Four-dimensional visualization with optical coherence tomography
The widespread development of improved imaging modalities dedicated to visualizing the anterior segment of the eye has regenerated interest in ophthalmic studies.1–3 Among methods such as slit-scanning topography, Scheimpflug imaging, and high-frequency ultrasonography, optical coherence tomography (OCT) has generated particular attention.
OCT emerged from investigations into low-coherence interferometry and optical reflectometry.4 As an echo technique, it can be thought of as similar to ultrasound. However, as OCT is based on light, it can deliver much higher resolution, similar to low-power microscopy. As an interferometric technique it can penetrate significantly deeper into tissue than microscopy. And by detecting only coherent light, the glare resulting from beam scattering is filtered out, resulting in clear images of subsurface features.
Applying Fourier-domain detection methods to OCT increases both its sensitivity and acquisition speed. Recent technological advances enable us to obtain two- and three-dimensional images with submicrometer resolution. Consequently, OCT has become a well-established noninvasive and noncontact approach used primarily in ophthalmology.5 From a clinical point of view, understanding the mechanisms of the eye's optical system is essential, and OCT offers new opportunities to investigate these complex processes.
We designed and constructed a spectral OCT instrument dedicated to anterior-segment imaging.6 The device's mechanical system is based on the SOCT Copernicus instrument (Optopol Technology S.A., Poland). High flexibility was achieved by employing a high-speed CMOS-based line-scan detector in the spectrometer (SPRINT, Germany). Our main goal was to design a high-speed spectral OCT apparatus capable of imaging the entire anterior segment of the eye with micrometer resolution. The resulting instrument can acquire 3D data within a 5mm axial range with a speed of up to 130k optical ‘A scans’ (1D depth measurements) per second and an axial resolution of 6.9μm. High-quality in vivo images reveal details of the corneal-lens morphology (see Figure 1).
The system's acquisition speed can be effectively used to image dynamic processes. Videos generated with the apparatus clearly show processes such as lenticular accommodation (see Figure 2 and video7). A fixation point was moved from nearby to infinity, inducing an accommodative response in the measured eye. A change in the curvature of the anterior surface of the lens is clearly visible.
In addition, we can acquire time-varying volumetric data (4D imaging) with a speed of approximately five volumes/s. We imaged blinking and pupillary reactions to light stimulus to test the feasibility of our spectral OCT system (see Figure 3 and videos8,9). Three-dimensional volumetric reconstruction yields outstanding views of the real-time dynamics of the human eye.
Ongoing studies will focus on extracting quantitative information from the OCT images. The biometric data and its dynamics during lenticular accommodation, for example, will enable us to assess the biomechanics of the eye.
The spectral OCT system was designed in collaboration with the team of Susana Marcos (Visual Optics and Biophotonics Laboratory, Instituto de Óptica ‘Daza de Valdés,’ Consejo Superior de Investigaciones Científicas, Madrid, Spain). This research is supported by EuroHORCs (European association of the heads of research-funding organizations)/European Science Foundation European Young Investigator (EURYI) award EURYI-01/2008-PL.
Maciej Wojtkowski received his MSc and PhD in physics from NCU. He is a research assistant professor and leads an independent junior research group. His interests include OCT and low-coherence interferometry applied to biomedical imaging. He is active in the field of OCT and its ophthalmic applications.
Ireneusz Grulkowski received his MSc in biomedical physics and PhD in experimental physics from the University of Gdansk, Poland. He also graduated from the Intercollegiate Faculty of Biotechnology of the University of Gdansk and the Medical University of Gdansk. He is an assistant professor in the Institute of Experimental Physics, University of Gdansk, and a postdoc at NCU.
Michalina Góra graduated in physics from NCU. At present, she is a PhD student and a Foundation of Polish Science scholar. As part of the ‘Ventures’ project she conducts research on sweep-source OCT and its biomedical applications. She is also involved in the implementation of OCT for structural imaging of artwork.
