Abstract:
Laser energy produced by a laser operating In the mid-infrared region (approximately 2 micrometers) Is delivered by an optical fiber in a catheter to a surgical site for biological tissue removal and repair. Disclosed laser sources which have an output wavelength in this region include: Holmium-doped Yttrium Aluminum Garnet (Ho:YAG), Holmium-doped Yttrium Lithium Fluoride (Ho:YLF), Erbium-doped YAG, Erbium-doped YLF and Thulium-doped YAG. For tissue removal, the lasers are operated with relatively long pulses at energy levels of approximately 1 joule per pulse. For tissue repair, the lasers are operated in a continuous wave mode at low power. Laser output energy is applied to a silica-based optical fiber which has been specially purified to reduce the hydroxyl-ion concentration to a low level. The catheter may be comprised of a single optical fiber or a plurality of optical fibers arranged to give overlapping output patterns for large area coverage.

Description:
This application is a continuation of application Ser. No. 08/411,581, filed Mar. 29, 1995, U.S. Pat. No. 5,843,073 which is a continuation of Ser. No. 08/049,147, filed Apr. 19, 1993 now abandoned, which is a divisional of Ser. No. 07/568,348, filed Aug. 15, 1990, which is a continuation of Ser. No. 07/257,760, filed Oct. 14, 1988, U.S. Pat No. 4,450,266 which is a continuation of Ser. No. 07/014,990, filed Feb. 17, 1987 now abandoned, which is a continuation Ser. No. 06/761,188, filed Jul. 31, 1995 now abandoned. The contents of all of the aforementioned application(s) are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to laser catheters and optical fiber systems for generating and transmitting energy to a surgical site in a living body for the purposes of tissue removal or repair. 
     BACKGROUND OF THE INVENTION 
     While lasers have been used for many years for industrial purposes such as drilling and cutting materials, it is only recently that surgeons have begin to use lasers for surgical operations on living tissue. To this end, laser energy has been used to repair retinal tissue and to cauterize blood vessels in the stomach and colon. 
     In many surgical situations, it is desirable to transmit laser energy down an optical fiber to the surgical location. if this can be done, the optical fiber can be included in a catheter which can be inserted into the body through a small opening, thus reducing the surgical trauma associated with the operation. In addition, the catheter can often be maneuvered to surgical sites which are so restricted that conventional scalpel surgery is difficult, if not impossible. For example, laser energy can be used to remove atherosclerotic plaque from the walls of the vasculature and to repair defects in small-diameter artery walls. 
     A problem has been encountered with laser surgery in that prior art lasers which have been used for industrial purposes often have characteristics which are not well suited to percutaneous laser surgery. For example, a laser which in conventionally used for scientific purposes is an excimer laser which Is a gas laser that operates with a gas mixture such as Argon-Fluorine, Krypton-Fluorine or Xenon-Fluorine. Another common industrial laser is the carbon dioxide or CO 2  laser. 
     Both the excimer laser and the CO 2  laser have been used for surgical purposes with varying results. One problem with excimer lasers is that they produce output energy having a wavelength in the range 0.2-0.5 micrometers. Blood hemoglobin and proteins have a relatively high absorption of energy in this wavelength range and, thus, the output beam of an excimer laser has a very short absorption length in these materials (the absorption length is the distance in the materials over which the laser beam can travel before most of the energy is absorbed). Consequently, the surgical site in which these lasers are to be used must be cleared of blood prior to the operation, otherwise most of the laser energy will be absorbed by intervening blood before it reaches the surgical area. While the removal of blood is possible if surgery is performed on an open area it is often difficult if surgery is to be performed via a catheter located in an artery or vein. 
     An additional problem with excimer lasers is that the output energy pulse developed by the laser is very short, typically about ten nanoseconds. In order to develop reasonable average power, pulses with extremely high peak power must be used. When an attempt is made to channel such a high peak power output into an optical fiber, the high peak power destroys the fiber. Thus, excimer lasers have a practical power limit which is relatively low. Consequently, when these lasers are used for biological tissue removal, the operation is slow and time consuming. 
     The CO 2  laser has other drawbacks. This laser generates output energy with a wavelength on the order of 10 micrometers. At this wavelength, the absorption of blood hemoglobin is negligible but the absorption by water and tissue is relatively high. Scattering at this wavelength is also very low. Although the CO 2  laser possesses favorable characteristics for surgical applications in that it has low scattering and high absorption in tissue, it suffers from the same drawback as excimer lasers in that the absorption length is relatively short due to the high absorption of the laser energy in water. Thus, the surgical area must be cleared of blood prior to the operation. 
     Unfortunately, the CO 2  laser also suffers from a serious additional problem. Due to the long wavelength, the output energy from the carbon dioxide laser cannot be presently transmitted down any optical fibers which are suitable for use in percutaneous surgery (present fibers which can transmit energy from a CO 2  laser are either composed of toxic materials, are soluble in water or are not readily bendable, or possess a combination of the previous problems) and, thus, the laser is only suitable for operations in which the laser energy can be either applied directly to the surgical area or applied by means of an optical system comprised of prisms or mirrors. 
     Accordingly, it is an object of the present invention to provide a laser catheter system which uses laser energy of a wavelength that is strongly absorbed in water, in bodily tissues and atherosclerotic plaque. 
     It is another object of the present invention to provide a laser catheter system which is capable of providing laser energy that can be transmitted through existing silica-based optical fibers. 
     It is a further object of the present invention to provide a laser catheter system in which optical scattering is minimized and which has a medium-length absorption length to confine the energy to the area of interest. 
     It is yet another object of the present invention to provide an optical catheter system with a laser that can be operated on either a pulsed mode or a continuous wave mode. 
     It is still another object of the present invention to provide a laser catheter system which can be used for biological material removal and biological material repair. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved and the foregoing problems are solved in one illustrative embodiment of the invention in which a laser catheter system employs a laser source operating in the wavelength region of 1.4-2.2 micrometers. Illustrative laser sources operating this region are Holmium-doped YAG, Holmium-doped YLF, Erbium-doped YAG, Erbium-doped YLF and Thulium-doped YAG lasers. 
     In the inventive laser system, the above-noted lasers are used with a specially-treated silica fiber that has been purified to reduce the concentration of hydroxyl (OH—) ions. 
     For biological tissue removal, the laser source may be operated in a pulsed mode with a relatively long pulse of approximately 0.2-5 milliseconds at an energy level of approximately 1-2 joules per pulse. With this time duration and energy level, the peak power of the laser pulse is approximately 1 kilowatt. This amount of power can easily be tolerated by the silica fiber, but is sufficient for rapid material removal. With a repetition rate in the range of 1-10 hertz, the average power delivered to a surgical site by such a laser will be under 10 watts. 
     Alternatively, for biological tissue repair, the laser source can be operated in a low power continuous wave mode to repair, by coagulation, of tissue by a process similar to “spot welding”. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a sketch of absorption of electromagnetic energy versus wavelength and electromagnetic energy scattering versus wavelength. 
     FIG. 2 shows an absorption versus wavelength plot for atherosclerotic plaque obtained in a carotid endarterectomy with the region of interest for the inventive laser sources (1.4-2.2 micrometers) outlined. 
     FIG. 3 of the drawing is a schematic diagram of a typical solid state laser construction used in the inventive laser sources. 
     FIG. 4 of the drawing is a plot of laser output intensity versus time for a typical pulse shape developed by a laser shown in FIG. 3 when used for tissue removal. 
     FIG. 5 is a schematic diagram of a laser catheter which employs a single optical fiber for transmitting laser energy to a surgical location. 
     FIG. 6 of the drawing is an enlarged cross-section of the probe tip the single fiber catheter shown in FIG.  5 . 
     FIG. 7 is a schematic diagram of a wire-guided catheter which employs four optical fibers to increase the area which can be irradiated with the laser light. 
     FIG. 8 of the drawing is an enlarged cross-sectional view of the probe tip of the catheter shown in FIG. 7 showing the four optical fibers. 
     FIG. 9 of the drawing is a schematic diagram of the beam pattern produced by the four-fiber catheter at the surgical location. 
     FIG. 10 is an end view of the probe tip of the catheter in the direction 10-10 of FIG.  9 . 
     FIG. 11 is a side view of the probe tip of the catheter of FIG.  9 . 
     FIG. 11A is a schematic diagram of a beam pattern as viewed in the direction 11A-11A of FIG.  11 . 
     FIG. 11B is a schematic diagram of a beam pattern as viewed in the direction 11B-11B of FIG.  11 . 
     FIG. 11C is a schematic diagram of a beam pattern as viewed in the direction 11C-11C of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The absorption and scattering characteristics versus output wavelength of a plurality of known laser systems are shown in FIG.  1 . FIG. 1 has a logarithmic scale representing the absorption coefficient in units of cm −1  along the vertical axis and the incident energy wavelength in micrometers along the horizontal axis. 
     From FIG. 1, it can be seen that excimer laser systems which utilize conventional gas mixtures such as Argon-Fluorine, Krypton-Fluorine and Xenon-Fluorine, and Argon gas lasers produce output energy which falls in the 0.2-0.5 micrometer wavelength region. In this region, the absorption of blood hemoglobin and proteins is very high. Consequently, the absorption length is very short (about 5-10 microns) and virtually no blood can be present between the fiber end and the surgical site during the operation. Thus, it is necessary to remove blood from the surgical area when these lasers are used for surgical purposes. 
     In addition, for lasers such as Argon, the absorption of water reaches a minimum at 0.5 micrometers so that it is necessary to use a higher power laser than is desirable to achieve sufficient power in the surgical area for material cutting and removal. Also, due to the low absorption of the laser output in water and hemoglobin, the absorption length is very long (approximately 1 mm). In addition, scattering for these lasers is relatively high, causing difficulty in controlling the laser energy and a danger of tissue damage outside the surgical area due to scattering of the laser energy. 
     At the other end of the wavelength spectrum shown in FIG. 1 are carbon monoxide and carbon dioxide lasers producing outputs at 5 and 10 micrometers, respectively. At these wavelengths scattering is negligible and absorption by water and tissue is relatively high and thus both lasers have good surgical properties. Unfortunately, due to the high absorption of water, the absorption length is relatively short (about 20 microns). Further, silica-based optical fibers in present use which are suitable for percutaneous surgical use have a practical “cutoff” in transmission which occurs approximately at 2.3 micrometers, and, thus, the output energy from carbon monoxide and carbon dioxide lasers cannot be transmitted through such an optical fiber. 
     In accordance with the invention, laser sources of interest are those that lie in the wavelength range of approximately 1.4-2.15 micrometers. As shown in FIG. 1, in this range, the energy absorption of water is relatively high whereas optical scattering is relatively low. Illustrative lasers which are useful with the present invention comprise Erbium-doped Yttrium Aluminum Garnet (YAG) with a wavelength of 1.55 micrometers, Erbium-doped Yttrium Lithium Fluoride (YLF) with a wavelength of 1.73 micrometers, Thulium-doped YAG with a wavelength of 1.88 micrometers, Holmium YLF with a wavelength of 2.06 micrometers and Holmium YAG at a wavelength of 2.1 micrometers. The absorption of the laser energy produced by lasers in this latter group by water is moderately high and, consequently, the absorption by biological tissues of such energy will also be relatively high. However, the absorption by water is not as high as the absorption of CO and CO 2  laser energy. Thus, the absorption length will be longer for the lasers operating in the 1.4-2.2 micron range. Typically, the absorption length In the body for these latter lasers is about 200 microns. Therefore, it is still possible to operate satisfactorily even with 10-30 microns of blood between the fiber end and the surgical site. of particular interest is the absorption of the laser energy by atherosclerotic plaque, since an important use of laser catheter systems is angioplasty, particularly the clearing of blocked arteries. FIG. 2 is a plot of the absorption by plaque of electromagnetic energy versus wavelength for energy in the wavelength range of 0.2-2.2 micrometers. As shown in FIG. 2, the absorption by plaque of electromagnetic energy reaches a minimum in the 0.8-1 micrometer wavelength range and generally increases with increasing wavelength in the wavelength region of 1-2.2 micrometers. 
     In the wavelength range from 1.4-2.2 micrometers, the wavelength range produced by laser in the above-mentioned group, the absorption by plaque is at a relatively high value. 
     A schematic diagram of a typical solid-state laser construction is shown in FIG.  3 . The laser assembly consists of a laser crystal  1  and an excitation device such as a flashlamp  3 . Typically, for the crystal compositions disclosed above, the laser crystal must be cooled to cryogenic temperature to provide low laser-threshhold operation. Cryogenic cooling is typically provided by enclosing crystal  1  in a quartz or fused-silica jacket  4  through which liquid nitrogen is circulated. Liquid nitrogen enters jacket  4  by means of an inlet pipe  5  and leaves by means of an outlet pipe  6 . The laser cavity is formed by a high-reflectivity concave mirror  10  and a partial reflector  12 . 
     Generally, the crystal is excited by optical pumping which is, in turn, accomplished by irradiating the crystal with light from a flashlamp  3 . A flashlamp which is typically used with the inventive laser compositions is a high-pressure Xenon flashlamp. Lamp  3  may also be surrounded by a quartz flow tube (not shown) through which coolant is pumped. 
     Crystal  1  and flashlamp  3  are enclosed in a reflector  2  which concentrates the flashlamp energy into the laser crystal. To maximize energy transfer from lamp  3  to crystal  1 , the inner surface of reflector  2  is coated with a material chosen to have high-reflectivity at the pumping wavelength of the laser crystal—illustratively, aluminum or silver, In order to provide thermal insulation to prevent condensation on the system optics, it may be necessary to evacuate the interior of reflector  2  or to provide a vacuum jacket around crystal  1 . 
     The construction of cryogenic solid-state lasers is conventional and described in a variety of sources; accordingly such construction will not be discussed further in detail herein. A more complete discussion of construction details of a typical cryogenic laser is set forth in an article entitled “TEM oo  Mode Ho:YLF laser”, N. P. Barnes, D. J. Gettemy, N. J. Levinos and J. E. Griggs,  Society of Photo - Optical Instrumentation Engineers , Volume 190—LASL Conference on Optics 1979, pp 297-304. 
     FIG. 4 of the drawing is a plot of the illustrative pulse shape developed by a laser in the preferred group when used in the “material removal” mode. FIG. 4 shows light intensity along the vertical axis increasing in the downward direction versus time increasing towards the right. Although, as shown in FIG. 4, the laser source has been adjusted to produce an output pulse of relatively long time duration, most of the output energy is released within approximately 1 millisecond of the beginning of the pulse. It should also be noted, as illustrated in FIG. 4, that lasers in the preferred laser group exhibit a “spiking” phenomenon caused by internal relaxation-oscillations in the laser crystal. The spiking behavior causes local increases in laser intensity which have a large magnitude, but a very short time duration. Similar “spiking” behavior has been found advantageous when lasers are used to drill metals and other materials for industrial purposes and it is believed that such “spiking” behavior enhances the laser usefulness for biological material removal. 
     FIG. 5 is a schematic diagram of a laser/catheter system employing a solid state laser of the type shown in detail in FIG.  3 . More specifically, the infrared output energy of laser  21  is focused by a conventional focusing lens onto the end of the optical fiber which is held in a conventional fiber optic connector  24 . Fiber optic connector  24  is, in turn, connected to a tube  27  which houses a single optical fiber. Tube  27  is connected to a conventional two-lumen catheter  30  by means of a bifurcation fitting  28 . 
     Illustratively, catheter  30  has two lumens passing axially therethrough to its distal end  34  so that an optical fiber can pass through one lumen and transmit laser energy from fiber optic connector  24  to lens tip  34 . As previously mentioned, the optical fiber which passes through the catheter is specially purified to reduce the hydroxyl ion concentration to a low level, thus preventing the laser energy which is transmitted down the fiber from being highly absorbed in the fiber material. A fiber which is suitable for use with the illustrative embodiment is a fused-silica optical fiber part no. 822W manufactured by the Spectran Corporation located in Sturbridge, Mass. 
     Advantageously, the mirrors and lenses ( 10 ,  12  and  22 ) which are used to form the IR laser cavity and focus the output energy beam are generally only reflective to energy with a wavelength falling within a narrow wavelength band and transparent to energy at other wavelengths. Consequently, the mirrors and lenses are transparent to visible light. An aiming laser  20  (for example, a conventional helium-neon laser) which generates visible light may be placed in series with IR laser  21  to generate a visible light beam. This light beam may be used to align mirrors  10  and  12  and to adjust focussing lens  22  so that the optical fiber system can be aligned prior to performing surgery. 
     Also, the optical fibers used to transmit the IR energy from laser  21  to the surgical area can also be used to transmit the visible light from the aiming laser  20  to the surgical area. Thus, when the inventive system is used in performing surgery where the surgical area is visible to the surgeon, the light produced by laser  20  passes through the optical fiber in catheter  30  and can be used to aim the probe tip before laser  21  is turned on to perform the actual operation. 
     The second lumen in catheter  30  is provided for transmission of a flushing fluid or to apply auction to the probe lens tip area to clear the area of blood during surgery. This latter lumen is connected through bifurcation fitting  28  to a second tube  29 . Tube  29  may illustratively be terminated by a standard Luer-Lok fitting  26  which allows connection of the catheter to injectors and standard flow fittings. Solutions injected into the catheter through tube  29  pass through the lumen in catheter  30  and exit at the distal end via a small orifice  32 . 
     Probe tip  34  consists of a lens arrangement which forms the laser energy into a beam  36  which is used to perform the surgical operations. An enlarged view of the probe tip in shown in FIGS. 6 and 7. 
     To ensure that the distal end of optical fiber  18  is spaced and oriented in a precise position with respect to the end of the probe, fiber  18  is mounted in a high-precision holder  58  which has a reduced diameter end  64  that forms a shoulder  68 . Shoulder  68 , as will hereinafter be described, is used to bold the probe tip assembly together. Bolder  38  has a precision-formed axial bore made up of two sections, including a large-diameter section  60  and a narrow-diameter section  63 . Bolder  58  may be made of glass, ceramic or other material capable of being formed to specified dimensions with precise tolerances. 
     In order to attach holder  58  to the end of fiber  18 , the fiber to first prepared as shown in FIG.  7 . More particularly, prior to insertion of fiber  18  into holder  58 , a portion of buffer sheath  61  is removed, exposing a length of optically-conductive core  65 . Care is taken when stripping buffer sheath  61  from the fiber not to damage the layer of reflective cladding  67  located on the surface of core  65 . After stripping, fiber  18  is inserted into holder  58  so that core  65  extends into the small-diameter bore  63  and sheath  61  extends into the large-diameter bore  60 . After fiber  18  has been inserted into holder  58 , it may be fastened by epoxy cement to permanently affix the components. To complete the assembly, the end of fiber  18  which protrudes beyond surface  62  of holder  58  may be finished flush with the surface by grinding the assembly or by carefully cleaving the fiber. 
     Referring to FIG. 6, holder  58  (with fiber  18  fastened inside) is mounted within a glass tube  51  to shield the assembly. The front surface,  62 , of holder  58  is spaced from the inner surface  142  of planar lens  144 , which may be comprised of glass or sapphire, by means of a spacing ring  154 . Ring  154  may illustratively be made of radiopaque material so that the catheter tip can be located inside the patient by means of a fluoroscope. 
     Glass tubing  51  is bent over shoulder  68  of holder  58  to form a constricted end,  65 , which holds the probe tip assembly together. A fillers  66 , which may be made of a plastic such as TEFLON (trademark of the DuPont corporation for polytetrafluoroethylene) fills the annular space between catheter, body  30  and end  65  of glass tube  51 . The outer diameter of the entire assembly from catheter body  30  to glass tube  51  is substantially the same, providing a smooth, uniform surface along the entire length of the catheter as indicated in FIG.  6 . 
     FIG. 8 shows a schematic diagram of a wire-guided, four-fiber catheter for use with the present invention. The laser system is set up as previously described with the infrared laser  21  constructed in accordance with the above disclosure. A visible helium-neon aiming laser  20  may also be used in line with laser  21  for aiming purposes as discussed with the single fiber catheter. The output of the infrared laser  21  is directed towards a set of four mirrors  60 - 68  arranged at a 45° angle with respect to the axis of beam  14 . 
     The first mirror,  60 , has a 25% reflective surface and directs approximately ¼ of the energy to focusing lens  70 . The second mirror of the set,  62 , is a 33% reflector which directs ¼ of the total energy to focusing lens  72 . Mirror  64  is a 50% reflector which directs ¼ of the total laser output to focusing lens  74 . The last mirror in the set, mirror  68 , is a 100% reflector which directs the remaining ¼ of the total energy to focusing lens  78 . Mirrors  60 - 68  and lenses  70 - 78  are conventional devices. Focusing lenses  70 - 78  focus the output energy from IR laser  21  onto four fiber optic connectors,  80 - 88 . Connectors  80 - 88  are connected, respectively, to tubes  90 - 96  which are all connected, via a branch connector  102 , to catheter  104 . Each of tubes  90 - 96  contains a single optical fiber which transmits ¼ of the total laser output energy through the catheter body to the catheter tip  108 . An additional tube  98  is provided which is connected to branch fitting  102  and to a conventional Luer-Lok connector,  100 . This latter tube is connected to a central lumen in catheter body  104  through which flushing solutions may be injected or through which a guide wire may be inserted through the catheter for purposes of guiding the catheter to the surgical area. 
     At catheter tip  108 , the four optical fibers which pass through the catheter are arranged symmetrically so that the beams  110  overlap to illuminate a larger area. Tip  108  also has a bole on the center thereof, through which guidewire  112  can protrude to direct the catheter to the proper location. 
     FIGS. 9 and 10 show detailed views of the illustrative four-fiber catheter tip. The four optical fibers  42  and the inner shaft  40  which holds the fibers, are held in a fiber holder  50  which is preferably formed from a radiopaque material such as stainless steel or platinum. Fiber bolder  50  is cylindrical and is provided with a central aperture,  54 , which communicates with a lumen  34  of approximately the same size that passes through the center of the catheter body  104 . Fiber holder  50  is provided with a plurality of longitudinally extending holes  56  which extend through the wall of holder  50  and receive, in a snug fit, the distal ends of the fiber cores  42 . The distal face  58  of the combined fiber cores  42  and holder  50  is polished flat to butt flush against optically transparent cap  52 . 
     Cap  52  is cylindrical and has the same outer diameter as catheter body  104  so that the two pieces define a smooth and continuous diameter. Cap  52  may be formed of a transparent substance such as pyrex glass or sapphire and has an enlarged bore  62  extending in from its proximal end. Bore  62  terminates at its end to form internal shoulder  60 . A smaller diameter central aperture,  64 , is formed in the distal end of cap  52  which aperture may have the same diameter as aperture  54  in fiber holder  50  and lumen  34  in catheter body  104  to provide a smooth and continuous lumen which opens at the distal tip of the catheter. However, the aperture  64  in tip  52  may also be somewhat narrower than aperture  54  and lumen  34  as long as sufficient clearance is provided to accommodate a guidewire without adversely interfering with fluid flow and pressure measurements. 
     Cap  52  is secured by an epoxy adhesive (placed on its inner surface  62 ) to the fiber holder  50  and also to the portion of the inner shaft  40  and fibers  42  which are disposed within the proximal end of the cap  52 . The distal end of the catheter body  104  is heat shrunk around the inner shaft  40  and fibers  42  to provide a smooth transition from cap  52  to catheter body  104 . 
     More complete construction details of a four-fiber catheter suitable for use with the illustrative embodiment are given in co-pending U.S. patent application entitled “Wire Guided Laser Catheter”, filed on May 22, 1985 by Stephen J. Herman, Laurence A. Roth, Edward L. Sinofsky and Douglas W. Dickinson, Jr. 
     FIG. 11 illustrates the output beam pattern developed by a four-fiber catheter, such as that described above, in which the four fibers are arranged in two diametrically-opposed pairs. The beam pattern from each of the four fiber ends is defined by a cone formed by the ray lines  70  in FIG.  11 . The beam from each individual fiber  42  is emitted from the distal face of the fiber  42  and enters the distal segment  72  of cap  52  through the face defining the shoulder  60 . The beam from each fiber is divergent and, in the illustrative embodiment, may have a half-angle in the range of 6°-16°, depending on the numerical aperture of the fibers which are used to construct the catheter. 
     The diverging beam from each of the fibers  42  exits from the distal emission face  74  at the end of cap  52 . FIGS. 11,  11 B and  11 C illustrate the overall beam pattern (in cross-section) which is formed by the output of the four fibers as seen along image planes  11 A,  11 B and  11 C in FIG.  11 . At plane  11 A, which is located at the emission face  74  of cap  52 , the four beams in the illustrative embodiment are still separate. At plane  11 B the diverging beams have spread further and have begun to overlap. At the plane indicated as  11 C, the beams have overlapped and define an envelop  73  having an outer diameter which is slightly greater than the outer diameter of the catheter body  104 . Preferably, at plane  11 C, beams  70  will have overlapped to merge and cover a continuous pattern. Illustratively, such a merger will have occurred within a distance from the distal face  74  of tip  52  which is approximately equal to the outer diameter of catheter  104  (a typical diameter is 1.5 millimeters).