Applying heat is a well-known technique for treating human tissue. Multiple thermal treatment techniques exist for tissue ablation and coagulation as well as for non-ablative tissue stimulation and regeneration. Those techniques include radio-frequency ablation (RFA), microwave, focused ultrasound and lasers. Lasers, and in some cases, other light sources such as intense pulsed light (IPL) sources are especially suitable for the delivery of the required thermal dose to the tissue being treated. Laser radiation energy is measured in Joules (J) and is directly proportional to the quantity of photons of the radiation. Power is the rate of energy exposure measured in Watts (W) where 1 W=1 J/s. Power Density, also known as Irradiance refers to the amount of power per area and is expressed in Watts per centimeter squared (W/cm2). The total amount of energy exposed to a surface is known as the Fluence=Power×Time/Area and is expressed in term of Joules per centimeter squared (J/cm2). In laser surgery, fluence determines the total volume of tissue damage.
In many medical and dermatological applications, the aim of the treatment is to deliver a predetermined amount of thermal energy to a desired target volume of tissue at a specific depth below the surface, while sparing the adjacent and overlying tissue.
One of the disadvantages of prior art laser techniques is its limited ability to deliver significant energy to a desired depth in the tissue, through superficial layers of the tissue, without exposing the superficial layers to significant levels of energy, which can cause undesirable effects to the superficial layers. Moreover, due to tissue attenuation effect, in order for the deeper tissue layers to receive sufficient amount of energy, the superficial layers are exposed to even higher levels of energy thus increasing these undesirable side effects.
Optical scanners have been proposed for addressing problems such as variable depth penetration, non-uniform exposure and consequent charring of tissue surface. In general, prior art scanners, such as those described in U.S. Pat. No. 5,743,902 to Trost, displace an optical treatment beam from one treatment spot to another in a controlled manner. Other scanners such as those described in U.S. Pat. No. 5,582,752 to Zair and U.S. Pat. No. 5,786,924 to Black et al, additionally provide focusing of the beam on the surface using refractive optics (such as lenses) or reflective optics (such as concave and convex mirrors) to provide homogeneous vaporization of the tissue. Although those scanners provide some improvement by allowing more uniform coverage of the surface, they don't address the problem of managing the desired level of energy delivery below tissue surface.
A known technique to address the problem of selective targeting while sparing the adjacent tissues is called Selective Photothermolysis (SP), in which selective tissue thermal damage can be achieved using optical energy with a wavelength that is specifically absorbed by a natural or artificially introduced chromophore in the target area. In addition, the process also involves choosing the duration of the energy pulse to maximize the temperature of the target before significant diffusion of heat to the surrounding tissue can take place. This technique, in the prior art, is limited and is applicable only to applications whereby the treated target has spectral absorption which is different from the adjacent tissue. This isn't the case in many applications.
For example, prior art optical technologies for long-term hair removal treatment are based on thermal destruction of the hair shaft and follicle using wavelengths that are specifically absorbed by the pigment melanin found in the hair follicle. One of the limitations of those technologies is the fact that the epidermis through which the light energy must penetrate is rich in melanin and therefore absorbs a major portion of the energy, resulting in inadequate heating of the hair follicles as well as damage to the epidermis. Using higher energy levels in order to generate sufficient heating of the hair follicles can cause charring and hyper-pigmentation. Although the chromophore being targeted may vary for different applications, the above limitations are common for all such applications.
Another problem with selective photothermolysis is that the wavelength selected for the radiation is generally dictated by the absorption characteristics of the chromophore utilized. However, such wavelengths may not be optimal for penetration deep enough to reach the target due to tissue scattering which depends on wavelength.
Various techniques have been used or proposed to assist in improving the efficiency of the process. These techniques include pre-cooling of the treated area, cooling during the process, selective cooling of the epidermis using millisecond cryogen spray, use of optical transmitting gels to improve coupling into the tissue, convex shaped applicators, and devices to draw folds of skin which may receive radiation from both sides.
Another approach, described in U.S. Pat. No. 5,735,844 to Anderson et al., in International PCT patent application published as WO 98/52481 to Colles and in U.S. patent application Ser. No. 10/033,302 to Anderson et al., use various types of refractive elements such as lenses to focus the optical energy at specific depth under the skin surface, thus increasing the energy fluence at the depth. There are several disadvantages associated with this approach:                1. Optical focusing in the tissue substantially increases the power density of the beam. Such increased power density at a focal point and its immediate proximity is very difficult to control, which may create a serious epidermis safety hazard. This is particularly true with low numerical aperture focusing systems, easily realizable with refractive optics.        2. There can be a significant thermal exposure to overlying tissue located above the target, especially that in close proximity to the target region at the focus.        3. Inability to simultaneously treat multiple, separate targets at different depths using different wavelengths and different light pulse durations.        4. Focusing of the beam using lenses, generally requires contact with the skin surface, which may complicate practical implementation of this method, for example, by precluding use of galvanic scanners.        
It must be emphasized that focusing techniques may also lack inherent accuracy and may be dependent on the particular properties of the patient's skin. Scattering of radiation inside the skin may preclude the possibility to attain a well defined focus as well as a well defined treatment depth.
Whereas there may be both advantages and disadvantages to varying degrees in all of these approaches, it is clear that there is a need for an improved light energy delivery system that addresses the problems of sub-surface thermal treatment.
It would therefore be desirable and advantageous to devise an effective method and apparatus for medical and aesthetic thermal treatment of tissue at the desired depth, while minimizing the thermal damage to the overlying layers of the tissue.
Accordingly, it is an object of the present invention to overcome the disadvantages of prior art methods and provide an improved method and apparatus for thermal treatment of a tissue. More specifically, it is an object of the invention to provide an apparatus for thermal treatment of confined tissue volumes at predetermined depths, while minimizing the damage to overlying, superficial tissue layers and surrounding tissues, thus significantly improving the safety of the procedure. Another objective is improving the efficacy of thermal treatment methods by delivering significantly higher fluence levels to the target, than achievable with prior art methods, still preserving safe fluence level at the tissue surface. Another objective is providing an apparatus capable of treating a variety of medical and dermatological conditions.
The foregoing objectives are attained by the apparatus and method of the present invention.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.