Liu Huiliang: Application of optical coherence tomography in the evaluation of coronary artery disease

Due to the ultra-high resolution of optical coherence tomography (OCT), its axial resolution can reach 10 μm and lateral to 25 μm, enabling it to accurately measure plaque fiber cap thickness, locate plaque rupture location, quantify the degree of macrophage aggregation, and usher in a new era of intracorular imaging technology. OCT technology can not only guide percutaneous coronary intervention (PCI), but also identify changes in the surrounding tissue structure after stent implantation, such as thrombosis, dissection, plaque protrusion, tissue prolapse, etc., and evaluate the intimal coverage after stent implantation; by accurately determining the morphology and nature of the lesion, follow up the clinical situation of different lesions, so as to develop a better treatment plan. For example, OCT is used to evaluate the long-term safety of stents and to guide dual antiplatelet therapy, and OCT can also be used to compare changes in tissue morphology after implantation of different stents to guide optimal stent selection.

"I. OCT's assessment of plaque composition

Oct is 10 times more resolution than intravascular ultrasound (IVUS) and can identify major plaque components such as fibrous plaque, lipid plaque, and calcified plaque. Fibrous plaques are formed by a large number of collagen fibers, smooth muscle cells and extracellular matrix, which are manifested as strong signal regions with uniform properties in OCT images; lipid plaques are composed of a large number of foam cells and extracellular lipids and necrotic debris, showing a uniform high signal in OCT images, and blurred low-signal or no-signal areas behind high signal shadows; calcified plaques are calcium salts deposited in extracellular lipids and cell fragments, fiber caps and even mid-membranes, and calcific points gradually expand to form calcilization deposits, and low signals are presented in OCT images. The calcified plaque is clearly demarcated from its surroundings.

Previous studies have confirmed that OCT technology is superior to IVAUS technology in judging lipid-rich plaques and calcified plaques. For lipid plaques, oct tests were 85% and 94% sensitive and specific, compared to 59% and 97%, respectively, for IVUS. Although OCT cannot pass through lipid plaques to show the tissue structure behind lipid plaques, measuring the size of the lipid arc can play a predictive role in non-recurrence. For calcified plaques, the vascular structure behind the calcified plaque cannot be visualized because the calcified lesions produce a back shadow under the IVAUS image, but OCT can clearly observe the boundaries of the calcified plaques and the tissue structure behind the calcified plaques can be observed.

2. Oct's assessment of vulnerable plaques

Vulnerable plaques are those that are unstable and have a tendency to thrombosis. The factors that contribute to the rupture of vulnerable plaques are manifold: inflammatory response, lipid content in the plaque, thickness of the plaque fiber cap, shear force on the plaque, and coronary artery tension at the plaque. About 60% to 80% of the large number of studies report that acute coronary syndrome (ACS) is due to rupture of coronary plaque and secondary thrombosis. It can be said that fragile plaques are the culprits of ACS. OCT can clearly distinguish between calcifications, fibers and lipid plaques, can measure the thickness of the fiber cap of lipid plaque, and can observe the size of the lipid core, macrophage aggregation or fibrin deposition, plaque cap fissures, etc. At the same time, the tearing of the fiber cap or the rupture of the atherosclerotic plaque can be observed in detail. At present, the main imaging criteria for evaluating plaque vulnerability by OCT technology include: 1. Active inflammation (mononuclear, macrophage or T cell infiltration); 2. Thin fiber cap thickness with large lipid core; 3. Endothelial exfoliation with surface platelet aggregation; 4. Plaque (cap) fissure; 5, severe stenosis greater than 90%. Secondary criteria include: 1, superficial calcified nodules 2, neovascular trophoblast vessels 3, intraplastic hemorrhage 4, endothelial dysfunction 5, positive remodeling, etc. The high-resolution features of OCT imaging are effective in identifying vulnerable plaques. The following is an octogram feature of fiber cap thickness, lipid core, microchannel, macrophage infiltration, superficial calcification points, and cholesterol crystallization.

(1) Thin fiber cap atheroma

Pathological examination of thin-cap fibroatheroma (TCFA) shows morphological similarities to plaque rupture and is considered a pre-plaque rupture. TCFA is lipid-rich plaque with a fiber cap thickness of less than 65 μm. On OCT imaging, the fiber composition shows high signal and low attenuation, and the lipid cell presents a uniform low signal region. The sharp contrast between the fiber cap and the lipid core high and low signals allows OCT to accurately measure the fiber cap thickness to identify TCFA.

(2) Lipid core

Plaques with large lipid cores are at high risk of plaque rupture and thrombosis. Prospect studies show that the size of the plaque load is an important predictor of future cardiovascular events caused by non-criminal plaques. OCT imaging defines lipid-rich plaque as lipid components that cover greater than or equal to two quadrants.

(3) Vascular microchannels

Vascular microchannels are defined on OCT as small pore structures within plaques displayed in drawdown imaging (images of 3 frames and above consecutively), with a diameter of 50-300 μm. Studies have shown that the emergence of vascular microchannels in criminal plaques is positively correlated with the incidence of plaque susceptibility traits such as TCFA and large lipid core. In non-offender plaques, vascular microchannels are associated with the vulnerable progression of the plaque. In addition, vascular microchannels of non-offender plaques are associated with resistance to statin therapy. Studies have shown that plaques containing vascular microchannels increased to a lesser extent than plaques without vascular microchannels in the case of the same decrease in blood lipid levels.

(4) Macrophage infiltration

Histological observations showed significant macrophage infiltration in coronary plaques in patients with ACS compared with patients with stable angina. Macrophages in paper plaques continue to secrete matrix metalloproteinase to degrade the matrix, so measuring the content of macrophages in the plate can indirectly evaluate the vulnerability of coronary plaques. In OCT imaging, macrophages present as large areas of high-reflex at the junction of the lipid pools of fibrous caps, where consistently oriented, linear, high-reflex, high-attenuation structures within the plaque can be seen.

(5) Calcification

The role of calcification in plaque susceptibility remains to be confirmed by more studies. Compared with the degree of plaque calcification, the morphology and area size of calcification are more predictive of plaque vulnerability. In OCT images, calcifications are presented as irregularly shaped high-reflection or low-reflection areas with low attenuation, and calcifications in OCT images that do not exceed one quadrant are defined as calcification points. The high resolution and low attenuation properties of OCT are expected to play a greater role in evaluating the progression and vulnerability of calcification points to plaques.

(6) Cholesterol crystallization

Cholesterol crystals are observed in the pathological analysis of coronary plaques, and the amount of crystals is related to the size of the plaque, plaque rupture, and the risk of thrombosis. Cholesterol crystals appear in OCT imaging as fine, linear, high-signal, low-attenuation regions. Cholesterol crystals in patients with stable angina are associated with the prevalence of plaque susceptibility such as calcification spots, vascular microchannels, and lipid plaques.

Third, the judgment of coronary artery dissection

The diagnosis of coronary artery dissection depends on coronary angiography, intravascular ultrasound, or OCT. Oct resolution is more than 10 times that of IVUS, can clearly distinguish the endumar, middle membrane, outer membrane, and even the components of arterial plaque - macrophages, foam cells and smooth muscle cells, in the diagnosis of coronary artery dissection has obvious advantages. Kubo et al. pointed out in OCT and IVUS examination of plaques in myocardial infarction patients that the probability of coronary artery dissection in OCT is 73%, which is significantly higher than that of IVUS 40%, and OCT can also measure the length of dissection tablets.

OCT can not only accurately determine the location, thickness, length, true and false cavity-related intramural hematomas and thrombosis associated with coronary artery rupture, but also accurately provide the location, length, and display of residual stent thrombus and distal vascular shadow. Oct detected coronary artery dissection more than coronary angiography, Tsimikas et al. used OCT to detect spontaneous coronary artery disease (atherosclerosis or thrombosis) in 5002 patients, and the remaining 11 had spontaneous coronary dissection.

Independent influencing factors of coronary artery dissection were detected by the OCT, including coronary atherosclerosis at the stent margin, fibrous cap calcification angle, minimum fibrous cap thickness, stent/vascular eccentricity, and vascular hyperelongation. The average length of the coronary artery dissection is 2.04±1.60 mm, and most of them are fibrous caps. THE OCT has a high detection rate of stent edge dissection, and stent edge dissection is mostly associated with atherosclerosis. The study found that the detection rate of coronary stent edge dissection of OCT was 37.8%, which was much higher than that of coronary angiography.

Fourth, the judgment of myocardial bridges

Coronary myocardial bridges are common in coronary angiography, but coronary angiography is often difficult to determine the nature of lesions at myocardial bridges, and OCT can more accurately measure the length and area of myocardial bridges, and at the same time can judge the structure of the vascular wall at the myocardial bridge at the histological level. In one OCT study, the vascular wall at the coronary myocardial bridge was thin, the systolic vascular circle layer was irregularly contracted, and the coronary lining of the proximal myocardial bridge had different degrees of hyperplasia, but obvious atherosclerotic plaques were found at the myocardial bridge and distal myocardial bridge, which may be related to abnormal intra-coronary blood flow impact caused by the myocardial bridge.