Coronary artery calcium scoring

# Overview
The presence of coronary artery calcification indicates underlying CHD. The coronary artery calcium (CAC) scan is a non-contrast CT scan used to visualize the extent of calcification in the coronary vessels. While there is a strong correlation between CAC burden and coronary plaque area, CAC scans do not identify noncalcified plaques that are capable of erosion or rupture and therefore may not be a good predictor of luminal obstruction. 
The role of CAC in patients with low or intermediate risk of developing a CHD event is uncertain. However, it is not indicated for patients at high risk of CHD as aggressive preventative measures would have already been initiated.

# Coronary Artery Calcium Scoring Systems

## Agatston Method
- The Agatston score is a scoring system that uses images obtained from a non-contrast CT.
- It is determined by the number of calcific lesions, the area of each lesion and the peak HU of each lesion detected. The score for every calcific lesion is based on its density score and area (mm2).
- The density score is determined by the peak HU and is as follows:
- 1 = 130 - 199 HU
2 = 200 - 299 HU
3 = 300 - 399 HU
4 = >399 HU
- 1 = 130 - 199 HU
- 2 = 200 - 299 HU
- 3 = 300 - 399 HU
- 4 = >399 HU
- If a lesion had a peak HU of 250 and an area of 4 mm2, it would receive a score of 8.
- The CAC is the sum of the scores assigned to each calcific lesion.
- The degree of calcification has also been shown to vary depending on certain demographic factors including age, gender and ethnicity.
- Taking this into consideration, the CAC score using the Agatston method can either be presented as an absolute value or as a percentile after adjusting for these three factors. Percentiles can be obtained from the Multi-Ethnic Study of Atherosclerosis (MESA) website.
The CAC can be stratified as the following:
Calcium Volume Score
- The calcium volume score is calculated by multiply the number of voxels with calcification by the volume of each voxel. This would include all voxels with a HU score of greater than 130.
Relative Calcium Mass Score
- The relative calcium mass score is calculated by multiplying the mean attenuation of the calcified plaque by the plaque volume in each image. 

# Role of CAC score in Clinical Practice
- The presence of CAC indicates underlying CHD.
- While there is a strong correlation between CAC burden and coronary plaque area, CAC scans do not identify noncalcified plaques that are capable of erosion or rupture. Therefore, CAC is not a good predictor of luminal obstruction.
- CAC scores have been shown to improve upon the current Framingham Risk Score, providing a significant increase in the accuracy of risk stratification. However, there is no prospective data that indicates that CAC screening results in a reduction of coronary events.
- CAC has also been demonstrated to be an independent predictor of major cardiovascular events.
CAC Scores in Asymptomatic Patients
- Various studies have shown that asymptomatic patients with a CAC score of zero have a low risk of CHD event in the long term.
- The use of the CAC score is not indicated in asymptomatic high risk patients as aggressive preventive measures would have already been initiated.
- Guidelines regarding the use of CAC in low and intermediate risk individuals have not been consistent.
- In 2010, the American College of Cardiology stated that the use of CAC is appropriate in asymptomatic, low risk individuals with a family history of CHD and in individuals with an intermediate risk of CHD (10%-20% 10 year risk of CHD). However, this has changed in the 2013 guidelines.
- The latest recommendation by the ACC is that CAC can be used to guide risk based decision making if after quantitative risk assessment, a risk based decision is uncertain.
- Due to concerns related to cumulative radiation exposure, routine serial CAC scans are not currently recommended.
CAC Scores in Symptomatic Patients
- The use of the CAC score alone is limited in symptomatic patients.
- The pretest probability of a CHD event should always be considered when interpreting a CAC score.
- The ACCF/AHA Expert Consensus suggests that the CAC score can be used to help rule out obstructive coronary disease in low risk patients presenting with atypical chest pain.
- The National Institute for Health and Care Excellence (NICE) recommends that a CAC score can be used in patients with chest pain and a 10% to 29% estimated likelihood of CAD (based on the modified Diamond and Forrester Criteria). If the CAC is:
0: Consider other causes of chest pain
1-400: Offer CT coronary angiography
400: Offer invasive coronary angiography.
- 0: Consider other causes of chest pain
- 1-400: Offer CT coronary angiography
- >400: Offer invasive coronary angiography.

# Latest Guidelines