Numerous studies 1
have suggested that acute cardiovascular events are most commonly triggered by the structural weakening and disruption of the thin fibrotic cap of fibroatheroma followed by thrombus formation. Generally, this process occurs in atherosclerotic lesions known as rupture-prone or vulnerable plaques. Plaque “vulnerability” has been associated with several pathologic features including a thin fibrous cap, a large lipid pool, and increased macrophage and other inflammatory cell infiltration within the cap. Early detection and treatment of such plaques before rupture depend upon the availability of tools able to characterize in vivo plaque composition through minimally-invasive and nondestructive ways. Most clinical techniques identify luminal diameter (stenosis), wall thickness, and plaque volume, but are inefficient in identifying the rupture-prone plaque 1–6
. New diagnostic techniques (including catheter-based) to localize and characterize vulnerable plaques are needed. Potential intravascular diagnostic techniques for assessment of plaque vulnerability include: magnetic resonance (MR) spectroscopy 6–8
, intravascular ultrasound (IVUS – including high-frequency, elastography) 6,9
, optical coherence tomography (OCT) 10–13
, thermography 14,15
, and optical spectroscopy methods (Raman, near-infrared (NIR), diffuse reflectance, and steady-state fluorescence) 5,16,17
. We report here an innovative time-resolved fluorescence spectroscopy approach that has potential to detect distinct markers of plaque vulnerability as a stand alone technique or to complement the other imaging or spectroscopy modalities in detection of vulnerable plaques.
This study describes a fiber-optic based time-resolved laser induced fluorescence spectroscopy (TR-LIFS) technique that can be used in vivo
to recognize the biochemical makeup of tissues including structural proteins, enzyme cofactors and lipid components. In previous studies, we validated this technique on fluorescent biomolecules constituent of normal and diseased arteries (elastin, collagens, free cholesterol, cholesteryl oleate and cholesteryl linoleate, LDL) 18–21
, in human coronary and aortic postmortem specimens 21,22
, and in vivo in an atherosclerotic rabbit model 23
. Depending upon the light excitation wavelength used and fiber optic excitation-collection geometry, TR-LIFS facilitates evaluation of tissue composition within small tissues volume (~0.6–1.5 mm diameter × 150–250 µm penetration depth), thus feasible for assessing the intimal composition of atherosclerotic plaques. Overall, TR-LIFS has potential for direct evaluation of relative changes of the elastin/collagen and collagen/lipid contents; and indirect assessment of cap thickness and infiltration of macrophage and other inflammatory cells affecting the collagen/lipid content within the excitation-collection tissue volume.
Carotid plaque represents an optimal study model for the validation of new optical devices for detection of plaque composition patients. This allows for direct access of plaque in vivo during carotid endarterectomy (CEA) using fiber optic probes (with or without distal rigidity) and without the need of intravascular procedures. The routine removal of the plaque during CEAs permits extensive TR-LIFS studies ex vivo in fresh plaque specimens that account for the plaque composition heterogeneity. Furthermore, it facilitates a direct validation of optical results against histopathological analysis. Consequently, we conducted this study in human carotid atherosclerotic plaque both in vivo in patients undergoing endarterectomy and ex vivo in the explanted plaques.
The goals of this study were 1) to use TR-LIFS to evaluate the composition of human carotid atherosclerotic plaques and to determine whether a TR-LIFS technique can distinguish fibrotic caps rich in macrophages and inflammatory cells and plaques with a necrotic/lipid core under a thin cap (rupture-prone) from plaques with caps rich in collagen (stable); and 2) to determine the TR-LIFS derived spectroscopic parameters that can be correlated to features of plaque vulnerability.