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Image-Guided Scalpels |
The whole concept of surgery is to identify a target, including its boundaries and margins, and take it out or kill it or localize it," says Ferenc Jolesz, Holman professor of radiology and director of the magnetic resonance imaging division and the image-guided therapy program at Boston's Brigham and Women's Hospital (BWH).
But in many cases, the surgeon does not have a clear view of the target because the naked eye cannot see past the surface of exposed tissues--and in some areas, particularly the brain, a tumor may look the same as normal tissue surrounding it. Furthermore, the minimally invasive procedures commonly performed today use keyhole incisions that add spatial constraints to the surgeon's purview. Without knowing the precise location, shape, and size of their target, surgeons may damage critical tissues and blood vessels, causing paralysis, blindness, excess bleeding, or other complications. Or, as happens in more than 50 percent of operations to remove malignant brain and breast tumors, reports Jolesz, surgeons leave pieces of tumor behind, reducing patients' long-term survival. "It's a tragedy," says the former neurosurgeon, "and no one wants to talk about it."
Views of a brain tumor, top to bottom: from the front; with brain made translucent; side view showing relationship of two tumor regions; view from above through transparent brain. The tumor is green, the blood vessels are red, and the ventricles blue.
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Imaging techniques like computed tomography (CT, also called computerized axial tomography, or CAT) and magnetic resonance imaging (MRI) routinely assist doctors with surgical planning by penetrating the body without cutting it. "Today, you see anatomy better on MRI than on autopsy," claims Jolesz, adding that MRI is currently the best imaging method for differentiating between normal and abnormal tissues. But because these imaging studies are usually done in the radiology department before surgery, they cannot take into account how tissues deform or shift during surgical manipulation--and are, therefore, misleading.
To solve this problem, Jolesz, in collaboration with colleagues at BWH, General Electric Medical Systems, and MIT, developed an open-magnet MRI system that lets surgeons and radiologists update the current location of the target and neighboring anatomical features using real-time imaging during the surgical operation. A conventional MRI machine has one huge, hollow cylindrical magnet within which a patient lies to be scanned. The new device splits this magnet in half, producing two doughnut-shaped sections with a gap that can accommodate two doctors. The patient lies on a table that goes through the doughnut holes.
MRI produces virtual slices of the surface and internal anatomy of the body part being treated and then combines the slices into a three-dimensional model. A surgeon standing in the gap of the open-magnet system can view this anatomical model on a screen mounted above the operating field.
Computers can also fuse the three-dimensional model with live video footage of the patient, thus merging the virtual with the real. "The composite picture is in perfect register with the patient's head as seen from the surgeon's vantage. It is as if he has developed 'x-ray vision,' for he can locate internal structures before he ever picks up a scalpel," wrote Jolesz and coauthors W. Eric Grimson, lecturer on radiology, Ron Kikinis, associate professor of radiology, and Peter Black, Ingraham professor of neurosurgery, in a recent Scientific American article. Computer programs assign a specific color to each tissue type: blood vessels may be red, tumors green. A probe topped with infrared light-emitting diodes enables surgeons to monitor the coordinates of their scalpels or other instruments at all times; they can also request fresh scans throughout the operation to see how tissues have shifted. The three-dimensional models and real-time scans help doctors plot the least damaging paths to their targets and improve their ability to excise tumors completely, including parts invisible to the naked eye. Surgical complications are less likely and, in some cases, operations once considered too risky are now possible.
The first General Electric double-doughnut MRI system (SIGNA SP) ever built was installed at BWH in 1993; it cost at least $30 million to develop. Currently, there are only five such scanners (still pricey at $2.5 million apiece) in the country and a small number abroad. Advanced image-guidance systems developed by other companies are in use at several centers but, unlike the GE version, they do not allow real-time scans during surgery.
So far, BWH doctors have used the system in hundreds of brain operations. They have also used it to ensure complete resection of breast tumors, and the safe removal of ruptured spinal discs and insertion of radioactive seeds in cancerous prostate glands. Guided by the open-magnet system, Jolesz and his colleagues have employed lasers to destroy tumors with heat; MRI is temperature sensitive, he explains, so you see the whole process of cooking. The doctors have also used the system when inserting cryoprobes (needle-like, low-temperature probes) into tumors to freeze them. As an ice ball forms around the probe, the MR image shows a black hole growing bigger and bigger. "When the tumor is not seen with color, it's done," Jolesz explains.
Under MRI guidance, the BWH team used cryotherapy on a major-league baseball player to freeze a tumor growing deep within his thigh. Surgery would have damaged key leg muscles, possibly ending the player's career. The patient went home the day after the procedure and images taken three months later show no evidence of tumor. Another man, diagnosed with a seizure-causing brain tumor, underwent image-guided surgery on a Monday. On Thursday, he was back at work and seizure-free.
Despite such success stories, Jolesz is reluctant to market the SIGNA SP because he has only one magnet and it is already loaded with work. "The brain and prostate alone are filling the schedule almost every day," he says. "We are looking for funding now to install a second magnet.
~ Kathleen Koman