X-Ray Interactions with Matter

Question 1

what are the 3 ways that photons interact with matter

Answer 1

transmission, absorption, scatter

Question 2

what is transmission

Answer 2

photon passes directly through tissue unchanged

Question 3

what is absorption

Answer 3

energy fully deposited into tissue and photon ceases to exist

Question 4

what is scatter and absorption

Answer 4

photon deflected by tissue
partial deposition of energy into tissue
photon continues in new direction

Question 5

what is attenuation

Answer 5

reduction in intensity of x-ray beam (can include absorption and scatter)

Question 6

what is x-ray beam intensity

Answer 6

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

Question 7

what is intensity proportional to

Answer 7

energy and number of photons

Question 8

what does minimal attenuation give

Answer 8

black image

Question 9

what does partial attenuation give

Answer 9

grey image

Question 10

what does complete attenuation give

Answer 10

white image

Question 11

what are predictions of x-ray photon interactions based on

Answer 11

physical properties of matter being exposed

Question 12

what are the 2 specific attenuation interactions

Answer 12

photoelectric effect and compton effect

Question 13

what is the photoelectric effect

Answer 13

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

Question 14

when does the photoelectric effect take place

Answer 14

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

Question 15

when does the photoelectric effect predominate

Answer 15

with lower energy photons

Question 16

what can a photoelectron do to tissues

Answer 16

ionise and damage adjacent tissues

Question 17

why does photoelectric effect lead to lighter areas on radiographs

Answer 17

it prevents x-ray photons reaching the receptor

Question 18

what is the photoelectric effect proportional to

Answer 18

atomic number cubed

Question 19

what is the compton effect

Answer 19

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

Question 20

when does the compton effect occur

Answer 20

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

Question 21

when does the compton effect predominate

Answer 21

with higher energy photons and outer shell electrons

Question 22

what can a recoil electron do to tissues

Answer 22

ionise and damage adjacent tissues

Question 23

what is scatter influenced by

Answer 23

energy of incoming photon

Question 24

where are higher energy photons scattered

Answer 24

more forward - forward scatter

Question 25

where are lower energy photons scattered

Answer 25

more backward - back scatter

Question 26

what is the majority of the scatter from an x-ray beam

Answer 26

forward scatter

Question 27

what is the reason for the controlled area to surround the patient

Answer 27

scatter

Question 28

what is the probability of the compton effect occurring

Answer 28

proportonal to the density of material

Question 29

what does collimation do

Answer 29

decrease surface area irradiated decrease volume of irradiated tissues decrease number of scattered photons produced in tissue decrease scattered photons interaction with receptor decrease loss of contrast on radiographic image

Question 30

what is the photoelectric effect on radiation dose

Answer 30

increase patient dose but is necessary for image formation

Question 31

what is the compton effect on radiation dose

Answer 31

increased patient dose but scattered photons do not contribute usefully to image

Question 32

what is the effect of lowering kV on x-ray unit

Answer 32

increased contrast between tissues with different atomic numbers but increased patient dose

Question 33

what is the effect of raising kV on x-ray unit

Answer 33

decreased dose absorbed by patient but decreased contrast between tissues with different atomic numbers

Question 34

what is the UK guidance for range of kV in intraoral units

Answer 34

between 60-70kV