Gert Nilsson

An instrument for measurement of tissue blood flow based on the laser Doppler principle was evaluated using a fluid model. A unique and linear relationship between flowmeter response and flux of red cells was demonstrated with red cell velocities and volume fractions within the normal physiological range of the microcirculatory network of the skin. Different degrees of oxygenation proved to influence the Doppler signal only to a minor extent. The study also shows that the Doppler signal is formed essentially by heterodyne mixing of light beams backscattered in static structures and moving red cells.

A new instrument for measurement of regional tissue blood flow based on the laser Doppler principle is reported. The theoretical background of light beating spectroscopy is discussed and a detection technique which makes possible the suppression of the adverse effects of laser-mode interference and wide-band beam amplitude noise is described. Instead of using a single square-law photodetector a differential detector technique is introduced that reduces common-mode noise to a negligible level, without influencing the blood flow related signal. The new instrument has proved to be highly stable and sensitive. Continuous recordings of tissue blood flow can be performed in the laboratory as well as at the bedside.

Imaging of tissue perfusion is important in assessing the influence of peripheral vascular disease on microcirculation. This paper reports on a laser Doppler perfusion imaging technique based on dynamic light scattering in tissue. When a laser beam sequentially scans the tissue (maximal area approximately 12 cm *12 cm), moving blood cells generate Doppler components in the back-scattered light. A fraction of this light is detected by a remote photodiode and converted into an electrical signal. In the signal processor, a signal proportional to the tissue perfusion at each measurement point is calculated and stored. When the scanning procedure is completed, the system generates a color-coded perfusion image on a monitor. A perfusion image is typically built up of data from 4,096 measurement sites, recorded during a time period of 4 min. This image has a spatial resolution of about 2 mm * 2 mm. A theory for the system inherent amplification factor dependence on the distance between individual measurement points and detector is proposed and correction measures are presented. The performance of the laser Doppler perfusion imager was evaluated using a flow simulator. The correlation coefficient between the estimated flow parameter and the perfusion through a mechanical flow simulator was calculated to r = 0.996. To assess the sampling depth of the laser beam, light scattering in tissue was simulated by a Monte Carlo technique. The average sampling depth for skin tissue was calculated to 200-240 microns, depending on the blood content.(ABSTRACT TRUNCATED AT 250 WORDS)