The coronary artery calcium (CAC) score is widely believed to be an important tool in determining the risk of developing heart disease. The measurement of this score has traditionally been based on using electrocardiography triggered computed tomography (CT). This confers an advantage over non-gated CT scanning by acquiring images during diastole, which reduces motion artefact and avoids missing areas of coronary artery calcification. Radiologists are, therefore, cautious when reporting CAC on non-gated CT scans due to concerns that it may not be accurate. This means that there is currently no obligation, from a radiology perspective, to report on the degree of CAC on non-gated CT scans. While this has been acceptable for a long time, emerging evidence may force us to change our practise.
Emerging evidence
In an issue of Circulation: Cardiovascular Imaging, Xie et al.1 performed a systematic review and meta-analysis to validate the prognostic importance of CAC scoring in non-triggered thoracic CT. The authors of this study performed a meta-analysis of five studies that compared CAC obtained using non-gated CT scans versus gated CT scans. This study demonstrated an excellent correlation between the two techniques with a pooled Cohen κ agreement being 0.89 (95% confidence interval [CI] 0.83–0.95). While this is promising, the authors have also highlighted some discrepancy between the two techniques, and this is important to note. Non-triggered CT scan will yield a false-negative calcium score in 8.8% of individuals and underestimated calcification in 19.1% of the individuals. Despite this difference, the authors recommend that we must not disregard CAC picked up using non-gated CT scans, and it must be reported like any other important incidental finding.
This is useful knowledge as it is well known that respiratory disease is an independent risk factor for developing heart disease. Therefore, patients coming in for lung cancer screening or follow-up for interstitial lung disease undergoing non-gated CT scanning must be screened for CAC. For example, preliminary data at our trust has revealed that 56% of patients who have undergone non-gated high-resolution CT (HRCT) scanning for follow-up of interstitial lung disease have severe CAC (CAC score >1,000). A significant proportion of these people are not on any secondary prevention medications either, despite having cardiovascular risk factors (hypertension, hyperlipidaemia and renal disease). A similar study by Jacobs et al.2 looking at adults referred for lung cancer screening corroborates our findings.
Another study (by Margolies et al.3) recently published in JACC: Cardiovascular Imaging has shown a link between breast arterial calcification (BAC) and CAC. This study included 325 women who underwent a non-gated CT scan and a digital mammogram within a year of the other. The degree of CAC and BAC was measured on a 12-point ordinal scale. Moderate CAC (≥4) was noted in 47.6% of women. The degree of BAC, based on the density of calcium in the lumen, number and length of vessels involved, was similarly scored on a 12-point ordinal scale: 42.5% of women had BAC, which was significantly associated with chronic kidney disease, high blood pressure and increasing age. More importantly, the authors report a highly significant agreement between BAC and CAC.
This is the first study of its kind to quantitatively link BAC and CAC. The authors of this paper hope that this study will encourage regular reporting of BAC in mammography reports as it will help identify women at risk of developing cardiovascular disease.
Conclusion
The emerging body of evidence that non-gated CT scanning may have a role in determining the degree of CAC compels us to consider how this can be applied to a clinical setting. We believe that there may be scope, for example, to mention the degree of CAC as a standard when reporting non-gated HRCT scans, positron emission tomography or single photon emission CT of the chest. This will allow us to risk-stratify patients and potentially refer them to cardiologists for further investigations, such as stress echocardiography. Moreover, patients can be commenced on medications, such as statins or aspirin, for primary prevention of cardiovascular disease. The images of calcified coronary arteries may potentially have a role in convincing people to make correct lifestyle choices.
It will take time for CAC scoring to fully penetrate the world of cardiology and radiology. There are questions that still remain unanswered and, ultimately, electrocardiogram (ECG)-gated CT scanning will remain the gold standard for picking up CAC. We are, however, fascinated by the findings reported by Xie et al.1 and Margolies et al.3 and hope that further studies will bring to light this novel topic.
We believe that a perceptual tool to quantify CAC should be developed. If such a tool can be validated against the standard quantitative method of using Agatston scores, then this will allow radiologists to very easily report the level of CAC within the limited time slot they have for reporting each scan. This may help to simplify a potentially cumbersome activity of measuring CAC using semi-quantitative tools, which will eventually mean that CAC score is more regularly incorporated in radiology reporting of non-gated CT scans.
Acknowledgements
We would like to acknowledge Dr Divya Raj and Dr David Zhu in the data collection process for the study conducted in our trust (unpublished data).
Conflict of interest
None declared.
References
1. Xie X, Zhao Y, de Bock GH et al. Validation and prognosis of coronary artery calcium scoring in non-triggered thoracic computed tomography: systematic review and meta-analysis. Circ Cardiovasc Imaging 2013;6:514–21. http://dx.doi.org/10.1161/CIRCIMAGING.113.000092
2. Jacobs PC, Gondrie MJ, van der Graaf Y et al. Coronary artery calcium can predict all-cause mortality and cardiovascular events on low-dose CT screening for lung cancer. AJR Am J Roentgenol 2012;198:505–11. http://dx.doi.org/10.2214/AJR.10.5577
3. Margolies L, Salvatore M, Hecht HS et al. Digital mammography and screening for coronary artery disease. JACC Cardiovasc Imaging 2016;9:350–60. http://dx.doi.org/10.1016/j.jcmg.2015.10.022
