Evaluation of Two Fiber Laser Doppler Velocimeter

Abstract

Laser Doppler velocimetry has been considered to be a promising new technique capable of measuring blood flow velocity accurately in a small sample volume [1,2]. Application of the LDV in medical and biological fields, however, had been restricted to blood velocity measurements in superficial fine vessels with a thin wall [3,4,5,6], since blood and vessel walls have relatively low transparency for laser light. To overcome this restriction, Tanaka and Benedek [7] introduced laser light into the blood stream through an optical fiber catheter and measured the average blood flow in the rabbit femoral vein by taking an autocorrelation of scattering light. However, pulsatile blood velocity could not be measured in real time by this method nor could reverse flow be differentiated from forward flow. The optical fiber was too thick (500 μm o.d.) for practical use. In order to apply LDV to real-time observation of phasic arterial and venous blood flow velocity, one should be able to measure the blood flow velocity with a high temporal resolution and also to discriminate the reverse from the forward component. To apply the LDV to real-time observation of phasic arterial and venous blood flow velocities, we developed a high-resolution LDV using a thin optical fiber [8,9,10,11]. Kilpatrick also developed an LDV with an optical fiber and demonstrated its utility for blood velocity measurements in the coronary vein [12]. We particularly intended to apply our method to an analysis of the blood flow velocity in the coronary vascular system. Our LDV with an optical fiber has the following advantages: 1) high spatial resolution — (100 μm); 2) high temporal resolution (−8ms), and 3) excellent accessibility with a flexible thin fiber sensor [8,9,10,13]. Blood velocity measurements can be performed satisfactorily in non-disturbed flows, accurate flow detection is difficult with a single fiber system, since the Doppler signal contains the flow information from the fiber tip (zero velocity) to the actual velocity. To widen the applicability of an optical fiber type LDV, e.g. blood velocity measurements in a disturbed flow field,we tested an LDV with two fibers [14,15], which extends the sensing field away from the fiber tip [16]. We used the system to measure blood velocities at poststenotic portions of the canine coronary artery.