interactions of radiation with matter

Question 1

what are the 3 things that can occur when x-ray passes through matter ?

Answer 1

-transmitted
- absorbed
- scattered

Question 2

Define aattenuation

Answer 2

reduction in the energy of the beam due to scatter and absorption of the photons

Question 3

define scatter

Answer 3

change in direction of a photon with some loss of energy

Question 4

define absorption

Answer 4

absorption of photon by the tissue

Question 5

define ionisation

Answer 5

removal of an electron from a neutral atom by a photon

Question 6

list the properties of photoelectric effect (same as Characteristic radiation, photon instead of e- ) 6 points

Answer 6

• Interaction between photon and inner shell electron
• TRANSFER of the photon energy

-Electron is ejected from its shell

• Ejected electron is known as photoelectron
• Emitted photon is known as characteristic photon
• PE more probable when energy of incident electron is similar to the binding energy

Question 7

where is the photoelectric effect more likely to happen

Answer 7

-more dense materials, as more atoms containing more electron
- so more chance of the photon hitting an electron.

Question 8

describe the compton effect simplified, ?

(hint p,p,e,e,p)

Answer 8

• photon hits outer shell electron

-causing the photon to change direction (scattered)

-Some of the photon energy is transferred to the electron

• removing the e- from the outer shell.
• Photon continues on with lower energy and at different angle

Question 9

when does the chances of the compton effect increase ?

Answer 9

• energy of x-ray beam increases (kV)
• amount of tissue irradiated increases

Question 10

what are the effects of scatter in the compton effect?

Answer 10

• results in noise, interferes with what you are trying to see
  -expose staff
  -exposes patient
• treat patient as source of radiatioin

Question 11

how do we reduce scatter ?

Answer 11

• reduce the volume of tissue irradiated
• reduce the kv to a point !
• need high enough kv to penetrate tissue
• not too much absorbed by patient
• balance between image quality and dose

Question 12

what is the equation for incident photon ?

Answer 12

E (incident photon) = E (scattered photon) + E (ejected electron)

Question 13

what happens between an Interaction between photon and nucleus ?

Answer 13

Energy of photon is converted into an electron and a positron

Question 14

how does scatter angle change with kv

Answer 14

As kV increases photons becomes more “forward scattered”

Question 15

what does it mean when you call an x-ray iosotopic

Answer 15

x-rays are emitted in all directions EQUALLY

Question 16

the greater the distance from the source of radiation…

Answer 16

the lower the dose

Question 17

if you double your distance from a source of radiation, what happens to the radiation dose ?

Answer 17

dose will quarter

Question 18

intensity is propotional too… (hint, an equation)

Answer 18

1/d-squared (check pp)

Question 19

why is the x-ray beam divergent

Answer 19

as x-ray leaves the tube it gets wider

Question 20

what is the result of an x-ray beam being divergent

Answer 20

image produced is bigger than the object (magnified)

Question 21

whats the equation for M

Answer 21

SID/SOD = IMAGE SIZE/OBJECT SIZE

Question 22

Why are chest x-rays done PA ?

Answer 22

• puts the heart as close as we can to the image receptor
• diameter of heart/diameter of chest* 100
  (should give value of 50%)
  -in ap the heart gets magnified so looks alot nore in large
  -in pa you get true reprenation of tru size