In this study, we report the successful integration of an SDOCT system with the operating microscope head. Imaging of both human and the porcine retinas was feasible. In addition, multiple surgical instruments were imaged with MMOCT while a surgeon performed intraoperative maneuvers. In our review of the literature, we did not find any published reports regarding MMSDOCT systems, the OCT characteristics of instrumentation, or OCT-based visualization of intraocular maneuvers. The prototype MMOCT device adds a 4-in. vertical increase to the axial distance from the surgical oculars and the surgical field. The extended-length oculars of the scope can be lowered to account for most of this height. For comfort, further modifications that lower the oculars may be useful for some surgeons using this system. Because the device is located above the objective lens and the wide-field viewing system of the microscope, it does not affect focusing. There were no difficulties related to the physical presence of the MMOCT device in performing the maneuvers or in focusing during the maneuvers.
Vitreoretinal surgery has made remarkable progress over the past few decades. Advances in surgical instrumentation, illumination, and vital dyes have all transformed the surgical landscape of the retina.3,5,14,15
The intraoperative integration of SDOCT may be the next major step in vitreoretinal surgery. Early research suggests that intraoperative OCT may yield critical information regarding disease states, impact of surgical maneuvers, and intraoperative anatomic configurations.
Intraoperative OCT has been used to examine multiple vitreoretinal diseases including optic pit-related maculopathy, ROP, ERM, macular holes, and retinal detachment. It provides the opportunity to achieve a greater understanding of the pathophysiology of vitreoretinal surgical pathology. For example, intraoperative analysis of pars plana vitrectomy for optic pit maculopathy has suggested a connection between the vitreous cavity and the macular retinoschisis.10
In addition, intraoperative OCT in ROP has revealed preretinal structures and retinoschisis that has not been identified clinically.7
In addition to providing information regarding pathophysiology, intraoperative OCT allows a surgeon to visualize the impact of a surgical maneuver on the tissues of interest. In macular holes, intraoperative OCT has demonstrated changes in hole configuration, successful removal of the internal limiting membrane, and visualization of retinal distortion and architecture changes related to surgical maneuvers (Ray JA, et al. IOVS
2010;51:ARVO E-Abstract 2962; Binder S, et al. IOVS
2010;51:ARVO E-abstract 268).8
After ERM removal, intraoperative imaging has revealed rapid improvement in retinal architecture and subclinical neurosensory retinal detachment in some cases (Baranano AE, et al. IOVS
2010;51:ARVO E-Abstract 269).8
Persistent subretinal fluid following perfluorocarbon liquid tamponade has been identified by intraoperative SDOCT in cases of macula-involving retinal detachment (Lee LB, et al. IOVS
2010;51:ARVO E-Abstract 6076). In addition to information on the maneuvers performed, intraoperative OCT may provide feedback that helps guide additional surgical maneuvers. One example is residual membranes that may not be appreciated clinically but could then be removed intraoperatively with OCT confirmation of removal (Baranano AE, et al. IOVS
2010;51:ARVO E-Abstract 269).9
Currently, there are no commercially available MMOCT units. A microscope-mounted prototype using the Cirrus (Carl Zeiss Meditec, Oberkochen) system has been presented, but to our knowledge, no reports have been published (Binder S, et al. IOVS 2010;51:ARVO E-abstract 268). Because of the lack of MMOCT systems, visualizing instrumentation and direct tissue manipulations with intraoperative OCT has not been possible. Using the MMOCT system, we described the wide-ranging OCT characteristics of intraocular instruments. Metallic instruments exhibited very high reflectivity with total shadowing. These characteristics make metallic instruments less than ideal for intraoperative manipulations while using OCT. Visualization of the underlying tissue and interfaces between instrumentation and tissues was limited. On the other hand, nonmetallic instruments (e.g., silicone, polyamide) provided improved visualization of the underlying tissues and the interactions between the instrument and the retinal surface. As OCT becomes more integrated into the operating room, new instrumentation with improved reflectivity profiles may have to be designed. For example, the current metallic forceps would limit OCT visualization of real-time membrane peeling due to severe shadowing. On the other hand, polyamide or PMMA-tipped forceps might allow real-time OCT visualization of intraocular maneuvers and instrument–tissue interactions.
An alternative to changing instrument materials to improve visualization may include real-time image processing of the intraoperative scans. Since the position of the surgical instrument would not necessarily remain constant over the several milliseconds required between sequential depth cross-sectional acquisitions, multiple spatially correlated B-scans may be co-registered and either stitched, mosaicked, or averaged to fill in any tissue regions obscured by instrument shadowing (Estrada R, et al. IOVS 2010;51:ARVO E-abstract 5928). These image-processing techniques take advantage of the fact that any motion of the instrument by the surgeon would be much larger than the lateral thickness of each depth cross-section; thus, shadowed regions of the tissue are decorrelated and can therefore be filled in rapidly by image processing. While potentially useful for removing shadowing artifacts, these techniques require real-time, high-resolution tracking and registration algorithms that may increase the computational cost of current data acquisition and visualization software.
A major limitation in intraoperative scanning of procedures is the efficient targeting of the OCT scan to the area of interest. The cross-sectional nature of B-scan OCT images necessitates precise targeting of the OCT device. Because of the dynamic nature of intraocular maneuvers, rapid targeting of the OCT scan is critical for the device to be fully integrated into surgical decision-making. There are multiple approaches to consider in facilitating scan targeting. One possibility is direct instrument modifications. One published report of instrument modifications includes direct integration of the OCT imaging fiber into the device.16
Although this approach would provide direct localization of the OCT scan, it may limit image quality due to instrument size and may limit the surgeon to using intraoperative OCT with only those instruments that have the built-in system. An alternative would be to create a dynamic aiming system with the tip of the instrument serving as a tracer for the OCT laser. This method would allow for rapid imaging at the tip of the instrument and for use of an MMOCT system. Finally, custom real-time image analysis may also help in guiding the SDOCT scanner to the area of interest. Various frameworks and algorithms have been described for tracking position and changes in scene during image processing.17–19
These processing algorithms may be used to help guide the location of the scanning beam to the location of the instrument and intraoperative maneuvers.
Current display systems for OCT imaging are also a limiting factor for its widespread use in the operating room. Currently, images are displayed on a monitor, causing the surgeon to look away from the microscope. When imaging intraoperatively after a surgical maneuver, this limitation is not problematic. However, for real-time visualization of surgical maneuvers, having the surgeon look away from the microscope may risk patient safety. This problem limits the feedback that the surgeon can receive from the real-time scan. Heads-up display systems have widespread use in automobiles and aviation. These displays reduce risk to the driver or pilot by allowing them to maintain a field of view in the environment while providing feedback of information, such as speed or altitude. In addition, in neurosurgery the use of heads-up displays has been adapted to surgical microscopes for integration of imaging information with real-time stereotactic surgery.20
A similar system could be used in the vitreoretinal operating room to allow for a heads-up display that enables simultaneous viewing of the real-time OCT images and the operative field. Such a display would provide the surgeon with rapid feedback of intraocular maneuvers through both OCT information and direct visualization. A critical component of this system would be the presentation of critical intraoperative data rather than the entire data set. Using a subset of the information that is critical for maneuvers would help to avoid information overload for the surgeon and reduce the potential distraction that too much information might provide.
Future applications of integrated MMOCT could include both quantitative and qualitative depth and tissue proximity information to the surgeon. The cross-sectional nature of the B-scan would allow for a qualitative display for the surgeon to view, to determine the proximity of the instrumentation to the tissue of interest. In addition, the use of automated segmentation algorithms may allow for real-time quantitative measurements of proximity of the surgical instrument to the retinal layers of interest.21
This real-time information could be integrated into the surgical procedure in various ways, including audio-based proximity alarms and measurement guide displays.
In this study, we report the successful integration of MMOCT into the surgical microscope. The platform was able to be used to image human volunteers as well as multiple instruments and surgical maneuvers in porcine eyes. The feedback from the system allowed visualization of instrument–tissue interactions during surgical maneuvers. The development of MMOCT is a major step toward integration of OCT in the vitreoretinal surgical environment. Further study of instrumentation design, OCT scanning protocols, means of capturing intraoperative motion, instrument tracking, display technology, image processing, and real-time intraoperative imaging in human subjects is needed.