Fun with lasers

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Chapter 8.3

Fun with lasers

M. Boord1 (Chairperson), C.S. Nett-Mettler2 (Secretary)

1 Animal Dermatology Clinic of San Diego, California, USA

2 – Dermatologie und Allergologie für Tiere, c/o ESK–Ennetsee Klinik fuer Kleintiere, Huenenberg, Switzerland

Mona Boord (USA) welcomed the audience to the workshop. She noted that she has used a CO2 laser since 1999 and finds that it allows her to perform procedures that could not be done with traditional surgery. She introduced three speakers (George Peavy, David Duclos and Claudia Nett) who would be presenting four different talks during the session on the use of lasers in medicine and surgery. The goal of the session was to help participants gain knowledge about the use of lasers in veterinary dermatology and help participants in deciding whether a laser would fit into their practice setting.

Claudia Nett-Mettler served as the secretary for this workshop.

Mona Boord also thanked Peter Vitruk (USA), a cofounder of Aesculight, who made it possible for the first speaker, George Peavy, to attend today. George Peavy is a Diplomate of the American Board of Veterinary Practitioners. He is a Project Scientist, Director of Comparative Medicine Programs, and Manager of the Military Photomedicine Program at the Beckman Laser Institute and Medical Clinic, College of Medicine at the University of California, Irvine. He is a leader in the field of laser biomedical research for veterinary and human applications.

Light, tissue and magic: a peek behind the curtain (G.M. Peavy)

George Peavy (USA) introduced how laser light interacts with tissue, to help the audience to understand some of the newer technologies currently under investigation that would be presented later in the session, and also to help members of the audience for whom lasers and laser tissue interactions are new concepts.

There are many types of different lasers, each named after the laser media comprising the laser. Different lasers emit light of different wavelengths. The wavelength is specific for the energy state of the laser media. Eximer lasers, which are excited dimer lasers, produce light in the ultraviolet spectrum.

Types of lasers:

  • argon, krypton and KTP laser (blue-green light)
  • dye and diode laser (577–780 nm wavelength)
  • helium neon (the old standard of red laser pointers before diode lasers entered the market)
  • gold vapour, ruby laser (red region)
  • Nd:YAG, holmium and near infrared diode lasers (910–2100 nm wavelength)
  • erbium:YAG (mid-infrared region)
  • CO2 (far infrared region).

Laser selection for a procedure is dependent upon the wavelength of the laser and optical properties of the target tissue. There are a variety of ways that light can interact with tissue. In the following, the basic laser tissue mechanisms are described.

Laser light and interaction with tissue

1 Reflection. Light can be completely reflected from tissue – that is why teeth are white: white light is composed of all the wavelengths in the visible spectrum, and all are reflected from the enamel surface.

2 Transmission. Light can be transmitted through the tissue (like light going through a glass with clear water in it). Putting a flashlight on one’s palm will result in some wavelengths being reflected; the blue and green light is absorbed by chromophores in the tissues of the hand, while the red light is transmitted through the palm and out the other side.

3 Scattering of light. Light can be scattered in the tissue. If some milk is added to a glass with clear water, the water becomes cloudy and soon it is impossible to see through because the milk fat molecules dispersed in the water scatter the light.

4 Absorption of light – photothermal reaction. Light is absorbed by chromophores in the tissue and transformed into thermal energy, causing a photothermal reaction, with changes to the tissue. This property of light is used to cut or ablate tissue.

5 Photodisruption – photomechanical action. High-energy short-duration pulses result in rapid heating and cooling of the tissue generating a shockwave. This is called a photomechanical action, a property that is used in laser lithotripsy.

6 Photochemistry. A chemical agent in the tissue is illuminated with a light, which the chemical absorbs. This energy stimulates a chemical reaction, which may be used to selectively destroy a target tissue such as cancer.

7 Fluorescence emission. Molecules in the tissue absorb photons of one wavelength, utilize some of that energy, and then release the excess energy in the form of another photon in a lower energy state (and therefore of a different wavelength).

Choosing the correct laser

There are four things that influence the outcome of applying a laser to tissue. Only three of those will be discussed here. These three things are important whether you are cutting or ablating tissue or whether you are using laser as a source of energy for a diagnostic application.

1 Wavelength selection. The wavelength needs to be selected so that the target tissue most efficiently absorbs it. There are a variety of chromophores in the body that will absorb light:

  • Melanin/haemoglobin. These chromophores absorb light in the blue and green but not in the red wavelengths. This is why haemoglobin is red: it absorbs the green, blue, orange and yellow light but not red light.
  • Water. Water is a chromophore that absorbs infrared light (it does not absorb in the visible spectrum -which is why it appears transparent). Lasers in the mid- and far-infrared spectrum will cut with more efficiency than a laser that is absorbed by melanin or haemoglobin, because soft tissues contain more water than either haemoglobin or melanin.

2 Photothermal reactions. When tissue is heated but does not reach 60°C, the tissue is only warmed up. If the heating is continued and reaches 60–65°C, denaturation of proteins occurs and tissue will no longer be viable. At 100°C, water goes through a phase change and vaporizes, which results in increased pressure within the tissue. Once the pressure exceeds the strength of confinement in the tissue, explosive vaporization occurs. If the heating continues for too long, all the oxygen, nitrogen and hydrogen elements are released, resulting in carbonization of the tissue.

3 Beam intensity (watts/cm2). Beam intensity (power density, irradiance) describes the rate at which photons are being delivered by the laser beam per unit area of tissue; this determines how efficient the laser exposure is going to be. A 10 W laser being put into a spot diameter of 2 mm will allow the delivery of approximately 320 W/cm2. This intensity results only in warming of the tissue. Vaporization does not necessarily require a higher power laser, instead the laser can be focused or adjusted into a smaller spot size. Using a 0.2 mm focal spot, the same laser at 10 W will be delivering 32 000 W/cm2. Changes of the spot diameter by a factor of two will change the intensity (e.g. the number of photons per unit area) by a factor of four. When using a CO2 laser, it is very important to have a power density of 4500–5000 W/cm2 in order to get good and efficient ablation. A 10 W laser beam delivered in a 0.8 mm focal spot delivers 2000 W/cm2, a setting that will allow cutting, but with reduced efficiency, resulting in thermal injury. By switching to a 0.4 mm focal spot diameter, the intensity will increase to 10 000 W/cm2.

4 Time domain of beam delivery. The longer it takes to cut or ablate tissue, the more time is allowed for heat to diffuse to the surrounding tissue, resulting in collateral thermal injury. Using higher power settings speeds cutting and ablating, thus resulting in less collateral thermal injury to surrounding tissue. Some lasers have a superpulse mode that delivers an ablation threshold level of energy in an individual pulse that is less than the thermal relaxation time of the tissue. Consequently the ablation is accomplished before there is enough time for heat diffusion into the surrounding tissue to cause thermal injury. The amount of thermal injury is thus both wavelength and time dependent

George Peavy explained that in his opinion there are two advantages of using a CO2 laser instead of traditional surgical techniques:

a Simultaneous haemostasis when excising tissue.

b Tissue can be removed with high precision moving between the focused and defocused regions of the beam, enabling the user to ablate cell layer by cell layer and remove tissue without damaging any sensitive structures.

5 Selective photothermolysis. This uses a technique where the target tissue, which is to be eliminated, has one optical property and the surrounding normal tissue has different optical properties. If using a laser that is absorbed by the target tissue but transmitted by the normal surrounding tissue, the target tissue is destroyed while sparing the surrounding tissue as it passes through. Examples for the use of selective photothermolysis are angiomatosis, distichiasis (individual pigmented hairs can be removed), and cyclophotocoagulation for treating glaucoma.

6 Photodynamic therapy. This is a process where an individual is administered a photochemical drug that localizes in cancer tissue. The area is then illuminated with a laser that produces a wavelength that excites that chemical agent (i.e. photochemical interaction) and produces singlet oxygen that destroys the tissue where the photochemical agent is localized. One typical clinical application is actinic keratosis in cats.

Mona Boord introduced David Duclos, the next speaker, as someone who is interested in CO2 lasers and has used them since 1992.

CO2 laser use in dermatology (D. Duclos)

David Duclos (USA) explained that when he first started to use a CO2 laser, his main goal was to find out whether there were areas where a laser would do a better job than a surgical knife. His talk would concentrate on indications for CO2 laser therapy – such as when lasers are the best or only option. Currently, David Duclos prefers using the laser for most standard operations and rarely uses a surgical knife.

David Duclos prefers the Aesculight CO2 laser. It is the only self-calibrating laser with calibration at the tip. This means that the rate of delivered energy is measured right at the tip of the laser; if set to 2 W, 2 W will be delivered from the tip of the laser.

He uses general anaesthesia in most cases; local anaesthesia is only used for single lesions.

Examples of procedures amenable to CO2 laser therapy were discussed, as follows.

Nodular sebaceous hyperplasia (multiple) and sebaceous gland tumours (‘senile warts’)

1 Cosmetic use of laser to ablate nodular glands causing neither pain nor pruritus. Therefore, animals rarely lick at the laser surgery sites.

2 No sutures needed, much shorter surgery time.

3 Laser settings:

a Typically uses the 0.8 mm tip or the paint brush tip.

b Power: starts at high power at 20 W and reduces to super pulse and 5 W at the base of the tumour.

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Jun 13, 2017 | Posted by in INTERNAL MEDICINE | Comments Off on Fun with lasers

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