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Echo > Physics > Flashcards

Physics Flashcards

1
Q

Specular reflection

A

Mirror-like
Structures >1 wavelength in diameter
Angle of incidence = angle of reflection

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2
Q

Specular reflection and frequency

A

Independent of frequency

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3
Q

Diffuse reflection

A

Omnidirectional
Structures <1 wavelength in diameter
Frequency dependent, more scattering with higher frequency

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4
Q

Interference

A

Algebraic sum of wave amplitude

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5
Q

Speckle

A

Noise in US images produced by interference pattern from multiple small reflectors (scatterers)

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6
Q

Refraction

A

Bending of wavefront as sound passes between media with different propagation velocities
Snell’s law

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7
Q

Snell’s law

A

Sin Angle I / Sin angle T = V I / V T

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8
Q

Diffraction

A

Spreading or divergence of sound beam

Greater with smaller source

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9
Q

Absorption

A

Sound energy converted to other forms of energy like heat

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10
Q

Attenuation

A

Loss of intensity as sound wave passes through medium

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11
Q

Soft tissue attenuation

A

0.5-1.0 dB/cm/MHz

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12
Q

Resonance

A

Air bubbles resonate within ultrasound field, ring like a bell
Persistent oscillation produces continued ultrasound signal

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13
Q

Nonlinear behavior

A

Distortion of sound wave by interaction with media

Can be described as sum of sine waves at frequencies that are multiples of principle frequency (harmonics)

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14
Q

Harmonics generated strongly by

A

Microbubbles

Compression and expansion not equal

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15
Q

Harmonic generated weakly by

A

Tissue

Velocity of sound higher during compression than rarefaction

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16
Q

Transducer components

A

Piezoelectric crystal
Backing block
Quarter-wavelength matching layer

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17
Q

Piezoelectric crystal

A

Vibrates with applied current, most efficient at resonant frequency
Transmitter and receiver

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18
Q

Backing block

A

Limits ringing of crystal to create discrete US pulses

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19
Q

Quarter-wavelength matching layer

A

Provides better acoustic impedance matching between crystal and skin

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20
Q

Mechanical transducer

A

Controlled movement of single element to produce 2D image

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21
Q

Phase array transducer

A

Multiple crystal elements to produce 2D or 3D image

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22
Q

Narrow bandwidth transducer

A

High Q factor
Crystals allowed to ring freely
More efficient / sensitive

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23
Q

Wide bandwith transducer

A 
Low Q factor
Wide range of frequencies sent and received 
Backing block, crystal impurities
Short pulses
Facilitates harmonic imaging
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24
Q

Continues imaging

A

Continuous
Separate send and receive transducers
CW doppler

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25
Q

Pulsed imaging

A

Single transducer alternately sends and receives
PW doppler
Grayscale imaging
Colorflow imaging

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26
Q

Colorflow pulsed imaging methods

A

Autocorrelation method

Time domain method

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27
Q

Colorflow autocorrelation method

A

Compares frequency differences from sequential pulses

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28
Q

Colorflow time domain method

A

Measures distance objects move between pulses separated in time by PRP
Velocity = change in distance./ PRP

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29
Q

Pulse repetition period

A

Time between pulses

Directly related to imaging depth

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30
Q

PRP Equation

A

PRP = depth * speed of sound

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31
Q

Pulse repetition frequency

A

Inversely related to PRP and imaging depth

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32
Q

PRF Equation

A

PRF = 1 / PRP

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33
Q

Duty factor

A

Fraction of PRP that transducer is emitting sound

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34
Q

CW doppler duty factor

A

1

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35
Q

Pulsed techniques duty factor

A

<1 (usually 0.001 to 0.01)

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36
Q

Duty factor equation

A

Pulse duration(s) / PRP (s)

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37
Q

Pulse duration

A

Time from start of a pulses to end of a pulse

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38
Q

Second harmonic imaging

A

Single sent out at nominal frequency, filter processes signals returned at 2f which are displayed as image

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39
Q

Types of resolution

A

Temporal
Contrast
Spatial

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40
Q

Temporal resolution

A

Ability to see object as it moves over time

How often you can see something

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41
Q

Temporal resolution determined by

A

Fram rate (frames / sec)
Depth
Line density
Use of multiple beam formers increase frame rate

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42
Q

Improving temporal resolution

A

Decrease image depth
Decrease image display width
Decrease # of focal points
Decrease line density

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43
Q

Contrast resolution

A

Ability to see differences in gray scale

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44
Q

Contrast resolution dependent on

A 
Intrinsic properties
Frequency
Gain
Compression
Color
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45
Q

Types of spatial resolution

A

Axial

Lateral

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46
Q

Axial resolution

A

Discern two objects in line of US beam

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47
Q

Axial resolution determined by

A

Spatial pulse length (set by manufacturer)

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48
Q

Axial resolution improved by

A

Higher frequencies

Fewer cycles

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49
Q

Axial resolution equation

A

(Cycles x wavelength) / 2

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50
Q

Lateral resolution

A

Discern two objects across image

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51
Q

Lateral resolution better with

A

Narrower beam

Use of focal point, near field

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52
Q

Ultrasound beam focusing

A

Mechanical beam focusing
Electronic beam focusing
Multiple focal zones

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53
Q

Mechanical beam focusing

A

Fixed, acoustic lenses, shaped crystals

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54
Q

Electronic beam focusing

A

Dynamic, delay in firing central elements

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55
Q

Multiple focal zones

A

Separate scan lines for each zone, images electronically spliced
Poor frame rate

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56
Q

Side lobes

A

Single element adjacent to main beam

57
Q

Grating lobes

A

Phased array lobes adjacent to main beam

58
Q

Echo ranging

A

Distance of object from transducer determined by time of flight and velocity (assumed 1540 m/s)

59
Q

Echo ranging equation

A

d = 1/2t * v

60
Q

Echo ranging 1 cm

61
Q

Transducer Output Power

A

Energy put out by machine (MI or dB)

62
Q

Higher transducer output power =

A

better signal to noise ratio

destroy contrast bubbles

63
Q

Amplification

A

Received signals made larger or smaller to optimize appearance
Noise and signal amplified
No effect on signal to noise ratio or bubbles

64
Q

Gain compensation

A

Corrects amplification for signal loss due to depth related attenuation (time-gain compensation)
or unequal signal strength across image (lateral-gain compensation)

65
Q

Compression

A

First information into display’s capacity

66
Q

Reject

A

Filters low or high intensity signal to improve image quality

67
Q

Wavelength

A

Distance between wave peaks

68
Q

Frequency (f) =

A

Number of waves / s (Hz)

69
Q

Ultrasound frequency

A

Above audible range (>20,000 Hz)

70
Q

Diagnostic US frequency range

71
Q

Velocity of sound

A

Varies with density and compressibility of medium

72
Q

Average soft tissue velocity of sound

73
Q

Wave equation

A

Velocity = frequency x wavelength

74
Q

Wavelength of 3 MHz sound

A

0.5. mm in soft tissue

75
Q

Frequency and wavelength

A

Higher f = shorter wavelength

76
Q

Ultrasound signal strength

A

Ampltiude
Power
Intensity

77
Q

Amplitude

A

Difference between max and min value of wave

78
Q

Power

A

Total energy produced each second (watts)

79
Q

Intensity

A

Power / cross-sectional beam area (W / cm2)

80
Q

Decibel (dB)

A

Units for describing difference between US intensities

81
Q

dB equation

A 
dB = 10 log (I / I0)
I = measured intensity
I0 = defined reference intensity
82
Q

3 dB change

A

Doubling in intensity

83
Q

30 dB change

A

1000 fold change in intensity

84
Q

100 d change

A

10^10 times more intense

85
Q

Mechanical index

A

Measure of potential to produce cavitation (formation f bubbles)

86
Q

MI equation

A

MI = peak negative pressure / sqrt(f)

87
Q

Bubbles destroyed when MI

88
Q

Harmonic signals weak when MI

89
Q

Acoustic impedance

A

Product of density (p) and velocity of sound (v) in medium
Differences between acoustic impedances directly related to % reflection
Z = p v

90
Q

Nyquist Limits (DF)

A

Highest doppler shift (velocity) that can be measured with pulsed doppler

91
Q

Nyquist Limit equation

A

DF = PRF / 2

92
Q

Higher Nyquist limit

A

Shallower depth

Lower frequency

93
Q

Velocities above Nyquist limit

A

Alias or wrap around, displayed as opposite direction

94
Q

Doppler effect

A

Frequency shift caused by relative motion between source and target

95
Q

Doppler shift increased by

A

Velocity of sample relative to source
Interrogation angle
Frequency of source

96
Q

Doppler shift decreased by

A

Velocity of sound in the medium

97
Q

How to raise nyquist limit

A

Decrease depth

No effect of sector width or range gate size

98
Q

How to decrease doppler frequency shift

A

Reduce transducer frequency (f0)

99
Q

Frequency means

A

Number of time particle in conducting medium vibrates per unit time

100
Q

Frequency vs period

A

Frequency = 1 / period

101
Q

Tissue with fastest loss of strength

102
Q

Materials that respond to acoustic waves and generate electrical signals

A

Piezoelectric crystals

103
Q

Doppler angle

A

Angle between the direction of flow and the ultrasound beam

104
Q

Doppler effect, higher frequency / pitch

A

Object moving towards you

105
Q

Positive doppler shift

A

Reflector moving so that angle between transmitted beam and direction of flow > 90 degrees

106
Q

Doppler shift of 0

A

Reflector is stationary or moving in direction perpendicular to the beam

107
Q

Time gain compensation corrects for different media…

A

Attenuation

108
Q

Attenuation is sum of

A

Scattering
Absorption
Reflection

109
Q

Strength of transmitted sound wave controlled by

A

Power control

110
Q

Control for what extent received signal is amplified

A

Gain control

111
Q

Compression

A

determines dynamic range of received signals used to create image

112
Q

Spatial resolution means

A

Smallest distance between two objects that allows distinction between them

113
Q

Spatial resolution =

A

Size of a pixel in the relevant direction

114
Q

Temporal resolution definition

A

Shortest time between two events that allows distinction between them
Shortest duration of event that can be detected

115
Q

Temporal resolution =

A

inverse of frame rate

116
Q

Dynamic range adjusted by

A

Compression control

117
Q

Frequency and depth

A

Higher frequency = smaller depth

118
Q

Increase to compensate for attenuation

119
Q

Decreasing what = better contrast

A

Dynamic range / compression

120
Q

Ghosting artifact

A

When imaging higher velocity regions, movement of cardiac structures produces low velocity signals which appear as color

121
Q

Removes ghosting artifact

122
Q

Time gain compensation and depth

A

Decreases signal in near field, increases in far field

123
Q

Better assess rapid structures

A

Higher frame rate
Narrowing sector
Decreasing depth

124
Q

Decreasing aliasing

A

Baseline shift away from direction of flow

125
Q

Persistence

A

Images averaged together to create smoothing effect

Lower = better temporal resolution

126
Q

Image resolution improved by

A

Increase the write zoom
Reducing sector width
Changing focal point

127
Q

Increasing line density effect

A

Improves spatial resolution

Decreases frame rate and temporal resolution

128
Q

Higher frequency and spatial resolution

A

Better spatial resolution

129
Q

Increase frame rate by

A

Decreasing depth

Reducing sector angle

130
Q

Reducing sector angle

A

Reduces scan lines -> higher frame rate

131
Q

M mode features

A

US reflections along single line over time
Higher temporal resolution
Simultaneous visualization of structures

132
Q

Spectral doppler features

A

Displays power spectrum of velocities along single line over time

133
Q

Rationale for echo contrast

A

Increased reflection by added gas-liquid interface

134
Q

Acoustic shadowing with contrast

A

Increased attenuation by contrast filled cavity

135
Q

Higher sweep speed

A

Detailed time estimates

136
Q

Lower sweep speed

A

Multicycle events like respiratory variation

137
Q

Higher harmonics ->

A

Reduces near field, enhances far field

138
Q

Smoothing

A

Averaging adjacent pixels to create a smoother image

Echo (30 decks)

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