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6. Echocardiographic principles > US physics > Flashcards

US physics Flashcards

1
Q

How does US works and can create images

A
  • Sound waves are to the body → reflected from soft tissue structures
    o Many continuous and rapid waves = generates many 2D images/min
     Moving image (real-time B-mode)
    o One continuous wave = generates image only with structures associated with that line
     M-mode
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2
Q

Define cycle

A
  • Molecules on a line of sound wave: compressed and refracted = 1 cycle
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3
Q

Define wavelength

A

Distance travelled by sound wave during a cycle = wavelength

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4
Q

Define frequency

A
  • # cycles/sec → 1 cycle/s = 1 Hertzo Transducer frequency = MHz (1 000 000 cycles/s)
    o Doppler frequency = KHz (1 000 cycles/s)
  • ↑ frequency = ↓ wavelength
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5
Q

Frequency affects

A

image resolution
tissue penetration
Doppler signal

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6
Q

Velocity of US in tissue

A
  • Speed that US travels the tissue
    o Depends on stiffness/density of medium
     ↑ density = travels faster (bone = 3000m/s)
     ↓ density = travel slower (air = 700m/s)
    o Velocity is constant in a homogenous substance
  • Average velocity in soft tissue = 1540m/s (myocardium, blood, liver, fat…)
    o Qualibrated to US machine → calculation of distance to cardiac structures
    o D = V x (T/2)
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7
Q

A-mode

A

Amplitude mode
* Amplitude of reflected US vs depth

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8
Q

M-mode

A

Motion mode
* Single line of sight
o PRF limited to the time to travel to maximal depth and back
o High resolution of structures possible
* Guided by 2D imaging to ensure appropriate angle btwn M line and cardiac structures

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9
Q

B-mode

A

2D echo

  • For each scan line: short pulses/burst of US emitted at a fixed pulse rate frequency (PRF)
    o PRF determined by time to travel to max image depth: ↓ at greatest image depth
    o Signal received by piezoelectric crystals → generate images
     Image formation = depend on time delay btw US transmission and return signal
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10
Q

Tissue harmonic imaging

A

o Based on harmonic frequencies generated by US waves propagation
 Non linear effects of US propagation
 Reduce near field and side lobe artifacts + improves endocardial definition
o Key properties
 Strength ↑ with depth propagation
 Maximal at typical cardiac imaging depth
 Stronger fundamental frequencies = stronger harmonics

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11
Q

Focusing of probe

A

o Unfocused: width equal to transducer and diverge as travel to tissues
 Distance from transducer → divergence = near field
* Near field length: α to beam diameter, iα to wavelength
* ↓ transducer diameter = ↓ divergence in far field
 Area beyond = far field
o Focused = gated acquisition

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12
Q

Artifacts

A
  • Patient movement/breathing artifacts
  • Side lobe artifact (beam width)
  • Reverberation/mirror image artifact/range ambiguity
  • Acoustic shadowing
  • Refraction
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13
Q

Side lobe artifact (beam width)

A

o Central + peripheral beam
 Peripheral beam directed laterally → can reflect sound waves back
 Transducer cannot differentiate central vs peripheral reflected waves
o Superimposition of lateral structures in central beam = side lobe artifact
o Most commonly in dilated chambers: empty space allows weaker side lobes to be displayed

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14
Q

Reverberation/mirror image artifact/range ambiguity

A

o Strong reflectors encountered in thorax
 Send strong echoes back = received + reflected from transducer
 Transducer receive same sound wave twice → perceived as taken twice the time cause travelled the heart again
o Mirror image below the first one
 Can also happen with Doppler: if both side of baseline
* Created by high gain settings
o Minimize in adjusting depth settings

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15
Q

Refraction

A

beam is deviated → reflected image assumed to come from original path = double image

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16
Q

Transducer physics

A
  • Contain piezoelectric crystals:
    o Deformed by electrical voltage → generate sound
    o Receive sound → convert to electrical E
  • Thickness of crystals: basic operating frequency of transducer
    o Wavelength = ½ of crystal thickness
    o ↓ thickness = ↓ wavelength = ↑ frequency
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17
Q

Pulse Repetition Frequency (PRF)

A
  • Pulsed US: transmit sound waves in short bursts → receive sounds
    o # pulses/sec = pulse repetition frequency (Hz)
     Determine max velocity w/o ambiguity
    o Usually 2-3cycles/pulse → controlled by damping material in transducer
  • Pulse length: ↓ if ↑f (shorter wavelengths)
  • Should ↓ for deeper structures
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18
Q

Instrument settings

A

Gain
Power output
Depth
Dynamic range

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19
Q

Gain

A

Adjust displayed amplitude of received signals
o Time gain compensation (TGC): differential adjustment of gain along length of US beam
 Compensate the effects of attenuation
 Near field gain can be lower, gradually increase gain as deepens

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20
Q

Power output

A

US energy delivered by transducer → ↑ amplitude of reflected signals

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21
Q

Depth

A

affects PRF and frame rate

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22
Q

Dynamic range

A

of grey level in the image
o Contrast btwn light and dark areas

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23
Q

Acoustic impedance

A
  • Opposition/resistance to sound propagation
    o Depend on density/stiffness of medium
     ↑ density = ↑ impedance (and ↑ velocity)
     Bone have higher impedance because inability to refract/compress easily
    o Independent of frequency
  • High acoustic impedance → high sound reflection at bony/air interface
    o Shadow on US image = lack of further sound transmission
24
Q

Impedance equation

A

Density x speed

25
Q

Reflection

A
  • Return of US signal from smooth tissue boundary
    o Interface with different acoustic impedance → portion of the sound reflected back to transducer
    o Greatest when beam perpendicular to tissue: 2D, M-mode images
26
Q

Reflection depend on

A

o Angle of incidence: angle at which the US strikes the surface = determines angle of reflection
o Acoustic impedance difference btw tissues (↑ = ↑ reflection angle)
o Size of structures: must be ¼ the size of wavelength to occur
 Higher f transducer = higher resolution images (can reflect smaller strctures)
* ↑ frequency = ↓ wavelength = ↑ resolution

27
Q

Refraction

A
  • Change of direction when change of medium
    o Acoustic impedance mismatch
    o Can result in artefacts: image deviated (deepest = ↑ deviation)
28
Q

Attenuation

A
  • Sound travelling weakened by reflection, refraction, scattering, absorption of heat in tissues
    o Loss of E = attenuation
  • ↑ frequency = ↑ attenuation = ↓ depth
    o Wavelengths interact w more structures
  • ½ power distance of a tissue = distance travelled before ½ of sound E is attenuated
29
Q

Scattering

A
  • Radiation of US in multiple directions from a small structure
    o When radius of target < wavelength
    o Basis of Doppler US: change in signal from moving target (RBCs)
30
Q

Extent of scattering depends on

A

Size of target
o # of particles
o US transducer frequency = #1 determinant
o Compressibility of RBCs and plasma

31
Q

Types of resolution

A

Axial
Lateral
Elevational
Temporal

32
Q

Axial resolution

A
  • Ability to identify 2 structures as different along length of US beam
    o = depth/longitudinal resolution
  • Smaller axial resolution = ↑ detail image
    o ↑frequency = ↑ resolution
  • Axial resultion = ½ pulse length (2 structures must be > ½ pulse length ditance)
    o ↑ frequency = ↑ resolution
  • Most precise: measures should be done on structures perpendicular to beam ideally
33
Q

Lateral resolution

A
  • Ability to identify 2 structures as different perpendicularly to US beam
  • Beam width: affected by
    o Focusing sound waves generated by transducer
    o Transducer diameter + frequency
  • ↓ beam width = ↑ resolution
    o Better in near field where beam width is narrowest (approaches axial resolution)
  • ↑ frequency = ↑ resolution
    o Longer near field
34
Q

Elevational resolution

A
  • Refers to the thickness of the image slice
    o Cardiac US usually have a thickness of 3-10mm
    o Strong reflectors adjacent to image plane can appear to be in the image
35
Q

Temporal resolution

A
  • Frame rate = # of real time images/min
    o Depend on PRF: ↑ frame rate = ↑ PRF
    o ↑ frame rate = ↑ temporal resolution
  • Rapidly moving structures = need fast frame rate
    o ↓ sector width + image depth = ↑ PRF (less time required to generate next frame)
36
Q

Information provided by Doppler analysis

A
  • Detection/analysis of moving RBCs/myocardium
    o Direction, velocity, character, timing of blood/muscle motion
37
Q

Doppler shift

A

change of frequency btw sound transmitted and received
o ↑ frequency when moving toward sound source
 RBCs moving toward probe = reflect higher number of sound waves
 Received frequency > transmitted frequency = positive frequency shift
o ↓ frequency when moving away from sound source
 RBCs moving away from probe = reflect lower number of sound waves
 Received frequency < transmitted frequency = negative frequency shift

38
Q

Doppler tracing

A

o Positive frequency shift = produce waves up from baseline
o Negative frequency shift = produce waves down from baseline

39
Q

Doppler equation

A

determine RBCs velocity
o V = speed of sound in tissues
o Fd = frequency shift, Fo = transmitted frequency by transducer
o Cos  = intercept angle
 Intercept angle: closer to // the transmitted wave is to blood flow = ↑ accuracy of velocity measure
 Interrogation angles > 15-20 = result in significant errors

V = ((C x fd))/(2 (fo)) x cos

40
Q

Nyquist limit

A

maximum Doppler shift = ½ PRF
o Limit which produces aliasing = signal ambiguity

41
Q

Nyquist limit is influenced by

A

transducer frequency
o Maximum velocity recorded at given depth is iα to frequency
o Higher velocity jets at any depth = better recorded with lower frequency transducer
o Exceeded sooner at deeper gates for any given frequency

42
Q

reduce aliasing

A

o Move baseline up/down
o Find imaging plane with less depth for structure of interest
o ↓ transducer frequency
o Switch to CW Dopper
o ↑ PRF for depth

43
Q

4 types of Doppler

A

PW
CW
Color flow
TDI

44
Q

PW doppler

A

o Site specific: receive signal for time interval of specific sample depth
o No sound wave transmission until received echoes from previous burst
o Limited capacity to detect higher frequency shifts

45
Q

Characteristic of PW Doppler envelope

A

o Spectral broadening on PW signal
 Blood flow: usually laminar
 Slower velocities at periphery vs center of flow stream
* Improper gain settings
* Large intercept angle
* Turbulent flow: many velocities, flow direction

46
Q

High PRF Doppler

A

use of range ambiguity to ↑ max velocity measured
 Signals twice as far will reach transducer during the next cycle = can be analyzed

47
Q

 Range ambiguity

A
  • Echo signal from earlier pulse cycle reach the transducer the next cycle
  • Deep structures appear closer to the transducer
  • Double image on vertical axis
48
Q

CW doppler signal

A

o Use 2 separate crystals: 1 transmit, 1 receive
o Continuously send and receive sound
 Detect frequency shifts along beam with no range of resolution
o Can detect higher frequency shifts
o Not possible to selectively interrogate at specific depths

49
Q

Characteristic of CW Doppler envelope

A

o Full envelops from multiple velocities = spectral broadening

50
Q

Color flow Doppler

A

o Form of pulsed wave Doppler: hundreds of interrogation lines/gates
o Perceived as moving towards or away from transducer (negative or positive frequency shift)
 Toward = red → from deep red for slow to yellow for fast
 Away = blue → from deep blue for slow to white for fast

51
Q

Color flow Doppler: quality

A

depends on PRF + frequency
 Intercept angle: no color if perpendicular flow

52
Q

Color flow Doppler: aliasing

A

mosaic/mixing of colors
 Can occur in normal flow with high frequency transducers
 Turbulent flow = green

53
Q

Color flow Doppler: frame rate

A

can be improved with reducing wedge size

54
Q

Optimize Color flow Doppler image

A

 ↓ frequency
 ↓ color sector width
 Eliminate real time image
 ↑ packet size (will ↓ frame rate)
 ↓ packet size: ↓ sampling time, good for high HR

55
Q

TDi

A

o Analyzes myocardial velocities
 Myocardial motion: ↑ amplitude signals at low velocities (opposite for RBCs)
 Bypass low velocity filter
o Narrow sector of color: PW gate placed over this sector
 Spectral trace of myocardial motion in real time
o Highest temporal and velocity range resolution

6. Echocardiographic principles (10 decks)

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