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(Radiographics. 1999;19:1409-1419.)
© RSNA, 1999

OPENING PLENARY SESSION: 1998

Opening Plenary Session: 1998 1

Coronary Artery Calcification as an Indicator of Preclinical Coronary Artery Disease

William Stanford, MD

1 From the Department of Radiology, University of Iowa Hospitals and Clinics and University of Iowa College of Medicine, 200 Hawkins Dr, Iowa City, IA 52242. From the Opening Plenary Session at the 1998 RSNA scientific assembly. Received April 22, 1999; revision requested June 11 and received July 2; accepted July 15. Address reprint requests to the author.

Index Terms: Computed tomography (CT), electron beam, 54.12119 • Computed tomography (CT), helical, 54.12115 Coronary vessels, calcification, 54.812 • Coronary vessels, CT, 54.12115, 54.12119, Coronary vessels, stenosis or obstruction, 54.76


   INTRODUCTION
Top
 INTRODUCTION
PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
 ELECTRON BEAM CT
 HELICAL CT
 CORONARY CALCIFICATION AND...
 CONCLUSIONS
 References
 
In 1995, the American Heart Association estimated that there were 1 million Americans who were victims of angina or heart attacks; of these, approximately 481,000 died (1). In addition, they reported that 30% of these heart attack victims were under 65 years of age and that 4% were younger than age 45 (1). Coronary artery disease is the leading cause of death in the United States today.

Traditional methods for detecting coronary artery disease rely on the evaluation of patient symptoms, the identification of elevated levels of cholesterol or abnormal blood lipids, physiologic responses to stress as identified on electrocardiograms or echocardiograms, and results of radionuclide perfusion imaging or coronary angiography. However, all of these methods, with the exception of tests for blood lipids, require the presence of established stenoses within the coronary artery to yield positive results.

Calcification, on the other hand, is a recognized marker for atherosclerotic coronary artery disease (2,3) and often occurs early in plaque development. Calcification can readily be identified with computed tomography (CT)—especially electron beam CT (Imatron, South San Francisco, Calif) and helical CT—and these modalities are highly sensitive in the detection of coronary calcification (4). This capability is important because the identification of coronary calcification provides a marker with which to guide the initiation of dietary and drug therapy before a cardiac event and with the hope of preventing or decreasing the chances for such an event. Thus, electron beam CT and helical CT are increasingly being used to evaluate coronary artery disease in the hope of detecting early atherosclerotic disease before cardiac events occur.

Consequently, it is imperative for physicians involved in imaging coronary arteries to understand the importance of coronary calcification as a marker for atherosclerosis. This article reviews the pathophysiology of coronary atherosclerosis; the imaging of coronary calcification, with emphasis on the use of electron beam CT and helical CT in the detection of calcified plaque; and the correlation of coronary calcification with clinical outcome.


   PATHOPHYSIOLOGY OF CORONARY ATHEROSCLEROSIS
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
IMAGING OF CORONARY...
 ELECTRON BEAM CT
 HELICAL CT
 CORONARY CALCIFICATION AND...
 CONCLUSIONS
 References
 
Coronary atherosclerosis is believed to be caused by an injury to the coronary artery endothelium. This injury allows circulating histiocytes to traverse the injured endothelium and to lodge in the vessel wall, where they are transformed into macrophages. As macrophages, the cells accumulate lipid and stimulate proliferation of smooth muscle cells. This lipid deposition is frequently seen as "fatty streaks" beneath the surface endothelium of the coronary vessel. As the lipid accumulates, it often becomes calcified. Because of remodeling (5), however, moderate to large amounts of lipid can accumulate within the vessel wall without encroaching on the arterial lumen (Fig 1). In these instances, a coronary angiogram may appear entirely normal. If the thin fibrous cap overlying these lipid deposits should rupture and allow circulating blood to come in contact with the lipid and smooth muscle cells in the vessel wall, an intense thrombogenic reaction will often result (Fig 2). Depending on the size of the artery, location of the plaque, and extent of collateral circulation, death or a myocardial infarction with resultant minimal, moderate, or extensive damage could ensue.



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  		Figure 1.   Photomicrograph (original magnification, x2; hematoxylin-eosin stain) of a coronary artery section shows a large amount of atherosclerotic plaque (p) deposited in the arterial wall. Because of remodeling, the plaque can distort and enlarge the coronary artery without compromising the lumen. Hence, the coronary angiogram may appear normal. (Reprinted, with permission, from reference 6.)

 


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  		Figure 2.   Photomicrograph (original magnification, x2; hematoxylin-eosin stain) of a coronary artery section shows total occlusion of the vessel by a large thrombus (t) formed by disruption of a thin fibrous plaque (p) over an accumulation of lipid in the arterial wall. Arrows = area of rupture of the fibrous cap (c).

 
With repeated episodes of thrombogenic reactions, the arterial lumen frequently narrows to produce a fixed stenosis (Fig 3) (7–10). The amount of plaque deposited in the coronary wall is variable, and rapid progression to severe stenosis or total occlusion can occur within a period of a few months (11).



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  		Figure 3.   Photomicrograph (original magnification, x2; hematoxylin-eosin stain) of a coronary artery section shows significant stenosis, with lipid and calcification (*) in the plaque (p). (Figs 2 and 3 reprinted, with permission, from reference 6.)

 
Calcium is deposited as calcium hydroxyapatite and is currently believed to be deposited in diseased coronary arteries through a process that is similar to active bone formation (12). Although rupture of the soft unstable plaque accompanying the more advanced calcified plaque is thought to be the most frequent cause of cardiac events (13), the detection of calcification is important because it is representative of the amount of soft plaque and hence the extent of the atherosclerotic burden. In fact, Rumberger et al (14) have determined with electron beam CT that calcified plaque volume is approximately one-fifth that of associated noncalcified plaque.


   IMAGING OF CORONARY CALCIFICATION
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
ELECTRON BEAM CT
 HELICAL CT
 CORONARY CALCIFICATION AND...
 CONCLUSIONS
 References
 
Coronary calcification is detectable with numerous imaging modalities, including plain radiography; fluoroscopy; conventional, helical, and electron beam CT; intravascular ultrasonography (US); magnetic resonance (MR) imaging; and transthoracic and transesophageal echocardiography. Fluoroscopy, electron beam CT, and helical CT are the more frequently used imaging modalities for demonstrating coronary calcification, with the remaining modalities having varying effectiveness.

Chest radiography has a poor sensitivity for detecting coronary calcification, and accuracies as low as 42% have been reported (15,16).

Fluoroscopy has been used for decades to detect coronary calcification. In one representative study, Loecker et al (17) reported on the correlation of fluoroscopically evident calcification with coronary stenosis in 613 asymptomatic male air crew members who had undergone coronary angiography because of one or more abnormal screening tests. These investigators found that coronary calcification had a 66.3% sensitivity and a 77.6% specificity for predicting angiographically significant (>50% diameter narrowing) coronary stenosis. The positive predictive value was 37.7%, and the negative predictive value was 91.9%. The authors concluded that the absence of calcification at fluoroscopy indicated a low likelihood for significant obstructive coronary artery disease, whereas the presence of calcification substantially increased the likelihood.

One must recognize, however, that the ability of fluoroscopy to demonstrate small calcified plaques is poor. In one study, Agatston et al (18) found that only 52% of calcific deposits seen on high-resolution electron beam CT images could be detected fluoroscopically (P = <.001). The mean attenuation value of calcific lesions detected with electron beam CT was +99 HU, whereas the mean value for lesions detected with fluoroscopy was +546 HU. Thus, only the larger, more highly calcified plaques were detectable with fluoroscopy compared with those seen at electron beam CT.

Fluoroscopy has several other disadvantages. The diagnostic effectiveness of fluoroscopy is highly operator dependent, and this, as well as the patient's body habitus, overlying anatomic structures, and calcifications in vertebrae and valve annuli may compromise fluoroscopic findings.

CT is extremely sensitive in detecting calcification because of the ability of calcium to attenuate the x-ray beam. In one study in which the detection of calcification at conventional CT was evaluated as a marker of significant angiographic stenosis, sensitivities of 16%–78% were reported (19). The specificities were 78%–100%, and the positive predictive values were 83%–100%. These data suggest that significant coronary artery disease may be likely when coronary calcification is seen at CT (19).

Results of conventional CT, fluoroscopy, and angiography in the detection of coronary calcification have also been compared. In a study of 47 patients with angina (mean age, 57 years), CT revealed calcified plaques in 62% of patients with significant stenoses at angiography, whereas fluoroscopy showed calcific lesions in only 35% (20). In a comparable group of patients without angina, coronary calcification was found at CT in only 4%, and no patient had significant stenosis at coronary angiography. In this study, CT demonstrated calcification in all patients in whom fluoroscopy showed calcified plaques and in all patients in whom angiography showed stenosis. Overall, CT showed calcification in 50% more vessels than did fluoroscopy.

Conventional CT, although superior to fluoroscopy in the detection of coronary calcification, has slow scan times and the potential for motion artifacts, volume averaging, and breathing misregistration. It also cannot quantify plaque volume. Thus, conventional CT is not as sensitive as electron beam or helical CT in the detection of calcified plaques. (Electron beam CT and helical CT are discussed in separate sections).

Intravascular US is useful for detecting the presence of atherosclerotic plaque in the coronary arteries (21,22). By using transducers with rotating deflectors mounted on the tips of catheters, it is possible to interrogate the wall of the coronary artery during cardiac catheterization. These sonograms provide information about the thickness and tissue characteristics of the arterial wall. Calcification is seen as a hyperechoic area with shadowing; fibrotic noncalcified plaques are seen as hyperechoic areas without shadowing (23). Mintz et al (24) compared intravascular US and angiography and found that angiography was significantly less sensitive in the detection of calcification at the site of an atherosclerotic lesion. This finding was confirmed by Tuzcu et al (25), who found that calcification could be visualized at US in up to two-thirds of patients without angiographically evident calcification at the target site.

However, intravascular US is invasive and is primarily performed in conjunction with selective coronary angiography. It demonstrates only a limited portion of the coronary tree, and, because of this, its role in screening for coronary artery disease is limited. The technique is clinically important, however, because it can depict atherosclerotic calcific deposits in patients with normal coronary angiograms (25). Intravascular US can also help define the morphologic characteristics of stenotic lesions prior to balloon angioplasty or coronary atherectomy.

At MR imaging, calcium is characterized by low signal intensity with T1-weighted, T2-weighted (26), and gradient-refocused pulse sequences (27,28). Because microcalcifications do not substantially alter the signal intensity of voxels within soft tissue, the sensitivity of MR imaging is low and the modality has little usefulness in evaluating coronary calcification.

Similarly, transthoracic and transesophageal echocardiography have limited effectiveness for this application. Because of limited acoustic windows, visualization of the coronary arteries with transthoracic echocardiography has been documented only on rare occasions. With transesophageal echocardiography, it is possible to visualize the proximal coronary arteries (29,30); however, neither technique has sufficient resolution nor appropriate acoustic windows to reliably define in situ coronary calcification.


   ELECTRON BEAM CT
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
 ELECTRON BEAM CT
HELICAL CT
 CORONARY CALCIFICATION AND...
 CONCLUSIONS
 References
 
The electron beam CT scanner, unlike conventional CT scanners, uses electromagnetically focused electrons to sweep stationary tungsten target rings to produce x rays. For coronary artery examinations, 20 contiguous, 100-msec, 3-mm sections are acquired during one or two breathholds. The images are triggered from the R wave of the electrocardiograph at 60%–80% of the R-R interval so that acquisition occurs during diastole; this technique minimizes motion artifact. Coronary calcific deposits are seen as bright white areas along the course of the coronary artery (Figs 4, 5).



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  		Figure 4.   Electron beam CT scan of a 65-year-old man shows multiple areas of dense calcification along the left main, left anterior descending, and circumflex coronary arteries. Calcifications are also seen within the aortic wall.

 


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  		Figure 5.    Electron beam CT scan of a 32-year-old man shows a small area of calcification in the left anterior descending artery (arrow). At angiography, the calcification was associated with an area of critical stenosis; the patient subsequently underwent a successful balloon angioplasty.

 
Scoring of Coronary Calcification
For scoring coronary calcific lesions, an arbitrary value of +130 HU and an area greater than 1.0 mm2 are often used. The +130 HU level was selected because it lies well above the +30–50 HU of unenhanced myocardium and soft tissue. Coronary calcification scoring, whether it be done with electron beam CT or helical CT, requires that regions of interest be placed around the areas of calcification. (With electron beam scanners, software has been incorporated into the scanner that deletes pixels below a threshold of +130 HU and allows only pixels above the +130 HU threshold to be seen.) Pixel clusters above +130 HU are displayed, and if the cluster lies over the course of a coronary artery and if the volume is greater than two contiguous pixels, the area is scored as a lesion (Fig 6). Individual pixels above +130 HU are commonly seen throughout the heart, and, to eliminate extraneous noise, the criteria above were selected to define a minimal area threshold. Most examination sites use a minimum of 2–4 contiguous pixels, and many use a 3-pixel minimum (which represents an area of 1.03 mm2 when a 30-cm field of view and a 512 x 512 matrix are used) as a compromise to optimize sensitivity and specificity while not being too restrictive.



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  		Figure 6a.   (a) Electron beam CT scan of a male patient shows multiple areas of calcification in the left main and left anterior descending coronary arteries (arrows). (b) On the electron beam CT scan, regions of interest have been placed around the lesions noted in the left main and left anterior descending arteries. Scoring is done by intrinsic scanner software. Lesion C (top rectangle) had a peak attenuation of +519 HU and an area of 11.59 mm2.

 


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  		Figure 6b.   (a) Electron beam CT scan of a male patient shows multiple areas of calcification in the left main and left anterior descending coronary arteries (arrows). (b) On the electron beam CT scan, regions of interest have been placed around the lesions noted in the left main and left anterior descending arteries. Scoring is done by intrinsic scanner software. Lesion C (top rectangle) had a peak attenuation of +519 HU and an area of 11.59 mm2.

 
Once the regions of interest have been placed, the scanner software displays the peak calcification, the attenuation in Hounsfield units, and the area in millimeters squared. The scanner will also display either the volume score or the Agatston score. The volume score is the area of the lesion, whereas the Agatston score takes into account lesion attenuation as well as area. In the Agatston scoring system, +131–200 HU lesions are multiplied by a factor of one, +201–300 HU lesions by a factor of two, +301–400 HU lesions by a factor of three, and lesions greater than 401 HU by a factor of four. The sum of the individual lesion scores is the score for that artery, and the sum of all lesion scores is the total calcification score. A similar methodology is used in helical CT scoring except that some examination sites prefer to use a +90 HU threshold rather than the traditional +130 HU threshold.

Histologic Correlation of Electron beam CT_detected Calcification
A number of studies have reported the association of calcification detected at electron beam CT with histologic plaque. Mautner et al (31) examined 1,298 coronary artery segments from 50 heart specimens and observed that coronary calcification had been detected with electron beam CT in 65% of segments with greater than 75% stenosis, compared with 50% of segments with 51%–75% stenosis, 18% of those with 26%–50% stenosis, and only 1% of those with 1%–25% stenosis. These investigators also found that electron beam CT–detected calcification was present in 41% of 1,426 segments from coronary arteries of patients with symptomatic coronary artery disease. In 1,535 segments from asymptomatic patients, calcific deposits were found in 24%, and in 1,337 segments from normal control patients, only 4% had calcification (32).

Review of Clinical Experience
In 1990, Agatston et al (18) reported results from the first large clinical series employing electron beam CT in the detection of coronary calcification. In this study, a total of 584 consecutive patients (mean age, 48 years) underwent electron beam CT (100-msec, 3-mm sections); 50 of these patients also underwent fluoroscopic examinations. Of the 584 patients, 109 had coronary artery disease, as established by a history of myocardial infarction (22 patients) or coronary artery narrowing of more than 50% at angiography (87 patients). The remaining 475 patients had no history of coronary artery disease. Patients with a history of coronary artery disease consistently had more calcific deposits, compared with comparably aged individuals with no history of coronary artery disease (P = <.001). A total calcification score of 300 had a sensitivity of 74% and a specificity of 81% in the detection of obstructive coronary artery disease. The negative predictive value of a zero calcification score was 98%. Overall, electron beam CT showed calcific deposits in 90% of patients, whereas fluoroscopy showed them in 52%. The authors concluded that electron beam CT appeared to be an excellent method for detecting and quantifying calcification of the coronary arteries.

In another representative study, Breen et al (33) evaluated 100 patients aged 23–59 years with electron beam CT and coronary angiography. The sensitivity for detecting calcific deposits in individuals with angiographically significant stenosis was 100%, and the specificity was 47%. In this series, eight patients without calcification had no angiographic evidence of coronary artery disease, whereas 28 patients with calcification had mild or moderate coronary artery disease. Significant obstruction was defined as greater than 50% narrowing on the angiogram.

The largest reported series was a multicenter study of 710 patients with symptoms of coronary artery disease, including 456 men and 254 women (mean age, 56 years) (34). In this group, the sensitivity of electron beam CT in the detection of calcific deposits as an indicator of significant stenosis (>50% narrowing) was 95%, and the specificity was 44%. The positive predictive value was 72%, and the negative predictive value was 84%.

In a recent article, Guerci et al (35) evaluated the calcification scores from electron beam CT examinations versus conventional risk factors in 290 individuals undergoing coronary angiography for clinical indications. For significant obstructive coronary artery disease, coronary calcification scores had a odds ratio of 34.12 versus 3.86 for age, 4.11 for total cholesterol–to–high-density lipoprotein ratio, 2.07 for male gender, and 3.16 for diabetes. They concluded that use of electron beam CT offers improved discrimination over conventional risk factors in determining angiographically significant obstructive coronary artery disease.

Despite the high sensitivity of electron beam CT, Bormann et al (36) found that calcification scores were not predictive of significant stenosis at the site of the calcification and that no receiver operator characteristic curve could be found that would suggest a clinically useful calcification score as an indicator of greater than 70% stenosis at that same anatomic site. In a later study, Stanford et al (37) examined data from 150 patients undergoing electron beam CT and coronary angiography in two institutions and found only one patient with greater than 50% stenosis with no coronary calcification. Thus, the absence of calcific deposits appears to negate the presence of significant luminal stenosis.

Calcification scores can show considerable variance in the detection of obstructive coronary artery disease. To better define sensitivity and specificity, Rumberger et al (38) compared the predictability of coronary calcification scores and found that a calcification score of 80 had a sensitivity of 84% and specificity of 84% for predicting stenosis somewhere within the coronary artery system. These investigators also found "cut points" in the calcification scores that would provide 90%–95% sensitivity and specificity in the detection of angiographically significant coronary artery stenosis (39).

Gender differences are important in the assessment of coronary calcification. In a study evaluating the prevalence of coronary calcification at intervals of 5–10 years in 1,396 male and 502 female asymptomatic subjects (age range, 14–88 years), Janowitz et al (40) found that the prevalence of coronary calcification in women was half that of men until age 60 years when the difference diminished. The distributions of coronary calcification in men aged 40–69 years were virtually identical to those in women aged 50–79 years.

Progression of calcified plaque was also assessed by Janowitz et al (41). Calcified plaque was evaluated in 25 symptomatic and asymptomatic individuals 406 days (mean) apart. Patients with angiographically proved obstructive coronary artery disease had a 48% increase in the calcification score at the second examination compared with a 22% increase in asymptomatic subjects. Patients with obstructive coronary artery disease also had 55 new calcific deposits at the follow-up examination versus 18 in the asymptomatic group. Although Janowitz et al concluded that electron beam CT may be useful for studying the natural history of coronary artery disease, they did not study the interscan differences that could result in measurement error.


   HELICAL CT
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
 ELECTRON BEAM CT
 HELICAL CT
CORONARY CALCIFICATION AND...
 CONCLUSIONS
 References
 
The scan times for helical CT are approaching 1 second, but helical CT scanners with acquisition times as fast as 0.5 second are being introduced (42,43). As in electron beam CT, calcific deposits are seen as bright white areas along the course of the coronary arteries (Figs 7, 8). Shemesh et al (44), who used dual-beam helical scanners to detect calcified plaque, reported that helical CT had a sensitivity of 91% and specificity of 52% compared with angiography in the detection of significant obstructive coronary artery disease. In another multicenter study, dual-beam helical CT had a sensitivity of 88%, specificity of 52%, and accuracy of 76% (45).



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  		Figure 7a.   (a) Electron beam CT scan shows coronary artery calcific deposits in the left main and left anterior descending coronary arteries. (b) On a helical CT 500-msec image, the coronary calcific deposits appear somewhat blurred because of the longer scan times and absence of triggering of the helical CT scanner. (Many newer helical CT scanners now have triggering capability.)

 


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  		Figure 7b.   (a) Electron beam CT scan shows coronary artery calcific deposits in the left main and left anterior descending coronary arteries. (b) On a helical CT 500-msec image, the coronary calcific deposits appear somewhat blurred because of the longer scan times and absence of triggering of the helical CT scanner. (Many newer helical CT scanners now have triggering capability.)

 


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  		Figure 8a.   Helical CT (a) and electron beam CT (b) scans demonstrate calcific lesions (arrow) in the circumflex coronary artery. Images of the circumflex artery lesions are often less blurred than those of left anterior descending artery lesions because of less motion.

 


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  		Figure 8b.   Helical CT (a) and electron beam CT (b) scans demonstrate calcific lesions (arrow) in the circumflex coronary artery. Images of the circumflex artery lesions are often less blurred than those of left anterior descending artery lesions because of less motion.

 
Other investigators have reported that calcific deposits imaged with 1-second scan times are often blurred because of cardiac motion, and small calcifications may not be seen (46). Recent reports, however, have shown good correlations in calcification scores when the faster helical CT scanners have been compared with electron beam CT (42,43).

Overall, helical CT remains superior to fluoroscopy and conventional CT in the detection of coronary calcification. The faster helical CT scanners, used with and without electrocardiographic triggering, have the potential to play an increasingly important role in the detection of coronary artery disease.


   CORONARY CALCIFICATION AND CLINICAL OUTCOME
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
 ELECTRON BEAM CT
 HELICAL CT
 CORONARY CALCIFICATION AND...
CONCLUSIONS
 References
 
Although the presence or absence of calcification appears related to the overall atherosclerotic plaque burden, it is coronary event data (occurrences of angina or myocardial infarction and necessity for percutaneous transthoracic coronary angioplasty or coronary artery bypass surgery) that are important in evaluating the true clinical significance of coronary calcification.

In 1980, Margolis et al (47) evaluated coronary event data in 800 patients with fluoroscopically detected coronary calcification who were referred for cardiac catheterization primarily for angina pectoris (90% of cases). These investigators found that symptomatic patients with calcification had a 5-year survival of 58% versus 87% for those without calcification.

In 1994, Detrano et al (48) studied a group of asymptomatic high-risk subjects with coronary calcification detected at fluoroscopy. These investigators evaluated 1,461 subjects with a greater than 10% risk of having a coronary event within 8 years. (A coronary event was defined as angina, documented myocardial infarction, myocardial revascularization, or death from coronary heart disease.) Coronary events occurred at 1 year in 5.4% of 691 subjects with coronary calcification versus 2.1% of 768 subjects without calcification (P = <.001) (two patients were lost to follow up). One-vessel calcification incurred an event risk of 5.4%; two-vessel calcification, 5.6%; and three-vessel calcification, 6.2%. Overall, they found that fluoroscopically detectable calcification was associated with an event risk 2.7 times greater than that in the group with no calcification. They also found that the presence of calcification was an independent predictor of at least one coronary event when controlled for age, gender, and other risk factors. The authors concluded that the presence of coronary calcification detected at fluoroscopy helped identify individuals at increased risk for a cardiac event at 1 year.

A 1996 multicenter study reviewed cardiac event data in 501 mostly symptomatic coronary artery disease patients who underwent both electron beam CT and coronary angiography (49). In this group, 1.8% of the patients died and 1.2% had nonfatal myocardial infarctions during a mean follow-up period of 31 months. A calcification score of 100 or greater was shown to be highly predictive in separating patients with cardiac events from those without events. In this study, logistic regression, which included calcification score, age, gender, and coronary angiographic findings as independent variables, showed that only the log calcification score predicted which patients had events.

A 1996 study by Arad et al (50) has also shown a strong correlation between coronary calcification and cardiovascular events in an asymptomatic group of individuals. In this study, 1,173 asymptomatic subjects were followed up for 19 months (average). In that time period, 18 subjects had 26 cardiovascular events, including one death, seven myocardial infarctions, eight coronary bypass operations, nine coronary angioplasties, and one nonhemorrhagic stroke. For a coronary calcification score of 680 (representing 50% stenosis), the sensitivity for indicating a cardiovascular event was 50%, and the specificity was 95%.

The ability to identify individuals at risk for acute coronary events is of tremendous value because of several large studies indicating that the introduction of "lipid-lowering" agents in patients with known or suspected coronary artery disease reduces the potential for cardiac events by 20%–30% (51–53).


   CONCLUSIONS
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
 ELECTRON BEAM CT
 HELICAL CT
 CORONARY CALCIFICATION AND...
 CONCLUSIONS
References
 
The accumulation of calcification in the coronary arteries is an organized, regulated process similar to new bone formation; it is not a passive precipitation of calcium phosphate crystals as once thought. The relationship of arterial calcification to the probability of plaque rupture is unknown, although the amount of coronary calcification correlates with the extent of atherosclerosis. Therefore, establishing clinically useful threshold values for coronary calcification is needed to make appropriate patient management decisions. There is also a need for additional follow-up studies to increase the understanding of this association.

Fluoroscopy, electron beam CT, and helical CT all can demonstrate calcified plaques. Electron beam CT and helical CT can reveal smaller and less dense calcific deposits. The absence of calcification implies the absence of angiographically significant coronary vessel narrowing; however, it does not imply the absence of unstable atherosclerotic plaque.

Although electron beam CT has been shown to be sufficiently accurate for predicting the presence of angiographic stenoses somewhere in the coronary arteries and has the potential for predicting the likelihood of cardiac events, the full importance of coronary calcification will have to await additional data that are currently being obtained.


   Acknowledgments
 
The author expresses appreciation to Suzanne Link and Heather Anderson for typing the manuscript.


   Footnotes
 
CME FEATURE This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician's Recognition Award. To obtain credit, see the questionnaire on pp 1633-1640.

LEARNING OBJECTIVES After reading this article and taking the test, the reader will: • Be able to summarize the pathophysiology involved in the development of coronary atherosclerosis. • Recognize the importance of calcification as being representative of the extent of atherosclerotic plaque burden. • Understand the importance of calcification as an indicator for the presence of coronary artery stenosis and as a predictor for the likelihood of sustaining a cardiac event.

See the commentary by McLoud following this article.


   References
Top
 INTRODUCTION
 PATHOPHYSIOLOGY OF CORONARY...
 IMAGING OF CORONARY...
 ELECTRON BEAM CT
 HELICAL CT
 CORONARY CALCIFICATION AND...
 CONCLUSIONS
 References
 

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