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MLS Master Class - Veterinary Imaging
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Laser Safety

Carl Gorman BVSc MRCVS

Key words : Laser, safety, Class 4 (or Class IV), eye protection, retina

 

Overview

It is important to consider safety precautions when working with high powered lasers such as the MLS laser.  All staff who will be involved in administering laser treatment should be familiar with the potential hazards and safety requirements, and they must be responsible for the safety of any patients treated and owners who may be present when treatment is given.

Because lasers produce a steady focussed beam of light energy, the accumulation of that energy can result in damage to tissues.

Staff should be aware of the class of laser which they are using, and the damage which might be caused by that laser.

Laser classes

By convention and agreement, lasers are classified as follows:

 

Class 1

Safe under all conditions of normal use - the maximum permissible exposure (MPE) cannot be exceeded when viewed with the naked eye or through optical equipment.

Class 1M

Safe under normal conditions unless viewed through magnifying optics such as a microscope or telescope.

Class 2

Generally safe because the blink reflex will limit exposure to under 0.25 seconds.  This category applies to visible light lasers (400-700 nm).  Damage could be sustained by deliberately looking at the laser source and suppressing the blink reflex.  Limited to 1 mW in continuous wave.

Class 2M

Safe because of the blink reflex, unless viewed through optical instruments.

Class 3R

These are considered safe if handled carefully.  The MPE can be exceeded with this class of laser, but with a low risk of injury.  Visible light lasers in this category are limited to 5 mW output.

Class 3B

These lasers are hazardous if the beam is viewed directly, but diffuse reflections (such as from paper, matt surfaces or skin) are not dangerous.  Protective eyewear is required where direct viewing of the laser beam may occur.  Class 3B lasers are required to be fitted with a key switch and a safety interlock.

Class 4

These are the highest class and most hazardous lasers.  They can burn skin or cause serious permanent eye damage when viewing the direct beam, or a reflection even from a matt surface or if diffuse.  Protective eyewear must be worn, and these lasers must be fitted with a key switch and a safety interlock.

Table 1  Laser Classes

 

The MLS laser is a class 4 laser.  Its power allows it to deliver treatment in a shorter period of time than Class 3B lasers, and the laser will penetrate deeper.  The power of the unit means that safety precautions must be used.

Damage to tissues

Lasers produce a focussed beam of energy.  This energy is absorbed by tissues, so when the energy absorbed is high enough, tissue damage can occur.  For the majority of body tissues this means thermal damage (i.e. burns). 

In practical terms the tissues most at risk in both patients and operators are the skin and eyes.  It is possible to cause thermal damage to tissues deep to the skin because high power lasers such as the MLS laser can deliver energy to cells several centimetres below the skin surface.  Normally pain and discomfort from the skin would cause an affected patient or operator to react before deep tissues are damaged.

Eyes are particularly vulnerable to laser damage for several reasons.  The cornea lacks a layer of dead keratinised cells covering it to absorb or deflect light energy.  In the far ultra-violet or infra-red spectrums, the cornea will absorb laser energy and may sustain damage.

The iris is also susceptible to thermal damage from wavelengths below 1300nm.  The rich blood supply of the iris helps to dissipate heat, while the very sensitive nerves in the iris are sensitive to slight changes in temperature and will respond to insult by a sensation of pain.  This leads to avoidance of the hazard by the blink reflex and so helps protect the iris and cornea.

The lens may also suffer damage especially at wavelengths between 315-400 nm. The damage may be cumulative, so that cataracts can develop gradually in response to repeated exposure, and these cataracts may develop years after the initial damage.

The retina is even more susceptible to damage because of the focussing effect of the lens.  Light wavelengths between 400 and 1400 nm are focussed in this fashion.  Both wavelengths of the MLS laser fall within this range.  If the eye is focussed in the distance, this will concentrate the light energy entering the eye by 100,000 times, concentrating it on a focal spot in the retina: the fovea centralis.  Thus a 10 watt beam entering the eye can have the effect of a 1 million watt beam on the central area of the retina. (Fig 1)

Safety Fig 1 Eye Absorbing Laser Energy

Fig 1 Laser energy is concentrated in the eye by the lens

The sensitivity of the eyes to laser injury means that it is mandatory to protect the operator and helpers' eyes by the use of appropriate protective glasses.

Figure 1 shows the areas of the eye which are susceptible to damage from different light wavelengths.  Lower class lasers deliver less energy and are considered less of a hazard because the blink reflex protects the eye from receiving too much energy.  0.25 seconds is considered to be the amount of time it takes a person to respond to a stimulus by blinking or averting their eyes.  For lower power lasers (e.g. Class 1 or 2) that doesn't allow sufficient energy to enter the eye to cause damage.  However Class 4 lasers are much more powerful and only a short time is needed to deliver a damaging dose of energy.

 

Wavelength

Area at risk

Injury

Note

 

 

 

 

Near Ultraviolet UVA (314-400 nm)

Lens

Cataracts

Effects are delayed and may not occur for years

Far ultraviolet UVB (280-315 nm) and UVC (100-280 nm)

Cornea

Keratoconjunctivitis

 

Visible (400-760 nm) and near infrared (760 - 1400 nm)

Retina

Flash blindness

Retinal burns and lesions

*** The MLS laser wavelengths fall in these ranges ***

Far infrared (1400 nm - 1 mm)

Cornea and iris

Burns

 

Table 2  Potential laser damage to eyes

Practical safety for the MLS laser

The MLS laser uses two wavelengths, 808 nm and 905 nm.

Both wavelengths are capable of causing damage to skin and eyes.  The 808 nm beam has a maximum power of 1 watt, and so is unlikely to deliver sufficient energy to the skin to cause thermal damage.  The 905 nm beam has a maximum power of 75 watts.  This energy is delivered in a pulsed manner which very much reduces the likelihood of causing thermal injury as the pulsing allows time for energy to dissipate in the target tissues.   Furthermore the pre programmed therapy settings will limit the energy delivery during any therapy session.

The potential for damage should not be ignored, and the danger of eye damage should always be considered.

Class 4 Laser devices have a Nominal Ocular Hazard Area (NOHA).  The NOHA for the MLS laser is 13 metres.  Any persons within the NOHA during the operation of the laser must wear appropriate protective eyewear.  It is important to realise that laser safety glasses are not interchangeable between different wavelengths of laser.  Glasses which protect against the specific wavelengths emitted by the laser the operator us working with must be worn. (Fig 2, Fig 3, Fig 4)

Safety Fig 2

Fig 2 Protective glasses

Safety Fig 3

Fig 3 Glasses must be appropriate to the wavelengths of laser energy being used 

Safety Fig 4

Fig 4 Protective glasses in use

Safety goggles are available for patients as well.  (Fig 5, Fig 6) Not all will tolerate wearing them, but many dogs and cats are not worried by the glasses.  If the patient won't accept them, consider the risk posed and decide on a safety strategy.  This may involve covering the head with a towel or ensuring that the handler keeps the animal's head and eyes facing away from the laser handpiece.

Safety Fig 5

Fig 5 'Doggles' are available for patients to wear

Safety Fig 6

Fig 6 Dog and cats are surprisingly tolerant of the eyewear

A laser safety officer should be appointed.  The officer should be responsible for familiarising themselves with the laser and the safety precautions required.  They should carry out a risk assessment, and ensure that all operators receive training in the safe use of the laser unit so that they are able to ensure their own safety, and the safety of any helpers and patients.

The device can be set up with a password at the setup screen.  This should be enabled to prevent unauthorised use of the unit.

Ideally a designated room or rooms should be allocated for laser use.  These should have warning signs at the entrances, and have lockable entrances to prevent accidental exposure of unprotected personnel. (Fig 7)  If for any reason a dedicated room is not available, then only the operator, patient and helpers should be within the NOHA (13 metres for the MLS laser).  It is possible to use a moveable screen to create a temporary laser area in a larger room. (Fig 8)

Safety Fig 7 Laser Warning Sign

Fig 7 A laser warning  sign to be displayed at the entrance to the controlled area

Safety Fig 8

Fig 8 Moveable screens can be used to create a temporary controlled area

Summary of laser safety

  • Avoid looking directly into the laser source, or directing the source at assistant's or patient's eyes
  • Always wear appropriate protective eyewear when the laser unit is switched on
  • Only use the laser in a controlled area
  • Only authorised persons and patients should be allowed access to the area when in use
  • Warning signs should be placed at entrances to the controlled area
  • Enable the password  to prevent unauthorised use
  • When not in use, use the master switch to turn the device off
  • Do not leave the device unattended when switched on
  • Ensure there are no reflecting objects within the range of the laser
  • Remove all metal or reflective objects from the treatment area (rings, watches, jewellery, mirrors)