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Ophthalmology Management

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Update on intra-operative OCT

Microscope integration makes real-time visualization of the tissue and tissue-instrument interactions possible.

Ophthalmology Management August 1, 2017

Noncontact methods of crossectional bioimaging with optical coherence tomography (OCT) have become an integral part of the evaluation, management and monitoring of a wide range of retinal pathologies.1 For example, spectral-domain OCT (SD-OCT) has provided improvement in resolution and speed of acquisition, which allows for more detailed visualization of vitreoretinal pathology.2

The role of OCT in our space seems unceasing in its expansion. In addition to its clinical management role, OCT imaging now has two more – in pre-operative surgical planning and postoperative evaluation, especially with regard to epiretinal membranes, macular holes, rhegmatogenous and tractional retinal detachments.3-8

OCT continues evolving and improving, becoming ever more helpful intraoperatively. Here’s a look at where the technology stands now.

THE ROAD TO INTRA-OPERATIVE OCT (iOCT)

The conventional table-top OCT unit has always required upright patient positioning and cooperation, so it cannot be used on supine patients in the operative suite.

That changed in 2007 when a portable, handheld SD-OCT scanner (Bioptigen, Inc.) came on the market, allowing imaging of supine patients. It is primarily used in operative suites during exams of anesthetized pediatric patients who have various conditions such as retinopathy of prematurity, albinism and shaken baby syndrome.9-12 Another commonly used handheld system is the iVue (Optovue).13-14

Intraoperative handheld OCT has allowed immediate evaluation of retinal contour, membranes and macular holes prior to and after surgical intervention. Multiple studies have reported feasibility of handheld iOCT to alter surgical decision-making, for example in cases requiring additional membrane peeling to achieve macular hole closure.15,16 However, this device configuration requires pausing of surgical procedures to acquire images, and that portends a higher risk of contaminating the surgical field.

The need to decrease image capture time, ensure sterility and improve reproducibility led to development of microscope-mounted iOCT devices.17-19 These devices allowed for easier alignment of the system, but real-time visualization of the tissue and tissue-instrument interactions were not possible until development of microscope-integrated iOCT (MiOCT) devices.19-21 The optical path of the MiOCT system incorporates into the common optical pathway of the surgical microscope, which allows improved targeting and tracking of the scan beam and achieves parfocal and coaxial OCT imaging with the surgical view.

MiOCT OPTIONS AND BENEFITS

In the first publication of MiOCT use in vitreoretinal surgery, Cynthia Toth, MD, at Duke University, and co-investigators described a custom prototype system: a research OCT integrated with a commercially available operating microscope.22 Other early prototypes include Cirrus SD-OCT (Carl Zeiss Meditec) using the Zeiss OPMI VISU 200 surgical microscope,18 and EnFocus (Leica Microsystems/Bioptigen) iOCT.23 Since 2010 we have seen significant advances in software and hardware of MiOCT systems. The FDA has approved three systems: EnFocus, used with a Leica surgical microscope,23 Haag-Streit iOCT, which is integrated through a microscope side-port,24 and RESCAN 700 (Zeiss), which is built on the Lumera 700 microscope platform.25-27

Figure 1. MiOCT (EnFocus) demonstrating subretinal fluid in the beginning of retinal detachment repair surgery (A). Confirmation of retinal re-attachment at the end of repair (B)
Figure 1. MiOCT (EnFocus) demonstrating subretinal fluid in the beginning of retinal detachment repair surgery (A). Confirmation of retinal re-attachment at the end of repair (B)
Figure 2. MiOCT (EnFocus) evaluation of tractional membranes in the beginning of tractional retinal detachment repair surgery (A). Real-time feedback during membrane peeling (B, C)
Figure 2. MiOCT (EnFocus) evaluation of tractional membranes in the beginning of tractional retinal detachment repair surgery (A). Real-time feedback during membrane peeling (B, C)
Figure 3. MiOCT (EnFocus) evaluation of epiretinal membrane in the beginning of surgery (A). Real-time feedback during membrane peeling (B)
Figure 3. MiOCT (EnFocus) evaluation of epiretinal membrane in the beginning of surgery (A). Real-time feedback during membrane peeling (B)

Recent studies examined MiOCT’s use for different conditions, such as epiretinal membrane, vitreomacular traction, macular hole and retinal detachment.28-31 During epiretinal membrane peels, MiOCT allows visualization of the distance growing between the RPE and the ellipsoid zone and the focal areas of retinal elevation at the peel initiation sites.31 However, it remains to be determined whether these intraoperative changes lead to significant functional changes. In macular hole surgery, anatomical success was associated with configuration of macular hole and changes in macular hole geometry seen with MiOCT.30 In vitreomacular traction repair procedures, MiOCT has allowed for assessment of the strength of vitreomacular adhesions and visualization of unroofed cysts, subclinical full-thickness macular hole development and incomplete peeling of membranes. Intraoperative identification of these subclinical changes may change the surgical approach, such as opting for a gas tamponade and potentially preventing the need for future reoperations.29 In retinal detachment surgery, MiOCT allows detection of residual subretinal fluid and can assist in completion of fluid air exchange. Real-time visualization of the planes may also help achieve more precise delamination and segmentation in tractional retinal detachment surgeries.

IT’S FEASIBLE

Many early studies of iOCT were small retrospective case reports. The PIONEER study was the first large prospective study to evaluate feasibility and utility of iOCT, specifically microscope-mounted SD-OCT (Bioptigen).32 Surgeons in this study reported that their approach in retinal membrane peeling procedures was changed by iOCT in 8% of cases.32

It showed that iOCT was feasible in 92% of cases. Per surgeon reports, it was most useful for membrane peeling procedures, providing valuable information, such as presence of residual membrane, in 65% of cases and leading to a change in surgeon’s decision-making in 35% of cases.23 The RESCAN 700 portion of the DISCOVER study similarly demonstrated the feasibility of real-time iOCT in ophthalmic surgery.27

In the RESCAN 700 system, the real-time OCT images are projected on a heads-up display, which allows the surgeon to manipulate scan length, location and angle through video monitor display or the foot pedal control. This study showed that imaging was successfully obtained in 99% of cases and provided new and different information to the surgeon in 19% of cases.27

STILL SOME KINKS TO WORK OUT

Current MiOCT systems have some limitations that require further software and hardware changes. Because there is a lack of OCT-compatible surgical instrumentation, most instruments lead to light scattering and shadowing, limiting to some degree the real-time visualization of instrument-retina areas of contact. The amount of shadowing varies, depending on instrument thickness, material and relative orientation to the optical axis of the OCT. Difficulties with visualizing the scan directly below the area of contact often leave tissue anatomy changes as a more valuable immediate feedback source.28 Development of instruments that minimize scatter and shadowing will allow for more precise tissue manipulation. Software algorithms may assist in software-based processing of the image to minimize shadowing as well as localize the beam to the area of interest. Spatial compounding of B-scans can enhance visualization of the tissue past the instrument.19

In systems with a heads-up display, surgical oculars limit the size of OCT images and the visual field. On the other hand, an external monitor with OCT images requires the surgeon to look away from the surgical field. Additionally, MiOCT systems achieve optimal scans of the macula and posterior pole, with deterioration of image while scanning peripheral retina. There are reports of intraocular probe-type OCTs that may be used to scan target tissue anywhere in the eye.33-36 However, these reports are limited to animal studies or pilot clinical cases.

A very recent development in MiOCT technology is real-time volumetric imaging that can achieve four-dimensional visual feedback of volumes through time (4D MiOCT).37 Instead of real-time iOCT imaging being limited by live B-scans, which prevents visualization of continuous and complete instrument motion, 4D MiOCT acquires, processes and renders volumes in real time. This allows for enhanced visualization of tissue deformation and decreases the need for constant tracking of the moving object. 4D MiOCT, developed at Duke University is not commercially available.

ADDING VALUE

Multiple studies demonstrate the feasibility and value of MiOCT in vitreoretinal surgery. As MiOCT systems offer the surgeon immediate image-guidance, they may improve the surgeon’s judgment, knowledge and decision-making. This technology will provide better understanding of effects of surgical manipulation on the tissues and possibly allow us to explain and predict variations in postoperative visual outcomes. Despite the changes in surgical decision-making reported in MiOCT studies, randomized control trials are needed to evaluate the effect of those changes on long-term patient outcomes. Furthermore, additional studies are needed to evaluate which surgical procedures would benefit the most from iOCT use. In addition to routine vitreoretinal surgical procedures, MiOCT may be beneficial in the future in regenerative and gene therapy, allowing for precise delivery of a therapeutic agent.

Improvement in OCT aiming and tracking, optimization of heads-up display or an external monitor to maximize feedback while minimizing distractions, and development of OCT-compatible instruments will all promote further integration of MiOCT and add value during surgical procedures. OM

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About the Author

Liliya Golas, MD, is in the Department of Ophthalmology and Visual Science, Vitreoretinal Service, The University of Chicago, Chicago, Ill.

Liliya Golas, MD, is in the Department of Ophthalmology and Visual Science, Vitreoretinal Service, The University of Chicago, Chicago, Ill.

Dimitra Skondra MD, PhD, is assistant professor of Ophthalmology and Visual Science director, J. Terry Ernest Ocular Imaging Center, Vitreoretinal Service, University of Chicago.

Dimitra Skondra MD, PhD, is assistant professor of Ophthalmology and Visual Science director, J. Terry Ernest Ocular Imaging Center, Vitreoretinal Service, University of Chicago.

Seenu Hariprasad, MD, is director of Clinical Research and Chief of the Vitreoretinal Service of University of Chicago, and professor of Ophthalmology & Visual Science. He is a consultant to Alcon, Alimera Sciences, Allergan, Bayer, Clearside Biomedical, Novartis/Sandoz, OD-OS and Regeneron. Contact him at: retina@uchicago.edu.

Seenu Hariprasad, MD, is director of Clinical Research and Chief of the Vitreoretinal Service of University of Chicago, and professor of Ophthalmology & Visual Science. He is a consultant to Alcon, Alimera Sciences, Allergan, Bayer, Clearside Biomedical, Novartis/Sandoz, OD-OS and Regeneron. Contact him at: retina@uchicago.edu.

Raj N. Patel, MD, MS, completed a vitreoretinal surgery fellowship at the University of Chicago before joining Carolina Ophthalmology, PA

Raj N. Patel, MD, MS, completed a vitreoretinal surgery fellowship at the University of Chicago before joining Carolina Ophthalmology, PA

Disclosure: Dr. Skondra has no financial disclosures.

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