Signal processor for laser Doppler tissue flowmeters

Abstract

Laser Doppler flowmeter for the assessment of tissue blood flow are generally equipped with a signal processor which generates the first moment of the unnormalised power spectral density as a continuous output signal. This signal is related to blood flow for low and moderate flow rates. At higher flow rates the interpretation of the output signal becomes ambiguous as a consequence of the multiple scattering in moving blood cells and the homodyne mixing of waves on the detector surface. The paper describes a new signal processor which takes these effects into account and establishes a linear relationship between the flowmeter output signal and blood flow for all flow rates. The performance of the signal processor was evaluated by an experimental fluid model which optically resembled the blood flow through the microvasculature. The transiently high blood flow in the initial phase of reactive hyperaemia, recorded from palmar skin, gave peak values which were almost double those recorded with the original signal processor of the flowmeter. In conjunction with these high peak values a high concentration of moving blood cells was recorded, indicating that the initially high flow rate is produced by an increased number of moving blood cells due to vasodulation rather than by a change in average velocity.