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VetRad Boards prep > Ultrasound physics > Flashcards

Ultrasound physics Flashcards

1
Q

What two things determine strength of sound?

A
  • Pressure amplitude
  • Intensity
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2
Q

What is/how do you explain a harmonic frequency?

A
  • A sinusoidal waveform is characterized by a single waveform (fundamental frequency).
    • Any other wave shape contains additonal freuencies that are even or odd multiples of that original frequency.
    • as the wave becomes less sinusoidal, the harmonics become stronger
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3
Q

How do you calculate beam intensity?

A

I = power (energy) ÷ area

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4
Q

How does intensity relate to pressure?

A

I is proportional to P2

(doubling pressure quadruples intensity)

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5
Q

Describe the relationship between speed of sound, wavelength and frequency?

Of these variabel, which are affected by the medium?

A
  • speed of sound (c) = wavelength x frequency (f)
  • frequency is unaffected by the propagation medium, unlike the other two
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6
Q

What are the frequencies for:

  • infrasound
  • audible sound
  • ultrasound
  • medical ultrasound
A
  • infrasound = <15 cyles/s (Hz)
  • Audible sound = 15 Hz- 20 kHz
  • Ultrasound = >20 kHz
  • medical ultrasound = 2-50 MHz
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7
Q

What is a period?

A

The time it takes for one cylce to occur

period = 1 ÷ frequency (f)

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8
Q

What two factors affect speed propagation and which is more important?

A
  • Tissue stiffness and tissue density
  • tissue stiffness effects>>tissue density
    • propagation speed increases with increased stiffness
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9
Q

What is attenuation coefficient?

A

relative intensity loss that occurs with each cm the sound travels (Att coeff = dB/cm)

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10
Q

How do you calculate atteunation?

A

a (dB) = u (dB/cm) x L (cm)

increases in attenuation coeff or path length will increas attenuation

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11
Q

What is the attenuation coefficient in soft tissue?

A

about 0.5 dB/cm for each MHz of frequency

a (dB) = 0.5 (dB/cm) x f (MHz) ÷ L (cm)

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12
Q

How do you calculate relative intensity?

A

Relativ intensity (dB) = 10 log [Iincident/Iecho]

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13
Q

What is the relation ship between acoustic impedance (Z), density (p) and propagation speed (c)?

A

Z (kg/m2sec) = p (kg/m3) x c (m/sec)

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14
Q

How do you calculate the reflection pressure coefficient? How can the Intensity Reflection Coefficient (IRC) be extrapolated from this? page 5

A

Rp = Pr ÷ Pi = [(Z2 - Z1) ÷ (Z2 + Z1)]

Since intensity (I) is proportional to P2, then:

IRC = Ir ÷ Ii = [(Z2 - Z1) ÷ (Z2 + Z1)]2

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15
Q

How does the intensity transmission coefficient (ITC) relate to the intensity reflection coefficient (IRC)?

A

ITC = 1 - IRC

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16
Q

What is the critical angle?

A

angle of incidence of the sound beam with a boundry b/w two media that when exceeded will cause total reflection.

Critical angle = SinØc = c1 ÷ c2

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17
Q

What is Snell’s law?

A

[sinØi ÷ SinØt] = C1 ÷ C2

If C2 > C1, then the angle of transmission is > the angle of incidence

If C2< C1, then the angle of transmission is < the angle of incidence

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18
Q

What determines the resonant frequency of a piezoelectric element?

A
  • The thickness of the piezoelectric element (0.2 - 1mm)
  • Propagation speed of the element material (4-6 mm/us)

f (MHz) = [Ct (mm/us)] ÷ 2 x thickness (mm)

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19
Q

What is the pulse repitition period?

A

Time from the beginning of one pulse to the beginning of the next: PRP = 1 ÷ PRF

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20
Q

How does dampening relate to bandwidth?

A

Shortening the pulse broadens the bandwidth

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21
Q

What are the effects of dampening and what are some advantages and disadvantages?

A
  • Reduces the spatial pulse length
  • Reduces the pulse duration

Advantages:

  • improves axial resolution in the near field
  • allows harmonic imaging

Disadvantages

  • reduces ultrasound amplitude, reducing efficiency and sensitivity of the system
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22
Q

What is Q factor?

A

Describes the bandwidth of the sound emanating from the transducer:

Q = f0 ÷ bandwidth

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23
Q

What is the difference between a High Q transducer and a Low Q transducer?

A
  • High Q transducer:
    • Narrow bandwidth (little dampening)
    • long spatial pulse length and decreased resolution
  • Low Q transducer:
    • Wide bandwidth (more dampening)
    • small spatial pulse length and increased axial resolution
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24
Q

What is Thermal Index (TI)?

A

TI = the ratio of acoustic power (W) produced by the transducer to the power required to raise the tissue in the beam area by 1°C

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25
Q

What is mechanical Index?

A

Value that estimates the likelihood of cavitation by the ultrasound beam

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26
Q

What is ISPTA? And what are the current FDA recommendations?

A

Spatial peak temporal average intensity (ISPTA) -

  • It is a good indicator of thermal US effects
  • FDA recommendations
    • ISPTA diagnostic ultrasound <100 mW/cm2
    • pulsed doppler <1000 mW/cm2
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27
Q

What are methods to decarease “exposure”?

A
  • Keep power low
  • Increase gain instead of increasing power
  • Choose scanned modes over unscnanned modes (B is less than M)
  • Decrease pulse length
  • Use appropraite transducer
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28
Q

What is I SPPA?

A

Spatial peak pulse average intensity -

  • indicator for potential mechanical bioeffects and cavitation
  • required by FDA
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29
Q

What are 3 ways harmonic ultrasound imporves image quality?

A
  • harmonic beam is narrower, flatter and has twice the frequency as the fundamental frequency
    • results in improved lateral resolution, elevational resolution and axial resolution
  • grating lobe artifacts are eliminated
  • the haroni beam is generated at a depth beyond where some of artefactual problems occur so the image degredation they cause is reduced or eliminated e.g superficial reverberation)
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30
Q

How do you increase axial resolution?

A
  • decrease the spatial pulse length
    • using higher frequency → decreases wavelength, thus decreasing SPL
    • increase dampening → lower Q, reduces pulse duration, decreasing SPL
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31
Q

What does beam width depend on?

A
  • aperture width
  • focal distance
  • wavelength
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32
Q

How do you determine focal spot width?

A
  • For single element transducers - focal spot width ~ 1/2 transducer diameter
  • for multiple array - determined by aperture width
33
Q

How do you increase focal length?

A
  • increase apertture width
  • increase frequency (decrease wavelength)
34
Q

How can you improve lateral resolution (for a rectangular aperture)?

A

df = 2 (wavelength x focal length) ÷ aperture

Lateral resolution is improved by decreasing df (focal beam diameter), thus:

  • increase aperture size
  • decrease focal length
  • decrease wavelength
35
Q

Which component of resolution contributes to partial volume artifact?

A

Elevational resolution (slice thickness) - occurs when the section thickness > size of the structure

36
Q

How do you narrow the focal zone?

A
  • increase frequency (decrease wavelength)
  • increase aperture width (d)
  • increase curvature (decrease focal length)
37
Q

What is PRF and how do you calculate it?

A
  • Pulse repetition frequency - number of pulses occuring in one second
  • PRF (kHz) = # pulses/sec
    • or: PRF (Hz) = n x LPF x FR (Hz)
    • n = number of focal zones
    • LPF = lines per frame
    • FR = frame rate
38
Q

What is pulse duration?

A

time that it takes for one pulse to occur

39
Q

How do you calculate pulse duration? How can you decrease pulse duration?

A
  • PD (us) = # cycles per pulse x period
  • Decrease by:
    • decreasing the numbr of cycles in a pulse
    • increase the frequency of the pulse (decreases period - inverse relationship)
40
Q

What is Duty factor and how do you calculate it?

A
  • DF = fraction of time that pulsed US is on
  • DF = PD(us) / PRP(us)
41
Q

WHat is pulse repetition period?

A

PRP (ms) = 1/ PRF(kHz)

time from beginning of one pulse to the beginning of the next

42
Q

What is the Doppler shift equation?

A

fD = 2 f0 v cosØ / c

can rearrange to solve for velocity; v = fD c / 2 f0 cosØ

43
Q

When does maximum Doppler shift occur?

A

when the Doppler angle = 0, cos 0 = 1

44
Q

What is the ideal range of Doppler angles and why?

A
  • 30-60°
  • <30° results in total reflection at the vessel wall
  • >60° results in unreliable measurement
45
Q

What are limitations of color flow Doppler?

A
  • Angle dependent
  • Lower frame rates (decreased PRF)
  • Lack of detailed spectral information
    • computes mean rather than maximal velocities
46
Q

How does poer Doppler differ from color flow Doppler?

A

Rather than assigning various hue, saturation and luminanec values to mean Doppler-shift frequency values, this technique assigns a sum value of all Doppler shift frequencies and presents a pixel intensity based on the sum rather than the mean

47
Q

WHat are advantages of power Doppler?

A
  • Free of aliasing
  • Not angle dependent
  • More sensitive to slow flow and flow in small vessels
  • Increased SNR
48
Q

What is spectral broadening and what are causes?

A
  • Def: vertical thickening of the spectral trace
  • Causes:
    • broad range of flow velocities - disturbed or turbulent flow
    • multiple Doppler angles
    • excessive Doppler gain and beam spreading
49
Q

What are advantages and disadvantages of continuous wave (CW) Doppler?

A
  • Advantages
    • highly accurate measurement of blood flow velocity in the heart or large arteries
    • No aliasing
  • Disadvantages
    • No depth resolution - signal results from entire volume - no directional info
    • Spectral broadening due to lack of directional info
50
Q

In PW Doppler, how is sample volume determined?

A
  • spatial pulse length
  • gate length = gate depth
  • gate width
51
Q

What is the Nyquist limit?

A
  • NL = the maximum Doppler shift (fmax)
    • determined by the PRF
    • if greater than the NL, then aliasing occurs
    • PRF must be set to at least 2x the fmax
52
Q

How do you correct aliasing?

A
  • Increase PRF
  • Baseline shift downward
  • Reduce depth of the range gate
  • Decrease transducer frequency (will decr Doopler shift frequencies) - reduced resolution
  • Switch to CW Doppler
  • Increase Doppler angle - BAD CHOICE
53
Q

What is resistive index and how is it calculated?

A
  • RI = (Vsytolic - Vdiastolic)/ Vsytolic
  • Expresses the resistance to blood flow within an arteriole that can be obtained by PW Doppler
54
Q

What is Pulsatility Index and how is it calculated?

A
  • PI = (Vsytolic - Vdiastolic) / mean
  • Can be used as an indicator of distal impedance
55
Q

What is the advantage of pulsatility index?

A

Takes into account the HR and BP alterations that alter the RI

56
Q

What is the normal RI in canine kidneys?

A

0.62 with a range 0.56-0.67

57
Q

In kidneys, an increased RI (over 0.70) is an indication of what?

A

active tubulointerstitial or vascular disease

58
Q

For plug flow, what does the spectral image look like and what type of vessel does it occur in?

A
  • Spectral: thin line in sytole that outlines a clear space called the spectral window
    • due to narrow range of velocities
  • Occurs in large arteries (e.g aorta)
59
Q

Regarding parabolic flow, what type of vessel does it occur in, what does the spectral image look like?

A
  • Occurs in small arteries (i.e renal arteries)
  • Spectral display: spectral window not seen during sytole due to the wide range of flow velocities
60
Q

Regarding blunted parabolic flow, what type of vessel does this occur in and what does the spectral image look like?

A
  • Middle sized arteries (e.g. celiac artery)
  • Spectral display: Intermediate range of velocites - small range of velocities are represented in peak systole resulting in a smaller spectral window than in an artery with plug flow
61
Q

What is the Reynold’s number and what factors influence it?

A
  • Predicts the onset of turbulent flow: Critical Re = 2000
  • Factors:
    • increased flow speed
    • increased viscosity
    • increased vessel diameter
    • increased density
62
Q

What is the Bernoulli effect?

A

describes the decreased pressure in regions of high flow speed → as flow energy increases, pressure energy decreases

63
Q

What is the modified Bernoulli equation?

A

P1-P2 = 4 (V22 - V12) ⇒ Delta P = 4(V2)2

V2 = the flow speed in the jet

It is most commonly used to calculate the pressure drop at stenotic heart valve.

64
Q

In the Bernoulli equation, why is V1 most often ignored?

A

In most clinical cases, V2>>V1 and V1 = ~1 m/s

65
Q

In AV valve flow, what is the normal E/A ratio and what influences it?

A
  • normally E/A >1
  • influences:
    • No A wave with A-fib
    • E &A begin to merge with increased heart rate because diastole is shortened
    • ventricular compliance and relaxation influence E/A ratio
66
Q

What is the normal velocity range for the tricuspid valve?

A

60-70 cm/s (E:86, A:60)

67
Q

What is the normal RV/TV systolic pressure gradient?

A

Delta P= 15-20 mmHg

68
Q

A jet velocity greater than what would indicate RV out flow obstruction (PS, PH, possibly ASD)?

69
Q

What is the normal velocity range for the mitral valve?

A

75-95 cm/s (E:90, A:63)

70
Q

What is the normal systolic pressure gradient at the mitral valve?

A

Delta P = 100 mmHg

71
Q

At the mitral valve, a regurgitant velocity >6 m/s would indicate what?

A

>6 m/s suspect systemic hypertension

72
Q

What is the normal peak systolic velocity measured at the aorta and the normal pressure difference?

A

Ao = 1-2 m/s

delta P = 4-16 mmHg b/w the LV and Ao)

73
Q

What is the normal Ao velocity range? and the normal diastolic gradient?

A

100-150 cm/s

50-60 mmHg (<3.5 m/s maximum regurgitant jet in a normal dog)

74
Q

What is the normal PV velocity? Diastolic gradient?

A
  • velocity: 85-120 cm/s
  • diastolic gradient: 10-15 cm/s (regurgitant jet <2 m/s)
75
Q

What is the cut offs for diagnosing pulmonary hypertension?

A

PH >30 mmHg (either at PV or TV)

Mild: 3.5-4.5 m/s (delta P 50-80 mmHg)

Severe > 4.5 m/s (delta P > 80 mmHg)

76
Q

How do you calculate fractional shortening? What is normal?

A

FS = (LVIDd - LVIDs) ÷ LVIDd x 100%

Dog: 35-45%, Cat: 45-55%

77
Q

How do you calculate ejection fraction? What is normal?

A

EF = SV ÷ EDV x 100%

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