The study of plaques of psoriasis using a scanning laser-Doppler velocimeter

A scanning laser-Doppler velocimeter, able to measure blood flux over a large area without contact with the skin surface, was used for the objective measurement of the vascular changes in psoriasis. Individual plaques were scanned, and tracings of the visible plaque edge taken on to a cellulose-acetate sheet from which area measurements were made using a digitizing tablet. Mean blood flux within plaques was increased fourfold compared with mean background flux. There was a highly significant linear correlation between plaque area measured by tracing and scanning, but area measured by scanning was greater. Detailed study of blood flux outside the visible plaque edge with the scanner, and with a conventional laser-Doppler instrument, showed this was due to a 2-4 mm rim of increased blood flux around plaques. The scanning laser-Doppler velocimeter allows rapid measurement of psoriasis in terms of mean blood flux and area of increased blood flux, and should be useful in the assessment of psoriasis severity and the response to treatment.