Optical fiber based distance sensors for vitreoretinal surgery robotics

Session

Poster Session

Authors

Radu-Florin Stancu (3), Manuel J. Marques (3), Ross Henry (1), Carlo Seneci (1), Lyndon da Cruz (2), Christos Bergeles (1), Michael Hughes (3), Adrian Podoleanu (3)

Affiliations

1. King's College London
2. Moorfields Eye Hospital
3. University of Kent

Keywords

vitreoretinal, fiber optics, distance sensor, low coherence interferometry, resolution

Abstract text

In this paper, we present the design and integration method of an entirely fiber-based sensor which is functioning in real time and that will be integrated in a robotic surgery tool designated for both eye disease diagnosis and interventions.

Vitreoretinal surgery represents a complex and challenging medical procedure, facing the spatial constraints of the human eye, the physiologic tremor, and the lack of positional stability. Recently, a number of robotic systems were proposed and employed in vitreoretinal surgery, integrating both non-invasive microscopy and optical coherence tomography (OCT) systems for imaging, and, respectively, micro-surgery tools as invasive procedures. The positional coordinates of the surgery tool relative to sensitive tissue, such as the retina, represent crucial parameters in ensuring real time safe intervention. 

The distance sensors were manufactured in house, using a Vytran GPX-300 glass processing system. They comprise three main fibre components: Hi1060 single mode fibre (SMF), Thorlabs FG125LA no core fibre (NCF), and a Thorlabs GIF-650 multimode graded index (GRIN) fiber. The NCF acts as a beam expander, while a smaller length of GRIN fiber acts as a focusing lens for the diverging beam to a point at a fixed coordinate in space. The sub-millimeter lengths of both NCF and GRIN sections are also important in mathematically determining the optical beam waist and the working distance of the sensor. 

Utilising the Fresnel equation for a fiber probe immersed in water, the power reflectivity measured is only 0.36%, thus the amount of returned power forming the reference signal for the common-path low coherence interferometer is insufficient for an efficient operation. The resulting signal is weak, when compared with the measurements performed for the same probe in an air environment, in which the reflectivity is about 4%. In order to improve the results described so far, a thin gold coating was applied through sputtering on the GRIN tip of the probe. The improvements obtained were considerable, as the reflectivity of the fiber end was increased several times, the power levels returned were by one order of magnitude higher, while independence of the refractive index of the medium that the probe is placed into was also achieved. The gold layer exhibited good adherence to the glass fibre and resilience in vitreous type fluid environments, despite being very thin to permit sufficient optical power transmission to the sample. 

In order to test and confirm its performance, the distance sensor was integrated into a low-coherence common-path interferometer, comprising as light source an Axsun swept source centred at 1060 nm, determining a coherence-limited OCT axial resolution of 8 µm in air, and a fast sweep rate of 100 kHz. Light is directed to and from the probe through a fiber coupler, with the returning light properties being measured with a balanced photodetector. The spectral interference patterns formed from the interference between the light scattered from the object of interest, such as the retina, with the reference reflection exhibits modulations at different frequencies. A Fast Fourier Transform (FFT) is applied to the measured channelled spectra, thus achieving an A-scan, in which individual depth-resolved layers of the object can be seen. The LabVIEW acquisition software was developed to determine the distance to the top layer of the object and relay this to the user in real time. The distance measured to the closest layer is displayed in two ways: either through a visual bar, or through a numerical value, both to a precision of 10 µm.  An additional layer of safety was introduced through sonification.

The sensor probes were effective so far in distance measurements performed on both non-organic multi layered samples and organic tissue (pig eye retina), at a maximum set range of 2 mm. The most effective probes developed generated a spot size of 42.9 µm at a working distance of 630 µm, and were gold sputtered for a duration of 5.0 s. Signal to noise ratio (SNR) measurements of the signal scattered from a multilayered target have demonstrated that the sputtered probe is effective in both air and water environments. In the experiments employing organic tissue, the sensor probes were inserted into a metallic tube, integrated inside a custom-made surgery tool; the tool was directly connected to the robotic infrastructure and manipulated with mechanical actuators. The distance sensor is programmed to acquire real time tissue proximity data, which will enable the safe use of other tools deployed in surgery. 

In conclusion, the distance sensor probe demonstrated good performance in a water environment and was subsequently integrated in a robotic infrastructure for further testing in an ex-vivo environment (pig eyes). Enhancing control and improving feedback to the surgeon when performing ophthalmic surgery remain areas of improvement in the near future. 

This work was supported by the National Institute for Health Research (Invention for Innovation, i4i; NIHR202879). The views expressed are those of the authors and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health and Social Care.

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