Intraoperative OCT: An Update on Current Technology

An evaluation of three devices surgeons can use during procedures

S. K. Steven Houston, III, MD • Audina M. Berrocal, MD • Timothy G. Murray, MD, MBA

Optical coherence tomography, as a noninvasive imaging technique for “virtual biopsy,” uses laser light, rather than sound waves, to obtain cross-sectional images of the retina. In this way, OCT is similar to echography, but with much greater resolution. OCT was developed in 1991,¹ with early devices measuring the time delay (TD) of light reflection and backscatter from the retina, referred to as TD-OCT. As a result of a small sampling area using only 512 A-scans, with the remaining determined by interpolation, image acquisition was hindered by subtle eye movements, thus potentially missing areas that were not directly imaged.²

Spectral domain (SD) OCT uses Fourier transforms to obtain 20,000 to 40,000 A-scans per second (Figure 1). The SD-OCT technology results in enhanced image resolution and minimization of motion artifacts, as well as more complete retinal scanning, with a resultant reduction in “skip” areas. Finally, SD-OCT technology allows for threedimensional image reconstructions, providing detailed topographic information of the retina.³

Spectral-domain OCT is the standard imaging modality for the diagnosis and management of many retinal diseases, including macular edema,⁴ AMD,⁵ macular holes^6,7 and vitreoretinal abnormalities such as vitreomacular traction and epiretinal membranes.⁸ Imaging in the clinic uses standard, FDA-approved devices, with patient cooperation and fixation aiding in image acquisition.

Conversely, the operative setting poses many more challenges to successful imaging of the retina using OCT technology. First, patients are usually sedated or under general anesthesia and are often unable to cooperate with fixation. Second, the surgical field is sterile, necessitating a noncontact device that does not risk contamination of the operating area. Next, surgical intervention requires precise measurements that must be consistent, reliable, and reproducible over time. Finally, imaging devices need to be operated easily and efficiently by the surgeon or by other operating room personnel.

The following article reports the status of current intraoperative OCT imaging technology, including our experience with three devices suited for use in the operating room setting. We also discuss future horizons in intraoperative OCT imaging.

Figure 1. SD-OCT technology results in enhanced image resolution and minimization of motion artifacts, as well as more complete retinal scanning.

BIOPTIGEN

The Bioptigen SDOCT instrument (Bioptigen, Inc.) is a handheld, FDA-approved device that is portable and able to produce high-resolution retinal images. The unit consists of a handheld imaging piece that is connected to the SD-OCT device via a 1.3-m flexible cable. The Bioptigen platform contains a flat-screen monitor, as well as other hardware, on an easily mobile cart (Figure 2a). Image calibration and focus correction are performed using a calibration knob on the unit and a focal distance adjustment on the handheld probe, ranging from +10 D to −12 D.

The Bioptigen unit was one of the first OCT imaging platforms to be used in an operative setting. The portable device has ventured out of the typical adult outpatient clinical setting, offering flexibility with current SD-OCT technology. With a convenient handheld scanner and mobile platform, the device adapts to a variety of clinical settings, including the clinic, laboratory, bedside and operating room.

Several investigators have reported successful image acquisition of patients in the pediatric clinic,^9-11 as well as imaging in the neonatal intensive care unit (NICU) for the evaluation of premature infants with retinopathy of prematurity.^12,13 Successful imaging of the fovea has been reported to be 74% for infants in the NICU vs 87% in the clinic. Common factors limiting successful image acquisition include poor patient fixation, motion and optics.¹²

The handheld Bioptigen is able to obtain high-resolution images secondary to the rapid image acquisition permitted with Fourier-domain technology. Acquiring 20,000 A-scans per second, the high-throughput spectral interferometer of the Bioptigen captures images with an axial resolution of 4 μm in less than six seconds. A recent report illustrated successful intraoperative use to image a pediatric patient with a traumatic macular hole. Perioperative images showed complete hole closure, which corresponded with follow-up OCT images at five days and one month.¹⁴ The unit is one of the first to pioneer this technology into exciting new areas.

Without the stability of a fixed platform, the handheld Bioptigen requires an experienced operator to obtain images of high quality. Subtle operator movements result in decreased image quality, thus necessitating the user to stabilize his or her hands on the patient or use a surgical support bar. As a result, the handheld probe poses a risk to the sterile field and may require the development of a sterile drape with an optical lens cover.

An additional drawback is the lack of a real-time fundus image to guide users to target locations, requiring operators to rely on axial images to find areas of interest. As a result, it may take additional time and experience to obtain intraoperative images efficiently, or development of new software may be necessary to display real-time fundus images.

As a result of poor patient fixation and cooperation during intraoperative examinations, as well as a lack of image-tracking features, images of precise locations are not always reproducible and consistent over time. However, the Bioptigen was one of the first handheld technologies to allow operative imaging, and we anticipate further modifications to address current shortcomings, making this device an attractive intraoperative OCT option.

HEIDELBERG INTRAOPERATIVE SPECTRALIS PROTOTYPE

The Spectralis (Heidelberg Engineering) is an FDAapproved device for SD-OCT imaging in the clinical setting. Designed similarly to operating microscope platforms, the intraoperative Spectralis is mounted with an adjustable arm that provides stability and controlled movements (Figure 2b). The unique platform is suited for the sterile operating field, as it has the option of hands-free foot pedal control to position the device precisely.

This added stabilization minimizes motion artifact, and when combined with eye-tracking technology, it acquires highquality, precise and reproducible images in the operative setting. The platform is mobile, but its size and bulkiness limit its effective motility and an efficient fit in an already crowded operating arena. A wide-screen, high-resolution display allows for easy viewing of representative images.

Spectralis combines scanning laser ophthalmoscopy (SLO) and OCT, resulting in an advanced eye-tracking technology effectively maintaining the OCT on the retina, thus providing high-quality, reproducible images that can be followed consistently over time. The eye-tracking technique also eliminates the need for patient fixation, an optimal feature for intraoperative use. With an advanced network, including operative platforms and clinical platforms, images can easily be viewed and compared over time throughout the perioperative period.

Spectralis performs 40,000 A-scans per second, providing an axial resolution of 4.5 to 7 μm. The aforementioned eye-tracking technology also contributes to the acquisition of high-quality images via dynamic noise reduction, as each line scan is performed repeatedly, up to 100 times.

However, this technology may also contribute to longer scanning times. In a head-to-head study of SD-OCT devices, the Spectralis was found to have the smallest coefficient of variation (0.5%) and least measurable change (1 μm).¹⁵ Finally, the Spectralis offers other imaging capabilities, including autofluorescence and fluorescein and indocyanine angiography.

The intraoperative SD-OCT platform developed by Heidelberg represents an important advancement in intraoperative OCT imaging. The system is designed around the existing Spectralis OCT, modified specifically for the operative setting. Although there are aspects that need further improvement, the platform is the first of its kind and is paving the way for intraoperative OCT to become the standard of ophthalmic surgical imaging.

OPTOVUE

The iVue (Optovue, Inc.) is an FDA-approved device contained in a compact platform, with the ability to obtain images in clinical settings via a slit-lamp control (Figure 2c). The device can also be detached and used as a handheld unit, suitable for use in the operative setting. The iVue platform has wheels for easy movement and a flat-screen monitor for image viewing, as well as a foot-pedal option for image acquisition. More recently, Optovue obtained clearance by the FDA for its iStand, a novel, multidirectional mounting stand that allows imaging of patients in the supine position, a modification that may enhance intraoperative imaging.

Figure 2. Three intraoperative OCT devices: the Bioptigen (A, left), the Heidelberg Spectralis prototype (B, center), and the Optovue iVue (C, right).

Using 26,000 A-scans per second, the iVue produces an image with an axial resolution of 5 μm and a transverse resolution of 15 μm. Similar to the Heidelberg platform, the iVue has live fundus imaging during use, allowing precise localization of areas to be imaged, as well as the ability to image peripheral lesions. The platform also allows for axial length measurements, as well as anterior-segment imaging, with the addition of a cornea-anterior module, included with the system. The iVue is ergonomic and compact, and it even comes with a foot pedal to control image acquisition, a useful feature for intraoperative use.

The drawbacks of the iVue for intraoperative imaging stem from the handheld nature of the device. The iVue proves fairly bulky and heavy when attempting to image a patient in the supine position. In addition, the photographer must stabilize the device manually, often resulting in an unsteady and fluctuating working distance that may affect the image quality. However, the new iStand will address most of these shortcomings.

Despite these limitations, the iVue is able to image retinal pathology reliably in an intraoperative setting via a readily available unit that can also be used to image patients in clinical settings. This dual utility allows for perioperative image comparison.

FUTURE HORIZONS

Intraoperative OCT imaging is a new phenomenon made possible by the recent developments of Bioptigen, Heidelberg and Optovue. Although these devices are FDA-approved in clinical settings, intraoperative imaging is an off-label use. Nevertheless, for the first time, a highresolution image of the retina can be obtained in the operative setting, with the potential to aid the surgeon in intraoperative management.

However, before OCT is widely accepted as an integral part of the modern ophthalmic operating room, further advancements and design specifications must be made. Modifications to the current platforms must be tailored to a real-world operating room with a minimal amount of extra space. It must be time-efficient and require a minimal number of operators. Images must also be easily exported, allowing fast image comparison. Finally, devices must be user-friendly, with minimization of operator-dependent variability.

On the horizon, an ideal intraoperative OCT device will be similar to an operating microscope or will perhaps even be incorporated into the technology of the operating microscope, with a foot pedal allowing easy movement and image acquisition. Microscope-mounted OCT is currently in development, with reports of in vivo efficacy for retinal imaging.^16,17 The authors reported on successful imaging of healthy volunteers and characterized the appearance of commonly used instruments, and they also reported on the appearance of tissue during manipulation in cadaver eyes.

Other microscope-mounted prototypes have been reported, using Cirrus HD-OCT combined with a Carl Zeiss Meditec operating microscope. Ideally, using an advanced network, images from the clinic and the operating room can be linked to allow the surgeon real-time assessment and comparison. This linkage will provide the surgeon with efficient and high-quality images, without disrupting the flow of the surgical procedure or placing the sterile field at risk.

Vitreoretinal surgery may be revolutionized with the added precision and detail provided by SD-OCT imaging. For the first time, surgeons can evaluate the retina in real time during the surgical procedure, allowing visualization of anatomic success in epiretinal membrane peels, macular hole repairs, vitreoretinal traction procedures, and repairs for retinal detachments. Intraoperative imaging may also prove important in pediatric evaluations, during exams under anesthesia, or during operative procedures.

Further studies are needed to determine whether this technology and imaging ability will correlate with improved surgical outcomes and visual results. As OCT imaging is incorporated into the surgical management of ophthalmic disease, improved anatomic and visual outcomes are anticipated, as well as a better understanding of pediatric retinal diseases.

CONCLUSIONS

Spectral-domain OCT technology has quickly advanced since its original development as TD-OCT in 1991, with the newest platforms providing high-resolution images of the retina. OCT has rapidly revolutionized the diagnosis and treatment of multiple retinal diseases, including AMD, macular edema, and vitreoretinal interface disorders. OCT is also being used frequently to assess the retinal nerve fiber layer in glaucoma, as well as to image the cornea.

Recent adaptations of current SD-OCT technology have enabled these devices to be used outside of the clinic, notably at the bedside and in the operating room. These modifications have opened a new realm of possibilities, as OCT imaging is being performed on infants, children, and surgical patients who could not be imaged in the past. The technology allows for high-quality images to be obtained in the operating room setting that are consistent, reproducible, and timely. At this time, it is unknown how intraoperative OCT imaging will change surgical management, but it may very well pave the way for OCT to become the gold standard for intraoperative imaging. RP

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