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Virtual Voyage Through the Body

A clever combination of detailed imaging and graphics software creates virtual endoscopy: a non-invasive peek inside the body
aging



J. W. Stewart
virtual colon
Image: Courtesy of SIEMENS
IMAGES OF THE COLON for a virtual endoscopy were made with a computed tomography (CT) scanner. Top left, a 2-D coronal (from head to toe) image. Top right, a 2-D axial (from belly to back) image. Bottom left, 3-D reconstruction of the colon, which indicates the location of the endoscope. Bottom right, 3-D virtual "fly through" of the colon to identify polyps. To experience a fly through, click here. (Note: the movie is in AVI format and 10MB in size.)
How to picture the dynamic goings-on inside body organs once was one of the hottest unsolved mysteries in medicine. Then, around 40 years ago, came endoscopes--fiberoptic-filled tubes that can be threaded through hollow organs to take still and video images. Millions of patients have since benefited from diagnosis or treatment with endoscopes, yet the technology isn't perfect: it is uncomfortable and can perforate organ walls, putting patients at risk of a deadly infection.

Now, medical imaging may be on the cusp of another revolutionary change: a new procedure called virtual endoscopy can film the insides of body organs without the unpleasant and risky tubes. First appearing in the early 1990s, the technique combines detailed imaging technology with advanced graphics software to make exquisite three-dimensional, highly accurate images of internal structures.

During virtual endoscopy, a patient receives a computer-aided tomography, or CT, scan, in which the CT machine transmits x-ray beams at different angles to create numerous, thin cross-sectional images of a body region. These images are then transmitted to a computer, where graphics software algorithms fuse them into a seamless three-dimensional representation of that region. Doctors can then "fly" at length through hollow organs to explore areas of concern, all without the worry of prolonging an unpleasant exam. The entire process¿from imaging to interpretation¿can take only minutes, causes little or no pain and carries about as much risk as a few chest x-rays.

Currently, most researchers are using virtual endoscopy to image the colon and airways, but the technology can picture any hollow body structure. "Anything that contains air or fluid is, at the current time, a candidate for virtual endoscopy," explains Ronald M. Summers of the National Institutes of Health, which has an area of its Web site devoted to the subject. In addition to diagnosing ailments, doctors are using virtual endoscopy to plan out many types of surgery.

Since its recent introduction, the technology has undergone some refinements. CT scanners are collecting thinner image slices of the body than before--making it possible to see smaller features--and the software used to interpret those images has gotten better at streaming it all together. As such, the newest incarnations of the most widely used techniques, virtual colonoscopy and virtual bronchoscopy, may be just as accurate as endoscopy.

Researchers estimate that thousands of patients across the country have undergone some type of virtual imaging, illustrating the most salient feature of the new technology: it is more comfortable than endoscopy for many patients, so they are more likely to choose it over standard imaging. Virtual endoscopy detects many diseases that are highly treatable if caught early; from a public health perspective, virtual endoscopy could save many lives.

"This is really what I kind of dream of--that we can eradicate colon cancer," says Arie Kaufman of the State University of New York at Stony Brook, one of the first researchers to develop virtual colonoscopy.

Into the Colon

polyp
Image: Courtesy of JUDY YEE
POLYP DETECTED during a virtual colonoscopy conducted by Judy Yee of the University of California, San Francisco.

Currently, the most widespread application of virtual endoscopy is in picturing the colon. Virtual colonoscopy detects colorectal cancer, the second leading cause of death from cancer, killing 60,000 people annually. Most cases arise from polyps that grow inside the colon. If the cancerous growths are detected early, patients have a 90 percent chance of surviving, but relatively few people undergo colonoscopy.

During colonoscopy, a gastroenterologist inserts a tube, or colonoscope, up a patient's rectum and through the colon, scanning the surface and snipping out anything that looks like a large polyp. Patients must be sedated, and the colonoscope can¿t turn around, so it must enter and leave the colon facing in the same direction, and it can miss polyps that are only visible from the other direction. If the colon is especially long or the patient is in too much pain, gastroenterologists can sometimes have trouble reaching the end of the colon, and so can miss outlying polyps.

Given the drawbacks and risks of colonoscopy, Kaufman suggests that all patients be screened using virtual colonoscopy. Any patients who are discovered to have abnormal findings (at least 10 percent of those screened) would then receive conventional colonoscopy to remove a polyp that is at a specific site in the colon. Focusing efforts on a specific region of the colon lets the doctor execute a colonoscopy quickly, possibly reducing the risk of complications. "If you want to have an invasive procedure," Kaufman reasons, "you want to have it only when you actually need to operate or remove a polyp, rather than just for routine screening."

The latest studies report that virtual colonoscopy may be just as good as colonoscopy at identifying polyps and distinguishing them from harmless structures. And creating images from a CT scan of the entire abdomen means that doctors may be able to find other abnormalities besides colon cancer, such as an abdominal aneurism or kidney cancer. The news is spreading: In March, Katie Couric experienced the procedure on the Today Show, and Kaufman says that the SUNY Stony Brook clinic is sometimes booked months in advance for a virtual colonoscopy. Many private clinics also now offer the technology.

Despite the pluses, virtual colonoscopy is not perfect. Patients still need to take laxatives the day before the exam because the CT scan can't distinguish stool from polyps. During the exam, doctors must still insert a small tube in the rectum and inflate the colon, which can be uncomfortable. In addition, CT scanners don¿t identify flat polyps well, which often stand out during a conventional colonoscopy because of their distinguishing color.

To address these flaws, researchers at SUNY Stony Brook and other facilities are investigating a drink that labels stool, allowing computers to subtract stool from the colon and eliminating the need for patients to undergo the unpleasant preparation of taking laxatives. Instead of using room air to inflate the colon, which can leave patients feeling bloated after the procedure, researchers are experimenting with using carbon dioxide, which is more easily absorbed into the body. In the future, also, doctors might inject agents that specifically label polyps to help detect flat lesions. Researchers at NIH, Stanford University and other facilities are developing software that helps identify polyps, which would improve detection and speed up the process. That might make the procedure more affordable--it can now cost more than $1,000 and, as a newer technique, is generally not covered by medical insurance. Colonoscopy generally costs more, but insurance companies will pay for the procedure.

Virtual Deep Throat

virtual bronchoscopy
Image: Courtesy of Drs. ERIC A. HOFFMAN and GEOFFREY McLANNAN of the University of Iowa and WILLIAM HIGGINS of Pennsylvania State University.

MONTAGE of virtual bronchoscopy work, completed at the University of Iowa, in collaboration with Pennsylvania State University.

Instead of sticking a bronchoscope down a patient's throat, some researchers are now generating three dimensional images of the lungs and airways using virtual bronchoscopy.

In addition to being much more comfortable for patients, virtual bronchoscopy can image structures outside the boundaries of the airways--something impossible with conventional bronchoscopy. Studies have found that this technique is just as capable of picking up some of the same abnormalities diagnosed by bronchoscopy, such as small masses, narrowing of the airways or blockages in some of the airway branches. Summers of the NIH estimates that perhaps several hundred virtual bronchoscopies have been performed in the U.S. alone.

Planning Surgery

The real benefit of virtual bronchoscopy may lie in its ability to help doctors plan and execute medical interventions. When tumors push against the airways, doctors often insert a stent to keep breathing passages open. The stent must be exactly the right size: too small, and it could be coughed out or sucked into the lung; too big, and it won¿t fully expand in the airways, and could block breathing even more. Using virtual bronchoscopy, researchers at the University of Iowa are trying to improve the success of their intervention by measuring the exact size of the airway before inserting the stent, to assure a perfect fit.

Some surgeons are now relying on virtual endoscopy to plan surgeries in advance. Because virtual bronchoscopy can image structures outside of the airways, when it finds outlying tumors, doctors can plan and practice different ways to biopsy masses from a bronchoscope confined to the airways.

Francois I. Luks and colleagues at Brown University recently published a paper in the American Journal of Obstetrics and Gynecology about how they used virtual imaging to save two sets of twin fetuses inside two mothers. In both cases, one twin was pumping some of its blood into the other; when blood exchange is unbalanced, the donor twin doesn¿t have enough resources to grow, so Luks decided to seal off the blood vessels connecting the twins. Normally, there is only a 50 percent chance both twins will survive the procedure, but Luks used the three-dimensional images of the mother¿s abdomen to carry out "what if" scenarios, allowing him to fine-tune the procedure beforehand.

And both sets of twins--like many other patients who've benefited from virtual endoscopy--survived.

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