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8/13/2019 Radilogic Physics Slides
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A
B
DC
Cross Section of Radiographic Film
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OvercoatEmulsion
Base
Adhesive layer
A = Overcoat = Protective covering of gelatin
B = emulsion = Silver halide grains that which can be sensitized to radiation or light to hold latent image
C = Adhesive base
D = Base =
Question: Films that have only a single emulsion layer ? (Convention radiography films have double emulsion
layers)
Answer : Laser and mammography films have only single emulsion layer . Why ? To prevent cross-over
KEY : Cross Section of Radiographic Film
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A
D
B
C
in contact with film
Cross Section of Intensifying Screen
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Base
Protective Coating
Reflective layer
A = Base = polyester base provides support for phosphor layer ; layer farthest from the film , 1 mm thick
B = Reflective layer = magnesium oxide or titanium dioxide ; redirects isotropically emitted light to phosphor layer
C = Phosphor layer = emits light when stimulated by x-rays ; materials include calcium tungstate, zinc sulfide,
barium lead sulfate and oxysulfides of rare earth metals e.g. gadolinium, lanthanum and yttrium
D = Protective coating = layer closest to film = transparent to light = makes screen resistant to abrasion
KEY : Cross Section of Intensifying Screen
Phosphor
in contact with film
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Question: What is intensification factor ?
Answer : Ratio of exposures, with or without intensifying screen, required to obtain a given film density.
exposure required without screen
intensification factor = -----------------------------------------------
exposure required without screen
Question: What is spectral matching ?
Answer : Matching the light (color , wavelength) emitted by the screen with the film sensitivity. Gadolinium oxysulfide emits
mainly green light. Lathanum oxybromide and Calcium tungstate emit mainly blue light.
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What is this interaction called ?
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KEY : Photoelectric Effect
Incident photon disappears and the K shell electron, now called the photo-electron, is ejected from the atom.
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Question: Name 5 X-Ray Interactions with Matter ? Which is most important in x-ray production ?
Answer :
1. Coherent Scattering - contributes slightly to film fog2. Photodisintegration
3. Pair Production
4. Compton Effect or scattering - reduces the contrast in an x-ray image.
5. Photoelectric Effect - most important in x-ray production
Question: What is the effect of increase in the atomic number of the absorber on the photoelectric effect ?
Answer: Increases proportionately with the cube of the atomic number, Z
3
Question: What is the effect of increase in the mass density of the absorber on the photoelectric effect ?
Answer : Increases proportionately with mass density
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What is this interaction called ?
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KEY : Photodisintegration
Need at least 10 MeV for photodisintegration. This is more energy than a
normal x-ray.
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What is this interaction called ?
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KEY : Compton Scattering
photon interacting with an electron, ejecting it and giving some of its
energy to the electron. The photon is scattered by an angle and the
wavelength is changed.
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Question: Name 5 X-Ray Interactions with Matter ? Which is most important in x-ray production ?
Answer :
1. Coherent Scattering
2. Photodisintegration
3. Pair Production
4. Compton Effect or scattering
5. Photoelectric Effect
Question: What is the effect of increase in the atomic number of the absorber on the compton scattering ?
Answer : No effect
Question: What is the effect of increase in the mass density of the absorber on the compton scattering ?
Answer : Increases proportionately with mass density
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What is this interaction called ?
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KEY : Coherent Scattering , Rayleigh scattering , Thomson scattering
incident photon undergoes a change in direction without a change in
wavelength
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Question: Name 5 X-Ray Interactions with Matter ? Which is most important in x-ray production ?
Answer :
1. Coherent Scattering
2. Photodisintegration
3. Pair Production
4. Compton Effect or scattering
5. Photoelectric Effect
Question: disadvantage of coherent scattering in radiography ?
Answer : Coherent scattering contributes slightly to film fog
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y
-a
xis
x - axis
A
B
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KEY : X-ray emission spectrum
A = Characteristic x-rays
B = Bremsstrauhlung x-rays
x-axis = x-ray energies (keV)
Y-axis = no. of x-rays
y
-axis
x - axis
A
B
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A
B
C
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KEY : Personal dosimetry film badge
the film badge sandwiches the film between at least three pairs of filters:
(a) 'thick plastic', (b) aluminum, and (c) tin with a thin lead foil
A
B
C