Total Systems Miniaturization

IMAGING

That crunching noise you hear these days is the sound of endoscopes shrinking. Medical diagnostic and therapeutic procedures are growing smaller, from neurology to podiatry and everything in between. Minimally invasive surgery (MIS) is shrinking to the point that incisions can heal without sutures, and the new words of the day are “endoluminal” and “NOTES” (natural orifice transluminal endoscopic surgery), both implying procedures done via natural body openings, with no external incisions.

Companies driving these trends include innovative small players like Endo-Optiks (retinal laser surgery) and Epitek (cardiothoracic), and industry giants like BioMet (cranio-maxillofacial and sports medicine) and Covidien (advanced MIS). And each new procedure seems to out-shrink the last. BioMet Microfixation has just released the OnPoint system for diagnostic arthroscopy of the temporomandibular joint, for example, requiring an incision smaller than 2 mm. Covidien has received 510(k) clearance to market its SILS Port multiple instrument access port for laparoscopic surgeries through a single umbilical incision. Some procedures, traditionally requiring hospital stays, are being addressed on an outpatient basis, often in a doctor's office or surgical center. Other procedures that once required weeks of hospitalization now require just a few days.
But how do the doctors performing these miracles see inside their patients? Various new and experimental technologies seek to give doctors with no direct visual access the equivalent of x-ray vision (see the sidebar “Paths to X-ray Vision”). But for most of these new procedures, the workhorse of visualization remains the venerable endoscope. The reasons for the use of endoscopy are legion, but some factors are fundamental.
Direct visualization via endoscopy provides the best, clearest image for doctors. It is the gold standard against which other technologies are weighed for effectiveness. For example, in comparisons between MRI and spectroscopy systems, arthroscopy has been used to judge the performance of each.¹ According to literature available, endoscopy demonstrates the following attributes:
• Offers a track record of procedural success.² • Enables real-time visualization to precisely guide placement, as well as use of instruments, vastly reducing potential for malpositioning or damage.³ It also enables doctors to see as a patient is manipulated (e.g., as a shoulder is rotated during surgery, via arthroscopy). • Provides greater portability than other similar technologies. Portability expands potential applications in doctor's offices, ERs, battlefields, and other places where space is at a premium.
Together, these factors mean endoscopy offers accuracy, flexibility, and breadth of use, particularly in therapeutic applications. Many studies bear out this claim. Magnetic resonance images (MRIs) of musculoskeletal joints including soft tissue, for instance, are notoriously problematic. Estimates of false positives for pathological knee MRIs reach as high as 20%, creating complications for patients and adding costs for insurance providers.⁴ In some studies, MRIs barely outperformed clinical examinations.⁵ In other areas, such as diagnosis of scapholunate ligament injury, MRI failures have been severe enough that studies have concluded they simply should not be used.⁶
Despite these well-known studies, endoscopy has historically been underused in many applications. MRIs still dominate advanced diagnosis of knee and shoulder pain. Why? Because traditional arthroscopy has been perceived as highly invasive, requiring fairly large scopes (2.7–4 mm diam), and thus full anesthesia in a hospital or surgical center, conversely MRIs are noninvasive. Some MRI centers are also linked with doctors' offices, owned by the doctors who prescribe using them. In some cases, such practices may add cost to the system without improving diagnoses. Given the perception that arthroscopy requires hospitalization or at least an advanced surgical center, doctors have not traditionally viewed arthroscopy as a comparable potential source of office revenue. But the perception of endoscopy as highly invasive is changing rapidly.
The Incredible Shrinking Endoscope: A Brief History
Endoscopes (essentially, minitelescopes adapted for use inside the human body) have a long history, stemming from the first attempts to look inside patients using catheters in ancient China and Egypt.⁷ Throughout this history, key innovations have created turning points that transformed the possibilities of endoscopy. Medieval Persian doctors, for instance, first focused light through catheters (using sunlight reflected off polished glass). Renaissance doctors lined catheters with telescopic lenses inspired by Galileo. In 1806 Phillip Bozzini, the “father of modern endoscopy,” added reflecting lenses, bouncing images from inside patients back to the doctor's eye. This innovation led to a technical explosion: by 1930, gastroscopes, laparoscopes, uteroscopes, and early arthroscopes were standard tools. They were generally rigid instruments made of solid lens relays of 10 mm or greater in diameter (large, rigid scopes are still used in some applications—today they have become commodity technology).
Further turning points include the development of semirigid and flexible “fiberscopes” using fiber optics, and crucially, the connection of endoscopes to video cameras and display systems in the 1970s by Camran Nezhat, MD, the “father of operative video-laparoscopy.”
Today, integrated endoscopic systems are going through another historic turning point, via technical innovations considered impossible even a few years ago. As recently as 2005, a 3.5 mm prototype was touted as the world's smallest endoscope.⁸ Today these innovations allow the creation of true endoscope systems at sizes less invasive than many hypodermic needles. These next-generation integrated microendoscopy systems are small enough to enable nonsurgical endoscopy, or endoscopy performed without full anesthesia or a fully equipped operating room. Such systems can be designed to use ordinary light, or to offer autofluorescence, laser guidance, and other technologies that actually enhance the doctor's vision beyond that of normal eyesight.
Nonsurgical endoscopy technology has advanced in the ability to make ever-smaller imaging systems, including scopes that measure 0.45 mm OD and, in some cases, smaller. With such small optical components, device manufacturers can concentrate on other procedural and therapeutic tools needed in the surgical channel, rather than on imaging. In arthroscopy, for instance, not long ago a 4-mm scope was considered small, and a 10-mm incision in the knee was expectable for patients. With the new sub-2-mm imaging systems now available, a 2-mm incision is normal.
Nonsurgical technology also has applications in major surgical procedures performed in minimally invasive fashion. For instance, one cardiothoracic procedure currently undergoing FDA review uses a nonsurgical endoscope inserted arterially, to enable clear imaging of the inside walls of a beating heart. This procedure has potential to revolutionize certain forms of heart surgery by allowing clear visualization for doctors who until recently used only static images generated via MRI.
As a result of these many advantages, nonsurgical endoscopy is rapidly becoming a key visualization tool enabling endoluminal and NOTES procedures.
Technical Challenges

Nonsurgical endoscopy is an exciting technology, but it has been hampered somewhat by technological challenges. When endoscopic systems are shrunk beyond the limits of traditional endoscopy (say, to sizes below 1.5 mm OD), pixelization and brittleness can occur. Microdiameters limit fiber size and the ability to carry light. Smaller systems mean less room for everything: light fiber, image fiber, coatings, and other materials that increase durability.
There are also challenges in trying to connect microendoscopes to the cameras that doctors use, and ultimately to the endoscope towers that offer video image display and capture options. Working with scopes in the 1-mm range has been likened to trying to control a strand of cooked spaghetti.
In addition, microendoscope systems can be difficult to manufacture reliably and with cost stability. Making a small number of experimental prototype microendoscopes that sell in the $7000 range is one thing. Finding reliable, repeatable manufacturing technologies that allow production runs in the thousands and scope prices in the $250–$500 range is a challenge. But it's the mass-produced scopes that can benefit doctors, patients, and insurance companies.
All of the difficulties associated with designing and manufacturing microendoscope systems must be resolved in an integrated way that provides seamless end-to-end imaging and service from the tip of the endoscope to the plug at the end of the image display and capture tower. There are several design challenges to overcome.
Light Sources Require Specialized Design for Small-Diameter Scopes. A standard xenon 300-W light source has a 5-mm fiber optic cable to carry the light to the scope. But a sub-1-mm scope with 80 light-carrying fibers of 30 μm each, won't carry most of that light to the end of the scope where it is needed.
Pixelization and Other Image Quality Problems Multiply at Small Diameters. Scopes must be properly integrated with digital display and image capture systems and must not be over- or undermagnified. Overmagnified optical images look pixelated (like looking through chicken wire) and dull. Undermagnified images are too small to use. It's critical to strike a balance between the size of the pixels in the fiber and the size of the pixels in the charge-coupled device (CCD) in the camera. This is because the active area of the CCD determines the overall size of the image. Because the magnification of any given scope is fixed, the size of the CCD determines the magnification of the image. A ½-in. CCD camera presents an image on a monitor half the size of a ¼-in. CCD. Without the proper balance, the final image looks pixilated and grainy.
Mismatches between Camera Power and Fiber Size Are Compounded at Microdiameters. Small fiberscopes have relatively few pixels in the image bundles compared with cameras. Magnifying the image to fill a display would show fiber pixels as overly large. There can also be interference between scope pixels and camera pixels, causing an unnerving moiré effect.
Ergonomic Concerns Multiply at Small Sizes. A tiny scope linked to a heavy traditional DIN coupler or handpiece can be unwieldy and tiring for a surgeon. In addition, marrying a tiny scope to a large surgical imaging tower greatly reduces fine control of the scope. One solution to the problem is to increase the length of the scope and fix the camera head and light cable, but that also increases the cost of the scope and makes it more fragile.
Materials Issues around Optics Become Critical. Many traditional smaller endoscopes are based on gradient index (GRIN) objective lens technology, which has poor optical performance compared with traditional lenses (often resulting in an image with poor aperture control and large chromatic aberration). Another alternative is solid GRIN rods, which offer high resolution, but are extremely brittle at small diameters—especially if integrated with a heavy camera.
Effective Sterilization becomes Challenging at the Microscale. Small scopes can be easily damaged when grouped with other heavier instruments in trays during either sterilization or transport from procedures, and sterilization validation remains complex.
 
The challenges presented by microendoscopy can be met with design and manufacturing techniques that fully integrate the microvisualization system, from the tip of the scope to the plug at the end of the image display system. The process must focus on the specialized needs of microendoscopy, including weight, digitization, pixelization, brittleness, and sterilization planning.
Light Source and Light Guide Optimization. The light delivery system for small scopes must be considered as a whole. Many small scopes suffer from difficulties in illuminating the field of view evenly, and thus ensuring a crisp image from edge to edge. This problem is addressed by matching the characteristics of the lamp, light cable, and light fibers, to minimize total system losses. Small scopes also have small illumination bundles, so a properly optimized low-power light source can be used as effectively as the high-power large sources found in traditional systems. Small light sources are at an advantage because they require less power than large lights, as well as generating less heat and using smaller fans. The operation is quieter and lamp replacements cost less than their larger counterparts. Keep in mind, however, that small scopes sometimes require specially designed light sources, which can add to the total cost.
Integrated Camera and Fiber Choices. The scope and camera systems must be considered in an integrated way to reduce the optical magnification issues associated with tiny light fibers. Today's surgical cameras have incredible image quality, in both resolution and color reproduction. Camera resolution goes up to 1080 pixels. The cameras are designed and programmed to work with solid glass optical scopes, from 2.7 to 10 mm. The requirements for 1-mm fiberscopes are much different in optical magnification, lighting, and camera programming. Again, appropriate matching of camera and optical fibers is required for balance of image magnification. The difficulty is that balancing image magnification often results in small images, to ensure a reasonable match to screen resolution. This can create a marketing challenge, in that doctors may resist images smaller than they are used to. However, because nonsurgical endoscopy often opens views into places where doctors could not see into at all before, most believe that even relatively small images are a big step forward. Smaller images also help brightness. Small scope images can seem dim if magnified too much.
Ergonomics. Surgeon fatigue should be considered, even though the small size of the scopes would seem to belie such concerns. But in fact, working extremely light and flexible scopes with heavy cameras and handpieces can be very fatiguing and can also be a significant cause of breakage. Hand-pieces, cameras, cables, connectors, and towers should all be designed specifically to match the needs of small scopes.
Materials Supplier Selection. Companies familiar with small scope technology understand the challenges of selecting appropriate materials. Currently, image fiber in the bundle sizes required for nonsurgical endoscopy comes from only two sources: Fujikura and Sumitomo. However, some larger-scale fiber suppliers are beginning to offer specialty fibers that fulfill particular scope needs.
OEMs should also consider alternatives to GRIN lenses. For example, advanced microoptics manufacturing technology allows some suppliers to develop multielement objective lenses using standard optical glass at sizes down to 0.45 mm. Finally, given the small margin for error in microsized scopes, extreme care is also critical in selection and use of epoxies, steel tubing, and light fibers.
Sterilization and Disposability. The challenges in sterilizing extremely small scopes call for additional measures. Such measures may include warnings, and tools (such as specially designed trays to protect the instruments during sterilization). Another option, if manufacturing costs can be controlled, is to offer disposable microendoscopes, eliminating the need for sterilization.
The Rubber Meets the Road

Effectively downsized systems tend to be lighter, more portable, and more cost-effective than their traditional counterparts. Integrated systems can also be sold as a package, so purchasers don't have to wade through separate camera, scope, light source, display tower, and image capture options. These factors may make integrated microvisualization systems applicable not just for less-invasive hospital-based surgical applications, but also for doctor's offices, surgical centers, ERs, and other environments in which space, cost, and weight are major considerations.
Indeed, despite the challenges, a growing number of companies are offering microendoscopic products and systems, with more efforts toward total systems miniaturization under way. Olympus, for example, a leader in capsule endoscopy, also provides a broad range of wired endoscopy systems, some as small as 3.2 mm, with a strong focus on videoscopes. Smith & Nephew advertises itself as the leader in microarthroscopy, offering drill guide systems and other tools for ever-less-invasive small joint procedures, most in the 2–4-mm range. Karl Storz offers scopes for minimally invasive specialties including neuroendoscopy, urology, and pediatric and spinal applications. Richard Wolf offers scopes as small as 2.7 mm for gynecological use. Stryker offers 1.9-mm miniendoscopes for maxillofacial, ankle, and wrist arthroscopy, and a 2.7-mm high-definition laparoscope and hysteroscope. BioVision Technologies has FDA approval on a fully integrated system linking a 1.2-mm microendoscope to a lunch box-sized endoscope tower that weighs less than 10 lb.
Looking Forward: Future Trends in Nonsurgical Endoscopy
The incredible shrinking endoscope cannot shrink indefinitely. Scopes have been produced (by Zibra) as small as 0.4 mm in diameter, but their medical uses are limited. In fact, below 0.45 mm, imaging begins to approach the theoretical limit of visible light manipulation. One key reason is that image fiber is approaching the practical limit of pixel density and core-to-cladding ratios. Each pixel is a separate fiber optic. As the thickness of the core decreases to increase pixel density, the cladding around the core that separates light-carrying elements is also reduced. If cladding becomes too thin, light bleeds between fibers, reducing overall image clarity (by contrast, large leached-bundle scopes, built with large individual fibers, produce very bright and crisp images, albeit with a “chicken wire” effect between pixels).
There is significant room to expand the use of autofluorescence, laser guidance, and other related vision-enhancing technologies in nonsurgical endoscopy, and various manufacturers are exploring these possibilities. Other improvements may come in the area of ergonomics, especially in the potential of wireless technology to eliminate cable clutter. Stryker Endoscopy, for instance, recently launched a wireless high-definition surgical display. Wireless technologies, as they continue to evolve, could play a large part in addressing ergonomic issues in the microvisualization industry.
But the biggest new developments in the field are likely to involve expansion of applications. Already, the technology is supporting commercial procedures in arthroscopy, cardiac surgery, ophthalmology, nephrology, and cranio-maxillofacial surgery. As the potential becomes better understood for such procedures in office and outpatient settings, including the benefits of presterilized single-use scopes, applications will broaden. This trend will further expand opportunities for medical device manufacturers to sell customized procedural kits. Procedures enabled by nonsurgical enoscopy often require specialized trocars, cannulae, stapling, or suturing, due to the smaller areas being investigated, the smaller incisions, and the minimal anesthesia required.
Doctors have been breaking the skin barrier in the office since the hypodermic needle. Today, the technology is the size of an 18 AWG needle and disposable, with image quality approaching that of a $3000, 2.7-mm glass arthroscope. The new procedures enabled by nonsurgical endoscopy offers the potential for a paradigm shift away from large traditional glass endoscopes. Innovative diagnostic and therapeutic applications will reduce costs for insurers, ease patient pain, and speed healing, while also increasing revenue for doctors and device manufacturers alike.
Rebecca Weiner supports global sourcing of optics for BioVision Technologies (www.biovisiontech.com; Golden, CO). She can be reached at [email protected].
References

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