interaction with matter

Question 1

what speed and path do x ray photons travel at

Answer 1

straight line at speed of light

Question 2

what energy can photons not exceed

Answer 2

cant exceed operating voltage e.g 70kv machine cant produce photons more than 70keV
(think continuous radiation - direct contact with nucleus)

Question 3

what are the 3 ways in which photons interact with matter

Answer 3

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

Question 4

what is attenuation and how is it seen on an x ray

Answer 4

reduction of an x ray beams intensity as it transverse matter
caused by absorption or scatter of the beam
minimal attenuation = black (oral cavity etc)
partial attenuation = grey (trabecular bone)
complete attenuation = white (restorations)

Question 5

how is x ray beam intensity measured

Answer 5

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

Question 6

what are the two effects that occur in patients tissues

Answer 6

photoelectric effect and comptom effect

Question 7

what is the photoelectric effect

Answer 7

occurs when photon from x ray beam interacts with an inner shell electron in the patient resulting in absorption of the photon and creation and release of a photoelectron

Question 8

at what energy does the photo electric effect occur

Answer 8

occurs when incoming photon energy is equal to or just greater than the inner shell electron binding energy
human tissues have low binding energies so low energy photons dominate the photoelectric effect for patient x rays

Question 9

what happens during the photoelectric effect

Answer 9

photon energy overcomes binding energy of electron ejecting it from its shell. This ejected electron no known as photoelectron. This can ionise and potentially damage adjacent tissues.
vacancy in inner shell is filled by outer shell electrons which produces light or heat
absorption via the photoelectric effect results in a more radiopaque area on the image

Question 10

what is the probability of the photoelectric effect occurring proportionate to

Answer 10

the atomic number (Z) of impacted tissue cubed
e.g human soft tissues , Z = 7 , probability of photoelectric effect occuring = 343

this explains why elements with a high atomic number e.g lead are good for shielding

Question 11

what is the compton effect

Answer 11

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

Question 12

at what energy does the compton effect occur

Answer 12

occurs when incoming photon energy is much greater than binding energy of electrons

Question 13

what happens during the compton effect

Answer 13

incoming photon reacts with outer shell electron. Some of photon energy is transferred to electron , now termed recoil electron, which can ionise and damage adjacent tissues.
The photon loses energy and is scattered. Direction of scatter is dependent on the energy

Question 14

what energies produce what direction of scatter in the compton effect

Answer 14

high energies = forward scatter
low energies = backwards scatter
majority of dental x ray scatter will be forwards

Question 15

why does the controlled area when taking an x ray involve whole circumference of the patient and not just the side where the beam is directed

Answer 15

due to potential backwards scatter from the compton effect
- photons of lower energies interacting with outer shell electrons in patients tissues

Question 16

what is the effect of scatter on an x ray image

Answer 16

can make image blurry as the scattered photons may still hit the receptor but at an angle to the tissue they passed through

Question 17

what is collimation and what are the benefits

Answer 17

decreases the size of the x ray beam to match the receptor
e.g round x ray tube beam collimated to a rectangle to match receptor
less SA irradiated, less scattered photons produced in tissues, less scattered photons interacting with receptor, decreased patient dose