According to the results of CCTA and CT-FFR, the patients were categorized into non-ischemia group (FFR >0.8) and ischemia group (FFR ≤0.8). Plaque characteristics were compared between the two groups, and the results showed that the differences in total plaque volume, calcified plaque volume, plaque length, and lumen stenosis >50% were significantly different. Moreover, different risk factors were investigated by logistic regression analysis, and the results indicated that total plaque volume, calcified plaque volume, plaque length, and lumen stenosis >50% were influencing factors for myocardial ischemia, especially lumen stenosis >50%, with an OR of 0.181 (p<0.001). Therefore, CCTA combined with CT-FFR has high diagnostic efficacy for coronary heart disease. A routine test can be used to observe anatomical stenosis and functional test. It is noninvasive and low-cost, which reduces the burden for patients.
Multiple studies demonstrated that CT-FFR has a higher diagnostic accuracy for coronary heart disease than traditional CCTA [7–9]. Studies have confirmed that about half of the patients with significant coronary stenosis on CCTA have lumen stenosis >50%, but without myocardial ischemia or only small patchy perfusion ischemia as detected by radionuclide imaging , and some patients receiving stent implantation have no significant symptom improvement after surgery . In this study, the results also showed that the patients with lumen stenosis >50% had CT-FFR >0.8, without obvious clinical symptoms. The reason for this may be that the CT-FFR value was not decreased due to collateral formation, while patients with mild stenosis (<50%) may suffer from myocardial ischemia, indicating that the anatomical stenosis does not match the actual functional ischemia. In most studies, FFR ≤0.8 is selected as the threshold, and FFR is considered as a guide . When FFR >0.8, there is no myocardial ischemia, so the therapeutic effect of medication is better. However, when FFR <0.8, suggesting the presence of myocardial ischemia, the probability of the end-point event of patients with PCI treatment is significantly lower than that of patients with drug treatment . Moreover, a portion of patients with CT-FFR slightly greater than 0.8 can also have more obvious clinical symptoms. This may be due to the fact that CT-FFR is a computer-derived data product, which has a higher correlation compared with the real-time data of invasive FFR but has differences, especially for patients with cut-off values.
The Chinese Guidelines for Percutaneous Coronary Intervention (2016) recommends therapeutic intervention when coronary artery stenosis is less than 90% and there is evidence of myocardial ischemia, or when FFR is less than 0.8, thereby greatly reducing the stent implantation rate and avoiding excessive medical treatment . For patients with myocardial ischemia, surgical revascularization is the first choice by clinicians [15, 16]. By observing patient's clinical symptoms and evaluating stenosis severity, FFR-guided CABG has a significantly lower mortality rate or probability of myocardial infarction after 6 years over ICA-guided coronary artery bypass grafting surgery .
Different types of atherosclerotic plaques have different stability. The pathogenic process is the vascular intima with reactive hyperplasia evolving into fibrous and calcified plaques . It is generally believed acute cardiovascular events mainly occur due to the rupture or erosion of high-risk plaques, and lumen stenosis caused by fibrous or calcified plaques can induce chronic myocardial ischemia . Consistent with previous studies, risk factors such as positive remodeling, napkin-ring sign, punctate calcification, and low-density plaque are characteristic of high-risk plaques, which can increase the occurrence of acute coronary syndromes. In contrast, simple calcified and fibrous plaques can only induce myocardial ischemia due to their stable nature (i.e., not easy to rupture).
To date, CT-FFR research has made considerable progress. However, CCTA image-based CT-FFR still has many uncertain factors for blood flow and arterial blood analysis, and the reasons are as follows: A) Different companies have different algorithms and varying calculation times. Although the processing time has shortened in recent years, it still takes a few hours at the fastest speed. B) The research data are relatively homogeneous. The main purpose of CCTA is to screen a large range of people, but there is no relevant evidence for patients with acute coronary syndrome. C) The CT-FFR algorithm simulates the filling state of blood vessels, which have individual differences, such as elasticity. The same model and formula will reduce accuracy, but for patients with microcirculation, the obtained CT-FFR value is too large . D) For evaluating patients with diffuse calcification, severe calcification will inevitably cause artifacts due to the limitation of CCTA images, and the data obtained will be lower.
In recent years, with the rapid development of CT-FFR, numerous experiments and evidence-based medicine verified that the technique has become the ''gatekeeper'' of ICA examination. ICA can characterize lumen stenosis, but cannot detect physiological ischemia. As a result, clinically significant lesions cannot be accurately treated and medical resources are wasted, while real lesions leading to myocardial ischemia go untreated .
There were many shortcomings in this study. Firstly, FFR was not used as the gold standard, but the accuracy of CT-FFR in diagnosing coronary artery stenosis has been confirmed in many previous studies. Moreover, CT-FFR had high correlation with traditional FFR [1–3], and the recognition was gradually improved. Secondly, this article was a retrospective study with a small sample size, and branch vessels were included in the study, which may lead to statistical biases against previous studies. Thirdly, patients who underwent stent implantation and coronary artery bypass grafting surgery were excluded from this study, so the research population needs to be comprehensively improved in future studies.