Optimizing the performance of Doppler blood-flow probes in extra-corporeal circuits

A theory is developed to improve the signal-to-noise (S/N) ratio of clamp-on Doppler blood-flow probes by optimizing their operating frequency for the tubing utilized in extra-corporeal blood circuits. The theoretical results can be used to increase the magnitude of both the S/N and Doppler frequency shift and to produce a greater tolerance of S/N to variations in the dimensions and acoustic parameters of Tygon tubes. It is shown that thin tube walls improve the S/N and reduce the effect of temperature variations on the magnitude of the S/N. Measurements were performed on various Tygon tubes and their acoustic attenuations obtained. These values were used to predict an optimum operating frequency of 1.78 MHz for the de facto standard tubing having 3/32-in wall thickness. If a 1/16-in wall thickness were substituted, then the optimum operating frequency would be 2.67 MHz and the S/N would increase by 14.54 dB.<>