2. XR Interactions with People

Question 1

Types of x-ray interactions

Answer 1

classical/coherent, compton, photoelectric, pair production, photodisintigration

Question 2

What energy level does compton scatter occur compared to classical/coherent interaction?

Answer 2

higher energy

Question 3

Describe the compton scatter interaction

Answer 3

1. incident X-ray frees an electron (compton e-)
2. atom is ionized (missing e-)
3. incoming x-ray is deflected (scattered photon)

Question 4

dominant force contributing to scatter/fog?

Answer 4

compton interactions, >100 keV

Question 5

major source of occupational exposure?

Answer 5

compton

Question 6

probability of compton depends on?

Answer 6

density of material or hydrogenous material/people (materials rich in hydrogen have increased probability)

decreased with increased x-ray energy

Question 7

mechanism of photoelectric interactions

Answer 7

x-ray strikes inner shell electron, displacing it and energy is absorbed; Auger electron is released

1. characteristic radiation
2. negative ion (photoelectron)
3. positive ion (atom missing an electron)

Question 8

what energy level does photoelectric interactions take place?

Answer 8

low energy; although present throughout diagnostic range (20-120)

Question 9

probability of PE?

Answer 9

• inversely proportional to incident photo energy 1/E^3
• must have minimum incident photon energy to free K shell electron
• probability increases as binding energy/incident photon energy are similiar
• proportional to the atomic number cubed (Z^3)

Question 10

how does Z and PE affect contrast?

Answer 10

small differences in Z (bone > soft tissue) are amplified (cubed); better soft tissue delineation

Question 11

why do xrays appear overexposed?

Answer 11

kVP is too high; not enough photoelectric effect/contrast

Question 12

lead and the photoelectric effect

Answer 12

lead has high Z; more absorption

Question 13

k shell of calcium

Answer 13

4 keV

Question 14

role of barium/iodine in secondary radiation

Answer 14

higher Z elements; generate secondary x-rays that can leave patient/fog radiation

Question 15

k-edge

Answer 15

photoelectric energy peaks around binding energy for the inner shell electron

Question 16

optimal kvP for contrast agents

Answer 16

65-90 kVp as barium (k edge 47), iodine (k edge 33)

Question 17

downside of PE interactions?

Answer 17

generate more dose than other types of radiation because ALL the energy of incident x-ray photon is absorbed

Question 18

x-ray production increases how with mA vs kVP

Answer 18

linearly with mA; increase kVp will double the intensity

Question 19

tactic to visualize low contrast objects

Answer 19

keep kVp constant, increase mAs; optimize PE

Question 20

lower dose but maintain exposure?

Answer 20

raise kVp 15%, lower mAs 50%

Question 21

optimal kVp for contrast agents

Answer 21

double the binding energy of contrast agent; if K edge of Iodine is 33, you want kVp of 66; barium kVp is 74 (k edge is 37 kev)

Question 22

what energy photon is needed to create pair production?

Answer 22

1.022 MeV, high energy photon on something with high Z

Question 23

mechanism of pair production

Answer 23

high energy photon interacts with nucleus and is absorbed; production of 1 e/positron. Positron will interact with an electron and annihilate giving off 2 511 keV photons 180 degrees apart

Question 24

what is the mechanism of photodisintegration

Answer 24

high energy photons (>10 MeV) strke nucleus and cause ejection of alpha particles

Question 25

dominant force below 30KeV

Answer 25

PE; low scatter

Question 26

30 keV dominant interaction

Answer 26

probability of compton and PE are equal; increased probability of penetration and reduction in total attenuation

Question 27

> 30 keV dominant interaction

Answer 27

reduced PE interaction (1/E^3) and compton scatter

Question 28

Increased tissue mass density results in?

Answer 28

decreased penetration, increased compton interactions, increased photoelectric interactions

Question 29

increase in atomic number

Answer 29

decreased penetration, no change in compton, increase in PE

Question 30

attenuation definition

Answer 30

xrays that interact with matter (compton/PE) and do not travel through an object

Question 31

linear attenuation definition

Answer 31

fraction of photons interacting per unit thickness; linear attenuating coefficient

Question 32

Factors that influence attenuation

Answer 32

more: denser objects, higher Z, at K-edge
less: higher kVp

Question 33

applied absorption theory

Answer 33

low keV the contrast is greatest, but photoelectric effects predominate at lower keV and increases absorption

Question 34

what is entrance skin dose

Answer 34

radiation absorbed by skin as beam strikes patient

Question 35

factors influencing entrance skin dose

Answer 35

tube current (mA), time of exposure (s), peak kilovoltage (squared), distance (inverse squared)