Physics Flashcards
1
Q
Specular reflection
A
Mirror-like
Structures >1 wavelength in diameter
Angle of incidence = angle of reflection
2
Q
Specular reflection and frequency
A
Independent of frequency
3
Q
Diffuse reflection
A
Omnidirectional
Structures <1 wavelength in diameter
Frequency dependent, more scattering with higher frequency
4
Q
Interference
A
Algebraic sum of wave amplitude
5
Q
Speckle
A
Noise in US images produced by interference pattern from multiple small reflectors (scatterers)
6
Q
Refraction
A
Bending of wavefront as sound passes between media with different propagation velocities
Snell’s law
7
Q
Snell’s law
A
Sin Angle I / Sin angle T = V I / V T
8
Q
Diffraction
A
Spreading or divergence of sound beam
Greater with smaller source
9
Q
Absorption
A
Sound energy converted to other forms of energy like heat
10
Q
Attenuation
A
Loss of intensity as sound wave passes through medium
11
Q
Soft tissue attenuation
A
0.5-1.0 dB/cm/MHz
12
Q
Resonance
A
Air bubbles resonate within ultrasound field, ring like a bell
Persistent oscillation produces continued ultrasound signal
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)
14
Q
Harmonics generated strongly by
A
Microbubbles
Compression and expansion not equal
15
Q
Harmonic generated weakly by
A
Tissue
Velocity of sound higher during compression than rarefaction
16
Q
Transducer components
A
Piezoelectric crystal
Backing block
Quarter-wavelength matching layer
17
Q
Piezoelectric crystal
A
Vibrates with applied current, most efficient at resonant frequency
Transmitter and receiver
18
Q
Backing block
A
Limits ringing of crystal to create discrete US pulses
19
Q
Quarter-wavelength matching layer
A
Provides better acoustic impedance matching between crystal and skin
20
Q
Mechanical transducer
A
Controlled movement of single element to produce 2D image
21
Q
Phase array transducer
A
Multiple crystal elements to produce 2D or 3D image
22
Q
Narrow bandwidth transducer
A
High Q factor
Crystals allowed to ring freely
More efficient / sensitive
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
24
Q
Continues imaging
A
Continuous
Separate send and receive transducers
CW doppler
25
Pulsed imaging
Single transducer alternately sends and receives
PW doppler
Grayscale imaging
Colorflow imaging
26
Colorflow pulsed imaging methods
Autocorrelation method
| Time domain method
27
Colorflow autocorrelation method
Compares frequency differences from sequential pulses
28
Colorflow time domain method
Measures distance objects move between pulses separated in time by PRP
Velocity = change in distance./ PRP
29
Pulse repetition period
Time between pulses
| Directly related to imaging depth
30
PRP Equation
PRP = depth * speed of sound
31
Pulse repetition frequency
Inversely related to PRP and imaging depth
32
PRF Equation
PRF = 1 / PRP
33
Duty factor
Fraction of PRP that transducer is emitting sound
34
CW doppler duty factor
1
35
Pulsed techniques duty factor
<1 (usually 0.001 to 0.01)
36
Duty factor equation
Pulse duration(s) / PRP (s)
37
Pulse duration
Time from start of a pulses to end of a pulse
38
Second harmonic imaging
Single sent out at nominal frequency, filter processes signals returned at 2f which are displayed as image
39
Types of resolution
Temporal
Contrast
Spatial
40
Temporal resolution
Ability to see object as it moves over time
| How often you can see something
41
Temporal resolution determined by
Fram rate (frames / sec)
Depth
Line density
Use of multiple beam formers increase frame rate
42
Improving temporal resolution
Decrease image depth
Decrease image display width
Decrease # of focal points
Decrease line density
43
Contrast resolution
Ability to see differences in gray scale
44
Contrast resolution dependent on
```
Intrinsic properties
Frequency
Gain
Compression
Color
```
45
Types of spatial resolution
Axial
| Lateral
46
Axial resolution
Discern two objects in line of US beam
47
Axial resolution determined by
Spatial pulse length (set by manufacturer)
48
Axial resolution improved by
Higher frequencies
| Fewer cycles
49
Axial resolution equation
(Cycles x wavelength) / 2
50
Lateral resolution
Discern two objects across image
51
Lateral resolution better with
Narrower beam
| Use of focal point, near field
52
Ultrasound beam focusing
Mechanical beam focusing
Electronic beam focusing
Multiple focal zones
53
Mechanical beam focusing
Fixed, acoustic lenses, shaped crystals
54
Electronic beam focusing
Dynamic, delay in firing central elements
55
Multiple focal zones
Separate scan lines for each zone, images electronically spliced
Poor frame rate
56
Side lobes
Single element adjacent to main beam
57
Grating lobes
Phased array lobes adjacent to main beam
58
Echo ranging
Distance of object from transducer determined by time of flight and velocity (assumed 1540 m/s)
59
Echo ranging equation
d = 1/2t * v
60
Echo ranging 1 cm
13 usec
61
Transducer Output Power
Energy put out by machine (MI or dB)
62
Higher transducer output power =
better signal to noise ratio
| destroy contrast bubbles
63
Amplification
Received signals made larger or smaller to optimize appearance
Noise and signal amplified
No effect on signal to noise ratio or bubbles
64
Gain compensation
Corrects amplification for signal loss due to depth related attenuation (time-gain compensation)
or unequal signal strength across image (lateral-gain compensation)
65
Compression
First information into display's capacity
66
Reject
Filters low or high intensity signal to improve image quality
67
Wavelength
Distance between wave peaks
68
Frequency (f) =
Number of waves / s (Hz)
69
Ultrasound frequency
Above audible range (>20,000 Hz)
70
Diagnostic US frequency range
1-20 MHz
71
Velocity of sound
Varies with density and compressibility of medium
72
Average soft tissue velocity of sound
1540 m/s
73
Wave equation
Velocity = frequency x wavelength
74
Wavelength of 3 MHz sound
0.5. mm in soft tissue
75
Frequency and wavelength
Higher f = shorter wavelength
76
Ultrasound signal strength
Ampltiude
Power
Intensity
77
Amplitude
Difference between max and min value of wave
78
Power
Total energy produced each second (watts)
79
Intensity
Power / cross-sectional beam area (W / cm2)
80
Decibel (dB)
Units for describing difference between US intensities
81
dB equation
```
dB = 10 log (I / I0)
I = measured intensity
I0 = defined reference intensity
```
82
3 dB change
Doubling in intensity
83
30 dB change
1000 fold change in intensity
84
100 d change
10^10 times more intense
85
Mechanical index
Measure of potential to produce cavitation (formation f bubbles)
86
MI equation
MI = peak negative pressure / sqrt(f)
87
Bubbles destroyed when MI
> 1
88
Harmonic signals weak when MI
<0.1
89
Acoustic impedance
Product of density (p) and velocity of sound (v) in medium
Differences between acoustic impedances directly related to % reflection
Z = p v
90
Nyquist Limits (DF)
Highest doppler shift (velocity) that can be measured with pulsed doppler
91
Nyquist Limit equation
DF = PRF / 2
92
Higher Nyquist limit
Shallower depth
| Lower frequency
93
Velocities above Nyquist limit
Alias or wrap around, displayed as opposite direction
94
Doppler effect
Frequency shift caused by relative motion between source and target
95
Doppler shift increased by
Velocity of sample relative to source
Interrogation angle
Frequency of source
96
Doppler shift decreased by
Velocity of sound in the medium
97
How to raise nyquist limit
Decrease depth
| No effect of sector width or range gate size
98
How to decrease doppler frequency shift
Reduce transducer frequency (f0)
99
Frequency means
Number of time particle in conducting medium vibrates per unit time
100
Frequency vs period
Frequency = 1 / period
101
Tissue with fastest loss of strength
Lung
102
Materials that respond to acoustic waves and generate electrical signals
Piezoelectric crystals
103
Doppler angle
Angle between the direction of flow and the ultrasound beam
104
Doppler effect, higher frequency / pitch
Object moving towards you
105
Positive doppler shift
Reflector moving so that angle between transmitted beam and direction of flow > 90 degrees
106
Doppler shift of 0
Reflector is stationary or moving in direction perpendicular to the beam
107
Time gain compensation corrects for different media...
Attenuation
108
Attenuation is sum of
Scattering
Absorption
Reflection
109
Strength of transmitted sound wave controlled by
Power control
110
Control for what extent received signal is amplified
Gain control
111
Compression
determines dynamic range of received signals used to create image
112
Spatial resolution means
Smallest distance between two objects that allows distinction between them
113
Spatial resolution =
Size of a pixel in the relevant direction
114
Temporal resolution definition
Shortest time between two events that allows distinction between them
Shortest duration of event that can be detected
115
Temporal resolution =
inverse of frame rate
116
Dynamic range adjusted by
Compression control
117
Frequency and depth
Higher frequency = smaller depth
118
Increase to compensate for attenuation
Gain
119
Decreasing what = better contrast
Dynamic range / compression
120
Ghosting artifact
When imaging higher velocity regions, movement of cardiac structures produces low velocity signals which appear as color
121
Removes ghosting artifact
Filtering
122
Time gain compensation and depth
Decreases signal in near field, increases in far field
123
Better assess rapid structures
Higher frame rate
Narrowing sector
Decreasing depth
124
Decreasing aliasing
Baseline shift away from direction of flow
125
Persistence
Images averaged together to create smoothing effect
| Lower = better temporal resolution
126
Image resolution improved by
Increase the write zoom
Reducing sector width
Changing focal point
127
Increasing line density effect
Improves spatial resolution
| Decreases frame rate and temporal resolution
128
Higher frequency and spatial resolution
Better spatial resolution
129
Increase frame rate by
Decreasing depth
| Reducing sector angle
130
Reducing sector angle
Reduces scan lines -> higher frame rate
131
M mode features
US reflections along single line over time
Higher temporal resolution
Simultaneous visualization of structures
132
Spectral doppler features
Displays power spectrum of velocities along single line over time
133
Rationale for echo contrast
Increased reflection by added gas-liquid interface
134
Acoustic shadowing with contrast
Increased attenuation by contrast filled cavity
135
Higher sweep speed
Detailed time estimates
136
Lower sweep speed
Multicycle events like respiratory variation
137
Higher harmonics ->
Reduces near field, enhances far field
138
Smoothing
Averaging adjacent pixels to create a smoother image
