There are many medical conditions, the treatment of which is substantially improved by being able to control the deposition of laser energy in a specific target tissue in order to damage that target tissue while sparing the adjacent tissue. While those in the past have utilized lasers, particularly in the port wine stain (PWS) syndrome, to destroy blood vessels, the problem associated with such systems is that the dwell time of the laser over the target produces significant thermal diffusion which damages not only the abnormal PWS vessels which are ectatic, i.e., dilated and filled, and strongly absorb the radiation, the ones producing the wine stain, but also damages a significant depth of the dermis such that scabbing and sloughing occurs as a consequence of treatment. Additionally, the use of an anesthetic is prescribed because of the amount of energy imparted to the target area which is painful to the patient. It will be appreciated that the desired treatment for port wine stains is to necrose only those vessels producing the stain while leaving most of the surrounding collagen and the normal vessels undamaged. This translates into control of thermal diffusion, which up until the present time has been difficult either because of the relatively long pulse lengths of the shuttered CW lasers utilized or because of the relatively short pulse lengths of the dye lasers which do not completely destroy the abnormal vessels. Consequently, no successful control has heretofore been exercised to limit the volume affected by thermal diffusion. The result of the lack of control is that lasers which dwell on a given target area for 20 milliseconds or more produce so much thermal diffusion that scabbing and sloughing of the epidermis and portions of dermis are produced regardless of wavelength, assuming any kind of therapeutic levels are introduced to the target area.
By way of further background, if it is desirable to destroy abnormal tissue contained within the volume of normal tissue and spare the overlying normal layer, differential absorption of the light is required. This can be obtained by either intrinsic optical qualities of the target or tagging with some exogenous chromophore. In the latter case, the wavelength or color of the laser must be selected on the basis of the absorption spectra of both the target and the surrounding tissue. In other words, it should be a wavelength where the target tissue is a good absorber and the surrounding tissue is a poor absorber. Moreover, the irradiation time must be selected on the basis of the thermal properties of the target and surrounding tissue and the geometric shape and dimensions of the tissue structure.
Thus, in some cases the target tissue is distinguished from surrounding tissue by a difference in absorption spectra either due to a naturally-occurring chromophore (absorbing molecule) or due to the selective deposition of some dye used in the treatment regimen. One important example of such a target tissue present throughout the body is the vasculature which contains erythrocytes. The erythrocytes contain hemoglobin, a naturally-occurring chromophore with a broad usable absorption band in the visible. The entire range of visible wavelengths shorter than approximately 600 nm (nanometers) and extending into the ultraviolet is available to purposely inflict damage to target tissues containing this chromophore. The specific wavelength selected depends on the relative effects of scattering, which varies with wavelength; the presence of other chromophores, such as melanin, in the adjacent or overlying tissues; and the availability of light sources.
In selecting the exposure time, one is limited by the time which will confine thermal damage due to heat transport to an acceptable distance from the target. For the treatment of port wine stains, it is desirable to be in the regime where the dominant mechanism for heat transport is conduction. A characteristic thermal diffusion length for heat conduction is given by EQU L.sup.2 =4Kt
where L=distance that heat diffuses; K=thermal diffusivity coefficient; and t=time allowed for diffusion.
This formula varies slightly with the geometry of the irradiated target in surrounding media, but the variations are not significant. A typical thermal diffusivity coefficient for biological tissues is 0.0015 cm.sup.2 /second.
Considering, for example, the treatment of port wine stains, a type of hemangioma that consists of hypertrophic capillaries in the dermis causing a pink, red or purple coloring of the skin, the pink and red lesions are high in erythrocytes carrying oxygenated hemoglobin (HbO.sub.2), while the purple lesions contain large quantities of deoxygenated hemoglobin (Hb). The lesions are characterized as consisting of abnormal capillary structure with the capillaries varing in diameter, the mean diameter being about 50 micrometers. The wall of the vessel, however, is only a few micrometers thick. The average vessel spacing is 100 micrometers. In order to damage the vessel containing the target hemoglobin, its wall, and a small portion of collagen surrounding the wall, the latter two being relatively free of chromophore, it is important to select an exposure time corresponding to thermal diffusion over a characteristic length slightly greater than the wall thickness. This, in general, refers to the delivery of radiation to a given target area of less than one millisecond. Those prior art devices which deliver 20 milliseconds or more cause damage due to thermal diffusion of heat to a distance even greater than the vessel spacing. Thus the entire tissue bulk is heated by the vessel network embedded within it and the damage is not at all selective.
J. L. Finley, S. H. Barsky and D. E. Geer in an article entitled "Healing of Port Wine Stains After Argon Laser Therapy," Archives of Dermatology, 1981, Volume 117, pps. 486-489, and J. L. Ratz, P. L. Bailin, and H. L. Levine in an article entitled "CO.sub.2 Laser Treatment of Port-Wine Stains: A Preliminary Report," J. Dermatol. Surg. Oncol. 1982, Vol. 8, No. 12, pps 1039-1044, describe both argon lasers and carbon dioxide lasers used in the clinical treatment of port wine stains. Neither of these provide an optimal treatment modality as the dwell time is not limited to prevent thermal diffusion.
It will be appreciated that the CO.sub.2 laser radiation whose wavelength is approximately 10 micrometers is very strongly absorbed in water and most proteins. In port wine stains, both the abnormal vasculature and the surrounding dermal tissues are approximately 90% water and consequently absorb the incident laser radiation and are heated to the point of thermal necrosis. Thus this treatment does not involve any specificity of damage. The necrotic tissue eventually sloughs off and is replaced via the normal healing process by scar tissue formation. Since the scar tissue formed is usually flat and white, it is often more acceptable to the patient that the original dark and, sometimes, hypertrophic lesion.
Present treatment of port wine stains with the argon laser is performed using comparatively long pulse times. Because of this, heat has time to diffuse to the surrounding tissue, and the effect observed is the same as for the CO.sub.2 laser in which radiation is uniformly absorbed in both vascular and surrounding tissue. The similarity of clinical results with the CO.sub.2 and the long pulse argon lasers has been noticed and documented in an article by J. W. Buecker, J. L. Ratz and D. F. Richfield entitled "Histology of Port Wine Stains Treated with CO.sub.2 Laser," Fifth International Conference of Laser Medicine and Surgery, Detroit 1983, in an abstract. Thus the nonspecificity of prior art laser treatment of port wine stains is both documented and explainable by the relatively long irradiation times causing massive long-distance thermal diffusion for argon lasers and the non-specific absorption for CO.sub.2 lasers.
By contrast ultrashort laser pulses have been used. Studies by R. R. Anderson and J. A. Parish entitled "Microvasculature Can Be Selectively Damaged Using Dye Lasers: A Basic Theory in Experimental Evidence in Human Skin," Lasers in Surgery and Medicine, 1981, Volume 1, pps. 263-276, show that when utilizing pulse dye lasers with fluence level on the order of 3 to 5 Joules/cm.sup.2 and exposure times of approximately 300 nanoseconds (ns) target specific damage may be produced in normal blood vessels. The wavelength used was 577 nanometers. While the above pulse width was short enough to restrict thermal diffusion to a small portion of the individual erythrocytes having typical dimensions of 7-15 micrometers carrying the hemoglobin, and thermal diffusion subsequent to the pulse could have allowed heating of the containing vessel without damage to the surrounding tissues, the short 300-nanosecond duration caused the vessels to burst and to spew forth blood. It is possible that a shock wave produced by the ultrashort pulse ruptured the blood vessels causing formation of purpura. Since the vessels are 50.mu. in diameter and the wall is about 1.mu. thick, the pulse is so short that only the hemoglobin itself (which is the optical absorber) and any spot on the inner edge of the wall which happens to be in intimate thermal contact with the hemoglobin bearing portion of an erythrocyte are heated during the pulse. After the pulse, the peak temperature achieved within the vessel decays as heat diffuses away. While regions outside the vessel are in fact heated as this diffusion occurs, it is not possible to achieve thermal damage to an adequate depth to insure permanent vessel necrosis.
Note that at the present time several microseconds is the longest pulse time available from commercial pulsed dye lasers. This is still too short to achieve the desired effect.
In summary, it will be appreciated that the difficulty in the prior art methods of utilizing argon laser treatment lies not in the wavelength, at least for low melanin skins, but in the exposure time utilized. The argon lasers are CW lasers which are mechanically shuttered to provide pulsewidths which may vary from 20 milliseconds to 100 milliseconds or more. Even the shortest of these exposure times, 20 milliseconds, results in thermal diffusion to a length of 100 micrometers which is equal to the average spacing between targets. Thus, even if the laser power is initially absorbed only in the target volumes, thermal diffusion during the laser pulse itself provides nearly uniform heating of the entire irradiated area. In order to achieve true specificity, damaging only the target vessels, the exposure time must be limited to about one millisecond or less, which is too short to be achieved with the mechanical shutters presently in use. Additionally, nanosecond pulses from dye lasers cause blood vessel rupture and causes only partial necrosis. Thus these techniques are not optimally useful in treating port wine stains.
Note the following U.S. patents deal with scanning lasers: U.S. Pat. Nos. 3,362,007; 3,642,007; 4,069,823; and 4,316,467; whereas U.S. patents dealing with coaxial bilaser beams include U.S. Pat. Nos. 3,456,651; 3,710,798; 3,769,963; 3,906,953; 3,910,276; 4,240,431 and 4,408,602. Finally, U.S. Pat. No. 3,434,476 deals with a plasma arc scalpel.