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VetRad Boards prep > Physics - rad > Flashcards

Physics - rad Flashcards

1
Q

What is the associated voltage ripple for each type of generator?

A
  • Single phase = 100%
  • Three phase
    • 6 pulses = 14% (13-25% on image)
    • 12 pulses = 4% (3-10% on image)
  • High frequcncy = 4-15%
  • Constant potential = 2%
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2
Q

How do Step up and step down transformers compare?

A
  • Step up: Ns>Np
    • increases voltage, decreases amperage
  • Step down: Ns
  • decreases voltage, increases amperage

This is because:

  • ratio of voltage output = the ratio of number of wire turns
  • N is the numbr of wire turns
  • Input power (voltage x current) = output power
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3
Q

How is power calculated and what are the si units?

A
  • Watt (W) is the si
    • W = J/s
  • power = voltage (v) x current (i)

Thus

  • 1W = 1 volt x 1 ampere (1 ampere = 1 coulomb(C)/s)
  • so, V=J/C via algebra
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4
Q

What is the SI unit of energy?

A

Joule (J) = 1W x 1 sec

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5
Q

How do you calculate heat unit (HU) for three phase and high frequency generators? What is the correction factor multiplier?

A

HU = kVP x mA x s x fcorr

the correction factor (fcorr) = 1.35-1.4

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6
Q

What is the relationship b/w the joule and the heat unit?

A

HU = 1.4 x heat input (J)

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7
Q

What is the federal leakage regulation?

A

< 0.1 R/hour @ 1 meter from focal spot

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8
Q

What is the relationship between the focal spot, focusing cup and filament length?

A

The focusing cup determind the focal spot width

The filament length determines the filament length

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9
Q

How do you decrease the heel effect?

A
  • Use a larger anode angle
  • use a smaller film or collimate more
  • Use a larger SID
  • position cathode over thicker tissue and anode side over thinner tissue
  • (potentially increase kVp according to Watson’s noted)
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10
Q

In regard to the space cloud, what does “space charge limited” and “emission limited” mena?

A
  • <40 kVp = space charge limited - not all electrons pulled to the anode, tube current insufficient to pull all electrons away from the cloud
  • >40 kVp = emission limited - all electrons pulled to anode, total # electrons (tube current) is determined by the filament current - further increase in kVp will not change tube current
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11
Q

What increases Bremsstrahlung w-ray production?

A

increased kVp and increased Z

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12
Q

What influcences x-ray quantity?

A
  • Directly proportional to mAs - mA double, quantity doubles
  • increase kVp, incr qauntity (and quality)
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13
Q

Accodring to WarMachine, what things increase quantity? page 40

A
  • Increased Z (more overall x-rays via Bremsstrahlung)
  • Increased kVp
  • Increased mAs
  • Increased voltage ripple (more overall Bremsstrahlung)
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14
Q

What influences quality?

A
  • increased kVp
  • increased filtration
  • decreased voltage waveform ripple (less ripple in tipple = better quality)
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15
Q

How do you decrease magnification?

A
  • Use a small focal spot
  • Use a small anode angle
  • Minimize the OFD(OID) → b
  • Maximize the FFD(SID) → a+b
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16
Q

How do you decrease the prenumbra?

A
  • use small focal spot
  • small object to image distance
  • large focus to object distance
  • place ROI on anode side

because,

P = ( F x OID)/SOD → F is the focal spot size

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17
Q

How do you calculate true magnification? And when does true mag (M) equal mag (m)?

A

M = m + (m-1)(f/d)

M= true mag

m=mag

f=focal spot size

d=object size

Note: when f<<<

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18
Q

WHat influences the probability of photoelectric effect occuring?

A
  • increases at energies just above the BE of the k shell (k-edge)
  • rapidly decreases at energies much above k-edge
  • Inversely proportional to the incident photon energy cubed: 1/E3
  • probability directly proportional to the anatomic number cubed: Z3
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19
Q

What is the k-edge for iodine?

A

33.2

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20
Q

What is tha k-edge for barium?

A

37.4

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21
Q

Does compton scatter occur with inner or outer shell electrons?

A

outer shell

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22
Q

How do you calculate wavelength change in compton scatter?

What is the relationship between the angle of deflection and the enegery of the scattered photon?

A
  • change wavelength = 0.024(1- cosø)
  • As the angle of defelction decreases, the energy retained by the scattered photon increases
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23
Q

What increases the probability of Compton scatter?

A
  • directly proportional to density
  • increases with low binding energy electrons
  • Dominates above 30 kEv
    • PE= compton at 25 kEv
  • proportional to 1/E
  • INDEPENDENT of Z
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24
Q

WHat is a major difference between linear attenuation coefficent and mass attenuation coefficient?

A

LAC is effected by density, MAC is not

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25
Q

What increases LAC (µ)?

aka what increases absorption?

A
  • Increased Z of absorber
  • increased physical density of absorber
  • decreased photon energy
  • K-edges
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26
Q

What is the half value layer?

A

thickness of material that attenuated the x-ray beam by 50%

(qauntifies the ability of the beam to penetrate tissue and therefore described the quality of the beam)

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27
Q

What factors affect subject contrast?

A
  • Increased kVp - lower contrast
    • lower kVp = higher contrast
  • Density difference = higher contrast
  • Z difference = higher contrast
  • Thickness difference = higher contrast
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28
Q

How does FWHM (full width at half max) relate to spatial resolution?

A

the smaller the FWHM, the better the spatial resolution

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29
Q

What is the approximate spatial resolution of film (no screen) and film w/ screen?

A
  • film no screen - 100 lp/mm
  • screen/film - 5-8 lp/mm
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30
Q
  • What is an advantage of film screens?
  • What are some disadvantages?
A

Advantage

  • captures 60-65% of x-rays, reducing mAs needed and dose to patient

Disadvantages:

  • loss of spatial resolution due to lateral liight diffusion
  • increased quantum mottle/noise (few photons)
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31
Q

What scintillator is now most commonly used in screens (film screen)?

A

Gadolinium oxysulfate (Gd2O2S)

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32
Q

How does light absorbing dye impact total conversion efficiency in screen/film?

A

reduced lateral speading of light (increases spatial resolution) but decreases conversion efficiency

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33
Q

How do reflective layers effect total conversion efficiency in screen film?

A

reduced spatial resolution but increase conversion efficiency

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34
Q

What is QDE?

A
  • quantum detection efficiency = abosrption efficiency
    • fraction of incident x-rays that are detected and interact with the screen
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35
Q

What is conversion efficiency (film screen)?

A

the fraction of the absorbed energy that is emitted as light

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36
Q

WHat factors affect QDE?

A
  • Z
  • E (k-edge)
  • packing fraction
  • phosphor coat weight
  • increased screen thickness - incr QDE (leads to reduced spatial resolution)
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37
Q

How do you increase film screen speed?

A

speed of system = QDE x CE

  • increased QDE
    • if achieve by using thicker screen, reduced resolution
  • increase CE
    • leads to more noise if increase CE
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38
Q

What is Modulation transfer function?

A

ratio of output modulation to the input modulation expressed as a function of spatial resolution (bascially what % of the input signal made it through the system)

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39
Q

WHat is the useful optical density range?

A
  1. 3 - 2.0
  2. 3= 50% transmittance
  3. 0 = 1% transmittance
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40
Q

How is OD related to transmittance?

A

OD represents the log reciprocal of transmittance (It/I0):

OD=log10 (I0/It)

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41
Q

Which OD/transmittance provides good contrast?

A

5% = 1.3 OD ⇒ good contrast

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42
Q

What is the OD of unexposed film?

A

0.11-0.15

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43
Q

For the film characteristic curves, how to the steep and flatter slopes affect contrast?

A
  • Steeper slope = narrow lattitude (narrower ranges of accepatble exposures
    • high contrast film
    • low kVp
  • Flatter slope = wider range/latitude
    • low contrast
    • high kVp
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44
Q

What effects does increasing the grid ratio have?

A
  • increases degree of collimation
  • cleans up more scatter
  • allows for more radiation dose to the patient
45
Q

What grid frequency needs to be used to avoid a Moire pattern?

A

>60 lp/mm

46
Q

How do you calculate grid ratio?

A

Heigh/interspace width

47
Q

What is the Bucky factor?

A

B = incident radiation onto grid ÷ transmitted radiation

48
Q

What affects B factor?

A
  • Increased grid ratio = increase B factor
    • e.g. 12:1 grid has B of 5, while 5:1 has B of 2
      • increased scatter absorption, need to incr mAs
  • Increased kVp= increased scatter = need for increased B factor

Increased B factor = Increased exposure factors = increased patient dose

49
Q

What factors influence the magnitude of lateral decentering grid cutoff?

A
  • increased grid ratio
  • decreased focal distance
  • increased lateral decentering distance

when exact centering not possible (such as with portable radiography) - use low ratio grid and long focal distance

50
Q

What increases the amount of grid cutoff?

A

high grid ratios and short grid-focus distances

51
Q

What is the difference between near and far focus grid cut off and which one has greater cut off?

A
  • Near focus grid - target is below the convergent line
  • Far focus grid - target is above the convergent line

Near focus will have higher magnitude of cutoff

52
Q

What does the resulting image look like with focus- grid distance decentering?

A

light periphery and unaffected center

53
Q

In combined lateral and focus-grid distance decentering, how do the images appear for near and far?

A
  • Lateral + far focus-grid decentering
    • cutoff is greatest under x-ray tube
    • so lighter film right under tube (the near side)
  • Lateral + near focus-grid decentering
    • cutoff is greatest on the oppposite side from the tube
    • so lighter film on the far side
54
Q

In lateral + focus-grid distance decentering, what affects the magnitude of cutoff?

A
  • cutoff propotional to:
    • grid ratio
    • decentering distance
  • cutoff inversely proportional to:
    • focal distance
55
Q

Increase in gap width increases blur (reduces spatial resolution). How can you compensate for this?

A

increase the SID (FFD)

56
Q

What are advantages of pulse mode?

A
  • improves temporal resolution (reduced blur)
  • reduces patient exposure
57
Q

In flouroscopy, what is the usual electron gain? (# of photons generated at the output phosphor for every photon generated at the input phosphor)

A

50

58
Q

In flouro, at the output phosphor, what type of light are electrons converted to? And what is the typical luminence gain?

A

Green light (530 nm) ⇒ 1 e- produces about 1000-2000 visible light photons

59
Q

What do the charged electrodes in the flouro vaccum do?

A
  • focus the electrons
  • accelerates the electrons to the anode creating and electron gain
  • intensifies and minifies the electron beam
  • Does NOT increase the # of electrons recived at the output phospher
60
Q

In flouro, when magnifying, what happens to dose, spatial resolution and brightness?

A
  • doubling magnification quadruples dose
  • increases spatial resolution
  • decreases brightness (need ABC to overcome)
61
Q

In flouro, what component is the limiting factor for spacial resolution?

A

the image intensifyer (about 4-5 lp/mm)

62
Q

How does QDE of an image intensifier compare to flat panel?

A

1.5 mmAl reduceds II QDE compared to the flat panel

63
Q

How does the contrast and temporal resolution of flouro compare to radiography?

A
  • contrast ⇒ low compared to radiography
  • temporal ⇒ high compared to radiography
64
Q

In flouro, what does the contrast ratio measure?

A

measures the veiling glare - 15:1 to 30:1 in most machines

65
Q

How does increasing kVp effect veiling glare?

A

Increased kVp will reduce contrast due to increased x-ray and light diffusion inside the II

66
Q

what does a Byte equal?

A

Byte = 8 bits (28) ⇒ 256 combinations

67
Q

How do you calculate pixel size?

A

pixel size = FOV ÷ # of pixels

68
Q

What are the ACR recommendations for minimum pixel number and minimum spatial resolution?

A
  • minimum 10 bits per pixel
  • minimum spatial resolution of 2.5 lp/mm
69
Q

How many bits per pixel are required for ultrasound?

A

6-7 bits

70
Q

How many bits per pixel are required for x-ray, CT and MR?

A

12 bits required to represent full range of CT numbers

71
Q

In CR, what is the photostimulable phospher detector compsed of?

A

Barium flourohalide (BaFBr and BaFl) doped with europium (Eu)

72
Q

When read out by the red laser, what light energy is released from the CR plate?

A

blue/blue-green light

73
Q

In indirect flat panel detectors, what determines spatial resolution?

A
  • largely determined by the size of the TFT
    • TFT determines the size of the pixel ⇒ the smaller the pixel, the greater the spatial resolution
  • also affected by lateral light diffusion in the scintillator
74
Q

What is fill factor and how does this effect contrast resolution?

A
  • ratio of light sensitive area to the entire area of each detector element
  • lower the fill factor, the lower the contrast resolution
    • smaller TFT (pixel size) causes increased SR but decreased SNR
75
Q

What accounts for Direct flat panels higher QDE?

A

thick selenium layer → much thicker than the silicon layers in the indirect systems

76
Q

What are advantages of direct DR over indirect DR?

A
  • High directionality of the electrons induced by the applied voltage reduces blurring (no lateral diffusion)
  • increased fill factor (incr SNR)
77
Q

What is DQE?

A
  • combines spatial resolution (MTF) and image noise to provide a measure of the signal to noise ratio of the various frequency components of the image
  • best objective indicator of image fidelity
78
Q

How do you calculate DQE?

A

DQE = SNRoutput/SNRinput

79
Q

What is the principal advantage of DR?

A

Decouples image acquisition and display ⇒ allows for a large dynamic range and linear response to x-ray exposure

80
Q

How do CR and DR compare as far as patient exposure/dose?

A
  • DR can reduse dose 2-3x in comparison to CR
    • due to DR’s incr. quantum absorption and incr conversion efficiency
    • at least equal to 400-800 speed film/screen systemts
81
Q

What are advantanges of flat panel detectors (IDR, DDR) in comparison to CCD DR?

A

Flat panel DR

  • incr dynamic range
  • inr spatial resolution (incr MTF)
  • dose reduction

CCD DR

  • increased SNR (100% fill factor)
  • small size of unit
82
Q

What processes involve raw datat manipulation?

A
  • Unsharp masking filter
  • Multiscale processing
83
Q

What artifact can occur with Unsharp masking?

A

edge enhancement artifacts (dark halo)

84
Q

How do unsharp masking and multiscale processing differ?

A

in MC, image is decomposed into multiple spatial frequncy bands (multiple sub images) based on sturcture size, unlike UM which decomposes into only 2 sub images

85
Q

Regarding kernal sizes, which one (1) results in edge enhancement, (2) results in dynamic range compression and better detail, (3) results in enhancemnt of very small structures?

A
  1. medium
  2. large
  3. small
86
Q

What tyoes of processing do NOT involve manipulation of raw data?

A
  • Histogram manipulation
  • LUT adjustments
87
Q

In regard to post porcessing windowing/leveling, what does viewing with a “wide window” or a “narrow” window really mean?

A
  • wide window - more latitude, less contrast
  • narrow window - less latitude, more contrast
88
Q

What is a ACR minimum requirement for monitor luminence?

A

171.3 cd/m2

89
Q

How does the spatial resolution of medical grade and commecial monitors compare?

A
  • medical grade - 2-5MP (1200x1600 to 2560x2048)
  • commercial - 0.75-2MP (1024x768 to 1200x1200)
90
Q

In regoard to monitors, what decreases contrast ratio (dynamic range?

A
  • increased ambient light
  • wider angle viewing
91
Q

What is the ACR recommended illuminence (ambient light)?

A

2-10 lx (lux)

92
Q

WHat does PACS stand for?

A

Picture archiving and communication systems

93
Q

What are DICOM interconectivity needs/requirements?

A
  • Internet protocol (IP) address
  • Application entitiy (AE) title
  • Communication port
94
Q

How do you calculate storage capacity?

A

matix size x #bytes/pixel x # images

95
Q

How do you calculate sensitivity?

A

TP/(TP + FN) x 100

96
Q

How do you calulate specificity?

A

TN ÷ (TN + FP) x 100

97
Q

What is sensitivity?

A

The ability to detect disease - a sensitive test has a low false-negative rate

98
Q

What is specificity?

A

The ability to identify the abscence of disease - a specific test has a low false-positive rate

99
Q

How do you calculate the PPV?

A

TP/(TP + FP) x 100

100
Q

How do you calculate the NPV?

A

TN/(TN + FN) x 100

101
Q

How do you calculate subject contrast and how does kVp effect this?

A
  • Cs = 1 - e-uz
  • Decreasing kVp ⇒ inc u (linear attenuation coefficient) ⇒ increases Cs
    • ​(decreasing x-ray energy will result in an increase in subject contrast)
102
Q

What factors affect subject contrast?

A
  • Density difference
  • Thickness difference
  • Atomic number
    • PE creates contrast, predominates at higher Z (and lower kVp)
  • Radiation quality
    • low kVp - high subject contrast→narrow latitiude
103
Q

OD base + fog should never be above what?

A

0.2 (usually between 0.13 - 0.18)

104
Q

What is the film gamma?

A

maximum slope of the H&D curve at its linear portion, primarily determined by film type and processing

105
Q

How do the characterisitc curves of DR and CR compare to film?

A

NOT sigmoidal

  • Flat panel = linear
    • contrast level is independent of exposure
  • CR = logarithmic
106
Q

How can you reduce scatter?

A
  • air gap
  • grid
  • reduced FOV
  • compression (decrease patient thickness)
  • narrow beam geometry
  • deacrease kVp
107
Q

What is sampling pitch?

A

refers to the spacing between discrete detectors (1/2 lp) from central points on the detector element

108
Q

What is the Nyquist frequency?

A
  • If you have a frequency (f), then in order to sample this frequency correctly the sampling frequency must be at least 2 x f
    • thus, maximum measurable frequency is half the sampling frequency (pitch)

Nyquist frequency = 1/(2 x sampling ptich)

109
Q
A

VetRad Boards prep (11 decks)

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