Characteristics of 308 nm excimer laser activated arterial tissue photoemission under ablative and non-ablative conditions

Abstract

The present study was designed to assess the characteristics of tissue photoemission obtained from normal and atherosclerotic segments of human postmortem femoral arteries by 308 nm excimer laser irradiation of 60 ns pulsewidth. Three ablative (20, 30, and 40 mJ/pulse) and three non-ablative (2.5, 5, and 10 mJ/pulse) energy fluences were employed. Both the activating laser pulses and the induced photoemission were guided simultaneously over one and the same 1,000 μm core optical fiber that was positioned in direct tissue contact perpendicular to the vascular surface. The spectral lineshape of normal arterial and noncalcified atherosclerotic structures was characterized by a broad-continuum, double-peak emission of relevant intensity between wavelengths of 360 and 500 nm, with the most prominent emission in the range of 400–415 (407 nm peak) and 430–445 nm (437 nm peak). Fibrous and lipid atherosclerotic lesions, however, exhibited a significantly reduced intensity at 437 nm compared to normal artery layers (P < 0.001), expressed as a 407/437 nm ratio of 1.321 ± 0.075 for fibrous and 1.392 ± 0.104 for lipid lesions. Normal artery components presented with approximately equal intensity at both emission peaks (407/437 nm ratio: intima, 1.054 ± 0.033; media, 1.024 ± 0.019; adventitia, 0.976 ± 0.021). Comparison of spectral lineshape obtained under various energy fluences within a group of noncalcified tissues disclosed no substantial difference using the 407/437 nm ratio (P > 0.05). In contrast, calcified lesions revealed high-intensity multiple-line (397, 442, 461, and 528 nm) emission spectra under ablative energy fluences, whereas a low-intensity broad-continuum, single-peak spectrum resulted from irradiation beyond the ablation threshold. Thus, these findings suggest fluorescence phenomena for broad-continuum spectra, and plasma emission for multiple-line spectra as an underlying photodynamic process. Regardless of the activating energy fluence, spectral analysis of 308 nm activated photoemission provides accurate information about the laser target under standardized in vitro conditions. It is demonstrated that direct contact ablation and simultaneous spectral imaging of the target tissue via the same optical fiber is feasible.