Published by the MIT News Office at the Massachusetts Institute of Technology, Cambridge, Mass.
Published by the MIT News Office at the Massachusetts Institute of
Technology, Cambridge, Mass.
PRECISE HEAT Laser Catheter to Aid Arterial Surgery An improved laser catheter system designed to perform precise microsurgery inside clogged coronary artery blood vessels to reopen them has been developed at MIT. Clinical trials on 10 patients will begin soon in Cleveland. The system, designed to minimize mechanical injury and trauma to the blood vessel during the procedure, involves: --Laser light at a specially selected wavelength which removes tissue from inside the blood vessel. --The use of spectroscopic signals to diagnose the nature of the tissue and to control the removal process. --A catheter tip which is specially designed to permit selective removal of diseased tissue. The system controls the delivery of heat so precisely that its laser can bore a hole in the head of a kitchen match without igniting the match. Professor Michael S. Feld, who led the development of the system at MIT, said the clinical trials will be conducted at the Cleveland Clinic Foundation by Dr. John Kramer, a cardiologist, who collaborated with MIT in designing the system. Also involved in the project is GV Medical, Inc. of Minneapolis, which holds an exclusive technology development license from MIT for the new system, and researchers at the Leonard Morse Hospital in Natick, Mass. The work at MIT has been centered in the George R. Harrison Spectroscopy Laboratory, which is headed by Professor Feld, a member of the Department of Physics. Professor Feld, noted for his research on the medical uses of lasers, said that by reducing mechanical injury to the delicate structures of the artery wall, the new system promises to minimize restenosis, the recurrence of vessel narrowing and reclogging after treatment. A high incidence of restenosis plagues balloon angioplasty, the currently used catheter-based method of treating atherosclerosis. Various catheter delivery alternatives employing laser light and rotating mechanical cutters, in various stages of experimental use, have so far failed to provide an improved treatment method, he said. Professor Feld described the MIT system recently at a special Harvard Medical School course on vascular surgery. The laser used in the MIT system is an Nd:YAG (neodymium yttrium aluminum garnet) solid state laser, tripled in frequency to a wavelength of 355 nanometers, which is routinely used in hospitals for many applications. (A nanometer is one billionth of a meter.) Professor Feld said that this wavelength was chosen to optimize the removal of calcified plaque deposits and eliminate the possibility of inducing mutagenic changes in arterial tissue. Professor Feld said that understanding the physics of laser tissue removal was very important in the design of the system. He said that calcified plaque is composed of tiny deposits of calcium salts imbedded in a matrix of soft lipid-based material. The laser light vaporizes the tissue by a thermal mechanism similar to that employed in the SDI laser defense system for destroying the aluminum skins of incoming missiles, he said. Only the soft material, and not the calcium, is vaporized, but the hard calcium particles are dragged away in the rapidly exiting vapor plume. Laser energy is deposited so fast and diseased tissue removed so rapidly that the deposited heat cannot diffuse to neighboring healthy tissue. This results in precise cutting with little damage to nearby tissue. During his Harvard Medical School lecture Professor Feld illustrated his system's ability to create heat without burning by showing a dramatic photograph: an extreme close-up of the head of an unignited kitchen match into which a hole had been bored by the laser light. The near ultraviolet wavelength of 355 nm was chosen, Professor Feld said, to minimize concern about photochemical damage to DNA in arterial cells, which is a potential cause of cancer. That is one reason, he said, why 308 nm radiation from a xenon chloride excimer laser was not selected. The excimer laser is currently in use in several commercial systems being developed to treat atherosclerosis. Numerous studies in cell lines, laboratory animals and human skin have shown light at this wavelength to be extremely mutagenic, and the MIT researchers wanted to avoid that potential problem. One problem with Nd:YAG lasers the MIT researchers encountered was their short pulse duration--7.5 nanoseconds--which, at the energies required to remove artery tissue, would damage the optical fibers used in the laser catheter. To overcome this, the MIT group developed a new concept which Professor Feld called stretched and sequenced pulses. In this process mirrors and lenses bounce the light back and forth a number of times, allowing pulses to be lengthened and shaped into any desired waveform. In the new system two short Nd:YAG input pulses are lengthened to over 200 nanoseconds each, and emitted in quick succession. This pulse waveform can be transmitted easily through the optical fibers of the catheter. Both laser and balloon angioplasty are performed by opening an artery--usually in the groin--and inserting a slender catheter into the blood vessel. In balloon angioplasty, the catheter carries a deflated balloon into the artery. When inflated, the balloon compresses the plaque against the walls of the vessel, providing more space for blood to flow from the heart. This process is extremely traumatic to the artery walls, Professor Feld said, creating conditions for restenosis, which frequently occurs, and often requires a repeat procedure or a subsequent coronary bypass operation. In the MIT system the catheter is tipped by a ring of 12 optical fibers surrounding a conventional guidewire and enclosed in a transparent enclosure called an optical shield. The guidewire positions the catheter in the artery. The shield, which is brought into contact with the blockage when tissue is to be removed, allows the cones of light emerging from the fibers to expand and overlap one another, forming a uniform distribution of light across the entire tip of the laser catheter. Professor Feld said that irradiation and removal of all of the diseased tissue at the catheter tip is essential. If any obstructing tissue remains, mechanical injury will result when the catheter is advanced, a probable cause of restenosis. He said that existing laser catheters remove only the central portion of the blocked tissue, often incompletely. In this case, the catheter must be pushed through this remaining tissue as it is advanced. The resulting trauma may be responsible for the high incidence of restenosis obtained with existing laser devices. In the MIT system, high power pulses of laser light are transmitted through some or all of the optical fibers, depending on the extent of the diseased tissue in contact with the catheter tip. Healthy tissue and blood are left undisturbed. The type of tissue being encountered at each part of the catheter tip at a given time is determined by a new diagnosis technique called laser-induced fluorescence. A weak pulse of blue-green diagnostic light at a wavelength of 480 nm is sent down each fiber, one at a time. The diagnostic light excites fluorescence light in the tissue, which is returned back through the fiber and analyzed spectroscopically. Professor Feld said that diagnosis and analysis for all 12 fibers takes about one second, and that the accuracy of the diagnosis was comparable to that obtained by conventional pathology, and therefore sufficiently accurate for control of tissue removal. After diagnosis, a computer displays a 12-element image of the tissue irradiated by the individual fibers to the physician, indicating the portions which are diseased, and readies the fibers pointing at diseased tissue for delivery of high-power laser light. If the physician concurs, ablation light is delivered, removing a small depth of tissue. The catheter is then advanced and another image presented. This sequence is continued until the entire blockage is removed. Professor Feld said that use of spectral diagnosis was the key element for improved performance of the MIT system. Advance knowledge of whether the tissue being irradiated by each fiber is healthy or diseased provides a margin of safety which allows the entire catheter tip, including the edges, to be illuminated, without the risk of perforation, he said.This makes it possible for the diameter of the reopened artery to be as large as that of the laser catheter itself, minimizing trauma to the artery wall, he said.