3.6.1 Theory of Selective Photothermolysis
3.6.1.1 Basic Principles
For many years, electromagnetic radiation (EMR) from lasers, lamps, and other sources (including microwave ones) has been used to treat a variety of medical conditions in ophthalmology, dermatology, urology, otolaryngology, and other specialties. For example, in dermatology EMR sources have been used to perform a wide variety of procedures including hair removal, treatment of various pigmented lesions, removal of unwanted veins, tattoo removal, and skin resurfacing. For all these treatments, a natural or artificial chro – mophore presented in the body is heated by absorption of either monochromatic or broadband EMR. Typical natural (endogenous) chromophores include water, melanin, hemoglobin, protein, lipid, etc. Exogenous chromophores can include dyes, ink, carbon particles, etc. For example, heating of a chromophore may result directly in the destruction of a tattoo or a pigmented lesion. In these cases, the treated target for destruction and the chromophore occupy the same area. These cases are well-described by the theory of selective photothermolysis (SP) [55].
The SP theory provides a quantitative description of optical treatments such as those mentioned earlier. The aim of SP is to provide a permanent thermal damage of targeted structures with the surrounding tissue held intact. The SP theory is based on three general principals:
1. Wavelength ofEMR has to be selected to provide maximum contrastofabsorption ofthe target vs. surrounding tissue and other competitive targets.
For example, for hair follicle, the SP target chromophore is the melanin in the hair shaft and hair matrix. Competitive chromophores are blood, water, and lipid. Competitive target is also epidermal melanin. In the first approximation, the optimum wavelength can be selected based on the analysis of spectra of absorption of competitive chromophores, as showed in Figs. 3.2-3.5. Additional factor for the wavelength selection is the location of the target and competitive target in different depth of the skin. Attenuation of light in skin is wavelength-dependent (see Fig. 3.8) and should be taken into account for optimum wavelength selection. For example, hair bulb has hair matrix located in subcutaneous fat at a depth of 2-5 mm, and stem cells are located at a depth of about 1 mm.
2. Pulsewidth ofEMRhas to be selected to provide maximal contrastofheatingofthe target versus surrounding tissue.
To satisfy this criterion, the EMR pulsewidth t must be small compared to the thermal relaxation time (TRT) of the whole target. The TRT is the time of cooling of the target with decreasing temperature of the target in e = 2.7 times after fast adiabatic heating. TRT ~ d/Fk, where d is the size of the target in mm or cm, k is the coefficient of thermal diffusion of tissue (k ~ 0.1 mm2/s = 0.001 cm2/s for dermis) and F is the geometrical factor. F = 8, 16, and 24 for planar, cylindrical, and spherical target, respectively. Actually, if the condition t << TRT is met, the heat generated within the target due to EMR absorption does not flow out of the structure until it becomes fully damaged (coagulated, injured). This approach provides both selective damage and minimum light energy deposition.
3. Fluence ofthe pulse has to be sufficientenough to provide coagulation orablation ofthe target.
The theory of SP is good for the first estimation of treatment parameters. But this theory has several limitations which correspond with the simplification of biological target. Real organ as blood vessel, hair follicle, and others have a complex distribution of chromophores and thermo-sensitive targets. For example, selection of the wavelength on the peak of spectrum of absorption of target can lead to nonuniform heating of the target. The absorption coefficient of blood with 75% oxygenation at 577 nm (peak of oxyhemoglobin absorption) is ga = 30 mm-1. The penetration depth of light in blood vessels can be estimated as h = 1/ga ~ 0.03 mm = 30 pm. For plexus vessels with diameter 7-30 pm, light will provide the uniform heating of the vessels. But for large vessels with a diameter of 100-200 pm, we can expect overheating just portion of a vessel and not complete coagulation of another portion. It can be a reason for creation of the so-named purpura effect. The usage of pulsewidths shorter than TRT of the target becomes inapplicable when the target absorption is nonuniform over its area and a part of the target exhibits weak or no absorption, but the other part exhibits a significant absorption. If this is the case, the weakly absorbing part of the target has to be damaged by heat diffusion from the highly pigmented/strongly absorbing one (hereafter called the heater or absorbed). An example of such a target is the hair follicle [56]. The highly absorbing areas consist of melanin-bearing structures, (i. e., the hair shaft and the matrix cells). The other follicular tissues including the stem cells do not contain an appreciable amount of any chromophore that absorbs in the red/NIR. These targets (tissues) can be damaged by heat diffusion from the hair shaft or the matrix cells to the surrounding follicular tissues. Another example is the treatment of telangiectasia or leg veins with a wavelength near the maximum hemoglobin absorption. Permanent closure of a vascular malformation or a vein probably requires coagulation of the vascular or vein wall [57,58]. In this case, coagulation of the wall requires heat diffusion from blood to wall. This consideration motivated Altshuler et al. to develop the theory of extended selective photothermolysis (ESP) [59].