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Managing Editor Scott Harris sharris@aamc.org Staff Writer Elissa Fuchs efuchs@aamc.org

Image Guided Surgery: Making “X-Ray Vision” A New Reality

Ask any surgeon what capability would make his or her job easier, and more than a few would likely answer, “X-ray vision.” This capacity, previously available only to fictional characters like Superman, is now becoming a reality for a growing number of surgeons using the tools of modern “image-guided surgery.”

“One of the most fundamental limitations of conventional surgery is that most tissues in the body are not transparent,” says Ron Kikinis, M.D., director of the Surgical Planning Laboratory at Brigham and Women’s Hospital in Boston, and associate professor of radiology at Harvard Medical School. “The surface is the only visible part of the body, so when surgeons begin cutting, they literally cannot see what they are doing. To remove that barrier is the underlying notion of image-guided surgery. That is, to use diagnostic imaging modalities to provide the surgeon with the capacity to see under the surface.”

Purveyors of this new technology also note that an inevitable side effect of conventional surgery, until recently taken for granted, is that healthy tissue often has to be sacrificed under the surgeon’s knife to access targeted areas. “A given of surgery for many years has been that you have to cut through the healthy parts of the body in order to see the diseased parts,” says Henry Fuchs, Ph.D., the Federico Gil professor of computer science and adjunct professor of radiation oncology and of biomedical engineering at the University of North Carolina at Chapel Hill. “After removing or repairing their target, surgeons also have to repair to the best of their ability the healthy parts of the body that they have inevitably damaged.”

Medical technology has tried to overcome these dual dilemmas — the visual limitations of the human eye and the barriers the human body presents to surgeons needing access to hard-to-reach areas — with imaging technology dating back to the x-ray, the first tool that allowed surgeons to “see through” the surface of the body. The limited ability of x-rays to provide detailed internal images of the body led to the development of more powerful imaging tools such as ultrasound, CT scans, and, most recently, MRIs.

These modern imaging techniques are the building blocks of today’s most advanced contributions to image-guided surgery. They are what has enabled Ramin Shahidi, Ph.D., assistant professor of neurosurgery and surgery and director of the Image Guidance Laboratories at Stanford University School of Medicine, to create what he refers to as the “virtual patient,” a three-dimensional digital model of an individual patient’s anatomy constructed from data derived from x-rays, MRI images, ultrasound, and other scans. Dr. Shahidi’s “virtual patient” transcends the boundaries imposed by minimally invasive surgery in which endoscopes equipped with cameras can give physicians only a limited view of the tissue directly in front of the device.

“The virtual patient overcomes the limitations of an endoscopic view with a realistic, fully three-dimensional digital model that the physician can interact with by selectively pulling back skin and other layers to not only analyze data but also to find the best path to a targeted area during a procedure,” explains Dr. Shahidi, who is now helping market his technology through a company he co-founded in 1999 called Cbyon, Inc.

Surgeons can use the “volumetric image” produced by the start-up company’s lead product, “Cbyon Suite,” to visualize internal structures that are behind or beyond surfaces visible with an endoscope, including pathologies such as tumors and the complex nerves, blood vessels, and other vascular structures in which they are often embedded.

Advanced computer graphics differentiate these structures with bright colors to aid surgeons when they plan their operational trajectories. Cbyon Suite also includes software that allows computers to track the position of surgical tools within the “virtual patient” in real time as surgeons move those tools inside the patient lying on the table.

“Once you marry the enhanced orientation provided by the virtual patient to the navigational part of it, you are dealing with technology that is very similar to Superman’s x-ray vision,” affirms Dr. Shahidi. “Physicians can see right through a patient’s body without a cut.”

Charting changing terrain

But once an incision is made, says W. Eric Grimson, Ph.D., a surgeon must take into account the movement that inevitably occurs inside a patient’s body. “The single biggest challenge in image-guided surgery is being able to do all of the processing in real time,” says the Bernard Gordon professor of medical engineering and associate director of the Artificial Intelligence Laboratory at MIT.

“You can create a very detailed reconstruction of the patient’s anatomy based on a preoperative image to enhance a surgeon’s visualization. But once the surgery starts, things move. Tissue is removed or may shift due to gravity or other factors, and the surgeon in essence needs to be able to constantly update that visualization and align it as accurately as possible with what the patient is undergoing,” Dr. Grimson says.

This problem became evident to Dr. Fuchs when his wife had to undergo amniocentesis when she became pregnant with their first child. The sight of the large sampling needle and the rapidity with which the obstetrician inserted it into his wife’s belly troubled the computer scientist. Dr. Fuchs watched as the physician used ultrasound guidance to identify the location of the baby, placed his thumb where the ultrasound transducer was, put the transducer down, picked up the sampling needle, and inserted it in the area marked by his thumb.

“He explained to me that you want to do the procedure fast because the baby may move and you don’t want to hit the baby; you want to get some fluid,” recalls Dr. Fuchs. “It seemed to me the equivalent of having to put down a flashlight when you’re about to do some task in the dark. We sometimes want a flashlight that stays in place, and that’s what we have work lights for.” Dr. Fuchs went on to explore the feasibility of equipping obstetricians with head-mounted displays that would allow them to see inside a pregnant woman’s abdomen while simultaneously inserting a sampling needle.

Just as the movement of a baby in the womb causes continual internal shifting in a pregnant woman, the movement of a surgeon’s tools causes often unpredictable internal “deformations” as soon as an incision is made — making the use of continually updated interoperative images necessary to maximize the potential of image-guided surgery systems. But how can a preoperative image be updated once a patient is on the operating table?

In 1994, Brigham and Women’s Hospital equipped its operating room with a “double doughnut,” or open-magnet MRI machine, which allows surgeons to operate inside an MRI unit. Such a configuration makes it possible for surgeons to periodically ask for fresh MRI scans while an operation is in process without moving the patient. Within seconds, the updated images are superimposed on the presurgical model and the surgeon is reoriented to the current internal landscape.

Such capability allows surgeons to compare the preoperative and current positions of pathologies such as tumors as well as discern any internal displace-ments that need to be taken into account. Dr. Kikinis, whose Surgical Planning Lab is part of the Image-Guided Therapy Program at Brigham and Women’s that helped pioneer the open MRI unit, says that the progress realized by his group is due in large part to its being an “integrated, multidiscip-linary program.”

“This type of activity transcends traditional boundaries between specialties,” explains Dr. Kikinis. “You have to be a surgeon in order to do surgery, but you have to be a radiologist to read those images. So you really need a team working together to accomplish your results. If you look worldwide — because we are by far not the only site that does this type of technology — you’ll find that those places where multidisciplinary teams work unhampered by internal politics are successful, whereas those places where too much internal politics come into play are not.”

Scientists’ integral role

Of course, image-guided surgery would not exist without those whose contributions have made possible the generation of these “images” in the first place — computer scientists. “For this type of activity, particularly for the imageprocessing, the work of computer scientists is absolutely critical,” emphasizes Dr. Kikinis.

“The advanced algorithms needed for the visualization element of image-guided surgery are very demanding, both in terms of the underlying mathematics and the implementation; you need to have very efficient implementation if you want the application to be practical,” says Dr. Kikinis. “So the same way the medical doctors in our field need cross-training in order to be able to communicate effectively with the computer scientists, we have a set of faculty-level computer scientists working in the Surgical Planning Lab so they can cross-train to interact with medical scientists and clinicians.”

Etta D. Pisano, M.D., professor of radiology and biomedical engineering at the University of North Carolina School of Medicine, Chapel Hill has worked closely with Dr. Fuchs in his attempts to use surgical guidance systems enabled by ultrasound to facilitate cyst aspirations and core biopsies in breasts. Her experience with testing Dr. Fuchs’ method of imposing an internal image on patients through the use of headgear that allows her to “see inside” patients’ breasts taught Dr. Pisano that physicians and computer scientists must cross disciplinary boundaries for this new technology to evolve.

“Physicians have a different skill set than computer scientists, and if we’re going to apply digital imaging technology to real clinical problems, we are going to have to work collaboratively with them,” Dr. Pisano says. “There are some radiologists who are quite sophisticated and able to do a lot of complicated image processing and programming. But I think the most likely thing to happen is for radiologists to work with people who spend their whole lives doing nothing but computer science, and I think that collaboration is definitely in the future.”

It goes without saying that enhanced medical technologies are just some of the myriad ways advancements in computer science have entered our lives. The sophisticated digital image rendering made possible with modern image-guided surgery can in fact be largely attributed to the incredibly lucrative video game industry.

“The image generation from synthetic data is much better now than it was when ‘Pong’ came out, largely because people are willing to spend literally billions of dollars to get better-looking computer games,” attests Dr. Fuchs.

Dr. Kikinis agrees. “The reason visualization has made so much progress in image-guided surgery is that we can get a ‘free ride’ from the generic technology, partially due to the gaming industry. We haven’t needed to do a whole lot of our own development in that area.” Dr. Shahidi adds that the video game industry’s accelerated development of cutting-edge graphics has also made current medical technology more economically feasible.

“The reality is that five years ago when I developed the first prototype of image-enhancement endoscopy, I had to use a $300,000 power processor,” says Dr. Shahidi. “If that was still the case, the system I am selling today would cost over a million dollars, an amount that our healthcare industry absolutely cannot absorb. Now, the $300 to $400 video game cards sold in retail stores are the same ones we use to get the enhanced, real-time rendering needed to reconstruct virtual patients for surgeons.”

Looking ahead

Dr. Grimson agrees with his colleagues that while neurosurgery is the surgical specialty that has benefited most from modern image-guided surgery techniques, the technology has advanced enough to be applicable to several other specialties, as evidenced by its use in sinus surgery, orthopedic surgery, and spinal surgery.

“In essence, any place where you want to hit a small target very accurately with minimal invasion is a place where image-guided surgery is going to be of value,” says Dr. Grimson. He adds that the technology, although still in its infancy, has been shown to reduce surgery time and make possible procedures previously regarded as inoperative, in essence because “it was just too hard to guarantee that you could get to the things you needed to without damaging nearby structures.”

Dr. Fuchs agrees that as image-guided surgery becomes more fine- tuned, it will be feasible in more and more cases. “So image-guided surgery, I think, will become the norm, while conventional, open-ended surgery will become the exception.” But such a view of the future, adds Dr. Grimson, does not mean that surgeons will be replaced by the tools that will continue to facilitate and fine-tune their work.

“While these technologies will dramatically leverage how surgical procedures are done in the future, they will not replace surgeons,” he affirms. “Surgeons are still crucial; you need that human experience and judgement that cannot be replaced by machines.”

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