Ultrasound Flashcards
Two types of waves?
Longitudinal and transverse
Definition of sound wave
Mechanical disturbance of a medium which passes through the medium at a fixed speed
What is rarefaction?
A region of negative pressure: particles in the medium move away from each other
What is compression?
A region of positive pressure: particles in the medium move towards each other
Definition of wavelength
Distance between two adjacent peaks or troughs in consecutive waves
What is wavelength measured in?
Greek letter: lambda
Typical wavelengths used for medical ultrasound imaging?
0.1-1mm
Frequency
Rate at which waves (a periodic propagating pattern) passes a fixed point per second
Definition of period
The time needed for one complete wave/cycle
It is the reciprocal of frequency
What is frequency measured in?
Hertz (Hz)
Typical frequencies used in medical ultrasound?
1-15 Mhz
Equation for wave speed?
Speed = frequency x wavelength
Units: metres per second
What does sound travel faster in, air, bone or liver?
Fastest in bone, then liver, then air
Definition of ultrasound?
Sound at a frequency above which the human ear can generally hear > 20 kHz
How is ultrasound generated?
Transducer is placed in contact with the skin, with a coupling gel in between. Beam of ultrasound pulses are transmitted into the tissue
Behaviour of ultrasound in tissue?
Waves propagate through tissue, partially reflecting or scatter from changes in acoustic impedance.
Detection of ultrasound
Reflected waves are detected by the probe and stored in the scanner
How is the image displayed in ultrasound
Depth information from tissue is calculated using speed of sound and time of flight. Stored data is used to form the image
Definition of Piezoelectric effect
When stress is applied to a certain material, a voltage is generated that is proportional to applied stress.
Is piezoelectric effect reversible or irreversible?
Reversible
Role of piezoelectric effect in the production of sound?
An electrical voltage causes the element to deform, releasing an ultrasound pulse
Role of piezoelectric in detection of sound?
Reflected echo from tissue exerts a pressure creating an electrical signal
What is the role of the piezoelectric plate?
For generation and detection of ultrasound signals
That is the piezoelectric plate made from?
Lead zirconate titantate (PZT)
How can PZT materials be damaged?
Mechanical damage: dropping
Heating
Exceeding electrical limits set by the manufacturer
How can the time of flight be calculated?
Distance = speed x time of flight / 2
Assumptions used in the pulse-echo principle?
Propagation in a straight line
A ‘thin’ beam
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
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
What is distortion?
When a force is applied to a material causing a change in shape or volume
Definition of density?
Mass of the medium per unit volume
Relationship between speed of sound and density?
Speed of sound is inversely proportional to the square root of density in the medium
Definition of compressibility?
Fractional decrease in volume when pressure is applied to the material.
Higher compressibility = easier to compress the material
Relationship between speed of sound and compressibility?
Speed of sound is inversely proportional to square root of compressibility
Relationship between bulk modulus and speed of sound?
Speed is directly proportional to square root of bulk modulus
Definition of bulk modulus
Negative ratio of stress and strain
Why is there a negative sign in bulk modulus?
A positive pressure causes a decrease in volume.
What does a large value for bulk modulus mean?
It is resistant to change when a force is applied
Relationship between bulk modulus, compressibility and speed of sound?
As bulk modulus increases, compressibility decreases and speed of sound increases
Relationship between compressibility and density?
Interdependent relationship: increase in density is coupled with a opposing change in compressibility
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
Relationship between angle of incidence and angle of reflection?
The two are equal
Definition of acoustic impedance (Z)
The response of particles in terms of their velocity to a given pressure
Small masses, weak springs leads to:
Material with low density and stiffness, and low acoustic impedance
Large masses and stiff springs leads to:
Material with high density and stiffness: high acoustic impedance
Equation for acoustic impedance
Acoustic impedance = density x speed of sound
What happens in impedance mismatch?
Causes a proportion of waves to be transmitted and reflected at the interface between two mediums
Reflection coefficient equation?
Reflected intensity (Ir) / Incident Intensity (Ii)
Equation for reflection coefficient in terms of material?
(Z2 - z1) / (Z2 + Z1) squared
Equation for percentage reflection?
Multiple reflection coefficient by 100
How do you calculate transmission coefficient?
1 - reflection coefficient
Equation for transmission coefficient?
4 Z2 Z1 / (Z2 + Z1) squared
Percentage transmission?
Transmission coeffient x 100
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
What happens with ultrasound waves at rough surfaces?
Diffuse reflection: A rough surface reflects the wave over a range of different angles
What happens when ultrasound waves encounter a small target?
Small targets will the scatter wave over a large angle.
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
What happens when scatterers is less than the wavelength?
Ultrasound is scattered isotropically in all directions
How does refraction occur?
If there is a change in speed at an interface between two media
In refraction, if the speed of sound increases…
The angle to the normal increases
If refraction, if the speed of sound decreases:
The angle to the normal decreases
What is snell’s law?
Relationship between angle of incidence, transmission and their respective wave speeds.
Diffraction is determined by what?
The width of the source (aperture) and the wavelength of the wave
In diffraction: if the sound wave is larger than the point source, what happens?
Wave spreads out as an expanding sphere
In diffraction: if the sound wave is smaller than the point source, what happens?
Waves are relatively flat (plane wave)
For unfocused ultrasound transducers: Near Field Depth equation?
NFD = Radius of transducer (squared) / wavelength
For focused ultrasound transducers: equation for beam width?
W = 1.4 x wavelength x focal length / Aperture (transducer)
What is focusing gain or degree of focusing?
Quantitative relationship between near field distance (Rayleigh length) to focal length (F)
Value of focusing gain in weak focusing?
0
Value of focusing gain in medium focusing?
2
Value of focusing gain in strong focusing?
Gain > 2pi
What happens in absorption?
Direct conversion of energy into heat
Relationship between ultrasound frequency and absorption?
Absorption increases linearly with ultrasound frequency
Definition of attenuation?
Loss of energy as an ultrasound wave propagates through a medium (Fraction of energy removed from a plane wave)
In A-mode imaging, what is plotted versus the depth of interface?
Amplitude of reflected signal
In A-mode imaging, what is represented by the height of the peak?
‘Strength’ of amplitude of reflected echo
What is B-mode imaging?
Brightness mode scanning, modulating the brightness of a spot indicating the amplitude of the reflected signal
In B-mode imaging, does a white or black spot indicate a stronger echo?
White
What is M-mode imaging?
A-line data is displayed as a function of time, used to measure changing dimensions of moving structures?
Purpose of Time Gain Compensation
To amplify signals from deeper tissues to compensate for the decrease in received echo size due to attenuation
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
Purpose of the Lens in transducers?
Electronically focused to produce focusing only in the elevation plane (perpendicular to scan plane)
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
Purpose of kerf?
Cut space in between the elements
Ideal Z value for matching layer?
Halfway between the Z of PZT and skin to prevent reverberations
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
When waves are generated from sources, what happens when they are in phase?
They interfere constructively forming a plane wave
What happens when waves are out of phase?
They interfere destructively
Three types of ultrasound array transducers in imaging?
Linear, curvilinear and phased arrays
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
Equation for minimum time for 1 line in an array?
2 x depth / speed of sound
Equation for time for one frame?
N x 2d/c
Equation for Frame rate?
Frame rate = c/2dN
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
What is pulse repetition frequency (PRF)
Number of pulses sent out per unit time (Hz)
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
What is the transmit power?
A user control which allows increase/decrease of the output power of the transducer
What is transmit beamforming?
Allows application of excitation pulses of different elements at different times to enable `steering or focusing of the ultrasound beam
What is an aperture?
Portion of piezoelectric plate used to generated ultrasound beam (Groups of elements)
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
Why is amplification needed in ultrasound?
Received echoes are too small in amplitude to undergo signal processing
What is needed first, signal amplification or TGC and why?
Amplification so that the signal can actually undergo processing. Then TGC can be applied
Purpose of the receive beamformer?
To delay received signal to maximise net signal associated with the reflector
What is the analogue to digital conversion (ADC)
Conversion of analogue echo signal to digital signal
What is the dynamic range?
Ratio of the largest to smallest signal that an ultrasound system is capable of processing
Is the dynamic range usually more or less than range of received echoes?
Less, these echoes need compression
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
What is a grey scale transfer curve?
Relationship between image pixel value and displayed grey level
Is pre pocessing destructive or constructive and give examples?
Destructive, TGC, depth, scale and compression
Is post processing destructive or non-destructive?
Non-destructive
Purpose of frame averaging?
To minimise noise by taking a weighted average of current and previous images
One disadvantage of frame averaging?
Can result in blurring of rapidly moving images
In the doppler effect, how does the frequency change when the source moves away from the observer?
Frequency of detected sound is lower
In the doppler effect, how does the frequency change when the source moves towards the observer?
Frequency of detected sound is higher
Definition of doppler effect?
Change in frequency of a wave for an observer moving relative to its source
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
In blood, how does the frequency change when the blood moves towards the transducer?
Received frequency is greater than transmitted frequency
In blood, how does the frequency change when the blood moves away from the transducer?
Transmitted frequency is greater than received frequency
In blood, what is the frequency change if both transducer and blood are stationary?
Transmitted and received frequency is equal
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
What is the detected doppler shift frequency? (fD)
Difference between transmitted and received frequency
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)
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
At what angle is the doppler shift frequency at its maximum?
At 90 degrees
Equation for velocity of blood in the medium?
Vb = c fd / 2ft cos (angle)
Two main display modes used in modern Doppler systems?
Spectral Doppler and 2D colour doppler
Use of spectral doppler?
Velocity of information detected from a signal location within the vessel in the form of frequency shift
Two modes of measurement in spectral doppler?
Greyscale indicates amplitude of detected ultrasound wave
Vertical shift from baseline corresponds to Doppler shift
Define 2D colour flow doppler?
Displayed as a 2D colour image that is superimposed onto the B-mode image
Definition of continuous wave (CW) doppler?
Doppler system that transmits continuously and needs separate elements to transmit and receive echoes
In CW doppler, where is the doppler signals obtained?
In the sensitive region: the region of overlap between transmit/receive elements
Definition of pulse wave doppler?
Doppler system that transmits short, ultrasound pulses, the same element is used to transmit and receive
One advantage and one disadvantage of pulsed wave doppler?
Doppler signals can be acquired from a known depth due to the gate length
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
Four types of signals received ultrasound consists of?
1) Stationary and moving tissue
2) Stationary and moving blood
Typical velocity and intensity ranges for tissue and blood?
Blood = high velocity, low signal intensity Tissue = low velocity, high amplitude
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
What is clutter?
Detection of ultrasound signals from stationary and moving tissues, this can be typically 30-40 dB greater than signals from blood
What happens after demodulation?
High frequency signals from transmit frequency is removed, leaving the Doppler shift signal from blood flow and clutter
Three stages of signal processor?
Demodulation, high pass filter and frequency estimator
Purpose of high pass filtering?
To remove the clutter signal and very low velocity blood velocities
Purpose of a spectrum analyser?
Calculates amplitude of all frequencies present in the signal
How does a spectrum analyser calculate amplitude?
Through fast fourier transform FFT
In spectral display, the brightness relates to what?
Power/amplitude of the Doppler signal at that Doppler frequency
Advantages of continuous wave doppler?
Increased sensitivity to detect slow flow
Operates at low acoustic power
Can discriminate small differences in low velocity
Disadvantages of CW Doppler
No depth information
May have overlying vessels and do not know vessel angle
Key difference between CW and PW Doppler?
Received US signal is not available continuously in PW systems
Three steps of signal processing?
Demodulation, high pass filtration and frequency estimator
How is a peak positive Doppler amplitude achieved?
Received and reference signal is in phase after mixing
How is a positive Dopper amplitude achieved?
Slightly out of phase mixed signal
How is zero Doppler amplitude achieved?
90 (pi/2) out of phase mixed signal
How is a negative Doppler amplitude achieved?
Slightly out of phase mixed signal
How is a peak negative Doppler amplitude achieved?
Out of phase mixed signal
Purpose of time domain Doppler?
Estimates velocity of a target by measuring change in time of flight between consecutive echoes
Time domain: how to calculate distance moved between two consecutive pulses?
Dm = c(t2 - t1) / 2
D1 = ct1/2 D2 = ct2/2
Time domain: how to calculate velocity of target?
V = distance moved / pulse repetition interval
= c(t2 - t1) x PRF / 2
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
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
What is aliasing?
When the pulse repetition frequency is too low so the Doppler frequency shift cannot be sampled properly.
Fd (max) = PRF / 2
Advantages of PW Doppler?
Depth information, can superimpose on B-mode image, good spectral information
Disadvantages of PW Doppler?
Aliasing limitis minimum PRF, gate needs to be set
Colour flow is a general term that covers what three modalities?
Colour Doppler
Power Doppler
Directional Power Doppler
Function of colour dopler?
Provides an image of mean Doppler frequency from blood, displayed in colour and superimposed onto B-mode image
Function of Power Doppler
Image in which power of Doppler signal is backscattered from blood and displayed in colour
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
Purpose of autocorrelation?
To calculate mean frequency detected within each sample volume
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
What does autocorrelation sacrifice?
Spectral information
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
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
Colour doppler can display what autocorrelator outputs?
Variance and mean frequency
Direction power Doppler can display what autocorrelator outputs?
Mean frequency, direction and power
Power Doppler can display what autocorrelator outputs?
Power
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
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
Turbulent flow of blood
Characterised by cross currents and multiple velocity components
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
Advantages of power doppler?
Can detect small volumes of blood flow
Better SNR than colour doppler
Disadvantages of power doppler?
No measure of turbulence or mean velocity
Insensitive to Doppler angle
3 mechanisms in which ultrasound may modify and/or destroy cells
1) Heating
2) Gas body activation
3) Radiation pressure
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
What interface is most at risk of ultrasound heating?
Soft-tissue bone interfaces
Heating depends on what?
Time averaged intensity and/or power
Definition of spatial peak temporal peak intensity?
Max value at the pulse in the beam where it is the highest
Spatial peak pulse average intensity?
Average value over pulse duration in the beam where it is highest
Spatial peak temporal average intensity
Temporal average intensity in the beam where it is highest
Spatial average temporal average intensity
Temporal intensity averaged over beam area
What is the thermal index?
Dimensionless unit that is closely associated with temperature rise associated with ultrasound exposure
What is a thermal test object (TTO)?
Device designed to measure temperature rise associated with ultrasound exposure in soft tissue or bone mimic material
Equation for thermal index?
Power emitted / W deg
W deg: power needed to cause a temperature increase of 1 degree in US beam
