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physics factoids > Xray and mammo > Flashcards

Xray and mammo Flashcards

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1
Q

Xray yield proportional to?

A

Z squared

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2
Q

Auger e heavy vs light elements

A

Heavy elements more likely x rays

Lighter elements more likely Auger

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3
Q

Secondary ionization electrons are called?

A

Delta rays

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4
Q

Why use plastic to shield beta emitters?

A

High Z = more bremstrahlung

Y90

Low Z plastic minimizes brems

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5
Q

mA vs kVp on intensity

A

mA = quantity, current at cathode

kVp = kinetic energy given to electrons, defines maximum energy

Xray production increases linearly with mA

increasing kVp by 15 % will double the intensity of spectrum

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6
Q

Heat math

tube power =

heat units =

A

Power = kV x mA

heat units = kVp x mA x seconds

130kv x 190mA = 24,700 watts or 24,700 J for a 1 second exposure

Watt = joule per second

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7
Q

“average energy”

affected by?

rough guess?

A

attenuation at target, exiting window, collimation

Standard Tungsten target with normal filtration, average energy is 1/3 - 1/2 the maximum energy (kVp)

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8
Q

kVp and mA spectrum changes

A

kVp moves the peak of the curve, increasing kVp increases average energy (and max energy)

mA increases size (area) of/under the curve

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9
Q

increased kVp and entrance skin dose

A

Entrance skin dose will change as the square of the change in kVp (tube voltage)

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10
Q

characteristic peaks move?

characteristic peaks go away?

A

Move = changing target material

Characteristic peaks disappear = kVp dropped below threshold for k shell e

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11
Q

HVL depends on?

DOES NOT DEPEND ON?

A

average photon energy (more energy = further)

higher Z target anode material GREATER HVL

More filtration, higher average energy GREATER HVL

Less filtration, lower average, LOWER HVL

mAs has no effect

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12
Q

10th HVL ?

A

“TVL”

attenuate 90%

used for shielding calcs

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13
Q

Average brems energy?

A

1/3 kVp selected

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14
Q

DEXA methods/trivia

relies on?

methods?

A

transmission measurements made at 2 different photon energies

filter that drops k-edge into middle of spectrum

switch tube voltage between low and high (70 and 140)

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15
Q

mammo focal spot size?

A

0.3 and 0.1 mm

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16
Q

general xray focal spot size?

A

0.6 and 1.2mm

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17
Q

limitation of portable xray anode?

A

usually stationary (doesn’t rotate), limits tube rating

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18
Q

Target angle and focal spot

A

SMALLER ANGLE = SMALLER FOCAL SPOT

(better spatial res)

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19
Q

heel effect worse on?

A

anode side

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20
Q

Heel effect worse with?

A

SMALLER angle

SMALLER SID

LARGER FOV (less uniform when spread out)

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21
Q

Mammo app of heel effect (position)?

A

Chest wall Cathode

Nipple Anode

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22
Q

mA/kVp and focal spot?

A

High mA, low kVp = WIDER

High kVp = SMALLER

more photons spread out more, blooming

higher kVp, moving faster, spread LESS

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23
Q

CLASSICAL/COHERENT interaction

what is?

energy change?

effect on dose?

A

low energy electron basically bouncing off an outer shell electron

NO ENERGY LOST

DIRECTION CHANGE

DON’T CAUSE IONIZATION

ADDS A TINY BIT OF DOSE

DOESN’T CONTRIBUTE TO IMAGE

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24
Q

Clinical setting of classical/coherent

A

LOW ENERGY, MOSTLY in MAMMO

15 % of photon interaction below 30keV

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25
Q

COMPTON “BAD GUY”

What is?

3 actors when its done

A

HIGH ENERGY incoming xray hits an outer shell e, knocks it out, loses some energy and heads off in new direction

IONIZED atom - bad

Free electron - bad, 2ary interactions

scattered photon bad - fogs image

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26
Q

COMPTON interactions depend on/probability of a ?

A

DOESNT depend on Z of the atom

DOES depend on density of material

DOMINANT contributor to scatter/fog

MAJOR SOURCE OF OCCUPATIONAL EXPOSURE

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27
Q

PE dominates at?

What is?

A

LOWER ENERGY RELATIVE to COMPTON

incoming xray gives all energy to inner shell e

ALL OR NOTHING, incomin xray is TOTALLY ABSORBED

–> characteristic xray or Auger (auger dominates in soft tissue, lower Z)

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28
Q

Energy of PE interaction?

A

need energy higher than inner shell binding

peak energy for PE is around this binding energy

Xrays with more energy pass through

INVERSE to THRID POWER

HIGHER Z makes PE more likely

ALSO INVERSE TO THIRD POWER

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29
Q

PE and image contrast

A

HIGHER Z = greater chance of PE = more xray absorption

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30
Q

Probability of PE and cubes

A

DIRECTLY PROPORTIONAL TO Z cubed

INVERSELY PROPORTIONAL TO ENERGY CUBED

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31
Q

K edge

A

spike in attenuation corresponding to the K shell binding energy

Lower energy are easily attenuated (no contrast)

Higher pass through everything (no contrast)

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32
Q

K edge barium iodine example

kVp selected?

A

K edge of barium = 37 keV, iodine = 33 keV

Select kVp of 65-90

average energy around these (1/3 - 1/2 kVp)

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33
Q

PEDS XRay

Grid?

kVp?

mAs?

A

NO GRID

lower kVp ~ 65 (kids are small, don’t need much to penetrate)

(adult cxr kVp = 120-140)

same or just less mAs

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34
Q

attenuation in tissue depends on 3 things?

A

Effective Z in tissue

X ray beam quality (energy)

Tissue density

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35
Q

ABOVE 30 kEv this interaction dominates?

BELOW?

A

Above = COMPTON

Below = PE

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36
Q

Noise

A

photon starvation

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37
Q

Factors that increase or decrease noise

Post processing?

A

Not a good answer. raw data is still crap

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38
Q

Factors that increase or decrease noise

Field size/collimation

A

smaller field/collimation decrease photons, increase noise

therefore mAs usually increased with collimation

(in this situation book says answer is collimation decreases noise)

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39
Q

Scatter primarily depends on ?

A

Collimation - less field less scatter

Thickness of body part

Energy of beam (Compton dominates >26kVp in soft tissue, 35kVp in bone)

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40
Q

Grid ratio

A

height to distance between

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41
Q

Grid effect on dose?

bucky factor?

A

increases dose (abc)

mAs with grid/mAs without (usually 2-3)

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42
Q

“Grid cut off”

A

too many photons blocked –> quantum mottle

grid not aligned/positioned correctly.

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43
Q

kVp and mAs with noise

A

both increase exposure but kVp going up will increase Compton scatter potentially more noise.

mAs better answer for decreasing noise

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44
Q

SDD and noise?

A

Increase in noise with increased SDD, described by inverse square law

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45
Q

noise vs mottle

A

noise includes scatter

mottle = photon starvation

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46
Q

Spatial resolution quant?

Spatial frequency?

Unsharpness?

A

line pairs per mm

spatial frequency = spatial resolution

unsharpness means loss of spatial res

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47
Q

Types of unsharpness

A

Motion (decrease exposure time)

System unsharpness = limiting factor of detector

Geometric unsharpness

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48
Q

System unsharpness

Film?

Computed radiography (CR) ?

Digital radiography (DR) ?

A

Film = size of the grain of photographic chemical

CR = size of laser used to read the phosphor plate

DR = size of individual thermoluminescent transistor

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49
Q

Geometric unsharpness (–> blur)

focal spot

SOD

ODD

MAG

A

focal spot - smaller = less blur

SOD - further = less blur

ODD - closer = less blur

mag –> blur

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50
Q

Magnification

formula?

A

SOD + ODD

SOD

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51
Q

MTF?

A

ability to maintain contrast as a function of spatial resolution

curve with MTF on Y and spatial res on X

As spatial res increases, MTF, ability to maintain contrast, decreases

52
Q

Nyquist frequency?

A

spatial resolution = half of nyquist frequency

How little you can sample something and still be able to tell what it is…

Harder and harder to keep up signal when looking at smaller and smaller line pairs (harder and harder to have contrast when looking at smaller things)

53
Q

Detective Quantum efficiency

A

estimate of required exposure level necessary to create an optimal image

“prediction of dose”

ideally = 1.0 (all radiation energy absorbed and converted to image) at any spatial resolution

Better the DQE, the less radiation needed to maintain spatial resolution

high DQE, low dose

54
Q

DQE factoids

A
  • Is a measurement of EFFICIENCY
  • Directly proportional to MTF
  • Inversely proportional to SNR
  • Better at LOW SPATIAL RES
  • Usually better, around 0.45 for DR, CR or film 0.25

HIGH DQE = HIGH DOSE LOW DQE = LOW DOSE

MORE JUICE PER SQUARE INCH to see fine details

55
Q

Spatial res and film

A

probably only advantage of film, made of a continuous thing, no pixels, small advantage in spatial res

56
Q

Random things that improve contrast

those that decrease scatter

A

Grid and collimation

57
Q

Image receptor contrast

digital imaging

A

Pixel depth = determined by number of bits

number of shades of gray available for imaging

more bits = more contrast resolution

58
Q

Window width and contrast

A

narrower window = more contrast

Level determines BRIGHTNESS

59
Q

Primary factor influencing contrast in plain film vs digital

A

Plain film primary factor for contrast is kVp

Digital systems primary factor for contrast is LUT

60
Q

Look Up Table?

A

histogram of known input intensities and corresponding grayscale

allows rescaling for adjustment to present optimal image (underexposed scaled darker, overexposed scaled lighter)

Means less important to adjust kVp when using digital

MEANS WIDER DYNAMIC RANGE IN DIGITAL SYSTEMS

61
Q

“Brightness”, film vs digital

A

FIlm, brightness means how many xrays are blocked

bones block a lot, they’re bright

Digital imaging, brightness = level

level up to see lung

62
Q

Dynamic range

A

narrow in film

easy to under or overexpose

Digital

larger dynamic range and linear response to exposure

63
Q

Dynamic range and dose

A

with digital

can underexpose a little and fix later

kVp less important, can go up by 15% and cut mA in half to decrease dose

64
Q

“pixel pitch”

A

distance from center of one pixel to another

65
Q

pixel pitch, density and spatial res

A

Better spatial res =

higher pixel density (pixels per unit area)

lower pixel pitch

66
Q

AEC mechanism

A

ionization chamber BETWEEN PATIENT AND RECEPTOR

charge produced in chamber, calibrated to know how much charge to terminate an exam based on body part

67
Q

AEC controls?

A

QUANTITY of radiation reaching receptor, NO effect on kVp

68
Q

Bit depth

A

number of bits determining number of shades of gray that can be displayed on a monitor

2 to the X number of bits = number of gray shades

69
Q

Classification of digital detectors

CR

DR

A

CR - storage phosphor (computer think laser), type of indirect

DR - Flat panel detectors direct or indirect

70
Q

Direct vs indirect

A

INDIRECT = SCINTILLATORS

XRAYS –> LIGHT –> CHARGE

DIRECT = photoconductors

XRAYS –> CHARGE

71
Q

CR = storage phosphor

mechanism

A

electons in phosphor interact with xrays, change to metastable state (in ‘conduction band’)

storage phosphor holds ‘LATENT IMAGE’

CASSETTE then docking station, readout

“photostimulable luminescense”

72
Q

Phosphor readout in CR

A

laser

red light laser liberates trapped electrons, return to their shells and release energy as BLUE-GREEN LIGHT

blue green light signal directed to a photodetector which then converts it to an electronic signal

signal digitized and divided into a matrix

73
Q

DR

no cassette

Flat panel detectors

A

what most people mean when they say “digital detector”

faster than CR or film

INDIRECT or DIRECT

74
Q

DR Indirect

A

INDIRECT THINK SCINTILLATOR

INDIRECT THINK LIGHT inbetween XRAYS and CHARGE

Xray activates Cesium Iodide

light emitted, converted by a photodiode into a charge

Charge captured and transmitted by Thin-Film-Transistor (TFT) to the workstation

75
Q

Lateral dispersion of light

INDIRECT method

A

LOSS of spatial res

phosphors

WORSE WITH GREATER THICKNESS OF CRYSTAL

76
Q

Types of indirect

A

CR = INDIRECT = PHOSPHOR

DR INDIRECT = CS scintillator (and photodiode, TFT)

77
Q

DIRECT think?

A

PHOTOCONDUCTOR

NO LIGHT IN BETWEEN

XRAY –> CHARGE

AMORPHOUS SELENIUM

78
Q

DIRECT rough mech

A

DIRECT THINK SELENIUM

charge applied across selenium, same direction as x rays

Xrays absorbed by selenium, electrons released, travel to surface of selenium and neutralize some of applied charge

NO LAT DISPERSION

charges are drawn to charge storage capacitor connected to TFT

pattern of charges scanned and converted to a digital signal

79
Q

“Fill Factor”

DR vs CR

A

Area of detector sensitive to Xrays (in relation to entire detector area)

HIGHER = more EFFICIENT

DR - electric field shaping allows near 100% fill factor

CR WORSE

80
Q

DQE

A

DR better fill factor = more efficient = HIGHER DQE

81
Q

Factors specific to Spatial Res of CR

SMALLER LASER?

THICKER PHOSPHOR?

SAMPLING FREQUENCY?

A

SMALLER LASER BETTER

THICKER PHOSPHOR WORSE

HIGHER SAMPLING FREQ = SMALLER PIXEL PITCH = BETTER

82
Q

Spatial res of DR

BETTER than CR WHY

A

avoid lateral dispersion

83
Q

Centralized vs Decentralized workflow

A

CR Centralized (Cassettes have to be brought somewhere)

DR Decentralized. image acquired, adjusted and transferred to PACS by the tech, in the room

84
Q

Ideal average energy for mammo

A

16-23 keV

therefore set kVp to 25-30 keV

85
Q

Mammo targets

A

molybdenum or rhodium (vs tungsten)

86
Q

K edge filtration

goal?

what’s filtered out?

A

goal = creating a mono-energetic beam

moly used, 20keV k edge, energies lower than 15 and higher than 20 filtered out

87
Q

Rhodium

A

similar, slightly stronger spectrum than moly

88
Q

Rh vs Moly

Denser breasts?

Mo anode with Rh filter?

A

Rh/Rh for denser breasts (higher energy)

Mo/Rh intermediate

89
Q

mammo focal spot size

effect on mA

A

.1 and .3 mm in mammo

general .6 and 1.2

smaller spot gets hotter, have to lower mA

mA 50 for 0.1 and 100 for 0.3

lower mA means longer exposure

90
Q

Spatial res trivia

Screen film mammo

A

15 lp/mm

91
Q

Spatial res trivia

digital mammo

A

7 lp/mm

92
Q

Spatial res trivia

Digital radiograph

A

3 lp/mm

93
Q

Spatial res trivia

CT

A

0.7 lp/mm

94
Q

Spatial res trivia

MRI

A

0.3 lp/mm

95
Q

Heel effect compensation in mammo

A

cathode side = chest wall

ANGLING TUBE UP TO ABOUT 20 degrees

96
Q

Effective anode angle = ?

A

Anode angle + tube angle

97
Q

mammo tube exit window vs general

A

General uses PYREX

Mammo uses Beryllium

PYREX attenuates energies used in mammo

98
Q

Compression

A

Less scatter = lower kVp can be used

lower kVp and less scatter = improved contrast

Thinner = less mAs, less dose

no motion

Closer to Bucky = less mag

less motion and mag = better spatial resolution

99
Q

mammo grid ratio

A

low kVp and compression already lower scatter, so lower grid ratio used

usually 4-5

standard xray 6-16

100
Q

Mammo and mag

A

distance to source cut in half –> double mag

101
Q

mag view air gap

A

NO GRID NECESSARY

increased object to detector distance means scatter scatters further away from detector

102
Q

mag view and focal spot

mA

exposure time

A

smaller focal spot used to improve spatial res

mA decreased so not to melt anode

exposure time increased, compensation for boob to detector distance

103
Q

Digital vs analog mammo dose

A

Digital - 1.6 mGy

analog - 1.8 mGy

104
Q

Dark noise

A

electronic fluctuations within the detector element

effect proportional to temp of detector

coolers

105
Q

flat field test

A

imaging a large piece of acrylic

improve image quality

calibrate digital detectors

106
Q

Digital artifacts

Ghosting

A

residual from prior exposure

lead is not allowed on flat panel digital systems

107
Q

Digital artifacts

Bad pixels

A

square or a streak

108
Q

PPV =

A

proportion of people who have a positive study and actually have breast cancer

PPV think all we care about is positive tests

positive test, has cancer

——————————————

positive test with cancer + positive test, no cancer

109
Q

PPV1 ?

benchmark

A

ANYTHING other than continued screening (BR0, BR3, BR4, BR5)

4.4% cancer within 1 year

110
Q

PPV 2 = ?

benchmark?

A

Cases where biopsy was recommended (BR4 or 5)

25.4% dx cancer within 1 year

111
Q

PPV 3 = ?

Benchmark

A

Results of biopsy, PBR pos biopsy rate

31%

112
Q

MAMMO MEM SHIT

target recall rate for audit

A

5-7%

113
Q

MAMMO MEM SHIT

cancers per 1000 screened

A

3-8

114
Q

MAMMO MEM SHIT

Processor QC

A

Daily

115
Q

MAMMO MEM SHIT

Darkroom cleanliness

A

DAILY

116
Q

MAMMO MEM SHIT

Viewbox conditions

A

WEEKLY

117
Q

MAMMO MEM SHIT

PHANOM EVAL

A

WEEKLY

118
Q

MAMMO MEM SHIT

Repeat analysis

A

Quarterly

119
Q

MAMMO MEM SHIT

Compression test

A

SEMI annually

120
Q

MAMMO MEM SHIT

darkroom fog

A

Semi annually

121
Q

MAMMO MEM SHIT

screen film contrast

A

SEMI annually

122
Q

MQSA

sites accredited and certified q?

MONEY TO?

A

3 years

MQSA = FDA

123
Q

Mammo education reqs

A

240 during a 6 month period in last 2 years

3 months of formal training

60 documented hours of mammo education

124
Q

Mean glandular dose

boob phantom spex

A

4.2 cm of compressed breast that is 50/50 adipose and glandular

125
Q

MQSA DOSE

A

UNDER 300 millirads

3 mGy

ONLY FOR PHANTOM

MEASURED WITH A GRID

WITHOUT GRID = 1mGy

126
Q

diagonal corduroy on a CR Xray

A

Moiré type artifact can be seen in computed radiography (CR) systems if grids with low grid frequencies are used that are oriented parallel to the plate readers scan lines.

127
Q
A

physics factoids (8 decks)

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