Surgery – Instruments – Light application
Reexamination Certificate
2000-02-10
2003-04-15
Shay, David M. (Department: 3739)
Surgery
Instruments
Light application
C606S009000, C606S013000, C607S089000, C607S090000
Reexamination Certificate
active
06547781
ABSTRACT:
BACKGROUND OF THE INVENTION
Vascular lesions, comprising enlarged or ectatic blood vessels, pigmented lesions, and tattoos have been successfully treated with lasers for many years. In the process called selective photothermolysis, the targeted structure, the lesion tissue or tattoo pigment particles, and the surrounding tissue are collectively irradiated with laser light. The wavelength or color of this laser light, however, is chosen so that its energy is preferentially absorbed by the target. Localized heating of the target resulting from the preferential absorption leads to its destruction.
Most commonly in the context of vascular lesions, such as portwine stains for example, hemoglobin of red blood cells within the ectatic blood vessels serves as the laser light absorber, i.e., the chromophore. These cells absorb the energy of the laser light and transfer this energy to the surrounding vessel as heat. If this occurs quickly and with enough energy, the vessel reaches a temperature to denature the constituents within the boundary of the vessel. The fluence, Joules per square centimeter, to reach the denaturation of a vessel and the contents is calculated to be that necessary to raise the temperature of the targeted volume within the vessel to about 70° C. before a significant portion of the absorbed laser energy can diffuse out of the vessel. The fluence must, however, be limited so that the tissue surrounding the vessel is not also denatured.
As suggested, simply selecting the necessary fluence is not enough. The intensity and pulse duration of the laser light must also be optimized for selectivity by both minimizing diffusion into the surrounding tissue during the pulse while avoiding localized vaporization. Boiling and vaporization lead to mechanical, rather than chemical, damage—which can increase injury and hemorrhage in the tissues that surround the lesion. This constraint suggests that for the fluence necessary to denature the contents of the vessel, the pulse duration should be long and at a low intensity to avoid vaporization. It must also not be too long because of thermal diffusion, however. Energy from the laser light pulse must be deposited before heat dissipates into the tissue surrounding the vessel. The situation becomes more complex if the chromophore is the blood cell hemoglobin within the lesion blood vessels, since the vessels are an order of magnitude larger than the blood cells. Radiation must be added at low intensities so as to not vaporize the small cells, yet long enough to heat the blood vessels by thermal diffusion to the point of denaturation and then terminated before tissue surrounding the blood vessels is damaged.
Conventionally, flashlamp-excited dye lasers have been used as the laser light source. These lasers have the high spectral brightness required for selective photothermolysis and can be tuned to colors at which preferential absorption occurs. For example, wavelengths in the range of 577 to 585 nanometers (nm) match the alpha absorption band of hemoglobin and thus are absorbed well by the red blood cells in the blood vessels. The absorption of melanin, the principal pigment in the skin, is poor in this range, yielding the necessary selectivity.
Flashlamp-excited dye lasers, however, present problems in the pulse length obtainable by this type of laser. Theory dictates that the length of the light pulse should be on the order of the thermal relaxation time of the ectatic vessels or other dermal target. Ectatic vessels of greater than 30 microns in diameter are characteristic of cutaneous vascular lesions. These large vessels have relaxation times of 0.5 milliseconds (msec) and longer and thus require pulse durations of this length. Commercially available flashlamp-excited dye lasers generally have maximum pulse lengths that are shorter than 0.5 msec. Brute force excitation of the dye gain medium can result in pulses as long as 1.5 milliseconds. As a result, selective photothermolysis treatment of ectatic vessels larger than 30 microns currently relies on higher than optimum irradiance to compensate for the pulse duration limitations. This leads to temporary discoloration of the skin, viz., purpura.
With shorter than desirable pulse durations, purpura, which is a bluish lesion that appears as black and blue spots, forms at the treated site. It is not medically harmful nor is it permanent, and lasts but a couple of weeks. Patients prefer not to have this cosmetically undesirable side effect. It is commonly believed that pulses longer than 5 msec will reduce the formation of purpura.
Dierickx, et al., “Thermal Relaxation of Port Wine Stain Vessels Probed In-Vivo: The Need for 1-10 Millisecond Laser Pulse Treatments,”
J. of Investigative Dermatology,
105, 709-714, (1995) report the data and histologic assessment of the vessel injury strongly suggest that pulse durations for ideal laser treatment are in the 1-10 millisecond region and depend on vessel diameter. No dermatologic laser presently used for port wine stain treatment operates in this pulse width domain. Commercial medical dye lasers with pulse durations of 1.5 msec are now available but these lasers do not show the needed improvement in the treatment of ectatic vessels. Moreover, the combination of two dye lasers was suggested to generate 4.5 msec pulses according to U.S. Pat. No. 5,746,735 and the output used in leg vein treatment. The results showed marginal improvement over pulses 1.5 msec long. See Alora M. B., et al., “Comparison of the 595 nm Long Pulse (1.5 ms) and 595 nm Ultra Long Pulse (4 ms) Laser in Treatment of Leg Veins,” American Society Laser Medicine 18th Annual Meeting Supplement 10, No. 158, (1998). It is therefore desirable to get to 10 msec and longer.
In dye lasers, it has been observed that the premature cessation of the lasing is caused primarily by the degradation of the dye solution. Improved dye solution_ formulations can yield some increases in pulse duration. Dye degradation, however, cannot be totally eliminated and other steps must be taken if pulse durations of 5 msec and longer and having the fluences for medical procedures are to be achieved.
One attempt at lengthening the pulse du ration utilizes a flashlamp-excited dye lasers that has a dye cell that permits rapid dye solution interchange during the laser excitation pulse. Specifically, the dye in the dye cell is replaced while the flashlamps are fired so that exhausted and degraded dye medium is removed from the resonant cavity and replaced with fresh dye medium during the excitation pulse, thereby facilitating the lengthening of the laser pulse. The approach is similar to that used to generate laser emission in cw dye lasers, albeit at the much higher energies required for these medical applications.
SUMMARY OF THE INVENTION
The batch replacement of dye solution and subsequent processing of the dye solution to lengthen the pulse duration in dye lasers has met with some success. Nonetheless, still longer pulses are required in some cases than currently appear practical using this technique.
The problem that appears to limit the practicality of this technique concerns the fact that it suboptimally uses the flashlamps. The low peak current may not be high enough to excite the dye gain medium well above threshold and the current below threshold is wasted making the long continuous pulse dye laser very inefficient. A preferred mode of operation for the dye laser to generate long, effective laser pulses is to use a sequence of short on and off flashlamp pulses. To generate long laser pulses without exceeding the explosion point of the flashlamp, the current through the lamp is limited to run safely without damaging the lamp. The short current pulses have peak currents that are well above the lasing threshold. The pulse duration of the individual pulse is short enough so as to be well below the explosion point of the flashlamp. If the time when the flashlamp is on and when it is off is shorter than the thermal relaxation time of the target to be heated, the heating effect is nearly the same as if
Cynsure, Inc.
Hamilton Brook Smith & Reynolds P.C.
Shay David M.
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