For Physicians

Coronary Calcium Scanning: A Guide for Physicians, Elizabeth Klodas, M.D.

Coronary artery disease (CAD) and its manifestations is the leading cause of death and disability in the United States in both men and women. Patients are usually first diagnosed with CAD when they develop symptoms, display an abnormal response to stress testing, or undergo coronary angiography. Unfortunately, by that time, the atherosclerotic process is relatively advanced, and many patients already have experienced myocardial infarction or activity limiting angina. In many ways, the opportunity for prevention has been missed or, in retrospect, delayed in these patients. Early detection of CAD could impact this scenario significantly by accelerating prevention efforts and positively impacting patient lifestyle choices, before the development of clinical manifestations of heart disease.

Coronary artery calcification scanning affords the opportunity to determine very accurately and noninvasively whether or not underlying coronary artery disease is present, as well as provide an estimate of the extent and severity of coronary disease. This information can then be utilized to optimize patient care, helping to appropriately tailor prevention goals and to determine further evaluation and follow up, if needed.

The purpose of this guide is to provide you with some background information regarding coronary calcium scanning. We hope that this will allow you to gain some appreciation of the technology itself, its capabilities and weaknesses, and its potential role in the milieu of already established testing modalities (from stress testing to angiography).

We will also provide you with an outline to help you interpret the results of the scan, as well as an algorithm to assist you in the further care of patients who have undergone this test.

According to 1996 estimates:

  • Nearly 59 million Americans have one or more forms of cardiovascular disease.
  • Coronary heart disease caused nearly 500,000 deaths in 1996 and is the single leading cause of death in America today.
  • From 1979 to 1996, the number of cardiovascular operations and procedures increased 355 percent according to the American Heart Association.

Normal: No Identifiable Plaque
Normal Reading

Moderate: Definite Plaque Burden
Moderate Reading

High: Extensive Atherosclerotic Plaque Burden
High Reading


Rationale for Coronary Calcium Evaluation

Atherosclerosis is the only process which results in the deposition of calcium within the walls of arteries. Calcification of the arterial bed is NOT a degenerative process and is NOT related to the aging process itself. Indeed, this is a very active metabolic process, in many ways similar to bone ossification. Early on in the atherosclerotic process, calcium deposits are very small and difficult to detect with conventional x-ray imaging, and this had created a significant impediment to the use of coronary calcium as a marker of CAD.
Over the past few years CT scanning has advanced to the point that detection of even minuscule calcium deposits can be accomplished quite easily. CT scanning, therefore, affords an opportunity to detect coronary calcium, and by inference, coronary atherosclerosis, early – before patients develop clinical manifestations of coronary disease.

The Technology Involved

Until recently, it was not possible to accurately assess the coronary arteries with CT scanning. This was related to the fact that the heart moves within the chest cavity with every cardiac cycle. Traditional CT scanners operate by having the image gantry rotate about the patient, then recocking after every image. This requires a finite amount of time for each tomographic slice to be acquired, far in excess of a single cardiac cycle, resulting in a blurred image. With helical (spiral) CT, the gantry rotates about the patient in a continuous loop, allowing the images to be acquired significantly faster, but still not at the speed needed to “freeze the heart”. Electron beam CT scanners (EBCT) utilize an electron gun (rather than mechanical parts) to acquire images, allowing for very rapid image acquisition (100 msec/slice), and, therefore, relatively straightforward assessment of coronary calcium deposits. However, EBCT scanners do not provide equivalent resolution for general radiologic studies, and they are very expensive, making their availability and utility quite limited.
Most recently, a simple modification of the helical CT technique, namely gating image acquisition to the ECG, has made possible the accurate assessment of coronary calcium with the more readily available, higher resolution helical CT scanners. By gating image acquisition, only those frames acquired during diastole (a time of relative cardiac standstill) are utilized for analysis of coronary calcium. This new development in imaging capability represents a significant breakthrough, because coronary calcium assessment can now be made available on a much wider scale, with equivalent accuracy to EBCT.

Heart CT at CMC is the noninvasive method to assess the extent of coronary artery calcification utilizing gated helical CT imaging.


Capabilities and Limitations of Coronary Calcium Scanning

Two basic pieces of information are provided by a Heart CT evaluation:

  • qualitative evaluation of the presence or absence of coronary calcium
  • quantitative evaluation of the degree/extent of calcification

The presence of ANY coronary calcium signifies that underlying CAD is present. The “calcium score” (an amalgamation of total size and density of the calcific deposits found throughout the coronary tree) provides a quantitative evaluation of extent of plaque burden.

In general, the higher the “score” the larger the plaque burden and the higher the risk of subsequent cardiac events in both symptomatic and asymptomatic patients. Although the relationship between the calcium score and severity of luminal narrowing has been found to be nonlinear, data regarding specific thresholds exist to help utilize the score in a clinically meaningful context.

A limitation of coronary calcium scanning is that although calcium deposition occurs relatively early in the atherosclerotic process, plaque material very initially is not calcified. Therefore, very minimal atherosclerotic changes may be missed by this technique. It is important to keep this point in mind – and the results of the scan should be viewed as only one, albeit powerful, component in the assessment and management of a particular patient.


Understanding the Calcium Score

A calcium score of 0 indicates absence of detected calcium, an extremely low likelihood of any obstructive CAD (negative predictive value 95-100% for stenosis >/=50%), and a good prognosis. In most studies, a score 400 implies the presence of extensive CAD, with a high likelihood (>90%) of at least one significantly obstructed vessel (>70% stenosis). Patients with scores >400, would be considered at high risk for subsequent development of symptomatic cardiac disease.

A score between 10 and 400 indicates a moderate plaque burden, and is associated with an intermediate, although significant risk of future cardiac events, especially when scores are >100. The odds ratio of developing symptomatic cardiovascular disease has been reported to be as high as 7:1 in patients with scores >50, 20:1 in patients with scores >100 and 35:1 in those with scores >160.

The risk stratification capability of coronary calcium scoring is especially significant when compared to the predictive powers of traditional risk factors in foretelling the development of symptomatic coronary disease: 1.8:1 for total cholesterol >240mg/dl; 1.8:1 for HDL400) should probably undergo stress testing to evaluate for inducible ischemia. Patients with scores in the intermediate range require individualized assessment of the need to undergo further testing (based upon age, clinical presentation, etc).

The clinical significance of a particular score is influenced by the patient’s age and gender. A score of 150 may be “average” for a 70 year old man, but would be considered markedly abnormal for a 40 year old woman. The correlation between calcium score and plaque burden is identical in men and women; however, just as clinical manifestations of CAD are delayed in women as compared to men, so is the development of coronary calcium. Table 1 displays expected percentile ranges of calcium scores stratified by sex and age.


Table 1 – Percentile Range of Coronary Artery Calcium Score in Asymptomatic Women and Men as a Function of Age

Percentile Rank
Women N=502
Men N=1.396
40-4950-5960-6940-4950-5960-69
10000001
250010215
500020130100
75138010150325
90104011840380700


Utilizing the Heart CT Results/Score

(1) The presence of any coronary artery calcification, alone, can impact care significantly in that secondary prevention goals may be more appropriate for patient management, affecting aggressiveness of lipid lowering, BP therapy, etc. The discovery of advanced or early coronary calcium may also prompt evaluation for less “traditional” risk factors, such as serum homocysteine, Lp(a), and risk factor screening of the patient’s family members. The discovery of any amount of coronary calcium may provide strong incentive for the patient to undertake healthy lifestyle modifications, potentially improving long-term prognosis.
(2) The calcium score should influence the decision about whether or not further cardiac testing is required. Patients with high calcium scores (>400) should probably undergo stress testing to evaluate for inducible ischemia. Patients with scores in the intermediate range require individualized assessment of the need to undergo further testing (based upon age, clinical presentation, etc).

In the absence of coronary calcium, no further functional testing is likely to be required.
These recommendations are summarized in Table 2:


Table 2 – Calcium Score Guidelines

Calcium Score
Plaque Burden
Probability of Significant Coronary Artery Disease
Implications for CV risk
Recommendations
0No identifiable plaqueVery low, generally 5%Very lowReassure patient, discuss general public health guidelines for primary prevention of CV disease
1-10Minimal identifiable plaque burdenVery unlikely, under 10%LowDiscuss general public health guidelines for primary prevention of CV disease
11-100Definite, at least mild atherosclerotic plaque burdenMild or minimal coronary stenoses likelyModeratelyCounsel about risk factor modification, strict adherence with primary prevention goals. Daily ASA.
101-400Definite, at least moderate atherosclerotic plaque burdenNon-obstructive CAD highly likely, although obstructive disease possibleModerately HighInstitute risk factor modification and secondary prevention goals. Consider exercise testing for further risk stratification. Daily ASA.
400+Extensive atherosclerotic plaque burdenHigh likelihood (90+%) of at least one significant coronary stenosisHighInstitute very aggressive risk factor modification. Consider exercise for pharmacologic nuclear stress testing to evaluate for inducible ischemia. Daily ASA.


Indications for Coronary Calcium Scanning

A Heart CT evaluation may be useful in those patients in whom the documentation of the presence of CAD would be expected to change or influence therapy.

  • Patients with borderline lipid levels or mild hypertension may be ideal candidates, helping stratify whether more aggressive and expensive secondary prevention therapies are clearly appropriate for these individuals.
  • Patients with a relatively early family history of coronary disease may benefit from the enhanced risk stratification offered by Heart CT. If premature CAD is detected, this may lead to a search for less “traditional” risk factors, such as homocysteine levels, Lp(a), and wider screening of family members for these and other cardiac risk factors.

  • In the setting of dilated cardiomyopathy, Heart CT may be utilized to noninvasively determine the extent of underlying CAD, helping to assess whether the cardiomyopathy is likely ischemic in etiology. A relatively low calcium score would suggest that the cardiomyopathy is probably idiopathic, viral or metabolic, and that the patient would not be expected to derive clinical benefit from undergoing coronary angiography.
  • Some centers have used coronary calcium scanning to help risk stratify patients who present with chest pain, especially young persons with atypical symptoms. This may represent another useful, cost effective application of the technology.
  • In general, most studies have evaluated patients 40-70 years of age, although younger individuals may be appropriate candidates depending upon their risk factor profile. Incremental clinical benefit is unlikely to be substantial in individuals over the age of 70, and generally Heart CT is not recommended in these individuals.


Contraindications:

Clinical
There is no data to support mass imaging of asymptomatic individuals at the present time. Patients who already have documented CAD are not appropriate for Heart CT evaluation. The results of the scan would not be expected to change management in this patient population.
Procedural
Scanning is accomplished at rest, without contrast administration, without the need for IV access, and in a nonfasting state. No changes in patient medication are required. There are no restrictions with respect to pacemakers or prosthetic devices.

Patients with arrhythmias (chronic atrial fibrillation, very frequent extrasystoles) or patients with relative resting tachycardia (HR>90-95 bpm) should not undergo Heart CT scanning, because adequate cardiac gating will be difficult to accomplish, compromising image quality. Because scanning does involve minimal x-ray exposure, women who are pregnant or potentially pregnant should not undergo this evaluation.


How does Heart CT fit in with other cardiac tests?

Heart CT’s major role is in providing extremely sensitive assessment of the presence of early CAD, and extent of plaque burden. Compared to other imaging modalities, Heart CT is not as helpful in defining location of significant coronary stenoses or in defining clinical prognosis.

The most powerful cardiology tool for defining clinical prognosis is nuclear stress testing, and, therefore, patients with significant coronary calcium deposition should be preferentially considered for an exercise or pharmacological nuclear stress testing for further evaluation.

In general, echocardiography is superior for evaluating valvular structures and valvular function. Although left ventricular function assessment is possible with CT scanning, this involves contrast infusion, a less than desirable requirement in view of the potential side effects of iodinated contrast administration.

Heart CT is not a replacement for coronary angiography. At the present time, coronary angiography represents the only reliable technology to accurately assess luminal narrowing within the coronary circulation.

Cardiac CT scanning may be utilized to evaluate the pericardium in patients with suspected constrictive pericarditis, and may be useful in evaluating the right ventricle in rare patients with suspected right ventricular dysplasia. For nearly all other indications, non-CT imaging modalities are more appropriate and/or clinically useful.


Bibliography

Wexler L, Brundage B, Crouse J, Detrano R, et al. Coronary artery calcification: pathophysiology, epidemiology, imaging methods, and clinical implications. A statement for health professionals from the American Heart Association. Circulation 1996;94:1175-1192.

Rumberger JA, Brundage BH, Rader DJ, Kondos G. Electron beam computed tomographic coronary calcium scanning: a review and guidelines for use in asymptomatic persons. Mayo Clin Proc 1999;74:243-252.

Janowitz WR, Agatston AS, Kaplan G, Viamonte M. Differences in prevalence and extent of coronary artery calcium detected by ultrafast computed tomography in asymptomatic men and women. Am J Cardiol 1993:72:247-254.

Simons DB, Schwarz RS, Edwards WD, Sheedy PF, et al. Noninvasive definition of anatomic coronary artery disease by ultrafast computed tomographic scanning: a quantitative pathologic comparison study. J Am Coll Cardiol 1992;20:1118-1126.

Guerci AD, Spadaro LA, Goodman KJ, Liedo-Parez A, et al. Comparison of electron beam computed tomography scanning and conventional risk factor assessment for the prediction of angiographic coronary artery disease. J Am Coll Cardiol 1998;32:673-679.

Rumberger JA, Behrenbeck T, Breen JF, Sheedy PF. Coronary calcification by electron beam computed tomography and obstructive coronary artery disease: a model for costs and effectiveness of diagnosis as compared with conventional cardiac testing methods. J Am Coll Cardiol 1999;33:453-462.

Comparison of Electron Beam and Helical CT in the Detection of Coronary Artery Calcification, K.D. Hopper, M.D., Hershey, PA, D.C. Strollo, M.D., D. Mauger, PhD. Radiologic Society of North America, 1998 Scientific Program.


Biography

Elizabeth Klodas, M.D., is Director of Cardiac Imaging at Center for Diagnostic Imaging (CDI). Dr. Klodas received her Doctorate from the University of Toronto in Ontario, Canada. She then completed a comprehensive medicine internship at Toronto General Hospital in Ontario, and an internal medicine residency with the Mayo Graduate School of Medicine in Rochester, Minnesota. She spent five years studying cardiovascular diseases during fellowships with both the Mayo Graduate School and Johns Hopkins School of Medicine in Baltimore, Maryland, focusing on non-invasive cardiology and echocardiography research. Prior to joining CDI, Dr. Klodas practiced at St. Paul Heart Clinic. During this time, she attained her board certification in Nuclear Cardiology, established the Nuclear Cardiology department and received the Certificate of Accreditation in Comprehensive Nuclear Cardiology Testing.
In the course of her career, Dr. Klodas has received numerous honors and awards including the Internal Medicine Outstanding Achievement Award from the Mayo Graduate School of Medicine, Physician of the Year Award from the St. Paul Heart Clinic, and has been recognized for outstanding presentations in her specialty by the American College of Chest Physicians. She has also authored and co-authored many articles and book chapters on cardiovascular diseases and diagnostic imaging.

Dr. Klodas’ professional affiliations include the American Heart Association, the American College of Cardiology and the American Society of Nuclear Cardiology, of which she is a founding member of the Upper Midwest Working Group.