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Since the advent of spectral-domain optical coherence tomography (SD-OCT), ophthalmologists have been able to visualize the eye and its disorders in amazing three-dimensional images generated at a speed and resolution not possible with time-domain OCT. The SD-OCT images create a virtual biopsy of the eye, providing details of pathology that help ophthalmologists—particularly retina specialists—improve diagnosis, monitoring and treatment of eye disease.
Even this welcome development has had a limit, however: Because of positioning and cooperation requirements, tabletop SD-OCT systems are not useful for capturing the eyes of three sorts of patients: squirmy kids, flailing babies and prone patients undergoing surgery. But the creation of a portable OCT probe has now made it possible to image neonates and young children without using anesthesia, and perhaps even enhance intraoperative visualization to the extent that surgical outcomes are improved.
Pediatrics: Wee, Wide-Awake Eyes
Handheld SD-OCT systems could obviously make imaging much easier in pediatric patients.
The device. The only portable SD-OCT system available on a commercial basis so far was developed by Bioptigen and costs around $100,000. Other systems are under development at other companies and at research institutions. Bioptigen’s system employs a handheld probe attached with a four-foot-long, fiber-optic cable to a wheeled cart that carries a spectrometer and computerized SD-OCT system. The combination of SD-OCT’s fast acquisition time, the movable cart and the portable scanning head makes imaging without anesthesia possible for rambunctious toddlers or uncomprehending infants, though some physicians also rely on a pacifier dipped in a sucrose solution to keep infants as still as possible.
The possibilities. With portable SD-OCT, researchers and pediatric eye surgeons hope to improve monitoring of pathologies such as retinopathy of prematurity, which often benefit from early intervention. Cynthia Toth, MD, was one of the first to prove that an SD-OCT unit with a portable probe could successfully image both sedated and nonsedated infants in a neonatal nursery. Dr. Toth is a professor of ophthalmology and biomedical engineering at Duke University. A paper published last year in Ophthalmology described how Dr. Toth and colleagues used a handheld SD-OCT system to image premature neonates with ROP without anesthesia at the bedside, or with anesthesia in the operating room.1 The portable SD-OCT helped identify subclinical pathology, including preretinal structures, retinoschisis and retinal detachment in premature, low birth-weight infants with ROP. Such images may aid early decision-making about when to intervene with surgery for ROP, the authors said.
In another study published this year in Investigative Ophthalmology and Visual Science, Dr. Toth and fellow researchers were able to define optical parameters for SD-OCT in children, and demonstrated that they could image most eyes in 42 children ranging in age from 31 weeks to 1.5 years. The optic nerve, fovea and posterior pole of the infants and children were imaged successfully in 74 percent of infants in an intensive care nursery and 87 percent of children in a pediatric clinic.2
At the Hospital for Sick Children in Toronto, imaging specialists and ophthalmologists have been using the handheld Bioptigen SD-OCT system for over a year to image children with retinoblastoma or glaucoma, as well as infants who have suffered retinal nerve fiber layer damage from the use of vigabatrin (Sabril) to treat infantile spasms. “When the handheld SD-OCT became available, we could image an entire population that could never have OCT before,” said imaging specialist Cynthia VandenHoven, BAA, CRA. “So any patient who couldn’t cooperate or couldn’t be imaged with a traditional SD-OCT tabletop system in the clinical setting—including newborns and children with developmental delays—could now have OCT.” The use of SD-OCT during assessment for retinoblastoma, to look for new or previously treated tumors, is particularly useful, Ms. VandenHoven said.
Extra benefits. If this imaging system continues to prove its worth, the considerable risks associated with using anesthesia in children might be decreased, said Dr. Toth. “We showed not only that we can image babies in a nursery, but that it didn’t have any negative side effects,” she said. “The vast majority of the time we were able to get a reasonable image of the macula in both eyes,” she said. Dr. Toth is currently working on research that will compare images taken with a portable-probe SD-OCT system to clinical findings in children with retinal pathology.
Imaging Eyes in Surgery
Portable-probe SD-OCT has also made it possible to visualize the effects of surgical interventions on the eye.
The possibilities. One example of intraoperative use of the handheld Bioptigen system is to image an eye immediately after epiretinal membranes are removed; the intraoperative images could obviously help a surgeon determine whether more membranes need removing, said Justis P. Ehlers, MD, staff physician in the vitreoretinal service at the Cleveland Clinic. Or in surgery for vitreomacular traction syndrome, an SD-OCT image might confirm whether all traction has been relieved and whether changes have occurred in the area following surgery, he said.
The ability to use SD-OCT intraoperatively has also provided a better understanding of the pathophysiology of various disease processes, such as optic pit-related maculopathy, and the direct impact of surgical interventions on the condition.3 “Using intraoperative OCT, we were able to identify the collapse of macular retinoschisis following air-fluid exchange with aspiration over the optic pit, suggesting a connection between the vitreous cavity and the intraretinal fluid,” Dr. Ehlers said. “On SD-OCT the following day, the retinal architecture had reorganized and the features of the area of collapse had changed. Without intraoperative SD-OCT, we would not have been able to identify the changes noted immediately following aspiration.” Dr. Ehlers has done OCT research using both the Bioptigen system and a prototype microscope-mounted system while working with Dr. Toth at Duke.
Risks and errors down, success up? Sunil Srivastava, MD, also on staff at the Cleveland Clinic, has used a portable SD-OCT system during surgery for about two years. He described several cases of surgery for epiretinal membrane in which intraoperative SD-OCT helped him know whether membranes remained or whether the tissue he saw in the microscope was instead a retinal fold. Although stains help a surgeon see tissue accurately during surgery, SD-OCT provides a three-dimensional image as well as identifies changes in anatomy that occur underneath membranes, out of range of the microscope, he said.
Additionally, intraoperative SD-OCT may allow a surgeon to better identify iatrogenic changes caused by the surgery and then adjust course appropriately, Dr. Ehlers said. For instance, intraoperative imaging might allow a surgeon to identify whether a cyst was unroofed or if a full-thickness hole was created while peeling an epiretinal membrane over a cystic fovea, and that will determine the need for a gas or air bubble.
Dr. Srivastava is also using intraoperative SD-OCT to understand microscopic changes that occur in the eye during a surgical procedure. “Certain manipulations, even fine manipulations of the epiretinal membranes, for instance, actually cause small retinal detachments,” he said. “Most of the time these resolve over a period of a few days or a few weeks, but we’re not sure what the clinical implications are. Studying these changes with SD-OCT will help us to understand if these changes make a difference in patient outcomes.”
Don’t forget the front of the eye. While intraoperative SD-OCT is currently being used just for retinal surgery, there is the possibility that it could also be used during anterior segment surgery, according to Anthony N. Kuo, MD, assistant professor of ophthalmology in cornea and refractive surgery at Duke. “SD-OCT could potentially give you important depth information in procedures such as DALK,” he said. In these surgeries, in which the cornea has to be dissected by almost 99 percent thickness, visual guidance from SD-OCT would be very helpful, he said.
Drawbacks of the Current System
The case for using a portable probe is easy to make for examining live-wire kids. But for intraoperative situations—in both adult and pediatric patients—there are at least three major challenges in using currently available portable systems:
- Experience may be needed. Surgeons usually need to acquire experience with the probe before they can accurately and quickly center it on the area of interest in the eye.
- Moving pictures a problem. When using a handheld probe, motion artifact can occur as the imager attempts to stabilize the probe over the pupil. During image acquisition, portable SD-OCT probes are typically stabilized by using a wrist rest raised to the level of the patient’s forehead or by resting the probe on several fingers placed on the forehead. In a pilot study at Duke, a team of surgeons was able to image eight patients during surgery and, with experience, each image took from four to five minutes.4 Under the direction of Dr. Toth, researchers at Duke are trying to improve on that expenditure of time by working on an SD-OCT system wrapped around a surgical microscope. Then the SD-OCT could be operated simultaneously with the microscope, and with the same controls.
Dr. Kuo has been working with a portable SD-OCT prototype to increase the actual speed of image acquisition. The advantage for anterior segment ophthalmologists would be the ability to more easily and more accurately capture information regarding corneal power and curvature, he said. (Any improvement in speed could, of course, apply to both tabletop and portable SD-OCT systems.)
For his part, Dr. Srivastava steadies the probe over the eye by attaching it to a surgical microscope mount he helped design with an engineer. Mounting the probe takes a few minutes but then he can use the microscope controls to quickly focus the probe on the same retinal landmarks that he sees in the operating microscope.
- Surgery grinds to a halt. Even with his mount, however, Dr. Srivastava must stop the surgery, mount the probe, assess the image and then take the probe off the mount to continue surgery. This raises the third and biggest challenge of intraoperative OCT: Even with the handheld probe, the surgeon must momentarily halt the surgery. Real-time images during surgery simply aren’t yet possible. In fact, taking an SD-OCT actually adds minutes to operating time.
To fix this, researchers are working on systems in which SD-OCT would be seamlessly built into operating microscopes, to offer real-time imaging from scalpel to sutures. But that sort of progress could pose yet more problems, Dr. Toth said. “Not only do you have to get the SD-OCT image, but you have to provide it to the surgeon in a way that doesn’t create information overload. If you are watching the eye through a microscope during surgery, pushing on foot pedals, and also adding a screen where one could see an OCT image, this might become a distraction from surgery. It’s going to be some time before we figure these problems out, and that’s why intraoperative SD-OCT is still in the early stages.”
One Advance at a Time
“The systems now available do not allow for true real-time imaging during surgery, but they have been a huge step forward and we are learning a lot from them,” Dr. Ehlers said. The attraction remains, for example, that the surgeon can examine the immediate impact of an intervention. “It does give you feedback during surgery after you’ve performed a maneuver—if you are peeling membranes, it can help the surgeon identify if the removal has been sufficient in the area of interest,” Dr. Ehlers said.
“But the eventual goal with intraoperative SD-OCT is to provide real-time feedback during surgery, and a technology that is seamlessly built into the microscope so that you can continue to operate while safely viewing the OCT images as needed.”
1 Chavala, S. H. et al. Ophthalmology 2009;116:2448–2456.
2 Maldonado, R. S. et al. Invest Ophthalmol Vis Sci 2010;51(5):2678–2685.
3 Ehlers, J. P. et al. Arch Ophthalmol Submitted for publication.
4 Dayani, P. N. et al. Retina 2009;29:1457–1468.
Dr. Ehlers, Dr. Srivastava and Ms. VandenHoven have no relevant financial disclosures. Dr. Kuo has intellectual property licensed by Duke to Bioptigen. Dr. Toth receives royalties for intellectual property licensed by Duke to Alcon, and has the potential for the same royalties with Bioptigen, and receives research support from Alcon, Genentech and Bioptigen.