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Imaging Review CCPM > Grad class notes IV > Flashcards

Grad class notes IV Flashcards

1
Q

what is matter

A

physical manifestation posessing mass
-can also possess charge

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2
Q

chemically inert atoms

A

have no vacancy in outer shell

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3
Q

cross section of radiation vs elastic scattering

A

-cross sections for radiation are 1/137 that of elastic scattering

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4
Q

radiative and collisional stopping power dependence

A

Irad ~ Z^2 T/A
I coll ~ Z/AT (falls to min at 1 MeV then increases slightly with T)
Irad/Icol = TZ/n, n is 700-800
Irad/Itotal = TZ/(TZ+n)

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5
Q

brems. yield for thick vs thin targets

A

thick- total energy is less than thin target but xray number is higher
-total brems energy = area under curve of radiant energy vs hv

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6
Q

brems angular distribution

A

-for T< 0.5 MeV - photon goes in any direction
-for T> 5 MeV- photon and electron go in direction of incident electron
-at lower energies, max intensity is at 90 degrees to incident electron

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7
Q

fluorescene yield, L and K

A

Yl < Yk/20

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8
Q

yield of auger electrons

A

1- fluorescene yield

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9
Q

auger

A

-transition to lower shell

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10
Q

coster kronig

A

transition between subshells in lower shell

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11
Q

super coster kronig

A

transitions between same shell

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12
Q

fluorescene photon angular distribution

A

isotropic

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13
Q

characteristic vs brems in beams

A

mammo and diagnostic- 30% is characteristic
therapy: few percent is characteristic

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14
Q

impact of scattered photons on image

A

-reduces image contrast
-resolution is the same

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15
Q

mean free path

A

1/mu
-after travelling this distance, 63% if incident photons have interacted with the material

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16
Q

homogeneity coefficient

A

HC= HVL1/HVL2
HC = 1 if monoenergetic beam

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17
Q

2 ways to create and accelerate electrons

A

-gas discharge
-heated cathode

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18
Q

therapy xray tube

A

-stationary anode
-copper shield stops secondary electrons
-W shield attenuates photons in non=patient directions
-oil surrounds tube for cooling and insulation
-Pb casing for further xray attenuation

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19
Q

rotating anode

A

-used for diagnostic: superior heat loading
-Mo surrounds tungsten and helps dissipate heat
-graphite mounts target and dissipates heat

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20
Q

recoil electrons in xray tube

A

-add dose, don’t contribute to image

capture of recoil electrons:
-decreases anode heat load
-improves image contrast
-reduce patient dose

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21
Q

what is tube current a function of?

A

-filament current
-operating kV

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22
Q

space charge cloud

A

-tubes below 40 kVp are space charge limited

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23
Q

effective focal spot

A

actual focal spot size * sin(theta)

larger filament = higher power loading, but larger effective focal size
smaller angle = smaller effective focal spot = better spatial resolution
smaller focal spot = smaller coverage

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24
Q

focal spot size distribution

A

focal spot length is shortest on anode side and longest on cathode side
-focal spot width is constant and depends on cathose width and focusing cup

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25
Q

heel effect

A

photons emitted towards anode side travel a larger distance therough anode and are therefore attenuated more
-thicker part of patient should be placed under cathode side
-beam at anode side is therefor harder

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26
Q

transformer

A

ferromagnetic core with windings
-AC through primary winding produces changing magnetic field, which produces AC on secondary winding

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27
Q

ideal transformer

A

-electrical power input into primary winding = electrical power output of secondary winding

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28
Q

types of transformer

A

-step-up
-isolation
-step-down

i2/i1= N1/N2 = V1/V2

N is number of windings

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29
Q

rectification

A

-current only flows when cathode is negative wrt anode

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30
Q

semiconductor diodes

A

-current flows if positive potential on p side and negative on n-side
-or else, current can’t flow
-current flows after depleted zone narrows to 0

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31
Q

bridge rectifier

A
  • a single diode uses only 1 pulse per AC cycle
    -bridge rectifier uses both AC lobes for xray
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32
Q

disadvantage of sine waves for xray tubes

A

-during portion of AC cycle when voltage is low, electrons are not accelerated sufficiently to produce xrays of high enough energies
-low energy xrays contribute heat, dose, and no info to exam
-mean intensity of radiation decreases since no useful radiation is generated for large part of AC cycle

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33
Q

3 phase current

A

-phase difference between any 2 of 3 lines is always 1/3 of variation of the voltage on a single line

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34
Q

DC voltage ripple

A

measure of how closely the xray tube’s potential mimics a DC potential
-for 3 phase, 12 pulse circuit, DC ripple is only 5 %

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35
Q

parts of xray power supply

A

AC- transformer - rectifier - smoothing- regulator

36
Q

what affects voltage ripple?

A

decreases with large capacitor
increases load current
decreases with frequency

output voltage changes with input voltage

37
Q

voltage regulator

A

keeps output voltage at constant value
-independent of load current and input voltage

38
Q

% ripple

A

(Vmax- Vmin)/Vmax
-single phase, 1 pulse, ripple = 100%
3 phase, 6 pulse, 13-25%
3 phase, 12 pulse, 3-10%
constant potential, < 2 %
medium-high frequency, 4-15%

39
Q

effect of kV ripple on xray spectrum

A

-average energy decreases as ripple increases

40
Q

6 factors that affect xray efficiency, quantity, quality, and exposure

A

-target material
-tube voltage
-tube current
-exposure time
-beam filtration
-generator waveform

41
Q

effect of kV on xray output

A

-exposure is proportional to kV^2
-exit exposure through a patient varies as 4th to 5th power of kV

42
Q

effect of tube current on xray output

A

-exposure is porportional to tube current

43
Q

effect of exposure time on xray output

A

of photons is product of current and exposure times

44
Q

what increases power rating?

A

-large focal spot
-high anode rotation speed
-massive anode
-large anode angle
-large anode diameter

45
Q

energy of generator

A

root mean square voltage X tube current X exposure time
Vrms = Vpeak/root2

46
Q

heat unit

A

-peak voltage X tube curent X exposure time X generator factor
-generator factor = 1 for single phase, 1.35 for 3 phase, 1.4 for constant potential

47
Q

falling load generator control circuit

A

-minimize exposure time while not exceeding tube’s capacity to absorb heat (decreases possibility of patient motion artifacts)
-works with AEC to deliver max possible mA for selected kV in shortest time

48
Q

AEC

A

-measures radiation and terminates exposure after treshold
-can prodive consistent exposure for differen patient size and attenuation
-backup timer ensures that if AEC fails, the patient doesn’t get large dose

49
Q

what is xray image map of?

A

photon survival probabilities

50
Q

5 factors that determine xray spectrum

A

-accelerating potential
-waveform
-target material
-tube and housing materials
-added filters

51
Q

electron density of bone vs muscle vs fat

A

electron density of bone is higher than muscle, and muscle higher than fat

52
Q

are narrow beam attenuation curves straight lines?

A

no, because of beam hardening

53
Q

f-factor

A

0.876 cGy/R * [uen/p) tissue to air

54
Q

skin entrance dose/BSF in diagnostic

A

skin entrance dose- largest dose
backscatter from inside patient needs to be accounted for

55
Q

magnification

A

SID/SOD

56
Q

size of off-axis radiation penumbra

A

F(M-1)
F= focal spot spize
M is magnification

57
Q

size of object motion unsharpness

A

delta m * magnification
m is motion

58
Q

size of focal spot motion unsharpness

A

delta m * (M-1)
m is motion
M is magnification

59
Q

size of blur at 3D contrast boundary

A

height * M/diameter
M is magnification

60
Q

3 zones of an image

A

shadow zone: object fully intercepts focal spot
light zone: none of focal spot is shadowed by object
penumbra zone: object intercepts a portion of focal spot

61
Q

focal spot vs magnification

A

-For M=1, focal-spot blur is insignificant, blur is due to detector
-For large M (every object detail is projected onto large area of detector), detector- induced unsharpness is negligible, focal spot blur dominates

62
Q

quantum detection efficiency

A

probability a photon incident on the receptor will be detected

63
Q

intensification

A

conversion of high energy xray photon (> 10 keV) into many light photons (2-3 eV)

64
Q

QDE of direct film (no screen)

A

< 0.25%
-with screen, QDE is > 50%

65
Q

latent image center

A

-introduce impurities into crystal structure to create sensitivity specks
-light photons or xrays interact with electrons in crystal
-electrons contact speck, Ag+ becomes Ag
-As more Ag atoms are formed, they combine as Ag2

66
Q

nucleation phase of image formation

A

-form Ag2

67
Q

growth phase of image formation

A

nucleation center acts as site for attracting other free electrons and mobile Ag+ ions, forming more Ag atoms

68
Q

film developer

A

-developer reduces Ag+ in latent image center to Ag

69
Q

4 stages of film development

A

-developing
-fixing
-washing
-drying

70
Q

what kind of recorder is film?

A

negative recorder
-region receiving most light appears darkest
OD = log(Io/I)

71
Q

high contrast film is what kind of latitude?

A

small latitude

72
Q

sensitometry

A

-chemical processing of film can result in underdevlopment or overdevelopment
-sensitometers ensure that development is working optimally for a given film type
-sensitometer exposes film to step series of different light intensities
-densitometer measures OD of film
-OD values compared against standard

73
Q

quenching

A

-non-radiative transition
-competes with luminescence

74
Q

quantum detection efficiency

A

-fraction of xray energy actually absorbed by screen to energy incident on screen (30-80%

75
Q

light conversion efficiency

A

-fraction of energy emitted as light photons to energy absorbed from incident photons (5-15%)

76
Q

light escape fraction

A

fraction of light photons created that escape the phosphor (50%)

77
Q

overall screen efficiency

A

product of QDE, energy absorption efficiency, light conversion efficiency, and light escape fraction

78
Q

intensification factor

A

-ratio of xray exposure w/o screen to that with screen for a given OD
-overall figure of merit

79
Q

impact factors on screen efficiency

A

-QDE increases as screen thickness increases
-QDE decreases as kVp increases (except for k edge)
-intensification factor increases as kVp increases
-energy absorption factor decreases as kVp increases

80
Q

how to increase QDE of screen

A

-increase phosphor thickness
-double emultion film between 2 screens

81
Q

how to increase light escape efficiency

A

-coat side of phosphor layer opposite of film with reflective coating to reflect light photons can toward film

82
Q

what happens to system’s contrast and resolution as object becomes smaller?

A
83
Q

rose model

A

SNR > 5 for detection
-SNR is proportional to contrast * diameter * square root of photon fluence

84
Q

minimum contrast required to see an object

A

10%

85
Q

why do scattered photons make the image worse?

A

-lost geometric correlation with focal spot
-energy discrimination not possible
-difficult to tell between scattered and primary photons

86
Q

for what energy is compton most probable interaction?

A

> 10 keV to 1 MeV

Imaging Review CCPM (11 decks)

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