Photoacoustic imaging of human coronary atherosclerosis in two spectral bands

Abstract

Spectroscopic intravascular photoacoustic imaging (sIVPA) has shown promise to detect and distinguish lipids in atherosclerotic plaques. sIVPA generally utilizes one of the two high absorption bands in the lipid absorption spectrum at 1.2 μm and 1.7 μm. Specific absorption signatures of various lipid compounds within the bands in either wavelength range can potentially be used to differentiate between plaque lipids and peri-adventitial lipids. With the aim to quantify any differences between the two bands, we performed combined sIVPA imaging in both absorption bands on a vessel phantom and an atherosclerotic human coronary artery ex vivo. Lipid detection in a human atherosclerotic lesion with sIVPA required lower pulse energy at 1.7 μm than at 1.2 μm (0.4 mJ versus 1.2 mJ). The imaging depth was twice as large at 1.2 μm compared to 1.7 μm. Adequate differentiation between plaque and peri-adventitial lipids was achieved at 1.2 μm only.

Keywords: Atherosclerosis; Intravascular imaging; Lipids; Spectroscopy; Tissue characterization; Vulnerable plaque.

Fig. 1

(a) Lipid and water absorption in the near-infrared wavelength region, showing the two high peaks in the lipid absorption spectrum around 1210 and 1720 nm. In these two optical windows, lipid absorption exceeds water absorption. Lipid absorption at 1720 nm is 5.5 times higher than at 1210 nm; water absorption is 5 times higher. Adapted from . (b) Transmission of light through water; computed based on data from http://omlc.ogi.edu/spectra/water/abs/index.html.

Fig. 2

Lipid containing vessel phantom and IVPA spectra of lipid inclusions. (a) Photograph of the phantom (top-view), filled with cholesterol (bottom), cholesterol oleate (right), cholesterol linoleate (top) and peri-adventitial tissue (left). (b) 6-Wavelength correlation coefficients between the spectra of the lipid inclusions. (c) Average, normalized PA spectra of the four lipid inclusions in the 1.2 μm wavelength range, and (d) in the 1.7 μm wavelength range. c, cholesterol; cl, cholesterol linoleate; co, cholesterol oleate; pl, peri-adventitial lipids.

Fig. 3

Lipid typing in a lipid-containing vessel phantom using sIVPA at 1.2 and 1.7 μm. (a) Lipid map based on 6-wavelength correlation with the cholesterol, and (b) with the peri-adventitial reference spectrum in the 1.2 μm wavelength range (1185, 1195, 1205, 1215, 1225 and 1235 nm). (c) Lipid map based on the 6-wavelength correlation with the cholesterol, and (d) with the peri-adventitial reference spectrum in the 1.7 μm wavelength range (1680, 1710, 1718, 1726, 1734 and 1751 nm). All lipid maps are shown overlaid on the corresponding IVUS image (dynamic range 65 dB). Plaque lipids, represented by cholesterol (bottom), cholesterol oleate (right) and cholesterol linoleate (top) are distinguished clearly from peri-adventitial tissue (left) at 1.2 μm, while lipid typing at 1.7 μm yielded an inferior result.

Fig. 4

Lipid detection in a lipid-containing vessel phantom using sIVPA at 1.2 and 1.7 μm. (a) 1205 nm and (b) 1235 nm combined IVPA/IVUS images (IVPA 50 dB, IVUS 65 dB) of PVA phantom filled with cholesterol (bottom), cholesterol oleate (right), cholesterol linoleate (top) and peri-adventitial tissue (left). (c) Lipid map based on 2-wavelength relative difference between the PA signal at 1205 nm and 1235 nm. (d). Co-registered 1710 nm and (e) 1680 nm combined IVPA/IVUS images (IVPA 50 dB, IVUS 65 dB) of the same cross section of the vessel phantom. (f) Lipid map resulting from the 2-wavelength relative difference between the PA signal at 1710 nm and 1680 nm. Both lipid maps are shown overlaid on the corresponding IVUS image.

Fig. 5

Relative difference of PA signal at high and low lipid absorption wavelengths of individual (unaveraged) IVPA spectra measured in the lipid containing phantom. (a) 5th to 95th percentile of the relative difference of the 1205 nm and 1235 nm PA signal strength of cholesterol, cholesterol oleate, cholesterol linoleate and peri-adventitial lipids (32, 128, 160 and 96 spectra, respectively). Data for elastin and collagen are obtained from . (b) 5th to 95th percentile of the relative difference of the 1205 nm and 1235 nm PA signal strength of same lipid components (same number of spectra). Elastin data obtained from ; collagen data from . c, cholesterol; cl, cholesterol linoleate; co, cholesterol oleate; pl, peri-adventitial lipids.

Fig. 6

Lipid detection in an atherosclerotic human coronary artery using sIVPA at 1.2 and 1.7 μm. (a) 1205 nm and (b) 1235 nm combined IVPA/IVUS images (IVPA 25 dB, IVUS 40 dB). (c) Lipid map based on 2-wavelength relative difference between the PA signal at 1205 nm and 1235 nm. (d). 1710 nm and (e) 1680 nm combined IVPA/IVUS images (IVPA 25 dB, IVUS 40 dB). (f) Lipid map resulting from the 2-wavelength relative difference between the PA signal at 1710 nm and 1680 nm. Both lipid maps are shown overlaid on the corresponding IVUS image. (g) Lipid histology stain (ORO); lipids are stained red; calcification is stained black (h) 5× magnification of the part of the atherosclerotic plaque indicated as lipid rich by the lipid stains (area outlined in black in (g)), shows larger extracellular lipid droplets, while the lipids in all other parts of the lesion are intracellular or contained in small extracellular droplets. (i) 4× magnification of area outlined in black in (h).