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Imaging of the lens capsule with an ultrahigh-resolution spectral optical coherence tomography prototype based on a femtosecond laser
  1. Bartlomiej J Kaluzny1,
  2. Michalina Gora2,
  3. Karol Karnowski2,
  4. Ireneusz Grulkowski2,
  5. Andrzej Kowalczyk2,
  6. Maciej Wojtkowski2
  1. 1Department of Ophthalmology, Collegium Medicum, Nicolaus Copernicus University, Bydgoszcz, Poland
  2. 2Institute of Physics, Nicolaus Copernicus University, Grudziadzka, Torun, Poland
  1. Correspondence to Dr Bartlomiej J Kaluzny, Department of Ophthalmology, Collegium Medicum, Nicolaus Copernicus University, Curie-Sklodowskiej 9, Bydgoszcz 85-094, Poland; bartka{at}by.onet.pl

Abstract

Aim To demonstrate the applicability of ultrahigh-speed, ultrahigh-resolution spectral optical coherence tomography (SOCT) to cross-sectional imaging of the capsular bag in vivo.

Methods The ultrahigh-speed and ultrahigh-resolution SOCT prototype was designed and constructed at Nicolaus Copernicus University (Torun, Poland). To obtain an ultrahigh speed up to 100 000 lines/s a new spectrometer with fast CMOS line-scan camera was built. A femtosecond laser with a central wavelength of 780 nm and Δλ=160 nm enabled imaging with an axial resolution of 2.3 μm and lateral resolution of 10 μm in tissue. Lens capsules of two healthy eyes were examined with the aid of the instrument using two- and three-dimensional scanning protocols.

Results The prototype provided ultrahigh-resolution tomograms composed of 8000 A-scans with an acquisition time of 0.16 s. The quality was sufficient to evaluate the capsular bag and to estimate its thickness. It was possible to visualise a separate layer of lens epithelium, to the authors' knowledge the first such visualisation. Three-dimensional data were used to produce lens-capsule thickness maps.

Conculsions Ultrahigh-resolution, ultrahigh-speed SOCT based on a femtosecond laser allows two- and three-dimensional evaluation of a capsular bag and lens epithelium. The instrument provides new information of scientific and clinical value.

  • Spectral optical coherence tomography
  • SOCT
  • OCT
  • lens capsule
  • capsular bag
  • lens and zonules
  • imaging
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Spectral optical coherence tomography (SOCT), also known as Fourier domain optical coherence tomography, is a novel imaging technique that can be used to visualise objects that weakly absorb and scatter light. It enables high-speed, high-resolution, non-contact optical biopsy of ocular structures in vivo.1 Due to its very short acquisition time and high sensitivity, SOCT is capable of providing tomograms of substantially better quality compared with conventional, time-domain OCT (TdOCT).2 3 Moreover, SOCT allows three-dimensional and real-time imaging.4 The application of SOCT to anterior segment imaging was presented in several papers,5–10 including posterior capsule opacification assessment.11 SOCT instruments with a central wavelength of 830 nm and Δλ=70 nm can achieve an axial resolution of 4–5 μm. The resolution can be further improved to 2.3 μm if a femtosecond laser with central wavelength of 780 nm and Δλ=160 nm is used.

The aim of this paper is to demonstrate the applicability of an ultrahigh-speed, ultrahigh-resolution SOCT prototype based on a femtosecond laser to cross-sectional imaging of the lens capsule in vivo.

Methods

SOCT measurements were performed with the prototype instrument constructed at the Institute of Physics, Nicolaus Copernicus University, Torun, Poland. Details of the design of this SOCT instrument are given elsewhere.6 For the purpose of this study, two changes were introduced to the system design: (1) a femtosecond laser (Fusion BB300, FemtoLasers Produktions GmbH, Austria Δλ=160 nm, central wavelength 780 nm) was implemented, enabling imaging with an axial resolution of 2.3 μm; and (2) a new spectrometer with a high-speed 12-bit CMOS line-scan camera (Sprint, Basler) was designed and constructed. This custom-designed spectrometer with a volume holographic grating DG (1200 grooves/mm) and a CMOS camera enables an acquisition line rate as high as 100 kHz for the ultrahigh-resolution mode. In this work, we illuminated the eye with 800 μW of optical power, achieving 94 dB sensitivity at 20 μs exposure time (50 kHz acquisition rate). The acquisition time of a tomogram composed of 8000 A-scans was thus 0.16 s. To reconstruct the lens capsule in three dimensions, we collected 4000×50 axial scans (X, Y). Three-dimensional scanning protocols provided data to create lens capsule thickness maps. To obtain information on the lens capsule thickness, we used a manual segmentation procedure, which enables delineation of the capsule. The maximum in-depth range of imaging is 1.5 mm. The transverse resolution of the system is 10 μm.

Lens capsules of two healthy eyes of two volunteers, aged 24 and 33, were examined with the aid of the instrument with two- and three-dimensional scanning protocols. The study was approved by the Ethics Committee of the Collegium Medicum in Bydgoszcz, Nicolaus Copernicus University in Torun, Poland.

Results

Figure 1 shows an SOCT tomogram of an anterior part of the crystalline lens and the pupillary margin of the iris. The quality of the image is adequate for evaluating the capsular bag and estimating its thickness. For the first time, to our knowledge, it is possible to visualise the lens epithelium as a separate layer. In the centre, below the lens epithelium, a number of highly reflective dots are detected.

Figure 1

Ultrahigh-resolution spectral optical coherence tomography (SOCT) tomogram of an anterior part of the lens and the pupillary margin of the iris. The insert shows a magnification of the selected area.

Three-dimensional data were used to reconstruct an en-face image of the iris and lens surface (figure 2A). Figure 2B shows an anterior lens capsule thickness map of the 33-year-old volunteer. As expected, the capsular bag is thinner in the centre and becomes thicker towards the periphery. The thickness of the anterior lens capsule in the centre is 12–14 μm, corresponding to 11–13 pixels of OCT image, whereas 3 mm away from the centre it is 20–24 μm, corresponding to 19–23 pixels of OCT image. To calculate the geometrical thickness, we assumed the refractive index value of 1.36.

Figure 2

(A) En-face image of the iris and lens surface reconstructed from three-dimensional data. (B) Anterior lens capsule thickness map (μm). The area of examination is 7 mm × 7 mm.

Discussion

Commercially available technologies are not capable of providing information for precise cross-sectional evaluation of the capsular bag, estimation of its thickness or visualisation of the lens epithelium in vivo. Axial resolution of the images provided by ultrasound biomicroscopies (25 μm),12 Scheimpflug camera, 1310 nm Td OCT (18 μm)12 and 830 nm SOCT (4.5 μm) is not adequate for such an assessment.6 In the field of OCT instruments, further improvements in axial resolution have been reported.3 13 To date, the highest axial resolution of 1.4 μm has been achieved by Linnola et al for intraocular lens and capsular bag imaging in vitro.14 Nevertheless, the tomograms presented there exhibit an inferior quality in terms of the number of details that can be distinguished, compared with the tomograms in the present study, probably because they used the much slower technology of time-domain OCT. The advantage of our 780 nm SOCT prototype with a femtosecond laser is not only the axial resolution of 2.3 μm but also the scanning speed of 100 kHz. However, the imaging depth of 1 mm might be a major limitation for its clinical use.

The images from our SOCT prototype based on a femtosecond laser confirm that this instrument is capable of providing detailed cross-sectional images of the lens and its capsule (figure 1). It is possible to evaluate the morphology, shape and thickness of the capsular bag, as well as the lens epithelium. The tomogram correlates well with histological images, with the exception of highly reflective structures detected in the centre, below the lens epithelium. These signals may be produced by lens sutures or the lens fibres that lay perpendicular to the light beam in this region of the lens, causing mirror-like reflections.15

Three-dimensional data were used to create a map of the capsule thickness. In the case presented, the thickness of the anterior lens capsule in the centre was 12–14 μm, whereas 3 mm away from the centre it was 20–24 μm. Historical, histological studies performed by Salzmann and Fincham revealed that the capsule thickness of a 30- to 40-year-old lens is 12–15 μm at the anterior pole and 20–22 μm in the anterior midperiphery. Several other recent papers reported that the anterior capsule thickness is 8–12 μm at the centre and 14–20 μm at the midperiphery at this age.16 Thus, our results are in agreement with those previously published.

The ultrahigh-speed, ultrahigh-resolution SOCT developed here for lens capsule imaging shows great potential for scientific and clinical use. It may bring important information in the field of presbyopia and accommodation research, including assessment of new designs of pseudoaccommodative intraocular lenses. Examination of capsular bag morphology may be useful for evaluating post-traumatic eyes with suspected capsule damage. Assessment of the lens capsule and estimation of its thickness may also be used to evaluate capsule-related cataract-surgery risk factors. Another potential area of investigation is the lens epithelium, its role in posterior capsule opacification and assessment of the strategies to prevent this process. The technology may help to differentiate pesudoexfoliation syndrome (PEX) from real exfoliation of the capsule. Moreover, it may be a valuable tool for early detection of PEX changes in vivo. There is evidence that symptomatic PEX is preceded by subclinical ultrastructural alterations in the anterior segment including the lens surface.17 18 Early detection of PEX has a special clinical significance, since PEX syndrome is the most common identifiable cause of open-angle glaucoma and is also an important risk factor for a number of ocular complications, especially during cataract surgery.

The SOCT prototype developed in this work for lens capsule imaging has a few limitations. First, due to the limited penetration of the 780 nm light into the eye, certain parts of the lens behind the iris cannot be visualised. Second, as the maximum depth range of imaging is restricted to 1.5 mm, it is not possible to examine the entire lens visible within the pupil.

In conclusion, we have demonstrated that an ultrahigh-resolution, ultrahigh-speed SOCT instrument based on a femtosecond laser allows two- and three-dimensional evaluation of the anterior capsular bag and lens epithelium.

Acknowledgments

We would like to acknowledge A Stingl, from FEMTOLASERS Produktions GmbH, for his kind support.

References

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Footnotes

  • Funding This work was supported by a European Young Investigator Award funded by European Heads of Research Councils and the European Science Foundation, Polish Ministry of Science grants for the years 2008–2012, the Foundation for Polish Science (projects Ventures and Homing).

  • Competing interests AK is a part-time employee of Optopol SA.

  • Ethics approval Ethics approval was provided by the Ethics Committee of the Collegium Medicum in Bydgoszcz, Nicolaus Copernicus University in Torun, Poland.

  • Patient consent Obtained.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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