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Medical Physics > Lecture 9 > Flashcards

Lecture 9 Flashcards

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1
Q

Properties of the tissue that light interaction depends on

A

Optical properties
Thermal properties
Chemical properties

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2
Q

Light interactions depend on

A

Laser irradiance (1j/cm^2 - 1000 j/cm^2)
Exposure time

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3
Q

In biological tissues absorption is caused by

A

Water molecules
Proteins
Pigments

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4
Q

Light tissue interaction mechanisms

A

Photochemical interaction
Thermal interaction
Photo ablation
Plasma-induced ablation
Photodisruption

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4
Q

Medical application of photochemical effect

A

Photodynamic therapy (PDT)

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4
Q

Chromophores capable of causing light-induced reactions in non-absorbing molecules

A

Photo sensitizers

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5
Q

Irradiance and exposure time of photochemical effect

A

Irradiance: 1 W/cm^2
Exposure time: seconds or continuous

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5
Q

PDT depends on

A

The excitation of oxygen molecules from the triplet ground state to a singlet excited state leading to the formation of ROS which is an aggressively reactive species that causes necrosis of affected tissues

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5
Q

Disadvantages of PDT

A
  • Long time of tissue decay and removal from body
  • Patients become photosensitive for several weeks or days
  • Might be painful and toxic
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5
Q

Clinical uses of PDT

A

Cancers: Lung/ skin/esophagus/ head and neck/post-radiation therapy
Non-cancer: Arterial diseases/ ophthalmology

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6
Q

PDT is similar to

A

SDT (sonodynamic therapy, uses ultrasound instead of light)

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7
Q

2 processes important for photothermal interaction to happen

A
  • Absorption –> excitation
  • Deactivation (nonradiative decay) –> inc in kinetic energy which leads to increasing tissue temperature
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8
Q

Irradiance and exposure time of photothermal interaction

A

Irradiance: 10-10^6 w/cm^2
Exp time: 1 min - 1 ms

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9
Q

Photothermal interaction depends on which tissue optical properties

A
  • Absorbed power density (H)
  • Probability per unit length of absorbed photon (absorption coeff)
  • Irradiance or fluence
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10
Q

Photothermal interaction depends on which tissue thermal properties

A
  • Heat capacity
  • Thermal conductivity
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11
Q

Leads to non-radiative processes such as internal conversion and vibrational relaxations

A

Thermal absorption

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12
Q

Thermal effects of the tissue depend on

A

Wavelength of beam and exposure time

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13
Q

Heating of tissue leads to

A
  • Coagulation
  • Vaporization
  • Carbonization
  • Melting
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14
Q

Irreversible necrosis without immediate tissue destruction

A

Coagulation

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15
Q

Temperature required for coagulation

A

50-100°C for even a second

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16
Q

Coagulation happens due to

A

Denaturation of proteins and collagen as a result of increase in temperature

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17
Q

A process which produces thermal ablation of the tissue

A

Vaporization

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18
Q

Temperature required for Vaporization

A

at least 100°C (as water turns into steam)

19
Q

Thermo-mechanical effect caused by pressure build-up due to steam formation

A

Thermal ablation

20
Q

A process where tissue chars and its organic constituents become carbon

A

Carbonization

21
Q

Temperature required for carbonization

A

at least 150°C

22
Q

Why should carbonization be avoided

A

As it leads to irreparable damage and has no benefit

23
Q

A process used for tissue welding

A

Melting

24
Q

Temperature required for melting

A

Must be higher than the melting point of the tissue (for milli or nanoseconds)

25
Q

Melting happens due to

A

High power density of a pulse laser beam for just a few milli or nanoseconds

26
Q

Lasers used for thermal applications depend on

A
  • Wavelength of the beam
  • Absorption coefficient for each tissue type
27
Q

Applications of photothermal interactions

A
  • Minor skin surgery
  • Cosmetic applications
  • Hair/ tattoo removal
  • Photothermal treatments in ophthalmology
28
Q

When the whole of the retina is coagulated

A

Panretinal photocoagulation

29
Q

Happens when the laser beam exceeds the bond energy between atoms of a molecule

A

Photoablation

30
Q

2 steps of photoablation

A

1- Excitation by UV light
2- Dissociation

31
Q

Irradiance and exposure time of photoablation

A

-Irradiance: 10^7 - 10^8 w/cm^2
-Exp time: nanoseconds

32
Q

Photoablation will only take place if

A

Intensity of absorbed light is higher than or equal the threshold intensity of photoablation

33
Q

A process by which cellular and extracellular components are photochemically decomposed by intense UV light

A

Photoablation

34
Q

Why is UV light used in photoablation

A
  • It has higher electric field intensity than the binding energy between molecules
  • Molecular bonds break and vaporize without generation of heat
35
Q

Difference between thermal ablation and photoablation

A

Thermal ablation: uses kinetic energy to increase temperature of the tissue
Photoablation: Uses kinetic energy to break the bonds between atoms and vibrate rather than cause heat

36
Q

Applications of photoablation

A

Refractive surgery (eye correction surgeries) –> reshaping cornea and implanting lenses to reduce dependency on eyeglasses and contact lenses

37
Q

Photomechanical effects

A
  • Plasma-induced ablation
  • photodisruption
38
Q

Gas is converted into plasma by

A

ionization

39
Q

The mixture of electrons and nuclei resulting from ionization

A

plasma

40
Q

Advantages of plasma-induced ablation

A

highly precise, well-defined removal of tissue

41
Q

Photoablation and photodisruption depend on

A

Optical breakdown

42
Q

Plasma generation by intense electric field

A

Dielectric breakdown

43
Q

Generation of electric field using light

A

Plasma-induced ablation

44
Q

Irradiance and exposure time needed for plasma-induced ablation

A

irradiance: 10^11 - 10^12 w/cm^2
exp time: 500 ns - 1 fs

45
Q

Application of plasma-induced ablation

A

Posterior capsulotomy after eye cataract surgery

46
Q

Causes cavitation bubbles and jet formation

A

Photodisruption

47
Q

Irradiance and exposure time needed for photodisruption

A

irradiance: 10^11 - 10^16 w/cm^2
exp time: 100 ns - 100 fs

48
Q

Difference between photodisruption and plasma-induced ablation

A

in photodisruption, laser beam is pointed inside the tissue, not on the surface leading to formation of cavitation bubbles that consist of gaseous vapors which diffuse again in surrounding tissue

49
Q

Jet formation is achieved when

A

cavitation bubbles collapse in fluids and near a solid boundary

50
Q

a kind of visible light waves medical application on tissues. It is a therapy that uses monochromatic light and adapted chromophores which are injected into the body and light may then trigger photochemical reactions, resulting in certain biological transformations

A

PDT

Medical Physics (1 decks)

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