Ultrasound Flashcards
(190 cards)
1
Q
Two types of waves?
A
Longitudinal and transverse
2
Q
Definition of sound wave
A
Mechanical disturbance of a medium which passes through the medium at a fixed speed
3
Q
What is rarefaction?
A
A region of negative pressure: particles in the medium move away from each other
4
Q
What is compression?
A
A region of positive pressure: particles in the medium move towards each other
5
Q
Definition of wavelength
A
Distance between two adjacent peaks or troughs in consecutive waves
6
Q
What is wavelength measured in?
A
Greek letter: lambda
7
Q
Typical wavelengths used for medical ultrasound imaging?
A
0.1-1mm
8
Q
Frequency
A
Rate at which waves (a periodic propagating pattern) passes a fixed point per second
9
Q
Definition of period
A
The time needed for one complete wave/cycle
It is the reciprocal of frequency
10
Q
What is frequency measured in?
A
Hertz (Hz)
11
Q
Typical frequencies used in medical ultrasound?
A
1-15 Mhz
12
Q
Equation for wave speed?
A
Speed = frequency x wavelength
Units: metres per second
13
Q
What does sound travel faster in, air, bone or liver?
A
Fastest in bone, then liver, then air
14
Q
Definition of ultrasound?
A
Sound at a frequency above which the human ear can generally hear > 20 kHz
15
Q
How is ultrasound generated?
A
Transducer is placed in contact with the skin, with a coupling gel in between. Beam of ultrasound pulses are transmitted into the tissue
16
Q
Behaviour of ultrasound in tissue?
A
Waves propagate through tissue, partially reflecting or scatter from changes in acoustic impedance.
17
Q
Detection of ultrasound
A
Reflected waves are detected by the probe and stored in the scanner
18
Q
How is the image displayed in ultrasound
A
Depth information from tissue is calculated using speed of sound and time of flight. Stored data is used to form the image
19
Q
Definition of Piezoelectric effect
A
When stress is applied to a certain material, a voltage is generated that is proportional to applied stress.
20
Q
Is piezoelectric effect reversible or irreversible?
A
Reversible
21
Q
Role of piezoelectric effect in the production of sound?
A
An electrical voltage causes the element to deform, releasing an ultrasound pulse
22
Q
Role of piezoelectric in detection of sound?
A
Reflected echo from tissue exerts a pressure creating an electrical signal
23
Q
What is the role of the piezoelectric plate?
A
For generation and detection of ultrasound signals
24
Q
That is the piezoelectric plate made from?
A
Lead zirconate titantate (PZT)
25
How can PZT materials be damaged?
Mechanical damage: dropping
Heating
Exceeding electrical limits set by the manufacturer
26
How can the time of flight be calculated?
Distance = speed x time of flight / 2
27
Assumptions used in the pulse-echo principle?
Propagation in a straight line
| A 'thin' beam
28
Quick summary of pulse-echo principle?
1) Electrical signal is applied to transducer, causing an acoustic signal to be transmitted into a medium
2) Change in material density and acoustic impedance causes a proportion of waves to be reflected back towards the transducer
3) This is detected by the ultrasound probe
29
Definition of elasticity
The ability of a material to return to its original shape and volume after a force is no longer acting on it
30
What is distortion?
When a force is applied to a material causing a change in shape or volume
31
Definition of density?
Mass of the medium per unit volume
32
Relationship between speed of sound and density?
Speed of sound is inversely proportional to the square root of density in the medium
33
Definition of compressibility?
Fractional decrease in volume when pressure is applied to the material.
Higher compressibility = easier to compress the material
34
Relationship between speed of sound and compressibility?
Speed of sound is inversely proportional to square root of compressibility
35
Relationship between bulk modulus and speed of sound?
Speed is directly proportional to square root of bulk modulus
36
Definition of bulk modulus
Negative ratio of stress and strain
37
Why is there a negative sign in bulk modulus?
A positive pressure causes a decrease in volume.
38
What does a large value for bulk modulus mean?
It is resistant to change when a force is applied
39
Relationship between bulk modulus, compressibility and speed of sound?
As bulk modulus increases, compressibility decreases and speed of sound increases
40
Relationship between compressibility and density?
Interdependent relationship: increase in density is coupled with a opposing change in compressibility
41
Definition of specular reflectors?
A sound beam directed at right angles to a smooth surface larger than the width of the beam: it will be partially reflected towards the sound source
42
Relationship between angle of incidence and angle of reflection?
The two are equal
43
Definition of acoustic impedance (Z)
The response of particles in terms of their velocity to a given pressure
44
Small masses, weak springs leads to:
Material with low density and stiffness, and low acoustic impedance
45
Large masses and stiff springs leads to:
Material with high density and stiffness: high acoustic impedance
46
Equation for acoustic impedance
Acoustic impedance = density x speed of sound
47
What happens in impedance mismatch?
Causes a proportion of waves to be transmitted and reflected at the interface between two mediums
48
Reflection coefficient equation?
Reflected intensity (Ir) / Incident Intensity (Ii)
49
Equation for reflection coefficient in terms of material?
(Z2 - z1) / (Z2 + Z1) squared
50
Equation for percentage reflection?
Multiple reflection coefficient by 100
51
How do you calculate transmission coefficient?
1 - reflection coefficient
52
Equation for transmission coefficient?
4 Z2 Z1 / (Z2 + Z1) squared
53
Percentage transmission?
Transmission coeffient x 100
54
When does acoustic shadowing occur?
When a high proportion of ultrasound beam is either reflected or attenuated by the target.
E.g. air/soft tissue interface
55
What happens with ultrasound waves at rough surfaces?
Diffuse reflection: A rough surface reflects the wave over a range of different angles
56
What happens when ultrasound waves encounter a small target?
Small targets will the scatter wave over a large angle.
57
What is scattering cause by?
By small scale variations in acoustic properties within organs:
constituting of small-scale reflecting targets of size comparable/smaller than ultrasound wavelength
58
What happens when scatterers is less than the wavelength?
Ultrasound is scattered isotropically in all directions
59
How does refraction occur?
If there is a change in speed at an interface between two media
60
In refraction, if the speed of sound increases...
The angle to the normal increases
61
If refraction, if the speed of sound decreases:
The angle to the normal decreases
62
What is snell's law?
Relationship between angle of incidence, transmission and their respective wave speeds.
63
Diffraction is determined by what?
The width of the source (aperture) and the wavelength of the wave
64
In diffraction: if the sound wave is larger than the point source, what happens?
Wave spreads out as an expanding sphere
65
In diffraction: if the sound wave is smaller than the point source, what happens?
Waves are relatively flat (plane wave)
66
For unfocused ultrasound transducers: Near Field Depth equation?
NFD = Radius of transducer (squared) / wavelength
67
For focused ultrasound transducers: equation for beam width?
W = 1.4 x wavelength x focal length / Aperture (transducer)
68
What is focusing gain or degree of focusing?
Quantitative relationship between near field distance (Rayleigh length) to focal length (F)
69
Value of focusing gain in weak focusing?
0
70
Value of focusing gain in medium focusing?
2
71
Value of focusing gain in strong focusing?
Gain > 2pi
72
What happens in absorption?
Direct conversion of energy into heat
73
Relationship between ultrasound frequency and absorption?
Absorption increases linearly with ultrasound frequency
74
Definition of attenuation?
Loss of energy as an ultrasound wave propagates through a medium (Fraction of energy removed from a plane wave)
75
In A-mode imaging, what is plotted versus the depth of interface?
Amplitude of reflected signal
76
In A-mode imaging, what is represented by the height of the peak?
'Strength' of amplitude of reflected echo
77
What is B-mode imaging?
Brightness mode scanning, modulating the brightness of a spot indicating the amplitude of the reflected signal
78
In B-mode imaging, does a white or black spot indicate a stronger echo?
White
79
What is M-mode imaging?
A-line data is displayed as a function of time, used to measure changing dimensions of moving structures?
80
Purpose of Time Gain Compensation
To amplify signals from deeper tissues to compensate for the decrease in received echo size due to attenuation
81
What is time gain control?
Ultrasound system control where operator adjusts amplifications of US signals from different depths to account for reduction in received US signals from deeper tissues
82
Purpose of the Lens in transducers?
Electronically focused to produce focusing only in the elevation plane (perpendicular to scan plane)
83
Purpose of backing material in the lens?
Designed to damp the motion of the element to prevent ringing of PZT plate by producing a short pulse
84
Purpose of kerf?
Cut space in between the elements
85
Ideal Z value for matching layer?
Halfway between the Z of PZT and skin to prevent reverberations
86
Purpose of matching layer?
1) Allows waves to efficiently enter the skin
| 2) Allows transmitted pulses to re-inforce and unwanted pulses to destructively interfere
87
When waves are generated from sources, what happens when they are in phase?
They interfere constructively forming a plane wave
88
What happens when waves are out of phase?
They interfere destructively
89
Three types of ultrasound array transducers in imaging?
Linear, curvilinear and phased arrays
90
When imaging with an array, what is the frame rate limited by?
```
Imaging depth (d) and number of lines (N) in an image
Speed of sound usually constant
```
91
Equation for minimum time for 1 line in an array?
2 x depth / speed of sound
92
Equation for time for one frame?
N x 2d/c
93
Equation for Frame rate?
Frame rate = c/2dN
94
Purpose of the system clock?
To send synchronising electronic pulses around the ultrasound system
Each electronic pulse represents a command to send a new ultrasound pulse around transducer
95
What is pulse repetition frequency (PRF)
Number of pulses sent out per unit time (Hz)
96
Purpose of the transmitter?
Responds to system clock by generating a high voltage pulse to excite transducer. This electrical signal causes an acoustic wave to be transmitted to the medium
97
What is the transmit power?
A user control which allows increase/decrease of the output power of the transducer
98
What is transmit beamforming?
Allows application of excitation pulses of different elements at different times to enable `steering or focusing of the ultrasound beam
99
What is an aperture?
Portion of piezoelectric plate used to generated ultrasound beam (Groups of elements)
100
Purpose of apodisation and how is it achieved?
To improve spatial resolution of a certain area.
| Achieved through non uniform excitation of individual elements, creating a longer focal zone and a broader main zone
101
Why is amplification needed in ultrasound?
Received echoes are too small in amplitude to undergo signal processing
102
What is needed first, signal amplification or TGC and why?
Amplification so that the signal can actually undergo processing. Then TGC can be applied
103
Purpose of the receive beamformer?
To delay received signal to maximise net signal associated with the reflector
104
What is the analogue to digital conversion (ADC)
Conversion of analogue echo signal to digital signal
105
What is the dynamic range?
Ratio of the largest to smallest signal that an ultrasound system is capable of processing
106
Is the dynamic range usually more or less than range of received echoes?
Less, these echoes need compression
107
Purpose of demodulation
Performed on received ultrasound signal to remove underlying RF signal
For B-mode imaging: used to extract envelope of received ultrasound signal
108
What is a grey scale transfer curve?
Relationship between image pixel value and displayed grey level
109
Is pre pocessing destructive or constructive and give examples?
Destructive, TGC, depth, scale and compression
110
Is post processing destructive or non-destructive?
Non-destructive
111
Purpose of frame averaging?
To minimise noise by taking a weighted average of current and previous images
112
One disadvantage of frame averaging?
Can result in blurring of rapidly moving images
113
In the doppler effect, how does the frequency change when the source moves away from the observer?
Frequency of detected sound is lower
114
In the doppler effect, how does the frequency change when the source moves towards the observer?
Frequency of detected sound is higher
115
Definition of doppler effect?
Change in frequency of a wave for an observer moving relative to its source
116
How does frequency change in the doppler effect?
If source is moving towards observer, frequency detected increases
If source is moving away from observer, frequency detected increases
117
In blood, how does the frequency change when the blood moves towards the transducer?
Received frequency is greater than transmitted frequency
118
In blood, how does the frequency change when the blood moves away from the transducer?
Transmitted frequency is greater than received frequency
119
In blood, what is the frequency change if both transducer and blood are stationary?
Transmitted and received frequency is equal
120
How many times is the ultrasound doppler shifted in blood?
Twice, once when it is scattered by the moving blood and again as a result of the motion of the blood
121
What is the detected doppler shift frequency? (fD)
Difference between transmitted and received frequency
122
What does the doppler shift frequency depend on?
Frequency of transmitted ultrasound, speed of sound in tissue and velocity of tissue, cos (angle between path of ultrasound beam and direction of blood flow)
123
How does the frequency doppler shift change as cos of angle increases?
As angle increases, fD value decreases until it reaches 90 degrees, which then it is zero
124
At what angle is the doppler shift frequency at its maximum?
At 90 degrees
125
Equation for velocity of blood in the medium?
Vb = c fd / 2ft cos (angle)
126
Two main display modes used in modern Doppler systems?
Spectral Doppler and 2D colour doppler
127
Use of spectral doppler?
Velocity of information detected from a signal location within the vessel in the form of frequency shift
128
Two modes of measurement in spectral doppler?
Greyscale indicates amplitude of detected ultrasound wave
| Vertical shift from baseline corresponds to Doppler shift
129
Define 2D colour flow doppler?
Displayed as a 2D colour image that is superimposed onto the B-mode image
130
Definition of continuous wave (CW) doppler?
Doppler system that transmits continuously and needs separate elements to transmit and receive echoes
131
In CW doppler, where is the doppler signals obtained?
In the sensitive region: the region of overlap between transmit/receive elements
132
Definition of pulse wave doppler?
Doppler system that transmits short, ultrasound pulses, the same element is used to transmit and receive
133
One advantage and one disadvantage of pulsed wave doppler?
Doppler signals can be acquired from a known depth due to the gate length
134
What is the gate depth and gate length and what doppler are they referring to?
Referring to PW doppler
Gate depth: specific depth or time that Doppler signals will be detected
Gate length: length of time over which Doppler signals are sampled
135
Four types of signals received ultrasound consists of?
1) Stationary and moving tissue
| 2) Stationary and moving blood
136
Typical velocity and intensity ranges for tissue and blood?
```
Blood = high velocity, low signal intensity
Tissue = low velocity, high amplitude
```
137
Outline the process of demodulation?
1) Reference and detected signal produces a mixed signal
2) A low pass filter is produced, removing the high frequency signal
3) Leaving the doppler shift signal
138
What is clutter?
Detection of ultrasound signals from stationary and moving tissues, this can be typically 30-40 dB greater than signals from blood
139
What happens after demodulation?
High frequency signals from transmit frequency is removed, leaving the Doppler shift signal from blood flow and clutter
140
Three stages of signal processor?
Demodulation, high pass filter and frequency estimator
141
Purpose of high pass filtering?
To remove the clutter signal and very low velocity blood velocities
142
Purpose of a spectrum analyser?
Calculates amplitude of all frequencies present in the signal
143
How does a spectrum analyser calculate amplitude?
Through fast fourier transform FFT
144
In spectral display, the brightness relates to what?
Power/amplitude of the Doppler signal at that Doppler frequency
145
Advantages of continuous wave doppler?
Increased sensitivity to detect slow flow
Operates at low acoustic power
Can discriminate small differences in low velocity
146
Disadvantages of CW Doppler
No depth information
| May have overlying vessels and do not know vessel angle
147
Key difference between CW and PW Doppler?
Received US signal is not available continuously in PW systems
148
Three steps of signal processing?
Demodulation, high pass filtration and frequency estimator
149
How is a peak positive Doppler amplitude achieved?
Received and reference signal is in phase after mixing
150
How is a positive Dopper amplitude achieved?
Slightly out of phase mixed signal
151
How is zero Doppler amplitude achieved?
90 (pi/2) out of phase mixed signal
152
How is a negative Doppler amplitude achieved?
Slightly out of phase mixed signal
153
How is a peak negative Doppler amplitude achieved?
Out of phase mixed signal
154
Purpose of time domain Doppler?
Estimates velocity of a target by measuring change in time of flight between consecutive echoes
155
Time domain: how to calculate distance moved between two consecutive pulses?
Dm = c(t2 - t1) / 2
```
D1 = ct1/2
D2 = ct2/2
```
156
Time domain: how to calculate velocity of target?
V = distance moved / pulse repetition interval
= c(t2 - t1) x PRF / 2
157
Which is more expensive, phase domain or time domain and why?
Time domain is more expensive as it needs to perform its calculations on radiofrequency data
158
What is the Nyquist limit?
The minimum amount a signal needs to be sampled to estimate the upper limit of the frequency
It must be sampled by at least half
159
What is aliasing?
When the pulse repetition frequency is too low so the Doppler frequency shift cannot be sampled properly.
Fd (max) = PRF / 2
160
Advantages of PW Doppler?
Depth information, can superimpose on B-mode image, good spectral information
161
Disadvantages of PW Doppler?
Aliasing limitis minimum PRF, gate needs to be set
162
Colour flow is a general term that covers what three modalities?
Colour Doppler
Power Doppler
Directional Power Doppler
163
Function of colour dopler?
Provides an image of mean Doppler frequency from blood, displayed in colour and superimposed onto B-mode image
164
Function of Power Doppler
Image in which power of Doppler signal is backscattered from blood and displayed in colour
165
Difference between power and directional power doppler?
Both display power of Doppler signals but Directional power shows separate colour coding of blood velocities towards and away from transducer
166
Purpose of autocorrelation?
To calculate mean frequency detected within each sample volume
167
What is a colour box?
Display showing doppler frequency within a limited region o the B-mode image.
Consists of a series of colour lines, made up of series of adjacent sample volumes
168
What does autocorrelation sacrifice?
Spectral information
169
Autocorrelator provides estimates of what three quantities?
1) Proportional to square of amplitude of Doppler signal
2) Mean Doppler frequency
3) Variance: quantification of variability of Doppler signal
170
Process of autocorrelation?
1) Compare lines in consecutive pairs
2) Divide each line into segments
3) Compare segment by segment
4) Use frequency content from segments
171
Colour doppler can display what autocorrelator outputs?
Variance and mean frequency
172
Direction power Doppler can display what autocorrelator outputs?
Mean frequency, direction and power
173
Power Doppler can display what autocorrelator outputs?
Power
174
How does blood have a high or low variance?
If they are moving at the same velocity, variance is low
| If moving at different velociteis, variance is high
175
Lamina flow of blood?
Fluid flow where neighbouring layers are not mixed, velocity profile incorporates slow moving components near vessel wall with faster components towards centre
176
Turbulent flow of blood
Characterised by cross currents and multiple velocity components
177
What is the blood tissue discriminator?
It's only possible to display either colour or B-mode information
Blood-tissue discriminator ensures isolation of blood info
178
Advantages of power doppler?
Can detect small volumes of blood flow
| Better SNR than colour doppler
179
Disadvantages of power doppler?
No measure of turbulence or mean velocity
| Insensitive to Doppler angle
180
3 mechanisms in which ultrasound may modify and/or destroy cells
1) Heating
2) Gas body activation
3) Radiation pressure
181
How is tissue heating through ultrasound exposure?
1) Local energy transfer from ultrasound wave by viscoelastic absorption
2) Heat transport from tissue to tissue (conduction)
3) Self heating from ultrasound transducer
182
What interface is most at risk of ultrasound heating?
Soft-tissue bone interfaces
183
Heating depends on what?
Time averaged intensity and/or power
184
Definition of spatial peak temporal peak intensity?
Max value at the pulse in the beam where it is the highest
185
Spatial peak pulse average intensity?
Average value over pulse duration in the beam where it is highest
186
Spatial peak temporal average intensity
Temporal average intensity in the beam where it is highest
187
Spatial average temporal average intensity
Temporal intensity averaged over beam area
188
What is the thermal index?
Dimensionless unit that is closely associated with temperature rise associated with ultrasound exposure
189
What is a thermal test object (TTO)?
Device designed to measure temperature rise associated with ultrasound exposure in soft tissue or bone mimic material
190
Equation for thermal index?
Power emitted / W deg
W deg: power needed to cause a temperature increase of 1 degree in US beam
