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Dr. Hossam Eldin Sayed Ali
Lecturer of medical Biophysics
The Research Institute of Ophthalmology
 Important Points to remember:
 Laser pumping is the act of energy transfer from an
external source into the gain medium...
 Population Inversion
 A state of the gain medium where a higher-lying
electronic level has a higher population than a l...
 Continuous wave (CW) Laser
Continuous-wave (cw) operation of a laser means that
the laser is continuously pumped and con...
 The term Pulsed Laser is most commonly used for Q-
switched lasers emitting nanosecond pulses. VIDEO
 Nanosecond pulses...
Excimer Laser
 Excimer stands for excited di meris, a family of lasers that have
repetitively pulsed output, emit powerfu...
 Semi conductor (diode) lasers
 An electrically pumped semiconductor laser in which
the active laser medium is formed by...
 The LED consists of a chip of semiconducting material
doped with impurities to create a p-n junction. As in
other diodes...
Holography
 How do Laser React With Tissue
 Laser as one of the electromagnetic waves, reacts in one of
the following ways when it ...
 The absorption of light energy, and the transformation
of the light into heat, is essential for the tissue
reactions to ...
Mechanisms
 Laser beam intensity is an important parameter for
the study of the effects of macroscopic and
microscopic in...
 An equal flow of energy can be supplied by changing
the exposure time and wavelength (λ) of the laser
radiation so that ...
 PHOTO-CHEMICAL laser causes chemical change or
response; the light has a chemical impact on the
tissue being treated. Ex...
 The interactions are based on the absorption of
radiation by :
 Water contained in the tissues.
 Hemoglobin in the blo...
 The photochemical (Photodynamic) interaction:
 occurs when the energy of photons is greater than that of
chemical energ...
 Fluence rates below the hyperthermia threshold can be
used for PDT (PhotoDynamic Therapy) , a two-step
modality in which...
 It is not possible to generate 1O2 by light of higher
wavelengths than about 800 nm. Thus, for PDT one
has to use radiat...
 Absorption of 10 light quanta can give as many as six 1O2
molecules, so the process is extremely efficient, and sub-
hyp...
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Medical applications of laser 2

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medical applications of laser

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  • Light and its nature have caused a lot of ink to flow during these last decades. Its dual behavior is partly explained by (1)Double-slit experiment of Thomas Young - who represents the photon’s motion as a wave - and also by (2)the Photoelectric effect in which the photon is considered as a particle. A Revolution: SALEH THEORY solves this ambiguity and this difficulty presenting a three-dimensional trajectory for the photon's motion and a new formula to calculate its energy. More information on https://youtu.be/mLtpARXuMbM https://www.slideshare.net/SalehTheory
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Medical applications of laser 2

  1. 1. Dr. Hossam Eldin Sayed Ali Lecturer of medical Biophysics The Research Institute of Ophthalmology
  2. 2.  Important Points to remember:  Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited state, a process known as population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier.  The pump power must be higher than the lasing threshold of the laser.  The pump energy is usually provided in the form of light or electric current, but more exotic sources have been used, such as chemical or nuclear reactions.
  3. 3.  Population Inversion  A state of the gain medium where a higher-lying electronic level has a higher population than a lower- lying level, this is achieved after adequate pumping  In simple cases, a laser transition, on which optical gain occurs as a result of stimulated emission, can occur only when the population of the upper laser level is higher than that of the lower level (i.e. more laser-active atoms or ions are in the higher level). This condition of the laser medium is called population inversion.
  4. 4.  Continuous wave (CW) Laser Continuous-wave (cw) operation of a laser means that the laser is continuously pumped and continuously emits light. The emission can occur in a single resonator mode (single-frequency operation) or on multiple modes. Pulsed lasers: Pulsed lasers are lasers which emit light not in a continuous mode, but rather in the form of optical pulses. The term is most commonly used for Q-switched lasers emitting nanosecond pulses. Versatile types of PL can be produced depending on: the pulse duration, pulse energy, pulse repetition rate and wavelength
  5. 5.  The term Pulsed Laser is most commonly used for Q- switched lasers emitting nanosecond pulses. VIDEO  Nanosecond pulses in the ultraviolet spectral region are generated with excimer lasers.  The picosecond or femtosecond pulses are usually generated with mode-locked lasers. VIDEO  milli = 1E-3 micro = 1E-6 nano = 1E-9 pico = 1E-12 femto = 1E-15 atto = 1E-18.
  6. 6. Excimer Laser  Excimer stands for excited di meris, a family of lasers that have repetitively pulsed output, emit powerful pulses lasting for nanoseconds or tens of nanoseconds, at wavelengths in or near the ultraviolet, and the lasing medium is a diatomic molecule, or dimer, in which the component atoms are bound in the excited state but not in the ground state.  Sometimes more correctly called an exciplex laser  The most important excimer molecules are rare gas halides such as XeF and XeCl.  Average output power (in watts) is the product of the pulse energy (in joules) multiplied by the number of pulses per second (repetition rate). Typical average powers range from under a watt to over 100W. 
  7. 7.  Semi conductor (diode) lasers  An electrically pumped semiconductor laser in which the active laser medium is formed by a p-n junction of a semiconductor diode similar to that found in a light- emitting diode.  The laser diode is the most common type of laser produced with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray Disc reading and recording, laser printing, laser scanning and increasingly directional lighting sources.
  8. 8.  The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.
  9. 9. Holography
  10. 10.  How do Laser React With Tissue  Laser as one of the electromagnetic waves, reacts in one of the following ways when it strikes a surface:  Reflection,  Absorption  Scattering  Reflection occurs as a consequence of the change of the refractive index between air and tissue, a small fraction of the radiation is reflected back.  Absorption and scattering of the remaining fraction occurs when the beam is propagated into the tissue.
  11. 11.  The absorption of light energy, and the transformation of the light into heat, is essential for the tissue reactions to occur.  The optical response of the tissue depends mainly on the effective absorption coefficient which takes account of absorption and scattering of the tissue at the used wavelength
  12. 12. Mechanisms  Laser beam intensity is an important parameter for the study of the effects of macroscopic and microscopic interaction of laser with matter and is defined as: The ratio of the emitted beam power (in watts) to the unit of irradiated area in square cm.  The laser intensities can be classified into:  High ( >1016 W/cm2),  Medium ( ~1010 W/cm2) and  Low ( <106 W/cm2). The latter in particular are used in medicine for diagnostic use, those intermediate for surgical use and those of high power mainly for research purposes.
  13. 13.  An equal flow of energy can be supplied by changing the exposure time and wavelength (λ) of the laser radiation so that we can reach the high intensities like 1016 W/cm2.  There are 4 types of interactions of the laser beam with tissues:  Photochemical.  Photothermal.  Photoablative.  Photomechanical interactions.
  14. 14.  PHOTO-CHEMICAL laser causes chemical change or response; the light has a chemical impact on the tissue being treated. Examples include: Photo Dynamic Therapy (PDT) Cancer treatment, ophthalmic treatments, and Laser therapy in pain relief.  PHOTO-THERMAL occurs when we use long pulses, biological effect due to heating such as hair removal, and most surgical lasers.  PHOTO-MECHANICAL: Short-pulsed (q-switched) lasers cause pressure waves that can stimulate the lymph draining system leading to dissolution of inflammatory mediators.  Photoablation such as in tattoo removal.
  15. 15.  The interactions are based on the absorption of radiation by :  Water contained in the tissues.  Hemoglobin in the blood  Pigments or chromophores in some tissues normally present (or externally administered).
  16. 16.  The photochemical (Photodynamic) interaction:  occurs when the energy of photons is greater than that of chemical energy bond, typically greater than 5 eV;  Occurs at very low levels of intensity or irradiance (I ~ 1 W/cm2)) and for high exposure times (duration time greater than a second).  The field of action is that of ultraviolet radiation that generates chemical fragmentation effects.  (If the energy density deposited is very high it is possible to achieve an ablation with high spatial control) (KrF laser, ArF). In this case the contours of the spots must be well- defined for the prevention of thermal spreads.  The energy absorbed in the tissue is used for structural modifications of the existing molecules, in fact the reaction: hν + A + B →(AB) …. a molecule A and another one B receives energy hν, a new product AB will be produced.
  17. 17.  Fluence rates below the hyperthermia threshold can be used for PDT (PhotoDynamic Therapy) , a two-step modality in which the delivery of a light activated and lesion-localizing photosensitizer is followed by a low, non-thermal dose of light irradiation to kill cancer cells.  Almost all photochemical reactions of biological relevance are dependent on generation of reactive oxygen species (ROS). The most important is 1O2  1O2 (Singlet oxygen) is a high energy form of oxygen  It is the lowest excited state of the dioxygen molecule. Its lifetime in solutions is in the microsecond range (3 µsec in water).  It undergoes several reactions with organic molecules.  It is believed to be the major cytotoxic agent involved in PDT.
  18. 18.  It is not possible to generate 1O2 by light of higher wavelengths than about 800 nm. Thus, for PDT one has to use radiation in the UV or visible range.  Because of the optical window of tissue ‘between’ 620 and 1100 nm, yielding optimal penetration depths (∼1– 3mmat 630 nm), red light is most frequently used for PDT.  The red band of heme proteins is at 620 nm, so chemicals (sensitizers) such as Porphyrins, chlorines and phthalocyanins that absorb beyond 620 nm are being used for laser PDT.  Several lasers can be applied, notably dye lasers and diode laser.
  19. 19.  Absorption of 10 light quanta can give as many as six 1O2 molecules, so the process is extremely efficient, and sub- hyperthermal fluence rates can be satisfactory.  Singlet oxygen has a short lifetime of 10–40 ns in cells and tissues, and its radius of action is only about 10–20 nm.  This means that PDT acts selectively on targets with high concentrations of sensitizer . PDT Tumor selectivity is based on this principle.  The tumor selectivity of sensitizer accumulation can be related to a number of factors:  Low tumor pH (several sensitizers protonate and get more lipophilic below pH 7),  High concentration of lipoprotein receptors in tumours (many sensitizers are bound to lipoprotein in the blood),  Presence of leaky microvessels with low lymphatic drainage in tumors, and a high concentration of macrophages (taking up aggregated sensitizers) in tumors

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