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

• Lasers– basic principles– common types

• Tissue optics & treatments– scattering & absorption– modes of interaction

• Safety

Contents

Spontaneous Emission

EnergyLevels Photon

Stimulated Emission and Absorption

Population inversion

Lasers

mirrors create illusion of a long tube

But diode laser “Collimation”

Low gain (say 10% per metre) requires long tube

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 useCO2 10.6 micronEr:YAG 2.94 mironHo:YAG 2.1 micronNd:YAG 1064 nmAr 488/514 nmKr 647 nmCu vapour 511/578 nm Dye eg 630/675 nmDiode eg 600 / 905 nmExcimer > 350 nm

Boxes

Laser/Tissue interaction

RefractionScatter 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.

Effect of power density on tissueP

ower

Den

sity

W/c

m2

Time (s)

Photoablative effect ofUV excimer laser light

Binding Energyapprox. 3.5 eV

PhotonEnergy

approx. 6.4 eV

Approx. 0.25 mCorneal Tissue

Excimer Beam

Corneal Tissue

Excimer Beam

Precision of the Excimer Laser

LASIK for Myopic ablation

Cornea

DesiredCorneal surface

Tissue to be removed

LLaseraser AAsssistedsisted

IIn n SiSitutu KKeratomileusiseratomileusis

LASIK in MyopiaLASIK in Myopia

LASIK in MiopiaLASIK in Miopia

LASIK in MyopiaLASIK in Myopia

Plasma formed by a focused Q-switch 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

AdvantagesThe 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 photochemical hazard

• <100s and >100s thermal hazard

• Nanosecond 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 SpectrumW

avel

engt

h (m

)

Gamma rays10-13

10-10

10-6

103

10-9

100

10-2

X rays

Ultra violet

Infra red

Radio frequency

UHFVHF

RadarFM radio

Broadcast

Power lines

Microwaves

Angstrom (Å)nano (nm)

micro (m)

kilo (Km)

centi (cm)1400 nm

700 nm

400 nm

visible

near IR

Ultraviolet RadiationUV-B and UV-C are absorbed by thecornea, over-exposure resulting inphotochemically induced photokeratitis(snow-blindness, arc-eye or welder's flash)and conjunctivitis.

UV-A (315-400 nm) is absorbed in thelens, over-exposure resulting in thephotochemical formation of an area ofopacity 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 RetinaThe thin highly absorbing layer makes the retina particularlysensitive 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, limitsthe 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