Non-invasive techniques for measuring dermal blood flow are important for clinical diagnosis and the treatment of peripheral vascular disease. Although techniques such as scanning laser Doppler imaging and laser speckle contrast imaging are both suitable for this task, the former requires long mapping times and the latter needs a priori knowledge of the blood velocity distribution.

Marcel Leutenegger and co-workers from Switzerland have now demonstrated a scanning laser Doppler imaging scheme that can be used to measure large-area blood perfusion in vivo and in real time (Biomed. Opt. Express 2, 1470–1477; 2011).

Credit: © 2011 OSA

A scanning laser Doppler imaging instrument typically consists of a monochromatic light source with a long coherence length, a fast detector and hardware and software for analysing the detected signal. When light from the laser illuminates a tissue sample, part is statically scattered by the tissue's structure and part is dynamically scattered by the red blood cells flowing through the arteries and veins. The dynamically scattered light incurs a small wavelength shift due to the Doppler effect. Coherent mixing of the statically and dynamically scattered light causes intensity beating at a frequency in the kilohertz range for typical blood flow speeds of a few millimetres per second. The blood flow can then be mapped based on the detected power spectrum.

In their scheme, Leutenegger and co-workers used a 150 mW, 808 nm laser as the light source and a high-speed CMOS camera chip and powerful field-programmable gated array chip for processing the captured images in real time. The processing unit is designed in such a way that the pixel values of the CMOS sensor are read during integration of the next frame, which allows for almost uninterrupted sensor exposure.

The researchers imaged blood perfusion in an area of 50 cm2 at 12–14 frames per second with 480 × 480 pixels per frame. The technique can be configured for smaller areas if a higher frame-rate is desired. The team also mapped the blood perfusion of human fingertips and skin samples of humans and small animals, which showed distinctive perfusion and/or microcirculatory responses. The researchers are confident that their approach will help assess health issues related to microcirculation.