High-speed intravascular photoacoustic imaging at 17 μm with a KTP-based OPO

Abstract

Lipid deposition inside the arterial wall is a hallmark of plaque vulnerability. Based on overtone absorption of C-H bonds, intravascular photoacoustic (IVPA) catheter is a promising technology for quantifying the amount of lipid and its spatial distribution inside the arterial wall. Thus far, the clinical translation of IVPA technology is limited by its slow imaging speed due to lack of a high-pulse-energy high-repetition-rate laser source for lipid-specific first overtone excitation at 1.7 μm. Here, we demonstrate a potassium titanyl phosphate (KTP)-based optical parametric oscillator with output pulse energy up to 2 mJ at a wavelength of 1724 nm and with a repetition rate of 500 Hz. Using this laser and a ring-shape transducer, IVPA imaging at speed of 1 frame per sec was demonstrated. Performance of the IVPA imaging system's resolution, sensitivity, and specificity were characterized by carbon fiber and a lipid-mimicking phantom. The clinical utility of this technology was further evaluated ex vivo in an excised atherosclerotic human femoral artery with comparison to histology. 503-505 (1994). 2. P. Libby, "Inflammation in atherosclerosis," Nature 420(6917), 868-874 (2002 385-390 (2014). 25. Y. Li, X. Gong, C. Liu, R. Lin, W. Hau, X. Bai, and L. Song, "High-speed intravascular spectroscopic photoacoustic imaging at 1000 A-lines per second with a 0.9-mm diameter catheter," J.

Real-time volumetric lipid imaging in vivo by intravascular photoacoustics at 20 frames per second

“…Partially due to ineffective coupling and scattering at the catheter tip, the optical exposure is less than 0.4 J/cm 2 at 1.7 µm, which is below the 1 J/cm 2 threshold specified by the ANSI laser safety standard [22]. Despite using a pulse energy which is one order of magnitude lower than that used previously in IVPA/US systems working at 1.7 µm, the PA SNR achieved in our imaging system was 20 dB, comparable to earlier reports [22,23]. We used pork lard as a lipid target for in vivo imaging, which has a different lipid composition, and thus a slightly different absorption spectrum, compared to lipid-rich plaque [20].…”

“…It is the first in vivo IVPA/US imaging in a coronary artery reported to date, acquiring images of coronary lipid in this highly mobile, challenging environment. Our IVPA/US imaging system significantly increased the imaging speed from 1 fps (the fastest imaging speed reported so far for a lipidimaging IVPA system) to 20 fps [21][22][23]31]. The operating parameters of our implementation of this technology match those desired of a clinical imaging system in terms of acquisition speed, operating wavelength, and scan method.…”

“…An intensive research effort is ongoing towards the further development of IVPA/US imaging systems. Imaging speed is primarily limited by laser pulse rate: with the introduction of 500 Hz to 2 kHz repetition rate laser systems, operating at the wavelengths of lipid absorption peaks near 1.2 µm or 1.7 µm, the IVPA/US lipid imaging speed has recently increased to about 1 frame per second (fps) [21][22][23]. Use of a more conventional laser wavelength facilitates speed but offers limited biological image contrast [24].…”

Highly Sensitive Intravascular Photoacoustic Imaging with a Collinear Catheter Probe

“…After validated for lipid absorption with a lab-built optical parametric oscillator at 1.7 µm [4], our imaging system was carried out for intravascular imaging of a fresh human coronary artery ex vivo at an imaging speed of 1 frame per second. To provide complementary information, intravascular ultrasound (IVUS) was performed simultaneously for the artery with the embedded transducer.…”

Real-time intravascular photoacoustic-ultrasound imaging of lipid-laden plaque at speed of video-rate level

“…Harnessing the superior penetration depth and chemical selectivity of PA imaging, IVPA channel maps the lipid deposition over the entire artery wall at 1.7 µm; while IVUS channel provides simultaneous and complementary morphological information. Thus far, several research groups are actively working on this technology to advance its clinical translation, which includes the identification of various tissue components [4][5][6], contrast mechanism [4,7], optical excitation sources [8][9][10], IVPA-US catheter designs [10][11][12][13], and ex vivo and preclinical validations [14,15]. However, all these works were limited by the use of slow imaging speeds as well as lack of real-time image display, which are necessary components for future in vivo applications where imaging must be at a sufficient speed to avoid motion artifacts from cardiac pulsation.…”