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Laser And Tissue Interactive
Radiative and non-radiative relaxation. Imagine an excited molecule that is alone, without any other nearby molecules to interact with. In this case, two things could happen. First,the energy gained by absorbing the photon, and initially stored in one mode, will begin to be shared out between all the modes in anon-radiative process of intra molecular redistribution until the molecule is in equilibrium (according to the equipartition theorem). However, the molecule could also jump abruptly to a lower energy state by emitting a photon.
If the radioactive life time of the molecule is shorter than the redistribution time, then it is likely thata photon will be emitted before the process of intramolecular redistribution has completed.
As some redistribution will always take place before a photon is emitted, the energy of there radiated photon will always be lower than the absorbed photon. There are two possible radiative processes: fluorescence and phosphorescence. During fluorescence there is a transition from a state to a similar state, eg. singlet-singlet, and is typically fast (ns or shorter).Phosphorescence occurs after an intra molecular inter-system crossinghas taken place, so the transition accompanying the radiation typically involves a change from a triplet to a singletstate which is much less likely to occur (according to quantum mechanics), and so the radiation is of lower energy and occurs over a much longer timescale (ms, seconds or even longer).All mechanisms that are notradiativeare by default non-radiative.
When the light absorption gives rise to an electronic transition, the more energetic electron will, on average, orbit the nuclei at a greater distance. As the attractive nuclear force falls o rapidly with distance, the electron will be less tightly bound, and will be able to form a chemical bond with another molecule more readily. This is the basis of photochemistry.
While an excited molecule is undergoing intra molecular redistribution it might collide with another molecule. Some of the vibrational energy in the excited molecule will transferred to the colliding molecule as translational kinetic energy. Molecular translational kinetic energy is what appears at a macroscopic level as a temperature rise so leads tophotothermal effects. This process of collisional relaxation will there by thermalise the absorbed photon energy in a matter of picoseconds, although the resulting macroscopic thermal effects occur over very much longer timescales (ms to s).
There are many different mechanisms by which laser light can interact with tissue, and these have been categorised in a number of different ways by different authors. For the purposes of these lectures, the most common interaction mechanisms for therapeutic and surgical applications will be divided into
1.Photochemical reactions: when a molecule absorbs a photon of sufficient energy, the energy can be transferred to one of the molecule's electrons. An electron with higher energy can more easily escape the nuclear forces keeping it close to the nucleus, and so excited molecules (which are molecules with an electron in a higher energy state)are more likely to undergo chemical reactions (exchanging or sharing of electrons) with other molecules. In photodynamic therapy, for instance, a photosensitising drug (aconcoction of molecules which, when they absorb light, cause reactive oxygen speciesto form) is used to cause necrosis (cell death) and apoptosis (`programmed' cell death). Photo dynamic therapy is increasingly widely used in oncology to destroy canceroustumours.
2. In photo thermal interactions, the energy of the photons absorbed by chromopores (aterm used to refer to any light-absorbing molecules) is converted into heat energy via molecular vibrations and collisions, which can cause a range of thermal effects fromtissue coagulation to vaporization. Applications include tissue cutting and welding inlaser surgery, and photoacoustic imaging.
3.In photoablation, high-energy, ultraviolet (UV) photons are absorbed by electrons, raising them from a lower energy `bonding' orbital to a higher energy `non-bonding' orbital,thereby causing virtually immediate dissociation of the molecules. This naturally leads to a rapid expansion of the irradiated volume and ejection of the tissue from the surface.This is used in eye (corneal) surgery, among other applications.
4. Inplasma-induced photoablationa free (sometimes called `lucky') electron is accelerated by the intense electric field which is found in the vicinity of a tightly focussed laser beam. When this very energetic electron collides with a molecule, it gives up some of its energy to the molecule. When sufficient energy is transferred to free a bound electron, a chain reaction of similar collisions is initiated, resulting in a plasma: a soup of ions and free electrons. One application of this is in lens capsulotomy to treat secondary cataracts.
5. The final set of related mechanisms, grouped under the termphotodisruption, are the mechanical effects that can accompany plasma generation, such as bubble formation, cavitation, jetting and shockwaves. These can be used in lithotripsy (breaking upkidney or gall stones), for example.
1.The type of molecules the tissue is made of and contains. These determine the energy levels - the energies of photons that can be absorbed - and the available de-excitation pathways, ie. the routes through which the energy leaves the state into which it was absorbed, to end up as heat or perhaps another photon.
2. The frequency (or wavelength) of the light, ie. the energy associated with each individual photon,
3. The power per unit area delivered by the laser,
4. The duration of the illumination, and repetition rate of the pulses for a pulsed laser. Because different interaction mechanisms dominate under different conditions (photoablation requires UV light, photodisruption requires very short duration pulses, etc), by carefully choosing the laser characteristics the interaction can be restricted to a specific mechanism, and therefore a specific effect on the tissue. Lasers are therefore useful for medical applications because:
a. the energy of the photons can be chosen, as each type of laser will emit photons of only one energy (one frequency or wavelength),
b. the power can be carefully controlled over a wide range of influence rates,
c. The beam shape can be well controlled (focused;, collimated, etc.,), and. the duration of thelaser pulses can range from as-long-as-you-like to less than 100 femtoseconds. (100 femtoseconds is really quite a short time. It is about the time ittakes light to travel the thickness of a human hair.)
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