Abstract:
This invention is a catheter-based device for generating an ultrasound excitation in biological tissue. Pulsed laser light is guided through an optical fiber to provide the energy for producing the acoustic vibrations. The optical energy is deposited in a water-based absorbing fluid, e.g. saline, thrombolytic agent, blood or thrombus, and generates an acoustic impulse in the fluid through thermoelastic and/or thermodynamic mechanisms. By pulsing the laser at a repetition rate (which may vary from 10 Hz to 100 kHz) an ultrasonic radiation field can be established locally in the medium. This method of producing ultrasonic vibrations can be used in vivo for the treatment of stroke-related conditions in humans, particularly for dissolving thrombus or treating vasospasm. The catheter can also incorporate thrombolytic drug treatments as an adjunct therapy and it can be operated in conjunction with ultrasonic detection equipment for imaging and feedback control and with optical sensors for characterization of thrombus type and consistency.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to the removal of blockages in tubular tissues and organs, and more specifically, it relates to the removal of intravascular occlusions such as atherosclerotic plaque or thrombus. 
     2. Description of Related Art 
     Ischemic strokes are caused by the formation or lodging of thrombus in the arterial network supplying the brain. Typically these occlusions are found in the carotid artery or even smaller vessels located still higher in the cranial cavity. Interventional cardiologists and vascular surgeons have devised minimally invasive procedures for treating these conditions in the vasculature elsewhere in the body. Among these treatments is ultrasound angioplasty whereby a micrc,catheter is directed to the site of an occlusion. An ultrasonic transducer is coupled to a transmission medium that passes within the catheter and transmits vibrations to a working tip at the distal end in close proximity to the occlusion. Ultrasonic catheters for dissolving atherosclerotic plaque and for facilitating clot lysis have been described previously. Improvements on these inventions have concentrated on improving the operation or function of the same basic device (Pflueger et al., U.S. Pat. No. 5,397,301). The vibrations coupled into the tissues help to dissolve or emulsify the clot through various ultrasonic mechanisms such as cavitation bubbles and microjets which expose the clot to strong localized shear and tensile stresses. These prior art devices are usually operated in conjunction with a thrombolytic drug and/or a radiographic contrast agent to facilitate visualization. 
     The ultrasonic catheter devices all have a common configuration in which the source of the vibrations (the transducer) is external to the catheter. The vibrational energy is coupled into the proximal end of the catheter and transmitted down the length of the catheter through a wire that can transmit the sound waves. There are associated disadvantages with this configuration: loss of energy through bends and curves with concomitant heating of the tissues in proximity; the devices are not small enough to be used for treatment of stroke and are difficult to scale to smaller sizes; it is difficult to assess or control dosimetry because of the unknown and varying coupling efficiency between the ultrasound generator and the distal end of the catheter. Dubrul et al., U.S. Pat. No. 5,380,273, attempts to improve on the prior art devices by incorporating advanced materials into the transmission member. Placement of the ultrasonic transducer itself at the distal end of the catheter has been impractical for a number of reasons including size constraints and power requirements. 
     A related method for removing occlusions is laser angioplasty in which laser light is directed down an optical fiber to impinge directly on the occluding material. Laser angioplasty devices have been found to cause damage or destruction of the surrounding tissues. In some cases uncontrolled heating has lead to vessel perforation. The use of high energy laser pulses at a low or moderate repetition rate, e.g. around 1 Hz to 100 Hz, results in nondiscriminatory stress waves that significantly damage healthy tissue and/or insufficient target-tissue removal when the independent laser parameters are adjusted such that healthy tissue is not affected. Use of high energy laser light to avoid thermal heating has been found to cause damage through other mechanisms associated with large cavitation bubbles and shock waves that puncture or otherwise adversely affect the tissue. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide means for dissolution of a vascular occlusion with a high frequency train of low energy laser pulses which generate ultrasonic excitation in the fluids in close proximity to the occlusion. 
     Energy transmission through a catheter is provided by using an optical fiber to guide laser pulses to the distal end. However, unlike laser angioplasty or laser thrombolysis, the present invention does not rely on direct ablation of the occlusion, but instead uses a high frequency train of low energy laser pulses to generate ultrasonic excitations in the fluids in close proximity to the occlusion. Dissolution of the occlusion is then prompted by ultrasonic action and/or by emulsification, and not directly by the interaction with the laser light. The key to inducing an ultrasonic response in the tissues and fluids lies in careful control of the wavelength, pulse duration, pulse energy and repetition rate of the laser light. The use of optical energy to induce an ultrasonic excitation in the tissue offers a number of advantages. Optical fibers can be fabricated to small dimensions, yet are highly transparent and capable of delivering substantial optical power densities from the source to the delivery site with little or no attenuation. Optical fibers are also flexible enough to navigate all vessels of interest. The present invention allows delivery of sufficient energy to generate the acoustic excitation through a small and flexible catheter, such as is required for stroke treatment. The method may also incorporate a feedback mechanism for monitoring and controlling the magnitude of the acoustic vibrations induced in the tissue. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a sketch of an application of the optical fiber-based opto-acoustic thrombolysis catheter of the present invention. 
     FIG. 1B depicts the ultrasonic dissolution of a blockage using an adjunct fluid. 
     FIGS. 2A-C depict the thermo-elastic operation of the present invention. 
     FIGS. 3A-C depict the superheated vapor expansion mode of the present invention. 
     FIG. 4A shows a fiber optic having a concave tip. 
     FIG. 4B shows a fiber optic having a convex tip. 
     FIG. 5 shows a bundle of fiber strands. 
     FIG. 6 shows a variable diameter fiber optic. 
     FIG. 7 shows a composite of a glass/plastic fiber. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention incorporates a catheter containing an optical fiber. The optical fiber is coupled at the proximal end to a high repetition rate laser system which injects pulses of light into the fiber. The light emerging from the fiber at the distal end is absorbed by the fluid surrounding the catheter. This fluid may be blood, a biological saline solution containing an absorbing dye, a thrombolytic pharmaceutical or thrombus itself The optical fiber functions as a means of energy transmission such that the optical energy produced by the laser is delivered to the end of the fiber. The laser light emerging from the distal end of the fiber optic has a pulse frequency within the range of 10 Hz to 100 kHz, a wavelength within the range of 200 nm to 5000 nm and an energy density within the range of 0.01 J/cm 2  to 4 J/cm 2 . In one embodiment, the pulse frequency is within the range of 5 kHz to 25 kHz. 
     Lysis of thrombus, atherosclerotic plaque or any other occluding material in the tubular tissue is facilitated by an ultrasonic radiation field created in the fluids near the occlusion. As an adjunct treatment, a working channel which surrounds or runs parallel to the optical fiber may be used to dispense small quantities of thrombolytic drugs to facilitate further lysis of any significantly sized debris (&gt;5 μm dia. particles) left over from the acoustic thrombolysis process. The conversion of optical to acoustic energy may proceed through several mechanisms that may be thermoelastic, thermodynamic or a combination of these. FIG. 1A shows an optical fiber 10 with a parallel working channel 12, where both the fiber 10 and the working channel 12 are both located within a catheter 14 which has been inserted into a blood vessel 16. The distal end of fiber 10 is placed near thrombus 18 and stenotic plaque 20 within blood vessel 16. A laser 2, controlled by laser controller 4, provides laser light 6 into fiber 10. In FIG. 1B, fiber 10 delivers laser light to produce a collapsing cavitation bubble 11 and the resulting expanding acoustic wave 13. A parallel working channel 12 in catheter 14 delivers an adjunct fluid 15 to aid in the removal of occlusion 17 from inside blood vessel 16. 
     As depicted in FIGS. 2A-C, in the thermoelastic mode, each laser pulse 22 delivers a controlled level of energy in the fluid 24 which creates a large thermoelastic stress in a small volume of the fluid. The expanding direction of this stress is indicated by arrows 25 in FIG. 2A. The volume of fluid 24 which is heated by the laser pulse 22 is determined by the absorption depth of the laser light in the fluid 24, and must be controlled to produce a desired size. For example, an appropriate size may be the fiber diameter, or a distance comparable to some fraction of the vessel containing the occlusion. This can be adjusted by controlling the laser wavelength or the composition of the fluid such that most of the laser energy is deposited in a fluid depth of the desired size. The laser pulse duration is short enough to deposit all of the laser energy into the absorbing fluid in a time scale shorter than the acoustic transit time across the smallest dimension of absorbing region. This is an isochoric (constant volume) heating process. For an absorption volume of approximately 100 μm in diameter the acoustic transit time is approximately 70 ns, so the deposition time must be significantly less than this, e.g., around 10 ns. 
     The absorbing fluid responds thermoelasticaly to the deposition of energy such that a region of high pressure is created in the fluid in the heated volume. The boundary of the high pressure zone decays into a pattern of acoustic waves: a compression wave propagates away from the energy deposition region (diverging wave front) and a rarefaction wave propagates towards the center of the energy deposition region (converging wave front). When the rarefaction wave converges on the center of the initial deposition region, it creates a region 26 of tensile stress that promotes the formation of a cloud of cavitation bubbles which coalesce to form a larger bubble 30. Eventually, the cavitation bubble collapses (32), resulting in an expanding acoustic wave 33. Collapse and subsequent rebound of the cavitation bubble will generate acoustic impulses in the surrounding fluid, which will carry off a portion of the energy of the cavity. The collapse and rebound processes take place on a time scale governed principally by the fluid density and the maximum size of the initial cavity. The first collapse and rebound will be followed by subsequent collapse and rebound events of diminishing intensity until the energy of the cavity is dissipated in the fluid. Subsequent laser pulses are delivered to repeat or continue this cycle and generate an ultrasonic radiation field at a frequency or frequencies determined by the laser pulse frequency. 
     To summarize, a device operating through the first mode produces an ultrasonic radiation field in the fluid by: (i) depositing laser energy in a volume of fluid comparable to the fiber dimension in a time scale of duration less than the acoustic transit time across this dimension (as controlled by choice of laser wavelength and absorbing fluid as the case may be); (ii) controlling the laser energy such that the maximum size of the cavitation bubble is approximately the same as the fiber diameter; and (iii) pulsing the laser at a repetition rate such that multiple cycles of this process generate an acoustic radiation field in the surrounding fluid; resonant operation may be achieved by synchronizing the laser pulse repetition rate with the cavity lifetime. Typical operation leads to a fluid-based transducer that cycles at 1-100 kHz with a reciprocating displacement of 100-200 μm (for typical optical fiber dimensions). This displacement is very similar to that found in mechanically-activated ultrasound angioplasty devices. 
     In the superheated vapor expansion mode, as shown in FIGS. 3A-C, in fiber optic 41, each laser pulse 40 delivers a controlled level of energy in the fluid within an absorption depth which is very small compared to the characteristic size of the vessel containing the catheter, or even small compared to the fiber diameter. The absorption depth may also be small compared to the distance that a sound wave travels in the duration of the laser pulse. The laser energy deposits a sufficient level of energy to heat all of the fluid within the absorption depth well above the vaporization temperature of the fluid at the ambient pressure. In the process of depositing the laser energy, a thermoelastically-generated acoustic wave is launched in the fluid, which propagates out from the heated region. On time scales longer than 1 μs, the superheated fluid 42 undergoes vaporization, which creates a bubble of vapor. As the fluid vaporizes, its volume 44 increases by a large factor, hence the need for involving only a small layer of fluid such that the ultimate size of the vapor bubble does not exceed, for example, the vessel diameter. 
     The laser pulse duration need not be restricted to times as short as in the thermoelastic mode since the bubble expansion is nearly an isobaric process; however, the laser pulse duration should be shorter than the bubble expansion time, and it should be much shorter than a typical thermal relaxation time for the superheated region. (According to the Rayleigh bubble collapse theory the bubble lifetime is approximately 25 μs for a 50 μm diameter bubble; thermal relaxation occurs on a few hundred microsecond time scale, so the laser pulse should be several microseconds or less in duration). The vapor bubble expands up to a maximum radius which depends on the vapor pressure initially created in the fluid. At the maximum bubble radius, the vapor pressure in the expanded bubble has dropped to well below the ambient pressure and the bubble 46 undergoes collapse, resulting in an expanding acoustic wave 48. Rebound and subsequent collapse events may take place following the first collapse. The bubble expansion and collapse couples acoustic energy into the fluid. Subsequent laser pulses are delivered to repeat or continue this cycle and generate an ultrasonic radiation field at a frequency or frequencies determined by the laser pulse frequency. Similar to the first mode, a resonant operation may be achieved by matching the laser pulse period to the lifetime of the vapor bubble. 
     To summarize, a device operating through the second mode produces an ultrasonic radiation field in the fluid by: (i) depositing laser energy in a small volume of fluid (as controlled by choice of laser wavelength and absorbing fluid as the case may be); (ii) controlling the laser energy such that the maximum size of the vapor bubble is approximately the same as or less than the diameter of the vessel being treated; and (iii) pulsing the laser energy at a repetition rate such that multiple cycles of the bubble generation and collapse process generates an acoustic radiation field in the surrounding fluid. Unlike the first mode, the delivery time is not a significant issue, so longer pulse duration lasers (up to several μs) may be useful. 
     For either mode of operation the laser wavelength, laser pulse duration and laser absorption depth must be precisely, controlled such that an adequate acoustic response is obtained with a minimum of laser pulse energy. For the first mode this entails matching the absorption volume to a characteristic dimension of the system such as the fiber diameter or some fraction of the vessel diameter, and using a short laser pulse (less than 20 ns). For second mode this entails depositing the laser energy in a very small absorption depth to achieve a sufficient level of superheat in a small fluid mass such as can be accommodated by a small energy budget and without creating a vapor bubble so large as to be damaging to the surrounding tissues. 
     These opto-acoustic modes of coupling laser energy into acoustic excitations in tissues include a number of features. Low to moderate laser pulse energy combined with high repetition rate avoids excessive tissue heating or intense shock generation. Localized absorption of the laser energy occurs. Laser energy may interact thermoelastically or thermodynamically with the ambient fluids. An acoustic radiation field is generated by repeated expansion and collapse of a bubble at the tip of the fiber. Resonant operation may be achieved by matching the laser pulse period to the lifetime of the generated bubble. Soft fibrous occlusions (thrombus) may be dissolved by generating the bubbles directly within the thrombus. 
     Control and/or manipulation of the spatial and temporal distribution of energy deposited in the fluid at the fiber tip can be used modify the near field acoustic radiation pattern, for example, to concentrate acoustic energy on an object in proximity to the fiber, or to distribute the acoustic radiation more uniformly. Techniques based on this strategy will be most successful for a special case of thermoelastic response (first mode) where the laser pulse duration is short and the fluid absorption is also relatively strong, such that the laser energy is deposited in a thin layer adjacent to the surface of the fiber tip. For example, by forming a concave surface on the fiber tip, the optical energy is deposited in the fluid in a similar shaped distribution. Acoustic waves emitted from this concave distribution will tend to focus to a point at a distance R from the fiber tip, where R is the radius of curvature of the concave surface. A planar fiber tip will generate an initially planar acoustic wavefront in proximity the fiber tip. A convex fiber tip will produce a diverging spherical wavefront which will disperse the acoustic energy over a larger solid angle. Another means of modifying the near field radiation pattern may be to use a fiber bundle through which the laser energy is delivered, and control the temporal distribution of deposited laser energy. The laser energy may be arranged to arrive at individual fiber strands in the catheter tip at different times, which, in combination with the different spatial positions of these individual strands, can be adjusted to control the directionality and shape of the acoustic radiation pattern, similar to phased-array techniques used in radar. FIG. 4A shows a modified fiber optic 50 having a concave distal end 52. FIG. 4B shows a fiber optic 50 with a convex distal end 54. FIG. 5 shows a modified fiber optic 56 consisting of a bundle of fiber strands 58, through each of which laser pulse energy is delivered at varying times. 
     Commercial fibers are usually jacketed to protect them from the environment. &#34;Bare&#34; or unjacketed fibers are available. It is helpful to use coatings on fibers to make them slide more easily through catheters. As shown in FIG. 6, a variable diameter optical fiber 60 allows for greater physical strength at the proximal end 62 and greater access at the distal end 64. This can be accomplished through modifying existing fibers (stripping the protective sheath from around the core) or by making custom fibers. Custom fabrication can be accomplished by varying the extrusion or draw rate for the fiber. Glass or plastic composition can be changed as a function of drawing the fiber so that greater control of the fiber from a distal end is achieved without sacrificing optical quality. One particular instance of this is to treat the tip so that it is &#34;soft,&#34; so the end will not jam in the catheter sheath. Also, shape memory in the tip allows steering of the fiber when it protrudes from the distal end of the catheter sheath. 
     FIG. 7 shows a composite of a glass/plastic fiber. Fiber 70 comprises a glass portion 72 with a relatively short plastic tip 74 which has a length within the range of a millimeter to a several centimeters. Due to the rigidity of the glass portion 72, a fiber optic having this configuration is easily pushed through vasculature. The softer plastic tip 74 is less likely to puncture a vein wall than a glass tip. This configuration could include an additional glass tip to increase the durability of the fiber optic. 
     Acoustic energy at many frequencies is generated in the present invention, and may be considered as a signal source for producing acoustic images of structures in body tissues. Any signal detection and analysis system which relies on a point source of acoustic radiation to produce the signal may be used with this invention. 
     Applications envisioned for this invention include any method or procedure whereby localized ultrasonic excitations are to be produced in the body&#39;s tissues through application of a catheter. The invention may be used in (i) endovascular treatment of vascular occlusions that lead to ischemic stroke (This technology can lyse thrombus and lead to reperfusion of the affected cerebral tissue), (ii) endovascular treatment of cerebral vasospasm (This technology can relax vaso-constriction leading to restoration of normal perfusion and therefore prevent further transient ischemic attacks or other abnormal perfusion situations), (iii) endovascular treatment of cardiovascular occlusions (This technology can lyse thrombus or remove atherosclerotic plaque from arteries), (iv) endovascular treatment of stenoses of the carotid arteries, (v) endovascular treatment of stenoses of peripheral arteries, (vi) general restoration of patency in any of the body&#39;s luminal passageways wherein access can be facilitated via percutaneous insertion, (vii) any ultrasonic imaging application where a localized (point) source of ultrasonic excitation is needed within an organ or tissue location accessible through insertion of a catheter, (viii) lithotriptic applications including therapeutic removal of gallstones, kidney stones or other calcified objects in the body and (ix) as a source of ultrasound in ultrasound modulated optical tomography. 
     The pulsed laser energy source used by this invention can be based on a gaseous, liquid or solid state medium. Rare earth-doped solid state lasers, ruby lasers, alexandrite lasers, Nd:YAG lasers and Ho:YLF lasers are all examples of lasers that can be operated in a pulsed mode at high repetition rate and used in the present invention. Any of these solid state lasers may incorporate non-linear frequency-doubling or frequency-tripling crystals to produce harmonics of the fundamental lasing wavelength. A solid state laser producing a coherent beam of ultraviolet radiation may be employed directly with the invention or used in conjunction with a dye laser to produce an output beam which is tunable over a wide portion of the ultraviolet and visible spectrum. Tunability over a wide spectrum provides a broad range of flexibility for matching the laser wavelength to the absorption characteristics of the fluids located at the distal end of the catheter. The output beam is coupled by an optical fiber to the surgical site through, for example, a percutaneous catheter. In operation, a pulsed beam of light drives the ultrasonic excitation which removes and/or emulsifies thrombus or atherosclerotic plaque with less damage to the underlying tissue and less chance of perforating the blood vessel wall than prior art devices. 
     Various other pulsed lasers can be substituted for the disclosed laser sources. Similarly, various dye materials and configurations can be used in the dye laser. Configurations other than a free-flowing dye, such as dye-impregnated plastic films or cuvette-encased dyes, can be substituted in the dye laser. The dye laser can also store a plurality of different dyes and substitute one for another automatically in response to user-initiated control signals or conditions encountered during use (e.g. when switching from a blood-filled field to a saline field or in response to calcific deposits). Suitable dyes for use in the dye laser components of the invention include, for example, P-terphenyl (peak wavelength 339); BiBuQ (peak wavelength: 385); DPS (peak wavelength: 405); and Coumarin 2 (peak wavelength: 448). 
     In yet another embodiment the pulsed light source may be an optical parametric oscillator (OPO) pumped by a frequency-doubled or frequency-tripled solid-state laser. OPO systems allow for a wide range of wavelength tunability in a compact system comprised entirely of solid state optical elements. The laser wavelength in OPO systems may also be varied automatically in response to user-initiated control signals or conditions encountered during use. 
     Catheters, useful in practicing the present invention, can take various forms. For example, one embodiment can consist of a catheter having an outer diameter of 3.5 millimeters or less, preferably 2.5 millimeters or less. Disposed within the catheter is the optical fiber which can be a 400 micron diameter or smaller silica (fused quartz) fiber such as the model SG 800 fiber manufactured by Spectran, Inc. of Sturbridge, Mass. The catheter may be multi-lumen to provide flushing and suction ports. In one embodiment the catheter tip can be constructed of radio-opaque and heat resistant material. The radio-opaque tip can be used to locate the catheter under fluoroscopy. 
     The invention can be used with various catheter devices, including devices which operate under fluoroscopic guidance as well as devices which incorporate imaging systems, such as echographic or photoacoustic imaging systems or optical viewing systems. For one example of a photoacoustic imaging system which can be specifically adapted for the catheter environment, see U.S. Pat. No. 4,504,727 incorporated herein by reference. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.