Ultrasound (Erin) Flashcards
1
Q
Velocity of USS Waves in a medium equation: C = what?
A
_________________________
_/(Bulk Modulus/density)
2
Q
C (velocity) = What? X What?
A
C (velocity) = frequency x wavelength
3
Q
We set frequency of USS Waves but the velocity depends on the …?
A
Medium. The wavelength changes to compensate.
4
Q
Diagnostic USS Waves travel at what frequencies in general?
A
2-15MHz
5
Q
Humans can only hear what frequency range?
A
20-Hz - 20kHz
6
Q
In soft tissue, Uss waves travel at what speed?
A
1540m/s
7
Q
Pulse Echo: Time taken for USS wave to go there an back = what?
A
2xDepth / velocity
8
Q
What relationship of frequency shift to Pulse Repetition Frequency causes Aliasing?
A
Aliasing occurs when PRF is less than half of the frequency shift.
ie Frequency shift > 1/2 PRF
9
Q
State the Doppler shift equation
A
2 x frequency x velocity of blood x cos(theta)
—————————————————
velocity of sound
10
Q
Thermal index refers to the power needed to increase temperature by how many degrees?
A
1 degree
11
Q
Mechanical Index must be less than what number to avoid risk of cavitation?
A
0.7
12
Q
True/false: mechanical bio effects are more likely to occur in the lung and intestines than soft tissues
A
True - risk of bruising and cavitations
13
Q
There is (increased/decreased) likelihood of acoustic cavitation at lower frequencies
A
Increased
14
Q
Thermal index should be kept below what number (generally)
A
1
15
Q
Thermal index should be kept below what number for obstetrics?
A
0.7
16
Q
Thermal index: temperature rise depends on…(4)
A
Frequency
Power
Duration of scan
Tissue type
17
Q
Mechanical index should be kept below what level for neonates?
A
0.3
18
Q
How safe are microbubble USS contrast agents?
A
Very safe. Can use in renal/liver failure, even in pts with allergies to iodine/gadolinium.
19
Q
What is the effect of USS Harmonics on:
- lateral resolution
- axial resolution
A
Improves lateral resolution
Reduces axial resolution
20
Q
USS Harmonics makes what structures appear clearer?
A
Cystic and fluid-filled structures
21
Q
Acoustic impedance equation?
A
Z = p x c
Acoustic impedance = density of tissue x speed of sound
22
Q
What is Snell’s law?
A
Angle of refraction at tissue boundaries.
sin(theta-1)/sin(theta-2) = c1/c2
23
Q
The near field/Fresnel zone of the USS beam can be calculated as…?
A
Diameter^2 / 4x wavelength
24
Q
What are the 3 methods of USS Beam focusing?
A
- Time-delays of firing waves from different transducer elements
- Curved crystals in transducer
- Lens to mould the beam
25
What is the speed of ultrasound in air?
300m/s
26
around x% of the signal is reflected at the kidney-perinephric fat boundary
1%
27
USS waves travel through Bone, Fat Muscle, liver and Air and in the following order from fastest to slowest:
Bone
Muscle
Liver/kidney
Fat
Air
28
True/false: The velocity of sound is independent of body temperature
False. Temp affects density.
29
T/F: Aliasing occurs if the Nyquist criteria are satisfied
False.
30
T/F: Aliasing occurs less at higher angles
True. higher angle = cos theta closer to 0 (between 0-90 degrees)
31
The wavelength of ultrasound in soft tissue is approximately...
0.1- 1.5 mm
32
33
Artefacts: Acoustic enhancement occurs when...
the sound travels through a structure which is heavily fluid-based.
34
Artefact: Ring down artefact is caused by...?
resonance within gas bubbles
35
Artefact: Reverberation happens close to...
strongly reflective structures
36
Artefact: Comet tail artefact is...
a kind of reverberation artefact seen underneath small foci of highly reflective material such as the crystals found in the gallbladder wall
37
Full destructive USS wave interference only happens if...
two waves are of the same wavelength and opposite phase when they meet
38
Mechanical Index is calculated using which 2 figures?
peak rarefaction pressure
Square root of Frequency
39
What is an acceptable frame per second rate?
30 fps
40
How many lines per frame makes a good ultrasound image
100 lines per frame
41
What percentage of the gas/tissue interface is reflected?
99.9%
42
What percentage of the bone-soft tissue interface is reflected?
Around 40%
43
What percentage of the fat to muscle interface is reflected
1%
44
What percentage of the liver to muscle interface is reflected
0.01%
45
What is the acoustic impedence equation
Acoustic Impedence (Z) = density (ρ) x speed of sound in the tissue (v)
46
What unit is acoustic impedence measured in
Rayls
47
In doppler what happens to frequency if the blood is travelling towards the transducer
Increase in frequency
48
In doppler what happens to frequency if the blood is travelling away from the transducer
Decrease in frequency
49
How is velocity measured in doppler ultrasound
Measure of the change of frequency
50
What transducer angle does doppler work best/worst at?
best at 0 degree (theoretically can't in actual practice)
Worst at 90 degrees
51
In Duplex Ultrasound what colour is given to higher frequencies (representing flow towards the transducer)
Red
(BART - blue away, red towards)
52
In Duplex Ultrasound what colour is given to lower frequencies (representing flow away from the transducer?
Blue
(BART - blue away, red towards)
53
Does duplex scanning produce quantitative date?
No
54
Define Thermal Index
The Thermal Index (TI) is a figure which compares the power being emitted to that which would cause a temperature rise of 1oC
55
At what level is the TI considered definitely safey
TI<0.5
56
At what TI should care be made to keep scanning time to a minimum
TI >0.7
57
What is the effect of a higher frequency on temperature rise in tissue
Higher effect
58
What is the effect of a lower frequency on temperature rise in tissue
Lower effect
59
How is MI calculated?
Calculated as the peak rarefaction (NEGATIVE) pressure divided by the square root of the frequency of ultrasound being used.
60
At what MI level does the risk of cavitation start?
MI >0.7
61
At what MI does the risk of Neonatal lung and intestinal damage occur?
MI >0.3
62
What is the risk with increasing mechanical index
Cavitation
63
At what TI is fetal scanning not recommended, no matter how brief?
TI >3
64
How is mechanical damage caused by ultrasound
At subcellular level, the acceleration of particles within cells can damage the cell wall
65
Alignment of subcellular structures is a mechanism of damage caused by USS T o r F?
True
Acoustic streaming – repeated passage of waves in one direction cause alignment of subcellular structures in the direction of the sound travel. This can have a negative effect on the cell membranes.
66
What does 1Hz stand for
1 Hz is 1 wavelength per second
67
How is acoustic impedence (Z) calculated?
Acoustic impedence (Z) = density (p) x speed of sound in that tissue (V)
68
How is the reflected portion calculated?
(Z1-Z2)2 / (Z1 + Z2)2
69
What is the unit of acoustic impedence Z?
Rayls
70
What is the velocity of USS waves in air?
330 ms-1 (approx)
71
What is the velocity of USS waves in soft tissue?
Approve 1540 ms -1
72
What is the velocity of USS waves in bone?
4000ms-1/5080 ms-1
73
Are USS waves faster in air or bone?
Bone
74
Are USS waves faster in soft tissue or bone?
Bone
75
Are USS waves faster in soft tissue or in air?
Soft tissue
76
How is USS velocity calculated?
_________________________
C = _/(stiffness/density)
Speed = square root of (stiffness/density)
77
If the stiffness increases what happens to the speed of USS waves
IncreasesI
78
If the density increases what happens to the speed of the USS waves
Decreases
79
What is the effect of increased temperature on the speed of ultrasound waves
Increases
80
Describe constructive interference
Interaction that occurs when the two waves are exactly in step and their amplitudes add up
81
Describe destructive interference
Interaction that occurs when the two waves are out of phase
Results in reduction in intensity
If equal and exactly out of phase they completely cancel each other out
82
What is frequency of a sound wave?
How many times per second the compression phases passes ant single point in the medium
83
Define acoustic impedence
Resistance experienced by an ultrasound beam in the medium
84
What is the main crystal type used in ultrasound?
Lead Zirconate Titanate (PZT)
85
What is the curie temperature for PZT?
350 degrees C
86
What is the effect of tissue harmonic imaging on lateral resolution
improves lateral resolution
87
What is the effect of tissue harmonics on axial resolution
Reduces axial resolution
88
What is the effect of using tissue harmonic imaging on resolution when scanning patients with a large BMI?
Improved resolution
89
What is the effect of tissue harmonics on reverberation and side lobe artefacts
Reduces side lobe and reverberation artefacts
90
What is the effect of tissue harmonics on imaging tissue structures
Makes tissue structures seem clearer
91
What is the wavelength of ultrasound in soft tissue
Around 0.1-1.0mm
92
How are contrast agents given in ultrasound?
IV injectiona
93
What is the contrast used in ultrasound?
Microbubbles
94
Is iodine used in ultrasound for contrast
No
95
What is the wavelength of 1.5MHz ultrasound wave in soft tissue?
1.0mm
96
What is the wavelength of 15MHz ultrasound wave in soft tissue?
0.1mm
97
Does bone have a high Z or low Z (acoustic impedence)
High Z
98
Does PZT crystal have a low or high Z (acoustic impedence)
High Z
99
Does air have a high or low Z (acoustic impedence)?
Low
100
Does lung have a high or low Z (acoustic impedence?)
Low
101
What causes scattering in USS?
Structures smaller than the wavelength of the beam scatter US rasiation in all directions
102
At what size of objects does scattering occur in ultrasound studies using 1.5MHz frequency?
If object <1mm
103
At what size of objects does scattering occur in ultrasound studies using 15MHz frequency?
If object <0.1mm
104
When does diffuse reflection occur?
If reflecting interface is rough or had undulations that are smaller than the wavelength of the radiation
105
When does specular reflection occur?
- the beam strikes a large smooth interface that is larger than the wavelength
- the acoustic impedence of the tissues on either side of the interface are non equal
106
What is the angle of reflection equal to?
The angle of incidence
107
How does refraction occur
If the speeds of sound of the tissues on either side of the interface are non equal
108
How does the intensity of any ultrasound beam change with increasing depth in tissue
Intensity of any ultrasound beam decreases exponentially with depth in tissue
109
What 3 factors cause attenuation of an ultrasound beam
Absorption
Scattering
Reflection
110
What unit are ultrasound attenuation co-efficients quoted in?
dB/cm/MHz
111
What is the pulse repetition frequency
Number of pulses emitted each second
112
What information does the time taken for the pulse to return provide?
Information on depth
113
How is the depth of an echo calculated
Return time x wave speed /2
114
How does TGC work
Echoes arriving later are from a greater depth and have suffered greater attenuation
In TCG pulses are amplified according to when they arrive at the transucer
All echoes from the same boundary will have the same brightness regardless of depth
115
What is the crystal thickness in ultrasound probes dependent on?
1/2 wavelength of emitted US waves
116
Do high frequency probes have thin or thick crystals?
Thin crystals
0.1mm at 7.5MHz
117
Do low frequency probes have thin or thick crystals?
Thick crystals
0.5mm at 1.5MHz
118
What are the 3 main types of ultrasound probes
Linear
Curvillinear
Phased array
119
What are linear array transducers useful for?
Scanning superficial structures i.e neck, breast, scrotum
120
What shape is the field of view produced by a linear array transducer
rectangular
121
Which has a higher FOV - linear or curvilinear transducers?
Curvilinear
122
Do curvilinear transducers operate at high or lower frequencies
Low frequencies
123
What are curvilinear transducers used for?
Abdominal, obstetric etc
124
In B mode what 3 factors does lateral resolution depends on?
Beam width
Number of scan lines per frame
Frequency of US radiation
125
How does beam width affect lateral resolution
Best resolution in the focal zone
(Narrowest zone)
126
Define lateral resolution
Ability to separate two structures side by side at the same depth
127
Define axial (depth) resolution
Ability to separate two interfaces along the same scan line
128
How does number of scan lines affect lateral resolution
More scan lines provide better lateral resolution
(but each frame takes longer to acquire)
129
How does frequency of US radiation affect lateral resolution
Resolution is better at higher frequency
130
What is the typical lateral resolution at 3MHz
2.5mm
131
What is the typical lateral resolution at 10MHz
1mm
132
What is axial resolution determined by? (In B mode)
Spatial pulse length
133
How is spatial pulse length calculated
Wavelength x number of waves per pulse
134
What is spatial pulse length?
Finite length of an ultrasound pulse
135
How is axial resolution calculated
SPL/2
136
How does axial resolution change with frequency
Improves with increasing frequency
137
Is axial resolution depth dependent?
No
138
In absorption:
Energy is transferred to the material it is travelling through as .....
Heat
139
Name 2 disadvantages of ultrasound scanning
Can't image through bone or gas
Very dependent on operator skill
140
Describe the advantages of ultrasound imaging
No radiation exposure
Non-invasive
Fast
Inexpensive
Real time imaging
Can measure velocity e.g of blood
Cross sectional imaging
141
How is the thickness of PZT crystals calculated
1/2 desired wavelength
142
How does a higher Q value affect vibration?
Vibrated for a long time (have a light dampening effect) and pulse persists for a long time
143
How does a lower Q value affect vibration?
Dampen the vibration quickly and the pulse lasts for a shorter time
144
How many crystal elements does a phased transducer normally contain
64-128 crystals
145
What is the doppler shift?
Change in frequency between the emitted and detected ultrasound wave
146
How is TI calculated
TI = W / Wdeg
where W is the acoustic power of the beam at a specified point, and Wdeg is the power required for a maximum temperature rise of 1°C anywhere in the beam
147
What is the restriction on exposure time for Thermal index (soft tissue) <0.7
No restriction
Generally considered safe TI<0.7
148
How is frequency related to wavelength?
Inversely proportional to one another
149
What is the speed of ultrasound in lung
600ms-1
150
What is acoustic impedence a measure of?
How a tissue resists the passage of the US wave
151
In ultrasound what does Z stand for
acoustic impedence
152
What percentage of the lung-soft tissue boundary is reflected back?
Around 50%
153
Why does refraction occur
Due to difference in ultrasound speed between two tissues
154
How does attenuation of an ultrasound beam alter with US frequency?
Attenuation increases with US frequency
155
What is the soft tissue attenuation coefficient at 1MHz and 5MHz
1MHz = 0.8 dB/cm= (17% reduction in intensity per cm)
If they quote between 0.5-1dB/cm in exam say true!!!!!!!
5MHz = 4dB/cm = (60% reduction in intensity per cm)
156
What is the water/blood attenuation coefficient at 1MHz and 5MHz
1MHz = 0.15 dB/cm= (3% reduction in intensity per cm)
5MHz = 0.45dB/cm = (10% reduction in intensity per cm)
157
What is the bone attenuation coefficient at 1MHz
1MHz = 15 dB/cm (97% reduction in intensity per cm)
158
What is the lung attenuation coefficient?
>30dB/cm/MHz
159
How does the Piezoelectric effect result in ultrasound production?
Alternating voltage applied to the end faces of the transducer crystal causes it to vibrate
160
How does the Piezoelectric effect result in Ultrasound detection?
Waves arriving at the crystal face cause it to vibrate and generate an alternating voltage
161
How does the near field alter with higher frequencies?
Near field is narrower at higher frequencies
162
How does the pulse echo determine the depth of a reflecting structure?
If velocity of pulse in tissue is known (or assumed)
Depth can be determined by
Depth of echo = return time x wave speed/2
163
How many wavelengths is a typical ultrasound pulse?
Length is typically 2-3 wavelengths
164
What is the typical duration of an ultrasound pulse
1 microsecond (us)
165
What is the typical Pulse Repetition Frequency?
3-6kHz
166
What does the vertical axis in A mode graph represent?
Amplitude of each echo
167
What does the horizontal axis of A mode graph represent?
Depth
168
Which specialty often uses A Mode scanning?
Opthalmology
169
What is image brightness proportional to in B mode?
Strength of echo
170
Which probe has better lateral resolution? Curvilinear or Linear
Linear
171
What does the horizontal axis on an A mode graph represent?
Depth
172
Phased array transducer: how are the signals fired?
All segments are fired at the same time
173
Continuous wave Doppler - how does it work?
Uses 2 separate transducers - 1 transmits and 1 receives
There is no depth information though
Often used in cardiac scanners or fetal heart scans
174
What is power Doppler? Advantage?
Displays magnitude of blood flow, not direction
No aliasing, good for finding low flow rates
175
How are segment fired in phased array transducers?
Fired simultaneously
176
What is spectral Doppler? Why do we use it?
Detailed flow information is displayed as a spectrum
Allows velocity calculation analysis
177
What 3 factors does lateral resolution depend on?
Beam width
Number of scan lines per frame
Frequency of US radiation
178
What is the effect of more scan lines per frame on lateral resolution
More scan lines provide better resolution
179
Is lateral resolution depth dependent?
Yes
180
At what depth (area/zone) is the lateral resolution the best?
Focal distance
181
What is axial resolution in B mode scans determined by?
Spatial pulse length
182
What is spatial pulse length?
Finite length of ultrasound pulse
183
How is spatial pulse length calculated?
Wavelength x no of waves per pulse
184
How is axial resolution calculated?
SPL/2
185
How does axial resolution change with increasing frequency?
Axial resolution improves with increasing frequency
186
Refraction Artefact: what is this?
Position of the object is incorrectly placed because the pulses are supposed to travel straight but this artefact occurs when the beam is obliquely incident on a tissue boundary with different speeds of sound (causing refraction)
187
Is axial resolution depth dependent?
No
188
How does axial resolution vary with depth?
Axial resolution is not depth dependent
189
What is the typical axial resolution at 3MHz?
1mm
190
What is the typical axial resolution at 10MHs
0.3mm
191
What is the effect on doppler frequency obtained is the beam is more aligned in the flow direction?
Higher doppler frequency
192
How many transducers does CW doppler require?
2 transducers
1 transmits continuously
1 receives continuously
193
Why is there no depth information in CW dopppler?
Because pulse-echo principle is not used
194
How does doppler signal arise in continuous wave doppler?
Arises from where the beams overlap
195
How is CW doppler detected?
Audible sound due to the Doppler shift being in the audible sound frequency range
196
What are the advantages of CW doppler? (3)
Cheap
Easy to use
Sensitive to flow
197
What are the disadvantages of CW doppler (3)?
Can't measure velocity
Usually combines arterial and venous signal
Can’t determine depth
198
What are the 3 types of pulse wave doppler?
Colour
Power
Spectral
199
What type of scanning mode does PW combine with?
B mode
200
What is the maximum detectable frequency shift equation for PW doppler?
1/2 PRF
201
When does aliasing occur in PW doppler?
When freq shift >1/2 PRF
202
What 2 things does colour flow doppler tell use about blood flow?
Direction and magnitude of blood flow
203
What 1 thing does power doppler tell us about blood flow?
Magnitude of blood flow
204
Does power doppler tell us about the direction of blood flow
No
205
Do aliasing artifacts occur with power doppler?
No
206
What is power doppler useful for?
Detecting low flow rates
207
What does spectral doppler display?
- A display of the spectrum of Doppler frequencies (flow velocities) on the vertical axis vs time on the horizontal axis.
- Examines detailed flow information from one site only. Detailed analysis of distribution of flow
208
What is the effect of a smaller SPL on axial resolution
Improved axial resolution
209
What is the effect of a low Q value (i.e greater dampening on axial resolution)
Improved axial resolution
Shorter pulse
210
What is the attenuation in dB/cm of water, blood, tissue, bone and lung
water = 0.0022
blood = 0.18
Average tissues = 0.7
Bone = 15
Lung = 40
211
What is more attenuating?
Water or bone
Bone
212
What is more attenuating bone or lung?
Lung
213
What is the effect of increasing probe frequency or increasing transducer size on far field divergence?
Reduced far field divergence
214
Describe vibration in materials with a low Q value
Dampen the vibration quickly
Pulse lasts for a shorter time
214
How is MI calcualted
Peak rarefraction/ (square root of) ultrasound frequency
214
How thick is the transducer matching layer (in terms of wavelength)
1/4 wavelength thick
215
Describe vibration in materials with a high Q value
Vibrate for a long time (light dampening effect)
216
What causes a longer PRF?
Deeper structures being imaged (the longer it takes to go and come back, the longer the listen phase of the pulse has to be)
More lines per frame
217
How is PRF calculated
Frame rate x line per frame
218
How is distance by beam calculated?
time x velocity x 0.5
219
Hows is depth of view calculated
0.5 x sound velocity/ PRF
220
How is near field calculated (as per radiology cafe)
D (squared) / (4 x wavelength)
D - diameter of transducer
221
What is the effect of a larger diameter of transducer on near field distance
Increased near field distance
222
What is the difference between phased and linear array
Phased array all segments are fired simultaneously
223
What are phased arrays useful for imaging?
Cardiac imaging
224
If the reflectors are separated by a distance >SPL/2 will they be resolved?
Yes
225
If reflectors are separated by a distance
No
The returning echoes will overlap and will appear as a single structure in the image
226
Does CW doppler use high Q or low Q?
High Q
With no backing block
To produce a precise narrow frequency bandwith with high output
227
What is the effect of focussing on lateral resolution
improved lateral resolution
228
What is the effect of smaller aperture on lateral resolution
Improved lateral resolution
229
What is the effect of increasing ultrasound intensity on echo amplitude at depth?
Increase echo amplitude at depth
230
What size are the microbubbles for contrast used in USS?
1-10 micrometres
231
Are microbubbles used as contrast in US destroyed by low or high ultrasound intensities?
High
232
How is doppler frequency shift related to the cosine of the angle of incidence
Doppler frequency shift is DIRECTLY PROPORTIONAL TO THE COSINE OF THE ANGLE OF INCIDENCE
233
How is doppler shift frequency related to the frequency of the incidental beam?
Directly proportional
234
What is the difference between linear array and phased array
Linear array = small group adjacent elements are simultaneously activated to create a scan line. Sequential activation of adjacent groups of crystal created a series of scan lines across the transducer surface
Phased array =all segments are fired simultaneous
235
How is near field calculated
d (squared) / 4 x wavelength
236
What is the effect of a longer diameter transducer on near field distance
Increased
237
What is the effect of increased frequency on near field distance
Increased
238
What is the effect of a wider diameter transducer on far field divergence
Reduced far field divergence
239
What is the effect of increased frequency on far field divergence
Reduced far field divergence
240
What is the effect of Low Q on side lobes
less side lobes
241
In what 2 ways can physical focusing be achieved
1. Curved PZT element (greater curvature)
2. Plastic or silicone acoustic lens
242
How does electronic focussing work
Outermost pair is energised first then each adjacent pair in succession ending with the centre
243
How does steering work
If small delays are introduced in rapid sequence along the transducer the pulses reinforce at an angles from the transducer face and destructively interfere in all other directions
Producing a steered plane wave
244
Low Q means ______ dampening leading to a _______ pulse
Heavy dampening
Shorter pulse
245
High Q means __________- dampening leading to a _________pulse
Lighter dampening
Longer pulse
246
What is the effect of Low Q/Shorter pulse on bandwidth?
Larger bandwidth
247
What is the effect of High Q/Longer pulse on bandwidth
Smaller bandwidth
248
What two factors can increase axial resolution
Higher frequency (shorter wavelength)
Low Q (greater dampening, shorter pulse)
Overall want a smaller SPL
249
Does lateral resolution = beam width
yes
250
Typical wavelength of ultrasound in abdominal scanning (in soft tissue) is 0.5mm T or F
True
