Assessment of Coronary Artery Disease by Cardiac Computed Tomography

Abstract

HomeCirculationVol. 114, No. 16Assessment of Coronary Artery Disease by Cardiac Computed Tomography Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBAssessment of Coronary Artery Disease by Cardiac Computed TomographyA Scientific Statement From the American Heart Association Committee on Cardiovascular Imaging and Intervention, Council on Cardiovascular Radiology and Intervention, and Committee on Cardiac Imaging, Council on Clinical Cardiology Matthew J. Budoff, Stephan Achenbach, Roger S. Blumenthal, J. Jeffrey Carr, Jonathan G. Goldin, Philip Greenland, Alan D. Guerci, Joao A.C. Lima, Daniel J. Rader, Geoffrey D. Rubin, Leslee J. Shaw and Susan E. Wiegers Matthew J. BudoffMatthew J. Budoff Search for more papers by this author , Stephan AchenbachStephan Achenbach Search for more papers by this author , Roger S. BlumenthalRoger S. Blumenthal Search for more papers by this author , J. Jeffrey CarrJ. Jeffrey Carr Search for more papers by this author , Jonathan G. GoldinJonathan G. Goldin Search for more papers by this author , Philip GreenlandPhilip Greenland Search for more papers by this author , Alan D. GuerciAlan D. Guerci Search for more papers by this author , Joao A.C. LimaJoao A.C. Lima Search for more papers by this author , Daniel J. RaderDaniel J. Rader Search for more papers by this author , Geoffrey D. RubinGeoffrey D. Rubin Search for more papers by this author , Leslee J. ShawLeslee J. Shaw Search for more papers by this author and Susan E. WiegersSusan E. Wiegers Search for more papers by this author Originally published2 Oct 2006https://doi.org/10.1161/CIRCULATIONAHA.106.178458Circulation. 2006;114:1761–1791Other version(s) of this articleYou are viewing the most recent version of this article. Previous versions: October 2, 2006: Previous Version 1 TABLE OF CONTENTSExecutive Summary…1761Introduction…17641. Coronary Artery Calcification and Epidemiology of Coronary Calcium…1764 1.1. Calcium Detection Methods …1765 1.1.1. EBCT Methods…1765 1.1.2. MDCT Methods …1766 1.2. Coronary Artery Calcified Plaque …1766 1.3. Speed/Temporal Resolution…1767 1.4. Studies Comparing EBCT and MDCT for Calcium Scoring…1767 1.5. Reproducibility and Validity of Calcium Scoring …1767 1.6. Radiation Dose for Cardiac CT…1768 1.6.1. Radiation Exposure During CT Angiography …17692. Clinical Utility of CACP Detection …1771 2.1. CT Coronary Calcium and Symptomatic Patients…1771 2.2. CT Coronary Calcification and Clinical Outcomes in Asymptomatic Individuals…1772 2.3. Limitations…1774 2.4. Recommendations of Professional Societies…1775 2.5. Utilizing Coronary Calcium Measure to Improve Outcomes…1776 2.6. Limitations of the Use of Coronary Calcium for Detecting Obstructive Disease in Asymptomatic Persons …17773. Future Directions…1777 3.1. Tracking Progression of Subclinical Atherosclerosis…1777 3.2. Hybrid Nuclear/CT Imaging …1779 3.3. Contrast-Enhanced CT of the Coronary Arteries…1779 3.3.1. Electron Beam CT …1779 3.3.2. Multidetector CT …1780 3.4. CT Angiography Applications in a Clinical Context…1782 3.4.1. Suspected CAD…1782 3.4.2. Follow-Up of Percutaneous Coronary Intervention…1782 3.4.3. Follow-Up After Bypass Surgery…1782 3.4.4. Anomalous Coronary Arteries …1782 3.5. Assessment of NCP …1782Conclusion …1783Disclosures …1784References …1784Executive SummaryThis scientific statement reviews the scientific data for cardiac computed tomography (CT) related to imaging of coronary artery disease (CAD) and atherosclerosis. Cardiac CT is a CT imaging technique that accounts for cardiac motion, typically through the use of ECG gating. The utility and limitations of generations of cardiac CT systems are reviewed in this statement with emphasis on CT measurement of CAD and coronary artery calcified plaque (CACP) and noncalcified plaque. Successive generations of CT technology have been applied to cardiac imaging beginning in the early 1980s with conventional CT, electron beam CT (EBCT) in 1987, and multidetector CT (MDCT) in 1999. Compared with other imaging modalities, cardiac CT has undergone an accelerated progression in imaging capabilities over the past decade, and this is expected to continue for the foreseeable future. As a result, the diagnostic capabilities at times have preceded the critical evaluation of clinical application. In this statement, the American Heart Association (AHA) Writing Group evaluates the available data for the application of cardiac CT for CAD.Cardiac CT uses natural contrast within subjects (utilizing the different brightness of fat, tissue, contrast, and air). Noncontrast CT is a low-radiation exposure technique and, even without premedication or intravenous contrast, can determine the presence or absence of CACP in <10 minutes. The amount of CACP can be measured to provide a reasonable estimate of total coronary atheroma including calcified and noncalcified plaque. The data supporting detection of CACP as a measure of CAD are extensive. Imaging applications that detect CACP include conventional chest radiographs, cinefluoroscopy, conventional and helical CT, EBCT, and MDCT.The majority of published studies have reported that the total amount of coronary calcium (usually expressed as the “Agatston score”) predicts coronary disease events beyond standard risk factors. Although some registries used self-reported risk factor data, data from EBCT reports using measured risk factors demonstrate incremental risk stratification beyond the Framingham Risk Score (FRS). These studies demonstrate that CACP is both independent of and incremental with respect to traditional risk factors in the prediction of cardiac events. Data from Greenland et al1 demonstrated that intermediate-risk patients with an elevated coronary artery calcium (CAC) score (intermediate FRS and CAC score >300) had an annual hard event rate of 2.8%, or a 10-year rate of 28%, and thus would be considered high risk. The best estimates of the relative risk (RR) from this study indicated that a CAC score >300 had a hazard ratio (HR) of about 4 compared with a score of 0. This would mean that the estimated risk in the intermediate-risk patient with a CAC score of 0 might be reduced by at least 2-fold while the risk of a person with a CAC score of 300+ would be increased by about 2-fold. Thus, the person with a high CAC score and intermediate FRS is now reclassified as high risk. CT information may then be used to guide primary prevention strategies, especially among individuals within the intermediate-risk category, in whom, as suggested by the AHA Prevention Conference V,2 clinical decision-making is most uncertain. Individuals determined to be at intermediate risk of a cardiovascular disease (CVD) event on the basis of traditional risk factors may benefit from further characterization of their risk through measurement of their atherosclerotic burden with cardiac CT. This AHA Writing Group agrees with the statement from the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III): “In persons with multiple risk factors, high coronary calcium scores (eg, >75th percentile for age and sex) denote advanced coronary atherosclerosis and provide a rationale for intensified LDL-lowering therapy.”3 Guidelines and expert consensus documents4 have extended the recommendation for use of coronary calcium measurements in clinically selected patients at intermediate risk for CAD (eg, those with a 10% to 20% 10-year FRS) to refining clinical risk predictions and to assessing whether more aggressive target values for lipid-lowering therapies are indicated for select patients.5 Asymptomatic persons should be assessed for their cardiovascular risk with such tools as the FRS. Individuals found to be at low risk (<10% 10-year risk) or at high risk (>20% 10-year risk) do not benefit from coronary calcium assessment (Class III, Level of Evidence: B). In clinically selected, intermediate-risk patients, it may be reasonable to measure the atherosclerosis burden using EBCT or MDCT to refine clinical risk prediction and to select patients for more aggressive target values for lipid-lowering therapies (Class IIb, Level of Evidence: B).When cardiac CT is used for CACP assessment, the AHA Writing Group strongly recommends a low-dose technique using prospective ECG gating. Although alternative techniques may provide improved resolution or increased precision in measurement, data to support an enhanced predictive ability given the higher radiation exposure are limited. A minimum CT-system configuration of EBCT C150 or more up to date or MDCT 4 channel with 0.5-second gantry rotation or faster is recommended. Although virtually all of the prognostic and epidemiological data derived for CACP have been performed with EBCT, several large prospective trials have documented that cardiac CT (both MDCT and EBCT) measurements can be similarly applied across multiple centers with equally high levels of patient satisfaction and acceptance.The utility of CACP in symptomatic patients has been widely studied and has been discussed in depth in a previous ACC/AHA statement,4 as well as in the AHA Cardiac Imaging Committee scientific statement “The Role of Cardiac Imaging in the Clinical Evaluation of Women With Known or Suspected Coronary Artery Disease.”5 The test has been shown to have a predictive accuracy equivalent to alternative methods for diagnosing CAD. These studies may have been subject to referral bias, as a positive test may have been the rationale for subjecting the patient to the invasive angiogram. More comparison work between modalities is clearly needed. A positive cardiac CT examination in which any CACP is identified is nearly 100% specific for atheromatous coronary plaque. CACP can develop early in the course of subclinical atherosclerosis and can be identified histologically after fatty streak formation. CACP is present in the intima of both obstructive and nonobstructive lesions, and thus, the presence of calcified plaque on cardiac CT is not specific to an obstructive lesion. Studies using intracoronary ultrasound have documented a strong association between patterns of CACP and culprit lesions in the setting of acute coronary syndromes.Cardiac CT studies correlating calcified plaque using EBCT technology and various methods of coronary angiography in more than 7600 symptomatic patients demonstrate negative predictive values of 96% to 100%, providing physicians with a high level of confidence that an individual without CACP (total calcium score=0) does not have obstructive angiographic CAD. The presence of CACP is extremely sensitive, albeit with reduced specificity, for diagnosing obstructive CAD (95% to 99%) in patients >40 years of age. A recent study of 1195 patients who underwent CACP measurement with EBCT and myocardial perfusion single photon emission CT (SPECT) assessment demonstrated that CACP was often present in the absence of myocardial perfusion scintigraphy (MPS) abnormalities (normal nuclear test) and that <2% of all patients with CACP <100 had positive MPS studies.6 This is supported by other published reports and is synthesized in a recent appropriateness criteria statement from the American Society of Nuclear Cardiology and the American College of Cardiology.7,8 CACP measured by cardiac CT has a high sensitivity and negative predictive power for obstructive CAD but markedly limited specificity. Because calcified plaque may be present in nonobstructive lesions, the presence of CACP in asymptomatic persons does not provide a rationale for revascularization but rather for risk factor modification and possible further functional assessment. Clinicians must understand that a positive calcium scan indicates atherosclerosis but most often no significant stenosis. With exceptions, high-risk calcium scores (such as an Agatston score ≥400) are associated with an increased frequency of perfusion ischemia and obstructive CAD. The absence of coronary calcium is most often associated with a normal nuclear test and no obstructive disease on angiography. Coronary calcium assessment may be reasonable for the assessment of symptomatic patients, especially in the setting of equivocal treadmill or functional testing (Class IIb, Level of Evidence: B). There are other situations when CAC assessment might be reasonable. CACP measurement may be considered in the symptomatic patient to determine the cause of cardiomyopathy (Class IIb, Level of Evidence: B). Also, patients with chest pain with equivocal or normal ECGs and negative cardiac enzyme studies may be considered for CAC assessment (Class IIb, Level of Evidence: B).Coronary calcium assessment for diagnosis of atherosclerosis and obstructive disease and for risk stratification for future cardiac events has undergone significant validation over the past 20 years. CT angiography is a noninvasive technique, performed by either EBCT or MDCT, to evaluate the lumen and wall of the coronary artery. Especially in the context of ruling out stenosis in patients with low to intermediate pretest likelihood of disease, CT coronary angiography may develop into a clinically useful tool. CT coronary angiography is reasonable for the assessment of obstructive disease in symptomatic patients (Class IIa, Level of Evidence: B). Several small studies have assessed the value of EBCT and MDCT for detecting restenosis after stent placement. At this time, however, imaging of patients to follow up stent placement cannot be recommended (Class III, Level of Evidence: C).Where MDCT is used for CT angiography, the AHA Writing Group currently recommends a minimum of 16-slice capability, submillimeter collimation, and 0.42-second gantry rotation with retrospective ECG gating. If EBCT is used, 1.5-mm slice thickness should be used. A limitation of EBCT relative to MDCT is its lower power, with EBCT limited to 63 or 100 milliamperes/second (mAs), depending on scanner generation, which becomes important in larger patients because image quality can be affected by noise. Another advantage of MDCT is thinner slice imaging, with section thickness as small as 0.5 mm, whereas EBCT is limited to 1.5 mm. An advantage of EBCT, however, is the lower radiation dose associated with this procedure (1.1 to 1.5 mSv), compared with MDCT angiography (5 to 13 mSv).9,10 The use of both CT modalities to evaluate noncalcified plaque (NCP) is promising but premature. There are limited data on variability but none on the prognostic implications of CT angiography for NCP assessment or on the utility of these measures to track atherosclerosis or stenosis over time; therefore, their use for these purposes is not recommended (Class III, Level of Evidence: C).CT technology is evolving rapidly, and these radiation dose estimates are likely to decrease with modification of the hardware and scanning protocols. The clinical relevance of the radiation dose that is administered with cardiac CT is unknown. However, higher radiation doses in general are associated with a small but defined increase in cancer risk later in life. The AHA Writing Group reviewing the available literature endorses the use of a prospective ECG trigger for measurement of CACP with a slice collimation of 2.5 to 3 mm for clinical practice. EBCT systems have an effective dose of 0.7 to 1 mSv (male) and 0.9 to 1.3 mSv (female), and MDCT systems have an effective dose of 1 to 1.5 mSv (male) and 1.1 to 1.9 mSv (female). Higher radiation exposures with retrospective gating for CACP assessment preclude its use for screening. Similarly, for CT angiography, the higher radiation doses (up to 1.5 mSv for EBCT and up to 13 mSv for MDCT) prohibit the use of this test as a screening tool for asymptomatic patients. CT coronary angiography is not recommended in asymptomatic persons for the assessment of occult CAD (Class III, Level of Evidence: C).The role of cardiac CT in measuring clinically or prognostically meaningful changes in calcified plaque over time and its correlation with other measures of coronary heart disease (CHD) is currently an area of intense investigation. Reductions in the test-to-test variability and improvements in the interreader reliability of the calcium score may allow for serial assessment of coronary calcium scores; however, more studies are required. It is difficult to justify the incremental population exposure to radiation and the cost associated with a repeat CT test to assess “change,” until it is better understood what therapies may be of benefit and how clinicians should utilize this data in clinical practice. There is conflicting evidence as to whether vigorous cholesterol-lowering therapy with statins retards the rate of progression of CACP. The AHA Writing Group concluded that this potential use of cardiac CT will require additional validation before any recommendation. Serial imaging for assessment of progression of coronary calcification is not indicated at this time (Class III, Level of Evidence: C).Cardiac CT technology is rapidly evolving. On the basis of the substantial validation data, EBCT remains the reference standard for CACP measurement.11 MDCT-64 is the current standard for coronary CT angiography and NCP characterization based on publications to date.12 The trend for improved image quality with cardiac CT is consistent. It is critical that the cardiac imaging scientific community continue to integrate evolving technological advances with best clinical practices for treatment and prevention of CVD.7,13An area of ongoing clinical research is the application of hybrid positron emission tomography CT (PET-CT) and SPECT-CT scanners that are currently available. This research will allow for the acquisition of metabolic and/or perfusion information as well as anatomic data, including angiographic data and data on coronary calcification. The incremental benefit of hybrid imaging strategies will need to be demonstrated before clinical implementation, as radiation exposure may be significant with dual nuclear/CT imaging. At this time, there are no data supporting the use of hybrid scanning to assess cardiovascular risk or presence of obstructive disease (Class III, Level of Evidence: C).In summary, cardiac CT has been demonstrated to provide quantitative measures of CACP and NCP. CACP, as determined by cardiac CT, documents the presence of coronary atherosclerosis, identifies individuals at elevated risk for myocardial infarction (MI) and CVD death, and adds significant predictive ability to the Framingham Score (an index of traditional CVD risk factors). Data suggest that cardiac CT may improve risk prediction, especially in individuals determined to be at intermediate risk according to the NCEP ATP III criteria and for whom decisions concerning prevention strategies may be altered based on the test results. The use of cardiac CT angiography for noninvasive assessment of lumen stenosis in symptomatic individuals has the potential to significantly alter the management of CAD and current diagnostic testing patterns. The assessment of progression of CACP and the detection of nonobstructive NCP by cardiac CT angiography warrant further investigation.IntroductionThe AHA has issued 2 prior statements on CAC scanning; one in 199614 and a second (in conjunction with the American College of Cardiology [ACC]) in 2000 specifically related to EBCT.4 The AHA also sponsored the Prevention V Conference, which focused on the identification of the asymptomatic high-risk patient and discussed the potential role of CAC scanning.2 In light of a rapidly evolving literature since the last ACC/AHA expert consensus statement (2000), the current statement will focus on new data available on using EBCT and MDCT to identify patients with coronary atherosclerosis defined by quantification of coronary artery calcification. EBCT is an especially fast form of x-ray imaging technology that can detect and measure calcium deposits in the coronary arteries.5 The amount of calcium detected by EBCT is related to the amount of underlying coronary atherosclerosis. During the past decade, there has been a progressive increase in the clinical use of both EBCT and MDCT scanners to identify and quantify the amount of calcified plaque in the coronary arteries. This approach has generated much interest and scrutiny for several reasons. Although coronary calcification can be quantified and calcium scores can be related to the extent and severity of atherosclerotic disease and improving CHD risk prediction, misuse or abuse of these methods as a broad-based “screening” tool has created considerable controversy.Recently, CT scanners with subsecond image acquisition and MDCT (also referred to as multirow or multislice) capability have been studied and proposed as an alternative approach to EBCT for detecting coronary calcification owing to the greater availability of such CT scanners. This scientific statement will compare MDCT and EBCT and serve as a clinical update for the use of CACP in clinical decision-making regarding evaluations for CHD in the asymptomatic individual. Current evidence regarding noninvasive angiography using CT, as well as the future role of these techniques in monitoring atherosclerosis over time and in detecting NCP, will be reviewed.1. Coronary Artery Calcification and Epidemiology of Coronary CalciumArterial calcium development is intimately associated with vascular injury and atherosclerotic plaque. CACP is an active process and can be seen at all stages of atherosclerotic plaque development.15–17 The long-held notion of so-called “degenerative” calcification of the coronary arteries with aging is incorrect. Since Faber18 noted in 1912 that Mönckeberg’s calcific medial sclerosis did not occur in the coronary arteries, atherosclerosis is the only vascular disease known to be associated with coronary calcification.4,11,14,19,20 Thus, CACP in the absence of luminal stenosis is not a “false-positive” result but rather evidence of coronary atherosclerosis.20Coronary calcification is nearly ubiquitous in patients with documented CAD21–23 and is strongly related to age, increasing dramatically after age 50 in men and after age 60 in women (Tables 1 and 2).24,25 However, coronary plaque and its associated coronary calcification may have only a weak correlation with the extent of histopathologic stenosis.26,27 The degree of encroachment on the vessel lumen by the atherosclerotic plaque is largely determined by individual variations in coronary artery remodeling. However, the presence of CACP is associated with atherosclerotic plaque size.26TABLE 1. Descriptive Characteristics of the Total Electron Beam Tomographic CAC Scores in Asymptomatic Men and WomenAge, yMenWomennTotal CAC ScorenTotal CAC ScoreMeanSDMedianMeanSDMedianAdapted from data presented in Hoff et al.24<40350412700.5641214040–4442382712011024897045–494940571753163418186050–544825121305162184291350.555–59347220341149183554189160–642288350972113133478250365–6912094647311807311473382470–7454066592130943622551555>74235836105347317425850775TABLE 2. Electron Beam Tomographic CAC Score Percentiles for Men and Women Within Each Age StratumAge, y<4040–4445–4950–5455–5960–6465–6970–74>74Adapted from data presented in Hoff et al.24Men (25 251)3504423849404825347222881209540235 25th Percentile00014133264166 50th Percentile1131548113180310473 75th Percentile39361032154105668921071 90th Percentile1459154332554994129917741982Women (9995)64110241634218418351334731438174 25th Percentile000000139 50th Percentile000013246275 75th Percentile11252357145210241 90th Percentile342265121193410631709Rumberger and colleagues28,29 examined 13 autopsied hearts and compared measures of CACP using EBCT as compared with direct histological plaque areas and percent luminal stenosis. These studies determined that the total area of CACP quantified by EBCT is linearly and highly correlated (r=0.90) with the total area of histological coronary artery plaque. Although the total atherosclerotic plaque burden was tracked by the total calcium burden, not all plaques were found to be calcified, and the total calcium area was approximately 20% of the total atherosclerotic plaque area. Baumgart et al30 and Schmermund et al31 compared direct intracoronary ultrasound measures during angiography with EBCT scanning and confirmed a direct association, in vivo, of CACP score with localization and extent of atherosclerotic plaques.The prevalence of CACP mirrors the prevalence of coronary atherosclerosis in both men and women.32 The data show the following: (1) the prevalence of CACP increases from only a small percentage in the second decade of life to nearly 100% by the eighth decade in men and women; (2) the prevalence of CACP in women is similar to that in men who are a decade younger; (3) the gender difference in prevalence with age is eliminated by approximately age 65 to 70, when the prevalence of coronary calcium in women is similar to that in men of the same age. The prevalence of CACP increases with age, paralleling the increased prevalence of coronary atherosclerosis with advancing age.1.1. Calcium Detection MethodsThis section will discuss methods related to CACP identification.1.1.1. EBCT MethodsEBCT is a tomographic imaging device developed nearly 20 years ago specifically for cardiac imaging. Although the technique can quantify ventricular anatomy and function33 as well as myocardial perfusion,34 it is currently best known for defining and measuring CACP. Over the past decade, there have been more than 1000 articles published regarding EBCT and coronary artery imaging.EBCT (also referred to as “EBT” and “Ultrafast-CT,” General Electric, South San Francisco, Calif) uses unique technology enabling ultrafast scan acquisition times currently of 50 ms, 100 ms, and multiples of 100 ms (up to 1.5 seconds) per slice (Table 3). There have been 3 iterations of EBCT systems since their clinical introduction in the early 1980s. The core imaging methods have remained unchanged, but there have been improvements in image acquisition; in data storage, manipulation, management, and display; and in spatial resolution. The original C-100 scanner was replaced in 1993 by the C-150, which was replaced by the C-300 in 2000. The current EBCT scanner, the “e-speed” (GE/Imatron, South San Francisco, Calif) was introduced in 2003. The e-speed is a multislice scanner and currently can perform a heart or body scan in half the total examination time required by the C-150 and C-300 scanners. In addition to the standard 50-ms and 100-ms scan modes common to all EBCT scanners, the e-speed is capable of high-resolution imaging speeds as fast as 50 ms. This very short acquisition time leads to fewer motion artifacts and improved contrast-to-noise ratios.35TABLE 3. Basic Description of CT System ComponentsEBCTMDCT*Heart rate limitations based on the prevalence of studies with significant coronary motion.Electron source (cathode)Electron gunTungsten filamentGantryFixed: Electron beam rapidly sweeps across tungsten ringsRotates: Tube and opposing detectors rotate within gantryImage reconstructionPartial scan/filtered back-projection Sharp kernelPartial scan/filtered back-projection Standard kernelBeam current, mAFixedUser selectableExposure time for coronary calcium50 or 100 ms (true prospective)≥220 ms Dependent on gantry rotation speed and postprocessingGating for CT angiographyProspective triggerRetrospective gatingExposure, mAsFixed mA × exposure timeUser-selectable mA × exposure timeHeart rate limitations*<110 bpm<65 bpmBest z-axis resolution1.5 mm0.5 mmEBCT uses a stationary multisource/split-detector combination coupled to a rotating electron beam and produces serial, contiguous, thin-section tomographic scans in synchrony with the heart cycle. EBCT is distinguished by its use of a scanning electron beam rather than the traditional x-ray tube and mechanical rotation device used in current “spiral,” single, and multiple-detector scanners. The electron beam is steered by an electromagnetic deflection system that sweeps it across the distant anode, a series of 4 fixed tungsten “target” rings. A stationary, single-level or dual-level arc of detectors lies in apposition to the tungsten target rings. In contrast, MDCT physically moves the x-ray tube in a circle about the patient; with EBCT, only the electron beam is moved.Standardized methods for imaging, identification, and quantification of CAC using EBCT have been established.4,36 The scanner is operated in the high-resolution, single-slice mode with continuous, nonoverlapping slices of 3-mm thickness and an acquisition time of 100 ms/tomogram.37 Electrocardiographic triggering is done during end-systole or early diastole at a time determined from the continuous ECG tracing done during scanning.Historically, the most common trigger time used is 80% of the R-R interval. However, this trigger occurs on or near the P wave during atrial systole, and the least cardiac motion among all heart rates occurs at 40% to 60% of the R-R interval.38 Therefore, it has been demonstrated that the protocol of triggering at 80% of the R-R interval is not optimal for imaging of the coronary segments near the right or left atrium. Mao et al39 compared 40% and 80% trigger delay (imaging during early compared with late diastole) and obtained an interscan variability of 11.5% versus 17.4%, respectively. For a more complete discussion on gating, see section 1.5.1.1.2. MDCT MethodsThe current generation of MDCT systems is capable of acquiring 4 to 64 sections of the heart simultaneously with ECG gating in either a pros