Essentials of ultrasound physics Flashcards
What is acoustic pressure and its units?
Quantification of the strength of the wave
- units = Pascals
What determines speed of sound through a material?
c = Sqrt(K / p)
What is the difference between characteristic impedance and acoustic impedance?
They are practically the same
What are the units for acoustic impedance?
Pressure per velocity per area
(or rayls)
What is acoustic impedance and its equation?
The acoustic pressure divided by the resultant particle velocity
Z = pc
What is Snell’s law?
The law for angle of refraction of an ultrasound beam
sin(theta2) / sin(theta1) = c2 / c1
What two conditions are required for refraction?
- Must be an incident angle between transducer and interface
- Must be 2 materials with different c
What is the difference between specular and diffuse reflection?
Diffuse = all directions
Specular = one direction
What are Rayleigh scatterers?
Scatterers that are much smaller than the wavelength of sound
- scatter sound in all directions
What is the 3dB rule?
Whenever intensity changes by 3dB this corresponds to a doubling of the intensity
What are the two sources of attenuation in the body?
- Reflection and scatter at interfaces
- Absorption (conversion to heat)
How is attenuation of ultrasound in tissue measured?
dB / cm
What is the purpose of the backing layer?
Allows shorter pulse duration
What is the equation for pulse duration and frequency?
PD = n / f
n = No. of cycles
f = frequency
What are both the near field length and the far field spread dependent on?
The transducer aperture
What is the interference of 2 waves 180 degrees apart?
Destructive - will cancel if they are the same sine wave
What are ultrasound transducer elements made of?
PZT (lead zirconate titanate)
How are PZT elements polarised?
They are heated above 365 degrees to allow particles to move.
A voltage is then applied across them and they are cooled with the voltage applied
What is the main determinant of the resonant frequency of a PZT crystal?
The thickness of the element
What is the purpose of backing material?
To provide damping needed for short pulses
What are Huygen outlets?
An ultrasound transducer with a collection of point sources that fire coherently to produce a beam
What is apodization?
Signal strength varies at different points on the transducer face - weaker at sides to reduce side lobe interference
Define focal zone?
The region over which the beam width is less than 2x the beam width at the focus
What are the two main advantages of using arrays?
- Electronic beam steering
- Electronic focusing and beam forming
How is electronic focusing achieved?
By exciting elements at different times - introducing delay
What determines slice thickness for linear, curved or phased array transducers?
The fixed focal length lens attached to the entire array
What is the duty factor?
The fraction of time the transducer actively transmits sound
What is demondulation?
When the receiver converts the amplified raw echo signal (pulses) into a single pulse spike
What is persistence?
Signals for each pixel location are combined with previous signals from the same location
What is the Doppler frequency?
fD = (2 v fT cos(theta)) / c
What is a possible explanation of ISB?
Differences in Cos(theta) due to the difference in angle from the middle and edges of the transducer
What are the 4 main pulsed doppler controls?
- Range gate position
- Gate or sample volume size
- Pulse duration
- Flow angle cursor
How is a digital signal converted to a spectral waveform?
Fast Fourier Transform
What can cause the filling of the spectral window?
- Turbulence
- Spectral broadening due to a wider range of doppler frequencies from the sample volume (wider range gate)
How do you calculate pulsatility index?
PI = (PSV - EDV) / average
How do you calculate resistive index?
RI = (max - min) / max
When does aliasing occur?
When the PRF < two times the maximum frequency
What is the Nyquist limit?
The condition when PRF = 2fD
- it defines the minimum sample rate for a signal before aliasing will occur
What is the equation for maximum velocity detectable with pulsed Doppler?
Vmax = ( c * PRFmax) / 4f
- as frequency is on the bottom, lower frequencies will allow a higher vmax to be detected
What does high PRF mode do?
Creates multiple range gates
What is PRF the same as?
Scale
What are colour packets?
The multiple pulse-echo signals that are required for colour flow imaging
What is colour hue?
The attribute of colour that allows it to be classified as red, yellow, green, blue or an intermediate
- hue is associated with wavelength of light
What is a pure hue?
A completely saturated colour with only 1 wavelength associated with it
What causes a less saturated colour?
The presence of more white light
- white light is composed of many wavelengths
What does the colour of power mode doppler depend on?
The strength of Doppler signal, number of reflecting moving targets
What are advantages of Power doppler?
- It is less impacted by angular effects
- It has more sensitivity than standard colour imaging
- If aliasing is present it does not affect energy mode display
What are disadvantages of power doppler?
- Image build up and image rates tend to be lower than colour
- There are greater amounts of flash artefact
What is a specular reflector?
Reflection at a smooth surface
What are diffuse reflectors?
Reflectors that scatter sound in all directions
What are 3 main assumptions of an ultrasound scanner?
- Reflectors giving rise to echoes lie along the beam axis
- The speed of sound is constant
- The echo strength indicates echogenicity
When can spectral mirroring occur?
- When gain is too high
- When Doppler beam approaches 90 degrees to the vessel axis
How can beam width artefacts occur?
- Small objects are broadened - appear as lines
- Smearing of echo information
In which direction is beam width widest?
Perpendicular to the image plane (in the slice thickness direction) has greater thickness than the image plane
What is acoustic power?
The rate at which energy is transmitted from the transducer to the medium being scanned
What is a hydrophone?
A device used to measure acoustic pressure and intensity
How does intensity relate to pressure?
Intensity is proportional to square of pressure
How do time average and pulse average intensities compare?
Pulse average are often much higher (1000x) than time average
What is thermal index?
TI is the ratio of acoustic power produced by the transducer to the power required to raise the temperature in tissue by 1 degree.
What does a TI value of 1 mean?
When the probe is stationary, it has the potential to raise tissue temperature by 1 degree
What is cavitation?
The activity of tiny gas bubbles in tissue in the presence of ultrasound waves
- ultrasound can generate tiny bubbles from dissolved gas in fluid
What is mechanical index?
The likelihood of a transducer causing cavitation
What is MI proportional to?
The peak rarefactional pressure
What is MI inversely proportional to?
The sqrt (f)
- if frequency increases, MI decreases
What are the advantages of ultrasound safety indices?
- Standardization of output specification information between manufacturers
- Presentation of output quantities relevant to potential bioeffects from ultrasound
- Information is available for users to implement ALARA
When can MI and TI be underestimated?
When a large fraction of the path between transducer and isonified region contains fluid e.g. bladder or amniotic fluid.
What is the biological effect concerned with TI?
Heating, caused by absorption of energy from the ultrasound beam
What are the two types of cavitation?
- Stable cavitation - creation of bubbles that oscillate with the ultrasound beam
- Transient (or inertial) cavitation - oscillations grow so strong that bubbles collapse - producing very intense, localised effects
What are non-cavitation mechanical effects?
Bioeffects caused by particle displacements
