Lasers in ENT

45 Lasers in ENT

45.1 Introduction

LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation. The possibility of laser light, which is monochromatic (same colour), coherent (intense and in phase) and collimated (parallel and unidirectional), and is therefore an extremely powerful and high-energy light beam, was first postulated by Einstein in 1917. It wasn’t until 1960 that Dr T Maiman produced the first laser light using synthetic ruby crystals.

Laser light is produced traditionally by having a laser medium within a resonating chamber which has a fully reflective mirror at one end and a partially reflective and partially transmitting mirror at the other. When this medium is heated to its excited state by an electrical current (rather like throwing copper granules into a flame), the medium rises to its excited state, then drops back down to its ground (stable) state, emitting a photon of light, the wavelength of which is characteristic to its own atomic structure. This is called fluorescence. Copper emits green light of a specific wavelength (510.6 nm). Einstein postulated that if a population inversion occurred, that is, if more than 50% of the atoms were excited instead of being in the usual ground state, then those photons emitted through fluorescence would interact with other excited atoms, and cause the emission of an identical clone of itself, as the excited atom descended to its ground state (stimulated emission). If a single clone could be encouraged, then this would create a beam of laser light. This occurs because the emitted photons bounce around within the inside of the resonating chamber and only those parallel to the long axis of the chamber are able to escape through the partially transmitting mirror.

Over the years, solid-state lasers such as the Neodymium doped Ytrrium Aluminium Garnet (Nd–YAG) have been developed, and more recently diode lasers using gallium aluminium arsenide chips rather like a silicon chip microprocessor. These are smaller, cheaper and more reliable than the original resonating chamber lasers. Excimer lasers use a combination of resonating chamber and excimer recombination.

There are an almost infinite number of compounds that can be used as a laser medium. imageTable 45.1 is a short, but not comprehensive, list.

Lasers are useful in medicine because of the effect laser light has on tissue. In essence, lasers are a heat beam which can be very accurately focused when using high-quality lenses. Very small spots with very high-energy densities are therefore possible. Because this heat beam is solely of one wavelength, the different penetration of light through tissue can be used to advantage. Picture yourself holding a bright white light on your finger tip. Look at the other side and you will just see red. Of the visible spectrum, only red light gets through, the rest is absorbed. See imageFig. 45.1a, b for a description of the characteristics of light transmission through tissue.

If a highly penetrating laser is required, for example, to slowly heat and coagulate large vascular tumours in a difficult area such as the trachea or main bronchi, choose a highly penetrating laser which won’t suddenly vaporise a big hole in a blood vessel, for example, the Nd–YAG laser. If you want to delicately nibble off a vocal cord polyp with a no-touch technique, choose a delicate non-penetrating system such as the carbon dioxide (CO2) laser.

45.2 Selective Photothermolysis and Thermal Relaxation

These two theories, developed in the 1980s, led to a great leap forward in the use of lasers clinically, particularly in ENT and dermatology.

Selective photothermolysis uses the fact that laser light can be selective. A laser light highly absorbed by oxidised haemoglobin, but poorly absorbed in other chromophores such as water and melanin should be chosen, and you have a wavelength that will vaporise blood vessels, but leave surrounding tissue such as skin, undamaged. Skin haemangiomas, where a tuneable dye laser, tuned to yellow light at 588 nm has been very effective in shrinking haemangiomas in children, is an example.

Thermal relaxation uses the fact that delivered energy can be confined to its target area without spreading and damaging surrounding tissue (thermal confinement). This happens if the laser light is delivered within a very short period of time, called the thermal relaxation time. Think of doing the ironing and touching the hot part of the iron. If you touch for the shortest time, your finger doesn’t burn. If you touch for a second, you will get a superficial burn, with some deeper dermal damage and scarring.

Shuttering (like a camera shutter) allows a short on–off time of laser beam delivery. The energy levels need to be sufficient to have its effect, so that a shuttered pulse will just heat the target tissue and won’t give an undesired effect of injuring surrounding tissue. For thermal confinement to be accurate and specific, a non-penetrating beam is most suitable. The CO2 laser was developed for this application and a new technique of super-pulsing the photon energy was used. Here separate laser pulses are stacked one on top of the other, to double the available energy but still deliver it within the thermal relaxation time of the target tissue (usually around 900–1,000 μs). This works well for vocal cord lesions or the skin. This same technique is used for cornea surgery, although here the risk of scarring is high and therefore an extremely non-penetrating laser is used, such as the argon fluoride excimer laser. This would be too delicate for the vocal cord; a nodule would take hours to remove.

Thermal confinement without superpulsing is still used in higher-powered laser systems for ENT use, the most modern of which is the Carl Zeiss computerised pattern generator (CPG).

45.3 Other Considerations

In some lasers, in particular, the CO2 laser, the light beam will not travel down a fibre, such as a quartz fibre, as it is absorbed by the quartz. This means it cannot be inserted into parts of the body that are relatively inaccessible. Light channels for the CO2 laser are not particularly effective. This confines its use mainly to skin and ENT surgery. Safety is always a concern, particularly with highly absorbed lasers such as the CO2, which can cause corneal scarring, or the more penetrating lasers of 500- to 1,200-nm wavelength, which can cause retinal damage. Also a laser beam has a great ability to set things on fire (an igniter), particularly in areas of high partial pressure of oxygen, such as the airway in general anaesthetic.

Mar 31, 2020 | Posted by in OPHTHALMOLOGY | Comments Off on Lasers in ENT

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