In vivo evaluation of signal processors for laser Doppler tissue flowmeters

The performance of different signal processors for laser Doppler tissue flowmeters was evaluated by the use of a well defined flow model comprising a segment of the feline intestinal wall. The processor that, apart from being based on the calculation of the first moment of the power spectral density, also takes into account the effect of multiple scattering in a number of blood cells gave an output signal that was linearly related to the intestinal wall perfusion as recorded independently by a drop-counting technique. At a recording bandwidth of 12 kHz, this linear relationship was valid for the entire flow range 0–300 ml min⁻¹ 100 g⁻¹ (r=0·98). The processor based on the first moment of the power spectral density alone under-estimated the highest flow rates by about 35 per cent, while within the flow range 0–100 ml min⁻¹ 100 g⁻¹ this processor also gave an output signal linearly related to flow at a recording bandwidth of 12 kHz (r=0·96). When the bandwidth was limited to 4 kHz, the output signals from both processors were linearly related to flow only within the range 0–100 ml min⁻¹ 100 g⁻¹ (r=0·90). The output signals recorded with the 4 kHz systems were, however, generally only about 65 per cent of those recorded with the 12 kHz systems.