Basics of laser physics Chap. 2: Matter acting on light Chap. 3

Mannheim BiomedicalEngineering Laboratories

Introduction / literature

Chap. 1: Basics of laser physics

Chap. 2: Matter acting on light

Chap. 3: Light acting on matter

Chap. 4: Medical laser applications

Chap. 5: Laser safety

Prof. Dr. Markolf Niemz

- Lecture notes -

- Exam preparation -


Electromagnetic

spectrum


LASER: Light Amplified by Stimulated Emission of Radiation

transition rates at photon density n and transition probabilities A12, A21 and B21:

A12nN1 B21N2 A21nN2


Laser beam characteristics

• monochromaticity

• collinearity

• spatial and temporal coherence


Matter acting on light

• reflection and refraction

• absorption and scattering


Reflection und refraction

law of reflection: θ = θ‘

law of refraction: n1 sin θ = n2 sin θ‘‘n1 n2


Absorption

tissue absorption according to Lambert-Beer: I(z) = I0 exp (−αz)


Scattering

Rayleigh scattering: I(λ) ~ λ− 4

Mie scattering: I(λ) ~ λ− x

with 0.4 < x < 0.5

Rayleigh scattering


Light acting on matter

process interaction type mechanism

non-ionizing processes photochemical interaction photokatalysis

thermal interaction increase in temperature

ionizing processes photoablation UV photodissociation

plasma-induced ablation plasma ionization

photodisruption shock wave generation


Laser-tissue interactions

examples:

• cataract

• keratomileusis

• sclerostomy

• coagulation

• photodynamictherapy


Photochemical interaction

photodynamic therapy (PDT)

−


Thermal interaction

temperature effect

37 oC −> 42 oC hyperthermia

> 60 oC coagulation

100 oC vaporization

> 100 oC carbonization


Extra: Laser-induced interstitial thermotherapy (LITT)

temperature effect

37 oC −> 42 oC hyperthermia

> 60 oC coagulation

100 oC vaporization

> 100 oC carbonization


Photoablation

excitation:

AB + hν (AB)*

deactivation:

(AB)* A + B + Ekin


Plasma-induced ablation


Photodisruption

pulse duration: 10−15 − 10− 9 sec

energy density: 1 − 1000 J/cm2

power density: 1011 − 1013 W/cm2

electric field strength: 107 − 108 V/cm

H2O


Laser applications in medicine

ophthalmology: retinal detachment, cataract, glaucoma, wrong-sightedness

dentistry: caries, feeling of pain

neurosurgery: brain tumor

gynecology: cervical intraepithelial neoplasia (CIN), endometriosis,

obstruction of the uterine tube, twin-syndrome

urology: bladder tumor, benign prostatic hyperplasia (BPH)

angioplasty: atherosclerosis, stenosis

dermatology: port wine stain, tattoo, skin cancer

orthopedics: arthrosis, herniated disc, otosclerosis

otorhinolaryngology: laryngeal tumor, otosclerosis


Laser applications in ophthalmology

indications:

• retinal detachment

• cataract

• glaucoma

• wrong-sightedness


Wrong-sightedness

index of refraction

air 1.00

cornea 1.37

aqueous humor 1.33

lens 1.42

vitreous 1.33

• cornea is responsible for 70 % of refractive power

• minimally invasive surgery of cornea is possible


Refractive power of the eye


Techniques in refractive corneal surgery

alternatives to glasses

and contact lenses

advantages:

• expensive eyewear becomes needless

• strongly deviating wrong-sightedness curable as well


Model of keratomileusis - part II

ablation depth: ca. 10 µm per diopter (ymax = 2.5 mm) ablation curve of the cornea (ArF laser, 193 nm)

∆D (dpt) Rf (mm) d(0) (µm)

1 7.965 9.0

2 8.137 17.9

3 8.316 26.8

4 8.504 35.7

5 8.700 44.6

6 8.906 53.4

7 9.121 62.2

8 9.347 71.0

9 9.585 79.7

10 9.835 88.4


fs-LASIK: Laser in situ keratomileusis with the femtosecond laser

advantages:

• original surface

• without scalpel

• without UV light

laser parameters:

• Nd:Glass laser (500 fs)

• 3 µJ / pulse


Laser applications in dentistry

indications:

• caries

• feeling of pain

(biostimulation)

pain caused by mechanical drill

dental surgery with laser light

• vibrations (typ. 100 − 1000 Hz)

• increase in temperature due to friction (at ∆T > 5 oC)

• without vibrations (contactless)

• increase in temperature negligible at ultrashort pulse durations


Painfree caries therapy: Investigated laser systems

Ho:YAG laser Er:YAG laser Nd:YLF laser

λ = 2.12 µm λ = 2.94 µm λ = 1.053 µm

τ = 250 µs τ = 60 µs τ = 30 ps

E = 300 mJ E = 50 mJ E = 1 mJ

n = 100 n = 2 n = 16 000


Caries diagnostics by plasma spectroscopy

LIBS: Laser-Induced Breakdown Spectroscopy


Plasma spectroscopy

healthy tooth

carious tooth


Laser applications in neurosurgery

indication:

• brain tumor

potential therapies

of a brain tumor

• radiotherapy (inaccurate deposition of energy)

• chemotherapy (difficult due to blood-brain-barrier)

• mechanical resection (not minimally invasive, bleeding)

• coagulation and ablation with laser light (under investigation)


Laser safety

spectral range eye skin

UVC: 100 − 280 nm keratitis (inflammation) erythema, skin cancer

UVB: 280 − 315 nm keratitis (inflammation) erythema, pigmentation

UVA: 315 − 400 nm cataract pigmentation

VIS: 400 − 780 nm retinal burn skin burn

IRA: 780 − 1400 nm retinal burn, cataract skin burn

IRB: 1.4 − 3.0 µm retinal burn, cataract skin burn

IRC: 3.0 − 10 µm corneal burn skin burn


Laser safety of the eye

VIS, IRA

IRB, IRC,UVB, UVC

UVA


Laser safety of the skin


Laser safety classes

class threshold laser types protection

1 < 0.4 µW capsuled lasers, low power lasers none

2 < 1 mW soft lasers (defined in VIS only) natural reflex of the eye lid

3a < 5 mW lasers for alignment purposes safety glasses

3b < 0.5 W medium power lasers safety glasses

skin protection (UV)

4 > 0.5 W high power lasers safety glasses

skin protection (UV)


Realized laser safety

• laser safety officer

• laser safety equipment:

1) warning sign and warning lamp

2) safety curtain

3) safety glasses


Determination of safety glasses

example: Nd:YAG laser (80 mJ, 15 nsec)

MPE = 5 × 10− 6 J/cm2 (from table)

H0 = 80 mJ / π r 2

= 80 mJ / π (0.35 cm) 2

= 0.21 J/cm2

OD = log (H0 / MPE)

= log (0.21 / 5E−6)

= 4.6

recommended filter density: 5


University Mannheim Technical University of Heidelberg Medical Center of Mannheim

Biomedical engineering: characterization and processing of biomaterialsinternet database for tissue parameters

Medical physics: design of medical applicators and sensorsbioelectrical multichannel measurement devices

Laser medicine: laser-tissue interactionsmedical applications of ultrashort laser pulses

Basics of laser physics Chap. 2: Matter acting on light Chap. 3

Documents