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1. PTE Masters > Physics & Hemodynamics > Flashcards

Physics & Hemodynamics Flashcards

1
Q

What is the Doppler Equation?

A

Change in frequency = blood flow velocity * Cosine theta * 2 * transmitted frequency / velocity of ultrasound in soft tissue

dF = VCos(theta)2*Ft/c

There is a 2 in the equation because two Doppler shifts occur.

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2
Q

What is the velocity of ultrasound in soft tissue?

A

1540 meters/second

1.54mm/microsec

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3
Q

What do wall filters do?

A

Wall filters filter out part of the doppler shift (filter velocities)

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4
Q

What is the high pass wall filter and what is it used for?

A

The high pass wall filter filters out low velocities and allows high velocities to pass (used for CFD).

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5
Q

What is the low pass wall filter and what is it used for?

A

The low pass wall filter filters out high velocities and allows low velocities to pass (used for TDI).

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6
Q

What signals are present in tissue doppler? Which wall filters are used?

A

Low velocity, high amplitude signals (high pass filter off, low pass filter on).

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7
Q

What signals are present in color flow doppler? Which wall filters are used?

A

High velocity, low amplitude signals (high pass filter on, low pass filter off)

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8
Q

What parameter is determined by both ultrasound source and medium?

A

Wavelength

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9
Q

Order of ultrasound speed through different tissues? What two parameters affect the velocity?

A

(fastest) Bone > soft tissue > fat > lung > air (slowest)

Velocity determined by density and stiffness of the medium (faster in stiffer media, slower in denser media)

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10
Q

What is power doppler? What information does it provide?

A

Only signifies the presence of a Doppler shift. Does not include information on direction or speed (i.e. no velocity information).

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11
Q

What are other names for power doppler?

A

Energy mode or color angio

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12
Q

What is the Nyquist limit?

A

The maximum doppler shift that can be measured before aliasing occurs

Aliasing occurs when the deltaF (Doppler shift) > 1/2 PRF

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13
Q

Equation for the Nyquist limit

A

Nyquist limit = 1/2 PRF

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14
Q

What are the following parameters for M Mode?
X-axis
Y-axis
Frame rate
Brightness

A

X-axis = time
Y-axis = depth
Frame rate = 1000
Brightness = strength of the returning signal (stronger reflector = brighter on screen)

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15
Q

What is wavelength?

A

The distance between two identical points in adjacent cycles of a waveform signal

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16
Q

What is spatial pulse length? What type of resolution does it determine?

A

The length of a single pulse (a measure of DISTANCE)

SPL = wavelength x number of cycles in the pulse

Axial resolution = 1/2 SPL

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17
Q

What parameter determines axial resolution?

A

Spatial pulse length

Axial resolution = 1/2 SPL

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18
Q

What are the synonyms for axial resolution?

A

Longitudinal, axial, radial, range, depth (LARRD)

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19
Q

What parameter determines lateral resolution?

A

Beam width

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20
Q

What are the synonyms for lateral resolution?

A

Lateral, angular, transverse, azimuthal (LATA)

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21
Q

What is spatial resolution?

A

The ability to accurately create images of small structures in their correct anatomic position

(The ability to distinguish the space between two individual points)

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22
Q

What is the frequency of ultrasound?

A

> 20,000 Hz (20kHz)

Diagnostic ultrasound is usually in the range of 2-15 MHz

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23
Q

What is frequency?

A

The number of cycles per second

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24
Q

What is temporal resolution? What mode of echo has the best temporal resolution?

A

The ability to accurately determine the position of a structure at a particular instant in time

M-mode has the best temporal resolution.

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25
Q

What is power? What is it proportional to?

A

Power is the rate of energy transfer by the ultrasound beam or the rate at which work is performed.

Measured in watts (J/sec).

Proportional to amplitude^2

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26
Q

What is intensity?

A

Power per unit area

This is what can cause tissue damage and is responsible for the bioeffects of ultrasound.

Proportional to amplitude^2

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27
Q

What is gain?

A

Signal amplification

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28
Q

What is compression?

A

A reduction in the differences between signals, leading to a smaller dynamic range

(Compression decreases dynamic range so that the signals can be processed by the TEE machine)

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29
Q

What is dynamic range?

A

The ratio between the largest and smallest values the signal can assume

I.e. the range of signals the ultrasound machine can process

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30
Q

What are the five functions of an ultrasound receiver (listed in order of occurrence)?

A

1) Amplification
2) Compensation
3) Compression
4) Demodulation
5) Rejection

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31
Q

What is rejection?

A

Elimination of very low amplitude signals in 2D ultrasound (reduces noise)

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32
Q

What are synonyms for rejection?

A

Threshold and suppression

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33
Q

What are 5 ways to decrease or eliminate aliasing artifact?

A

1) Use CWD instead of PWD
2) Use a lower transmitted frequency (Ft)
3) Decrease depth of the sample gate
4) Increase PRF
5) Change baseline

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34
Q

What is an L wave?

A

Wave between the E and A waves indicated impaired relaxation and elevated LA pressure on MVI PWD

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35
Q

What is pressure half time?

A

The amount of time it takes to go from the maximum pressure gradient to 1/2 that maximum pressure (~71% decrease in velocity)

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36
Q

Which ultrasound parameters are depth dependent?

A

Pulse repetition period and pulse repetition frequency (dependent on listening time)

Nyquist limit is therefore depth dependent (1/2 PRF)

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37
Q

Which mode of ultrasound creates the greatest thermal intensity?

A

Pulse wave Doppler (maximum heating spatial peak temporal average)

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38
Q

What is mechanical index? What is the equation for it?

A

The strength of the US beam and the beam’s ability to produce cavitation of contrast material. High MI = strong beam.

MI = peak negative pressure / √frequency

(MI indicates the beam’s ability to cause cavitation-related bioeffects; thought a reasonable proxy for micromechanical damage)

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39
Q

What mechanical index will generate harmonics?

A

MI >0.1

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40
Q

What is quality factor?

A

QF = RF/BW

Lower QF = better image quality

Bandwidth = range of frequencies emitted in a pulse (6-11MHz = 5)

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41
Q

What is SPTA? What is the maximum SPTA recommended to avoid thermal injury for an unfocused US beam? For a focused beam?

A

Spatial peak temporal average intensity

Unfocused: < 1W/cm2
Focused: <100mW/cm2 (0.1W/cm2)

42
Q

What 3 processes cause attenuation?

A
  • reflection
  • scattering
  • absorption
43
Q

What is Rayleigh scattering?

A

Form of organized scatter when the beam encounters objects smaller than its wavelength and is redirected equally in all directions.

Is increased with increasing frequency to the 4th power.

44
Q

What is the attenuation coefficient?

A

The number of decibels of attenuation when sounds travels 1cm

Units: dB/cm

In soft tissues, the attenuation coefficient = 1/2 frequency

AC = 1/2 F

45
Q

What has the highest acoustic impedance?

A

The piezoelectric crystal

Piezoelectric crystal > matching layer > gel > skin

46
Q

What are the units of acoustic impedance?

A

Rayls (Z)

Density * velocity

Z=p*V

47
Q

What is the maximum allowable mechanical index per the FDA?

A

1.9

48
Q

What type of wave is sound waves?

A

Sound is a mechanical longitudinal wave.

Sound travels via a series of compressions and rarefactions in molecules of the medium it travels through.

49
Q

What is amplitude? How are power and intensity related to amplitude?

A

The difference between the average acoustic variable and the peak acoustic variable

Power and intensity are related to amplitude^2

50
Q

What mechanical index creates the strongest harmonics?

A

MI>1

51
Q

What beam frequency and strength (high vs low) causes:
1) a low mechanical index 2) a high mechanical index

A

Low mechanical index: high frequency transducer, low beam strength

High mechanical index:
low frequency transducer, high beam strength

MI = peak negative pressure / √frequency

52
Q

What determines wave properties involving time?

A

Only the source of the beam

Remember: SECONDS = SOURCE

53
Q

What is the pulse repetition period?

A

PRP = pulse duration + listening time

(Time from beginning of one pulse to the beginning of the next pulse)

54
Q

What ultrasound time parameters are ALSO dependent on depth?

A

Pulse repetition period and pulse repetition frequency (because they include listening time, and listening time is depth dependent)

Less depth = less listening time = more pulses can be sent per second = higher pulse repetition frequency = higher Nyquist limit and better temporal resolution

55
Q

What is frame rate? What does it depend on?

A

Frame rate = number of frames per second (number of images per second)

Higher PRF = higher frame rate

Higher frame rate = better temporal resolution

56
Q

What determines the velocity of ultrasound in tissue? How does stiffness and density affect velocity?

A

The medium only

  • As stiffness increases, velocity increases (i.e. as elasticity decreases)
  • As density increases, velocity decreases

A less dense, less elastic tissue will have a higher US velocity

57
Q

What are the advantages and disadvantages of higher frequency ultrasound?

A

Advantage: better axial resolution

Disadvantages: greater attenuation, less harmonics

58
Q

What is impedance?

A

Acoustic resistance to sound traveling through a medium

Impedance = density x velocity

Z=PxV

Determined by the medium only

Unit: Rayls

Velocity is determined by density and elasticity, and elasticity is inversely related to stiffness. So Z is affected by velocity, density, elasticity, AND stiffness

59
Q

What is the Curie temperature for ultrasound?

A

Around 360 C

Beyond this point, the material becomes depolarized and loses its piezoelectric properties (i.e. the transducer no longer works)

60
Q

What does backing material do in the transducer?

A

Acts as a damper to decrease ringing, allowing the probe to generate short, clean pulses.

  • Decreases spatial pulse length and pulse duration (improves axial resolution)
  • Increases bandwidth
  • Decreases quality factor (good!)
  • But also decreases sensitivity to reflected echoes
61
Q

What is the purpose of the matching layer?

A

To prevent a large reflection at the interface of the mucosa and PZT

  • Has an acoustic impedance between that of mucosa and the PZT
62
Q

For imaging, should the quality factor be higher or lower?

A

Lower quality factor used for imaging

63
Q

What is bandwidth?

A

The difference between the lowest and highest frequencies in a pulse

64
Q

What is the equation for resonant frequency for pulsed US?

A

RF=V/2T

V= velocity
T = thickness

65
Q

Where does the best lateral resolution in an ultrasound beam occur?

A

At the point of focus

66
Q

What is the ultrasound beam focus point?

A

Focus = location of the minimum diameter of the beam

67
Q

What is the near field? What is it also called?

A

The distance from the transducer to the point of focus

Aka: Near Zone, Fresnel Zone

68
Q

Equation for length of the near field (Ln)

A

Ln=r^2/lambda

r=radius of crystal
lambda=wavelength

69
Q

What is the far field? What is it also called?

A

The distance past the point of focus

Aka: Far Zone, Fraunhofer Zone

70
Q

How is beam divergence affected by:
- crystal diameter
- crystal frequency

A

Crystal diameter: larger diameter = less divergence (deeper focal length)

Crystal frequency: higher frequency = less divergence (deeper focal length)

71
Q

What are the two steps of demodulation?

A

Rectification and Smoothing

Rectification: turns all negative voltages positive

Smoothing: places an envelope around signals to smooth them out

72
Q

What is a reverberation artifact?

A

Caused by the beam ricocheting between two strong reflectors, creating multiple, equally-spaced reflections in the image

Aka: ringdown artifact, comet tail

73
Q

What is a refraction artifact?

A

When the beam is bent prior to striking the reflector

The machine assumes the beam travelled in a straight line, so the image will be placed to the side and deeper than the true reflector

74
Q

How is scattering related to frequency?

A

Scattering is directly related to frequency (higher frequency beam = greater scattering)

75
Q

What is the equation for Snell’s law?

A

n1sin(theta1) = n2sin(theta2)

The ratio of the sine of the angles of incidence and transmission is equal to the ratio of the refractive index of the materials at the interface.

76
Q

When refraction occurs, if the incident velocity is higher than the transmitted velocity, how is the incident angle related to the transmitted angle?

A

If Vi>Vt, then the incident angle is > transmitted angle

V1/V2 = sine(theta 1)/sine(theta2)
Sine increases as angle increases

77
Q

What conditions must be met for refraction to occur?

A

Oblique incidence (not 90 degrees) AND the transmitted velocity must be different from the incident velocity

78
Q

What is line density? How is it related to temporal resolution?

A

Line density = number of scan lines/image

As line density increases, temporal resolution decreases.

79
Q

What is the most common composition of a piezoelectric crystal in a TEE probe?

A

Lead zirconate titanate (PZT)

80
Q

What determines the frequency of CWD? What about PWD?

A

For CWD, frequency = frequency of electrical excitation voltage applied to the to the crystal.

For PWD, frequency = V/2T (need to know the thickness of the crystal and the velocity of sound in the crystal)

81
Q

What is the intensity reflection coefficient?

A

The % of ultrasound intensity that is reflected at the interface of two media

= (incident intensity-transmission intensity)/incident indensity x100%

Affected by acoustic impedance, medium density, stiffness, and the velocity of ultrasound

82
Q

What is duty factor?

A

A unitless number describing the amount of time that the ultrasound machine is producing sound.

Increasing PRF increases DF.

83
Q

What is responsible for Doppler determinations of blood flow velocities?

A

Rayleigh scattering

84
Q

What is the effect of focusing an ultrasound beam on:
- focal zone
- far field divergence
- beam diameter
- focal depth

A
  • smaller focal zone
  • increased far field divergence
  • reduced beam diameter in near field
  • shallower focal depth

Note: narrower beam = better lateral resolution

85
Q

What two factors determine beam divergence? Are the relationships inverse or proportional?

A

Crystal diameter and frequency of sound (which is determined by V/2T)

Crystal diameter and frequency are both INVERSELY related to beam divergence

86
Q

What is the optimal thickness of the PZT crystal?

A

1/2 wavelength

87
Q

What is the optimal thickness of the matching layer?

A

1/4 wavelength

88
Q

What is the piezoelectric effect and reverse piezoelectric effect?

A

Piezoelectric effect: conversion of sound to electrical signal

Reverse piezoelectric effect: conversion of electrical signal to sound

Reverse piezoelectric effect happens first, piezoelectric effect happens second

89
Q

What is the ALARA principle?

A

“As low as reasonably achievable”

Minimize patient exposure to ultrasound

  • If image is too dark, increase GAIN first
  • If image is too bright, decrease POWER first
90
Q

What is the shortcut for calculating wavelength from MHz?

A

Divide 1.54 by the frequency in MHz

Velocity = 1.54mm/microsecond
1 MHz = 1 cycle/microsecond

91
Q

Equation for duty factor

A

DF = pulse duration / PRP x100

92
Q

Order of attenuation in tissues (greatest to least)

Main mechanism of attenuation?

A

Air > Lung and bone > Muscle > Soft tissue > Fat > Blood/fluid > Water

Absorption is primary mechanism in air, lungs, bone

93
Q

Beam divergence equation

A

Sin(beam divergence angle) = 1.2 * lambda / d

d=diameter

Beam divergence increases with smaller diameter crystal and lower frequency sound

Remember shorter focal length = greater divergence with depth

94
Q

What is Huygen’s principle?

A

A large active element can be thought of as millions of tiny distinct sound sources

Each point on a wavefront is the source of spherical waves

95
Q

What is backing material composed of?

A

May be composed of tungsten powder and araldite

96
Q

How do you calculate Qp/Qs?

A
  • Calculate Qp (use radius main PA and VTI through PA)
  • Calculate Qs (radius LVOT and VTI through LVOT)

Then divide Qp by Qs (stroke volume through pulmonary artery / stroke volume through LVOT)

97
Q

How do you calculate AR regurgitant volume using stroke volume?

A

Total volume into LV = volume out through AV + regurgitant volume

Therefore, regurgitant volume = stroke volume LVOT - stroke volume MV

98
Q

How do you calculate stroke volume (general equation)?

A

Volume = area x distance

Therefore, SV = area x VTI

99
Q

What is the equation for regurgitant fraction?

A

RF = regurgitant volume / stroke volume through valve

100
Q

Simplified Bernoulli’s equation

A

Driving pressure - pressure in receiving chamber = 4V^2

101
Q

How is dp/dt calculated?

A

Slope of LV pressure rise from 4-36mmHg

(slope from MR velocity = 1 to MR velocity = 3)

Therefore, dp/dt = 32/dt

102
Q

What is the equation for velocity of circumferential shortening?

A

VcF=FS/ejection time

1. PTE Masters (14 decks)

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