A fluorescence lifetime imaging classification method to investigate the collagen to lipid ratio in fibrous caps of atherosclerotic plaque

Background and Objective This study describes a novel fluorescence lifetime imaging (FLIM) classification method to determine the ratio of collagen to lipid content in the fibrous cap of atherosclerotic plaques. Additionally, an analytical process to assess risk of plaque rupture based on this ratio is proposed. Collagen to lipid ratio has been shown to be an important parameter to evaluate structural integrity of the fibrous cap. FLIM and other time-resolved fluorescence techniques have recently been applied to the study of atherosclerosis based on the ability to assess biochemical composition. Study Design/Materials and Methods Autofluorescence of specimens retrieved during carotid endarterectomy procedures was measured through three optical filters, F377: 377/50?nm, F460: 460/66?nm, and F510: 510/84?nm (center wavelength/bandwidth). A Laguerre deconvolution technique was used for the evaluation of fluorescence decay dynamics. The resulting decay parameters (average fluorescence lifetime and 4 Laguerre coefficients at each of the recorded bandwidths) were used for sample characterization. Linear discriminant analysis (LDA) was used to classify each image into collagen or lipid-rich regions based on these parameters. Ultimately, a risk-level was assigned based on the ratio of collagen to lipid on the surface of the fibrous cap. Results FLIM images were acquired in 18 carotid plaque specimens at 43 locations. Classification of collagen and lipid-rich regions within the fibrous cap was performed with sensitivity and specificity of 80% and 82%, respectively. Conclusions Results from this study show that an LDA method of classifying regions of FLIM images of carotid plaque into collagen and lipid-rich regions is capable of being automated and used to rate the risk of plaque rupture based on autofluorescence decay dynamics and without the need for fluorescence intensity or contrast agents. Lasers Surg. Med. 44: 564-571, 2012. (c) Wiley Periodicals, Inc.