Advances in endoscopic visualization and surgical navigation

Endoscopic visualization: Historical context

Today, endoscopic endonasal surgery is an important component of skull base surgery. This development has only been possible because of constant evolution of visualization capabilities together with deeper knowledge of anatomy and refinement of surgical tools and techniques. High-fidelity visualization of the internal nasal structures has been a longstanding goal to enable diagnosis and treatment of rhinological and skull base pathologies. ^, Historically, the first known transnasal procedures were performed by the ancient Egyptians when the intracranial contents were removed through the intranasal corridor as part of the mumification process. Illumination through the long, narrow nasal corridor has always been a major challenge, and the earliest procedures were done with the patient’s nose tilted toward sunlight to allow the passage of light. Almost 2 millennia later, Dr. Antonin Jean Desormeaux first described the use of an endoscope to perform a urologic examination, with the device providing continuous illumination along with a lens to narrow and intensify the field of light. After World War II, Harold Hopkins developed flexible fiberoptics using silica that allowed less light loss and better image quality. Hopkins also developed the rod-lens system in 1959 that allowed even better image quality and brightness, a wider field of view, and a smaller diameter lens, changes that were promptly incorporated by the German equipment manufacturer Karl Storz in 1965. ^, Using the Hopkins endoscope, Dr. Guiot from Paris, who also introduced video fluoroscopy in pituitary surgery, performed the first endoscopic approach to the sella using a fiberoptic endoscope. However because of rapid advancements in the operating microscope, the microscopic transsphenoidal approach became the gold standard visualization tool for transsphenoidal and transcranial approaches to skull base lesions. ^, However, in 1997, Dr. Ricardo Carrau and Dr. Hae-Dong Jho from Pittsburgh described the first case series of fully endoscopic endonasal pituitary surgery. The technique was revolutionary because it allowed a wider field of view and the ability to look around corners, expanding the possibility of transnasal skull base surgery. Since then, endoscopic endonasal surgery is widely adopted as visualization technology, and surgical instrumentation has greatly improved along with improved understanding of endonasal surgical anatomy and reconstruction techniques. To understand technological advancements in endoscopic visualization, we must define optimal imaging characteristics in terms of illumination, brightness, and resolution. We will also discuss aspects related to stereoscopic three-dimensional (3D) images and adjuncts such as fluorescence.

Illumination

Illumination of the intranasal corridor is essential in endoscopic endonasal surgery. There are a variety of light sources with different spectral emissions and illumination power. In a traditional incandescent light source, the light originates from heating a wire filament, and the color of the light varies with the temperature of the source, with higher temperatures resulting in brighter light and more accurate images (blue specter). The advent of halogen allowed higher temperature and longer bulb duration. Xenon provided the next advancement in illumination. Xenon sources work by combining an electrical charge with a xenon gas emitting light. Currently, most endoscopes, microscopes, and exoscopes use a 300-W xenon valor (6000 K) that provides excellent illumination for visualization and photo documentation because of the high content of blue spectral light.

Although the xenon source light has been commonly called “cold light” compared with traditional halogen light sources, a significant amount of heat is still generated with the potential to cause thermal injury to tissue as has been well described in the literature. ^, For example, in a study by Kurita et al. using a 300-W xenon source microscope in an experiment of illumination of a free flap model in rats, it was found that prolonged light on the vascular pedicle resulted in a significantly higher number flap failures from venous thrombosis, with the temperature in the tissue reaching 80ºC. This study highlights the need to balance image quality with the potential negative consequences of excessive heat generation. Newer systems use light-emitting diode (LED) light sources that generate less heat (maximum temperatures of ~46ºC at the tip of an endoscope) while maintaining light quality ( Fig. 40.1 ).

Brightness

Another aspect of illumination that is relevant to endoscopic surgery is brightness, that is, the perception elicited by the luminance of a visual target. In other words, brightness is the intensity of the light transmitted back into the endoscope from the visual target and is a function of the intensity of the light source and the distance to the visual target. Doubling the distance between the endoscope and the visual target increases the dispersion of light and reduces brightness fourfold. The surgeon must maintain a balanced surgical distance to reconcile adequate brightness and resolution with adequate ergonomics and avoidance of instrument conflict. ^,

Resolution

Resolution is a concept specific to camera and monitor capabilities and defines the complexity of the image detail. Resolution is measured in DPI (dots per inch), in which the higher the dot density, the higher the resolution of the image. The DPI measurement has a direct correlation between the horizontal screen resolution in pixels (720, 1080, 2160) and the size of the screen. ^, Current endoscope systems are high definition (HD) (1960 × 1080 pixels). Recently, new 4K video systems, also called ultra HD (UHD; 3840 × 2160 pixels), were developed with the intention to enhance surgical and anatomic visualization. However, the viewing distance and screen size are imperative to understand the benefits of increasing resolution as perceived by the human eye. Figure 40.2 demonstrates the capability of the human eye to appreciate higher resolution as a function of viewing distance and screen size. It is reported that the average distance between a surgeon and the viewing surgical monitor is 3 to 8 feet. If using a 30-inch monitor, for example, the surgeon would only notice any improved resolution from a 4K system when standing less than 4 feet away from the monitor. Conversely, considering a fixed viewing distance of 5 feet, a surgeon would only notice improved resolution from a 4K system when using a screen size larger than 40 inches. In general, a larger viewing monitor is desired to be able to perceive the advantages of higher resolution. ^,

There are few reports in the literature scientifically evaluating 4K endoscopic systems. As discussed, 4K systems provide nearly fourfold increase of information per frame compared with standard HD images. Such detail may potentially be valuable when evaluating very fine detail such as the tumor–mucosal interface and intradural anatomy. One other potential advantage is the ability to increase digital zoom while still achieving sufficient resolution. This may reduce instrument conflict with the endoscope during delicate two-handed intradural dissection through small corridors (e.g., optic chiasm–pituitary gland window in craniopharyngioma surgery) by allowing the endoscope to sit farther back in the sphenoid sinus while using digital zoom to crop and magnify the region of interest with sufficient resolution. Another advantage of this system is that there is no learning curve, and it does not change the perspective of surgical visualization. In addition to the expense, possible disadvantages are the need of larger monitors that require space in the operating room (OR) and the fact that the 4K video files tend to be quite large, delaying video transfer and requiring a large amount of hard drive space. ^{, ,} In the near future, UHD 8K monitors may be on the market, providing 16-fold higher resolution than HD.