X-Ray Interaction with Matter Flashcards

1
Q

what are the three types of ways in which photons interact with matter

A

transmission - passes through matter unaltered
absorption - stopped by matter
scatter - changes direction

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2
Q

what happens when a photon is both scattered and absorbed

A

photon is deflected by tissue but there is partial dposition of energy into tissue

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3
Q

what is attenutation

A

reduction in intensity of x-ray beam due to absorption or scattering

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4
Q

what is x-ray beam intensity

A

quantity of photon energy passing through a cross-sectional area of the beam per unit time

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5
Q

what is intensity proportional to

A

energy of x-ray beam and number of photons

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6
Q

how can you increase the energy of the x-ray beam

A

increase average photon energy

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7
Q

what colour will tissue that have minimal attenuation be

A

black

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8
Q

what colour will tissue be that has partial attenuation

A

grey

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9
Q

if there is complete attenuation what colour will show up on the image

A

white

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10
Q

what are the two specific attenuation interactions

A

photoelectric effect - complete absorption
compton effect - absorption and scatter

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11
Q

what is the photoelectric effect

A

photon in x-ray bea, interacts with inner shell electron in subject, resulting in absorption of the photon and creation of photoelectron

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12
Q

what energy do photo-electrons move off with

A

kinetic

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13
Q

when does the photoelectron effect occur

A

when energy of incoming photon is equal to or just greater than the binding energy of inner shell electron

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14
Q

what does the absorption by radiooelectric effect cause on the x-ray image

A

it prevents photons reaching the receptor and causes a lighter area on the image

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15
Q

what is the relation between the photoelectric effect and atomic number

A

small increases in atomic number lead to large jumps in absorption

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16
Q

what is the photoelectric effect proportional to

A

the atomic number cubed

17
Q

why are gold and lead goo shields

A

high atomic numbers = huge proportion of photoelectric effect

18
Q

what is the compton effect

A

photon in x-ray beam interacts with outer shell electrons in subject, resulting in partial absorption and scattering of the photon and creation of recoil electron

19
Q

when does compton effect occur

A

when the energy of incoming photon is much greater than binding energy of electron

20
Q

what occurs with the recoil electron from compton effet

A

can ionise and potentially damage adjacent tissues

21
Q

in which direction do higher energy photons deflect

A

forward scatter

22
Q

in which direction do lower energy photons deflect

A

back scatter

23
Q

why does the control area need to completely surround the patient

A

due to scatter

24
Q

how does scatter effect the radiographic image

A

photons scattered backwards, sideways or obliquely will not reach receptor
photons scattered slightly forwards may still reach receptor but interact with the wrong area which causes fogging of image

25
Q

why should decreasing photon energy reduce fogging of images

A

because then there will not be forward scatter - only back scatter which does not reach the receptor and does not affect image quality

26
Q

what are the 5 aspects of collimation

A

reduce surface area irradiated
reduce volume of irradiated tissue
reduce number of scattered photons produced in the tissue
reduce scattered protons interacting with receptor
reduce loss of contrast on radiographic image

27
Q

what effect does the photoelectric effect have on radiation dose

A

deposition of all x-ray photons energy into tissue
increases patient dose but necessary for image formation

28
Q

what effect does the compton effect have on radiation dose

A

deposition of some x-ray photon energy into tissue
increase patient dose but scatter photons do not contribute usefully to image

29
Q

what is the average effective dose of periapical/ bitewing radiographs

A

4 microSv

30
Q

what happens when you decrease the kilovoltage on x-ray unit (kV)

A

lower x-ray tube potential difference which causes overall lower energy photons produced so there is increase in photoelectric effect and increase contrast between tissue but the patient also absorbs a lot

31
Q

what occurs when you raise the kilovoltage (kV) dose on x-ray unit

A

higher x-ray tube potential difference which causes overall higher energy photons produced which decreases photoelectric effect and causes forward scatter - decreases dose absorbed by patient but image quality is poor