High-Speed Intravascular Spectroscopic Photoacoustic Imaging at Two Spectral Bands

Abstract

The rupture of atherosclerotic plaques is the leading cause of acute coronary events. The detection of lipid content and its distribution within the plaques is critical for identifying vulnerable plaques that are prone to rupture [1]. Intravascular spectroscopic photoacoustic technology is capable of imaging the composition and structure of atherosclerotic plaques [2, 3]. To achieve accurate spectroscopic IVPA imaging, an ns-pulsed, wavelength tunable laser [e.g., optical parameter oscillator (OPO) laser] is critical. However, in previously reported IVPA systems, laser pulse energy in the order of mJ was usually required to produce decent photoacoustic signals, so the choices of laser sources were usually limited to OPO lasers of a pulse repetition rate of 10–20 Hz. The B-scan rate at a single wavelength with such lasers was 0.05–0.1 f/s (assuming 200 A-lines/B-scan); for spectroscopic imaging, the image acquisition would take several times longer. Overall, the imaging speed is becoming one of the major challenges for translating IVPA into clinical applications [4, 5]. In this study, a catheter of 0.9 mm in diameter with a novel quasi-focusing light illumination scheme is designed and developed, smaller than the critical size of 1 mm required for clinical translation. With this design, the laser fluence in the targeted imaging region was increased, which produced detectable signals with laser energy as low as ~30 μJ/pulse. As a result, a 1-kHz-repetition-rate, ns-pulsed optical parametric oscillator (OPO) laser was able to be utilized to achieve high-speed IVPA imaging, working at both the 1.2 and 1.7 μm spectral bands for lipid detection. Specifically, a B-scan acquisition rate of 5 Hz was achieved, ~100-fold faster than conventional IVPA systems operating at the similar tunable range. With the system, multi-wavelength (five wavelengths) spectroscopic IVPA imaging of both a lipid-mimicking phantom and peri-adventitial adipose tissue from a porcine vessel was demonstrated at both the 1.2 and 1.7 μm spectral bands. The significantly improved imaging speed, together with the reduced catheter size and multi-wavelength spectroscopic imaging ability, suggests that the developed high-speed IVPA technology is of great potential to be further translated for in vivo applications.