The introduction and continuous evolution of cardiac computed tomography (Cardiac CT) in the last two decades represents one of the most fascinating recent innovations in clinical medicine. The high accuracy and reproducibility of this rather new method together with exceptional predictive power makes cardiac CT a game changer in modern cardiology.
After years of stenosis-driven treatment decisions in coronary artery disease (CAD), characterised by the search for coronary luminal narrowing exceeding 70 per cent, to subsequently revascularise this stenosis through coronary bypass graft or coronary stent placement, the chapter of ischemia-centred treatment approach was opened. It became obvious that the pure anatomical approach might not be able to paint the full picture for optimised therapy, whereas information about the situation at the myocardium becomes more important.
The dynamic interplay between anatomy and function has driven the development of more sophisticated techniques to diagnose and manage coronary artery disease (CAD). Initially, cardiac CT has proven to have a very high negative predictive value (NPV) for the diagnosis/rule out of CAD. In cases of good diagnostic image quality — nowadays achievable in almost all patients thanks to the high technical performance of modern CT scanners — and normal coronary arteries, a CAD is excluded with an NPV of 97 – 99 per cent. It can be simplified that a negative CT safely excludes CAD.
Even the presence and severity of coronary artery stenoses can be detected and assessed with very high diagnostic accuracy. In other words, the prerequisites for anatomical imaging have been fulfilled by this non-invasive imaging technique. As proof of this development, “coronary CTA is recommended as the initial test to diagnose CAD in symptomatic patients in whom obstructive CAD cannot be excluded by clinical assessment alone”, according to the recent guidelines of the European Society of Cardiology (ESC) for the diagnosis and management of chronic coronary syndromes.
However, in times of ischemia-driven treatment decisions, this morphology-only approach represented by the rule out/diagnosis of relevant stenosis alone is not sufficient anymore as mentioned above. Cardiac CT has shown to be able to non-invasively provide the biomarkers needed for such ischemia-driven patient management by several unique features.
The accuracy of coronary stenosis assessment can be further increased by adding CT-derived Fractional Flow Reserve (CT-FFR) providing functional information about the pressure gradient among a stenosis. Additionally, possible myocardial perfusion differences can be detected in stress as well as in rest by adding CT perfusion. Last but not least, the possibility to characterise coronary plaques and predict future events has been introduced recently.
The underlying principle of CT-FFR involves simulating blood flow within the coronary arteries and calculating the pressure drop across stenotic lesions. By combining anatomical information from CCTA with computational modelling, CT-FFR generates a virtual fractional flow reserve value, reflecting the severity of coronary obstruction and its impact on blood flow. This innovative approach offers a more comprehensive understanding of coronary physiology, bridging the gap between anatomy and function.
One of the primary advantages of CT-FFR is its non-invasive nature, eliminating the need for invasive coronary angiography or pressure wire measurements. This not only reduces patient discomfort but also minimises the associated risks and complications. Moreover, CT-FFR provides a more detailed assessment of coronary lesions, enabling clinicians to differentiate between haemodynamically significant and insignificant stenoses.
The integration of CT-FFR into clinical practice has been supported by robust research demonstrating its diagnostic accuracy and clinical utility. Multiple studies have shown that CT-FFR has a high sensitivity and specificity for identifying functionally significant coronary stenoses when compared to invasive fractional flow reserve measurements. This evidence has contributed to the growing acceptance of CT-FFR as a valuable tool in the diagnostic armamentarium for CAD.
Myocardial CT perfusion (CTP) provides detailed information about myocardial blood flow and perfusion, aiding in the identification and characterisation of ischemic heart disease. The primary objective of myocardial CTP is to evaluate the blood supply to the heart muscle during rest and stress conditions. By utilising contrast-enhanced computed tomography scans, this imaging modality can visualise the distribution of contrast material within the myocardium, highlighting areas with compromised blood flow. During stress imaging, typically induced through pharmacological agents, the test can reveal regions of the heart that may exhibit reduced perfusion, indicative of obstructed coronary arteries.
One of the significant advantages of myocardial CTP is its ability to provide a comprehensive assessment of both anatomy and function in a single examination. The information obtained from myocardial CTP is valuable for guiding treatment decisions. Furthermore, myocardial CTP has shown promise in risk stratification and prognostication for individuals with suspected or known CAD. The ability to assess myocardial perfusion adds an important dimension to the diagnostic process, assisting clinicians in tailoring management strategies to individual patient needs.
Recently, cardiac CT has shown an ability to not only predict but also improve the outcome of patients. The Scot-Heart trial, a landmark trial published in 2018, and its subsequent studies proved the relationship between plaque-related biomarkers, identified by cardiac CT, and events during follow-up. Additionally, this study could also show that the addition of cardiac CT to normal patient care has the potential to improve the outcome by direct reduction of events and cardiac-related deaths. Remarkably, this was not achieved by an increased number of coronary revascularisations, but by optimising the medical care of the patients based on CT-based risk estimation.
The story of cardiac CT is still ongoing with an exceptional direct impact on patient care and outcome. The chapter of ischemia-driven treatment decisions will be followed by the just opened one of outcome-driven decisions, facilitated by non-invasive cardiac CT. The potential of plaque visualisation together with anatomical information and myocardial assessment is not yet fully explored but will for sure change patients’ pathways in the future.
References available on request.
Dr. Christian Loewe is the Chairman of the Division of Cardiovascular and Interventional Radiology, Department of Bioimaging and Image-Guided Therapy at the Medical University of Vienna, Austria.
