4. Physics of Ultrasound Flashcards

1
Q

Define frequency, wavelength and velocity?

A

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)

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2
Q

What is the range of human hearing? What is the typical range of diagnostic US?

A

Human hearing: 20-20,000Hz

US: Anything above this. (Infrasound = below this)

Diagnostics: 2-15 MHz

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3
Q

What effect do stiffness and physical density have on US transmission?

A

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)

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4
Q

What is the veloctiy of sound in soft tissue?

A

1540 m/s

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5
Q

What is acoustic impedance?

A

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

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6
Q

Formulas for calculation of reflection / transmission of sound wave

A

% transmitted = 100 - % reflected

Where Z = acoustic impedance of different substances

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7
Q

Different sound reflection at interfaces?

A
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8
Q

What is the angle of incidence? How does it affect image formation?

A
  • Angle of US beam relative to reflector. If perpendicular, 180deg reflection
  • If > perpendicular return angle matches. >3 degress likely not detected by transducer
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9
Q

What is attenuation? What is the approximation of this value?

A
  • 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

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10
Q

What are PRF and SPL?

A

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

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11
Q

How is axial resolution measured / quantified?

A

Ability to distinguish two reflectors along direction sound is travelling:

= HALF the SPL

NB: Higher freq, smaller SPL, therefore > axial resolution

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12
Q

What factor determines lateral resolution of US?

A

Width of US beam -> Best at focal point, as beam narrowest.

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13
Q

What is harmonic imaging ? Why does it improve image quality?

A
  • 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
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14
Q

What is spatial compounding?

A
  • 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

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15
Q

How is distance calculated by US machine?

A

Pulse - echo principle

Rate (mm/µs) x Time (µs) = Distance (mm)

NB: Assumes 1540m/s

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16
Q

Why does acoustic enhancement occur?

A
  • decreased attenuaion of beam distal to e.g. fluid filled structure
17
Q

What is reverberation artefact?

A
  • SOund wave reflecting between two strong reflectors -> multiple hyperechoic foci occuring at regular intervals
18
Q

Whatt are comet tail / ring down artifacts?

A
  • 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

19
Q

What is Mirror image artifact?

A
  • Duplication of normal structure opposite sides of strong reflector
  • See diagram for explanation
20
Q

What are side lobes / grating lobes?

A
  • 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.

21
Q

What is pseudosludge, and how can it be identiifed / corrected?

A
  • Slice thickness artifact, typically in UB and GB
  • SUrface often curved
  • Image from different angle
22
Q

What causes edge shadowing?

A
  • Refraction of sound hitting curved reflector tangentially.
  • Refractionartefacts can also result in lateral displacement of objects, affecting e.g. measurment
23
Q

What is doppler shift? And what is the doppler equation?

A
  • 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

24
Q

How do continuous wave and pulsed wave doppler compare?

A
  • 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.

25
Q

What does spectral broadening refer to?

A
  • Thicke spectral trace in PW / CW doppler -> heterogeneous flow velocities e.g. turbulence, or if close to vessel may have multiple US angles contributing
26
Q

What is colour doppler?

A

Type of PW doppler

  • Colour coded MEAN velocity
  • Angle dependent
  • Determine absence, direction, speed and character of flow
27
Q

What should doppler angle be?

A

<60 degrees

28
Q

What is power doppler?

A
  • Analyses total strength while ignoring direction
  • Detects very low velocty blood, and small vessels
  • Angle independent
  • Not prone to aliasing
29
Q

What is aliasing?

A
  • When PRF too low in pulsed doppler, portion of tracing wraps around to opposite direction.
  • COlour: Addition of colour from opposite end of scale

SAMPLING RATE NEEDS TO BE 2x THE HIGHEST DOPPLER SHOFT FREQUENCY OF THE BLOOD FLOW

Fix:

Increase PRF (Scale)

Decrease depth of sampling gate

Reduce frequency

Decrease angle

30
Q

What is range ambiguity?

A

If PRF too long - Lagging echoes may not return before new echo sent. Incorrectly plotted closer to transducer