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How and why Cardiologists use different types of CT to diagnose coronary heart disease

Posted on Friday March 15, 2024 in Health Screening

An article by Dr Edward Leatham, Consultant Cardiologist

Coronary heart disease (CHD) is a significant health concern worldwide, leading to premature death and increased mortality rates in developed nations. Understanding the early stages of CHD is crucial, as the disease begins long before its symptoms become apparent, often starting as early as one’s 20s. The initial changes are fatty streaks in the coronary arteries, visible to pathologists during autopsies, particularly in young individuals who have died in accidents.

It is widely acknowledged that atherosclerosis is a chronic inflammatory disease where a combination of inflammatory cells ( foam cells within the artery’s intima) interacting with LDL Cholesterol build up over many decades within focal areas of the coronary arteries, evolving into non-calcified plaques at specific locations, with gradual increase in bulk of each plaque particularly in the proximal sections of the coronary arteries.  At this stage these can only be detected by intravascular ultrasound and high resolution CT imaging.  Non-calcified plaques grow and, through chronic inflammation, begin to calcify, transforming from soft, toothpaste-like material into hard, calcified plaques. This calcification is a part of the plaque stabilization process, making them less likely to cause acute cardiac events. It is notable that is only at this point that coronary plaques can be detected using non contrast CT, visible as small specks of calcification  in the area of the three coronary arteries. Thus, by the time coronary calcification is visible using the most basic test coronary artery calcification (CAC) score, the disease has been present for many years.

Further plaque growth eventually exceeds the capacity of the artery to compensate by growing outwards and the plaque grows into the arterial lumen  leading to focal areas of flow limiting ‘scar tissue’ – at which point symptoms start to occur – including angina of effort and acute coronary syndromes. It is only at this relatively late stage in the process that many of the standard tests used by cardiologists such as stress ECG, stress echo, nuclear and perfusion scans will show any abnormality.

Recent advancements in imaging technology, particularly computed tomography (CT), have revolutionized our understanding and management of CAD.  One notable innovation is the CaRi heart score, developed by researchers at Oxford. This score utilizes CT to measure the fat attenuation index (FAI) using AI and machine learnt analysis of the CT grey scale images of the tissue adjacent to each of the coronary arteries, which offers a validated technique and indirect marker of arterial inflammation. The FAI, example shown in the figure below where blue and red indicate abnormally high inflammation around one coronary artery seen in longitudinal and cross section, coupled with patient age and risk factors, helps predict cardiovascular mortality over eight years. This groundbreaking approach allows cardiologists to assess and manage patients’ risks more effectively, even before significant plaque or calcium accumulation is detected.

The evolution of cardiac imaging, particularly CT, has significantly influenced the screening tests used in cardiology. Traditional calcium scoring, which assesses the risk of coronary events based on the presence or absence of coronary calcification, has been complemented by CT angiography. This newer method is particularly useful in identifying patients with severe non-calcified coronary disease, offering a more comprehensive assessment of cardiac risk.

The integration of CT angiography with FAI has expanded the scope of non-invasive cardiac assessments, allowing for more nuanced risk stratification and management strategies. This is particularly beneficial for individuals in their 40s and 50s, where traditional imaging might not fully capture their cardiovascular risk profile. In practice, the choice of cardiac CT varies based on the patient’s age, symptoms, and risk factors. For individuals under 40 without symptoms, CT angiography is generally not recommended. However, for those with symptoms suggestive of coronary artery disease, a CT angiogram with a visual calcium score is advised to minimize radiation exposure while providing essential diagnostic information. For asymptomatic individuals or those with atypical symptoms but concerned about their future coronary risk, the FAI-based CT angiogram offers a valuable tool, especially for detecting inflammation and potential risk in younger patients without visible plaque or calcium.

Ultimately, the approach to cardiac CT screening is highly individualized, reflecting the diverse presentations and risk factors among patients. By employing a sophisticated pre-consultation process, cardiologists can tailor their assessment and management plans to best suit each patient’s unique needs, enhancing the precision and effectiveness of cardiac care.


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