Noninvasive real-time laser Doppler flowmetry in perfusion regions and larger vessels

This paper describes an enhanced noninvasive continuous laser Doppler system for real time blood flow measurement in perfusion regions and larger vessels. The system consists of a miniaturized sensor, a fast digital signal processing (DSP) unit and a PC for signal visualization. The sensor comprises an IR laser diode to illuminate tissue through a short optical fiber with a variable spot size of 400 micrometers to 1000 micrometers and a photodetector which can be positioned 3 mm to 7 mm from the laser spot. The DSP of the system uses a parametric estimation of the laser Doppler power spectrum density based on a first order autoregressive process model AR (1) to calculate the first weighted moment. This algorithm is approximately ten times faster and as accurate as equivalent FFT-based algorithms. With a sampling rate of 390 kHz, it is possible to calculate and visualize 85 flow values per second. Model measurements prove very high linear correlations (r >= 0.99) between calculated first moments and flow velocities in a range from 1 mm/s up to 120 mm/s. Furthermore, in vivo measurement of blood flow both in perfusion regions and larger arteries, such as the a. radialis, were successfully performed in real time.