Rainbow scale in computer-aided sinus surgery.

The introduction of functional endoscopic sinus surgery in the early 1980s revolutionized the surgical treatment of sinonasal disorders. This procedure also produced unexpected surgical complications. The surgical creation of drainage sites in the paranasal sinuses brings a concomitant risk of injuries to the vital surrounding structures of the skull base, the brain, and the orbit.

The development of computer-aided navigation systems has added another helpful tool to the operating surgeon's armamentarium. [1-3] The tip of an instrument can now be localized in real time to a computed tomography (CT) representation on a computer screen in the operating room with reasonable resolution.' The technique can be performed with a probe, a suction tip, or a microdebrider blade.2 Although this has not replaced the need for experience and a thorough knowledge of anatomy, the use of image-guided systems for endoscopic sinonasal surgery can help to educate the novice, give additional information to the experienced surgeon, and prevent orbital and intracranial complications.

A recent innovation in the visual output for image-guided surgery is the use of a color scale to represent the CT image (figure 1). This provides a different level of contrast from traditional gray-scale imaging. It is called the rainbow scale.

The rainbow scale was intergrated into the StealthStation System by engineers at Sofamor Danek (Memphis, Tenn.) as a method for enhanced visualization for their computer image-guided system for endoscopic sinus surgery. The rainbow scale is a processing standard that has been applied to density data to improve visual clarity. In conventional gray-scale CT imaging, the different tissue densities are presented in shades from black to gray to white (figure 2). In the rainbow scale, these differences are applied across the visible spectrum. This can improve visualization because it can be easier to differentiate between two different colors than between two shades of gray. The rainbow-scale colors are red, orange, yellow, green, blue, indigo, violet, and white.

In the case presented here, several representative trends in color are seen (figure 1). The patient was found to have fungal sinusitis that involved the sphenoid, posterior ethmoid, and maxillary sinuses on the right. In the patient's nonpathologic sinuses, the mucosal surfaces appear as red. The diseased mucosa appears as blue. The area of fungal debris in the sphenoid, posterior ethmoid, and maxillary sinuses is green. It is interesting to note that the brain is also green. As is true in gray-scale images, the bone density is white and the air is black. Inspection of the CT image shows a significant qualitative improvement in the resolution when viewing the borders of adjacent structures such as normal mucosa over underlying bone. This is more easily appreciated as a red line over white rather than represented in gray.

The potential benefits of the rainbow scale have yet to be elucidated. At the very least, it is a visually pleasing representation that makes viewing CT images much easier. It is possible that color differences may correspond reasonably well to disease states, adding a diagnostic dimension. These possibilities will be determined as experience with the rainbow scale increases.

From the Halifax Medical Center, Daytona Beach, Fla. (Dr. Christmas and Dr. Mirante), the Department of Otolaryngology, University of South Florida College of Medicine, Tampa (Dr. Christmas), and the Southern New England Ear, Nose, Throat, and Facial Plastic Surgery Group, New Haven, Coon., and the Section of Otolaryngology, Yale University School of Medicine, New Haven (Dr. Yanagisawa).

References

(1.) Metson R, Gliklich RE, Cosenza M. A comparison of image guidance systems for sinus surgery. Laryngoscope 1998.;1O8:1164-70.

(2.) Gunkel AR, Freysinger w, Martin A, et al. Three-dimensional image-guided endonasal surgery with a microdebrider. Laryngoscope 1997;107:834-8.

(3.) Anon JB. Computer-aided endoscopic sinus surgery. Laryngoscope 1998;108:949-61.