Abstract:
A method and apparatus for performing a laser treatment of a patient includes applying a positive pressure impulse on a predetermined target site on the patient with sufficient positive pressure arising from the momentum flux of sprayed material incident on the target site to momentarily lessen pain sensation during irradiation during or proximate in time to irradiation of the target site. The predetermined target site is cooled by applying a predetermined amount of coolant or cryogen onto the target site. The target site is radiated with energy to produce heat in tissue at the target site while leaving a superficial part of the target site substantially undamaged due to dynamic cooling of the superficial part of the target site by the coolant. Mediation of the pain sensation arising from the radiation is at least partially masked or lessened by the positive pressure impulse and/or by the temperature of the coolant.

Description:
[0001]    The present application is a continuation-in-part application of copending U.S. patent application Ser. No. 11/845,503, filed on Aug. 27, 2007, which in turn is related to U.S. Provisional Patent Application Ser. No. 60/840,867, filed on Aug. 28, 2006, both of which are incorporated herein by reference and to which priority is claimed pursuant to 35 USC 120. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to the field of therapeutic treatment of skin or tissue by exposure to light or other electromagnetic radiation in combination with cryogen cooling of the irradiated tissue. 
         [0004]    2. Description of the Prior Art 
         [0005]    An important issue in laser treatment of cutaneous lesions is to protect the epidermis from thermal damage. This heating, which is primarily caused by light absorption in the melanosomes, can easily bring the temperature of the basal layer above the threshold damage value of 65-70° C. Precooling of the epidermal basal layer from the normal value of 35° C. to 0° C. increases the optical radiant exposure that can be safely delivered by a factor of two. 
         [0006]    Selective epidermal cooling can be obtained by exposing the skin surface to a cryogen for an interval of time corresponding to the thermal diffusion time from the stratum corneum through the epidermis and down to the basal layer. Thus, the upper layers are cooled while leaving the temperature of dermal and subcutaneous layers unchanged. Currently, selective epidermal cooling is achieved using a liquid spray of the cryogen R-134a (tetrafluoroethane) for 30-100 ms immediately before laser exposure. A typical procedure is to spray the surface for 30-50 ms, and then expose the skin to laser irradiation 20-30 ms after the end of the cryogen spurt. 
         [0007]    Tetrafluoroethane (H 2 FC—CF 3 ) is a chlorine free hydrofluorocarbon (HFC), thus representing no damage to the ozone layer. However, recent studies suggested that the non-CO 2  greenhouse gases such as methane (CH 4 ), nitrous oxide (N 2 O), hydrofluorocarbon (HFC), perfluorocarbon (PFC) and sulphur hexafluoride (SF 6 )) can make a significant contribution to global warming in comparatively low concentrations. 
         [0008]    Table 1 gives the global warming potential (GWP) for frequently used gases. (The GWP of CO 2  is defined as unity) 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Major GWP Gases in the United States (100-year global 
               
               
                 warming potentials) a   
               
             
          
           
               
                   
                   
                 Atmospheric 
                   
               
               
                 Gas 
                 GWP 
                 Lifetime 
                 Source of Emissions 
               
               
                   
               
             
          
           
               
                 HFC-23 
                 11,700 
                 264 
                 HCFC-22 Production, 
               
               
                   
                   
                   
                 Fire Extinguishing 
               
               
                   
                   
                   
                 Equipment, 
               
               
                   
                   
                   
                 Aerosols, Semiconductor 
               
               
                   
                   
                   
                 Manufacture 
               
               
                 HFC-43-10mee 
                 1,300 
                 17.1 
                 Solvents 
               
               
                 HFC-125 
                 2,800 
                 32.6 
                 Refrigeration/Air 
               
               
                   
                   
                   
                 Conditioning 
               
               
                 HFC-134a 
                 1,300 
                 14.6 
                 Refrigeration/Air 
               
               
                   
                   
                   
                 Conditioning, Aerosols, 
               
               
                   
                   
                   
                 Foams 
               
               
                 HFC-143a 
                 3,800 
                 48.3 
                 Refrigeration/Air 
               
               
                   
                   
                   
                 Conditioning 
               
               
                 HFC-152a 
                 140 
                 1.5 
                 Refrigeration/Air 
               
               
                   
                   
                   
                 Conditioning, Aerosols, 
               
               
                   
                   
                   
                 Foams 
               
               
                 HFC-227ea 
                 2,900 
                 36.5 
                 Aerosols, Fire 
               
               
                   
                   
                   
                 Extinguishing Equipment 
               
               
                 HFC-236fa 
                 6,300 
                 209 
                 Refrigeration/Air 
               
               
                   
                   
                   
                 Conditioning, Fire 
               
               
                   
                   
                   
                 Extinguishing 
               
               
                 SF6 
                 23,900 
                 3,200 
                 Electricity 
               
               
                   
                   
                   
                 Transmission/Distribution; 
               
               
                   
                   
                   
                 Magnesium 
               
               
                   
                   
                   
                 Production; 
               
               
                   
                   
                   
                 Semiconductor 
               
               
                   
                   
                   
                 Manufacturing 
               
               
                 PFCs (primarily 
                 6,500- 
                 10,000- 
                 Aluminum Smelting, 
               
               
                 CF4, C2F6) 
                 9,200 
                 50,000 
                 Semiconductor 
               
               
                   
                   
                   
                 Manufacture, 
               
               
                   
                   
                   
                 Fire Extinguishing 
               
               
                 PFC/PFPEsb 
                 7,400 
                 3,200 
                 Solvents 
               
               
                   
               
               
                   a Note that this table lists major commercial gases and sources; other minor gases and uses such as lab applications are not listed here. The GWPs and atmospheric lifetimes are taken from Climate Change 1995, the IPCC Second Assessment Report. 
               
               
                 bPFC/PFPEs are a diverse collection of PFCs and perfluoropolyethers (PFPEs) used as solvents. 
               
             
          
         
       
     
         [0009]    Tetrafluoroethane R-134a, which is non-toxic, non-flammable, and non-ozone depleting, has a boiling point (b.p.) of −26° C. at 1 atm. These properties make it an excellent choice for cryogen spray cooling of human skin. However, R-134a has a comparatively high GWP of 1,300. Therefore, substitutes for R-134a for use in large scale applications such as air conditioners for cars, home appliances and manufacturing of thermally insulating polystyrene foams are being developed. As a result, carbon dioxide based air conditioners are being introduced for the car industry and non-fluorinated, but flammable, gases are used in home appliances today. The lowest GPW value (GWP=140) in table 1 is that of R152a (difluoroethane, F 2 HC—CH 3 ), but this compound autoignites at 455° C. 
         [0010]    The GWP values for non-fluorinated hydrocarbons such as propane (C 3 H 8 , R-290, b.p. −42.1° C.), butane (C 4 H 10 , R-600, b.p −0.5° C.) and isobutene (R-600a, b.p −12° C.) are very low because they are rapidly broken down in the atmosphere. The global warming potential of propane is GWP=3 and that of butane is less than 10. 
         [0011]    Potential non-flammable candidates for cryogen spray cooling could be carbon dioxide (CO 2 , R-744) or nitrous oxide (N 2 O, R-744a) but, unfortunately, these compounds do not form boiling liquids at atmospheric pressure. Liquid CO 2  results in dry-ice formation immediately after leaving the outlet nozzle. The skin surface will therefore be covered with a layer of dry-ice crystals. Possible reduction in the heat transfer coefficient due to build up of a porous layer of dry ice crystals can be avoided by having an adequate high momentum flux of the spray. 
         [0012]    Liquid nitrogen (N 2 , b.p. −195.8° C.) is an environmentally safe, non-toxic and liquid forming cryogen which has been extensively used in dermatology. However, the evaporation loss is high, and the cryogenic equipment for delivering liquid cryogen spurts in the 100 ms range might be technically very cumbersome. 
         [0013]    Interesting candidates are also in the group of flammable hydrocarbons, which are used today in medical applications. The commercial product Histofreezer®, which is used for the treatment of cervical bleeding and removal of warts, is composed of a mixture of dimethyl ether (C 2 H 6 O, b.p. −22° C.), propane and isobutene. Further on, these compounds are also used as propellants for hair lacquers, etc. 
         [0014]    The use of a flammable cryogen during laser exposure for, e.g., hair removal requires special precautions. The pulse energy, which typically can be in the range of 20-70 J/cm 2 , can ignite the hair above the skin surface. Thus, the combined use of flammable cryogen and high laser energy might induce burns to the skin. 
         [0015]    The dynamic cooling principle as defined in U.S. Pat. Nos. 6,669,688; 6,248.103; 6,171,301; 5,997,530; 5,979,454 and 5,814,040, which are incorporated herein by reference, represent a controllable, reliable and efficient method, denoted as “dynamic cooling”, for protection of the epidermis, and for some applications such as pulsed laser treatment of port wine stain lesions it represents the only choice for efficient protection. Port wine stain is a congenital lesion with a frequency of one in two hundred births, often resulting in a high psychological burden for the child. 
         [0016]    What is needed is to find an efficient substitute for tetrafluoroethane that is compatible with requirements for low global warming potential. 
       BRIEF SUMMARY OF THE INVENTION 
       [0017]    The illustrated embodiment of the invention is a method for performing an energy beam treatment of a patient comprising the steps of applying a pressure impulse on a predetermined target site on the patient with sufficient pressure arising from the momentum flux of sprayed material incident on the target site to momentarily lessen pain sensation during irradiation of the target site during or proximate in time to irradiation of the target site. The predetermined target site is cooled by applying a predetermined amount of coolant or cryogen onto the target site, such as by use of a method developed by the Beckman Laser Institute known as “dynamic cooling”. The target site is radiated with energy to produce heat in tissue at the target site while leaving a superficial part of the target site substantially undamaged due to dynamic cooling of the superficial part of the target site by the coolant. The energy is preferably delivered through a laser beam, but the invention expressly contemplates any source of tissue or dermal heating. As such, the scope of the invention includes any source of electromagnetic energy which is effective to heat tissue, whether it is coherent or not. The sensation of pain arising from the radiation is at least partially masked or lessened by the pressure impulse and/or by the temperature of the coolant or cryogen or by their application. For example, in addition to pressure and temperature sensations which are intermixed in the mind of the patient, there may also be aural sensations which accompany the application of the cryogen which factor into the subjective sensation of pain in any given individual. The subjective sensation of pain is not perfectly understood and individual variation of pain sensation to both somatic stimuli as well as cognitive stimuli can interact in complex ways. 
         [0018]    In the illustrated embodiment the step of applying the pressure impulse on the predetermined target site on the patient comprises applying a momentum flux of sprayed liquid carbon dioxide expanded or nearly adiabatically expanded through at least one nozzle applicator to form carbon dioxide snow impinging on the target site. 
         [0019]    In one embodiment the step of applying a momentum flux of sprayed liquid carbon dioxide comprises applying a momentum flux of a slurry of carbon dioxide snow and liquid carbon dioxide. In another embodiment the step of applying a momentum flux of sprayed liquid carbon dioxide comprises applying a momentum flux of substantially all carbon dioxide snow in a solid state. 
         [0020]    Preferably the step of cooling the predetermined target site by applying the predetermined amount of coolant onto the target site comprises cooling the predetermined target site with the same material used to apply the pressure impulse to the target site. However, the invention expressly includes the embodiments where the pressure may be applied by more than one agent or gas impulse. Namely, the step of cooling the predetermined target site by applying the predetermined amount of coolant onto the target site comprises cooling the predetermined target site at least in part with a different material than that used to apply the pressure impulse to the target site, such as shielding gas. 
         [0021]    The step of applying a pressure impulse on a predetermined target site to momentarily lessen pain sensation during irradiation of the target site during or proximate in time to irradiation of the target site further comprises applying a cooling temperature impulse to the predetermined target site during irradiation of the target site during or proximate in time to irradiation of the target site. In the illustrated embodiment, the step of applying a cooling temperature impulse to the predetermined target site during irradiation of the target site during or proximate in time to irradiation of the target site comprises applying a temperature impulse by means of a spurt of cryogen snow. 
         [0022]    Similarly, the step of applying a temperature impulse by means of a spurt of cryogen snow comprises applying a spray of frozen or nearly adiabatically frozen liquid carbon dioxide expressed through at least one nozzle. 
         [0023]    In one embodiment the step of applying the pressure impulse on a predetermined target site comprises applying 0.005 to 0.15 MPa of pressure at the target site. 
         [0024]    In another embodiment the step of cooling the predetermined target site by applying the predetermined amount of coolant onto the target site comprises disposing the coolant on the target site in a solid state so that pooling of any substantial amount of coolant and consequent uneven cooling is substantially avoided. Typically, the cryogen or coolant is not sprayed onto the target site in a direction perpendicular to the surface of the target site, but at an inclined angle. The laser or energy beam source may be occupying a position wherein the beam is directed perpendicularly onto the site, thus requiring the cryogen spurt to be applied onto the site at an angle. As result coolant or cryogen is applied to the site in an elliptical pattern as shown in the photograph of  FIG. 17  in the case of R134a cryogen 260 ms after initiation. The edges of the pattern may tend to build up a pool or slight excess of coolant or cryogen as compared to the center or interior portions of the pattern. The downstream side of the pattern may tend to be blown further downstream from the impact site with the upstream side maintaining the pooling. As a result, the cooling pattern at the target site can be asymmetric or inhomogeneous. In the case of carbon dioxide snow as the cryogenic agent, little or no pooling occurs because the excess accumulated snow has little surface tension and is more uniformly blown away from the edges of the target site or bounces off the skin in all directions or at all portions of the site periphery regardless of the angle of inclination of the spurt onto the site. 
         [0025]    In still another embodiment the step of operating the high pressure valve comprises operating the valve at liquid carbon dioxide storage pressures at room or elevated temperature with openings controlled to an accuracy within 0.1 to 25 msec of a predetermined time duration. 
         [0026]    The illustrated embodiments of the invention also include an apparatus for performing laser or other thermal treatment of a biological tissue comprising a laser or other energy source directed to deliver energy to a selected location of the biological tissue or skin and a source of liquid carbon dioxide. A controller is coupled to the laser and a spray applicator having at least one nozzle communicated with the source of liquid carbon dioxide. The controller controls the spray applicator in coordination with the energy source to apply a predetermined amount of liquid carbon dioxide through the at least one nozzle in order to apply a pressure impulse on a predetermined target site on the biological tissue or skin. The pressure impulse arises from the momentum flux of sprayed carbon dioxide incident on the biological tissue sufficient to depress the tissue or skin by more than one millimeter at the target site. Any pain sensation arising from the radiation is at least partially masked by the pressure impulse and/or by the temperature of the carbon dioxide snow, or other cognitive stimuli also delivered. 
         [0027]    In the illustrated embodiment the spray applicator applies a pressure impulse of up to 0.15 MPa on a predetermined target site on the biological tissue. 
         [0028]    In the preferred embodiment the apparatus is for performing laser treatment of a biological tissue or skin and comprises a laser directed to deliver energy to a selected location of the biological tissue, a source of liquid carbon dioxide, a controller; and a spray applicator having at least one nozzle communicated with the source of liquid carbon dioxide. The spray application and laser are coupled to and controlled by the controller to spray a predetermined amount of liquid carbon dioxide through the at least one nozzle to expand or nearly adiabatically expand the liquid carbon dioxide to create a controlled amount of carbon dioxide snow which impinges onto the selected location of the biological tissue in coordination with laser irradiation onto the selected location. The carbon dioxide snow is substantially all deposited on the biological tissue in a solid state. The deposition of the carbon dioxide snow on the biological tissue substantially all in the solid state avoids pooling of any substantial amount of liquid state carbon dioxide and consequent uneven cooling of the biological tissue. 
         [0029]    In another embodiment the apparatus is for performing laser treatment of a biological tissue or skin and comprises a laser directed to deliver energy to a selected location of the biological tissue; a source of liquid carbon dioxide under high pressure at room or elevated temperature; a controller to accurately control a predetermined time period of liquid carbon dioxide release from the source; and a spray applicator having a high pressure valve and at least one nozzle communicated with the source of liquid carbon dioxide, coupled to and controlled by the controller to spray a predetermined amount of liquid carbon dioxide through the at least one nozzle to expand or nearly adiabatically expand the liquid carbon dioxide to create an accurately controlled amount of carbon dioxide snow which impinges onto the selected location of the biological tissue, so that precise control of the duration of the time-controlled pulses of carbon dioxide are achieved by use of the high pressure valve. 
         [0030]    It is to be expressly understood that while the illustrated embodiment describes liquid carbon dioxide which is adiabatically cooled to form a sprayed snow, that any liquid cryogen can be substituted and adiabatically cooled to form a sprayed cryogenic snow to be applied and used in a substantially similar manner for a substantially similar result. 
         [0031]    While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a diagram illustrating a method of liquid cryogen cooling using an inert shielding gas. 
           [0033]      FIG. 2  is a diagram illustrating a method of using a mixture of liquid flammable cryogens and carbon dioxide supplied from a pressurized container. 
           [0034]      FIG. 3  is a phase diagram of carbon dioxide. 
           [0035]      FIG. 4  is a diagram illustrating an apparatus for providing carbon dioxide supplied from a pressurized container as a solid cryogen sprayed onto the skin. 
           [0036]      FIG. 5  is a graph of temperature of a target surface as a function of distance from the center of the impingement as measured in the apparatus of  FIG. 4  for a spray duration of 100 ms provided by a nozzle bore 0.7 mm at a 30 mm distance from the target surface. 
           [0037]      FIG. 6  is a graph of temperature of a target surface as a function of distance from the center of the impingement as measured in the apparatus of  FIG. 4  for a spray duration of 100 ms provided by a larger nozzle bore of 1.6 mm at a 30 mm distance from the target surface. 
           [0038]      FIG. 7  is a graph of disk temperature versus spray time for a nozzle bore 0.7 mm, 30 mm distance in a heat transfer measurement device. 
           [0039]      FIGS. 8   a - 8   c  are simplified side cross sectional views of three embodiments using multiple nozzles or nozzle orifices. 
           [0040]      FIGS. 9   a  and  9   b  are graphs of the disk temperature versus spray time for propane using a nozzle bore 0.7 mm at 30 mm distance and 50 mm distance respectively. 
           [0041]      FIG. 10  is a diagrammatic depiction of the structures within human skin, showing the location of some nerve endings. 
           [0042]      FIG. 11  is a photograph of a laser and spray applicator in which the illustrated embodiments may be implemented. 
           [0043]      FIG. 12  is a side cross sectional view of a diagram of a prior art vacuum chamber applying negative pressure to the skin. 
           [0044]      FIG. 13  is a side cross sectional view of a diagram of positive pressure being applied to the skin using a nozzle or jet. 
           [0045]      FIG. 14  is a side cross sectional view of a diagram of a sphere in which positive pressure is used to apply a momentum flux density. 
           [0046]      FIG. 15  is a graph of the skin impact pressure induced by spurts using R134a and liquid CO 2 . 
           [0047]      FIG. 16  is a bar graph of the pain scores of five subjects to which laser irradiation was applied followed by dynamic cooling using R134a and liquid CO 2 . 
           [0048]      FIG. 17  is a photograph of pooling on the surface of an epoxy skin phantom at t=260 ms using R134a which is incline 30° from the vertical. 
           [0049]      FIG. 18  is a graph of temperature of the center C of the spot of  FIG. 17  and the radial point X 1  shown as a function of time using R134a. 
           [0050]      FIG. 19  is a graph of temperature of the center C of the spot of  FIG. 17  and the radial point X 1  shown as a function of time using liquid CO 2 . 
           [0051]      FIG. 20  is a graph of temperature of the center C of the spot of  FIG. 17  for a 50 ms spurt shown as a function of time using liquid CO 2  and R134a. 
       
    
    
       [0052]    The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    The principles presented below provide a solution to the problem of finding an efficient substitute for tetrafluoroethane that is compatible with requirements for low global warming potential. In order to prevent cryogen ignition during pulsed laser exposure, one or more of the following methods are used. 
         [0054]    Shielding Gas 
         [0055]    The method of using a shielding gas to prevent oxygen from arriving at the heated spot is well established in electrical welding. Oxygen from the surrounding air is prevented from oxidizing the welding spot by using a shield of inert gas. A circular orifice mounted concentrically with the electrode can deliver the shielding gas. Examples of this technique are MIG (metal-inert-gas) welding where an inert gas surrounds the electrode, or TIG (tungsten-inert-gas) welding where a tungsten electrode heats up the welding materials. Typical shielding gases are inert gases such as argon, but nitrogen and carbon dioxide can also be used. This principle could be used to prevent ignition of flammable cryogens during dynamic cooling such a diagrammatically depicted in  FIG. 1 . Liquid cryogen from a source  10  is supplied under pressure to a central tube  16  while a shielding gas from a source  12  is supplied to a concentric tube  14 . The liquid cryogen exits from a nozzle or orifice  22  which is located downstream from a concentric outer nozzle or orifice  20  through which the shielding gas exits. What results is a shielded spray  16  which impinges on skin or tissue  18  in which the contingently inflammable cryogen is surrounding by a shielding cone of noncombustible gas. Laser light or another energy source which is then directed to the same impingement point on skin  18  thus does not ignite the cryogen even though ignition temperatures might be temporarily reached, because the cryogen is shielded from access to oxygen in the ambient. 
         [0056]    The arrangement shown in  FIG. 1  enables the use of flammable cryogens such as dimethyl ester, propane, isobutene or any mixture of these compounds. The inert shielding gas can be argon or carbon dioxide gas. Two separate gas valves, one for the liquid cryogen and a second one for the shielding gas must be used. In principle, the shielding gas valve should be opened before the cryogen valve and left open until after the laser energy has been delivered. The flammability ranges of hydrocarbons in air are quite narrow as the range limits for ignition is 2.2 to 9.6% in a propane/air mixture, 1.5 to 8.5% for butane/air mixture and 3.4 to 17% for dimethyl ether/air mixture. The flame extinguishing concentrations for n-heptane (C 7 H 16 ) flames in air have been reported to 33.6, 43.3 and 22% for, respectively, N 2 , Ar and CO 2 . The required amount of shielding gas for flame-extinguishing or ignition-preventing properties is dependent on the details in the mixing process of hydrocarbon, shielding gas and air. 
         [0057]    Mixture of Liquid Flammable Cryogen with Shielding Gas 
         [0058]    Another embodiment of the method is based on mixing a flammable cryogen with an inert gas before application of the mixture. The technique of mixing nonflammable propellants with flammable ones is frequently used, e.g. R-134a is mixed with a flammable propellant such as propane and butane to prevent ignition. In the present case, carbon dioxide can be mixed with liquid cryogen in the same pressurized container and sprayed onto the skin surface through the same valve and nozzle as depicted in the diagram of  FIG. 2 . The premixed gases are contained with a pressurized container  24  and controlled by a valve  26  to a nozzle  28  from which exits a spray  30  of the gas mixture impinging on skin  18 . 
         [0059]    Adiabatic expansion of liquid carbon dioxide when leaving the nozzle  28  will not cool the constituents of the flammable mixture below their respective freezing points, since the freezing points for propane, butane, isobutane and dimethyl ether are −190, −138, −159 and −142° C., respectively. The properties of a mixture of propane and carbon dioxide are also strongly dependent on the mixing ratios. A pressurized mixture of, e.g., 50% liquid propane and 50% liquid carbon dioxide at room temperature has a pressure of 3.7 MPa, i.e., which is 35% lower than for 100% CO 2 . Further, propane at less then 9.4% in carbon dioxide is by federal regulation classified as a non-flammable mixture. 
         [0060]    Liquid Carbon Dioxide Cooling. 
         [0061]    The use of flammable cryogens with shielding gas requires separate FDA approval for use in medical treatments. Therefore, it would be advantageous if all flammable compounds could be avoided. A potential candidate, which would satisfy the requirements for low global warming potential and is completely non-flammable, is carbon dioxide, i.e., R744. Carbon dioxide is toxic in high concentrations, i.e. above 10% in air and might be lethal in concentrations above 20%, but it is completely safe and harmless in low concentrations, e.g., exhaled air contain up to 4%. 
         [0062]    Carbon dioxide has a triple point where liquid, solid and gas coexist at −56.4° C. at 5.11 atm. pressure as graphically depicted in the phase diagram of  FIG. 3 . Below this pressure only the solid and gas phase coexist, and solid carbon dioxide, i.e. dry ice, will sublimate directly into gas. The gas pressure for the liquid at room temperature is 5.72 MPa (56.4 atm.). When liquid CO 2  is released into air at atmospheric pressures it evaporates adiabatically forming dry ice at −78.5° C. with a latent heat of sublimation of 571.3 kJ/kg. 
         [0063]    In an experimental setup as diagrammatically illustrated in  FIG. 4 , a high-pressure steel container with 5 kg liquid CO 2  was connected to a valve via a 1 m long high pressure flexible tube  34  connected to a valve  36 , which in the illustrated embodiment was a Parker Hannifin Corp. Fairfield N.J. series 99, 8.62 MPa (85.1 atm.), 0.8 mm orifice. In order to fill the entire connecting system with liquid carbon dioxide, the steel container  32  was mounted upside down and the valve  36  was thoroughly purged. The valve  36  could be equipped with several different nozzles made by cylindrical steel tubes  38  having a distal outlet nozzle  42 . The bore of the tubes  38  were 0.25, 0.5 and 0.7 mm in diameter and the length was 39 mm. Additionally, experiments were done without tubes  38 . In that case, the outlet nozzle  44  comprised a mounting nut with 1.6 mm diameter bore and about 5 mm length. The length of spray  40  or the distance from the nozzle opening  42  or  44  to the object or skin  18  being cooled was variously set at 30 or 50 mm. The temperature distribution of a sprayed object was measured with a film-microthermocouple mounted on a 10 mm thick epoxy slab. The thermal properties of epoxy are close to those of skin, thus the epoxy slab served as a tissue phantom. 
         [0064]    The results from spaying with the 0.7 mm diameter bore nozzle at 30 mm distance is shown in the graph of  FIG. 5 . The graph shows the temperature distribution during and after a 100 ms long spurt at five different distances from the center of the spray, namely curves  46 - 56  correspond to distances from the center of 0-5 mm in 1 mm increments respectively. The temperature at the center exhibits somewhat erratic temperature fluctuations, which is believed due a tendency to build up dry ice in that region. At 1 mm from the center the temperature fluctuations are significantly reduced. The temperature at 1 mm from the center drops down to −47° C. after 50 ms and further down to −52° C. at 100 ms. The corresponding values at 5 mm from the center are −20° C. and −31° C. at 50 and 100 ms respectively. 
         [0065]    The corresponding results for the 1.6 mm bore nozzle  38  are given in the graph of  FIG. 6 . This nozzle  38  gives better uniformity. As in the case with the graph of  FIG. 5 ,  FIG. 6  shows the temperature distribution during and after a 100 ms long spurt at five different distances from the center of the spray, namely curves  58 - 68  correspond to distances from the center of 0-5 mm in 1 mm increments respectively. The temperature drop at 1 mm from the center after 50 ms and 100 ms are, −40° C. and −48° C. The corresponding values at 5 mm from the center are −30° C. and −38° C. respectively. 
         [0066]    The cooling efficiency of the 0.5 mm bore nozzle  38  was somewhat less than for the 0.7 mm, but still had acceptable performance. However, the 0.25 mm nozzle exhibited poor performance. This nozzle  38  produced very little dry ice and the surface was primarily hit by cold CO 2  gas. 
         [0067]    The efficiency of the nozzles  38  at a distance of 50 mm was also investigated. However, neither the lateral coverage of the spray  40  nor the temperature drop was improved. If a wider area than about 10 mm diameter needs to be cooled, application of multiple simultaneously spraying nozzles  38  could be employed. 
         [0068]    The heat transfer was measured by spraying a cylindrical silver disk embedded in the surface of an epoxy slab as described in U.S. Pat. No. 6,669,688, which is incorporated herein by reference. The diameter and height of the disk were, respectively, 7.2 mm and 0.42 mm. The specific weight of silver is 10,500 kg/m 3  and the specific heat is 232 J/kgK. Thus, the heat capacity of the disk is 0.042 J/K. The temperature drop versus spraying time for the 0.7 mm bore nozzle  38  at 30 mm distance is given in the graph of  FIG. 7 . Spay was directed normal to the disk and centered at the disk. The heat transfer coefficient H between the dry ice at −78.5° C. and the silver disk at initially 23° C. can be determined from the thermal relaxation time τ of the silver disk using 
         [0000]    
       
         
           
             
               
                 
                   H 
                   = 
                   
                     
                       ρ 
                        
                       
                           
                       
                        
                       Cd 
                     
                     τ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0069]    where ρ, C and d are, respectively, the specific weight, specific heat and the thickness of the disk. The results of  FIG. 7  are summarized in table 2. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Spray duration 
               
             
          
           
               
                   
                 50 ms 
                 100 ms 
                 150 ms 
                 200 ms 
               
               
                   
                   
               
             
          
           
               
                 Heat transfer 
                 4500 W/m 2 K 
                 3600 W/m 2 K 
                 3100 W/m 2 K 
                 3700 W/m 2 K 
               
               
                 coefficient 
               
               
                 Total heat 
                 0.8 J 
                 1.2 J 
                 1.5 J 
                 1.8 J 
               
               
                 extraction 
               
               
                 Average heat 
                 2.0 J/cm 2   
                 3.1 J/cm 2   
                 3.8 J/cm 2   
                 4.4 J/cm 2   
               
               
                 extraction 
               
               
                 Required dry 
                 1.4 mg 
                 2.1 mg 
                 2.6 mg 
                 3.1 mg 
               
               
                 ice 
               
               
                 consumption. 
               
               
                   
               
             
          
         
       
     
         [0070]    The heat transfer coefficient for tetrafluoroethane, R134a, sprayed onto a 10 mm diameter, 1 mm thick silver disk with a 0.7 mm bore nozzle at 50 mm distance has been reported to H=7500 W/m 2 K for a spurt duration of 100 ms. The measured heat transfer coefficient for carbon dioxide was about 50% less than this value. One contributing factor to this reduction might be the nonuniformity of the temperature distribution for the 0.7 mm bore nozzle  38 , where the temperature drops for 50-100 ms spurts were about 30% less at 4-5 mm distance as compared to the center as shown in the graph of  FIG. 5 . The heat extraction for the 1.6 mm bore nozzle  38 , which had more uniform temperature distribution, was somewhat higher than for the 0.7 mm nozzle  38 , e.g., the heat extraction after a 50 ms spray with the first nozzle  38  was about 15% higher than for the latter one. However, the much lower sublimation point of dry ice of −78.5° C. might partly compensate for the reduction in heat coefficient as compared to tetrafluoroethane since the boiling point of the latter is −26.5° C. at 1 atm. 
         [0071]    The measured heat extraction is, however, quite adequate for cooling the 60-100 μm thick epidermis of normal human skin from ambient skin temperature of 35° C. to 0° C. This corresponds to a heat extraction in the range of 0.9-1.5 J/cm 2 , which according to the results given in Table 2 can be obtained by spurts in the range of 25-45 ms duration. 
         [0072]    The liquid carbon dioxide consumption and the amount of dry ice in the spray was evaluated by collecting the mixture of gas/dry ice in a closed plastic bag. The volume of the content increased more then 10 times when all dry ice had sublimated after about 30 s as compared to the volume immediately after the spurt. Thus, the cryogen spray contained more than 90% of dry ice by weight. The consumption for the various nozzles  38  for a 100 ms spurt are summarized in Table 3. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Nozzle bore 
                   
               
             
          
           
               
                   
                 1.6 mm 
                 0.7 mm 
                 0.5 mm 
                 0.25 mm 
               
               
                   
                   
               
             
          
           
               
                   
                 CO 2   
                 0.7 g 
                 0.7 g 
                 0.45 g 
                 0.15 g 
               
               
                   
                   
               
             
          
         
       
     
         [0073]    The carbon dioxide consumption for the 1.6 mm and the 0.7 mm nozzles  38  were about the same. The reason was that maximum output was limited by the 0=8 mm diameter orifice of the valve  36  itself as depicted in  FIG. 4 . The required dry ice formation for cooling a tissue surface  18  is a very small fraction of the liquid cryogen consumption, e.g. the required amount of dry ice for cooling a 100 μm thick epidermal layer of 5 mm radius from 37° C. to 0° C. is about 2 mg. Thus, the CO 2  consumption could in principle be reduced by proper optimization of the nozzle  38 . However, the present consumption might be quite acceptable, e.g. spraying with the 1.6 mm or the 0.7 mm bore for 50 ms corresponds to a cooling capacity of about 3000 spurts per kg carbon dioxide. 
         [0074]    In the case of laser irradiation of larger spots, e.g. 18 mm diameter, multiple nozzles or specifically designed nozzles might be necessary.  FIGS. 8   a - 8   c  show sketches of various nozzles design for improving the uniformity of the spray over large spot sizes.  FIG. 8   a  is an embodiment where a plurality of nozzles  38   a  are provided in a parallel array with a corresponding pattern of parallel sprays  40   a .  FIG. 8   b  is an embodiment where nozzle  38   b  is diverging or provided in the shape of an inverted cone to provide a diverging spray  40   b .  FIG. 8   c  is an embodiment where nozzle  38   c  is provided with a plurality of orifices communicated to the same tube to provide a patterned spray  40   c  as determined by the pattern and angulation of the bores defined in the nozzle  38   c  like a showerhead. Similarly, nozzle  38  may be provided with an adjustable spray head so that the user may change the spray pattern from one use to another. It is to be understood that these embodiments do not exhaust the possibilities for nozzle arrays or designs, all of which are expressly contemplated as being within the scope of the invention. 
         [0075]    Consider also the acoustical noise generated by the adiabatic expansion of carbon dioxide. This noise level, which can be disturbing to a patient, can be reduced by a conventional silencer, if desired. Similarly, the force of the spray  40  onto the skin surface  18  can be varied not only by means of the delivery pressures, but by use of various types of conventional gas diffusers combined with nozzle  38 . In some embodiments as described below, there is an advantage to having both a distractive type and level of sound created and to apply a distractive level of pressure to the target site when the laser pulse is applied to mask or lessen the perceived pain created by the laser pulse. 
         [0076]    Liquid Propane Cooling. 
         [0077]    The cooling efficiency with propane was investigated with the same silver disk detector as discussed above for carbon dioxide measurements. The propane was of commercial quality for soldering/heating application marketed in a pressurized canister containing 0.4 kg liquid propane as sold by Bernzomatic, of Medina N.J. Typical results are shown in the graphs of  FIGS. 9   a  and  9   b . The results are summarized in table 4. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 Spray duration/distance 
               
             
          
           
               
                   
                 50 ms/30 
                 100 ms/ 
                   
                   
               
               
                   
                 mm 
                 30 mm 
                 50 ms/50 mm 
                 50 ms/50 mm 
               
               
                   
                   
               
             
          
           
               
                 Heat transfer 
                 15 000 
                 17 000 
                 3900 W/m 2 K 
                 4500 W/m 2 K 
               
               
                 coefficient 
                 W/m 2 K 
                 W/m 2 K 
               
               
                 Total heat 
                 1.5 J 
                 2.3 J 
                 0.5 J 
                 1.0 J 
               
               
                 extraction 
               
               
                 Average heat 
                 3.8 J/cm 2   
                 5.7 J/cm 2   
                 1.2 J/cm 2   
                 2.5 J/cm 2   
               
               
                 extraction 
               
               
                   
               
             
          
         
       
     
         [0078]    The heat transfer coefficient for a 50 ms long propane spurt at 30 mm distance was about twice as large as reported for tetrafluoroethane under the similar conditions. One reason for the high heat transfer coefficient could be that the propane spray adhered very well to the surface  18 . It is not clear whether this phenomenon was due to propane itself or to the sulphuric odor compound added to propane by the manufacturer for leak detection. The efficacy dropped off rapidly with distance, and the coefficient dropped down by about 75% at 50 mm distance, i.e., to about the same value as for carbon dioxide at 30 mm distance. (See Table 2). 
         [0079]    The gas consumption was very low. A 100 ms long spurt of propane delivered by the 0.7 mm bore nozzle  38  was 80 ml gas at atmospheric pressure, which corresponds to 0.14 g liquid propane. Propane has, as discussed before, a range of flammability of 2.2-9.6% by volume in air. Thus, to remain below the range of flammability the propane spurt has to be mixed with about 4 l of air. 
         [0080]    Human Skin Exposure. 
         [0081]    Evaluation of possible damage to human skin during spray was examined by spraying human forearms for spurt duration up to 200 ms with the 1.6 mm bore nozzle at 30 mm distance. No damage was observed either immediately or after several days of follow up. The maximum force onto the skin for the 1.6 mm nozzle  38  covering a 10 mm diameter spot is up to 2.6% of the pressure in the container, i.e., about 0.15 MPa (1.4 atm). The cryogen impact on the skin was monitored with a high speed video camera (Fast Cam PCI R2, Photron, San Diego Calif.). This momentum flux depressed the skin surface by several millimeters, but the skin returned to original position less than 10 ms after the spurt. This bouncing back of the skin also removed all debris of ice from the surface. This is in contrast to spurts of liquid cryogen where a liquid deposit and ice formed from the ambient air frequently remain for several tens of milliseconds after the spurt. 
         [0082]    A compression of the dermis could, in principle, force blood out of the vessels. This phenomenon might be an advantage in treating deeper located targets such as the bulb of the hair follicles since the optical absorption of the upper layers is reduced. However, it might be a disadvantage in treating shallowly located blood vessels such as in the case of port wine stains. This effect was studied by evaluating the purpura introduced by laser exposure typically used during treatment of port wine stains. The laser (Sclero-Plus, Candela Corp. Inc. Wayland Mass.) was set either to 7 J/cm 2  or to 8 J/cm 2 , pulse duration 1.5 ms and wavelength 585 nm. The results from exposure to normal skin of Caucasian forearms (palmar side) are summarized in Table 5. The diameter of the irradiated spot was 7 mm, and the cooling of the spot was aligned with carbon dioxide spray  40  from a 1.6 mm bore nozzle  38  at 30 mm distance from the skin. See nozzle performance in the graph of  FIG. 6 . The results were evaluated at 22 h and 50 h after exposure. Purpura was evaluated visually on a scale from 1 to 10 where 10 was set to the value for the noncooled control. 
         [0000]    
       
         
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
             
             
               
                   
                   
               
               
                   
                 Delay 
               
             
          
           
               
                   
                 10 ms 
                 30 ms 
                 50 ms 
               
               
                   
                   
               
             
          
           
               
                   
                 Spurt duration 
                 8 (9) 
                 7 (4) 
                 6 (3) 
               
               
                   
                 25 ms 
               
               
                   
                 Spurt duration 
                 7 (4) 
                 6 (3) 
                 6 (3) 
               
               
                   
                 50 ms 
               
               
                   
                 Spurt duration 
                 8 (9) 
                 6 (4) 
                 0 (0) 
               
               
                   
                 75 ms 
               
               
                   
                 Spurt duration 
                 8 (9) 
                 4 (3) 
                 0 (0) 
               
               
                   
                 100 ms 
               
               
                   
                   
               
             
          
         
       
     
         [0083]    All exposed spots exhibited a well-demarked edema 2 h after exposure. Purpura developed during 12 h after exposure. However, the results indicate that purpura only was minor affected for the shortest cooling duration and delay, i.e., from a maximum score of 10 for the non-cooled control site to 7-10. This seems to indicate that the blood remains in the vessels during the cooling spray. The smaller score values are observed for the spots irradiated 125-150 ms after onset of the spurt. This time correspond to the delay for a thermal wave to propagate through an epidermal layer of about 100 μm. Thus, the results indicate that the time dynamic is too fast to force blood out of the capillaries in papillary/reticular dermis, and that the reduction of purpura for long delays is due to protection of these vessels by the cooling. 
         [0084]    In summary, with respect to the global warming potential propane (GWP=3) seems to be an excellent replacement for tetrafluoroethane (GWP=1300). However, the high flammability of propane is believed to be unsatisfactory for medical laser treatments when used without a shielding gas. A commercial brand of liquid propane adheres very well to the surface, resulting in a high heat transfer coefficient. This very high transfer coefficient might indicate that the cooling efficiency might be acceptable even when reduced by the presence of a shielding gas. 
         [0085]    A premix of propane/butane with a non-flammable compound before spraying rather than using a shielding gas has several advantages. A pressurized mixture of, e.g., 50% liquid propane and 50% liquid carbon dioxide at room temperature have a pressure of 3.7 MPa, i.e., 35% lower than for 100% CO 2 , and a mixture of propane at less than 9.4% in carbon dioxide is by federal regulation classified as a non-flammable mixture. Secondly, the requirements for an efficient adiabatic expansion of carbon dioxide for forming dry ice after leaving the nozzle are reduced. A slurry consisting of a mixture of dry ice crystal and liquid propane has a very good wetting of the skin surface, resulting in a high heat transfer coefficient. Thus, the momentum flux transferred to the skin surface is reduced. These properties are expected to be preferred for treating lesions as port wine stains in regions with thin skin such as in the ocular orbit. 
         [0086]    Liquid carbon dioxide (GWP=1) can also be used as the only cryogen. However, the momentum flux of the spray is high, and the skin surface is somewhat depressed. This is an advantage in treating deeper located target such as the bulb of the hair follicle, but it might represent a problem for shallowly located targets such as the ectatic capillaries of port wine stain. However, preliminary measurements on normal skin indicate that the blood remain in the capillaries for spurt durations in the range of 50-100 ms. All residual dry ice or ice from ambient air humidity bounced off the skin surface as it relaxed back about 10 ms after the spray, leaving a clean skin surface. No skin damage was observed due to the cryogen spurts up to 200 ms duration. Finally it should be noted that although carbon dioxide by definition has a global warming potential equal to one, the real value GWP is zero for CO 2  made from sources other than burning of fossil fuels. Industrial CO 2  production is, e.g., made from fermentation of corn in alcohol production. Thus, the use contemplated by the invention represents a recycling of CO 2  made from solar irradiation. 
         [0087]    Pain Management 
         [0088]    It has become apparent that use of liquid carbon dioxide with dynamic cooling of the skin results in pain reduction during laser hair removal and potentially in all dynamic cooling and laser or energy beam applications. The illustrated embodiment disclosed below relates to preliminary testing of the pain reduction experienced by the patients during hair removal with an Alexandrite laser at 755 nm wavelength (GentleLase® Candela Corp. Wayland, Mass.). Significant reduction in pain was observed with liquid CO 2  cooling, dispensed as CO 2  snow or slurry, as compared to R134a cryogen cooling and as compared to no cooling. 
         [0089]    Consider now the effect of pain masking or lessening in more detail. An important issue in laser treatment of cutaneous lesions is to protect the epidermis from thermal damage and minimize the discomfort to the patient. This heating, which is primarily caused by light absorption in the melanosomes, can easily bring the temperature of the basal layer above the threshold damage value of 65-70° C. Pre-cooling of the epidermal basal layer from the ambient value of 35° C. to 0° C. increases the optical radiant exposure that can be safely delivered by a factor of two. Selective epidermal cooling can be obtained by exposing the skin surface to a cryogen for an interval of time corresponding to the thermal diffusion time from the stratum corneum through the epidermis and down to the basal layer. Thus, the upper layers are cooled while leaving the temperature of dermal and subcutaneous layers unchanged. This heating/cooling protocol is included within the definition of “dynamic cooling”. 
         [0090]    Currently, selective epidermal cooling or dynamic cooling is achieved using a liquid spray of the cryogen R-134a (tetrafluoroethane) for 30-100 ms immediately before laser exposure. A typical procedure is to spray the surface for 30-50 ms, and then expose the skin to laser irradiation 20-30 ms after the end of the cryogen spurt. Tetrafluoroethane is a chorine free hydrofluorocarbon (HFC), thus representing no damage to the ozone layer. However, the global warming potential (GWP) of tetrafluoroethane, which is defined as ratio for the contribution to global warming over a 100 years period as compared to the same amount of CO 2  (carbon dioxide), is comparatively high. (GWP=1300 for R134a). 
         [0091]    An alternative and probably a better choice of cryogen for medical applications, which would satisfy the requirements for low global warming potential and is completely non-flammable, is liquid or solid carbon dioxide, i.e., R744. Carbon dioxide is toxic in high concentrations, i.e. above 10% in air and might be lethal in concentrations above 20%, but it is completely safe and harmless in low concentrations, e.g., exhaled air contain up to 4%. Carbon dioxide has a triple point where liquid, solid and gas coexist, at −56.4° C. at 5.11 bar pressure. Below this pressure only the solid and gas phase coexist, and solid carbon dioxide, i.e. dry ice, will sublimate directly into gas. The gas pressure for the liquid at room temperature is 5.72 MPa (57.2 bar). When liquid CO 2  is released into air at atmospheric pressures it evaporates adiabatically to form dry ice at −78.5° C. with a latent heat of sublimation of 571.3 kJ/kg. If disposed in atomized or droplet form through one or more nozzles, the dry ice forms as carbon dioxide snow. 
         [0092]    The illustrated embodiment is based on preliminary testing of pain reduction experienced by the subjects during hair removal with an Alexandrite laser at 755 nm wavelength (GentleLase® Candela Corp. Wayland, Mass.). The skin of healthy volunteers was exposed to cooling with R134a and with CO 2  shortly before exposure to the laser beam. Human skin has several kind of sensory receptors, which respond to mechanical, thermal and chemical exposure. There are four main types of mechanoreceptors in the glabrous skin of humans: Pacinian corpuscles, Meissner&#39;s corpuscles, Merkel&#39;s discs and Ruffini corpuscles. Pacinian corpuscles are responsible for sensitivity to deep pressure touch and Meissner&#39;s corpuscles (or tactile corpuscles) are a responsible for sensitivity to light touch. The Merkel nerves are found in the basal layer of glabrous skin, such as on the finger tips and in hair follicles. The Merkel cells (along with Meissner&#39;s corpuscles) occur in the superficial skin layers, and are found clustered beneath the ridges of the fingertips whereas Pacinian corpuscles and Ruffini endings, are found primarily in tissue. 
         [0093]    The mechanoreceptors have myelinated Aβ-fiber axons with fast conduction velocity. The sensory receptor that responds to temperature, i.e. a thermoreceptor, primarily responds to temperatures within the innocuous range. In the peripheral nervous system warm receptors are thought to have unmyeiinated C-fiber axons with a slow conduction velocity lower than 2 m/s, while those responding to cold have thinly myelinated Aδ-fiber axons with a faster conduction velocity of 2 to 30 m/s. Nociceptors, which respond only to noxious stimuli, allow the organism to feel pain in response to damaging pressure, excessive heat, excessive cold and a range of chemicals, which are damaging to the tissue surrounding the receptor. The nociceptors are found in external tissues, such as skin. The cell bodies of these neurons are located in either the dorsal root ganglia or the trigeminal ganglia. There are several types of nociceptors and they are classified according to the stimulus modalities to which they respond, i.e., thermal, mechanical or chemical. Some receptors respond to more than one of these modalities and are consequently designated polymodal. Thermal nociceptors are activated by noxious heat or cold, temperatures above 45° C. and below 5° C., and mechanical nociceptors respond to excess pressure or mechanical deformation. Nociceptors may have either Aδ-fiber axons or C-fiber axons. 
         [0094]    Thus, pain often comes in two phases, and acute pain mediated by the fast-conducting Aδ fibers and slow burning pain associated with C fibers. The majority of Aδ fibers and C fibers end as free nerve endings (FNE), which are unencapsulated and have no complex sensory structures. They are located in the skin, and also protrude into the epidermis and end up in the stratum granulosum. The FNE in epidermis is indicated by the double line arrow on the left hand side of  FIG. 10 . 
         [0095]    Turn now and consider the materials and methods used to demonstrate pain masking or lessening in the illustrated embodiment. The laser was a 755 nm Alexandrite laser  100  with 3 ms pulse duration. (Candela GentleLase®) as shown in  FIG. 11 . The laser  100  was equipped with a hand piece  102  for cooling with tetrafluoroethane (Dynamic cooling device DCD®), and the beam diameter at the skin surface was approximately 12 mm. An identical hand piece  102  was used for the CO 2 , and the high pressure CO 2  container  24  was mounted upside down, thus allowing liquid carbon dioxide to reach the high pressure CO 2  valve  26  diagrammatically shown in  FIG. 2 . The valve  26  (Parker Hannifin Corp. Fairfield N.J.) was equipped with 39 mm long nozzle tube  28  of 0.5 mm inner diameter, and the end  22  of the nozzle orifice was positioned 30 mm above the skin  18 . It is to be expressly understood that the illustrated embodiment described here includes the non-shield configuration of  FIG. 2  as well as the shielded configuration of  FIG. 1 . 
         [0096]    The pain was assessed by the patients on a scale from 0-10, where 0 is no pain, 1-2 is mild pain, 3-6 is moderate pain and 7-10 is strong pain. The results from exposure to the forearm of two healthy volunteers are shown in Table 6. The R134a cooling was set at the typical ratings used in the clinic, i.e. 50 ms cryogen spray and 30 ms delay. In case of the CO 2  cooling, no liquid is deposited on the skin, but snowflakes of dry ice are deposited. This dry ice snow is carried away with the high-velocity CO 2  gas flow very quickly, enabling a delay of 10 ms to be enough to clear the skin from deposits. The adiabatic cooling also cools the snow below the boiling point. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Exposure to right forearm, palmar side, 
               
             
          
           
               
                   
                 R134a cooling 
                 CO 2  cooling 50 ms 
                   
               
               
                   
                 50 ms spurt, 
                 Spurt, 
               
               
                 No cooling 
                 30 ms delay 
                 10 ms delay 
               
             
          
           
               
                   
                 Skin 
                   
                 Skin 
                   
                 Skin 
                   
               
               
                 Pain 
                 response 
                 Pain 
                 response 
                 Pain 
                 response 
                 Fluence 
               
               
                 score 
                 at 6 h 
                 score 
                 at 6 h 
                 score 
                 At 6 h 
                 J/cm 2   
               
               
                   
               
               
                 *7-8 
                 Slight 
                 5-6 
                 Very 
                 0-1 
                 Very 
                 40 
               
               
                   
                 edema 
                   
                 slight 
                   
                 slight 
               
               
                   
                   
                   
                 edema 
                   
                 edema 
               
               
                 **n/a 
                 n/a 
                 4-5 
                 n/a 
                 0-1 
                 n/a 
                 40 
               
               
                 **n/a 
                 n/a 
                 3-4 
                 n/a 
                 1 
                 n/a 
                 30 
               
               
                   
               
               
                 *Patient: North European, Fitzpatrick skin type II-III, melanin index 143 
               
               
                 **Patient: Asian, Fitzpatrick skin type III 
               
             
          
         
       
     
         [0097]    After 24 hours all edema was gone in all spots. No erythema was seen in the cooled spots, whereas a very slight erythema remained at the spots with no cooling.
       i. Corresponding results for exposure to the dorsal side of the forearm is given in Table 7. The arm was carefully shaved before exposure to the laser  100 .       
 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Exposure to left forearm, dorsal side, melanin index 191 
               
             
          
           
               
                   
                 R134a cooling 
                 CO 2  cooling 
                   
               
               
                   
                 50 ms spurt, 
                 50 ms Spurt, 
               
               
                 No cooling 
                 30 ms delay 
                 10 ms delay 
               
             
          
           
               
                   
                 Skin 
                   
                 Skin 
                   
                 Skin 
                   
               
               
                 Pain 
                 response 
                 Pain 
                 response 
                 Pain 
                 response 
                 Fluence 
               
               
                 score 
                 at 6 h 
                 score 
                 at 6 h 
                 score 
                 At 6 h 
                 J/cm 2   
               
               
                   
               
               
                 *7-8 
                 Strong 
                 6-7 
                 minor 
                 1-2 
                 minor 
                 40 
               
               
                   
                 edema 
                   
                 edema 
                   
                 edema 
               
               
                   
               
               
                 *Patient: North European, Fitzpatrick skin type II-III 
               
             
          
         
       
     
         [0099]    The pain and the skin response in this case were systematically more pronounced, as compared to the palmar side. This is believed to be due to higher melanin content. After 24 hours all edema was gone and only a moderate erythema remained at the spots with no cooling. The erythema remained in the non-cooled spots after 3 days, whereas only signs of burned hair follicles were seen in the cooled spots at this time. No differences were observed in the spots exposed to CO 2  or R134a. 
         [0100]    Thus skin response for the two cooling modalities was very much the same, but a striking reduction in pain score for the CO 2  was systematically observed. The overall reduction in the sensation of pain with cooling is believed due the pain gate effect, where incoming stimuli from the mechanical exposure to the skin blocks the stimuli from the thermal response in neural synapsis in the posterior horn. The pain gate effect is observed on both Aδ-fiber and C-fiber axons. In order to evaluate the duration of the pain blocking effect the delay between exposure to the CO 2  spray and the laser exposure was varied. The results are shown in Table 8. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Exposure to right forearm, palmar side. 
               
             
          
           
               
                   
                 Delay between 
                   
               
               
                   
                 end of CO 2  spray 
               
               
                   
                 and laser 
               
               
                   
                 exposure (ms) 
                 Pain score 
               
               
                   
                   
               
               
                   
                  10 
                 0 
               
               
                   
                  20 
                 0 
               
               
                   
                  30 
                 0-1 
               
               
                   
                 100 
                 1-2 
               
               
                   
                 200 
                 1-2 
               
               
                   
                 400 
                 1-2 
               
               
                   
                 1000  
                 3-6 
               
               
                   
                 2000  
                 3-4 
               
               
                   
                 No cooling 
                 7-8 
               
               
                   
                   
               
               
                   
                 *Patient: North European, Fitzpatrick skin type II-III. 
               
               
                   
                 Fluence 40 J/cm 2 , liquid CO 2  50 ms, spot 12 mm diameter 
               
             
          
         
       
     
         [0101]    No erythema or edema observed after 1 hour. At 24 hours no erythema or edema observed. The pain score was no pain for delays up to about 0.1 s, a mild pain up to about 0.5 s and moderate pain for delays larger than 1 s. In the no-cooling case, a strong acute pain was followed by a milder pain relaxing down in 2 s. 
         [0102]    In order to investigate the dependence of the pain reduction delay effect on different anatomical locations the same experiment was done on the back. The results are summarized in Table 9. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Exposure to upper back, 100 mm from spine 
               
             
          
           
               
                   
                 Delay between 
                   
               
               
                   
                 end of CO 2  spray 
               
               
                   
                 and laser 
               
               
                   
                 exposure (ms) 
                 Pain score 
               
               
                   
                   
               
               
                   
                  10 
                 0 
               
               
                   
                  20 
                 0 
               
               
                   
                  50 
                 0-1 
               
               
                   
                 100 
                 0-1 
               
               
                   
                 200 
                 1-2 
               
               
                   
                 400 
                 2-3 
               
               
                   
                 1000  
                 4-5 
               
               
                   
                 2000  
                 6-7 
               
               
                   
                   
               
               
                   
                 *Patient: North European, Fitzpatrick skin type II-III. 
               
               
                   
                 Fluence 40 J/cm 2 , liquid CO 2  50 ms, spot 12 mm diameter 
               
             
          
         
       
     
         [0103]    The results indicate that the pain response was about the same as in the case of the forearm, and the pain relaxation effect was rather independent on the distance from the spine. 
         [0104]    In order to investigate the effect on pain removal from the gas flow alone the gas container was positioned to allow only CO 2  gas leave the valve  26  on the flask  24 . This strongly reduces the cooling effect as well as reducing the momentum flux of the gas hitting the skin. The results are summarized in Tables 10 and 11. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Exposure to right forearm, palmar side 
               
             
          
           
               
                   
                 Delay between 
                   
               
               
                   
                 end of CO 2  spray 
               
               
                   
                 and laser 
               
               
                   
                 exposure (ms) 
                 Pain score 
               
               
                   
                   
               
               
                   
                  10 
                 0-1 
               
               
                   
                  20 
                 2 
               
               
                   
                  50 
                 1-2 
               
               
                   
                 100 
                 1-2 
               
               
                   
                 200 
                 2-3 
               
               
                   
                 400 
                 3-4 
               
               
                   
                 1000  
                 4-5 
               
               
                   
                 2000  
                 6-7 
               
               
                   
                   
               
               
                   
                 *Patient: North European, Fitzpatrick skin type II-III. 
               
               
                   
                 Fluence 40 J/cm 2 , CO 2  gas 50 ms, spot 12 mm diameter. 
               
             
          
         
       
     
         [0105]    The pain score was somewhat higher than in case of liquid CO 2 , e.g. the pain score at 10 ms delay was increased from zero pain to mild pain. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 11 
               
             
             
               
                   
               
               
                 Exposure to forearm, dorsal side 
               
             
          
           
               
                   
                   
                   
                 Gas CO 2  cooling 
                   
               
               
                   
                   
                   
                 50 ms 
               
               
                   
                   
                   
                 Spurt, 
               
               
                   
                 No cooling 
                   
                 10 ms delay 
               
             
          
           
               
                   
                 Skin 
                   
                 Skin 
                   
               
               
                 Pain 
                 response 
                 Pain 
                 response 
                 Fluence 
               
               
                 score 
                 at 2 h 
                 score 
                 At 2 h 
                 J/cm 2   
               
               
                   
               
               
                 *7-8 
                 Mild 
                 6-7 
                 Mild 
                 40 
               
               
                   
                 edema 
                   
                 edema 
               
               
                   
               
               
                 *Patient: North European, Fitzpatrick skin type II-III 
               
               
                 Fluence 40 J/cm 2 , liquid CO 2  50 ms, 10 ms delay, spot 12 mm diameter 
               
             
          
         
       
     
         [0106]    The temperature at the skin surface during cooling was about the same for the R134a cryogen and the CO 2  cooling. This was also corroborated by the observation that the skin response was the same the two cases. The momentum flux density of the cryogen in the present settings is, however, about 10 times larger for CO 2  than for R134a cryogen, i.e. 43.1 kPa (328 mmHg) and 3.6 kPa (28 mmHg) respectively. Momentum flux density of CO 2  typically spans or includes the range of 40 kPa (300 mm Hg) to 67 kPa (500 mmHg). The decrease in pain is believed caused by mechanical stress stimuli and the total pain suppression is assumed to be a result of the combined mechanical and cooling stimuli rather than the cooling alone such as in the case of R134a cryogen. It is also not impossible that some masking of the pain sensation results from distraction or psychosomatic effects arising from the louder popping or explosive discharge sounds, which tend to accompany adiabatic expansion of liquid carbon dioxide as compared to R134a cryogen. The observation of a relaxation time of 1-2 s for the pain reduction effect corresponds well to the time required for the signals from C-fiber axons propagate over about 1 m distance from the forearm to the spinal ganglion. However, this assumption was not supported from measurements close to the spine, as the relaxation time was approximately the same at a location 0.1 m from the spine. 
         [0107]    It is also noteworthy that the momentum flux density of the CO 2  spurt of 340 mmHg is in the same range as a hypobaric pressure of 300-500 mmHg which has been reported for pain reduction with a suction device. 
         [0108]    The acute pain in the non-cooled case and in the case of R-134a cooling ranges from severe (score 7-8) to moderate (score 4-7), respectively. No pain at all or very mild pain was experienced with CO 2  cooling. (Score 0-2). Thus, a significant reduction in pain scores was observed with CO 2  cooling. 
       Pain Management Using Positive Pressure 
       [0109]    The use of negative pressure or partial vacuum to mask pain in dermal treatments using lasers or intense pulsed light in cosmetic dermal mediation is well known and is discussed in Lask et. al., “Psnuematic Skin Flattening (PSF): A novel technology for marked pain reduction in hair removal with high energy density lasers and IPLs” J. of Cosmetic and Laser Therapy, 2006; 8:76-81; Slatkine, US Patent Publications 2005/0215987; 2006/0259102; and 2006/0293722. However, the negative pressure of a vacuum device and the positive pressure of a CO 2  flow introduce different stress distributions in skin. The stress tensor τ xx , τ yy , τ zz , τ xy =τ yx , τ xz =τ zx , τ yz =τ zy  has 6 different elements. If you consider a cube, the stress tensor component τ xx  represents the force in the x-direction normal to the surface. Correspondingly, τ yy  and τ zz  are the forces normal to the surface in the y and in the z directions, respectively. With the convention that the normal is positive when directed outward from the surface, the forces can be either negative (compression) or positive (tension). The other three tensor components represent the shear forces, i.e. τ xy  represents the force tangential in y direction to the surface with surface normal in x-direction. 
         [0110]    The stress distribution with a negative pressure pneumatic skin flattening device are primarily compressive nature as diagrammatically illustrated in  FIG. 12 . A sapphire chamber  50  through which the radiation is aimed onto skin  52  is provided with a negative pressure or partial vacuum in space  54 . The ambient internal tissue pressure, represented symbolically by arrows  56  in the skin and its lower layers, applies a pure compressive force from below onto the irradiated portion of the skin  52 . The stress distribution in case of CO 2  momentum flux is diagrammatically shown in  FIG. 13  and is more similar to that found in a bending beam, namely the components of the stress tensor will vary with location. The stress tensor in the case of  FIG. 13  is comprised of a combination of shear and compressive and tensile stresses. 
         [0111]      FIG. 14  is a simplified model showing a sphere  58  in cross section. The sphere  58  has a wall thickness equal to the irradiated dermal thickness, δ. The stress, σ o , within the dermis of thickness, δ, is tensile in nature if the interior of the sphere  58  is provided with a positive pressure of gas or CO 2 , and is approximately given by, 
         [0000]      Π d   2 σ/4=σ 0   Πd δ, thus σ 0   =σd/ 4δ 
         [0000]    where d is the diameter of cavity  54  created by the positive pressure or CO 2  flow onto the tissue  52  exposed to a momentum flux density σ of CO 2 , which cavity is assumed for the sake of simplicity to be spherical. The diagram of  FIG. 13  showing spurt depressing a skin spot can be considered to be or approximated by half the spherical model of  FIG. 14 . Thus, the stress within the skin or tissue  52  can easily be higher than the momentum flux density, e.g., if the exposed region is 10 mm in diameter and the dermis is 1 mm thick, the stress will be about 2.5 times larger than the momentum flux. Further on, the stresses within the dermis are not limited by the air pressure of 1 bar, but can be well above it. Therefore, it can be readily understood that the use of positive pressure to create stress distributions in skin is not equivalent to that caused by negative pressure. Further  FIG. 15  is a graph which shows the impact pressure on skin from delivery of R134a cryogen as compared to CO 2  through identical nozzles. R134a cryogen and liquid CO 2  are both kept in pressurized containers at room temperature. In order to insure that delivery pressure through the nozzle is maintained at a constant value without the need for an external pump and controller to maintain pressure, both R134a cryogen and liquid CO 2  are maintained at a saturated state in the storage container, i.e. gas and liquid are in equilibrium. The saturation pressure at room temperature is much lower for R134a cryogen than it is for liquid CO 2  with the consequence that skin impact pressures at a spray distance of 30 mm is ten times higher for liquid CO 2  than for R134a cryogen. The higher impact pressure of  FIG. 15  translates into lower pain scores as shown in  FIG. 16  where five subjects reported pain measures immediately after laser irradiation of a 12 mm spot on the skin in combination with dynamic cooling using R134a cryogen or liquid CO 2 . The R134a cryogen cooled spot was adjacent to the liquid CO 2  cooled spot and the laser&#39;s radiant exposures were the same for both spots but radiant exposure for different body location (palmar or dorsal arm) was different (30 or 40 J/cm 2 ). The average pain score was almost 4 times lower with liquid CO 2  than with R134a cryogen. Pooling was also significantly less with liquid CO 2  than with R134a cryogen.  FIG. 18  is a graph which shows the measured surface temperature of an impact spot on skin phantom created with R134a cryogen by a spray nozzle 30 mm vertically above the skin surface and inclined at a 30° angle relative to the normal of the skin surface. The center point as shown in  FIG. 17  is denoted in  FIGS. 20 and 21  by the letter C and a point 2 mm from C is denoted by the letter X 1 .  FIG. 18  shows that the R134a cryogen pooled on the surface for much more than 500 ms.  FIG. 19  is a graph showing the temperature impact of liquid CO 2  for the same nozzle configuration as in  FIG. 18  but shows substantial evaporation of the coolant at both C and X 1  within 200 ms. High speed videos of the impact area show that no dry ice snow was adhered to the cooled target surface due to the force of the high velocity gas CO 2  phase. 
         [0112]    The measured surface temperature (T s ) variations with time for R134a and CO 2  sprays are shown in  FIG. 20 . Although T s  for R134a drops faster than CO 2  spray, the minimum T s  for CO 2  spray is lower because of the lower temperature of the dry ice snow. If the spurt duration is longer than 50 ms, T s  for CO 2  spray will decrease further. In contrast, T s  for R134a spray will keep nearly the same. Studies using high-speed video camera reveal that CO 2  spray can be better controlled than R134a spray. Because of the higher saturation pressure of liquid CO 2 , CO 2  spray takes less time to become fully developed than R134a spray. The developing or closing time for CO 2  spray is less than 1 ms, while it is 3-4 ms for R134a spray. Therefore, the same valve can produce a nearly perfectly step-shaped temporal envelope for CO 2  spray with a minimum duration of 5 ms. In contrast, the minimum duration of R134a spray is ˜11 ms. This feature is especially beneficial for laser treatment using multiple cryogen spurts and multiple laser pulses. 
         [0113]    Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments. 
         [0114]    Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. A teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other, but may be used alone or combined in other combinations. The excision of any disclosed element of the invention is explicitly contemplated as within the scope of the invention. 
         [0115]    The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
         [0116]    The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination. 
         [0117]    Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
         [0118]    The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.