Multispectral fluorescence lifetime imaging system for intravascular diagnostics with ultrasound guidance: in vivo validation in swine arteries

Fluorescence lifetime technique has demonstrated potential for analysis of atherosclerotic lesions and for complementing existing intravascular imaging modalities such as intravascular ultrasound (IVUS) in identifying lesions at high risk of rupture. This study presents a multimodal catheter system integrating a 40 MHz commercial IVUS and fluorescence lifetime imaging (FLIm) using fast helical motion scanning (400 rpm, 0.75 mm/s), able to acquire in vivo in pulsatile blood flow the autofluorescence emission of arterial vessels with high precision (5.08 ± 0.26 ns mean average lifetime over 13 scans). Co-registered FLIm and IVUS data allowed 3D visualization of both biochemical and morphological vessel properties. Current study supports the development of clinically compatible intravascular diagnostic system integrating FLIm and demonstrates, to our knowledge, the first in vivo intravascular application of a fluorescence lifetime imaging technique. Keywords: time resolved fluorescence spectroscopy, fluorescence lifetime imaging, intravascular ultrasound (IVUS), intravascular catheter, atherosclerosis 1. Introduction Coronary artery disease is the leading cause of death in western societies [1]. Although many diagnostic tools have been developed to help predict which patients will suffer cardiovascular complications, current diagnostic techniques have significant limitations [2]. New techniques, including intravascular methods, are needed in order to make a significant impact on this public health challenge. Point time-resolved fluorescence spectroscopy (TRFS) and fluorescence lifetime imaging microscopy (FLIM) have demonstrated ability to assess a plurality of diagnostic features associated with arterial wall pathologies including atherosclerotic plaques at risk of rupture [3]. However, the overall TRFS and FLIM potential for atherosclerotic plaques analysis was primarily evaluated ex-vivo in arterial tissue specimens with intima directly exposed to the fiberoptic probe interrogation. Practical intravascular application of these techniques requires development of TRFS systems and catheters capable of rapid acquisition of robust time-resolved fluorescence data in vivo from the luminal surface of the artery in the presence of pulsa-tile blood flow. A novel implementation of a TRFS apparatus with the ability to rapidly (<1 μs per point measurement) spectrally and temporarily resolve tissue autofluorescence [4] has the potential to address such a problem. Using a fiberoptic for fluorescence excitation-collection coupled to this apparatus we demonstrated in tissue phantoms the potential of this approach for continuous recording in helical scanning (rotation and pull-back) of TRFS data from luminal surfaces in multiple spectral bands [5], thus enabling fluorescence lifetime imaging (FLIm) of the luminal surface. However, the slow, scanning speed of this apparatus (10 rpm, 0.01 mm/s) was found impractical for in vivo intravascular application. The main goal of this study is to demonstrate how an improved version of this instrument allowed us to perform, to our knowledge, the first in vivo intravascular application of FLIm. The scanning parameters are compatible with intravascular clinical applications. A second goal is to conduct these experiments in conjunction with intravascular ultra-sound (IVUS) which is the most commonly used intravascular imaging technique in patients [6]. In addition to guiding the TRFS measurement, IVUS provides complementary information on arterial morphology and anatomy. Thus, an intravascular rotational catheter combining FLIm with IVUS detection can provide a simultaneous evaluation of the biochemical and morphological features of the arterial vessels. In current study, we adapted a bi-modal catheter previously tested in tissue phantoms [7] to in vivo intravascular utilization and performed simultaneous acquisition of biochemical and structural features of arterial vessels.