We report a method to visualize lipid distribution in axial and lateral direction within arterial vessel walls by
spectroscopic spectral-domain Optical Coherence Tomography (OCT) at 1.7 μm wavelength for identification of lipidrich
plaque. An OCT interferometric spectrum, after divided into several subbands for each of which A-scan profile is
calculated, are fitted to a model accounting the contributions from absorption and scattering, with assumption that lipid
content changes slowly in space, and lipid distribution is visualized. We investigated numerically the effects of analyzing
parameters on the performance of lipid detection using simulated OCT data, and have found optimum parameters.
We have developed a near infrared (NIR) angioscope that takes multi-wavelength images in 1.7μm band for visualizing
lipid-rich coronary plaques. The angioscope comprises light source, camera, and angioscopic catheter. The light source,
containing a supercontinuum source and a switching optical filter, emits 1.60, 1.65, 1.73 and 1.76μm wavelengths
sequentially in synchronization to the camera frame. The supercontinuum is seeded by 1.55μm wavelength pulses,
whose spectrum is spread by an optical fiber with ring loops for reducing peak power so that light in 1.7μm band is
generated efficiently. The switching filter contains 1×4 fiber-optic path switches and interferometric band-pass filters.
The camera detects NIR images by an InGaAs/GaAsSb type-II quantum well sensor at 100 frames/s. The source
wavelength and the camera frame are synchronized with each other by an FPGA. The angioscopic catheter, based on a
silica-based image-guide designed for 1.7 μm wavelength, transmits 1300-pixel NIR images and has 0.73 mm outer
diameter, which is compatible with the conventional angioscope and suited for continuous flushing to displace blood. We
have also developed image processing software that calculates spectral contribution of lipid as lipid score at each pixel
and create lipid-enhanced color images at 12 frames/s. The system also includes conventional visible light source and
camera, and takes visible light images synchronously with the lipid-enhanced images. The performance of the
angioscope for detecting lipid-rich plaque has been verified in bench tests using a plaque model made by injecting lard
into excised swine carotid arterial vessel. The plaque models are imaged in water at working distances of 0 to 2 mm, and
significantly distinguished from normal vessels.
We demonstrate visualization of lipid distribution in in-vitro artery model by 1.7-μm spectroscopic spectral-domain optical coherence tomography (SD-OCT). In the demonstration, we measure spectral fringes by a spectrometer with an extended InGaAs line sensor and a super-continuum (SC) light source whose spectrum is arranged to have its maximum intensity in 1.7-μm band. The OCT system has an axial resolution of 21μm, a measurement range of 5mm and a sensitivity of 108dB with an A-scan rate of 0.96kHz, which is limited by the noise of the available SC light source. The in-vitro model is made by injecting lipid into swine carotid artery, which is compared to intact artery. We perform Bscan of the model in water by connecting an OCT probe to the OCT system and pulling the probe back at 0.027mm/sec with a rotation rate of 112rpm. For visualizing lipid distribution, we adopt a spectroscopic OCT algorism where the detected spectral fringe is divided into six sub-bands, the set of the sub-band A-scans are fitted to a model accounting absorption characteristics of lipid with its peak at 1726nm, and the content of lipid is estimated as lipid score. As a result, the p-value of the lipid score between normal artery and plaque one is less than 1E-10 in 1-mm depth from the surface, which is significant of visualization of lipid distribution.
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