2.1.

Position-Guided Surgical Needle and M-Mode Optical Coherence Tomography

We developed a position-guided surgical needle, which equips an embedded OCT that can acquire the positional contexts of the underlying tissue. For a compact and minimalistic structure, our surgical needle employed a stepwise transitional core (STC) fiber [Fig. 1(a)], which produces an output beam without using a lens element.³² It was composed of three different optical fibers with an outer diameter of $125\mu \mathrm{m}$, spliced in series to the standard single-mode fiber of light delivery. The large core fiber with its diameter of $29\mu \mathrm{m}$ at the distal end directly determined the characteristics of the output beam while keeping the effective single-mode guidance through the STC structure. To bend the beam perpendicularly to the fiber axis, a 43 deg-tilted reflector plane was made by polishing the end of the fiber structure. This structure is advantageous in its compactness, ease of fabrication, and operational reliability, though with a moderate sacrifice of the imaging range. The STC fiber supported an effective imaging range of 0.9 mm with beam diameter of $\sim 25\mu \mathrm{m}$ in corneal tissue ($n=1.35$),³² so was sufficient for our application to corneal imaging. The fabricated fragile fiber structure was reinforced by a metal jacket with an outer diameter of $200\mu \mathrm{m}$. This fabricated fiber was finally integrated into a 26-G syringe needle as shown in Fig. 1(b). The inner and outer diameters of the position-guided needle were 260 and $460\mu \mathrm{m}$, respectively. The fluid channel was formed between the metal-jacketed fiber and the inner side of the syringe needle.

Fig. 1

Schematics of (a) STC fiber, (b) STC fiber integrated position-guided needle, and (c) customized optical coherence tomography system assembled with position-guided needle. FC, fiber coupler; CIR, circulator; PC, polarization controller; RM, reference mirror; and BPD, balanced photodetector.

In this study, to acquire the depth information from the needle, we utilized the home-built swept source OCT (SS-OCT) imaging system as shown in Fig. 1(c). The swept source laser (Axsun Technologies Inc.) with 1310-nm center wavelength and 105-nm bandwidth was incorporated into the SS-OCT system, providing average 20 mW of output power and an axial resolution of $\sim 7.6\mu \mathrm{m}$. A 90:10 coupler splits the light emitted from the swept source laser into the mirror in the reference arm and sample arm. The position-guided needle was attached to the sample arm in our system to achieve optical signals from the inside of the tissue. The power from the sample arm was measured $\sim 10\mathrm{mW}$. Reflections from each arm were combined and generated the spectral interference signals at the 50:50 coupler. A balanced photo detector (PDB450C, Thorlabs Inc.) and digitizer (ATS9350, Alazar Technologies Inc., Canada) were utilized to detect the interference signals. A fast Fourier transform was computed to achieve the position profile and image was updated based on time order with $\sim 0.01\mathrm{s}$ of update speed (M-mode). Our customized SS-OCT system facilitates 50-kHz A-line acquisition.

2.2.

Corneal-Mimicking Phantom Preparation

To verify the functionalities of the developed position-guided surgical needle, we prepared the corneal-mimicking phantom, which has two layers, representing the cornea and the anterior chamber, respectively. The top layer was prepared by blending an agarose powder and distilled water, and a scatterer was added to the mixture to increase the optical scattering while imitating the stroma texture. The bottom layer had a low concentration of mixture compared with the top layer to realize the transparency similar to that of the anterior chamber. Solidifying the subsequently poured mixtures of differently concentrated agarose develops the corneal-mimicking phantom. The top layer of corneal-mimicking phantom was fabricated with 1-mm thickness and then utilized to evaluate the functions of the developed position-guided needle.

2.3.

Ex Vivo Animal Study Preparation

The rabbit eyes were enucleated from two New Zealand rabbits weighing 2.0 to 2.5 kg that were euthanized in accordance with institutional guidelines and approved Institutional Animal Care and Use Committee at the College of Medicine, the Catholic University of Korea. The rabbit eye was held in an anterior chamber manufactured by a 3-D printer for an optimal surgical procedure. The anterior chamber consisted of a plastic cradle with two nozzles, which controlled pressure using water and air to fasten the eye tightly. After setting the eye in the cradle, a position-guided needle was inserted into the cornea, which measured the remaining thickness of the cornea in real-time when the needle end was moving toward a border between the cornea and the anterior chamber. When the needle end reached the target location inside the cornea, air was blown through the needle to separate two layers.

3.1.

Evaluation of Phantom Study

To identify the performance of the surgical needle as a position sensor, the corneal-mimicking phantom was utilized, as shown in Figs. 2(a) and 2(b). For this experiment, an OCT image was taken to monitor the needle insertion in the phantom and calibrate the position information acquired from the needle. Figure 2(c) shows an OCT image of the phantom without a needle and clearly delineates the two solidified agarose layers with different concentrations and scatters. When the surgical needle is inserted into the phantom, an OCT image and position information from the needle were simultaneously acquired, as presented in Figs. 2(d) and 2(e). The A-line profile indicating the position information of the needle is directly converted and visualized as an M-mode OCT image. Video 1 presents demonstration of M-mode OCT when the position-guided needle is applied to the cornea phantom and approaches the desired location. The M-mode OCT image, which was updated every 10 ms, denotes the time sequential signals along the depth from the end of the STC fiber. As the needle penetrated into the phantom, the thickness of the M-mode OCT image gets thinner while showing the residual thickness of the top layer. In this experiment, we confirmed that a position-guided needle enables us to clearly distinguish the two layers, and offers position location with high accuracy. The sensing speed was also fast enough to apply to real surgical settings while guiding the placement of a needle.

Fig. 2

Phantom study evaluating the feasibility of the position-guided needle combined with M-mode OCT. (a) Schematic diagram of phantom study. (b) Corneal-mimicking phantom developed with double layers of different concentrated agarose. (c), (d1), and (d2) regular OCT image acquired while needle punched through the surface of agarose gel and passed by the middle of first layer at 1.68 s, and came closer to second layer at 4.14 s, respectively. (e) M-mode imaging from position-guided needle in real-time (Video 1). Red and yellow arrows denote the residual thickness between the needle distal end and the boundary (assuming the DM and endothelium) within the phantom, respectively (Video 1, MPEG, 3.42 MB [URL: http://dx.doi.org/10.1117/1.JBO.22.12.125005.1]).

3.2.

Evaluation of Animal Study

The M-mode OCT needle was applied to the DALK procedure with an ex vivo rabbit animal model with manual scanning. In this experiment, a position-guided needle with the bevel facing downward was carefully penetrated tangentially into a paracentral rabbit cornea. The entire DALK procedure was recorded by video and M-mode OCT as shown in Video 2. Figure 3 shows how the surgical needle is inserted and guided to approach near the DM. A time-sequential A-line profile acquired from the distal end of the needle was continuously monitored while updating the current location. The real-time feedback from the inserted needle was visualized on the display to help the clinician perceive the position of the needle. The whole cornea thickness was visualized at the epithelial layer [Fig. 3(a)] and became thinner when the needle approached the paracentral cornea [Fig. 3(b)]. The green line shown in the Video 2 indicates the current residual thickness between the DM and the needle end, which provide fast and intuitive information to the clinician.

Fig. 3

Animal study demonstrating the position-guided surgical needle and M-mode OCT (Video 2). (a1) Animal surgery using position-guided needle when the tip of needle was inserted into the surface of cornea. (a2) The corresponding M-mode OCT image from the needle when its tip was maintaining the corneal layer with residual thickness of around $300\mu \mathrm{m}$. (b1) Next medical procedure when the clinician was aided with the needle to quantitatively find out the desired position of the cornea. (b2) The corresponding M-mode OCT image was captured when the needle approached toward lowermost of the enucleated rabbit eyeball and detected the rest of the cornea thickness within $100\mu \mathrm{m}$ (Video 2, MPEG, 6.32 MB [URL: http://dx.doi.org/10.1117/1.JBO.22.12.125005.2]).

We further evaluated the feasibility of bubble formation induced by our needle using SS-OCT imaging system. $512\times 500\times 500$ voxel of OCT images were captured immediately after the needle was placed at $100\mu \mathrm{m}$ above the DM layer and after performing the air injection at the same position. Figures 4(a) and 4(b) show the DALK process including needle insertion and the various bubble formations. As shown in Figs. 4(c) and 4(d), the 3-D OCT image presents the whole structure of the cornea with the needle. In DALK surgery, a clinician usually lays the needle parallel to the corneal surface to avoid the risk of perforation under the DM layer. This method may also help to easily separate the corneal layer with less damage in stromal layer when the big-bubble technique is used. 2-D and 3-D OCT images clearly identify the needle location inside the cornea in both success and failure cases of big-bubble formation. The blue and red lines in Figs. 4(c) and 4(d) show the cross-sectional image perpendicular and parallel to the needle, respectively. After the big-bubble technique was applied, it was found that more than half of the stroma layer was detached and air entered underneath, as shown in Fig. 4(d). From Fig. 4, we realized that DALK was successfully operated without any tissue damage by means of the position-guided needle.

Fig. 4

Feasibility study of bubble formation with position-guided needle through 3-D OCT images on rabbit eyes. (a1)–(a3) Process of inserting a needle before air injection into the rabbit eye. (b1), (b2) Failed big-bubble cases after air injection due to over- and under-penetration of the needle, respectively. (b3), (b4) Successful big-bubble cases after air injection $\sim 100\text{-}\mu \mathrm{m}$ position. (c1)–(c4) 3-D OCT image, en-face image, and cross-sectional images in different views before air injection, (d1)–(d4) after air injection.