CT Coronary Calcium Score

The coronary artery calcium (CAC) score is a non-contrast, electrocardiogram gated CT of the heart that measures calcified plaque in the coronary arteries.

The scan covers the heart from the carina to the diaphragm in 3 mm axial slices (or 2.5 mm on modern scanners) acquired in mid-diastole with prospective gating, in a single breath hold of about 10 seconds, at 120 kV.

A lesion is counted if it has a density over 130 HU and an area of 1 mm2 or more (at least 3 contiguous pixels).

The score is calculated from area and a density weighting factor for each lesion (1 for 130 to 199 HU, 2 for 200 to 299, 3 for 300 to 399, and 4 for 400 HU or more). The scores are summed over all lesions in the four main coronary arteries (left main, left anterior descending, left circumflex and right coronary artery).

Scores are read as 0, 1 to 99, 100 to 399, and 400 or more, or as age, sex and ethnic group centiles. Effective dose is low, about 0.5 to 1.5 mSv, and the examination takes 10 to 15 minutes with no contrast, no cannula, and no heart rate drug in most cases.

Coronary calcium reflects atherosclerotic plaque burden, correlates with total plaque (but not with the degree of stenosis) and strongly predicts cardiovascular events, independent of conventional risk factors.

A score of zero carries a very low 10-year risk of events (under 1% in most groups), with a negative predictive value of over 95 to 98% for obstructive disease in asymptomatic people. Scores over 400 carry a 10-year risk of coronary events over 10 to 20%, and over 1000 a greater risk.

Prevalence of calcium increases with age: present in about 50% of men over 55 and women over 65.

Indications. Asymptomatic adults with intermediate cardiovascular risk (for example, a 10-year risk of 5 to 20%), or borderline risk with risk enhancing factors, to decide whether to start or intensify statin treatment. Asymptomatic adults with a strong family history of premature coronary disease. Selected patients with diabetes aged 40 to 75 years.

Patients reluctant to take statins or hesitating about treatment, in whom a zero score may allow deferral. Stable chest pain with low pretest probability, as an alternative to a stress test in some centres. Ancillary use: assessing the coronary calcium on routine chest CT (every non-gated chest CT should report visible coronary calcification).

Not for patients with known coronary disease, symptomatic patients needing diagnostic testing, low risk young people, or very high risk people already on treatment.

Contraindications and precautions. No absolute contraindications; pregnancy is a relative contraindication to any ionising radiation. Patients must be able to lie flat and hold their breath for 10 seconds. The result does not rule out non-calcified plaque, and a zero score should not be taken as lifelong protection.

The patient should understand that the scan is a risk assessment test with possible incidental findings that need follow up. Caffeine and smoking before the scan are avoided if possible, and a regular heart rate under 80 to 90 beats each minute gives acceptable results.

Preparation and technique. No fasting and no contrast. Three electrocardiogram electrodes are placed, and the patient is positioned supine, arms raised over the head. A scout identifies the heart.

Prospective electrocardiogram triggering at 60 to 80% of the R-R interval (or at 40 to 50% for faster rates) acquires the heart in 3 mm slices. The calcium is quantified with dedicated software that identifies calcified lesions in each artery, and the reader checks that lesions are coronary and not valve, aortic wall, or mitral annulus calcification.

The score is reported as the total score, scores for each artery, number of vessels, and the age, sex and ethnic group percentile. Volume score and mass score can also be reported. Iterative reconstruction and low tube voltage settings (100 kV) are possible, with calibration checks for consistency of the score between scanners.

Normal appearances. No calcified plaque in any coronary artery (score 0). The coronary arteries are not seen individually on this non-contrast scan except for calcium within them, and the epicardial fat and heart are normal. Calcification is absent in most young adults, and the aortic valve, mitral annulus, aortic root and pericardium are free of calcification in a normal scan.

A score of zero in a person over 60 occurs in about 20 to 30% of women and 10 to 15% of men.

Interpretation.

• Score 0: no identifiable plaque; very low risk of events over the next 5 years (annual event rate under 0.1%), and the chance of obstructive disease is under 5% in asymptomatic people.
• Statins may be deferred if there are no other strong indicators, although a repeat scan in 5 to 7 years can be considered.
• Smokers, diabetics and those with a strong family history do not gain full reassurance.
• Score 1 to 99 (mild): confirmed atherosclerosis, with a relatively low but not negligible risk; lifestyle measures and statin therapy are favoured, particularly over age 55 and in those with other risk factors.
• Score 100 to 399 (moderate): clear atherosclerosis and a moderately increased risk (annual risk 1 to 2%); statin and aspirin discussion, and intensive risk factor control.
• Score 400 or more (severe): high risk of events and of obstructive disease (annual risk over 2 to 3%); high intensity statin therapy, blood pressure and glucose control, and consideration of functional testing if symptomatic.
• Score 1000 or more: very high risk, comparable to established disease, with cardiology review.
• Age, sex and ethnic group percentile: a score over the 75th percentile is high for age and warrants earlier and more intensive prevention even if the absolute score is moderate; a low score for age is reassuring.
• Distribution: left main or multivessel involvement carries higher risk than single vessel disease; calcium density is inversely related to risk of rupture, so dense calcium is more stable than a similar volume of spotty low density calcium.
• Non-coronary findings: aortic valve and mitral annular calcification, aortic root and thoracic aorta calcification and dilatation, pericardial calcification, pulmonary nodules, emphysema, interstitial abnormality, hiatus hernia, breast and thyroid lesions, and upper abdominal organs; these are reported and followed up on their own merits.
• Progression: annual increase of 15 to 20% is usual in untreated disease; rapid progression (over 15% each year or over 100 units) predicts events; statins increase calcium density, so a rising score on treatment does not mean failure.

Critical values and findings needing immediate action.

• Score 400 or more or 1000 or more with symptoms: cardiology review for functional testing or CT coronary angiography.
• Large coronary calcification in a young person (under 45 years) or any score with familial hyperlipidaemia: lipid specialist and cardiology review.
• Extensive left main calcification: cardiology review.
• Findings suggesting acute pathology (aortic dissection flap, large pericardial effusion, pulmonary embolism): immediate action.
• Incidental lung mass or large nodule: referral for diagnostic CT and oncology review.
• Severe aortic valve calcification with symptoms: echocardiography and cardiology review.

Limitations and artefacts.

• The score reflects calcified plaque only; young people and some high risk groups (smokers, diabetics, familial hypercholesterolaemia, HIV) may have non-calcified plaque with a zero score, and a zero score does not exclude obstructive disease in symptomatic patients (about 2 to 5% have stenosis).
• A high score does not show how severe the stenosis is or whether ischaemia is present.
• Motion from fast heart rate, ectopic beats and breathing blurs and misplaces calcium; image noise in obese patients and metal (pacemaker leads, sternal wires) hide lesions.
• Calcification of the aortic wall, valve and mitral annulus can be wrongly counted; the reader must exclude them.
• Variation between scanners, slice thickness and software changes the score by 10 to 20% on repeat scans; the same method must be used for follow up.
• The scan results in radiation and may lead to incidental findings and further tests; use only when the score would change management.
• Scores for age, sex and ethnic group are based on reference populations that may not match the patient (the data are from specific populations).
• Results should be integrated with risk calculators and clinical judgement rather than used alone.

Special groups.

• Women: calcium appears 10 years later than in men, and the risk at any score is similar; the test helps in intermediate risk women.
• Younger adults (under 40) and those with premature family history: a low or zero score may mislead; clinical risk factors count more.
• Diabetics: higher scores and risk; a zero score gives less reassurance than in non-diabetics.
• Patients with chronic kidney disease: very high calcium due to mineral disorder, with limited prognostic precision.
• Patients on statins: score will not fall; do not repeat within 5 years unless a change in management is planned.
• Pregnancy: avoid.
• Regions: coronary disease in younger people is increasing in urban populations, but access to scoring is limited; clinical risk assessment and lipid and glucose testing are used to decide on treatment, and calcium scoring can be useful in borderline cases where available.

Follow up tests. CT coronary angiography or functional testing (stress echocardiography, nuclear perfusion imaging, stress MR) if symptomatic, lipid profile (total, LDL and HDL cholesterol, triglycerides, lipoprotein(a)), glucose and haemoglobin A1c, blood pressure measurement, renal function, high sensitivity C-reactive protein, family screening in familial hyperlipidaemia,

electrocardiogram and exercise test, repeat score in 3 to 5 years if management would depend on it, and referral to cardiology, preventive cardiology or lipid clinic.