The analysis of speckle contrast in a time-integrated speckle pattern enables visualization of superficial blood flow in exposed vasculature, a method we call laser speckle imaging (LSI). With current methods, LSI does not enable visualization of subsurface or small vasculature, because of optical scattering by stationary structures. In this work we propose a new technique called photothermal LSI to improve the visualization of blood vessels. A 595 nm laser pulse was used to excite blood in both in vitro and in vivo samples. The high absorption coefficient of blood at this wavelength results in efficient conversion of optical energy to thermal energy, resulting in an increase in the local temperature and hence increased scatterer motion, and thus a transient decrease in speckle contrast. As a result, we found that photothermal LSI was able to visualize blood vessels that were hidden when imaged with a conventional LSI system.
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