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Core > Physics: Rapid Review > Flashcards

Physics: Rapid Review Flashcards

1
Q

What is the approximate average energy of an xray beam?

A

1/3 kVp

Note: The beam is mostly (80%) due to Bremsstrahlung interactions creating xrays.

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2
Q

Which electron is ejected from an atom during characteristic xray production?

A

An inner shell electron

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3
Q

What is the binding energy for Tungsten that creates a characteristic xray peak?

A

70 KeV

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4
Q

What is the purpose of the glass enclosure/envelop of an xray tube?

A
  • Maintain a vacuum
  • Allow the amount and speed of the electrons to be controlled independently
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5
Q

What does the xray tube cathode do?

A

It is the filament that emits electrons to be accelerated towards the anode

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6
Q

What does the xray tube focusing cup do?

A

Help the electron beam strike an appropriately sized focus on the anode

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7
Q

What does the anode do?

A

Provide the target material (e.g. tungston) to be hit by the electron beam to make xrays

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8
Q

What happens if you increase the target anode atomic number (Z)?

A

You increase the quantity and quality of xrays produced

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9
Q

What happens if you increase the xray tube kVp?

A

You increase the quality and quantity of xrays produced

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10
Q

What happens if you increase the xray tube mAs?

A

You increase the quantity of xrays produced (but not their average energy)

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11
Q

What happens if you increase the voltage ripple in an xray tube?

A

You decrease the quantity and quality of xrays produced

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12
Q

What happens if you add filtration to an xray tube?

A

You increase the quality but decrease the quantity of xrays produced

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13
Q

On which side should you place thicker parts to minimize heel effect?

A

The cathode side

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14
Q

How should you change the xray tube angle to minimize heel effect?

A

Increase angle

Note: Smaller angles are associated with worse heel effect.

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15
Q

How does the focus to film distance (FFD) affect the heel effect?

A

Smaller FFD means less heel effect

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16
Q

How does the field of view affect the heel effect?

A

Smaller field of view (film) means less heel effect

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17
Q

For mammography, you should put the chest side on the…

A

Cathode side (to minimize heel effect)

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18
Q

How does increasing mAs affect the average energy of xrays produced?

A

It doesn’t change the average energy (it only results in a higher quantity of xrays produced)

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19
Q

How can you determine the kVp for a given target curve?

A

It is the maximum energy of xray photon produced (where the curve falls back to 0 on the x axis)

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20
Q

You lower the kVp and now no longer see a characteristic peak on the target curve…

A

You lowered the kVp below the binding energy for the target (no characteristic xrays can be produced)

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21
Q

What happened if the target curve now has a different characteristic xray peak?

A

You changed target material

Note: Tungston will always have a characteristic peak at ~70 KeV (as long as the kVp is set to 70 or above).

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22
Q

How does k shell binding energy change with atomic number?

A

Higher atomic number means higher k shell binding energy (need a higher kVp to produce characteristic xrays)

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23
Q

Compton interactions are a main contributor to…

A

Scatter/noise

Note: Compton scatter involves the outer shell electrons, whereas the photoelectric effect (which contributes to desired contrast) involves inner shell electrons.

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24
Q

Magnification is proportional to

A

SID/SOD

Note: Increased source-image distance and decreased source-object distance both increase magnification.

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25
Q

How can you decrease blur in radiographs?

A
  • Smaller focal spot
  • Bring detector as close as possible to pt (and source as far away as possible)
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26
Q

Detective quantum efficiency

A

An estimate of the required exposure level necessary to produce an optimal image (a measure of how efficient a detector is compared to an ideal detector)

Note: Higher DQE detectors will result in decreased doses to the pt.

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27
Q

Modulation transfer function

A

A measure of the relationship between sharpness (i.e. edge detection) and resolution (i.e. ability to discriminate between two close points)

Note: The higher the MTF, the higher the detective quantum efficiency of the detector.

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28
Q

As detective quantum efficiency increases, signal to noise ratio ______

A

Decreases (inversely proportional)

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29
Q

Detective quantum efficiency is better at low/high spatial resolution

A

DQE is better at LOW spatial resolution

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30
Q

What is the approximate detective quantum efficiency of digital radiography?

A

0.45 (almost half of the xrays hitting the detector are recorded)

Note: The DQE for plain film is worse (0.25).

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31
Q

If a tech wants to noticeably increase the brightness of a radiograph, they should…

A

Increase mAs by 30%

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32
Q

How does changing the kVp change the radiograph?

A

Lower kVp -> higher contrast

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33
Q

How does changing the mAs change the radiograph?

A

Higher mAs -> higher radiographic density (brighter image)

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34
Q

If you increase mAs by 50%, what should you change to maintain the same radiographic density?

A

Decrease the kVp by 15%

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35
Q

What is the main downside to using a grid in radiography?

A

Increased dose to the pt (more xrays needed to produce the same exposure)

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36
Q

What are common scenarios where you would not use a grid for radiography?

A
  • Babies
  • Extremities
  • Magnification (in mammography)
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37
Q

How can you reduce scatter (improve contrast in radiography)?

A
  • Collimate
  • Compress the body part
  • Lower kVp
  • Use a grid or air gap
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38
Q

What are the benefits of using collimation in radiography?

A
  • Increased contrast
  • Decreased scatter
  • Decreased dose (kerma area product)

Note: The main down side to collimation is decreased field of view.

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39
Q

What is the typical standard of care resolution for a digital display in radiology?

A

3 Mega pixels (3,000,000 pixels)

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40
Q

What is the major determinate of image contrast in film radiography?

A

kVp (lower kVp -> higher contrast)

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41
Q

What is the major determinate of image contrast in digital radiography?

A

Look up tables

Note: kVp still influences contrast, but since digital detectors have such a high dynamic range you can adjust the contrast by windowing.

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42
Q

What is the difference between the detector response curves in film vs digital radiography?

A

DR has a linear response curve (high dynamic range)

FR has a curvilear response curve (has a hard time discriminating between really low energy and really high energy photons, good contrast is only achieved in a narrow range of photon energies)

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43
Q

What are the major determinates of spatial resolution in digital radiography

A

Better resolution if:

  • Smaller pixels (detector elements)
  • Decreased pixel pitch (distance between pixels)
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44
Q

Indirect digital detectors

A

Indirect detectors (scintillators) convert the xrays into visible light, which then gets converted into electrical charge

Note: Direct detectors (photoconductors) convert xrays directly into electrical charge.

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45
Q

What is used as the scintillator in indirect digital detectors?

A

Thallium doped Cesium Iodide (CsI)

Note: This converts the xrays into visible light. The added step also decreases resolution (the thicker the crystal the more the light can scatter before actually reaching the digital detector).

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46
Q

What component converts xrays to electrical charge in direct digital detectors?

A

Amorphous selenium

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47
Q

What factors can improve spatial resolution for computed radiography?

A
  • Smaller laser size
  • Thinner phosphor plate (less light spreading)
  • Increased sampling frequency (which results in a smaller pixel pitch)
  • Smaller imaging plate (for a fixed matrix size CR system)

Note: Increasing the number of xrays available will not improve maximum spatial resolution.

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48
Q

Which has better spatial resolution: computed radiography or digital radiography?

A

Digital radiography (because the pixel detector is built into the DR flat panel)

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49
Q

What factors can improve spatial resolution for digital radiography?

A

Solely dependent on the size of the detector elements (smaller detector elements, better resolution)

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50
Q

What is the fill factor for direct conversion digital radiography?

A

Nearly 100%

Note: Indirect detectors only have a moderate fill factor.

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51
Q

What is the kVp used for mammography?

A

Low energy (25-25 kVp)

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52
Q

What is the kVp used for general radiography?

A

High energy (50-120 kVp)

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53
Q

What is the most common anode used in mammography?

A

Molybdenum

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54
Q

What is the most common anode used in general radiography?

A

Tungston

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55
Q

What is the mAs used in mammography?

A

100 mAs (low tube current)

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56
Q

What is the mAs used in general radiography?

A

500 mAs (high tube current)

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57
Q

What is a typical exposure time for mammography?

A

1 second/1000 ms (long)

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58
Q

What is the typical exposure time in general radiography?

A

50 ms (short)

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59
Q

How does the receptor air kerma differ in mammography compared to general radiography?

A

Higher receptor air kerma (100 micro Gy)

Note: General radiography is 5 micro Gy.

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60
Q

Instead of the pyrex glass window used in general radiography, mammography uses…

A

A beryllium window

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61
Q

How does the focal spot differ in mammography compared to general radiography?

A

Smaller focal spot than used in general radiography

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62
Q

How does the grid ratio differ in mammography compared to general radiography?

A

Lower grid ratio (5:1) than used in general radiography (10:1)

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63
Q

How does the optic density differ in mammography compared to general radiography?

A

Higher optic density than used in general radiography

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64
Q

How do the view boxes differ in mammography compared to general radiography?

A

Brighter (3000 cd/m^2) than in general radiography (1500 cd/m^2)

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65
Q

How do processing times differ in mammography compared to general radiography?

A

Longer processing times than in general radiography

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66
Q

What is used to reduce scatter in mammography?

A
  • A grid
  • Air gap without grid (for mag views)
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67
Q

What is the focal spot used for mag views in mammography?

A

0.1 mm (smaller than the 0.3 mm used for general mammography)

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68
Q

What changes for mag views in mammography?

A
  • Air gap instead of grid
  • Smaller focal spot (0.3 mm -> 0.1 mm)
  • Lower mAs (100 mAs -> 25 mAs)
  • Increased exposure time (1 sec -> 3 sec)
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69
Q

What target/filter should you use for larger/dense breasts?

A

Rh/Rh or Mo/Al

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70
Q

What target/filter should you use for intermediate density breasts?

A

Mo/Rh

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71
Q

What target/filter should you use for low density breasts?

A

Mo/Mo

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72
Q

What target/filter combination should never be used?

A

Rh/Mo

Note: Rh produces characteristic xrays of 21 kEv, which would get filtered by a molybdenum filter (K edge of 20 KeV).

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73
Q

MQSA requirement for PPV1 (abnormal screener call back)

A

Benchmark 4.4% (acceptable range is 3-8%)

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74
Q

MQSA requirement for PPV2 (recomendations for biopsy)

A

Benchmark 25.4% (acceptable range is 15-40%)

Note: The acceptable range increases if there is a palpable mass (to 25-50%).

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75
Q

MQSA requirement for PPV3 (percentage of biopsies performed that show cancer)

A

Benchmark 31% (acceptable range is 20-45%)

Note: The acceptable range increases if there is a palpable mass (to 30-55%).

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76
Q

What body is tasked with MQSA enforcement?

A

FDA

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77
Q

MQSA QA frequency for processor quality control

A

Daily

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78
Q

MQSA QA frequency for darkroom cleanliness

A

Daily

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79
Q

MQSA QA frequency for viewbox conditions

A

Weekly

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80
Q

MQSA QA frequency for phantom evaluation

A

Weekly

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81
Q

MQSA QA frequency for repeat analysis

A

Quarterly

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82
Q

MQSA QA frequency for compression test

A

Semi-annually (twice yearly)

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83
Q

MQSA QA frequency for darkroom fog

A

Semi-annually (twice yearly)

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84
Q

MQSA QA frequency for screen-film contrast

A

Semi-annually (twice yearly)

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85
Q

What is the appropriate target range for MQSA recall rate for a medical audit?

A

5-7%

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86
Q

What is the appropriate target range for MQSA # of cancers/1000 screened?

A

3-8

87
Q

How many mammograms do you have to read during training for MQSA?

A

240

88
Q

How long is the MQSA formal training requirement for mammography?

A

3 months (and 60 documented hours)

89
Q

Describe the MQSA phantom breast

A
  • 50% glandular tissue (50% fat)
  • 4.2 cm thick (compressed)
90
Q

What is the target dose for the MQSA breast phantom?

A

< 3 mGy (300 millirads)

Note: The phantom dose is performed with a grid.

91
Q

What is the typical mAs used for fluoro?

A

0-5 mAs

Note: This is much less than the 200-800 mAs used for general radiography.

92
Q

What is the typical kVp used for fluoro?

A

50-120 kVp (same as for general radiography)

93
Q

What is the typical focal spot used for fluoroscopy?

A

0.3-0.6 mm (smaller than the 1.0-1.2 mm used for general radiography)

94
Q

Fluoroscopy uses ______ exposure times compared to general radiography

A

Fluoroscopy uses longer exposure times

95
Q

Compared to a spot film, a “last image hold” in fluoroscopy has…

A
  • Lower dose to pt
  • More quantum mottle (less photons)
  • Lower spatial resolution (~2 line pairs/mm compared to ~3 line pairs/mm for a spot film)

Note: A digital spot is the same as a film spot (but doesn’t need a cassett).

96
Q

How do you achieve geometric magnification?

A

You decrease source-object distance

Note: SID/SOD (you could also increase source-image distance).

97
Q

How does radiation dose change as you move the pt closer to the source?

A

Increases by the square of the change in distance (1/3 the distance means 9x the radiation)

98
Q

How does operator dose change if you move the pt closer to the source?

A

Increases (scatter radiation isn’t blocked as efficiently)

99
Q

How does geometric magnification affect image quality?

A
  • Increased blurring
  • Decreased scatter (due to added air gap)
100
Q

How do you achieve electric magnification (i.e. zoom)?

A

You project a smaller field of view on the same matrix of detector elements (resulting in minification gain)

101
Q

How does electronic magnification affect pt radiation dose?

A

Increases air kerma (due to automatic brightness control compensating for fewer photons)

Note: Air kerma product does not change.

102
Q

How does electronic magnification affect image quality?

A

Increased resolution

Note: There is no increased blurring like there is with geometric magnification.

103
Q

What is the best pt positioning for fluoroscopy?

A

As close to the detector (II/image intensifier) as possible

As far away from the source (xray tube) as possible

104
Q

Where should the operator stand during fluoro?

A

On the same side of the pt as the detector (II)

i.e. opposite side as the xray tube/source

105
Q

How does doubling your distance from the xray source change your radiation dose?

A

Decreases it by a factor of 4 (inverse square law)

106
Q

What is the normal air kerma limit for fluoroscopy?

A

87 mGy/min (10 roentgens per min)

107
Q

What is the air kerma limit if using high level control fluoroscopy (for very large pts)?

A

176 mGy/min (20 roentgens per min)

Note: This is double the normal limit.

108
Q

What must you have in addition to normal precautions when using high level control fluoroscopy?

A

Audible or visual alarms (in addition to the normal timer alarm)

109
Q

What is pulse fluoro?

A

Using pulses of xrays rather than a continuous current (less radiation and less motion blur)

110
Q

Pulse fluoro results in less radiation dose as long as…

A

Frame rate is below 30 frames/sec

111
Q

If you lower the fluoroscopy frame rate from 30 to 15 fps, then radiation dose…

A

Decreases by 30%

Note: It is not a 1:1 relationship because lower frame rates have higher mAs per pulse.

112
Q

What is the best kVp to use during IR cases using IV contrast?

A

60-80 kVp (average beams hit the k-edge for iodinated contrast well)

113
Q

To maximize spatial resolution during IR fluoro cases…

A
  • Smaller focal spots
  • Smaller anode angles
114
Q

Are grids used for IR fluoroscopy?

A

Usually (not if pediatric pts or extremities)

115
Q

50% of the radiation dose during IR fluoroscopy is…

A

Delivered to the superficial 3-5 cm of skin/fat

116
Q

What is the concept of dose spreading?

A

Changing the angle of the gantry during fluoroscopy cases in order to spread out the radiation dose over a larger volume of tissue

117
Q

How does magnification affect radiation dose to the pt during IR fluoroscopy?

A

Increases air kerma (but not air kerma product)

118
Q

What percentage of the radiation dose received by the pt does an interventional radiologist receive standing 1 m away from the pt (not wearing lead)?

A

0.1% (1/1000)

119
Q

How do the mAs used in CT differ from general radiology?

A

Higher (up to 1000 mAs) compared to general radiography (200-800 mAs)

120
Q

How do the kVps used in CT compare to general radiography?

A

Higher but similar (80-120 kVp) compared to general radiography (50-120 kVp)

121
Q

What focal spot sizes are used for CT?

A

0.6-1.2 mm, often smaller than that for general radiography (1.0-1.2 mm)

122
Q

What kind of xrays are used for CT?

A

Highly filtered, high keV xrays (average energy is 75 keV)

123
Q

Why are bow tie filters used in CT?

A
  • Compensate for uneven filtration
  • Reduces scatter
  • Reduces dose to the pt
124
Q

What are the “septa” used in CT?

A

A grid

125
Q

What determines the minimal slice thickness for a CT?

A

The detector element aperture width

126
Q

How can you calculate the pixel size that a CT will create?

A

field of view/matrix size

127
Q

How will decreasing the kVp from 140 to 80 change the HU of a contrast-enhanced vessel?

A

Increased HU

Note: This increased HU with increased kVp is only seen with high atomic number substances (e.g. iodine) due to the higher k-edge.

128
Q

CT pitch of 1

A

No overlap between slices

129
Q

CT pitch greater than 1

A

Gaps between slices (the table moves faster than the xray beam), resulting in decreased spatial resolution

130
Q

CT pitch less than 1

A

Some overlap between slices (table moves slower than the xray beam), resulting in increased spatial resolution and increased radiation dose to the pt

131
Q

What windowing value should you change to change image brightness?

A

Window level (midpoint of the gray scale display)

Note: Set the window level at the attenuation level of the thing you want to see (e.g. high level for bone).

132
Q

What windowing value should you change to change image contrast?

A

Window width

Note: Narrow windows are better for comparing things of similar attenuation (e.g. white and gray matter).

133
Q

How do you increase contrast by windowing?

A

Make the window width more narrow

134
Q

Brain window values

A

Level +40 (width 80)

135
Q

Stroke window values

A

Level +30 (width 30)

136
Q

Lung window values

A

Level -400 (width 1500)

137
Q

Abdomen window values

A

Level +50 (width 400)

138
Q

Bone window values

A

Level +500 (width 1600)

139
Q

What is the benefit of prospective cardiac gating?

A

Reduced radiation dose (CT is only on at specific points in the cardiac cycle)

140
Q

What are the cons of prospective cardiac gating?

A
  • More motion artifact
  • No functional imaging
  • Need a slow heart rate
141
Q

Can you use a helical CT for cardiac gating?

A

Not for prospective cardiac gating (only axial CT)

142
Q

What dose of metoprolol is used for cardiac CT to reduce heart rate?

A

2.5-5.0 mg IV metoprolol

143
Q

What dose of nitroglycerine is used for prospective cardiac CT to dilate coronary arteries?

A

0.8-1.2 mg glycerol trinitrate

OR

5 mg isosorbide dinitrate

144
Q

What is the required heart rate for prospective cardiac CT?

A

50-65 bpm

145
Q

Contraindications for metoprolol use in cardiac CT

A
  • Bradycardia (HR < 60)
  • SBP < 100
  • Decompensated heart failure
  • Asthma (and on beta-agonist inhaler/albuterol)
  • Active bronchospasm
  • Severe COPD
  • 2nd or 3rd degree AV block
146
Q

Treatment for metoprolol overdose during cardiac CT

A
  • Fluids (careful if CHF)
  • Atropine
  • Glucagon
147
Q

What dose of atropine should be used for metoprolol overdose during cardiac CT?

A

0.5 mg IV atropine (can be repeated up to a total of 3 mg)

148
Q

What dose of glucagon should be used for metoprolol overdose during cardiac CT?

A

50 micrograms/kg IV loading dose, followed by 1-15 mg/hour infusion

149
Q

Contraindications for nitroglycerine during cardiac CT?

A
  • Viagra/cialis within 48 hours
  • Severe aortic stenosis
  • Hypertrophic cardiomyopathy
150
Q

PDE-5 inhibitors should be held for ______ prior to a cardiac CT

A

48 hours (2 days)

e.g. sildenafil/tadalafil

151
Q

How will decreasing the CT field of view affect resolution?

A
  • Improved spatial resolution
  • Decreased contrast resolution (fewer photons per detector element)

Note: Smaller FOV means smaller pixel size (pixel size = FOV/matrix size).

152
Q

How do reconstruction filters affect resolution?

A

Bone algorithm gives better spatial resolution

Soft tissue algorithm gives better contrast resolution

153
Q

A ______ focal spot will increase spatial resolution

A

Smaller focal spot

154
Q

A ______ detector element width will increase spatial resolution

A

Smaller detector element width

Note: This determines spatial resolution in the craniocaudal dimension.

155
Q

A ______ CT pitch will increase spatial resolution

A

lower (<1) CT pitch

Note: If >1 there will be gaps between slices.

156
Q

A ______ CT pitch will increase slice sensitivity profile

A

higher (>1)

157
Q

What is the downside to a lower CT pitch?

A

Increased radiation dose (more slice overlap)

158
Q

How can you improve contrast resolution by changing tube voltage and current?

A
  • Increase mAs (more signal, less mottle)
  • Decrease kVp (less scatter, less noise)
159
Q

Decreasing CT mAs by 50% will ______ radiation dose by ______

A

Decrease radiation dose by 50%

160
Q

Increasing CT pitch by 50% will ______ radiation dose by _______

A

Decrease radiation dose by 50%

161
Q

Which type of reconstruction method allows for reduced pt radiation doses?

A

Iterative reconstruction (handles noise better allowing for lower dose protocols)

162
Q

Increased CT rotational speed leads to _______ radiation dose

A

Decreased radiation dose

163
Q

How do you fix this artifact?

A

Call manufacturer to replace the detector (Ring artifact is a detector problem)

164
Q

How can you decrease partial volume artifact?

A

Thinner slices (decrease beam width, increase beam collimation)

165
Q

How can you decrease stair step artifact?

A
  • Thinner slices (decrease beam width, increase beam collimation)
  • Reconstruction with overlapping intervals
166
Q

How can you fix this artifact?

A
  • Reposition (e.g. move arms up)
  • Increase kVp

Note: Beam hardening artifact is due to the selection for higher energy xrays as the beam moves through the pt.

167
Q

How can you decrease metal artifact?

A
  • Increase kVp
  • Thinner slices
  • Interpolation software
168
Q

How can you decrease photon starvation artifact?

A
  • Increase automatic tube current modulation (i.e. increase mAs)
  • Use adaptive filtration (to smooth the data)

Note: Photon starvation artifact is increased quantum mottle because not enough xrays are reaching the detector.

169
Q

How can you decrease motion artifact?

A
  • Decrease scan acquisition time
  • Over scan (add an extra 10% to each 360 degree rotation and take the average)
  • ECG gating
170
Q

Ultrasound reflection occurs at

A

The interface between tissues with different acoustic impedences

171
Q

Ultrasound refraction occurs at

A

The interface between tissues with different speeds of sound

172
Q

Ultrasound refraction is dependent on…

A
  • The change in speed of sound
  • The angle of incidence
173
Q

How do higher frequency probes affect scatter?

A

More scatter

174
Q

How do higher frequency probes affect attenuation?

A

More attenuation (can’t see as deep)

175
Q

The functional part of the ultrasound probe is the…

A

Piezoelectric crystal

176
Q

A thicker piezoelectric crystal results in a…

A

Lower frequency US probe

177
Q

Why would you use a high Q dampening block for ultrasound?

A

To preserve velocity information during Doppler ultrasound

178
Q

Why would you use a low Q dampening block for ultrasound?

A

For higher axial resolution (heavy dampening with broad bandwidth)

179
Q

What does the matching layer do in ultrasound?

A

Minimizes the acoustic impedence differences between the transducer and the pt

180
Q

A higher frequency transducer has a ____ near field

A

Longer near field

181
Q

The focal zone on an ultrasound machine maximizes…

A

Lateral resolution

182
Q

How can you improve tissue penetration during ultrasound?

A

Switch to a lower frequency probe

183
Q

How can you brighten deep structures during US?

A

Increase time gain compensation

184
Q

How can you improve axial US resolution?

A
  • Shorter pulses (smaller spatial pulse length)
  • Greater dampening (Low Q)
  • Higher frequency probe
185
Q

How can you improve lateral ultrasound resolution?

A
  • Adjust focal zone
  • Increase the line density of the probe
  • Higher frequency probes (less beam spreading)
186
Q

How can you improve elevational ultrasound resolution?

A
  • Use a thinner crystal
  • Minimize slice thickness
  • Use a fixed focal length across the surface of the array
187
Q

What is the purpose of the stand off pad in ultrasound?

A

Helps place superficial things in the focal zone (improving lateral resolution)

188
Q

Axial resolution depends on…

A

Spatial pulse length

189
Q

Lateral resolution depends on…

A

Transducer element width

190
Q

Elevation resolution depends on…

A

Transducer element height/thickness

191
Q

What type of ultrasound resolution gets worse the deeper you go?

A

Lateral resolution (due to beam spreading)

Note: Axial resolution stays the same.

192
Q

What type of resolution is improved by using harmonics?

A

Lateral resolution

193
Q

How does ultrasound harmonics work?

A

Transmitting at one frequency and receiving at another (harmonic) frequency

194
Q

How does ultrasound compound imaging work?

A

Using electronic US beam steering to image an object in multiple different directions

195
Q

Where will US harmonics not be helpful?

A

Near field (US waves haven’t traveled far enough)

196
Q

What is the downside to using US harmonics?

A

Decreased depth penetration (higher frequencies attenuate more quickly)

197
Q

What technique can you use to decrease posterior acoustic shadowing?

A

Compound imaging

Note: Compound imaging will also make a cyst look more solid.

198
Q

How does ultrasound compound imaging affect edges?

A

It sharpens them

199
Q

What is the ultrasound thermal index?

A

The maximum temperature rise in tissue secondary to energy absorption

200
Q

What is the ultrasound mechanical index?

A

How likely that cavitation will occur (considering peak rarefaction pressure and frequency)

201
Q

What should be avoided with neonatal imaging?

A

Pulsed spectral doppler (high risk of cavitation)

Note: Use M-Mode instead to document fetal heart rate.

202
Q

The ultrasound thermal index should be under…

A

1.0 (some say under 0.7)

203
Q

What is the ideal Doppler angle?

A

30-60 degrees (but the lower the better)

204
Q

Doppler angle of 90 degrees

A

Will not show any flow

205
Q

If you are looking for slow flow using Doppler…

A
  • Use power Doppler
  • Use a low pulse repetition frequency
206
Q

Power doppler does not depend on…

A

The Doppler angle

207
Q

What artifact will not be demonstrated on power Doppler?

A

Aliasing

Note: Color and spectral Doppler both can show aliasing.

208
Q

When does ultrasound aliasing occur?

A

When the doppler shift is too high (greater than the Nyquist frequency)

209
Q

How can you reduce ultrasound aliasing artifact?

A
  • Increase the scale
  • Increase the pulse repetition frequency
  • Use a lower frequency transducer
  • Increase the Doppler angle (closer to 90)
210
Q

Reverberation artifact is due to

A

Two parallel highly reflective surfaces

211
Q

Comet tail artifact is due to…

A

Two parallel highly reflective surfaces that are very close together (<1/2 the spatial pulse length)

212
Q

Ring down artifact is due to…

A

Fluid trapped between a tetrahedron of air bubbles

213
Q

Mirror image artifact is due to…

A

US waves getting trapped behind a strong reflector

Core (98 decks)

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