## 2 Coronary Artery Calcium Score
### Imaging Acquisition, Reconstruction, and Quantification of CAC  
For CAC assessment, acquisition and reconstruction settings are standardized. Data are usually acquired using prospective electrocardiographic triggering in late diastole, with a section thickness of 2.5 mm, a section interval of 1 mm and without contrast material administration, using 120 kV tube voltage [38].  
Electron-beam computed tomography (EBCT) and multi-detector computed tomography (MDCT) have been the main CT methods for CAC measurements, using fast scan speeds to reduce motion artifacts, with MDCT being associated with improved spatial resolution and largely replacing EBCT in practice [40].  
Section thickness in the axial soft-tissue window and axial lung window settings for reconstruction is set to 2.5 mm. The CAC score is then determined using vendor-provided software, often measured with the use of the Agatston method, with calcification typically being defined as high attenuation (130 HU) and an area \(\geq\) 1 mm [38; 41]. The overall plaque area and a cofactor based on the attenuation of the plaque calcium in Hounsfield units are combined to get the Agatston score [38; 42; 43]. The area of the lesion (Ai) is multiplied by a weighting factor (wi) based on the maximal CT number (CTmax) in the segment of interest to determine the calcium score for each desired segment (Csi): CSi = wi x Ai, where wi is 1 if 130 HUCTmax <200 HU, 2 if 200 HU \(\leq\) CTmax < 300 HU, 3 if 300 HU \(\leq\) CTmax < 400 HU, 4 if 400 HU \(\leq\) CTmax. Agatston score for each artery, each calcification, or the entire heart, also known as total calcium score (TCS), is calculated by summing the respective values for the regions of interest [41; 43].  
The Agatston score can be reported either as an absolute value with predetermined cut-offs or as a percentile compared to age-, sex-, and ethnicity-matched individuals using the Multi-Ethnic Study of Atherosclerosis (MESA) database [44]. As an alternative to total plaque burden, the following description of the total Agatston calcium score and the corresponding risk category is suitable: no coronary calcium, 1-9: minimal, 10-99: mild, 100-399: moderate, and \(\geq\)400: severe (Figure 4) [45]. However, the absolute measurement strategy outperformed the age-gender-ethnicity method, according to the MESA trial, even though both approaches produced effective risk categorization. In that study, even the lowest CAC (Agatston score of 1-10) was associated with a 3-fold increased risk of CAD than patients with an Agatston score of 0, and a score greater than 100 was associated with a 10-fold increased risk [44].  
Another method for CAC assessment was developed based on volume scoring that represents the volume of the calcification; this score has become popular due to its relative resistance to slight variations in noise and its robust interscan reproducibility [46]. It is expressed with the product between the volume of one voxel and the number of voxels in the volume dataset related to the calcification. Nevertheless, there are several limits including the overestimation of the CAC grade in areas of strong attenuation and the vulnerability to partial volume averaging of this method. Another scoring system is the mass score, which is derived using the plaque volume, plaque attenuation, and a calibration factor that considers the water attenuation. Even though it is an up-and-coming method based on the measurement of a true mineral mass of calcium hydroxyapatite in the plaques, it requires complex postprocessing, and limited supporting data have been available so far [41; 46]. Finally, [PERSON] et al. proposed another score for clinical use, the calcium coverage score, defined as the percentage of coronary arteries affected by calcified plaque, which was highly associated with coronary heart disease events and provided information about cardiovascular occurrence beyond the calcium burden defined by the Agatston or calcium mass score [47].