Fig. 29.1
(a) The prototype of the first surgical microscope-integrated iSD-OCT system based on Cirrus HD-OCT™ source and running the 4.5.1.11 Cirrus HD-OCT™ acquisition software adapted to the optical pathway of a Zeiss OPMI VISU 200™ SM (Carl Zeiss Meditec, Oberkochen, Germany). (b) iSD-OCT scan of an epiretinal membrane with a vitreomacular traction
29.4 Equipment
29.4.1 Intraoperative Optical Coherence Tomography Probes
A wide range of OCT systems provide imaging of eye structures that are perpendicular to the scanning beam, including the cornea, iris, lens, macula, optic nerve, retinal nerve fibers, and choroid. Straight beam delivery, however, lacks the ability to image peripheral structures. Additionally, the OCT signal becomes degraded and distorted in eyes with dense media, such as mature cataracts or vitreous opacities, which result in low-quality images (Fercher et al. 2003). The ability of forward-imaging intraocular OCT probes to provide real-time cross-sectional scanning of inner ocular structures gives this method a unique value during eye microsurgery. The main advantage of the iOCT probe is that it can scan almost any part of the anterior and posterior eye segments depending on the placement through the bypassing mechanism with a direct view of structures that are unreachable using external OCT systems (Li et al. 2000a, b; OKT 1300-E BioMedTech; Sun et al. 2014).
According to the direction of the optical scanning mechanism, the OCT probes can be divided into two main types: side-imaging probes and forward-imaging probes (Sun et al. 2014). The three main probe parameters are field of view, scanning range, and scanning rate. OCT side-scanning probes with different sizes have been designed to examine tissues within tubular structures. Those OCT probes are being successfully utilized in imaging of the esophagus, coronary arteries, etc., with a lateral resolution up to 10 μm. The first side imaging, 27-gage needle probe OCT, was designed by Li et al. in 2000 for interstitial imaging of different organs (Li et al. 2000a, b). A forward-imaging OCT probe, with relatively large diameter, has also been used to image bladders (Feldchtein et al. 1998). Usually, two-dimensional (2D) data imaging is provided by forward-imaging probes. By using 2D imaging with a combination of probe rotation or linear movement, three dimensional (3D) images can be obtained. A forward-imaging probe would be most desirable to examine the inner structures of a hollow organ such as the eye, in order to evaluate the retina.
The capability of OCT probes for real-time cross-sectional extraocular and intraocular imaging is considered to be effective for intraoperative diagnosis and surgery. The diameter of the probe is also an important issue, as it has to fit through a standard surgical entry port, preferably 23- or 25-gage. An important issue that is specific to the probe is that the tip must be positioned within close proximity to delicate tissues, such as the retina (Binder et al. 2011). A wide range of iOCT probes has been developed to visualize posterior pole and peripheral inner eye structures (Li et al. 2000a, b; OKT 1300-E BioMedTech;Stolyarenko et al. 2011; Sun et al. 2014). The average scanning range is 1–2 mm with an axial resolution of 4–6 μm and a lateral resolution of 25–35 μm. The distance between imaged structures and the probe tip has to be 1.6–4 mm, and the acquisition quality is dependent on the surgeon’s ability to hold the probe steadily.
In 2007, at the Posterior Eye Segment Diagnostics and Surgery Center in Moscow, Russia, G. Stolyarenko and his group started to use the OKT 1300-E (BioMedTech, Russia) – a universal OCT system that can be used with both anterior and posterior segment probes (23- and 25-gage) (Fig. 29.2). The OKT 1300-E has scan depth of 1.8–2 mm, a depth resolution of 10 μm in air, with an acquisition rate 20,000 A-scans/s (Table 29.1) (OKT 1300-E BioMedTech; Stolyarenko et al. 2011). The main application for anterior segment use was the differentiation of malignancies of the conjunctiva, localization of the site of biopsy, visualization of the cornea, iris, and anterior chamber, as well as intraoperative control during tumor resection. During posterior segment surgery, it was used intraoperatively to detect epiretinal membranes, assess vitreomacular traction, and control surgical maneuvers and outcome (Fig. 29.3).
Fig. 29.2
iOCT system OKT 1300-E (BioMedTech, Russia) (a) with a 23-gage posterior segment endoprobe (b). Acquisition of the iOCT with endoprobe during pars plana vitrectomy (c) (Photos printed with the permission of Prof. G. Stolyarenko)
Table 29.1
Technical characteristics of the OCT systems that are used during ophthalmic surgery
Characteristics | OKT 1300-E (BioMedTech, Russia) | Bioptigen SDOIS (Bioptigen, Morrisville, NC, USA) | iVue scanner (Optovue, Fremont, CA, USA) | iOCT® (Optomedical Technologies GmbH, Lübeck, Germany) | Rescan 700 (Zeiss, Oberkochen, Germany) |
|---|---|---|---|---|---|
OCT source | Spectral domain | Spectral domain | Spectral domain | Spectral domain | Spectral domain |
OCT image | 20,000 A-scan/s | 32,000 A-scans/s | 28,000 A-scan/s | 10,000 A-scans/s | 27.000 A-scans/s |
Frame rate | 20 frames/s | 256–1024 A-scan/frame | 10 frames/s | ||
Depth resolution | 10 μm in air | 3.3 μm in air (2.4 μm in tissue) | 5.0 μm in tissue | 10 μm in air | 5.5 μm in tissue |
Traverse resolution | 15 μm (retina) | 15 μm (retina) | |||
Scan range depth | 1.8–2 mm | 3.4 mm (2.5 mm in tissue) | 2–2.3 mm (retina) | 4.2 mm | 2.0 mm |
Scan beam wavelength | 1310 nm | 830 ± 30 nm | 840 ± 10 nm | 800 nm | 840 nm |
Exposure power at pupil | 700 μW (1500 μW max) | 750 μW | |||
Lateral scan width | 1.6–2.4 mm | 13 × 9 mm external image | 5–30 mm dependent on microscope zoom | 3–16 mm (scan rotation 360°) | |
Recording options | Videos, snapshots, and 3D images | Videos, snapshots | Videos, snapshots, 3D en face analysis upgrade | Videos, snapshots, and 3D images | Videos, snapshots |
Working distance | 4 mm | 22 mm/15 mm | |||
Motorized focus range | +10D to −12D | −15D to ± 12D |
Fig. 29.3
iOCT imaging of the retina (a, b) with a 23-gage probe (OKT 1300-E (BioMedTech, Russia) in comparison to the preoperative OCT (color scale, a). A 2D (c) and a 3D (d) visualization of the retina with the OKT 1300-E endoprobe (Photos printed with the permission of Prof. G. Stolyarenko)
In 2013, K. Joos and J. Shen reported the use of the smallest self-contained 25-gage handheld forward-imaging B-scan OCT probe for ophthalmic examination (Joos and Shen 2013). This new iOCT probe was adapted to an SD-OCT system Bioptigen Inc.1which delivered 700 μW of power, central wavelength of 870 nm, with 2,000 A-scans per B-scan. Optimal working distance was 3–4 mm from the probe tip to the targeted tissue. The intraocular OCT probe was meant to permit evaluation of structures beyond the central posterior pole of the eye, such as the peripheral retina and ciliary body. The possibility of iOCT probes to scan the peripheral structures of the eye is a very promising capability for the understanding of certain eye diseases. However, the narrow field of the OCT image and interruption of the surgical workflow in order to perform the scanning limit its use.
29.4.2 Handheld Optical Coherence Tomography Systems
29.4.2.1 Bioptigen Envisu SDOIS
Their Bioptigen Envisu Spectral Domain Ophthalmic Imaging System (SDOIS) was developed to capture, process, display, and save depth-resolved images of ocular tissue microstructures based on a spectral domain OCT source Bioptigen Inc.1 (BioptigenEnvisu 2015) Bioptigen SDOIS is a noncontact, portable, handheld SD-OCT system. The SD-OCT engine is connected via a 1.3-m flexible cable to the imaging handpiece that is used by the operator. The OCT unit is mounted on a trolley cart to increase the mobility in the operating room, as well as a screen, keyboard, and printer. A reference arm position adjuster with digital readout is also included into the OCT system. The handheld probe has a focus adjustment system and a diopter scale bar with a range of +10 to −12 D. The custom software (Bioptigen, Durham, NC, USA) enables imaging, analyzing, and archiving of the acquired data. The technical characteristics of Bioptigen SDOIS are a wavelength of 830 ± 30 nm, an axial resolution of 3.3 μm (2.4 μm in tissue), a scan depth of 3.4 mm (2.5 mm in tissue), and an acquisition rate of 32,000 A-scans/s (Table 29.1).
In 2009, C. Toth and her group reported the use of an FDA-approved portable, noncontact, handheld SD-OCT unit by Bioptigen Inc.1 (Bouma and Tearney 1999; Scott et al. 2009). The OCT system consisted of an imaging handpiece connected to an SD-OCT engine and mounted on a trolley cart. To adjust the reference arm position, a calibrated knob was used. The focus correction adjustment of the handheld probe ranged from +10 to −12 D. J. Ehlers and his group at the Cole Eye Institute reported the use of two different prototype microscope-mounted systems. In the first one, the handheld system was fixed below the lens of the microscope. The surgeon was therefore able to use an undraped microscope. In the second system, a modified plate was used to position the OCT probe to the side of the scope, and the entire system had to be draped (Ehlers et al. 2011a, b; Srivastava 2010; Srivastava et al. 2011).
29.4.2.2 iVue® SD-OCT
The iVue® SD-OCT (Optovue, Fremont, CA, USA) is a commercially available, portable, compact OCT system with the capability to perform a quantitative analysis of the obtained data (Optovue, iVue. SD-OCT 2015). The iVue includes a spectral domain source and provides a complete posterior and anterior segment scanning with an axial depth resolution of 5 μm, a traverse resolution for retina of 15 μm, and an OCT A-scan acquisition rate of 28,000 A-scans/s (Table 29.1). The depth of the OCT scan is 2–2.3 mm. Minimum pupil diameter required is 2.5 mm. The working distance is 22/15 mm. The motorized focus can be adjusted in a range from −15D to ± 12D (Optovue). The software enables the creation of retina, glaucoma, and cornea scans and reports live video and en face images. The iVue® SD-OCT software can be upgraded with ganglion cell complex analysis which provides the ability to identify ganglion cell loss. The 3D en face analysis upgrade provides virtual dissection of the retina and the optic disk with 67 million data points within a 512 × 128 dense cube and high-density 3D volume for visualization and analysis of the eye tissue structures. The iVue can be mounted to the iStand, a rolling floor stand option which enables scanning of the patients in different positions, including the supine position and moving the OCT system around.
The use of handheld OCT systems leads to a number of difficulties. For example, it is difficult to reach a proper stabilization of the units in order to obtain good images. The use of handheld systems can adversely affect the sterility of the surgical field with the risk of potential complications. The surgeon has to interrupt the surgery, remove the microscope, and adjust the OCT system, making the procedure time-consuming. Even with the custom microscope mount, the microscope has to be removed from the surgical field. In addition, with handheld OCT systems, it is impossible to image surgical maneuvers themselves. The quantitative data analysis also has to be improved.
29.4.3 Microscope-Integrated Intraoperative Optical Coherence Tomography Systems
29.4.3.1 Microscope-Integrated OCT (Mi-OCT)
A custom microscope-integrated OCT (Mi-OCT) system was developed by the team led by Toth and Izatt at Duke Eye Center, by implementing the OCT scanner into the surgical microscope (Hahn et al. 2013a, b). For OCT imaging, a commercially available SD-OCT engine Bioptigen Inc.1 (Table 29.1) with a near-infrared light source, 700 μW of power, and a 5.2 μm axial resolution in air was integrated into the surgical microscope Leica M844 (Leica Microsystems, Switzerland) using a Leica digital imaging color module (DI C800). The Mi-OCT system enables simultaneous, wide-field noncontact, real-time, cross-sectional iOCT imaging of retinal structures including intraocular surgical manipulations without impairing the surgeon’s view through the surgical microscope. Recently, the Mi-OCT was upgraded with a swept source OCT (SS-OCT) engine and customized with tracking hardware and software to target the OCT beam to the area of interest. The SS-OCT makes it possible to obtain many individual scans in the same time needed to obtain only a single scan using SD-OCT. With the new software, 3-dimensional real-time visualization of the retina is possible.
29.4.3.2 iOCT® OPMedT
Universal iOCT® camera by OPMedT (Optomedical Technologies GmbH, Lübeck, Germany) can be mounted to the standard camera port on a surgical microscope (HS Hi-R Neo 900A NIR, Haag-Streit Surgical GmbH) (iOCT®) (iOCT. Haag-Streit 2015). An additional M.DIS touch screen is available. The system has an SD-OCT source, 800 nm wavelength, 10,000 A-scans/s, and 10 μm axial resolution in air. As the iOCT engine itself is not directly integrated into the microscope stand, future upgrades and multiple screens are possible (Table 29.1). The iOCT images are injected into both oculars where they are superimposed over the microscopic view. The system ensures the same focus for iOCT and microscope. The zoom in the microscope can also magnify the iOCT image. The scan window depth is 4.2 mm. The second microscope-mounted display is installed close to the oculars. The microscope and iOCT adjustment is enabled via foot control for which the settings can be customized and saved. It can be used in both anterior and posterior segment surgeries, as well as in ENT and neurosurgery (Fig. 29.4). But up to the date, the use of OPMedT OCT system is widely described in the field of keratoplasty (iOCT®). The software enables the standard recording and editing of the surgery films and snapshots. Also, the generation of 3-dimensional images out of 30 consecutive images is possible.
Fig. 29.4
iOCT imaging of a healthy cornea (a) lens, (b) retina, (c) and 3D visualization of the retina (d) with iOCT® camera by OPMedT (Optomedical Technologies GmbH, Lübeck, Germany) (Photos printed with the permission of Prof. C. Cursiefen and Prof. J. Szaflik)
29.4.3.3 Rescan 700
The Carl Zeiss Meditec Rescan 700 is the first commercially available CE approved iOCT system which is fully integrated into the foot stand of the operating microscope OPMI LUMERA 700 Carl Zeiss Meditec.2 (Fig. 29.5a) (Rescan 700 Carl Zeiss Meditech 2015, Ehlers et al. 2014b). The system has a spectral domain OCT engine with a wavelength of 840 nm and a scanning speed of 27,000 A-scans per second and is incorporated into the head of the microscope without causing any loss of function. The main scan parameters are the following: A-scan depth (2.0 mm in tissue), axial resolution (5.5 μm in tissue), scan length adjustable from 3 to 16 mm, and scan rotation adjustable 360° (Table 29.1). Scan modes for live film acquisition and snapshot capture are available (1-line, 5-line, cross hair). The Callisto eye system includes a touch screen and a 500 GB hard drive. The Callisto eye software adapts the iOCT acquisition to the different optics used with the Resight viewing system (60 D, 128 D lenses are included) and contact lenses for macular surgery. Due to the live iOCT images being directly injected into one of the surgeon’s oculars, the surgeon does not need to change the direction of his gaze during surgery in order to see the iOCT images (Fig. 29.5b). The surgeon can easily control the key options of the Rescan 700 OCT system from the microscope’s foot pedal, quickly turning it on and off, triggering OCT videos, and capturing SD-OCT images without looking up or interrupting the surgery.
Fig. 29.5
iSD-OCT system Rescan 700 fully integrated into the foot stand of OPMI LUMERA 7002 (Rescan 700 (Carl Zeiss Meditec, Oberkochen, Germany)). (a) Head-up display built into the ocular of OPMI LUMERA 7002 (Rescan 700 (Carl Zeiss Meditec, Oberkochen, Germany)) (b)
29.5 Intraoperative Optical Coherence Tomography Imaging
While performing iOCT, the surgeon needs to be aware of the two main scanning modes: filming and snapshots. Both modes allow the surgeon to observe, analyze, and process the data in real time and after the surgery.
29.5.1 Acquisition
Anterior segment iOCT visualization usually can be achieved directly by handheld iOCT systems or through the objective lens of the ophthalmic surgical microscope into which the iOCT is integrated (Fig. 29.6a). By using the macular surgery contact lens, a wider and better visualization of the anterior segment can be achieved (Fig. 29.6b). The two most important prerequisites to obtain high-quality posterior segment iOCT are clear optical media and normal intraocular pressure (IOP). To obtain an iOCT image using a probe, it has to be placed close to the desired ocular structure at a distance of 1.6–4 mm. In handheld and microscope-integrated OCT systems, posterior segment iOCT scanning is possible without any additional optics but with an inferior quality and magnification. The wide angle, noncontact viewing systems, such as Resight 700 (Carl Zeiss Meditec, Germany), EIBOS II (Haag-Streit Surgical GmbH, Germany), and Oculus BIOM 3 (OCULUS Optikgeräte GmbH, Germany) with wide-field ophthalmic lens, can be used together with iOCT units. Any contact lens for macular surgery could also facilitate iOCT capturing with a higher magnification (Fig. 29.6c). The type of optics used has to be taken into consideration and adjusted in the iOCT systems settings if possible.
Fig. 29.6
iSD-OCT imaging of healthy cornea through the objective lens of the ophthalmic surgical microscope (a). Anterior segment iOCT visualization with the use of a contact macular lens (b). iOCT scanning of the ERM through a contact lens for macular surgery (c). Shadowing from the end-gripping forceps (c, arrow). Visualization of the vitreoretinal adhesion enhanced by triamcinolone acetonide (d, arrow). iOCT images made with iSD-OCT system Rescan 7002 (Rescan 700 (Carl Zeiss Meditec, Oberkochen, Germany))
For imaging posterior retina, the best quality of iSD-OCT scans can be obtained in patients with clear cornea, pseudophakia, and no posterior capsule or vitreous opacities. The intraoperative swelling of the lens or cornea can influence on the quality of iOCT scans. Therefore, caution has to be maintained while manipulating close to them. Additionally, the surgery duration has to be optimal. It was shown that enhancement of the visualization of semitransparent tissues and tissue planes with triamcinolone acetonide or indocyanine green (ICG) may provide further improvement in surgeon feedback (Fig. 29.6d), Video 29.1 (3) (Ehlers et al. 2010, 2014c).
Any type of anesthesia is suitable for iOCT scanning. In the case of local drop anesthesia, the patient can be asked to fixate or follow the illuminated target point produced by iOCT machine if the preoperative vision is sufficient. The best conditions for iOCT imaging are complete akinesia and straight alignment of the eye. In addition, the centering of the iOCT image can be adjusted by centering the eye with the forceps or a fixation ring. Acquisition of the iOCT images usually is controlled via foot pedal or with help of an assistant. After activating the iOCT, a real-time video of OCT imaging starts running. Snapshot mode allows the acquisition of separate scans of a selected area. All data can be recorded and post-processed with supplementary software.
Besides the ocular tissues that can be visualized with iOCT, any anterior or posterior segment instrument that is introduced into the eye can produce hyperreflectivity and shadowing in the iOCT image (Fig. 29.6c) (Hahn et al. 2013a, b). The ability to image the instruments while using them within the eye is a unique feature that can help to control the safe distance to the ocular tissues and prevent any undesirable contact that may result in iatrogenic trauma. Tracking the instruments with iOCT during the entire surgery would be challenging as the surgeon rarely stays steady with the hands while performing a lot of tiny maneuvers.
The visualization of ophthalmic surgical maneuvers could also be improved with the use of iOCT enhanced with special additional lenses and mechanical enhancements. Tao and his group have demonstrated the use of an integrated electrically tunable lens that allows rapid focal plane focusing and iOCT imaging of ocular tissues of anterior and posterior segments in enucleated porcine eye (Tao et al. 2014). The real-time surgical maneuvers were visualized by acquiring spatially compounded volumetric data sets which allowed segmentation of the surface of a diamond-dusted scraper and retina on individual cross-sectional images (Hahn et al. 2013a, b). The study data provide integrative visualization of the membrane scraper position relative to the retina surface.
29.5.2 Calibration
The lack of calibration in iOCT systems limits the analysis of acquired data. The problem of iOCT system calibration is due to the constantly changing optical media during the surgery. The system would have to be recalibrated every time a different lens is used during surgery as well as at every change in magnification of the microscope. Nevertheless, some graphic software does offer substantial measurements of the tissues that can be used to show the difference, time dynamics in length, width, height, or volume. Due to the calibration issue, these measurements are only relative and not absolute.
Toth used a handheld SD-OCT unit by Bioptigen Inc.1 to image the retina of infants. The iSD-OCT system was complemented with a calibrated knob to adjust the reference arm position with a digital readout. A manufacturer-supplied calibration factor was used to convert the readout units to optical distance in millimeters (Maldonado et al. 2010).
29.5.3 Post-processing
iOCT data post-processing is a very important step that usually is time-consuming but also reveals additional information about the intra-surgical tissue behavior. The main aim of post-processing of iOCT data is a deeper analysis obtained by the use of special software and algorithms (Hahn 2014; Koprowski and Wróbel 2011). Since the iOCT images have a low signal to noise ratio and movement artifacts, the post-processing also serves to improve data presentation. Each microscope-integrated iOCT system is supplemented with hardware and software with a built-in camera system to facilitate the generation of images for information, documentation, teaching, and presentation purposes.
29.5.4 Three-Dimensional Visualization and Animation
With the goal of enhancing the clarity of subtle structures inside the iOCT data, two different post-processing and 3D visualization systems can be used. Both of them employ ray-traced shading, which enhances small structures by throwing a shadow from the virtual light source or light sources onto the data behind or below the illuminated structure, making the structure stand out from its surroundings and thereby increasing the contrast of the visualization. The first system is voxel-based and has relatively low preprocessing time and can if necessary use unprocessed raw data directly from the iOCT. The raw iOCT data sets are exported, z-aligned, and noise-reduced in post-processing. The resulting data is imported into Cinema 4D™ and rendered using a custom-made plug-in which renders the data as voxels with ray-traced shading and a single customizable light source direction. The second system is based on triangle mesh representation of the data. This requires full nonplanar segmentation of the data, and a large amount of preprocessing is therefore necessary. Segmented meshes are created in Cinema 4D™ using a fluid dynamics particle generator/mesher and rendered using a CUDA GPU (Fig. 29.7, Video 29.2).
Fig. 29.7
Triangle-meshed CUDA-based 3D iOCT visualization of an ERM after peeling. A residual membrane “sail” is still visible. The raw iOCT data is on the left. The reconstructions are on the right
29.6 Application of Intraoperative Optical Coherence Tomography
29.6.1 Anterior Segment Application
The iOCT scanning of the cornea and anterior segment structures is the most used examination. The images are the highest quality images the iOCT system can offer, because these structures are the first ocular tissues that scatter the light beam. The iris blocks the OCT scan penetration, limiting the examination of the ciliary body and the lens. The iOCT can be used for all anterior segment surgery. While being used for different types of keratoplasties (DMEK, DSAEK, DALK, PKP), it can assist the surgeon to orient the donor’s graft by assessing its position (Fig. 29.8). It can also aid in the search for residual fluid, viscoelastic, or air in between the host and donor corneal tissues, as well as improving the sutures. In cases with opaque cornea, the use of an iOCT probe can help to discover and recognize the structures behind the cornea and to find the most convenient entry side. The iOCT provides the surgeon with precise and accurate imaging of the entire anterior chamber including detailed information about apposition of the graft (De Benito-Llopis et al. 2014; Juthani et al. 2014; Miyakoshi et al. 2014; Steven et al. 2013, 2014; Xu et al. 2014).
