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Laser in MedicineNeurosurgery

Er:YAG

Student: Luis R. CastilloProfessor: PhD. Carlos Treviño

INAOE

04/22/04

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• Introduction• Laser Physics• Medical Laser• Interaction of Laser Beam and Material• Laser Interaction with Tissue• Thermal Interaction between Laser and

Tissue• Laser System in Neurosurgery• Complication in the use of Laser System• Conclusion• Bibliography

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Agenda

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- Light has been used for diagnostic and therapy proceduresthroughout the years.

Greeks and RomansAncient Egypt

- It was clearly understood by physics and doctors that the patients would benefit enormously if they diagnosed and treated the diseases of the patients, in side or out side of their body, using non surgical instruments.

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Introduction

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- One of the first attempts for diagnostic proposewas the development of optical instruments to look:

EyesEarsMouth etc...

- With the availability of lasers, laser crystals, fiber optic. A complex instruments became a powerful tools for medical applications

- An example is the endoscope integrated system.From the Greek endo means within and skopien,to view

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Introduction

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- The new integrated systems such as:

Laser Catheter,Laser endoscopeFiberscope etc..

- They have been the cause of revolution in many fieldsof medicine such as:

Cardiovascular diseasesOdonthologyOphthalmology,Neurology.

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Introduction

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Laser Physics

Property Laser Ordinary light source

Directionality Collimated (parallel beam) Non collimated (light emitted in all directions)

Color Monochromatic (one color) Polychromatic (many colors)Comment: coherent beam Comment: non coherent beam(i.e., ordered in time & space) (i.e., non ordered)

Power output Can be high Medium or low

Temporal Can produce very short and Typically long and low-energyenergetic pulses pulses

Power density High; can be focused to a low; relatively large focal spotvery small spot (of diameterd=lambda)

Laser versus Ordinary Light Source

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Laser Physics

Comparison of a Laser and an ordinary light source

Coherent(in space and time) Non coherent

inte

nsit

y

Monochromatic Polychromatic

I I

inte

nsit

y

Collimated beam Divergent beam

laser Ordinary lamp

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Laser Physics

CW and pulsed laser beams

Time

PowerContinuous wave (CW)

Time

Power

Time

Power

Long pulses, high rep rate, low peak power

Short pulses , low rep rate, high peak power

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Laser Physics

Schematic drawing of the laser

Pump Fiber (as a gain medium)

LASER

Pump

Crystal (as a gain medium) LASER

Pump

or Pump

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Laser Physics

Atomic transitions

Absorption

inte

nsit

y

I I I

hE

in

hE

o

out

Spontaneous emission Stimulated emission

E2E

1E

2N

1N

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Laser Physics

3 level and 4 level

population

Upperlevel

EnergyLevel

Pump Band

Lowerlevel

UpperlevelEnergy

Level

population

Pump Band

Ground

level

Lowerlevel Ground

level

Relax

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Laser Physics

A basic illustration of the allowed photon emission processes

Energy0

1s1

2s 2p2

3s 3p3 3d

4s 4p4 4d 4f

5s 5p5 5d 5f

= 0 = 1 = 2 = 3

13.6eV

n

Photon

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Laser Physics

Example of an energy diagram for Er3+ ion in the glass fiber medium

Energy of the Er3+ ionin the glass fiber

E10

1.27 eV

0.80 eV E2

E3

1550 nm 1550 nm

InOut

980 nm

Non-radiative decayPump

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Laser Physics

Example of an energy diagram for Erbium

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Laser Physics

Program

The program computes the energy and the width of the emitted pulse by a laser Er:YAG

LASER RATE EQUATIONS

Accumulative build-up time

n1

1

g01

g01

Einj

Ep A c dt

n1

==> initial values

==> rate of change of inversion densityni 1

i 1

g0i 1

ni

1 2 c i dt

i 1 c ni

w1 dt

ni

==> rate of change of photon density

==> change in gain

0 5 10 15 200

0.05

0.1

0.15

0.2

0.25

g0i

Trti

5 10 15 20

0.005

0.01

0.015

0.02

0.025

Erti

Trti

Laser Er:YAG

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Laser Physics

Gain Medium Parameters

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Laser Physics

Pump Laser Parameters

2W High Power Laser Diode 810 ± 10

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Laser Physics

Gain Medium

- Change of host material makes small differences in laser radiation frequency

- Change of dopant ion makes large changes in laser radiation frequency

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Laser Physics

Beam

TEMo1

TEM11

TEMoo

TEM1o

Intensity distribution is not just a mathematical curiosity; it is extremely important for laser-tissue interaction and in particular for laser neurosurgery.

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Laser Physics

Real Beam Profile

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Medical Laser

Popular Lasers

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Medical Laser

Field Power Duration Depth Medical Example

Density of Penetration Application

Diagnosis Very low Long Shallow Blood diagnosis HeCdTissue Charact.

Therapy Low Long Deep Bio stimulation HeNeMedium Long Medium Tissue welding Nd:YAG

Blood coagulation Ar ionMedium Long Deep Laser hyperthermia Nd:YAG

Phototherapy Au vaporHigh Short Laser lithotrispy Dye

Surgery High Long Shallow Cutting CO2V. High V. Short Shallow Ablation without Excimer

thermal damage Er:YAG

Some Medical Applications of Lasers

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Ir reflected irradiance

Is scattered irradiance

Ia absorbed irradiance

It transmitted irradiance

Ii = Ir + Is + Ia +It

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Interaction of Laser Beam and Material

Transmission of laser beams through materials

2

1

1

n

n

Ii

IrxeIxI )0()(

xeIxI )0()(

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Interaction of Laser Beam and Material

Laser beams through materials

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Interaction of Laser Beam and Material

Absorption not always result in generation of heat

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Interaction of Laser Beam and Material

Material processing by laser beams

- If heating effects of laser beam are take place

- If the scattering effect are ignored

- If the beam is totally absorbed in a distance L

- If the laser beam is applied for a period t

- If thermal losses are ignored

- If mechanics heat transfer is viewed as macroscopic - The material removal rate is given u

HTc

I

dt

dxu

u = vaporization rate p = density of the materialdx = layer thicknessI = power density ( irradiance)c= heat capacity T= change of temperatureH = latent heat of vaporization

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- It has been the subject of extensive

experimental and theoretical work ( Welch 1984, 1991; Mckenzic

1990; Jacques 1993,1996).

- Er:YAG lasers showed minimal thermal damage When a sample is heated by a heat source, its temperature T rises and it is possible to calculate the spatial and temporal change of T.

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Thermal Interaction between Laser and Tissue

Thermal damage

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Thermal Interaction between Laser and Tissue

Spatial and temporal change of T

ctZYXQtZYXTktZYXt

T /,,,,,,,,, 2

cv cs

danvqdvt

q

t

Q )(

(Eq.1)

(Eq.2)

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Thermal Interaction between Laser and Tissue

Additional assumptions….

-If one dimensional case is ….

-If flat sample whose surface is the xy plane

-If A Gaussian laser beam is directed a long z axis

- in this case the absorbed energy generates heat and rate of heat is given by:

Iz

IQ

when the last function is substituted in Eq. 1, it may be solved numerically for each irradiation conditions such as:

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-Laser wavelength for which there is deep penetration into tissue and strong scattering.

- Laser wavelength that are strongly absorbed with no scattering

- Long pulses (or CW) , where heat dissipation occurs via conduction during the pulse

- Short pulses where there is practically no dissipation during the pulse

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Thermal Interaction between Laser and Tissue

Additional assumptions….

T(z,t) rises quickly to a final value Tm that is proportional to the deposited energy

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Thermal Interaction between Laser and Tissue

Tissue effects

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Thermal Interaction between Laser and Tissue

Tissue effects

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Laser System in Neurosurgery

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- The laser beam may be focused to a small area, it is possible view it by a integrated Microscope.

- The focal spot is easily moved with a mirror or lens system.

- The laser beam vaporizes or coagulates tissue in the target area without mechanical Contact and damage to adjacent areas.

- Laser radiation has been used to vaporize tumors in sensitive locations in the brain.

- Once the exact location of such tumors is determined by scan system (CT or MRI) , include deeply tumors, can be treated successfully.

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Laser System in Neurosurgery

Advantages

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- Focusing a high power laser beam into a thin optical fiber presents a problem.

- An optical feedback and control mechanism that prevents damage at the output face of the fiber due to high power densities. - It is difficult to asses how much laser energy has reached a target tissue

and how much has actually been absorbed in the tissue.

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Complication in the use of Laser System

Optical problems

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- Optical fibers tend to break upon repeated bending.

- Thick optical fibers (power transmission) are a little bit difficult to use.

-The mechanical devices that connect fibers to holders or to attach the tips are still bulky in contrast with the thin catheters and endoscopes.

- Due to de above comment it can not be easily inserted and guided inside the body but must be inserted through guide wire.

- Exits difficult for angulations for the tips for a target point of view A cool system is needed to the power fibers sterilization is difficult.

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Complication in the use of Laser System

Mechanical problems

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Conclusion

Your own conclusion !!!

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Bibliography

[1] Abraham Katzir, Laser and Optical Fibers in Medicine, Academic Press.

[2] Kuo-Cheng Cheng, Effects of Laser Pulse Shape and Beam Profile OEIT, PhD Thesis

[3] Max Born & Emil Wolf, Principles of Optics. Pergamon Press.

[4] Samuel C. Barden, Fiber Optics in Astronomy, Astronomical Society of the Pacific.

[5] Michael A. Morgan II, David V. Guerra,"An introduction to laser modeling studies with nitrogen pumped dye laser",Am. J. Phys. 67 (9), september 1999

[6] Carlos B. Roundy, Current Technology of Laser Beam Profile Measurements, Spiricon, Inc.

[7] Sony Corporation

[8] Saint-Gobain Crystals & Detectors KK Corporation

[9] Carlos Treviño, Laser Course Notes, http://www-optica.inaoep.mx/investigadores/dr_trevino.htm

[10] S.J. Heyes, http://www.chem.ox.ac.uk/icl/heyes/LanthAct/lanthact.html 1997-8

[11] Manuel Forcales Fernandez, Two Color Spectroscopy of Energy Transfers in Si:Er PhD, Thesis

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• Introduction• Laser Physics• Medical Laser• Interaction of Laser Beam and Material• Laser Interaction with Tissue• Thermal Interaction between Laser and

Tissue• Fiber Optic Laser System in Neurosurgery• Complication in the use of Laser System• Conclusion• Bibliography

04/22/04

Questions or Comments