Transcript The Physics of Lasers as Applied to Medicine

The physics of lasers as applied
to medicine
Sandy Mosse
Medical Physics, UCLH
National Medical Laser Centre, UCL
Light
Amplification by
Stimulated
Emission of
Radiation
Contents
• Lasers
– basic principles
– common types
• Tissue optics & treatments
– scattering & absorption
– modes of interaction
• Safety
Spontaneous Emission
Energy
Levels
Photon
Stimulated Emission and
Absorption
Population inversion
Lasers
Low gain (say 10% per metre) requires long tube
mirrors create illusion of a long tube
But diode laser
“Collimation”
Power / Energy
• Power (Watts) =
rate at which energy
is delivered
• Energy (Joules) = amount of energy
delivered
• 1 Watt = 1 Joule per second
• Energy = power x time, J = Wt
Some lasers in medical use
CO2
Er:YAG
Ho:YAG
Nd:YAG
Ar
Kr
Cu vapour
Dye
Diode
Excimer
10.6 micron
2.94 miron
2.1 micron
1064 nm
488/514 nm
647 nm
511/578 nm
eg 630/675 nm
eg 600 / 905 nm
> 350 nm
Boxes
Laser/Tissue interaction
Refraction
Scatter
Absorption
Laser penetration depths
Photothermal - low power
density
Coagulation:
 Thermal denaturation of proteins at T > 42.5 oC
 At 60 oC, soft tissue is coagulated in 1 second
 Disruption of tri-helical structure of collagen
 Tissue whitening due to increased optical scattering
 Tissue shrinkage
 Thrombus formation
 Haemostasis
Mesenteric vessel photothermally
coagulated by a 60W Nd:YAG laser
Photothermal - high power
density. Ablation
Tongue before & after CO2
Charring:
 Carbonisation of residual cellular components
 c. 4000C, tissue blackens and smokes
 5000C, tissue burns and evaporates
Hyperthermia:
 Cell death caused by long exposure to T > 42.50C
Welding:
 Thermal bonding of vessels and nerves.
Power Density
W/cm2
Effect of power density on tissue
Time (s)
Photon
Energy
approx. 6.4 eV
Photoablative effect of
UV excimer laser light
Excimer Beam
Binding Energy
approx. 3.5 eV
Excimer Beam
Corneal Tissue
Corneal Tissue
Approx. 0.25 mm
Precision of the Excimer Laser
LASIK for Myopic ablation
Desired
Corneal surface
Tissue to be removed
Cornea
Laser Assisted
In Situ Keratomileusis
LASIK in Myopia
LASIK in Miopia
LASIK in Myopia
Plasma formed by a focused Qswitch laser
Laser fragmentation of a kidney
stone
Pulse dye laser fragmentation of
a kidney stone
Photodynamic Therapy - PDT
Taken from
http://omlc.ogi.edu
Dougherty’s pet cat
Modern times
Photosensitising Drugs
• Based on porphyrins or similar structures
• Absorbed photon energy is transferred to
form singlet oxygen
• 1O2 highly reactive, reacts within ~ 20 nm
• Sufficient cell damage causes cell death
• Should concentrate in the tumour
Kills, rather than annihilates, cells.
• Direct photodamage: 1-2 log kill of tumour
cells1. Treatment needs 6 - 8 log
• Most cells die as a result of ~
– Damage to microvasculature
– Inflamatory reaction
– Immune response
Advantages
The alternatives;~
Surgery
Chemotherapy is systemic
Radiation serious side effects. Once only.
PDT kills cells but does less damage to collagenous tissue
structures, and normal cells will repopulate these structures.
PDT is potentially a low-cost minimally invasive localised
treatment.
PDT is repeatable, unlike radiation therapy.
Allows wide area shallow treatment
Disadvantages
•
•
•
•
Light sensitive patient
Poorly defined volume of treatment
Access for light
Variable potency
Light sensitivity
Achilles’ heal
Laser safety
• Many types of laser
– Local safety rules - written for each laser in
each location.
• Laser light is just light,nothing magic, but
may be HIGHLY COLLIMATED
– Travel a long way without diverging
– Focus to a tiny spot - high power density
Laser hazards
• Exposure >100s
• <100s and >100ms
• Nanosecond
photochemical hazard
thermal hazard
photoacoustic shock
Laser hazards
• Skin
• Electrical - Hazard for engineers, not for medics
• Fire - In particular down endoscope
• Eye - Retinal or corneal burns
– Maybe no sensation of damage at the time
– Symptoms include
• headache
• watering
• “floaters”
Eye
Laser (visible & NIR)
Electromagnetic Spectrum
10-13
Wavelength (m)
Angstrom (Å) 10-10
nano (nm) 10-9
Gamma rays
X rays
Ultra violet
400 nm
visible
micro (mm) 10-6
Infra red
centi (cm) 10-2
Microwaves
100 UHF VHF
Radar
FM radio
Broadcast
kilo (Km) 103
Radio frequency
Power lines
near
IR
700 nm
1400 nm
Ultraviolet Radiation
UV-B and UV-C are absorbed by the
cornea, over-exposure resulting in
photochemically induced photokeratitis
(snow-blindness, arc-eye or welder's flash)
and conjunctivitis.
UV-A (315-400 nm) is absorbed in the
lens, over-exposure resulting in the
photochemical formation of an area of
opacity i.e. a cataract.
Eye - visible (400-700 nm) light
• Focussed to a point - intensified x100,000
• Blink protects but takes ~ 1/4 second
• Safe limit = 1mW,
– bright London sunlight  ½ mW/mm2
• Fovea only 3-4% of area of retina
Eye - Invisible light
• NIR transmitted to the retina. No deliberate
focussing but, relaxed eye will focus
anyway.
• IR (>1400 nm) absorbed by cornea, lens etc.
– glass blower’s or furnace man’s cataract
• NO BLINK RESPONSE
Damage to the Retina
The thin highly absorbing layer makes the retina particularly
sensitive to damage from burns.




Burn to the peripheral retina  peripheral blind spot
Burn to fovea  loss of central vision (major vision loss)
Burn on exit of optic nerve  total or partial blindness
Haemorrhage & debris in the vitreous humour 
permanent impairment of varying degree
Photochemical damage is possible but less common.
The blink reflex, effective for 400 nm<  < 700 nm, limits
the maximum retinal exposure time to 0.25 s.
Eye Injury
Nd:YAG (1064 nm) - permanent damage.
Victim saw white flash, heard a click, then immediately a dark spot in visual field
Eye Injury
Lasers which destroy retinal tissue can cause haemorrhaging into the vitreous this will eventually clear but the retinal damage is permanent
Safety goggles
CHECK THEY ARE FOR THE
CORRECT WAVELENGTH
Usually 4-6 O.D. (1 OD = 1 log reduction = 10% transmission)
Fumes & debris
• Plume of smoke during laser ablation
– may contain viruses etc.
• Always use a fume extractor as close as
possible to the treatment area