Hybrid Imaging Technique Elucidates Coronary Plaques

Combining optical coherence tomography (OCT) with near-infrared autofluorescence (NIRAF) could enable more accurate identification of coronary artery plaques that are most likely to rupture and cause a heart attack.

OCT provides images of tissue microstructure but not of its chemical and molecular composition, said researchers from the Wellman Center for Photomedicine at Massachusetts General Hospital (MGH). Now they have combined OCT with NIRAF to create a powerful tool for investigating coronary pathology.

A 3D visualization of OCT-NIRAF (optical coherence tomography-near-infrared autofluorescence) imaging data acquired from a patient's coronary artery. The gray portion shows microstructural information provided by OCT, and the overlaid color represents NIRAF signal intensity, which was elevated only over portions of the artery that contained plaque with high-risk microstructural features. Courtesy of Giovanni J. Ughi/Wellman Center for Photomedicine, Massachusetts General Hospital.

The detailed images provided by OCT are created by bouncing NIR light off the internal surfaces of blood vessels and can identify plaques that have the appearance of rupture-prone, vulnerable plaques with the potential to cause a heart attack or sudden cardiac death.

Fluorescence imaging techniques like NIRAF illuminate an artery with a specific wavelength of light to excite certain molecules, which respond by emitting different wavelengths. Since only certain molecules respond, the resulting signal provides information on the molecular composition of analyzed tissue.

A team led by Dr. Gary Tearney has been investigating whether the additional data provided by NIRAF could identify rupture-prone sites within arterial plaques, particularly fibroatheromas — advanced lesions consisting of a core of dead cells covered by an often-thin fibrous cap, which are particularly prone to rupture.

In a previous study using coronary artery segments from cadavers, the investigators showed that the NIRAF signal was elevated in fibroatheromas and highest in those with thin fibrous caps. The current study was the first to investigate the use of NIRAF in living patients.

The study enrolled 12 patients receiving cardiac catheterization at MGH between July 2014 and January 2015. In addition to the clinical procedures conducted to diagnose and/or treat the patients' cardiac disease, Dr. Farouc Jaffer director of MGH Coronary Intervention and co-senior author of the paper, used a novel device developed by the Wellman/MGH team that acquired both OCT and NIRAF data to construct images of coronary arterial segments. The investigational procedure was identical to that used for conventional OCT imaging.

The team reported that no additional time was required to image using the integrated system, and that the clinical success of OCT-NIRAF could pave the way forward for targeted NIR fluorescence molecular imaging using injectable molecular- or cellular-specific agents.

The primary results of the study were confirmation that the procedure was as safe and as feasible to perform as conventional OCT. The OCT-NIRAF images revealed that the NIRAF signal was elevated in areas in which OCT results suggested the presence of a fibroatheroma, and even higher in lesions with thin caps or at sites of plaque rupture and clot formation.

Several aspects of the NIRAF signal were different from the patterns produced by other coronary vascular imaging modalities, and more investigation is needed to determine the molecular underpinnings and clinical significance of NIRAF signal results. NIRAF was also elevated in sites showing evidence of inflammation, another potential biomarker of plaques likely to rupture.

"Overall we believe that the combined OCT-NIRAF examination provides information on molecules within arterial plaques and other features associated with a higher risk of an acute coronary event," Tearney said. "But right now this is a hypothesis, and our findings need to be borne out in larger studies, which we plan to have underway later this year."

The research was published online in the Journal of the American College of Cardiology Cardiovascular Imaging (doi: 10.1016/j.jcmg.2015.11.020).