Photoacoustic-induced vascular tissue dissection resulting from irradition with a Q-switched frequency-doubled Nd:YAG laser

摘要

Abstract: A Q-switched frequency-doubled Nd:YAG dye laser, tuned to 577 nm, was used to study the effect of nanosecond pulsed light on vascular tissue. Different reactions such as vasospasm, vessel expansion and vessel rupture were observed in living rats and were seen to be correlated with increasing fluence up to 3 J/cm$+2$/. When it occurred, localized vessel rupture was seen on the irradiated side of the blood vessel, as well as on the opposite side. It was hypothesized that the damage on the backside of the blood vessel is the result of intense acoustic waves produced by strong absorption of the laser radiant energy in the first 30 $mu@m of blood. Experiments were performed in vitro using cuvettes filled with diluted hemoglobin on which the 532 nm radiant energy produced by a Q-switched frequency-doubled Nd:YAG laser impinged. High-speed imaging of the irradiated air- blood interface using a time-delayed pulsed nitrogen-dye laser did not show evidence of cavitation micro-bubbles but did show the formation of a large, slowly expanding vapor bubble. Measurements of the acoustic waves produced with 12 mJ pulse in a spot size estimated to be 0.25 mm gave pressures up to 74 bars. Measurements at different positions with respect to the irradiation spot showed differences in acoustic amplitude that could not be explained by absorption attenuation. It is hypothesized that these differences are a result of differential diffraction of the frequency components of the acoustic wave, components of which extend up to a maximum of about 4 MHz. It is the highly directional high frequency acoustic energy that could be causing the damage on the side of the blood vessel opposite the point of irradiation.!6