NOT so long ago, a blocked artery called for major surgery. That meant patients spent many hours under anesthesia, endured large and potentially traumatic incisions, and required months of recovery time.
Today, surgeons are using minimally invasive medical procedures that are less traumatic and more cost-effective. Most of these newer procedures incorporate thin catheters-hollow flexible tubes-that surgeons insert in small incisions in a major artery, such as the femoral artery in the thigh. The surgeon snakes the catheter through the arterial network to the problem area. Millimeter-size tools can then be guided through the tube to fix the medical condition.
Use of catheter-based procedures is growing rapidly. In the United States alone, over 700,000 of these minimally invasive surgeries are performed annually. A key advantage of this technique is that patients recover in days, not months. For example, balloon angioplasty (in which a small balloon is snaked through the catheter and, once at the end, inflated to open up a blocked artery) is now routinely conducted on an outpatient basis.
Still, there is room for improvement. Surgeons need better ways of navigating and positioning the catheter. In most cases, they rely on x rays to provide a snapshot of the arterial system as they manually push and pull the catheter into position. In addition to this limited view, there's also the problem that the catheters can be 2 meters long and 800 micrometers across, and maneuvering these thin soft tubes of plastic is like pushing on a string.
At Lawrence Livermore, a team of researchers (backed by three directorates-Laser Programs, Engineering, and Defense and Nuclear Technologies-and by the Laboratory Directed Research and Development Program) has developed a prototype catheter that shows promise in meeting these challenges. The advanced imaging catheter, as envisioned by physicist Luiz Da Silva and his team, will have a number of optical fibers embedded in the catheter wall to produce a stream of images-essentially a video-of the surrounding fluid and arterial wall.





3D Brought Inside
The advanced imaging catheter builds on unique technologies developed at the Laboratory, including a birefringence-insensitive optical coherence tomography (OCT) system. OCT is a noninvasive, noncontact optical technique that uses infrared light to image through highly scattering media such as blood and the vascular wall. "OCT is similar to ultrasound imaging, but it can achieve significantly higher spatial resolutions and is sensitive to differences in optical rather than acoustic properties of tissue," explains Da Silva.
This same technology lies at the heart of an R&D 100 Award-winning system that images teeth and dental tissue with near-infrared light. (See S&TR, October 1998, pp. 10-11, for more information about OCT and the optical dental imaging system.)
The three-dimensional imaging makes it easier for the physician to identify the location of the medical condition in an artery and guide the catheter to it. For instance, the figure at bottom right shows an x-ray image of a cerebral aneurysm. Because an x ray is a two-dimensional image of a three-dimensional structure, its views often leave the surgeon unsure which way to navigate the catheter. Moreover, the amount of chemical dye injected to provide contrast in the x ray must be limited, so the dye is not used continuously and the surgeon at times must work "blind" without even this two-dimensional visual clue.
"I observed an operation where the surgeon refused to proceed because he was uncertain that the catheter was in the correct position," said Da Silva. "Physicians told us that if a compact catheter could produce three-dimensional images and allowed them to actively guide it in the body, these procedures would be quicker, less traumatic, and have a much higher success rate."

Three Areas to Tackle
The team is focusing on three areas of development for this next generation of catheters: the fabrication technology needed to place optical fibers within the thin polymer wall of a catheter, the materials and techniques required for actively controlling the catheter, and radiation transport modeling.
"There are two ways to place optical fibers in the walls of the catheter," says Da Silva. "One way is to embed the optical fibers into existing commercial catheters. A key question regarding this approach is how it would affect the flexibility of the catheter. Another possibility is to extrude the catheter polymer with the optical fibers already embedded in the walls. We're pursuing both possible solutions." Along with a commercial collaborator, the team has developed tubing that can hold 10 optical fibers and is 1.7 millimeters in diameter with a wall 0.2 millimeters thick.
To find a better way of "pushing on a string," the team looked at Laboratory-developed microactuators and so-called smart materials. One possible solution might come from a shaped-memory polymer material being investigated at Livermore that can be activated by optical heating. "By using multiple wavelengths and different polymers," says Da Silva, "it may be possible to actively control and manipulate the tip of the catheter." Alternatively, shaped-memory alloy materials that can be made to move when heated could be placed near the catheter tip. A surgeon could then guide or position the tip by altering the temperature of the shaped-memory alloy, causing the catheter tip to bend. A microrudder scheme has also been proposed, which would steer the catheter tip using blood flow as a means of propulsion.
Radiation transport modeling is key to understanding OCT and developing simulation and analytical tools for optimizing the catheter's imaging system. Laboratory researchers recently adapted the Livermore-developed code LATIS (a two-dimensional laser-tissue interaction code) to simulate what occurs when photons from the fiber optics scatter from blood and biological tissue and return to the catheter's collecting lens. (For more information about LATIS, see S&TR, March 1999, pp. 23-25.) Besides helping in the design of the final system, the modified LATIS code can help researchers interpret OCT imaging in a variety of applications.





Results Are Promising
The team has manufactured and tested a prototype single-fiber catheter with encouraging results. "The OCT imaging system has an order of magnitude higher resolution than state-of-the-art ultrasound," says Da Silva. The team has also been working on miniaturizing the electronics and has managed to shrink the device from a large (60 by 90 by 90 centimeters) rack-mounted electromechanical device to a box about half a meter on each side-a size much more compatible with operating room conditions.
In the process, the team made significant improvements to the system, increasing its sensitivity by a factor of 30. It can now penetrate through 2 to 3 millimeters of biological tissue such as arteries.
The team sees other possible uses for the catheter as well. For instance, it could be used to examine the structures of composite materials and to search for signs of delamination in plastic explosives. The team is also evaluating whether the imaging catheter might be used to characterize wall- and ice-layer thickness in laser targets for the National Ignition Facility.
"The other challenge we must keep in mind, if our imaging catheter is to become a commercial reality, is cost." continues Da Silva. "The added cost of the device needs to be reasonable. Right now, we're looking at adding $10 or less to catheters that normally cost between $500 and $1,000. We're talking with the National Institutes of Health, industry, and physicians as we continue our development. Everyone's interested and sees cost-effectiveness as an important step toward the next-generation catheter."
—Ann Parker

Key Words: advanced imaging catheter, LATIS, medical technology, optical coherence tomography (OCT), radiation transport modeling.

For further information contact Luiz Da Silva (925) 423-9867 (dasilva1@llnl.gov).


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