Abstract:
A method and apparatus for treating lesions and tattoos sites. The method includes the steps of irradiating said lesion by a first single pulse Q-Switched laser beam directed onto the site; and again irradiating the lesion by a second single pulse Q-Switched laser beam directed onto the same lesion site within a time interval of less than about 100 μs.

Description:
This application claims the benefit of Provisional Application No. 60/273,165 filed Mar. 3, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to laser apparatus and more particularly to laser apparatus for the treatment and/or removal of lesions and tattoos. 
     2. Prior Art 
     Pigmented lesions are common conditions of the skin of humans. Those lesions may be dermal lesions and include nevus of ota, nevus of ito, or epidermal lesions, including solar lentigenes, and freckles, liver or age spots, and birth marks. 
     Pigmented lesions include tattoos. These may be human caused tattoos and traumatic tattoos which are the result of an accident or mishap such as a scrape or an abrasion or the like where some foreign material becomes embedded under the skin. In each case, the skin becomes pigmented and scarred. The tattoos are made by dyes or inks which are deposited into the skin by a needle to create coloration and patterns on the skin of an individual. Tattoos are usually created by a vibrating needle by which colored pigments are introduced into the skin, usually to the papillary layer of the dermis. Tattoos may be green, blue, brown, black, bluegreen, aqua and red, yellow or orange. At some point in the lives of individuals having such lesions or tattoos, a decision is made in the attempt to remove those same colorations from their skin. Treatment of such pigmented lesions in the field of dermatology often involves a short pulse Q-Switched laser. Absorption of the energy of a short pulse from a Q-Switched laser effects a rapid heating and high pressure in the target tissue which is exposed to the laser radiation, resulting in an efficient breakup of that tissue structure. The disrupted structure begins to clear up by the normal immunological response. The tattoo is such a structure which is treated by means of the short pulse Q-Switched laser. Such lasers may include the ruby laser, the Alexandrite laser and the Nd:YAG laser. 
     Q-Switched Alexandrite lasers are commonly limited to an output of about one joule/pulse. The fluence required to treat a tattoo effectively then limits the area that can be treated from a single pulse to the order of about 3 mm diameter. Larger output is possible by the use of amplifier stages or by the use of large volume laser rods. These methods are complex and expensive and have limited commercial appeal. 
     The prior art such as found in U.S. Pat. No. 5,217,455 and 5,290,273, both issued to Oon T. Tan disclose Q-switched Alexandrite laser arrangements for the treatment of tattoos. Such treatment utilizes a single pulse of laser radiation to a chosen site, with multiple treatments applied over a period of weeks and/or months. 
     It is an object of the present invention to provide a method and apparatus for the treatment for pigmented lesions and tattoos which is an improvement over the prior art. 
     It is a further object of the present invention to provide a Q-Switched laser having improved output pulses and output energy for the treatment of lesions and tattoos than does the prior art. 
     It is still a further object of the present invention, to provide a Q-Switched laser which permits larger spot sizes at high repetition rate for treatment for lesions and tattoos than does the prior art. 
     It is still yet a further object of the present invention to provide a Q-Switched laser which minimizes the overall treatment time necessary for lesions and tattoos. 
     It is still yet a further object of the present invention to provide a Q-Switched laser arrangement which will minimize the number of treatments necessary for lesions and tattoos. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention involves a laser arrangement for the treatment of pigmented lesions or tattoos. In a typical treatment of a pigmented lesion or tattoo, the area of the lesion is irradiated with the laser. The laser output is characterized by short pulses of high peak power often in excess of 10 M watt. Immediately following a laser pulse, the area irradiated undergoes blanching caused by vaporization of tissue in the neighborhood of the pigment. A second laser pulse delivered to this area after vaporization would experience a great deal of scattering, thus preventing the laser light from reaching the intended target and consequently not being very effective at providing any further breakup of the pigmented structure. The time frame for the tissue vaporization to occur is or the order of 100 μs. If a second Q-Switched pulse is delivered to the pigmented lesion within that time interval, it would not experience the large scattering and therefore that second laser pulse can be very effective in treating that lesion. 
     It is common in the treatment of tattoos to increase the laser fluence as the lesion clears. The increase is needed since as the tattoo clears, there is less pigment to absorb the laser energy and, increased fluence (energy per unit area provided by a laser beam at a target site) is needed to insure that sufficient energy is absorbed by the pigment. If two pulses are delivered to the tissue in a window of less than about 100 μs, then the amount of pigment available to absorb the laser will be the same for both pulses. 
     A typical flashlamp excited solid state laser such as an Alexandrite laser uses a gain medium typically in the shape of a cylindrical rod. A flashlamp provides radiation needed to excite the rod, and a reflective chamber is used to insure that the radiation from the lamp reaches the laser rod. The radiation from the flashlamp excites the gain medium. Spontaneous radiation is emitted from the excited gain medium. Some of this radiation is reflected back into the gain medium by a pair of carefully aligned mirrors that form a resonator. This radiation experiences amplification as it traverses the gain medium. One of the mirrors is partially transmitting thus allowing a useful output from the laser. Stable laser oscillation begins when the round trip gain experienced by the radiation exactly balances the round trip losses, including the amount of radiation that exits the laser as useful output. 
     If a large loss is introduced in the laser resonator, the gain needed to achieve laser oscillation will be very large. A large amount of energy can be deposited in the laser rod achieving a large gain without laser oscillation taking place. If the losses are removed very rapidly while the gain in the medium is very high, the resulting laser will be well above threshold. The radiation in the resonator will grow very rapidly and a giant, short pulse will be developed and a portion of the energy previously deposited in the rod will be its output. This process is called “Q-Switching”. 
     The fraction of the energy stored in the rod that is extracted in a Q-Switched pulse depends on a number of factors including how much above threshold the laser is immediately after the losses are removed from the resonator, the energy resident in the rod at that time, and an inherent quantity of the gain medium commonly called the “saturation” fluence. This is the fluence that must be present in the resonator in order to extract a large fraction of the energy stored in the rod. The saturated fluence is the ratio of the emitted photon energy divided by the stimulated emission cross-section of the material. For the case of Alexandrite, the stimulated emission cross-section is very small, resulting in a large saturation fluence. As a result, in a common Alexandrite laser under flashlamp excitation, only a small fraction of the energy stored is extracted in the Q-Switch process. A significant fraction remains stored in the rod, If the resonator losses are restored, and the flashlamp excitation is extended past the time where the Q-Switch pulse was extracted, the stored energy in the rod will once again increase. Since a significant amount of energy is already present in the rod, the stored energy will reach a level equal to the level at which the first pulse was extracted with less energy from the flashlamp. That is, a second pulse whose energy is equal to that of the first may be extracted from the rod and will require less additional energy than the first one. The process may be continued for additional pulses. 
     To achieve efficient two pulse operation, the time between the two pulses should not exceed the radiative lifetime of the excited medium. For the case of Alexandrite, this time of the order of 100 μs. It follows then that a dual pulse Alexandrite laser can meet the requirements set forth for a dual pulse tattoo treatment. 
     There are many benefits to this type of operation. First, it is clear that the laser output will be twice that of a typical Q-Switched laser without the need for amplifiers or a large expensive laser rods. Because the peak power is not increased, the laser delivery optics are not stressed more than they are in a singe pulse case. This is particularly important when the laser delivery uses optical fibers. In many cases these fibers operate close to their damage point and doubling the output energy in a single pulse can otherwise seriously reduce their reliability. 
     The “pulse timing” of the present invention may, in further embodiments be utilized in a treatment with a catheter-based light generation (Alexandrite laser) application where similar limited-time constraints on the order of 1 millisecond or less (such as for example as low as 0.1 millisecond) are needed. Those applications may include the opening of occluded lumens, the breakup of objects within the body or in body lumens. Such examples may include the treatment of arthroscopic plaque, kidney stones or calculi or the like. 
     The present invention thus comprises a method of treating a lesion or tattoo site on tissue of a human patient in a lasing procedure by the steps of: irradiating the lesion by a first independent single pulse Q-Switched laser beam directed onto the site; and irradiating the lesion by a second independent single pulse Q-Switched laser beam directed onto the site within a time interval of less than about 100 μs after the first pulse. 
     The method may also include the steps of: irradiating the lesion again by a further single pulse Q-Switched laser beam directed onto the site within the time interval of less than about 100 μs. The first pulse and the second pulse may each have a range of pulse duration of about 50 nanoseconds to about 75 nanoseconds. The first pulse and the second pulse may each have preferably a pulse duration of about 60 nanoseconds. The Q-switched laser may be comprised of Alexandrite. The Alexandrite laser may have a wavelength of about 755 nm. The Alexandrite laser may have a maximum fluence of about 30 Joules/cm 2  at 2.4 mm. The Alexandrite laser may have a maximum fluence of about 20 Joules/cm 2  at 3.0 mm. The Alexandrite laser may have a maximum fluence of about 7 Joules/cm 2  at 5.0 mm. 
     The present invention may also comprise a flashlamp excited Alexandrite laser arrangement with a driven laser rod for the treatment of pigmented lesions and tattoos. The arrangement may include: a Q-Switch driver for effecting Q-Switching of the laser resonator to output single laser pulses onto a site to be treated wherein at least two single pulses are enabled to be applied to the site within a time window equivalent to about 100 μs. Each of the single laser pulses may preferably have a pulse duration in a range of about 50 nanoseconds to about 75 nanoseconds. Each of the single laser pulses may have a pulse duration of about 60 nanoseconds. The Alexandrite laser may have a wavelength of about 755 nm. The laser may have a maximum fluence of about 30 J/cm 2  at 2.4 mm. The laser may in another embodiment have a maximum fluence of about 20 J/cm 2  at 3.0 mm. The laser in a further embodiment have a maximum fluence of about 7 J/cm 2  at 5.0 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and advantages of the present invention will become more apparent when viewed in conjunction with the following drawings in which: 
     FIG. 1 is a schematic representation of a laser apparatus for the treatment of pigmented lesions and tattoos; and 
     FIG. 2 discloses traces of pulses associated with the laser apparatus for treatment of pigmented lesions and tattoos. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in detail and particularly to FIG. 1, there is shown the present invention which comprises a laser arrangement  10  for the treatment of pigmented lesions or tattoos  12  on a mammalian patient “P”. The laser arrangement  10  comprises a laser rod  16 , of in a first preferred embodiment for example Alexandrite, excited by a flashlamp  18 . 
     Further preferred embodiments of the laser rod  16  may be comprised of a Ruby laser or a laser rod comprised of Nd:YAG. The flashlamp  18  for these embodiments is powered by a proper circuit  20  including a high voltage power supply  22 . The laser rod  16  has a 100% reflective mirror  23  at a rear end thereof, and a partial reflective mirror  25  at its output end as may be seen in FIG. 1. A Q-Switch  24 , controlled by an interconnected Q-Switch driver  26  governs the pulse generation and output of the laser  10 . In a typical treatment of a pigmented lesion or tattoo  12 , the area of the lesion is irradiated with the laser beam  14 . The laser output is characterized by a high peak power often in excess of 10 M watt. The preferred pulse duration (the time interval during which a laser beam strike&#39;s its target site, typically of nanosecond length) for the present invention may extend for a range of about 50 to 75 nanoseconds, and is preferably about 60 nanoseconds. Almost immediately following the laser pulse, the area irradiated undergoes blanching caused by vaporization of tissue in the neighborhood of the pigment. A second laser pulse delivered to this area of the lesion or tattoo  12  after tissue vaporization would experience a great deal of beam pulse scattering and not be very effective at providing any further breakup of the pigmented structure. The time frame for the vaporization to occur is or the order of about 100 μs. If a second Q-Switched pulse of about 60 nanosecond duration is delivered to the pigmented lesion within that critical time interval of about 100 μs from the first pulse, that second Q-switched pulse would not experience the large scattering and that second laser pulse would be very effective, as has been demonstrated clinically. The second pulse thus improves the effect and treatment realized in a single clinician visit. 
     It is common in the prior art treatment of tattoos to increase the laser fluence as the lesion clears. The increase is needed since as the tattoo clears, there is less pigment to absorb the laser energy and, increased fluence is needed to insure that sufficient energy is absorbed. 
     In the present invention however, at least two Alexandrite laser pulses, each pulse of about 60 nanosecond duration, are delivered to the tissue target site in a “time window” of less than about 100 μs, so that the amount of pigment available to absorb the laser will be the same for each pulse. That is, both pulses can be equally effective. Further embodiments contemplate a combination of multiple pulses which may become even more effective depending upon the number of pulses within that 100 μs “window”. 
     As may be seen in FIG. 2, pulse traces are displayed for the laser treatment arrangement  10  shown in FIG.  1 . Trace “a” represents the current pulse that drives the laser flashlamp  18 . As indicated on trace “c”, at the time the flashlamp  18  is fired, a drive signal from the Q-Switch driver  26  is sent to the Q-Switch  24  to deter laser oscillation. Trace “b” shows the laser gain. The gain rises as a result of excitation caused by radiation from the flashlamp  18 . Near the peak of the gain, the signal from the Q-Switch drive  26  is removed as indicated on trace “c” at time t 1 . 
     A large first output pulse is generated by the laser rod  16 , as indicated at t 1 , in trace “d”. The laser gain drops as energy is output as is evidenced in trace “b”. The Q-Switch driver  26  signal is restored and the gain rises once again. When the gain has recovered to its near maximum, the Q-Switch drive  26  is again removed and a further (second) large output pulse is produced from the laser  16  at time t 2 . 
     A typical flashlamp excited solid state laser such as an Alexandrite laser uses a gain medium typically in the shape of a cylindrical rod, as may be seen in FIG.  1 . The flashlamp  18  provides radiation needed to excite the laser rod  16 , and the reflective chamber  19  is used to insure that the radiation from the lamp reaches the laser rod. The radiation from the flashlamp excites the gain medium. Spontaneous radiation is thus emitted from the excited gain medium. Some of this radiation is reflected back into the gain medium by a pair of carefully aligned mirrors  23  and  25  that form a resonator. This radiation experiences amplification as it traverses the gain medium. One of the mirrors  25  is partially transmitting thus allowing a useful output from the laser rod  16 . Stable laser oscillation begins when the round trip gain experienced by the radiation exactly balances the round trip losses, including the amount of radiation that exits the laser  10  as useful output. 
     If a large loss is introduced in the laser resonator, the gain needed to achieve laser oscillation will be very large. A large amount of energy can be deposited in the laser rod  16  and a large gain achieved without laser oscillation taking place. If the losses are removed very rapidly while the gain in the medium is very high, the resulting laser will be well above threshold. The radiation in the resonator will grow very rapidly and a giant, short pulse will be developed and a portion of the energy previously deposited in the rod  16  will be its output. This is the process identified hereinabove as “Q-Switching”. 
     The fraction of the energy stored in the rod  16  that is extracted in a Q-Switched pulse depends on a number of factors including how much above threshold the laser is immediately after the losses are removed from the resonator, the energy resident in the rod at that time, and an inherent quantity of the gain medium commonly called the “saturation” fluence. This is the fluence that must be present in the resonator in order to extract a large fraction of the energy stored in the rod  16 . The saturated fluence is the ratio of the emitted photon energy divided by the stimulated emission cross-section of the material. For the case of Alexandrite, the stimulated emission cross-section is very small, resulting in a large saturation fluence. As a result, in a common Alexandrite laser under flashlamp excitation, only a small fraction of the energy stored is extracted in the Q-Switch process. A significant fraction remains stored in the rod. If the resonator losses are restored, and the flashlamp excitation is extended past the time where the Q-Switch pulse was extracted, the stored energy in the rod will once again increase. Since a significant amount of energy is already present in the rod, the stored energy will reach a level equal to the level at which the first pulse was extracted with less energy from the flashlamp. That is, a second pulse whose energy is equal to that of the first may be extracted from the rod. The generation of this second pulse will require less additional energy than that of the first one. The process may be continued for additional pulses. 
     To achieve efficient two pulse operation, the time between the two pulses on trace “d” should not exceed the radiative lifetime of the excited medium. For the case of flashlamp excited Alexandrite, this time “window” is of the order of 100 μs or less. It follows then that a dual pulse Alexandrite laser having a wavelength of about 755 nm and maximum fluence of about 30 J/cm 2  at 2.4 mm; about 20 J/cm 2  at 3.0 mm; or about 7 J/cm 2  at 5 mm can meet the requirements set forth for a dual pulse tattoo or pigmented lesion treatment.