Optimizing Visualization in Endoscopic Spine Surgery

Abstract Given the inherent limitations of spinal endoscopic surgery, proper lighting and visualization are of tremendous importance. These limitations include a small field of view, significant potential for disorientation, and small working cannulas. While modern endoscopic surgery has evolved in spite of these shortcomings, further progress in improving and enhancing visualization must be made to improve the safety and efficacy of endoscopic surgery. However, in order to understand potential avenues for improvement, a strong basis in the physical principles behind modern endoscopic surgery is first required. Having established these principles, novel techniques for enhanced visualization can be considered. Most compelling are technologies that leverage the concepts of light transformation, tissue manipulation, and image processing. These broad categories of enhanced visualization are well established in other surgical subspecialties and include techniques such as optical chromoendoscopy, fluorescence imaging, and 3-dimensional endoscopy. These techniques have clear applications to spinal endoscopy and represent important avenues for future research.

Given the inherent limitations of spinal endoscopic surgery, proper lighting and visualization are of tremendous importance.These limitations include a small field of view, significant potential for disorientation, and small working cannulas.While modern endoscopic surgery has evolved in spite of these shortcomings, further progress in improving and enhancing visualization must be made to improve the safety and efficacy of endoscopic surgery.However, in order to understand potential avenues for improvement, a strong basis in the physical principles behind modern endoscopic surgery is first required.Having established these principles, novel techniques for enhanced visualization can be considered.Most compelling are technologies that leverage the concepts of light transformation, tissue manipulation, and image processing.These broad categories of enhanced visualization are well established in other surgical subspecialties and include techniques such as optical chromoendoscopy, fluorescence imaging, and 3-dimensional endoscopy.These techniques have clear applications to spinal endoscopy and represent important avenues for future research.

KEY WORDS: Endoscopic fusion, Endoscopic discectomy, Endoscopic visualization, Endoscopic lighting, Spinal endoscopy
Operative Neurosurgery 21:S59-S66, 2021 DOI: 10.1093/ons/opaa382 W ith modern endoscopic techniques utilizing smaller surgical corridors and cannulas, the development of novel techniques to maximize and manipulate light is increasingly critical.This is especially important in spinal surgery.Most endoscopic procedures are performed within either an existing 3dimensional (3D) cavity or the enlargement of a potential space (such as in endoscopy of the gastrointestinal tract, nasal sinuses, cerebral ventricles, thoraco-abdominal compartments).In contrast, spinal endoscopy (apart from thoracic endoscopy) involves surgical manipulation within confined spaces.The situation is further complicated by the proximity of nondeformable anatomic structures (bone) and structures at great risk of injury from deformation (neural tissue).
2][3][4][5] This learning curve is secondary to not only the lack of an existing potential space but also: r limited field of view and lack of resolution, making identification of anatomic structures difficult, r disorientation given the indirect nature of visualization, resulting in the surgeon being unable to accommodate for geometric orientation and angulation of viewpoint (perspective), r limitations in effectors because of the small working cannula (instruments cannot utilize all 6-degrees of freedom in movement).
Enhanced optical systems and refinements in surgical instruments have been suggested to combat these challenges. 6In this paper, we address the concept of enhancing endoscopic visualization by utilizing technologies that involve (1) transformation of lighting utilizing the visible spectrum, (2) tissue manipulation with light transformation, or (3) enhancement or postprocessing to improve image perception.

THE PHYSICS OF LIGHT-TISSUE INTERACTIONS
Light and its properties (namely whether it is a wave or particle) have been the subject of vigorous scientific debate for centuries.While a detailed discussion of the history is beyond the scope of this paper, it is worthwhile to note that the concept of light as a wave prevailed until the mid-1800s, when Einstein suggested that light was composed of Lichtquanten (today referred to as photons), which he later described as having wave-like properties-meaning light has both particle and wave behavior. 7his theory referred to as the "dual nature of light" predominates today.
This dual nature of light is critical to understanding how we perceive and interact with light, and therefore how we can manipulate it to improve visualization."Visible light" refers to wavelengths of electromagnetic radiation perceptible to the human eye-approximately 380 to 750 nanometers (nm). 8The wave nature of light dictates why we see different colors.Color of light perceived is determined by the wavelength of the light source and the absorptive and reflective characteristics of the object it illuminates. 8As light strikes an object, only certain portions of the mixed wavelength are absorbed.The remaining light (which is reflected) determines the color perceived. 8Stains and dyes absorb specific light wavelengths, and optical filters can alter perception of certain wavelengths of reflected light.Fluorescence, the emission of light resulting from absorption of light of a different higher energy wavelength, can also be utilized to enhance visualization. 9

LIGHT AND ENDOSCOPIC SURGERY Transmission of Light and Images
In endoscopic surgery, the surgical field is visualized through illumination from a high-powered light source that is transmitted through a fiber optic cable through total internal reflection (Figure 1). 10 Total internal reflection occurs when light strikes the fiberoptic wall at a greater angle (with respect to the normal) than the critical angle, allowing it to propagate with nearly the same intensity as prior to reflection.Light then passes through a series of apertures within the rigid endoscope en route from the fiber optic cable to the target tissue.Aperture width dictates the amount of light energy that is passed on to the target. 11Illumination refers to the light power per area unit of the image and increases with dilation of the light apertures of the endoscopic system. 11

Image Visualization Within Target Tissues
Images pass through a series of lenses within the rigid endoscope prior to transmission through the outgoing fiberoptic cable.Optical angle and focus length are 2 variable features of the endoscope that play a role in image acquisition in endoscopy.The field of view of the endoscope refers to the cone of visualization centered at the camera (Figure 2). 12ptical angle of the endoscope refers to the angle between the camera middle axis and endoscope axis (Figure 2).The optical angle of spine endoscopes is generally between 0 • and 30 • depending on the level of the spine at which the endoscope is used, the purpose of the instrument, and the manufacturer of the endoscopic system (Figure 3). 13Endoscopes with a larger optical angle allow for a more flexible field of vision, as rotation of the endoscope permits the surgeon to view different areas of the target tissue. 14However, because humans are accustomed to a 0 • optical angle when viewing the world unassisted, a higher optical angle can lead to greater disorientation.
Focus length refers to the distance between the endoscope camera and the point of convergence of light rays prior to entering the endoscope.Most endoscopes have an adjustable focus length of 10 to 40 mm.A wider focus length range allows the surgeon to maintain the sharpness of the image without repositioning of the endoscope to keep an identical point of convergence on the target tissue. 15

Image Processing
In modern endoscopes, the light image undergoes digital processing to allow for projection onto a screen, image manipulation, and recording.The 4 principles that determine image video quality are resolution, refresh rate, minimum required luminance, and signal-to-noise ratio.Resolution is measured in pixels per inch, with a higher resolution corresponding to greater image sharpness.Refresh rate is a measure of the number of distinct images captured and joined to produce a video.Minimum required luminance is the minimum brightness at which the target must be illuminated to be detected on the camera system. 16Signal-to-noise ratio compares the magnitude in intensity of the detected image relative to the uncertainty in image transmission. 17

Challenges
Each component of modern endoscopic equipment represents a potential avenue of improvement.However, since the production of high-intensity light and high-resolution imaging is no longer a major technological challenge, the next stages of advancement will rely on transformational methods for enhanced visualization and anatomic discrimination.

LIGHT TRANSFORMATION Optical Chromendoscopy
One technology that offers promise for endoscopic spinal surgery is optical chromendoscopy.Examples of this technology include narrow band imaging (NBI), flexible spectral color imaging enhancement (FICE, Fujinon), and i-scan (Pentax). 18o understand how these lighting modalities diverge from classic means of endoscopic illumination, we must recall that endoscopic imaging traditionally relies on white light which is produced using various methodologies.For example, operating microscope illumination has traditionally utilized plasma arcs for illuminance rather than metal (carbon, tungsten) filaments.Plasma arc lamps (mercury, xenon) are 10 to 100 times brighter than incandescent bulbs. 19Mercury arc lamps (HBO) provide excellent illumination over certain specific wavelengths in the visible spectrum with wavelength peaks at 405 nm (violet), 436 nm (indigo), 546 nm (green), and 579 nm (yellow), whereas xenon arc plasma lamps (XBO) lack prominent emission lines in the visible spectrum and instead have peaks between 800 and 1000 nm, and are therefore more suited to quantitative electron microscopy. 20ather than changing the physical medium used to produce light (as has previously been done and described above), NBI uses filters to alter the spectrum of emitted light, thereby achieving a desired spectral output. 18NBI narrows the red-green-blue bands while increasing the blue band intensity. 18This targeted alteration of light is particularly useful in highlighting microvascular differences resulting from preferential blue light absorption by hemoglobin. 18Improved visualization of microvascular architecture has clear oncologic applications, and as such has been leveraged by diverse subspecialties including neurosurgery and urology (Figure 4). 21,22n cranial videoscopic surgery, NBI uses an optical filter that only allows 2 narrow wavelength bands. 21Filtered blue light (415 nm) has more superficial penetration and highlights surface architecture, while green light (at 540 nm) provides better imaging of deeper tissue. 21Images produced from these 2 bands are integrated via a processor, producing a single, sharp image. 21ICE and i-scan have similar end-user output to NBI, but rely on computer-based filters, rather than physical filtering of emitted light. 18echnologies such as NBI and FICE have obvious applications to endoscopic spine surgery.While they have not yet been tested expressly for this purpose, it seems evident that the vasculature of the dura will differ significantly from the surrounding soft tissue (muscle, connective tissue, etc).Considering this fact, these technologies should be able to provide improved contrast between these 2 media.This enhanced contrast could be used to (1) identify and therefore protect critical neural elements during complex or re-operation procedures and (2) verify adequate neural decompression.

TISSUE MANIPULATION WITH LIGHT TRANSFORMATION-FLUORESCENCE IMAGING
"Fluorescence" refers to emission of light after the absorption of light of a higher energy (shorter wavelength).Cranial neurosurgery harnesses this property through the usage of certain chemical substrates.The 3 most commonly used agents are 5-aminolevulinic (5-ALA), indocyanine green (ICG), and fluorescein.

5-Aminolevulinic
4][25] This technology leverages the fact that 5-ALA causes synthesis of fluorescent porphyrins in malignant gliomas. 24,25atients are administered 5-ALA preoperatively.Once the surgical exposure has been performed, a microscope and filter are used to view fluorescence and identify residual tumor. 24Normal neural tissue demonstrates no fluorescence, while neoplastic tissue exhibits red fluorescence (Figure 5). 25,26This methodology is more specific than previous attempts at evoking tumor fluorescence since the fluorescent porphyrins are being synthesized endogenously by the tumor (rather than a fluorescent material being delivered via a disrupted blood-brain barrier). 24he potential applications of this technology to endoscopic spinal surgery are multiple.First, there is clear utility in spinal oncology.In this regard, the use of 5-ALA to aid in the resection of spinal tumors has previously been described in regards to meningiomas, ependymomas, hemangiopericytomas, and drop metastases of central nervous system tumors. 27,28Once could envision the use of 5-ALA in combination with endoscopy to perform a tumor biopsy or debulking for separation surgery.While these types of oncologic surgeries have not traditionally been performed endoscopically, it can be inferred that this is at least partially due to issues with differentiating normal and neoplastic tissue through a small endoscopic cannula.0][31] The use of fluorescence-based imaging has the potential to mitigate the risks associated with such procedures and aid in adequate decompression.

Indocyanine Green
ICG has become a mainstay in open neurosurgical vascular surgery. 32ICG is a near infrared dye, with 2 key attributes that make it attractive for vascular neurosurgery: (1) when administered intravenously, ICG binds to globulins keeping it in the intravascular compartment, and (2) the absorption and emission peaks of ICG lie within a range in which absorption from  endogenous chromophores is low. 32Therefore, when ICG is administered and the operative field is illuminated by light with a wavelength within the ICG absorption band, ICG fluorescence occurs. 32Like with 5-ALA, fluorescence then is detected using a camera with a filter that blocks ambient and excitation light, allowing for visualization of vessels of interest (Figure 6). 32,33erhaps more relevant to this discussion is ICG usage to identify peripheral nerves.This application leverages ICG's affinity for intravascular globulins to highlight the vasa nervorum of peripheral nerves.Specifically, it has been described in the identification and decompression of the superior cluneal nerve.The identification of this nerve is classically challenging due to its small caliber and because it is contained within a layer of fat. 34y using intraoperative ICG with an operative microscope, the nerve can not only be identified and targeted, but the adequacy of the decompression can then be assessed. 34This use is extremely compelling with regard to endoscopic spinal surgery.

Fluorescein
Intrathecal fluorescein has long been a mainstay of endoscopic skull-based surgery and is used predominantly to identify cerebrospinal fluid (CSF) leaks.When injected intrathecally at low doses, it causes a yellow pigmentation of the CSF, allowing for direct visualization of the fistulous point (either under ambient OPERATIVE NEUROSURGERY room lighting or traditional white-light microscopic illumination).While there are complications associated with higher doses (seizures), its safety profile intrathecally has been well established in the literature.This traditional use of fluorescein, however, only partially leverages its full capability.
More recently, there have been technological advances that allow the surgeon to take advantage of the fluorescent properties of fluorescein. 35Carl Zeiss Meditec developed a fluorescence module integrated into the microscope to visualize fluorescence while maintaining near-natural coloration of surrounding tissues. 35This module, coined YELLOW 560 (Carl Zeiss Meditec, Oberkochen, Germany), is optimized for fluorescence stimulation in the 460 to 500 nm wavelength and detection in the 540 to 690 nm range: ranges that align with the fluorescent properties of fluorescein. 35While this technology has not yet been applied to spinal endoscopic neurosurgery, the potential value in highlighting critical neural elements is apparent.
As an example, fluorescein could be injected into the subarachnoid space preoperatively, and an endoscopic camera equipped with the YELLOW 560 module could then be used to detect the nerve root sleeve and thecal sac intraoperatively.Alternatively, fluorescein could be used to assist in the endoscopic repair of dural tears causing pseudomeningoceles.While dural tears are typically repaired at the time of injury, persistent pseudomeningoceles may require CSF diversion or re-operation and exploration. 36,37Although these re-operation surgeries are typically performed in an open fashion, endoscopic repair of such injuries should be considered.One of the major challenges of an endoscopic repair, however, would be identification of the fistulous point.Indeed, while there is already literature on the endoscopic repair of such dural injuries, this literature typically refers to repair at the time of injury-where the site of leakage is clear. 38Identifying these leaks endoscopically at a later date poses significant challenge and may be aided by the use of fluorescein.

IMAGE PROCESSING Three-Dimensional Endoscopy
0][41][42] One benefit of 3D endoscopy is depth perception to estimate relative distances, which can only be accomplished in traditional endoscopy by moving the endoscope. 40hree-dimensional endoscopy differs from traditional endoscopy in the way in which images are obtained, processed, and visualized.Whereas 2D endoscopy involves capturing, processing, and displaying a single image, the same is performed for 2 sets of images in 3D endoscopy, termed stereoscopic encoding.Capturing a 3D image requires the use of at least 2 distinct cameras to yield 2 separate images.These 2 cameras are oriented at different angles to converge at the point equivalent to the focal length of each camera.The 2 images are then visualized with the image from the left camera projected into the left eye and the image from the right camera projected into the right eye, usually with 3D glasses or a virtual reality headset. 39lthough stereoscopic encoding is widely used in 3D endoscopy, there are several drawbacks.Because the point of convergence of the 2 endoscopic cameras must match the focus length of both lenses, tissues within the periphery of the image can appear out of focus, which can make intraoperative orientation and navigation challenging.

DISCUSSION
The development of new, enhanced lighting and visualization techniques is critical to the progression and continued safety of endoscopic approaches to the spine.Expansion of indications and applications of endoscopic spine surgery necessitates a critical appraisal of existing technology and its applications.While optimal trajectories and use of navigation have been described, there has been little attention given to the use of advanced technologies to enhance visualization.
One of the primary challenges, specifically for the early practitioner in endoscopic surgery, is identifying and protecting critical neural structures.This is especially difficult in re-operations, where scar tissue can alter and distort normal anatomy.It is here that we can look to past neurosurgical usage of 5-ALA, ICG, and fluorescein for a conceptual framework of how to proceed.While the intrathecal safety profile of 5-ALA and ICG is not well discussed in the literature, fluorescein, in low doses, has long been used to detect CSF leaks in neurosurgical patients. 43,44The use of fluorescein to aid in endoscopic CSF repairs of the spine is particularly compelling and should be possible with current endoscopic equipment.Additionally, the use of the YELLOW 560 module in conjunction with intrathecal fluorescein has the potential to improve visualization of critical neural elements during complex endoscopic cases.
Likewise, the use of other fluorescent agents, such as 5-ALA and ICG, offers opportunities for enhanced visualization in spinal endoscopy.ICG has already been leveraged to identify the vaso nervorum of peripheral nerves, 34 and there is no reason that this technology could not be similarly applied to endoscopic spine surgery to validate effectiveness of decompression.Unlike fluorescein, these agents can be administered intravenously, and therefore do not require intrathecal access to harness their benefits.The usage of 3D endoscopy in spine surgery is also compelling and may have a synergistic effect with existing technologies.
This paper represents an effort to synthesize existing enhanced visualization technologies and suggest how they might be applied to endoscopic spinal surgery.Needless to say, these techniques will require rigorous investigation before they can be adopted and integrated into endoscopic practice.We do, however, believe that a discussion of these techniques, even ahead of formal validation, is both appropriate and relevant and will hopefully lead to further scientific discourse.

CONCLUSION
As endoscopic spine surgery becomes increasingly prevalent, it is critical that technological advancements are made to improve the safety and efficacy of these procedures.In this regard, there are several enhanced imaging technologies such as fluorescence imaging, optic-based chromoendoscopy, and endoscopic tattooing that offer significant promise.Each of these technologies offers their own challenges, and future research should be directed towards the safety and efficacy of these techniques with regard to spinal neuroendoscopy.

FIGURE 1 .
FIGURE 1.Total internal reflection A occurs when the angle of incidence with respect to the normal (θ ) is greater than the θ c , the critical angle B. Total internal reflection will not occur if the angle of incidence with respect to the normal is equal to or less than the critical angle C.

FIGURE 2 .
FIGURE 2. Image demonstrating field of view and optical angle of an endoscope.

FIGURE 3 .
FIGURE 3. Characteristics of the commonly utilized commercially available endoscopic systems.

FIGURE 4 .
FIGURE 4. Intraoperative endoscopic view of a bladder tumor from Diorio et al22 demonstrating the view of a tumor using a conventional endoscope white light (left), compared to the view using NBI where the tumor and surrounding vasculature are clearly highlighted in a green-blue color (right).Reprinted with permission from: Adjunctive use of narrow band imaging during transurethral resection/vaporization of bladder tumors to aid in identifying mucosal and sub-mucosal hypervascularity; Diorio GJ, Canter DJ; Can J Urol.; volume 62, issue 2, pages 7763-7766; 2015.22

FIGURE 5 .
FIGURE 5. Intraoperative photo from Schatlo et al 26 study demonstrating fluorescent tumor tissue (pink/red) as visualized under violet-blue light with a filter.Normal surrounding tissue can be seen as blue/violet.Reprinted from: 5-aminolevulinic acid fluorescence indicates perilesional brain infiltration in brain metastases; Schatlo B, Stockhammer F, Barrantes-Freer A, et al 26 ; World Neurosurg., volume 5, page 100069; C 2020.With permission from Elsevier.

VOLUME 21 |FIGURE 6 .
FIGURE 6. Intraoperative photos from Senko et al 33 demonstrating the normal anatomic view of an aneurysm and surrounding vasculature (left), compared to the enhanced visualization of vessels following administration of ICG as seen under a microscope using a camera with a filter (right).Images taken prior to aneurysm clipping are given on the top and after aneurysm clipping are given on the bottom.Reprinted from: Intraoperative rupture cerebral aneurysm and computational flow dynamics; Senko I, Shatokhin A, Bishnoi I, et al; Asian J Neurosurg., 33 volume 13, issue 2, pages 496-498; C 2018 Asian Journal of Neurosurgery, reprinted under CC-BY-NC-SA-4.0 license (https://creativecommons.org/licenses/by/4.0/).