The infrared absorption spectra ranging from 2.5 to 10 μm in wavelength at various layers of both the human normal abdominal aorta wall and the fibrofatty atheroma were measured by conventional transmission spectrophotometry. Optical specimens 7 μm in thickness were prepared using a cold microtome. Pathological examination was simultaneously performed to identify tissue type of the optical specimen. We found that the presence of characteristic absorption peaks at 5.75 and 3.4 μm was restricted to the atheromatous (fibrofatty) layer of the aorta wall. These peaks may be attributed to the accumulated cholesterol deposits. We also discovered that the normal media layer had strong absorption at 6.05 μm. The discrimination between normal media, fibrofatty atheroma, and aged intimal tissue was made possible by the normalized peak ratio which was based on the ratio of the 5.75‐and 6.05‐μm absorption peaks. These results may be significant for selective laser ablation of atheromatous tissue, as well as tissue diagnosis, to prevent artery perforation during laser angioplastic procedures.
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