The detection of calcium in the coronary arteries by CT scanning can be used to assess the presence of coronary artery disease in patients in a non invasive manner.

Rationale for Coronary Calcium Evaluation

There is a strong correlation between the amount of calcium in the coronary arteries and the total burden of atherosclerotic plaque. Lipids accumulate in the vessel wall of the coronary arteries and the lipids in stable plaques often calcify.

Calcified plaques are evidence for the presence of coronary atherosclerosis and are considered to indicate the risk of the patient for a future coronary event. Calcified coronary atherosclerotic plaques are believed to be relatively stable as opposed to unstable lipid-rich non-calcified coronary atherosclerotic plaques. It is also well known non calcified unstable soft plaque is more prone to rupture and can lead to an acute cardiac event such as coronary artery thrombosis.

The amount of calcification present is representative of the amount of soft plaque and gives an indication of the atherosclerotic burden. There is about 5 times the amount of soft plaque present compared to calcified plaque. The progression of these calcified plaques also indicates the progression of coronary atherosclerotic disease. Calcified plaques are also often not the sites of significant stenoses. Instead, hemodynamically significant stenoses are often correlated to the presence of non-calcified plaques and located distant to calcifications.

The Technology Involved

Most of the studies published in the literature over the past 10 years have related to use of electron beam CT (EBCT), a type of ultrafast CT scanner that utilizes an electron gun and which can scan very rapidly (100msec/slice) so that images will not be distorted by cardiac pulsations. However with the advent of helical CT scanners especially the newer multislice scanners, scanning times have been reduced to as low as 300msec/slice and it is now possible to scan the heart for calcium using these scanners together with prospective ECG gating.

It is currently the common understanding that The "Gold-Standard" modality for diagnosis of non-calcified plaques is Intravascular Ultrasound (IVUS). Initial results show that contrast enhanced ECG-gated multi-slice CT is able to non-invasively visualize non-calcified atherosclerotic plaques in the coronary arteries. This finding is currently being investigated in ongoing clinical studies .

Calcium Scoring

So-called "Calcium Scoring" is used for identification and quantification of calcified lesions in the coronary arteries. By scanning the coronary arteries, it is possible to obtain a qualitative as well as quantitative evaluation of the presence or absence of coronary calcium.

A calcium score is a calculation based on the total amount and density of the calcific deposits found in the coronary arteries and provides a quantitative evaluation of the extent of plaque burden.

The commonest calcium score used is the Agaston Score which is calculated using a formula suggested by Dr Agaston in 1990. However there is now also possible to calculate a volumetric score based on the volume of calcium present and initial studies show this may be a more reliable and reproducible score.

Value of Coronary Calcium Scoring

There are now many studies that show the prognostic value of the presence of coronary artery calcium in predicting future coronary events.

Arad Y et al Prediction of coronary events with electron beam computed tomography. J Am Coll Cardiol 2000 Oct;36(4):1253-60

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Wong ND, Hsu JC, Detrano RC, Diamond G, Eisenberg H, Gardin JM Coronary artery calcium evaluation by electron beam computed tomography and its relation to new cardiovascular events. Am J Cardiol 2000 Sep 1;86(5):495-8

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He ZX, Hedrick TD, Pratt CM, Verani MS, Aquino V, Roberts R, Mahmarian JJ Severity of coronary artery calcification by electron beam computed tomography predicts silent myocardial ischemia. Circulation 2000 Jan 25;101(3):244-51

EBCT also outperforms stress testing in detection of coronary artery disease

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while calcium scores are predictive of coronary artery disease even in asymptomatic patients. The sensitivity of EBCT at detecting obstructive CAD is 97.0% with a specificity of 72.4% in a recent paper by Dr Peyser ( Circulation 2000:102:308-385

Conversely the negative predictive value of calcium scoring is also well documented as the absence of calcium on CT can be predictive of angiographically normal coronary arteries in elderly patients ( Radiology 1996:199:665-668

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Coronary Artery Calcification: Pathophysiology,Epidemiology, Imaging Methods, and Clinical Implications A Statement for Health Professionals From the American Heart Association

Lewis Wexler, et al Circulation. 1996;94:1175-1192.)

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Recently the American Heart Association and the American College of Cardiology released an Expert Consensus Document on EBCT for the Diagnosis and Prognosis of Coronary Artery Disease

(American College of Cardiology/American Heart Association expert consensus document on electron-beam computed tomography for the diagnosis and prognosis of coronary artery disease J Am Coll Cardiol 2000:36:326-40

This document stated that they agreed that:

1. A negative EBCT test makes the presence of atheroscelortic plaque, including unstable plaque, very unlikely.
2. A negative test is highly unlikely in the presence of significant luminal obstructive disease
3. Negative tests occur in the majority of patients who have angiographically normal coronary arteries
4. A negative test can be consistent with a low risk of a cardiovascular event in the next 2 to 5 years
5. A positive EBCT confirms the presence of a coronary atherosclerotic plaque
6. The greater the amount of calcium, the greater the likelihood of occlusive CAD, but there is not a 1-to-1 relationship, and the findings may not be site specific.
7. The total amount of calcium correlates best with the total amount of atherosclerotic plaque, although the true “plaque burden” is underestimated.
8. A high calcium score may be consistent with moderate to high risk of a cardiovascular event within the next 2 to 5 years.

Indications for Calcium Scoring

The American Heart Association in the above mentioned report states that:

1. Calcium scoring should not be used to screen symptomatic patients in the general population, unless other risk factors are present. Patients with several other risk factors, such as family history of heart disease and slightly elevated blood pressure, for example, could be treated more aggressively if a scan revealed a high calcium score.
2. Calcium scoring can be useful for stratifying moderate – risk and high risk patients.
3. Monitoring progression of atherosclerosis in patients under therapy. To follow the progression of coronary calcification, coronary scanning to determine a significant progression of coronary calcification should be performed in reasonable intervals. Extensive calcification may influence and guide the results of interventional procedures such as angioplasty, stenting, atherectomy or rotor ablation.

Current indications can include:

1. Evaluation of high risk patients with one or more of the recognised risk factors for CAD such as elevated cholesterol, hypertension, family history of CAD etc
2. Patients with atypical chest pain and requiring further evaluation
3. Patients with known coronary artery disease and requiring further assessment of treatment

Understanding the Calcium Score

Calcium Score Interpretation Guidelines:

The calcium score is stratified into five progressive categories:

A Score of 0
No identifiable atherosclerotic plaque (a negative examination). Adherence to general guidelines on diet and exercise are stressed. No recommendations are made for clinical action.

Score of 1 to 10
Minimal identifiable plaque. Significant coronary artery disease is unlikely. Recommendations are to follow the general guidelines on cardiovascular risk reduction.

Score of 11 to 100
Definite but mild plaque. Risk factor modification is recommended, including daily aspirin prophylaxis and strict adherence to National Cholesterol Education Program (NCEP) guidelines. Clinical follow-up is necessary.

Score of 101 to 400
Definite, moderate plaque. Aggressive risk factor modification is recommended with non-invasive stress testing preparatory to an exercise program. Daily aspirin and use of a statin medication to reduce the LDL cholesterol

Score over 400
Significant plaque burden indicating a high likelihood of "significant" coronary stenosis. Aggressive risk factor modification is recommended (including aspirin and a statin medication) with non-invasive stress testing and, possibly, angiography.

If CACS is greater than the 75th percentile for age or gender or involves 2 or more vessels, advice for the category above that indicated by score alone is suggested.              

Is calcium scoring with Multislice CT the same as with EBCT?

Most of the research published previously has been with EBCT. However with multislice CT scanners, which are more widely available, calcium scoring can also be done. Initial reports show that the results are comparable with EBCT with correlation of 96%.

Becker CR et al AJR 2000:174:543-547

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Carr JJ et al. Evaluation of subsecond gated helical CT for quantification of coronary artery calcium and comparison with electron beam CT. AJR 2000:174:915-921

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Useful sites:

www.heartct.com

www.heartinformation.com

http://www.heartworkup.com/

http://www.northshoremrict.com/ct.htm

SAMPLE REPORT

HISTORY:

The patient is 57 years old.

REPORT

High-resolution, cardiac gated Computed Tomography of the chest with attention devoted to the coronary arteries was performed by Toshiba Aquilion Scanner. Coronary calcification is analyzed using ScImage's volumetric calcified plaque analysis software. Interpretation of coronary calcification is provided below:

RESULTS:

In general, the lower the percentile rank the lower the cardiac risk. This percentile rank assumes the absence of symptoms and does not account for risk factors or for the number of calcified vessels. In general, the greater the symptoms, the more the risk factors, the more vessels demonstrating calcification, the greater is the likelihood of significant coronary artery disease as seen on angiogram. The percentile ranking is based on a study performed at the University of Illinois on greater than 19000 patients without symptoms.

This diagram demonstrates the LOCATION of coronary artery calcifications only, but DOES NOT NECESSARILY INDICATE THE PRESENCE, ABSENCE OR LOCATION OF A STENOTIC LESION

The Calcium Score should be interpreted in the context of several factors:

• Clinical decision-making requires the Calcium Score to be weighed along with other factors, i.e. the number of calcified vessels, the patient's age, gender, symptoms and risk factors.
• "Normal" score for any age is ideally zero. The Calcium Score has greater significance when it is above the 75th percentile of age and sex group, or if calcium is present in 2 or more vessels.
• Since coronary artery calcium is a definite marker for the presence of atherosclerosis which is a progressive disease process, periodic scans (1-3 years) will be useful in assessing the rate of progression of atherosclerosis.
• A score of zero indicates no coronary artery calcification and this implies the absence of significant angiographic coronary narrowing in 99% of cases. It does not absolutely rule out the presence of soft non-calcified plaque, especially in younger patients and those who smoke heavily.