Transcript Laser in Medicine Neurosurgery Er:YAG Student: Luis R. Castillo Professor: PhD. Carlos Treviño INAOE04/22/04 Agenda • • • • • • • • • • Introduction Laser Physics Medical Laser Interaction of Laser Beam and Material Laser Interaction with Tissue Thermal Interaction.

Laser in Medicine
Neurosurgery
Er:YAG
Student: Luis R. Castillo
Professor: PhD. Carlos Treviño
INAOE
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Agenda
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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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Introduction
-
Light has been used for diagnostic and therapy procedures
throughout the years.
Greeks and Romans
Ancient Egypt
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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
-
One of the first attempts for diagnostic propose
was the development of optical instruments to look:
Eyes
Ears
Mouth etc...
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With the availability of lasers, laser crystals, fiber
optic. A complex instruments became a powerful
tools for medical applications
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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 endoscope
Fiberscope etc..
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They have been the cause of revolution in many fields
of medicine such as:
Cardiovascular diseases
Odonthology
Ophthalmology,
Neurology.
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Laser Physics
Laser versus Ordinary Light Source
Property
Laser
Ordinary light source
Directionality
Collimated (parallel beam)
Non collimated (light emitted
in all directions)
Color
Monochromatic (one color)
Comment: coherent beam
(i.e., ordered in time & space)
Polychromatic (many colors)
Comment: non coherent beam
(i.e., non ordered)
Power output
Can be high
Medium or low
Temporal
Can produce very short and
energetic pulses
Typically long and low-energy
pulses
Power density
High; can be focused to a
very small spot (of diameter
d=lambda)
low; relatively large focal spot
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Laser Physics
Comparison of a Laser and an ordinary light source
laser
Ordinary lamp
I
Divergent beam
intensity
intensity
Collimated beam
I
Monochromatic
Polychromatic
Coherent
(in space and time)
Non coherent
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Laser Physics
CW and pulsed laser beams
Continuous wave (CW)
Power
Time
Long pulses, high rep
rate, low peak power
Power
Time
Short pulses , low rep
rate, high peak power
Power
Time
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Laser Physics
Schematic drawing of the laser
Fiber (as a gain medium)
Pump
LASER
Pump
Crystal (as a
gain medium)
or
Pump
LASER
Pump
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Laser Physics
Atomic transitions
I
intensity
I
 in 
E2
E
h

I
E
o 
h

 out

N2
E
E1
Absorption
N1
Spontaneous emission
Stimulated emission
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Laser Physics
3 level and 4 level
Pump Band
Pump Band
Upper
level
Energy
Level
Lower
level
population
Upper
level
Energy
Level
Lower
level
Relax
Ground
level
population
Ground
level
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Laser Physics
A basic illustration of the allowed
photon emission processes
Energy n
0
l
l
=0
=1
l
=2
l
=3
5 5s
4 4s
5p
5d
5f
4p
4d
4f
3 3s
3p
3d
2 2s
13.6eV
l
Photon
2p
1 1s
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Laser Physics
Example of an energy diagram for
Er3+ ion in the glass fiber medium
Energy of the Er3+ ion
in the glass fiber
1.27 eV
E3
Non-radiative decay
980 nm
Pump
0.80 eV
1550 nm
E2
Out
In
0
1550 nm
E1
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Laser Physics
Example of an energy
diagram for Erbium
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Laser Physics
Program
0.25
Laser Er:YAG
0.2
0.15
g0i
0.1
0.05
LASER RATE EQUATI ONS
Ac cumulativ e build-up t ime
 g 01 


 n1   




==> initial v alues
Ei nj 
 1   


 g 0   Ep A  c d t 
 1
  n 1 


==> rat e of c hange of inv ersion densit y
 n i 1   n i  1  2  c i d t 

 
==> rat e of c hange of photon dens it y
 i 1     i  1  c  n i  w 1  d t 
 g0  

 n
==> c hange in gain
 i 1  
i

 


0
The program computes the energy
and the width of the emitted pulse
by a laser Er:YAG
0
5
10
Trti
15
10
Trti
15
20
0.025
0.02
0.015
Erti
0.01
0.005
5
20
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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
Intensity distribution is not just a mathematical curiosity; it is extremely
important for laser-tissue interaction and in particular for laser neurosurgery.
TEMoo
TEM1o
TEMo1
TEM11
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Laser Physics
Real Beam Profile
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Medical Laser
Popular Lasers
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Medical Laser
Some Medical Applications of Lasers
Field
Power
Duration
Density
Depth
Medical
of Penetration
Application
Example
Diagnosis Very low
Long
Shallow
Blood diagnosis
Tissue Charact.
HeCd
Therapy
Low
Medium
Long
Long
Deep
Medium
Medium
Long
Deep
High
Short
Bio stimulation
Tissue welding
Blood coagulation
Laser hyperthermia
Phototherapy
Laser lithotrispy
HeNe
Nd:YAG
Ar ion
Nd:YAG
Au vapor
Dye
High
V. High
Long
V. Short
Cutting
Ablation without
thermal damage
CO2
Excimer
Er:YAG
Surgery
Shallow
Shallow
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Interaction of Laser Beam and Material
Transmission of laser beams through materials
Ir
reflected irradiance
Is
scattered irradiance
Ia
absorbed irradiance
It
transmitted irradiance
Ii = Ir + Is + Ia +It
Ir  n  1 


Ii  n  1 
2
I ( x)  I (0) e
 x
I ( x)  I (0)e x
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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
dx
I
u

dt cT  H
- If thermal losses are ignored
- If mechanics heat transfer is viewed as
macroscopic
u = vaporization rate
p = density of the material
dx = layer thickness
I = power density ( irradiance)
c= heat capacity
T= change of temperature
H = latent heat of vaporization
- The material removal rate is given u
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Thermal Interaction between Laser and Tissue
Thermal damage
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It has been the subject of
extensive
experimental and theoretical work
( Welch 1984, 1991; Mckenzic 1990;
Jacques 1993,1996).
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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
Spatial and temporal change of T


(Eq.1)
Q
(q )
 
 
dv   q v  n da
t cv t
cs
(Eq.2)
T
X ,Y , Z , t  k  2 T X ,Y , Z , t   Q X ,Y , Z , t  /  c
t
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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:
I
Q
 I
z
when the last function is substituted in Eq. 1, it may be solved
numerically for
each irradiation conditions such as:
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Thermal Interaction between Laser and Tissue
Additional assumptions….
-Laser wavelength for which there is deep penetration into
tissue and strong scattering.
- Laser wavelength that are strongly absorbed
with no scattering
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Long pulses (or CW) , where heat dissipation occurs
via conduction during the pulse
- Short pulses where there is practically no dissipation
during the pulse
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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Laser System in Neurosurgery
Advantages
- 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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Complication in the use of Laser System
Optical problems
- 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
Mechanical problems
- 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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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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Questions or Comments
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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
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