Date Published: March 28, 2019
Publisher: Public Library of Science
Author(s): Jan Philip Kolb, Wolfgang Draxinger, Julian Klee, Tom Pfeiffer, Matthias Eibl, Thomas Klein, Wolfgang Wieser, Robert Huber, Bang V. Bui.
Surgical microscopes are vital tools for ophthalmic surgeons. The recent development of an integrated OCT system for the first time allows to look at tissue features below the surface. Hence, these systems can drastically improve the quality and reduce the risk of surgical interventions. However, current commercial OCT-enhanced ophthalmic surgical microscopes provide only one additional cross sectional view to the standard microscope image and feature a low update rate. To present volumetric data at a high update rate, much faster OCT systems than the ones applied in today’s surgical microscopes need to be developed. We demonstrate live volumetric retinal OCT imaging, which may provide a sufficiently large volume size (330x330x595 Voxel) and high update frequency (24.2 Hz) such that the surgeon may even purely rely on the OCT for certain surgical maneuvers. It represents a major technological step towards the possible application of OCT-only surgical microscopes in the future which would be much more compact thus enabling many additional minimal invasive applications. We show that multi-MHz A-scan rates are essential for such a device. Additionally, advanced phase-based OCT techniques require 3D OCT volumes to be detected with a stable optical phase. These techniques can provide additional functional information of the retina. Up to now, classical OCT was to slow for this, so our system can pave the way to holographic OCT with a traditional confocal flying spot approach. For the first time, we present point scanning volumetric OCT imaging of the posterior eye with up to 191.2 Hz volume rate. We show that this volume rate is high enough to enable a sufficiently stable optical phase to a level, where remaining phase errors can be corrected. Applying advanced post processing concepts for numerical refocusing or computational adaptive optics should be possible in future with such a system.
Optical Coherence Tomography (OCT)  is a non-invasive imaging modality, which uses usually near-infrared light to create three dimensional images with μm-scale resolution. Typically, it features 1–2 mm penetration depth for scattering tissue. Also, it is perfectly suited for retinal imaging [2, 3], as the lens and the vitreous body of the human eye consist mostly of water, nearly transparent to some parts of the near-infrared spectrum . Here, OCT’s unique capability of creating detailed cross-sectional views is very valuable for diagnosis of retinal diseases . While ophthalmic imaging is still the main application for OCT, it also has found many other applications, for example intravascular imaging , dermatology  or even non-destructive testing of materials .
For the investigation of the live 4D OCT with focus on surgical guidance, we use a slightly different setup than for the non-live OCT imaging focusing on potential applications regarding phase stable imaging. An overview of the two configurations can be found in Table 1. In this chapter, we describe in detail our setups by starting with the FDML laser itself. Next we will present our interferometer and the scanning optics, then we will discuss our data acquisition and processing and finally we will describe our synchronization strategies for both setups.
This section presents the imaging results. First, we performed non-live imaging with focus on the phase stable application, then live imaging was used to evaluate a possible use in surgery. In each subsection, results are presented and discussed focusing on possible challenges. All in vivo retinal imaging experiments were performed in accordance to the tenets of the Declaration of Helsinki. The ethics committee of the University of Lubeck approved the experiments. Verbal informed consent was obtained from the volunteers prior to the measurements. This was in January 2015 for the non-live imaging and in September 2017 for the live imaging. Data of four volunteers (all members of the group) in total was recorded, but not all are presented here for redundancy reasons.
This paper investigated the present and possible challenges related to non-live and live 4D-OCT imaging of the posterior eye with a MHz-FDML laser. We gave detailed information on our setup including the laser source, interferometer, scanning protocols, data acquisition and the necessary synchronization. Videos for both cases were presented and the individual challenges discussed.