LASER-DOPPLER PERFUSION IMAGING OF SKIN BLOOD-FLOW USING RED AND NEAR-INFRARED SOURCES

At present, scanning laser Doppler imaging uses a 633-nm helium-neon l aser (RED) as the only light source, but this restricts its ability to measure blood flow (i) at darkly pigmented skin and (ii) from deeper or subdermal structures. Because near-infrared (NIR) light is known to penetrate deeper into tissue and to be less absorbed than RED, two im agers were adapted to include a NIR laser diode source (one of 830 nm for UK studies; one of 780 mm for leprosy field trials) in parallel wi th the existing RED source. In human hands representing a range of ski n pigmentations, RED scans were unobtainable at the darkest areas of s kin, but intact MR scans could be collected in all cases. In experimen ts at the rat knee and the dorsal human hand, MR and RED values were s imilar on normal skin, Over underlying vessels, however, NIR values gr eatly exceeded RED values, an effect abolished by occlusion, Similarly , in patients with leprosy and in healthy controls in Spain, finger-pu lp NIR values exceeded RED values to the greatest degree when thermore gulatory flow was highest, i.e., when the deeper-lying arteriovenous a nastomoses were open. Over areas of experimental inflammation, NIR gav e higher values and also exhibited a greater degree of spatial heterog eneity than RED, We conclude that some current limitations of laser Do ppler imaging technology can be overcome by the use of NIR laser diode sources.