A new side-projected catheter for in vivo blood flow measurements has been developed and tested.This catheter is designed to measure blood flow in both the forward (towards the catheter tip) andreverse (away from the catheter tip) flow directions. It consists of two multimode optical fibres withcore diameter of 50μm and cladding diameter of 125 μm. One fibre transmits the laser beam intoblood and the other receives the back-scattered light from the erythrocytes within the probe volume.The experimental plots of the dominant frequency (frequency with the highest amplitude) against theflow velocity are found to be linear in both the forward and reverse flow directions. This result wasreaffirmed by the numerical flow simulation along the fibre catheter side wall. In the simulation, itwas found that the flow within the boundary layer is indeed laminar, and hence the relationshipbetween the Doppler shift frequencies and the flow velocities is linear, thereby making the linearcalibration possible for predicting the free stream flow velocity. The analysis of the experimental andthe computational results indicated that the boundary layer along the side wall of the fibre cathetervaries with both the flow velocity and flow direction. Furthermore, using the fibre optic catheter withan increasing fibre core separation, it was observed experimentally and numerically that thepercentage of the free stream flow velocity increases, suggesting that the projection of the measuringprobe volume is indeed moving towards the free stream domain.
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