4. Physics of Ultrasound Flashcards
Define frequency, wavelength and velocity?
Freq = number of times a wave is repeated per sec (Hertz)
Wavelength = distance travelled by a sound wave in 1 cycle (mm)
Velocty = Rate at which sounds travels through an acoustic medium, determined by physical density (mass per unit volume) and stiffness (hardness)
Velocity (mm/µs) = Freq (MHz) x Wavelength (mm)
What is the range of human hearing? What is the typical range of diagnostic US?
Human hearing: 20-20,000Hz
US: Anything above this. (Infrasound = below this)
Diagnostics: 2-15 MHz
What effect do stiffness and physical density have on US transmission?
Stiffness: Increase will lead to increased velocity (if density constant)
Density: Increase will lead to decreased velocity (if stiffness constant)
Travel fastest in bone, slowest in gas (lung)
What is the veloctiy of sound in soft tissue?
1540 m/s
What is acoustic impedance?
Acoustic Impedance (Z) = Velocity (v) x Tissue density (p)
=> UNIT = Rayls
Differences in impedance between tissues determine how much of a sound wave is reflected / transmitted
Amplitude of echo PROPORTIONAL to difference in acoustic impedance in two adjacent substances i.e. no echo if identical impedance
Formulas for calculation of reflection / transmission of sound wave
% transmitted = 100 - % reflected
Where Z = acoustic impedance of different substances
Different sound reflection at interfaces?
What is the angle of incidence? How does it affect image formation?
- Angle of US beam relative to reflector. If perpendicular, 180deg reflection
- If > perpendicular return angle matches. >3 degress likely not detected by transducer
What is attenuation? What is the approximation of this value?
- Loss of energy through a medium
approximately 0.5 dB/cm per MHz over a round trip distance
=> High freq attenuated more. Occurs by absorption, scatter and reflection
What are PRF and SPL?
PRF = Pulse repition frequency, number of us pulses per sec
SPL = Spatial Pulse Length = length of space in one pulse of US. E.g. if wavlength is 0.5mm and 3 waves per pulse, SPL = 1.5mm
=> SPL IMPORTANT FOR AXIAL RESOLUTION
NOTE: >99% of time probe is LISTENING
How is axial resolution measured / quantified?
Ability to distinguish two reflectors along direction sound is travelling:
= HALF the SPL
NB: Higher freq, smaller SPL, therefore > axial resolution
What factor determines lateral resolution of US?
Width of US beam -> Best at focal point, as beam narrowest.
What is harmonic imaging ? Why does it improve image quality?
- Use integer multiples of fundamental freq; E.g. 4MHz, 8MHz, 12MHz etc. Created by compression and expansion of objects
=> usually only 2nd harmonic used as the rest absorbed.
- Improved quality because:
1) Narrow primary beam -> Better lat res
2) Reverb, side lobe and grating lobe reduced or eliminated
3) Harmonic beam generated at depth beyond where some artifactual problems occur, reducing image degradation and noise in near field
What is spatial compounding?
- Composite image generated by combining multiple frames from US beams in different directions. Beam steering can contribute
Adv:
Improved spatial res (border definition)
Improved contrast resolution
Increased SNR
Can help with acoustic enhancement and shadowing artefacts
Dis
Decreased temporal res (takes more time toa acquire
How is distance calculated by US machine?
Pulse - echo principle
Rate (mm/µs) x Time (µs) = Distance (mm)
NB: Assumes 1540m/s
Why does acoustic enhancement occur?
- decreased attenuaion of beam distal to e.g. fluid filled structure
What is reverberation artefact?
- SOund wave reflecting between two strong reflectors -> multiple hyperechoic foci occuring at regular intervals
Whatt are comet tail / ring down artifacts?
- Variants of reverberation artefacts:
Comet tail - two closely spaced, discrete, highly reflective surfaces. e.g. gas bubble, pellet. Diverging thin line.
Ring down = US reverberates tihin fluid trabbed in a tetrahedrn of air bubbles
What is Mirror image artifact?
- Duplication of normal structure opposite sides of strong reflector
- See diagram for explanation
What are side lobes / grating lobes?
- Secondary sound beams that emanate in a different direction than primary
Side: All transducers
Grating: Only array
=> must encounter ++ reflective surface to be of sufficeitn intensity to be noticed.
What is pseudosludge, and how can it be identiifed / corrected?
- Slice thickness artifact, typically in UB and GB
- SUrface often curved
- Image from different angle
What causes edge shadowing?
- Refraction of sound hitting curved reflector tangentially.
- Refractionartefacts can also result in lateral displacement of objects, affecting e.g. measurment
What is doppler shift? And what is the doppler equation?
- Doppler shift is the frequency difference between the incident sound wave and reflected sound wave (typically hitting e.g. RBC)
fD = (2 x f0 x vRBC) / c
Where fD = frequency of doppler shift, c is speed of sound in ST (1540m/s), f0 = original frequency
Frequency shift is in Hz, so hence audible range.
THIS EQUATION ASSUMES PERPENDICULAR RELATIONSHIP BETWEEN SOUND AND REFLECTOR
In actuality, need to control for Doppler angle otherwise shift is underestimated:
vRBC = (fD x c) / (2 x f0 x Cos angle)
Cannot measure shift if angle is 90 becaus cos = 0, and division by 0 impossible.
SHOULD BE < 60DEG
How do continuous wave and pulsed wave doppler compare?
- Continuous:
Always sending and recieving
HIGHLY ACCURATE FOR SHIFT
Can record higher velocities
Spectral trace produced
Con = all blood flow measured, so cannot distinguish e.g. velocity from two vessels
- Pulsed
Used with B mode (duplex scanning)
Gate placed in vessel
Spectral trace produced
Spectral trace = velocity and direction as function of time.
