Radilogic Physics Slides

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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KEY : Photodisintegration

Need at least 10 MeV for photodisintegration. This is more energy than a

normal x-ray.


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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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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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KEY : Coherent Scattering , Rayleigh scattering , Thomson scattering

incident photon undergoes a change in direction without a change in

wavelength


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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

Radilogic Physics Slides

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