We developed a laser Doppler velocimeter (LDV) using a single optical fiber and succeeded in measuring coronary blood velocities. However, it suffers from a broad Doppler spectrum, since the single fiber system senses lower blood velocities in the stagnation field in the vicinity of the fiber tip. Accordingly, it could not provide a good measure for the analysis of blood velocities with disturbed flow.
To overcome this problem, we modify the LDV using two fibers which extends the sensing spot away from the fiber tip. The two fibers were placed side by side. The He-Ne laser beam was transmitted through the fiber into the bloodstream with a spreading angle of 5°. The backscattered light from red blood cells were detected by the other fiber with a receiving angle of 17.3°. The sample volume is approximately given by the cross over volume of the transmittable and the receivable regions. The latter relates to the receiving angle and maximum detectable distance of back-scattered light (about 300μm).
To obtain a suitable sample volume and position, we tested the combinations of two different graded-index multimode fibers with cladding diameters of 62.5μm and 125μm (core diameter is 50μm for both fibers). Evaluation of the Doppler signal was performed by using an annular open channel blood flow on a rotating turntable. The powers of Doppler signals of 62.5/125μm fiber pickup were too small to detect the signal, but the 62.5/62.5μm fiber pickup shows a sharp peaked pattern. The Doppler shift frequencies showed an excellent linearity with the known blood velocities on the turntable. Then, we applied our method to measurements of post-stenotic coronary artery flow. The phasic Doppler spectra of non-stenotic blood velocity showed a peaked pattern, whereas the signal from post-stenotic blood velocities showed a broad Doppler spectrum with the degree of the broadening being especially prominent during diastole.
In conclusion, the dual-fiber LDV is promising for measuring blood velocity accurately with a small sample volume and capable of blood velocity measurements even with flow disturbance.
