probably an elusive goal with current imaging technology. Nevertheless, MSCT can detect obstructive/nonobstructive lesions, evaluate location/distribution of CAD, and estimate left ventricular function. For instance, MSCT might allow a patient to be reclassified from the low-risk category to a higher risk category when three-vessel disease, involvement of the left main stem, or impaired LV function is seen.

It was recently shown22 that mixed plaques were independent predictors of acute events in patients with CAD. Mixed plaques may contain a necrotic component (necrotic core) and be associated with higher risk of rupture than densely calcified plaques. The detection and differentiation of noncalcified plaque components from calcified plaque components is feasible with MSCT; the distinction between necrotic areas and fibrous areas, conversely, is more challenging. Nevertheless, some studies have explored the ability of MSCT coronary angiography to differentiate large necrotic cores from fibrous lesions by measuring the CT attenuation values within the plaques.23-25 Although these studies report significant variation of attenuation values within plaques of the same type, plaques with a necrotic core demonstrated by IVUS consistently showed lower CT attenuation values than fibrous plaques (Figure 2).

Bugiardini et al26 evaluated a large cohort of non-ST-elevation acute coronary syndrome patients drawn from international multicenter trials.27-33 This study showed that most of the patients classified as low-risk had nonobstructive CAD or normal coronary arteries. The presence of nonobstructive CAD detected by conventional angiography was associated with a higher risk of events (one-year follow-up) than that associated with normal coronary angiograms.

Hoffmann et al34 performed MSCT in 103 chest pain patients presenting at the emergency department with no ischemic ECG changes and negative initial biomarkers. This study found that the absence of coronary artery plaque or obstructions at MSCT had a high negative predictive value for the subsequent diagnosis of acute coronary syndrome. Moreover, the presence of any plaque or coronary obstruction provided information incremental to conventional risk factors in predicting the risk of acute coronary syndrome.

Pundziute et al22 reported on the prognostic value of MSCT coronary angiography in symptomatic patients with known or suspected CAD and a high prevalence of conventional risk factors. At 16 months, a 100% event-free survival was noted in patients with normal MSCT coronary angiography. Patients with coronary atherosclerosis identified on MSCT had a worse prognosis. In keeping with the previous studies, although the risk of events was the highest in patients with obstructive CAD, the risk associated with nonobstructive CAD (Figure 3) was higher than that associated with a normal MSCT study.

The study by Bugiardini et al shows that low-risk patients might be a heterogeneous group and that risk stratification should be improved in these patients. All three of these studies support the concept that detecting the presence of CAD, whether associated with obstructive lesions or not, may be used for improving risk definition within the low-risk patient category.

More hard data are needed to define the clinical and prognostic value of specific MSCT findings in patients with acute coronary syndrome. Large trials will have to prove the clinical meaning of mixed/noncalcified plaques, plaque location, and plaque distribution in both the short and the long-term.

In spite of tremendous technical developments, the resolution of multislice CT is still far from what is needed to visualize individual structures of the atheroma such as the fibrous cap, which is regarded as the pathological marker of vulnerable plaques. The reproducibility and inter-reader agreement in MSCT coronary plaque imaging will also have to be tested in the real world beyond the academic context.

Nevertheless, the available data consistently support the excellent negative predictive value of a normal multislice CT in patients with suspected acute coronary syndrome. Using these data may consistently reduce the number of unnecessary hospital admissions. The finding that not only obstructive lesions but also nonobstructive CAD and mixed plaques are associated with a worse prognosis22,26 is in keeping with the notion that many patients are entirely asymptomatic up to their first acute event, and that plaque composition, in addition to stenosis severity, contributes to the occurrence of events.

Although the routine use of multislice CT in patients with suspected acute coronary syndrome is not supported yet by practice guidelines, the concepts discussed here lay the foundation for use of MSCT in these patients. In the near future, MSCT might find a role in the diagnostic algorithm of low-risk patients with non-ST-elevation acute coronary syndromes to rule out obstructive CAD, reducing times and costs of care and improving risk stratification.

Dr. Pugliese is a radiologist and Dr. Krestin is the radiology chair, both at Erasmus MC University Medical Centre Rotterdam in the Netherlands.

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If multislice CT can rule out obstructive coronary artery disease, time and cost of care would be reduced and risk stratification improved

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Author | Francesca Pugliese, MD

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