X-Ray Interaction with Matter

Question 1

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

Answer 1

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

Question 2

what happens when a photon is both scattered and absorbed

Answer 2

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

Question 3

what is attenutation

Answer 3

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

Question 4

what is x-ray beam intensity

Answer 4

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

Question 5

what is intensity proportional to

Answer 5

energy of x-ray beam and number of photons

Question 6

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

Answer 6

increase average photon energy

Question 7

what colour will tissue that have minimal attenuation be

Answer 7

black

Question 8

what colour will tissue be that has partial attenuation

Answer 8

grey

Question 9

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

Answer 9

white

Question 10

what are the two specific attenuation interactions

Answer 10

photoelectric effect - complete absorption
compton effect - absorption and scatter

Question 11

what is the photoelectric effect

Answer 11

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

Question 12

what energy do photo-electrons move off with

Answer 12

kinetic

Question 13

when does the photoelectron effect occur

Answer 13

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

Question 14

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

Answer 14

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

Question 15

what is the relation between the photoelectric effect and atomic number

Answer 15

small increases in atomic number lead to large jumps in absorption

Question 16

what is the photoelectric effect proportional to

Answer 16

the atomic number cubed

Question 17

why are gold and lead goo shields

Answer 17

high atomic numbers = huge proportion of photoelectric effect

Question 18

what is the compton effect

Answer 18

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

Question 19

when does compton effect occur

Answer 19

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

Question 20

what occurs with the recoil electron from compton effet

Answer 20

can ionise and potentially damage adjacent tissues

Question 21

in which direction do higher energy photons deflect

Answer 21

forward scatter

Question 22

in which direction do lower energy photons deflect

Answer 22

back scatter

Question 23

why does the control area need to completely surround the patient

Answer 23

due to scatter

Question 24

how does scatter effect the radiographic image

Answer 24

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

Question 25

why should decreasing photon energy reduce fogging of images

Answer 25

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

Question 26

what are the 5 aspects of collimation

Answer 26

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

Question 27

what effect does the photoelectric effect have on radiation dose

Answer 27

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

Question 28

what effect does the compton effect have on radiation dose

Answer 28

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

Question 29

what is the average effective dose of periapical/ bitewing radiographs

Answer 29

4 microSv

Question 30

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

Answer 30

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

Question 31

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

Answer 31

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