An improved sensor head for cross-correlation intravital velocimetry

A new sensing unit has been designed for intravital velocimetry using cross-correlation techniques. Using a photodiode and low-noise operational amplifier, residual noise has been greatly reduced. At low light levels with equal-frequency passbands, signal-to-noise ratios are improved by greater than a factor of 10. Part of the improvement has been traded for increased frequency range that will allow accurate measurements of velocity 10 to 100 times greater than the unit that was used previously. The precise upper limit for flow velocity measurement with the modified unit will depend on factors such as the spatial fine structure of the RBC column and the digital sampling of the cross-correlation unit, but a cursory examination indicates that most flow velocities in the microcirculation can be measured. The increased frequency range combined with light-level-independent characteristics will greatly improve the accuracy of RBC velocity measurements made in vivo.