Spatially Weighted Calcium Score Beyond Power of Zero: Worth the Effort?

Abstract

Coronary artery calcification (CAC) score has evolved as a very robust marker of subclinical coronary atherosclerosis. It has proven to be that CAC is a consistent test to predict coronary heart disease and cardiovascular events across a wide range of patient profiles.1,2 Due to this consistency in detecting highand low-risk individuals, CAC has recently been incorporated in primary prevention guidelines to aid in the risk stratification and decision making to allocate risk reducing strategies.3–5 In those guidelines, the power of zero (ie, CAC=0) is now accepted as a negative or low-risk marker. However, this classification is based on the traditional Agatston CAC score, which considers the area and density of CAC calcifications detected on the coronary arteries in an ECG-gated noncontrast computed tomography with well-defined protocols with include thick 3-mm slices and a fixed threshold to define vessel calcifications.6 Yet, it is well known that thick slices lead to volume averaging effects, which can obscure small calcifications, while less dense calcifications might not be picked up due to the threshold effect. Prior studies have shown that use of a larger slice thickness misses approximately one-third of calcified lesions and, hence, can potentially limit power of zero favorable prognostic value. For example, in one of earlier CAC studies that used a 6-mm slice thickness rather than the standard 3-mm collimation, the event rate in CAC zero was significantly higher than other studies. As a result, any opportunity to overcome the missing subthreshold calcifications by traditional methods will be a welcoming advance in our ability to more reassuringly de-risk individuals, especially at a higher baseline risk spectrum.7 In the current study, Shea et al,8 propose a novel strategy to potentially further refine identification handle such limitations of false-negative CAC scores in the current issue of Circulation: Cardiovascular Imaging. The authors evaluated >3000 participants in from the MESA study (multi-ethnic study of atherosclerosis) who had an initial CAC of zero and evaluated if a spatially weighted CAC score (SWCS) could provide additional risk stratification information in this overall lowrisk group. The calculation of SWCS has been described in a previous publication from the same group.9 In short, each coronary voxel is given a weight from a weighting function derived from a scan’s phantom to calibrate and weight each voxel. Then, each voxel was assigned a score according to the weight assigned to it and its neighbor to reduce the impact of noise. With this technique, CAC becomes a valid continuous measure without a single threshold. There are a few noteworthy findings. First, in the 15-year follow-up, only 2.9% individuals with CAC zero had any coronary heart disease event, confirming the wellestablished power of zero notion. Second, the authors provided insights that even © 2021 American Heart Association, Inc. Circulation: Cardiovascular Imaging