Physics Ch 2 XR Interactions

Question 1

XR –> interact w ppl –> 5 ways?

Answer 1

• classical
• compton
• photoelectric
• pair production
• photodisintegration

Question 2

what is classical (elastic, coherent, thomson) interaxn?

Answer 2

xray –> strike orbital electron –> bounce off –> change direction

Question 3

classical (elastic, coherent, thomson) –> what energy level?

Answer 3

low energy –> <10 keV

Question 4

classical (elastic, coherent, thomson) –> cause ionization –> T/F?

Answer 4

F: too low energy

Question 5

classical (elastic, coherent, thomson) –> how much contribute to…

• image?
• pt rad dose?

Answer 5

image: no sig

pt rad dose: tiny/small

Question 6

Compton scatter –> what energy level?

Answer 6

high

Question 7

what is compton scatter?

Answer 7

XR –> strike outer shell electron –> eject electron –> XR lose some energy –> change direction

Question 8

compton scatter interaxn –> main results? (3)

Answer 8

• freed electron –> compton electron
• atom ionized
• XR –> lose energy & change direction –> “scattered photon”

Question 9

compton scatter interaxn –> what happen to…

• image?
• pt rad dose?

Answer 9

image –> foggy

rad dose: increase

Question 10

diag energy (>100keV) –> soft tissue –> MC interaxn?

Answer 10

compton

Question 11

diag energy range –> what can decrease compton scatter?

Answer 11

inc XR energy

Question 12

image –> scatter/fog –> MC contributor?

Answer 12

compton scatter

Question 13

occupational exposure –> MC source?

Answer 13

compton scatter

Question 14

inc atomic# (Z) –> decrease compton scatter –> T/F?

Answer 14

F: compton scatter not depend on Z

Question 15

inc materal density –> inc compton scatter –> T/F?

Answer 15

T

Question 16

photoelectric interaxns –> occur at what energy?

Answer 16

diag range (20-120)

Question 17

low energy –> photoelectric interaxn vs compton scatter –> which predominates?

Answer 17

photoelectric

Question 18

photoelectric interaxns –> MOA?

Answer 18

xray –> strike inner shell electron –> eject electron

Question 19

photoelectric interaxns –> what 3 products?

Answer 19

• charact xrays or Auger electron
• ejected electron (photoelectron)
• positive ion (original atom –> now missing 1 electron)

Question 20

photoelectric interaxns –> highest probability –> at what energy level?

Answer 20

near binding energy

Question 21

probability of photoelectric interaxn relative to kVp energy (incident photon energy) –> formula?

Answer 21

1/E^3

Question 22

probability of photoelectric interaxn relative to atomic# –> formula?

Answer 22

Z^3

Question 23

which interaxn develops image contrast?

Answer 23

photoeletric

Question 24

probability of photoelectric interaxn –> affected by what 2 factors?

Answer 24

• kVp

- Z

Question 25

which interaxn contributes most to pt rad dose?

Answer 25

photoeletric

Question 26

general practice --> select highest kVp possible --> why?

Answer 26

photoelectric interaxn: - higher kVP --> less absorbed --> less rad dose to pt - but not too high kVp --> enough image contrast --> diagnostic image

Question 27

more photoelectric interaxn --> what happen to contrast?

Answer 27

increase

Question 28

what is K-edge?

Answer 28

energy at which there is peak photo-electric

Question 29

to visualize low-contrast tissues --> how to change... - kVp - mA

Answer 29

- kVp constant | - mA inc

Question 30

to lower dose but maintain constant exposure --> how to change... - kVp - mA why?

Answer 30

- kVp inc 15% - mA dec 1/2 higher kVp --> more penetrate --> less absorbed by pt (less photoelectric)

Question 31

iodine --> K-edge 33 keV --> minimum kVp should be?

Answer 31

66 kVp

Question 32

barium --> K-edge 37 keV --> minimum kVp should be?

Answer 32

74 kVp

Question 33

classic (coherent, elastic) --> causes ionization --> T/F?

Answer 33

F too low energy to cause ionization

Question 34

differeniate: probability of photoelectric vs compton --> at <30 keV?

Answer 34

- PE: high prob | - compton: low

Question 35

differeniate: probability of photoelectric vs compton --> at 30 keV?

Answer 35

PE & compton --> equal prob

Question 36

differeniate: probability of photoelectric vs compton --> at >30 keV?

Answer 36

- PE: sig dec prob (1/E^3) | - compton: dec prob (but not as much as PE)

Question 37

differeniate: probability of photoelectric vs compton --> inc tissue mass density?

Answer 37

- PE: inc | - compton: inc

Question 38

differeniate: probability of photoelectric vs compton --> inc Z?

Answer 38

- PE: inc | - compton: no change

Question 39

relationship bw mass attenuation & linear attenuation --> formula?

Answer 39

mass attenuation coefficient = linear atten coeff / density

Question 40

what is linear attenuation coefficient?

Answer 40

material's probability to attenuate XR beam over a set distance

Question 41

what is mass attenuation coefficient?

Answer 41

rate of XR energy loss as travel thru material

Question 42

linear attenuation coefficient --> affected by what factor? (1)

Answer 42

material density

Question 43

mass attenuation coefficient --> affected by what factor? (2)

Answer 43

- Z | - photon energy

Question 44

inc kVp --> what happen to... - linear atten coeff - half value layer

Answer 44

- linear atten coeff --> dec | - half value layer --> inc

Question 45

inc density of tissue --> what happen to... - linear atten coeff - half value layer

Answer 45

- linear atten coeff --> inc | - half value layer --> dec

Question 46

entrance skin dose --> affected by what factors? (4) by how much?

Answer 46

- mA --> proportional - time of exposure (s) --> proportional - kVp --> ^2 - distance --> 1/^2

Question 47

if dose = 28 mR at 25mA --> what is dose at 35mA?

Answer 47

28 x 35 / 25

Question 48

if dose = 20mR at 75kVp --> what is dose at 125kVp?

Answer 48

20 x 125^2 / 75^2