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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
Reflection
* 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
Reflection depend on
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
Refraction
* Change of direction when change of medium o Acoustic impedance mismatch o Can result in artefacts: image deviated (deepest = ↑ deviation)
28
Attenuation
* 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
Scattering
* 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
Extent of scattering depends on
Size of target o # of particles o US transducer frequency = #1 determinant o Compressibility of RBCs and plasma
31
Types of resolution
Axial Lateral Elevational Temporal
32
Axial resolution
* 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
Lateral resolution
* 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
Elevational resolution
* 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
Temporal resolution
* 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
Information provided by Doppler analysis
* Detection/analysis of moving RBCs/myocardium o Direction, velocity, character, timing of blood/muscle motion
37
Doppler shift
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
Doppler tracing
o Positive frequency shift = produce waves up from baseline o Negative frequency shift = produce waves down from baseline
39
Doppler equation
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
Nyquist limit
maximum Doppler shift = ½ PRF o Limit which produces aliasing = signal ambiguity
41
Nyquist limit is influenced by
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
reduce aliasing
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
4 types of Doppler
PW CW Color flow TDI
44
PW doppler
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
Characteristic of PW Doppler envelope
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
High PRF Doppler
use of range ambiguity to ↑ max velocity measured  Signals twice as far will reach transducer during the next cycle = can be analyzed
47
 Range ambiguity
* 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
CW doppler signal
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
Characteristic of CW Doppler envelope
o Full envelops from multiple velocities = spectral broadening
50
Color flow Doppler
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
Color flow Doppler: quality
depends on PRF + frequency  Intercept angle: no color if perpendicular flow
52
Color flow Doppler: aliasing
mosaic/mixing of colors  Can occur in normal flow with high frequency transducers  Turbulent flow = green
53
Color flow Doppler: frame rate
can be improved with reducing wedge size
54
Optimize Color flow Doppler image
 ↓ frequency  ↓ color sector width  Eliminate real time image  ↑ packet size (will ↓ frame rate)  ↓ packet size: ↓ sampling time, good for high HR
55
TDi
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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