Coherence-gated Doppler: a fiber sensor for precise localization of blood flow

Abstract

Miniature optical sensors that can detect blood vessels in front of advancing instruments will significantly benefit many interventional procedures. Towards this end, we developed a thin and flexible coherence-gated Doppler (CGD) fiber probe (O.D. = 0.125 mm) that can be integrated with minimally-invasive tools to provide real-time audio feedback of blood flow at precise locations in front of the probe. Coherence-gated Doppler (CGD) is a hybrid technology with features of laser Doppler flowmetry (LDF) and Doppler optical coherence tomography (DOCT). Because of its confocal optical design and coherence-gating capabilities, CGD provides higher spatial resolution than LDF. And compared to DOCT imaging systems, CGD is simpler and less costly to produce. In vivo studies of rat femoral vessels using CGD demonstrate its ability to distinguish between artery, vein and bulk movement of the surrounding soft tissue. Finally, by placing the CGD probe inside a 30-gauge needle and advancing it into the brain of an anesthetized sheep, we demonstrate that it is capable of detecting vessels in front of advancing probes during simulated stereotactic neurosurgical procedures. Using simultaneous ultrasound (US) monitoring from the surface of the brain we show that CGD can detect at-risk blood vessels up to 3 mm in front of the advancing probe. The improved spatial resolution afforded by coherence gating combined with the simplicity, minute size and robustness of the CGD probe suggest it may benefit many minimally invasive procedures and enable it to be embedded into a variety of surgical instruments.

Keywords: (170.3340) Laser Doppler velocimetry; (170.4500) Optical coherence tomography; (280.1415) Biological sensing and sensors.

Fig. 1

(A) CGD system (B) Doppler OCT system (C) LDF system. SLD: superluminescent diode, LS: light source, BW: wavelength band-width, FC: fiber coupler, C: collimator, M: mirror, PM Phase modulator, ADC: analog to digital converter, SM: single-mode fiber, GM, gradient-index fiber, EM: emission fiber, RE, receiver fiber. The detection volume of each system is shown in the probe drawing to the right in red.

Fig. 2

A CGD probe during insertion into sheep brain. The inset shows a CGD probe placed in a 30 G needle.

Fig. 3

Characterization of the linearity of flow speed measurement within two ranges.

Fig. 4

(A) Signals measured while scanning the CGD probe (0.4 mm away from the top surface of capillary) laterally across a capillary tube immersed in 2% Intralipid solution. The inset shows a DOCT B-scan image of the phantom. The flow speed in the capillary is 1.5 mm/s parallel to the CGD probe. (B) Axial detection range. Moving a CGD probe axially away from a capillary immersed in 2% Intralipid solution.

Fig. 5

(A) Exposed rat femoral vessels and CGD probe inside a large outer guide. CGD probe is 3 mm away from the sample. Voltage waveform (Media 1) from (B) rat femoral tissue, (C) vein, and (D) artery. Arrow indicates bulk motion.

Fig. 6

The CGD waveform (Media 2), autocorrelation coefficient and spectrogram of (A) advancing probe in brain tissue, (B) vein, and (C) artery in sheep brain in vivo. The red arrows indicate the time point when the CGD probe is static relative to the brain tissue.

Fig. 7

Ultrasound monitoring with Doppler detection of the CGD probe as it approaches a vessel (yellow arrows) in a sheep brain in vivo (Media 3). The CGD probe tip appears as the bright white spot (red arrows). The target vein is the blue spot in the insets in top panel. (A) Bulk motion away from the vein, (B) approaching the vein, and (C) pressing on the vein resulting in the disappearance of the ultrasound Doppler signal as well as the CGD signal.