© RSNA, 2008
Coronary Calcium Coverage Score: Determination, Correlates, and Predictive Accuracy in the Multi-Ethnic Study of Atherosclerosis
Appendix E1
Evaluation of the Spatial Distribution of Calcium
As part of routine CT reading in the MESA, a cardiologist or radiologist reader identified the LM/LAD, LCx, and RCA on CT images by indicating waypoints along the visualized or anatomic course of the arteries. The coronary ostia (for the RCA and LM/LAD) or origin (for the LCx) was recorded as the starting position of each artery. The cardiac apex was defined as the ending position for the LAD and RCA. A point at the level of the coronary sinus was used as the ending position of the LCx. The marked waypoints defined the trajectory of each artery and allowed calculation of the three-dimensional distance from the ostium to the calcified lesions.
The MESA reading protocol (1) defined a calcified region as at least four contiguous voxels in a section with an attenuation level greater than 130 HU and located along the expected anatomic course of the coronary arteries as defined by the waypoints and not judged to be artifact or noncoronary calcification. The information stored for each calcified region consisted of the x, y, and z coordinates of the voxel with the highest attenuation level in that region, the attenuation level of that voxel, the mean attenuation level of the voxels that composed the region, the area of the region, and the artery in which the region was located. The attenuation levels of the voxels in the CT images analyzed here were adjusted according to the phantom attenuation levels as described by Nelson et al (2).
The Figure shows a three-dimensional plot of a participant's traced trajectory for the LCx, with the waypoints numbered consecutively from the ostium of the artery. The lines connecting the waypoints specify the trajectory of the artery. The × symbol marks the coordinates of the center of a calcified region as defined by the voxel in the calcified region with the highest attenuation level. Note that the center of a calcified region is not necessarily along the trajectory of the artery and does not necessarily mark a geometric center of the calcified region. The centers of the calcified regions were then mapped back along a line perpendicular to the trajectory of the artery to determine the closest corresponding point (x*, y*, z*) along the arterial trajectory:
 |
(E1) |
where
 |
(E2) |
and (xm, ym, zm) and (xp, yp, zp) are the coordinates of the two waypoints closest to the center of the calcified region.
 |
| Illustration of how distance to a calcified region is calculated and subdivisions are defined for one subject. In both plots, the numbers represent the consecutive waypoints marking the trajectory of the artery. The top plot shows how these points are traced to construct a representation of the trajectory of the artery (solid lines). The × symbol represents the center of the calcified lesion. The red circle indicates the closest point along the trajectory of the artery to the lesion and is labeled a. The bottom plot shows the artery stretched along its length. The 5-mm subdivisions are defined by the dashed lines. |
The distance from the arterial ostia to each calcified region along the arterial trajectory was calculated using the following algorithm:
- Any two consecutive waypoints (xj, yj, zj) and (xj+1, yj+1, zj+1) defined a linear segment of length:
 |
(E3) |
where j labels the linear segments and J is the total number of segments.
- The distance from the arterial ostia to the mapped calcified region (x*, y*, z*) was the sum of the lengths of all (k − 1) segments proximal to the calcified region plus the distance from (x*, y*, z*) to the kth (k < J) waypoint; so the total absolute distance to the calcified region was
 |
(E4) |
The relative distance to each calcified region is given by d/L, where L is the total arterial length and is calculated as
 |
(E5) |
Using the Figure as an example, we see that 12 waypoints (numbered 1,...,12) were identified along the length of the LCx. In this example, there is one calcified region whose center is shown by ×. The closest waypoints to this calcified region are labeled 3 and 4. The closest point to the center of the calcified region along the trajectory of the artery is marked as a red circle and labeled a. To calculate the distance along the arterial trajectory to point a, we first calculate l1 and l2, the distance between points 1 and 2 and points 2 and 3, respectively. The distance to point a is then calculated as l1 + l2 added to the distance from waypoint 3 to point a.
References
1. Carr JJ, Nelson JC, Wong ND, et al. Calcified coronary artery plaque measurement with cardiac CT in population-based studies: standardized protocol of multi-ethnic study of atherosclerosis (MESA) and coronary artery risk development in young adults (CARDIA) study. Radiology 2005;234:35-43.
2. Nelson JC, Kronmal RA, Carr JJ, et al. Measuring coronary calcium on CT images adjusted for attenuation differences. Radiology 2005;235:403-414.
