Skip to content
Home » Stone » LASER

LASER

Light amplification by the stimulated emission of radiation.

  • Application of initiating energy (usually electricity) to a lasing medium (Ho:YAG, CO2, KTP)
  • Energy causes electrons to be raised to higher energy level (pumping)
  • Electrons spontaneously return to resting state, emitting a photon of energy and wavelength characteristic of the emitting atom (spontaneous emission)
  • Emitted photons may strike other atoms which are already in an excited state, emitting further photons (stimulated emission)
  • Multiple photons sharing the same wavelength travel in the same direction, with resonant cavities (within the laser machine) allowing light to pass around and reflect off mirrors before escaping through a user-controlled aperture in a focussed coherent beam

 

Characteristics of laser light (differentiating it from white light):

  • Monochromatic – same wavelength, and therefore same colour
  • Collimated – photons travel in parallel
  • Coherent – waves travel in the same phase

 

LASER effects on tissue and stone:

  1. Thermal
    1. Dependent on wavelength of laser, power of energy delivered, tissue factors (content of haemoglobin and pigmentation)
    2. Absorption of laser energy is converted to heat in the target tissue
    3. Enough heat will causes coagulation and subsequent vaporisation
    4. Depth of thermal transmission determined by absorption length of specific laser
  2. Mechanical
    1. High power density at surface of stone results in freeing of a column of electrons -> formation of a plasma bubble -> collapse of bubble results in pressure waves which fragment stone (cavitation)
    2. Secondary MOSES effect – now exploited by high power lasers – with plasma bubbles providing an ideal medium for more efficient transmission of laser beam, pulses are modulated to create bubble then pass laser through
  3. Photochemical ablation
  4. Tissue welding

 

Types of LASER in urology

Holmium:YAG (ytrrium-aluminium-garnet)

  • 2100 – 2140 nm wavelength (invisible / infra-red)
  • High absorption in water – must contact stone for effect
  • Penetration depth 0.4 mm
  • Solid state
  • Pulsed energy
  • Works by photothermal effect (direct heat fragmentation) and less so photoacoustic (weak shockwaves from cavitation of bubble)
  • High powered laser (100 – 120 W) – MOSES settings – modulated pulses to create bubble then direct laser beam through bubble to maximise effect
  • Can be used for stones, HoLEP, TCC

Pulsed dye laser

  • First historic laser
  • 504 nm wavelength
  • Not effective on harder calcium or cystine stones

KTP (greenlight)

  • Potassium titanyl phosphate
  • 532 nm
  • Visible green light
  • Doubled frequency Nd:YAG passed through KTP crystal (which halves wavelength and reduces tissue penetration)
  • Absorbed by haemoglobin hence useful for coagulation
  • Low absorption by water
  • Optical penetration of 0.8 mm – can damage cornea
  • Coagulation depth / tissue penetration 1 – 2 mm
  • Side firing laser for PVP
  • Photothermal effects – vaporises tissue with necrosis surrounding by coagulated tissue

CO2

  • 9000 – 10 000 nm wavelength
  • Infrared / invisible
  • 1 mm tissue depth absorption
  • Strongly absorbed by water
  • Used for skin lesions / warts / penile cancers

Thulium

  • 2000 nm wavelength
  • Solid state
  • Energy generation is by laser diodes instead of specific electricity – more efficient, can use normal power source, less heavy and quieter
  • Absorbed by water
  • Tissue absorption 0.2 mm
  • Continuous wave and pulsed both available
  • Potentially less retropulsion, less stone treatment time

 

Laser safety

Visible spectrum lasers (wavelength 400 – 1400) can damage the retina and cause severe visual injury. These are KTP/greenlight and Nd:Yag

Holmium and thulium are non visible / infra-red lasers – with wavelengths 2000 or 2140 nm – and therefore do not cause retinal injury. They can theoretically cause corneal abrasions.

Whilst holmium laser has never been reported to cause eye injury, it can cause burns if fired outside scope on to skin.

General laser safety measures:

  • Designated laser-safe theatre
  • Clearly signed and indicated when laser in operation
  • Minimise staff entry and exit
  • Windows blocked
  • Trained laser safety officer running the laser
  • Appropriate glasses mandatory when using visible spectrum lasers (KTP greenlight, Nd:YAG)
  • Patient’s eyes covered
  • Appropriate training for staff using laser
  • Regular inspection, maintenance and servicing of machine
  • Follow local protocols, guidelines or restrictions

Can also cause expensive damage to scope – keep flexible ureteroscopes straight.