OCT Parameter Tracks Atherosclerosis Progression

LONDON — The histopathological status of an atherosclerotic plaque can not only be quantified but also tracked over time by using a parameter calculated from optical coherence tomography (OCT) images, researchers from the Chinese PLA General Hospital in Beijing, China, have found.

Called the index of plaque attenuation (IPA), the parameter is a measure of a tissue’s optical properties. Using the IPA, the researchers were able to identify increasingly pathologic stages of atherosclerotic plaques, from fibrous and fibrocalcific to thick- and thin-cap fibroatheromas. They were also able to determine the likely stability of the plaques.

IPA values reflect the dynamic progression of coronary atherosclerotic plaques and show a significant positive correlation with their histological classification, explained study investigator and associate physician, Shanshan Zhou, MD, here at the European Society of Cardiology (ESC) Congress 2024 on September 1.

Distinct variations were observed across different stages and types of plaques, she noted, and macrophage infiltration was a major contributor to higher IPA values.

“The translational perspective is that maybe we can provide qualitative data to help physicians assess coronary lesions more rapidly, accurately, and consistently,” Zhou explained. This real-time imaging could enable clinical tracking of coronary plaque grading, she suggested during the “Smaller Trials and Other Studies on Atherosclerosis” late-breaking research session at the congress.

Study Information

The study builds on previous knowledge that OCT can differentiate altered tissue structures and organization that is reflected in its optical properties or attenuation coefficient. According to Zhou, these changes may not be visible with conventional imaging techniques.

She explained that a higher attenuation coefficient indicated tissue instability, areas of dead cells (necrotic cores), and macrophage infiltration. Conversely, lower attenuation values indicated calcification and fibrous tissue.

To quantify and comprehensively characterize coronary atherosclerotic plaques using OCT attenuation imaging, the team at the Chinese PLA General Hospital used 10 autopsied human hearts. The hearts had been snap-frozen in liquid nitrogen after removal and stored at −80°C. They took 21 sections of the coronary arteries from 30 vessels within these hearts and prepared 359 blocks of tissue for histological examination. Of these, 288 were suitable for both histological and OCT examination.

IPA values were calculated from the OCT images as the ratio of the number of pixels in the image greater than a certain threshold (ie, between 8 and 12 mm⁻¹) to the total number of pixels present, then multiplied by 1000.

Plaques were classified histologically according to American Heart Association criteria published in 1995, where type I-II lesions showed increasing infiltration of fat and other molecules, and type IV-VI showed increasing formation of atheroma and fibrotic lesions.

Plaques were considered “significant” if they were fibrous, had pathologic intimal thickening, were fibrocalcific, a fibroatheroma, or a thin-cap fibroatheroma.

Key Results

Results showed that the IPA value was significantly correlated with the pathological staging of plaques, with an IPA value of 10 found to be the most optimal for detecting advanced plaques. The area under the receiver operating characteristic (AUROC) curve was 0.844 (P < .001).

Combining the IPA₁₀ with the percentage of stenosis seen in the coronary artery sample allowed for an even more accurate identification of advanced plaques, with an AUROC value of 0.088 (P < .001) and corresponding sensitivity and specificity values of 91.4% and 80.8%, respectively, and positive and negative predictive values of 79.1% and 92.2%, respectively.

Zhou pointed out that notable variations in IPA values were observed among different types of plaques. AUROC values were 0.88 for significant plaques as a group, 0.81 for fibrous/fibrocalcific plaques, 0.89 for thick-cap fibroatheromas, and 0.97 for thin-cap fibroatheromas. All were significant (all P < .001).

Moreover, in advanced plaques, high optical attenuation was associated with high-risk features, indicating the possibility of instability and an increased likelihood of rupture. This included the infiltration of lipids and, most importantly, macrophages.

Looking more closely at macrophage-infiltrated lesions revealed that the IPA values were different depending on the type of plaques being studied. More importantly, the IPA value increased with the pathological staging of the plaque, from 264 for fibrous plaques, 548 for atheromatous plaques, and 676 for thin-cap atheromatous plaques.

Validates OCT Value for Plaque Characterization

Co-chairing the session in which Zhou presented the findings was Stephen Nicholls, MD, professor of cardiology at Monash University in Melbourne, Australia. He told Medscape Medical News: “It validates the use of OCT as an imaging tool to show us about the composition of the plaque” and “shows that OCT can distinguish between a stable and an unstable plaque.”

Nicholls added that the data could have implications for both clinical practice and research. The implication for the cardiac catheterization laboratory would be, perhaps, “intervening more intensively in patients who might have more unstable disease.” With regards to clinical trials, he said, “OCT is being used in clinical trials to evaluate the effect of lipid-lowering therapy, for example, so I think this is a nice pathology validation for those imaging studies that have been done.”

Plaque Quantification Important

The quantification of the plaque is important, Erik Rafflenbeul, MD, a consultant interventional cardiologist from Schoen Clinic Hamburg in Hamburg, Germany, said in a separate interview.

“It’s not only the coronary angiogram that you need to look at,” he said. “You need a deeper dive in some stenosis cases even if it’s not highly stenotic, because the plaques that rupture, causing myocardial infarction, are not the ones with stenosis at 90% or 80%; it’s the plaques from 30% to 50%,” he explained.

The use of OCT in cath labs across Europe is variable, Rafflenbeul added. Some recent data from the United Kingdom suggest that the use of intravascular imaging generally, which includes both OCT and intravascular ultrasound, is at a rate of around 30%, he said, but the rate in Germany is about 5%; “so very low.” 

However, this is a situation that perhaps needs to change as the 2024 ESC Guidelines for the management of chronic coronary syndromes recommend that OCT and IVUS are used both before and after percutaneous coronary intervention for complex lesions.

“So we must do it,” Rafflenbeul said. “This is why it’s important to understand how to quantify.”

Translation Into Clinical Practice and Future Work

Zhou said in an interview with Medscape Medical News that the IPA value increases the higher the stage of the plaque. “It’s interesting. That means maybe we can use the IPA value to diagnose which stage the plaque is really at in clinical practice.”

She acknowledged that this was a small study and said that the team at the Chinese PLA General Hospital was continuing to include more samples from additional autopsied human hearts. So far, they have included a further 10 hearts in their studies, bringing the total to 20, but they want to get to 30 hearts and 1000 OCT images before they consider publishing their data.

The study was independently supported. Zhou reported no relevant financial relationships. Nichols has led multiple industry trials that have used a range of different imaging modalities to evaluate the effect of medical therapies.