Laser tissue coagulation and concurrent optical coherence tomography through a double-clad fiber coupler

Abstract

Double-clad fiber (DCF) is herein used in conjunction with a double-clad fiber coupler (DCFC) to enable simultaneous and co-registered optical coherence tomography (OCT) and laser tissue coagulation. The DCF allows a single channel fiber-optic probe to be shared: i.e. the core propagating the OCT signal while the inner cladding delivers the coagulation laser light. We herein present a novel DCFC designed and built to combine both signals within a DCF (>90% of single-mode transmission; >65% multimode coupling). Potential OCT imaging degradation mechanisms are also investigated and solutions to mitigate them are presented. The combined DCFC-based system was used to induce coagulation of an ex vivo swine esophagus allowing a real-time assessment of thermal dynamic processes. We therefore demonstrate a DCFC-based system combining OCT imaging with laser coagulation through a single fiber, thus enabling both modalities to be performed simultaneously and in a co-registered manner. Such a system enables endoscopic image-guided laser marking of superficial epithelial tissues or laser thermal therapy of epithelial lesions in pathologies such as Barrett's esophagus.

Keywords: (060.2340) Fiber optics components; (170.2150) Endoscopic imaging; (170.3880) Medical and biological imaging; (170.3890) Medical optics instrumentation; (170.4500) Optical coherence tomography.

Fig. 1

(a) Schematic diagram of the DCFC achieved by asymmetrically fusing the imaging fiber (top) to the transition segment of the injection fiber (bottom). The dashed box represents the section along which the fibers were fused. (b) Schematic diagram of cross-sections of, from left to right, the injection fiber, the fused section and the imaging fiber. (c) Single-mode transmission spectrum (smoothed using a moving average filter) of the core signal from Port 1 to Port 2.

Fig. 2

Experimental setup combining OCT imaging and coagulation laser (RFL) through a DCFC. Ex vivo and in vivo images were acquired using the imaging arm while the calibration arm was used to acquire sensitivity measurements. DCF lengths are indicated in green. BD: beam dump; G: galvanometer-mounted mirrors; SL: scanning lens. S_1-4 indicates fusion splices.

Fig. 3

Sensitivity analysis. OCT M-mode images of a mirror (a-c: Sample 1) and of a scattering sample (d-f: Sample 2) using a SMF imaging arm (a & d) or a DCF imaging arm (b-c & e-f). Black arrow indicates the position of the samples while curly brackets highlight the regions used for artifact assessment. In (c) and (f), sample power was further attenuated by 8 and 12 dB, respectively. Graphs (g) and (h) present signal (upper curves) and noise (lower curves) levels for different positions of Samples 1 and 2 across the imaging range. Solid lines were obtained using a spline interpolation.

Fig. 4

In vivo OCT images of human skin (a-b) without and (c-d) with index-matching gel. The sample was imaged at different positions within the imaging range. The position of the focal plane is constant relative to the sample across images. Curly bracket highlights ghost lines. Scale bar: 1 mm.

Fig. 5

OCT images of swine esophageal tissue before (a & e) and after (c & g) laser coagulation are shown along with simultaneous M-mode acquisitions (b & f) showing real-time monitoring of coagulation processes. The M-mode in (b) has been down-sampled by a factor of 1,000. Upper and lower panels show results of irradiation performed using pulse energies of 5.7 mJ and 8.6 mJ, respectively. Red arrows highlight individual coagulation laser pulses. Yellow arrows indicate reaching coagulation threshold. Right panels (d & h) show microscope photographs (4x) of the coagulation marks. Attenuation curves for (i) lower and (j) higher power settings shown at different time points. Scale bars: 1 mm.