Minimally invasive surgical techniques have been in use for more than 100 years, so what advances can surgeons expect within the next decade?
The first recorded use of a laparoscopic instrument in a human is generally credited to Hans Christian Jacobaeus in 1910. As is often the case, the understanding of the concepts involved in minimally invasive surgery (MIS) came well before the technologies and equipment that would make it a standard procedure. In fact, more than 60 years after Jacobaeus’s first human laparoscopic surgery, Kurt Semm was under fire from his colleagues for promoting the use of laparoscopic surgery. As related by The New York Times: “In 1970, after Dr. Semm became the chairman of obstetrics and gynecology at the University of Kiel, his co-workers demanded that he undergo a brain scan because, they said, 'only a person with brain damage would perform laparoscopic surgery,' Dr. Mettler said.”1
Although the theory of minimally invasive surgery has been available for more than a century, it is only within the last 30 years that it has become accepted within the field. As a young field surrounded by ever expanding technological advances, there are many exciting developments in process and on the horizon. This article examines one of those developments, the advent of stereoscopic (3-D) endoscopy.
We begin by examining some of the most recent findings about the efficacy of 3-D endoscopy. We then discuss some of the unique challenges that are introduced by adopting this technology (challenges that are inherent to stereoscopic imaging as a whole, as well as some equipment specific challenges). Finally, we consider how the same trends that enable stereoscopic endoscopy may prove critical to another currently debated technique, natural orifice surgery (NOS, or scarless surgery).
Operating principle of a single-objective 3-D endoscope system. The viewer is looking down towards the objects imaged on the sensor. Each of the colored rays represents the center of mass of the cone of light that reaches the sensor as the optical modulator switches between the right view and the left view states. Note that the optical modulator selects different angles for the light rays in each view, creating separate viewpoints within the single lens. Projecting the right view image to the right eye and left view image to the left eye generates a stereoscopic image. Click image for larger view.
An Introduction to 3-D Endoscopy
At first glance, introducing depth information to minimally invasive surgical procedures would seem to bring a host of benefits to those procedures. After all, surgeons work in a 3-D space within the patient, yet standard monitors show only a 2-D plane, requiring the surgeons to learn to interpret the nonstereo depth cues available to intuit the third dimension while performing surgeries.
However, such a task is not unique to surgery. People are trained to do something similar when watching television or movies, a medium that asks us to understand a 3-D landscape via a 2-D display. As a whole, people are remarkably good at understanding gross-depth relationships even when presented on completely flat display surfaces.
To understand why this is the case, we must consider that while there are 14 cues that our brains use to determine depth and depth order, only three of these require stereoscopic vision. The other 11 can be determined and interpreted without left-eye/right-eye image disparity. 2
What those 11 monoscopic cues don’t provide, however, is the ability to accurately judge distances between objects on the z-axis (i.e., judging how far ahead or behind an object is from another). Judging distances on the x- and y-axis in 2-D is fairly easy, as the objects on screen provide easy reference points to measure against one another, but there is very little depth information provided.
Efficacy of Stereoscopic (3-D) Endoscopy
Perhaps not surprisingly, evidence pointing towards the efficacy of introducing 3-D to surgical procedures is mixed. Many studies report a subjective preference for 3-D, though few show a statistically significant performance increase in any objectively measurable area among experienced surgeons. The often small sample sizes of the studies also present problems, with some studies using as few as six experienced surgeons in the testing groups.
There is one common theme; while there are no statistically significant performance improvements among experienced surgeons, novice surgeons may experience lower error rates in some tasks. In addition, all groups report improved depth perception (as would be expected).
For example, in a study involving 21 novice and 6 experienced surgeons, a report comparing 2-D and 3-D camera systems in laparoscopy stated, “The 3-D system provided significantly greater depth perception than the 2-D system. The errors during the two tasks were significantly lower with 3-D system in novice group, but performance time was not different between the 2-D and 3-D systems. The novices had more dizziness with the 3-D system in first two days. However, the severity of dizziness was minimal (less than 2 of 10) and overcome with the passage of time. About 54% of the novices and 80% of the experienced surgeons preferred the 3D system.”3
Similarly, a study involving 13 patients undergoing endonasal endoscopic transsphenoidal surgery found “no significant differences in operative time, length of stay, or extent of resection compared with cases in which a 2-D endoscope was used. Subjective depth perception was improved compared with standard 2-D scopes.” 4
While various studies report different reactions to 3-D endoscopy among experienced surgeons, almost all note increased reporting of depth-perception accuracy among novice and experienced surgeons alike, greater task performance among novices, and no performance reductions when using 3-D as opposed to 2-D endoscopes.
Nelson Oyesiku, MD, PhD, FACS of Emory University addresses the subjective benefits, stating “The reason why three dimensional is important is that it tells you where you are in space. That could be critical in an area where you have a lot of stuff around, like in the pituitary, where you’re surrounded by blood vessels and nerves, and the last thing you want to do is to stumble into an artery because you just didn’t know that it was that close. So the 3-D takes from the 2-D endoscope and it takes from the 3-D microscope and marries the best features of both in one instrument.” 5
How 3D Works
Creating a 3-D image is a fairly simple two-step process.
Step One. Capture two images of the same scene with an offset between them (often referred to as a stereoscopic image pair, or just an image pair). These images are referred to as the “left eye” and “right eye” images respectively (see Figure 1).
Step Two. Display the image pair in a way that enables one to view the pair as a single image. This process is accomplished by having each eye see a separate image in the pair (i.e. your left eye will see the left eye image, and your right eye will see the right eye image) and can be achieved in one of three ways:
- Screens working with active glasses display images at twice the normal rate, and the glasses block one of your eyes in synch with the TV display. While the screen displays the left eye image, your glasses block your right eye. These systems are the least expensive to manufacture, because they simply require screen refresh rates that are high enough to accommodate the image switching.
- An alternate option is a passive screen paired with polarized glasses. In these screens, the two images are displayed simultaneously but at different polarizations. The glasses you wear with these screens work by using different polarized filters on each eye, allowing you to see only the right-eye and left-eye images through those respective eyes.
- A third method is known as autostereopsis, or glasses-free 3-D. These screens can work by using technologies like eye tracking, parallax barrier, or lenticular lenses on the screen. These technologies are still in their infancy with respect to autostereoscopic 3-D.
|Figure 1.ISee3D’s core technology consists of modifying a lens system to enable the capture of stereoscopic image pairs through a single lens. By occluding a portion of the lens, it is possible to shift the effective center of the lens–by doing this in sequence with different occluded areas (see images above--left eye, left; right eye, right), it is possible to capture stereoscopic image pairs with a fixed interaxial distance. Click images for larger versions.
The Unique Problems of 3-D
One issue noted with using 3-D technology was a feeling of dizziness that occurred among a small sample of the surgeons in their trial.3 In addition, 3-D systems introduces the risk of dizziness or nausea among some viewers even in short duration exposure (such as a movie or TV show). This issue must be addressed when considering 3-D for use in the operating theater with longer exposure times and much greater repercussions for viewer discomfort.
End-viewer discomfort caused by 3-D has two primary causes—the source material itself (capture errors) and problems unique to the display environment (display issues). Below we’ll consider the causes of end-viewer discomfort and ways to mitigate both of these areas.
Problems with Capturing Images
When capturing images using two cameras or two lenses, it is critical that the lenses are perfectly aligned and calibrated. The most common cause of 3-D discomfort (including dizziness and nausea) is from lens misalignment. Vertical misalignment is the most egregious offender, though horizontal or rotational misalignment can also lead to discomfort.
Mismatches in lens focal lengths, color filters, and aberrations found in one lens of a pair can also cause artifacts that lead to discomfort when viewing a stereoscopic image pair (for example, trying to interpret a scene that has an object visible only to the left eye).
Problems with Display
Three key factors that cause viewer discomfort can be primarily attributed to the display. Two of these are directly related to the 3-D glasses (weight and bulk, and flickering), while the third is an issue of synch between the screen and glasses (crosstalk).
The issue of flickering glasses is constrained to active shutter glasses. These glasses alternate occlusion of the left and right eye in synch with the display of the right and left eye images on the display. This interaction produces a faint but noticeable flickering, which is obvious when looking at anything other than the display. The flickering can cause eye fatigue and may result in end-viewer discomfort.
The second issue, which relates to the weight and bulk of 3-D glasses, is primarily an active shutter issue; tthese glasses generally require a battery, an infrared synch, and liquid crystal elements, causing them to be larger and generally bulkier than their polarized lens counterparts.
Crosstalk, or ghosting, occurs when there is not a complete separation between the left and right images—you may see some parts of the left image “leak over’”to the right side, or vice versa, resulting in what appears to be a translucent object on the screen (hence the term ghosting). This is the result of an imperfect synch between active shutter glasses and the screen.
Addressing Display Issues
Given that the three major causes of display-dependent viewer discomfort come from active glasses and monitors, it is fair to ask why these systems are considered in the first place. The answer is resolution. Given that they display sequential full frames, active shutter systems offer true HD resolution. Passive systems, which must display both the left and right images simultaneously, will always be presenting at reduced resolution (on an HD screen, either at 960 × 1080 or 1920 × 540 pixels).
However, a detailed article by Raymond Soneira, MD recently showed that viewer testing among passive and active systems almost universally showed a preference for passive systems in all areas relating to 3-D quality.6 It is reasonable to conclude that until autostereoscopic displays advance sufficiently to enable wide viewing angles at high resolution, the best display solution is a passive screen.
Addressing Capture Issues
Capture issues are trickier to address, because many of the errors that occur are built into the nature of dual lens capture. Synchronization, alignment, and intensity and chroma calibration will be issues whenever there are multiple optical channels.
One approach to this problem is to eliminate the second optical channel altogether, by using an approach that enables the capture of 3-D through a single lens. For example, one company takes this approach and has developed a single lens 3-D endoscope that measures less than 4 mm in diameter.
The general physics of such an approach are relatively easy to understand—by occluding a portion of the lens, you create a new “center viewpoint” of the lens closer to the edge of the nonoccluded side. If you block the left half of the lens and capture an image frame, then block the right half of the lens and capture an image frame, you’ll have two images from different viewpoints—ideal conditions required to create a stereoscopic image pair.
Since such an approach uses only a single lens, there are no issues with calibration, synchronization, or differences in focal length or color filtering. The image pairs are perfectly matched, enabling high quality, comfortable 3-D without the discomfort issues associated with 3-D captured via dual lens methods.
The movement to a single lens to capture 3-D has the additional benefit of enabling much smaller 3-D endoscopes as well as offset scopes, opening up many 3-D minimally in MIS procedures that would be difficult to accomplish with larger 3-D endoscopes. This advance enables endoscopes to follow the general trend of all electronic technology, which is to become smaller and less expensive while including more processing power than preceding generations.
Applicability in Natural Orifice Surgery
NOS is a proposed MIS technique that uses natural orifices as instrument entry points, as opposed to the current practice of creating an incision to allow the instruments to enter the patient. It is still in very early stages, with some of the first reported successful human natural orifice transluminal endoscopic surgery (NOTES) taking place in 2007.
Since NOS is still in such early stages, there have been no known studies on the efficacy of 3-D versus 2-D endoscopes in NOS. However, we can safely assume that the preference for smaller instruments will be as pronounced or more in NOS as it is in MIS. If 3-D endoscopes are to be used in NOS, they must be able to meet these size requirements.
As evidenced by Oyesiku’s quote about the importance of depth information while performing MIS in areas that are “crowded,” the introduction of 3-D may become more helpful as more surgeries take place in narrower or more crowded environments (as will be the case with many NOS procedures).
Technological Effect on Manufacturing
Advances in technology in general, and endoscopy specifically, have dramatically changed the landscape of MIS. As supporting technologies become smaller in size, more powerful (increases in processing power, resolution, and reliability) and less expensive to implement, device manufacturers are presented with both the opportunity and the imperative to innovate.
As an example, consider field-of-capsule endoscopy. Prior to 2001, it was effectively impossible for medical professionals to examine the entire length of the digestive tract. The emergence of miniaturized camera components enabled the development of a camera that could be swallowed by a patient and then record images at every point in the digestive tract. Capsule endoscopy was approved by FDA in 2001 and has presented a significant opportunity for both endoscope manufacturers and equipment suppliers.
Similarly, the field of robot-assisted surgery has become extremely popular. It has provided opportunities and challenges for manufacturers to redesign existing components to be seamlessly integrated with the robotic surgery system, as well as the requirement for new types of equipment that are not feasible in traditional MIS but required for robot-assisted surgeries.
This trend holds true even outside the field of robot assisted surgeries. As MIS is performed in smaller environments (ENT, neurosurgeries, NOTES, etc.), not only do components need to become smaller, in many cases they need to be completely redesigned. A recent report by BCC Research suggests that NOTES surgery will require single instruments that can perform many of the functions traditionally associated with multiple instruments. Adapting and redesigning existing equipment to fit the size and functionality needs of the emerging MIS climate will provide opportunities for existing and emerging companies to add value to this field.
3-D endoscopy is a valuable evolution that has proven to reduce error rates in novice surgeons, as well as providing subjective improvements to depth perception in both novice and experienced surgeons alike.
While care must be taken to avoid end-viewer discomfort (which can have root causes both in the capture and display of 3-D), these issues are known and can be accommodated. By using a passive display system (as opposed to active), and ensuring proper calibration and alignment of the 3-D endoscope (or, in time, switching to a single lens 3-D endoscope so that alignment and calibration are nonissues), surgeons can reap the benefits of enhanced depth perception during surgeries without facing the discomfort that is sometimes associated with 3-D.
More research must be done on 3-D endoscopy, but early results suggest that there are no ill effects when the display and capture issues are taken into account, and that there may in fact be objective positive effects for noviceand experienced surgeons alike.
Taking a wider perspective, advances in technology are allowing rapid advancements in MIS. Within the past decade we have seen the emergence of capsule endoscopy, robot assisted surgery, and high-quality 3-D visualization introduced to the field of MIS. NOS holds promise in the near future. As with most areas of technology, the development of components that are smaller, more powerful, and less expensive will continue to drive technology advances and entirely new procedures.
2. Mark Schubin, “What is 3-D and Why It Matters,” NAB Show (Las Vegas, April 2010).
3. SH Kong and BMo Oh, “Comparison of Two- and Three-Dimensional Camera Systems in Laparoscopic Performance: A Novel 3D System with One Camera,” Surgical Endoscopy 24 (2009): 1132-1143.
4. A Tabaee and V Anand, “Three-Dimensional Endoscopic Pituitary Surgery,” Neurosurgery 64 (May2009): 288–295.
||Shawn Veltman is Product Manager for Vancouver-based ISee3D Inc. The company develops proprietary 3-D image capture products based on a patented single lens technology. While the technology has direct application in several verticals, ISee3D is currently focused on the medical imaging space as it pertains specifically to the $9 billion annual expenditure in the areas of microscopy and endoscopy.
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