In Search of Vulnerable Plaque

OCT-NIRS Imaging for Detection of Coronary Plaque Structure and Vulnerability

A combination optical coherence tomography and near-infrared spectroscopy (OCT-NIRS) coronary imaging system is being developed to improve the care of coronary patients. While stenting has improved, complications continue to occur at the stented site and new events are caused by unrecognized vulnerable plaques. An OCT-NIRS device has potential to improve secondary prevention by optimizing stenting and by identifying vulnerable patients and vulnerable plaques. OCT is already in widespread use world-wide to optimize coronary artery stenting. It provides automated lumen detection and can identify features of coronary plaques not accurately identified by angiography or intravascular ultrasound. The ILUMIEN IV study, to be completed in 2022, will determine if OCT-guided stenting will yield better clinical outcomes than angiographic guidance alone. While the superb spatial resolution of OCT enables the identification of many plaque structural features, the detection by OCT of lipids, an important component of vulnerable plaques, is limited by suboptimal specificity and interobserver agreement. In contrast, NIRS has been extensively validated for lipid-rich plaque detection against the gold-standard of histology and is the only FDA-approved method to identify coronary lipids. Studies in patients have demonstrated that NIRS detects lipid in culprit lesions causing coronary events. In 2019, the positive results of the prospective Lipid-Rich Plaque Study led to FDA approval of NIRS for detection of high-risk plaques and patients. The complementarity of OCT for plaque structure and NIRS for plaque composition led to the sequential performance of NIRS and OCT imaging in patients. NIRS identified lipid while OCT determined the thickness of the cap over the lipid pool. The positive results obtained with OCT and NIRS imaging led to development of a prototype combined OCT-NIRS catheter that can provide co-registered OCT and NIRS data in a single pullback. The data will provide structural and chemical information likely to improve stenting and deliver more accurate identification of vulnerable plaques and vulnerable patients. More precise diagnosis will then lead to OCT-NIRS guided treatment trials to improve secondary prevention. Success in secondary prevention will then facilitate development of improved primary prevention with invasive imaging and effective treatment of patients identified by non-invasive methods.

Near-infrared spectroscopy combined with intravascular ultrasound in carotid arteries

Limited insights into the pathophysiology of the atherosclerotic carotid stenosis are available in vivo. We conducted a prospective study to assess safety and feasibility of intravascular ultrasound (IVUS) combined with near-infrared spectroscopy (NIRS) in carotid arteries. In addition, we described the size and the distribution of lipid rich plaques in significant atherosclerotic carotid stenoses. In a prospective single centre study 45 consecutive patients (mean age 66 ± 8 years) with symptomatic (≥50 %) or asymptomatic (≥70 %) stenosis of internal carotid artery (ICA) amendable to carotid stenting were enrolled. A 40 mm long NIRS-IVUS pullback through the stenosis was performed. IVUS and NIRS data were analyzed to assess minimal luminal area (MLA), plaque burden (PB), remodeling index (RI), calcifications, lipid core burden index (LCBI), maximal LCBI in any 4 mm segment of the artery (LCBImx) and LCBI in the 4 mm segment at the site of minimal luminal area (LCBImxMLA). NIRS-IVUS pullbacks were safely performed without overt clinical events. LCBImx was significantly higher than LCBImxMLA (369.1 ± 221.1 vs. 215.7 ± 2589; p = 0.004). Conversely, PB was significantly larger at the site of MLA (87.4 ± 4.8 % vs. 58.3 ± 18.2 %; p < 0001). Distance of the NIRS-IVUS frame with the highest LCBI from the site of MLA was 6.5 ± 7.7 mm. Eighty percent of frames with maximal LCBI were localized within 10 mm from the site of MLA and 67 % proximally to or at the site of MLA. This study suggested safety and feasibility of the NIRS-IVUS imaging of the carotid stenosis and provided insights on the distribution of lipids in the carotid stenosis. Lipid rich plaques were more often located in the sites with a milder stenosis and smaller plaque burden than at the site of MLA.

Detection by near-infrared spectroscopy of large lipid core plaques at culprit sites in patients with acute ST-segment elevation myocardial infarction

OBJECTIVES

This study sought to describe near-infrared spectroscopy (NIRS) findings of culprit lesions in ST-segment elevation myocardial infarction (STEMI).

BACKGROUND

Although autopsy studies demonstrate that most STEMI are caused by rupture of pre-existing lipid core plaque (LCP), it has not been possible to identify LCP in vivo. A novel intracoronary NIRS catheter has made it possible to detect LCP in patients.

METHODS

We performed NIRS within the culprit vessels of 20 patients with acute STEMI and compared the STEMI culprit findings to findings in nonculprit segments of the artery and to findings in autopsy control segments. Culprit and control segments were analyzed for the maximum lipid core burden index in a 4-mm length of artery (maxLCBI(4mm)).

RESULTS

MaxLCBI(4mm) was 5.8-fold higher in STEMI culprit segments than in 87 nonculprit segments of the STEMI culprit vessel (median [interquartile range (IQR)]: 523 [445 to 821] vs. 90 [6 to 265]; p < 0.001) and 87-fold higher than in 279 coronary autopsy segments free of large LCP by histology (median [IQR]: 523 [445 to 821] vs. 6 [0 to 88]; p < 0.001).Within the STEMI culprit artery, NIRS accurately distinguished culprit from nonculprit segments (receiver-operating characteristic analysis area under the curve = 0.90). A threshold of maxLCBI(4mm) >400 distinguished STEMI culprit segments from specimens free of large LCP by histology (sensitivity: 85%, specificity: 98%).

CONCLUSIONS

The present study has demonstrated in vivo that a maxLCBI(4mm) >400, as detected by NIRS, is a signature of plaques causing STEMI.