Abstract:
A device for irradiating tissue includes a fluorescent element for receiving pump radiation and responsively emitting radiation having different spectral characteristics than the pump radiation. A redirector receives emitted radiation promulgated in a direction away from a tissue target and redirects the radiation toward the target. The pump radiation may be supplied, for example, by a flashlamp or frequency-doubled neodymium-doped laser. Use of the device provides an inexpensive and effective alternative to conventional dye laser-based systems for various medical therapies, including treatment of vascular and pigmented lesions, tattoo and hair removal, and photodynamic therapy (PDT).

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to optical devices, and more particularly to devices for irradiating tissue for use in medical procedures. 
   BACKGROUND OF THE ART 
   A variety of medical procedures utilize a laser or other radiation source to irradiate a tissue target. Examples of such procedures include dermatological therapies such as treatment of vascular lesions and removal of tattoos and unwanted hair, as well as non-dermatological procedures such as photodynamic therapy (PDT) for treatment of tumors. In procedures involving irradiation of a tissue target, it is usually desirable to match the spectral characteristics of the light produced by the radiation source with the absorption characteristics of the target. This matching promotes efficient absorption of the radiation by the target (which is necessary to effect the localized heating or ablation of the target) and may minimize thermal damage to adjacent tissue. 
   To facilitate matching of the spectral characteristics of the radiation source with the absorption characteristics of the target, some medical procedures employ a dye laser as the radiation source. An example of one such dye laser is described in U.S. Pat. No. 5,066,293 to Furomoto (“Light Amplifier and Method of Photothermolysis”). The output wavelength of the dye laser is controlled by means of the choice of dye and/or adjustment of a tuning element such as an intracavity rotatable birefringent filter. Further, dye lasers are typically capable of delivering radiation having output energies and pulse durations suitable for a range of medical applications. 
   Disadvantages associated with dye lasers include their high expense and complexity. Misalignment of or damage to optical components, malfunctioning of the dye recirculation system, and/or problems with control circuitry may cause the tunable dye laser to become partially or fully inoperative, leading to downtime and substantial repair or replacement costs. Further, owing to their relative complexity, it may be necessary to provide extensive training and practice to clinicians before they are able to competently operate dye laser-based systems. 
   SUMMARY 
   According to one embodiment of the invention, a device for irradiating tissue is provided having a fluorescent element positioned to receive incident pump radiation. The fluorescent element may comprise, without limitation, a laser dye compound dispersed in a solid medium such as polyvinyl toluene, an encapsulated liquid dye solution, or a laser crystal such as ruby. Responsive to receipt of the incident pump radiation, the fluorescent element fluoresces and emits radiation having spectral characteristics substantially different from the spectral characteristics of the pump radiation. 
   Because the fluorescent element emits radiation in a diffuse manner, i.e., without a preferred direction, at least a portion of the emitted radiation travels in a direction away from the tissue target. The device is therefore provided with a redirector for redirecting toward the tissue target the portion of emitted radiation initially directed away from the target. In one embodiment, the redirector comprises a diffuse reflector having an elongated frustro-conical shape. Emitted radiation entering the redirector undergoes multiple reflections in a random-walk fashion and eventually exits the redirector travelling in the direction of the target. The device may be additionally provided with a transparent window having a first face positioned proximal the fluorescent element and a second face held in contact with the target. The window may be cooled to minimize thermal damage to tissue adjacent the target. 
   Devices of the foregoing description may be utilized to irradiate tissue for a number of medical procedures, including without limitation selective photothermolysis of vascular lesions, tattoo removal, treatment of wrinkles and stretch marks, and PDT. In practice, a clinician performing a procedure may simply select a device having a florescent element which emits radiation having spectral characteristics appropriate to the procedure and the absorption characteristics of the target tissue and connect the device to a source of pump radiation. Because the device utilizes fluorescence rather than lasing to generate the emitted radiation, the device can be manufactured inexpensively, is significantly less prone to malfunction, and is relatively easy to use when compared to prior art systems utilizing dye lasers. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     In the accompanying drawings: 
       FIG. 1  is a system for irradiating tissue employing the fluorescent device of the present invention; 
       FIG. 2  is a cross-sectional view of a first embodiment of the fluorescent device; 
       FIG. 3  depicts a fragmentary view of an exemplary reflective surface utilized in the fluorescent device; 
       FIG. 4  is a cross-sectional view of a second embodiment of the fluorescent device; 
       FIG. 5  depicts absorption and emission spectra of a representative fluorescent dye utilized in the fluorescent device; 
       FIG. 6(   a ) is a cross-sectional view of a third embodiment of the fluorescent device; and 
       FIG. 6(   b ) is a top plan view of an entrance face of the third embodiment. 
   

   DETAILED DESCRIPTION 
   Referring initially to  FIG. 1 , there is depicted in block form a system  100  for irradiating a tissue target  102  utilizing a device  104  of the present invention. System  100  generally comprises a pump radiation source  106  configured to generate pump radiation and supply the pump radiation to device  104 . Various light-emitting devices may be employed for pump radiation source  106  including, without limitation, a frequency-doubled neodymium-doped solid-state laser, an arc lamp or a flashlamp. In a typical mode of operation, pump radiation source  106  generates and supplies pulsed radiation having a wavelength of  532  nanometers and a pulse duration of between  0 . 1  and  500  milliseconds. Alternatively, pump radiation source  106  may generate CW or quasi-CW radiation. U.S. Pat. No. 5,151,909 to Davenport et al. (“Frequency Doubled Solid State Laser Having Programmable Pump Power Modes and Method for Controllable Lasers”) describes an example of a Nd:YAG laser which may be utilized for pump radiation source  106 . 
   Pump radiation source  106  is optically coupled to device  104  by optical fiber  108 , which delivers the pump radiation to a fluorescent element (not shown in  FIG. 1 ) held within a housing  110  of device  104 . Optical fiber  108  is flexible and thereby allows device  104  to be freely positioned relative to pump radiation source  106 . Alternatively, an articulated arm extending between pump radiation source  106  and device  104  may be utilized for optical coupling in place of optical fiber  108 . In other embodiments of the invention, pump radiation source  106  may be integrated within housing  110  of device  104 , obviating the need for optical fiber  108  or equivalent means of delivering the pump radiation. 
   System  100  may optionally include a coolant recirculation system  112  for removing heat from a window  116  of device  104 . Window  116  is fabricated from an optically transparent material and has a distal face  118  which is held in thermal contact with tissue target  102  during operation of system  100 . As is discussed further hereinbelow, cooling of window  116  beneficially reduces damage to non-targeted tissue and resultant scarring. Coolant recirculation system  112  will conventionally comprise heat exchanger-based or evaporative chiller for removing heat from a liquid coolant (which may consist of water or a water/glycol mix) and a pump for delivering the chilled coolant to thermally conductive tubing contacting surfaces of window  116 . Other well-known techniques for cooling tissue target  102  may be substituted for or used in conjunction with coolant recirculation system  112 . 
   Reference is now directed to  FIG. 2 , which shows a cross-sectional view of device  104 . Device  104  includes housing  110  in which is disposed a fluorescent element  202 . A redirector  204 , which operates to redirect radiation emitted by fluorescent element  202  toward tissue target  102 , includes a conically or frustro-conically shaped diffuse reflector  206  surrounding a cavity  208 . One end of housing  110  is adapted to admit a corresponding end of optical fiber  108 , which delivers the pump radiation from pump radiation source  106 . Radiation leaving optical fiber  108  is thereafter directed through cavity  208  onto fluorescent element  202 . 
   Fluorescent element  202  may be fabricated from any one of a number of materials having fluorescent properties. The fluorescent element material may be selected to provide desired spectral characteristics of the emitted fluorescent light. In one embodiment, fluorescent element  202  is fabricated from a solid material consisting of a fluorescent dye compound (also known as a fluorochrome), such as Rhodamine 6G dispersed in a polymeric matrix, such as polyvinyl toluene (PVT) or polymethyl methacrylate (PMMA, commonly known as Plexiglas). Those skilled in the art will recognize that materials of the foregoing description may be formed by dissolving or dispersing the fluorescent dye in a monomer prior to polymerization. 
   Other materials which may be used to fabricate fluorescent element  202  include, without limitation, a laser dye dispersed in the interstitial voids of porous glass (also known as “thirsty glass”) or unconsolidated Vicor, phosphors, and laser crystals such as ruby. In still other implementations of the invention, fluorescent element  202  may comprise a static or recirculating encapsulated laser dye solution. 
   As is depicted in  FIG. 2 , fluorescent element  202  may be constructed in a disk-like shape sized to be received within one end of housing  110  of device  104 , although other geometries and configurations may be utilized without departing from the scope of the invention. Fluorescent element  202  is positioned to receive incident thereon the pump radiation delivered via optical fiber  108  At least a portion of the pump radiation is absorbed by fluorochromes (dye solution molecules) within fluorescent element  202 . Absorption of the pump radiation results in excitation of the fluorochromes (i.e., boosting of an electron from a ground to an excited state). Excitation of fluorochromes causes fluorescent element  202  to emit radiation, the emitted radiation having substantially different spectral characteristics from those of the absorbed (pump radiation). Emission and absorption spectra of a representative fluorochrome are depicted in  FIG. 5  and discussed below. 
   It will be recognized that fluorescent element  202  will emit radiation in all directions, and that in the absence of structures for redirecting radiation emitted by fluorescent element  202  in a non-preferred direction (i.e., away from tissue target  102 ), a substantial portion of the emitted radiation would be wasted. Device  104  is therefore provided with redirector  204  for redirecting toward tissue target  102  radiation emitted by fluorescent element  202  in a non-preferred direction such that substantially all of the radiation emitted by fluorescent element  202  reaches tissue target  102 . 
   In the embodiment depicted in  FIG. 2 , redirector  204  comprises an elongated frustro-conically shaped diffuse reflector  206  surrounding cavity  208 . The walls of reflector  206  are relatively widely spaced proximal to fluorescent element  202  and become progressively narrower in the direction extending away from fluorescent element  202 . Radiation emitted by fluorescent element  202  in a non-preferred direction, as represented by ray  210 , enters redirector  206  and undergoes multiple reflections from the walls of reflector  206  in a random-walk fashion. Ray  210  is eventually oriented such that it travels through cavity  208  in a direction substantially parallel to the central longitudinal axis of redirector  206  and subsequently passes through fluorescent element  202  and window  116  onto tissue target  102 . Of course, radiation emitted by fluorescent element in a preferred direction (represented by ray  212 ) will travel directly from fluorescent element  202  to tissue target  102  through window  116 . It is noted that redirector  206  may serve the additional function of collecting and redirecting toward tissue target  102  fluorescent radiation which is reflected by tissue target  102 . 
   Those skilled in the art will recognize that redirector  206  may be constructed in other shapes (e.g., hemispherical), and hence the invention should not be construed as being limited to a conically- or frustro-conically shaped redirector. 
   Window  116  may be formed from glass, sapphire, or other suitable material which is substantially transparent in the wavelengths of the radiation emitted by fluorescent element  202 . Window  116  terminates in a distal face  118  which is maintained in contact with tissue target  102  during operation of system  100 . It has been found that undesirable collateral thermal damage caused to non-targeted tissue during procedures such as selective photothermolysis may be eliminated or substantially reduced by cooling the irradiated tissue (for a discussion of this benefit, reference may be made to U.S. Pat. No. 5,057,104 to Chess, entitled “Method and Apparatus for Treating Cutaneous Vascular Lesions”). To achieve cooling of tissue target  102 , window  116  may be provided with thermally conductive tubing  216 , arranged about the periphery of window  116 , and through which is circulated chilled coolant supplied by coolant recirculation system  112 . Tubing  216  may be held in good thermal contact with window  116  by means of an appropriate adhesive. Because glass or other optically transparent materials used to form window  116  typically have high thermal conductivities, cooling of window  116  effective cools tissue target  102  via conduction. Cooling of tissue target  102  may be optimized by applying a thermally conductive gel to tissue target  102  prior to bringing window  116  in contact therewith. Other techniques for cooling tissue target  102  are well known in the art and may be substituted for or used in conjunction with the technique described above. 
     FIG. 3  is a fragmentary view of a portion of a wall of reflector  206 . Reflector  206  may be adapted with surface irregularities or protrusions  302  which effectively scatter light rays incident thereon, thus producing the random-walk behavior discussed above and depicted in  FIG. 2 . Various shapes and sizes of surface irregularities may be used to cause scattering. Reflector  206  may comprise a reflective coating deposited on a supporting substrate, or alternatively be directly machined from a block of metal or other reflective material. 
     FIG. 4  depicts a second embodiment of a device  400  for irradiating tissue  102 . The device housing has been omitted in  FIG. 4  for the purpose of clarity. Device  400  is provided with a fluorescent element  402  and window  404  of similar description to fluorescent element  202  and window  116  of the  FIG. 2  embodiment. Redirector  406  consists essentially of a mirror  408  having a coating  410  which selectively reflects wavelengths corresponding to the radiation emitted by fluorescent element  402  while being substantially transparent to wavelengths corresponding to the pump radiation. Coating  410  may conventionally comprise a plurality of dielectric layers, the number, thicknesses, and composition of dielectric layers being chosen to produce the desired selective reflectance behavior. As indicated by ray  412 , radiation emitted by fluorescent device  402  in a non-preferred direction (away from tissue target  102 ) is reflected by coating  410  and thereby redirected toward tissue target  102 . Window  404  may be provided with cooling means as discussed above in connection with  FIG. 2 . 
     FIG. 5  depicts absorption and emission spectra of a representative fluorochrome (rhodamine 6G) which may be used in fluorescent element  202  or  402 . It can be seen that the absorption spectrum exhibits a peak at approximately  530  nanometers, while the emission or fluorescence spectrum peaks at approximately  560  nanometers. The difference between the peaks of the absorption and emission spectra, called the Stokes shift, varies among different fluorochromes. A clinician may therefore match the emission spectra to the absorption characteristics of target tissue  102  by selecting a device having a fluorescent element which produces the desired Stokes shift. 
     FIG. 6(   a ) is a cross-sectional view of a device  600  for irradiating tissue  102  according to a third embodiment of the invention. Device  600  is provided with a fluorescent element  602  and window  604  of similar description to fluorescent element  202  and window  116  of the  FIG. 2  embodiment. Redirector  606  comprises a waveguide  608  having a reflective entrance face  610  and reflective walls  612  extending from entrance face  610  to fluorescent element  602 . The core of waveguide  608  may be fabricated from glass, sapphire or other suitable high refraction index material. Referring to  FIG. 6(   b ), which shows a top plan view of entrance face  610 , it is seen that entrance face  610  includes a central aperture  614  through which pump radiation from radiation source  106  is admitted into waveguide  608 . Central aperture  614  is configured to be substantially transparent in the wavelength(s) of the pump radiation. Device  600  may include a divergent lens or similar element  616  positioned between the end of optical fiber  108  and aperture  614  to distribute the radiation emitted by fiber  108  over fluorescent element  602 . 
   Entrance face  610  has an outer annular region  618  which is coated with a dielectric or metallic reflective coating to redirect toward tissue  102  radiation emitted by fluorescent element  602 , as shown in  FIG. 6(   a ). To minimize loss of emitted radiation through aperture  614 , the area of aperture  614  is preferably a small fraction of the area of outer region  618 . Walls  612  may also be provided with a dielectric or metallic reflective coating to redirect emitted radiation. However, in a preferred embodiment, walls  612  comprise a boundary between the core of waveguide  608  and a cladding  620  (depicted in phantom). Cladding  620  is constructed from a material, such as Teflon, having an index of refraction substantially lower than the index of refraction of the waveguide  608  core, which causes the emitted light to undergo total internal reflection at the boundary or walls  612 . Radiation emitted by fluorescent element  602  in a non-preferred direction is thereby redirected toward tissue  102 . 
   While waveguide  608  is depicted as having a downwardly tapering duct shape, it is not to be construed as limited thereto and may instead be constructed, for example, in a conical or cylindrical shape. Further, although waveguide  608  is shown as being circular in cross-section, polygonal and other cross-sectional shapes may be substituted. 
   It should be recognized that fluorescent devices of the foregoing description may be utilized for numerous therapeutic applications. Examples of therapies for which the devices may be advantageously employed include (without limitation) photothermolysis of vascular and pigmented lesions, tattoo removal, hair removal, and photodynamic therapy (PDT) for treatment of tumors. 
   While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, as defined by the appended claims.