Different from ordinary light sources, lasers have the following physical characteristics:
Due to the restriction of the resonant cavity on the direction of light oscillation, the laser can only amplify the stimulated radiation oscillation along the axis of the cavity, so the laser has high directivity. Therefore, the laser can spread the beam in parallel to a long distance and still ensure sufficient intensity.
The wavelength range of visible light that causes visual color, that is, the spectral line width, is a measure of the monochromaticity of a light source. The narrower the spectral line width, the better the monochromaticity. Natural light has a wide wavelength range. For example, after sunlight is split by a prism, spectral bands composed of multiple colors can be seen. Laser is produced by stimulated radiation of atoms and has extremely narrow spectral lines, so it has high monochromaticity.
Coherence is divided into temporal coherence and spatial coherence. Temporal coherence describes the phase relationship of each point in the propagation direction of the light beam and is related to the monochromaticity of the light source. The spectral line width of the laser is very narrow and the monochromaticity is high, so it has high temporal coherence. Spatial coherence describes the phase relationship between points on the wave surface perpendicular to the direction of beam propagation. It refers to the coherence of different spatial points in the light field at the same time and is closely related to its directionality. The high directivity of laser determines its high spatial coherence. Laser is a kind of coherent light. The motion frequency, phase, polarization state and propagation direction of each photon are the same. Single-mode laser can be completely coherent.
The monochromatic brightness of a light source refers to the optical power emitted by the light source within unit area, unit frequency bandwidth and unit solid angle. The characteristics of laser such as high directivity and monochromaticity allow its energy to be better focused in space and time, so it has extremely high monochromatic directional brightness.
When laser acts on biological tissue, it produces heat, pressure, actinic and electromagnetic fields, etc., which is called the biological effect of laser. Factors such as the wavelength and intensity of the laser and the reflection, absorption and heat conduction characteristics of the laser at the irradiated part of the biological tissue all have an impact on its biological effects. At present, it is believed that the biological effects of laser are mainly reflected in the following aspects: thermal effect, light effect, electromagnetic field effect, pressure and shock wave effect.
1. Thermal effect: The essence of laser is electromagnetic wave. If the frequency of its propagation is equal to or similar to the vibration frequency of tissue molecules, its vibration will be enhanced. This molecular vibration is the mechanism that generates heat, so it is also called thermal vibration. Under certain conditions, the laser energy acting on tissue is mostly converted into heat energy, so the thermal effect is an important factor in the effect of laser on tissue.
The wavelength of molecular thermal motion is mainly shown near the infrared band. Therefore, the infrared laser output by the carbon dioxide laser has a strong thermal effect on the tissue. When a certain type and power of laser irradiates biological tissue, it can produce 200~1000℃ and above in a few milliseconds. High temperature, this is because laser, especially focused laser, can concentrate great energy in a tiny beam. For example, a ruby laser of tens of joules focuses on a micro-area of a tissue and can generate a high temperature of hundreds of degrees Celsius in the area within a few milliseconds, destroying the proteins in the area and causing burns or vaporization. Ordinary light of tens of joules is fundamental. It has no such effect. In addition, it was also found that when the irradiation is stopped, the temperature rise caused by the laser decreases slower than the temperature rise caused by any method. For example, it takes dozens of tens of joules for the temperature rise caused by the ruby laser to drop to the original normal temperature. minute.
2. Light effect Biological tissues have a certain degree of coloration and can selectively absorb the 300~1000nm spectrum. Pigments in living organisms include melanin, melanoidin, hemoglobin, carotene, iron, etc. Among them, melanin has the greatest absorption of laser energy. Reduced hemoglobin has clear absorption bands at 556nm, oxyhemoglobin has clear absorption bands at 415nm, 542nm, and 575nm. Carotene has an absorption band at 480nm. Melanin and melanoidin have the strongest absorption in the 400~450nm band. Whether it is a normal cell or a tumor cell, there are many melanin granules in the cytoplasm and between cells. They absorb laser energy so that the energy accumulates on the pigment granules and becomes a heat source. The energy is conducted and diffused to the surroundings, causing damage to surrounding tissue cells.
The transparency of tissue cell components to laser is relative. For example, Lowndes et al. proved that reduced nicotinamide adenine nucleic acid is transparent to ruby laser with a wavelength of 694.3nm, but it can absorb ultraviolet light with a wavelength of 330~350nm. Absorption occurs when a ruby laser beam acts on a concentrated solution of prototypic nicotinamide adenine nucleic acid. Biological macromolecules have broad and strong absorption bands in the visible spectrum, so there is a certain probability of multi-photon absorption when strong laser radiation interacts with biological substances. Biomolecules can be excited after absorbing photons, and the energy is either converted into heat, or partially re-radiated in the form of phosphorescence or fluorescence, or the energy is used to accelerate chemical reactions.
In addition to the various properties of the laser itself, the degree of coloration of the tissue or the type of photoreceptor (pigment) plays an important role in the light effect of laser on living tissue. Complementary colors or near-complementary colors have the most obvious effect. Skin of different colors, organs or tissue structures of different colors may have significantly different absorption of laser light. The greater the tissue's transmittance and absorption of laser light of different wavelengths, the more obvious its corresponding light effects will be. After the tissue absorbs the laser quanta, it can produce photochemical reactions, photoelectric effects, electronic transitions, stimulate radiation of other wavelengths (such as fluorescence), thermal energy, free radicals, and ultra-microluminescence of cells, which can cause tissue decomposition and ionization, ultimately affecting the irradiated tissue. structure and function, and even cause damage.
3. Electromagnetic field effect Under the action of laser with normal intensity, the electromagnetic field effect is not obvious; only when the laser intensity is extremely high, the electromagnetic field effect is more obvious. After focusing the laser, when the light energy density at the focus reaches 106W/cm2, it is equivalent to an electric field strength of 105V/cm’. The electromagnetic field effect can cause or change the quantized movement of molecules and atoms in biological tissues. It can cause atoms, molecules, and molecular groups in the body to produce excitation, oscillation, thermal effects, and ionization. It can catalyze biochemical reactions, generate free radicals, and destroy cells. Change the electrochemical properties of tissues, etc.
Which reaction or reactions are caused after laser irradiation has an important relationship with its frequency and dose. For example, free radicals can only be formed when the electric field intensity is higher than 1010V/cm?. Laser light can be measured using electron spin resonance
Free radicals produced by beam irradiation of tissues such as black skin and melanoma. Due to the special properties of lasers, laser technology has been used in many aspects in biological research and medical applications. For example, flash photolysis and Raman spectroscopy are used to study the rapid biological reaction process and the structure of complex molecules, and the laser knife is used to cut tissues and coagulate small blood vessels and nerves during surgery.
4. Pressure and shock wave effect The light pressure of ordinary light is negligible. However, when the energy density at the focus of the focused laser beam reaches 10MW/cm', the pressure will be about 4kPa, which will cause considerable primary pressure on biological tissues. . When the laser beam is focused to a light spot below 0.2mm, the pressure can reach 20kPa; when a 107W giant pulse ruby laser is used to irradiate human or animal skin specimens, the actual pressure generated is measured to be 17.58MPa.
When a laser beam irradiates living tissue, due to the large pressure per unit area, the pressure on the surface of the living tissue is transmitted into the interior of the tissue, that is, part of the laser energy radiated on the tissue becomes a mechanical compression wave, and a pressure gradient appears. If the pressure of the laser beam is large enough to evaporate the particles on the surface of the irradiated tissue, the living tissue particles will be ejected, causing a mechanical pulse wave (reverse shock) in the opposite direction of movement of the ejected particles - a shock wave. This shock wave It can make living tissue eject different numbers of particles layer by layer, and finally form a conical "crater"-like cavity.
In addition to the above-mentioned shock waves formed by the backlash pressure caused by strong radiation pressure, the thermal expansion of tissue may also generate shock waves. Because the temperature rises sharply in a short period of time (milliseconds or less), the heat released instantly has no time to diffuse, resulting in accelerated body thermal expansion. For example, when a 60% ruby laser is used to irradiate the abdominal wall of a mouse, a half-shaped abdominal wall forms in a few milliseconds. A round protrusion, which is an explosive thermal expansion of the body in the irradiated subcutaneous tissue. The pressure and recoil pressure formed in the tissue due to thermal expansion of the body can produce elastic waves that propagate to other parts. They initially form ultrasonic waves, gradually turn into sound waves due to deceleration, and then turn into mechanical waves in the form of subsonic waves, and finally stop propagating. In the microcavity liquid layer of the tissue, cavitation can occur while ultrasonic waves are propagating. The accumulation of cavitations can cause obvious tissue collapse, and sometimes a large compression shock wave can be generated. This series of reactions are all Can cause damage. The scope of the laser thermal effect is very limited, and the tissue damage caused by the pressure effect can spread to parts far away from the illuminated area. For example, when a ruby laser was used to irradiate the head of a mouse, it was found that the scalp was slightly damaged, the skull and the dura mater of the brain were not damaged, but the brain itself suffered from large-scale bleeding and even death. The electrostrictive phenomenon of tissue in the extremely strong electric field caused by a strong laser beam can also generate shock waves and other elastic waves.
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