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PHYSICS of modalities > Ultrasound > Flashcards

Ultrasound Flashcards

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1
Q

what are the 3 categories of sound

A
  • audible sound: 20Hz-20kHz
  • ultrasound: >20kHz
  • diagnostic ultrasound: 1MHz - 20MHz
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2
Q

what are the 2 categories that waves are classified

A
  • mechanical waves
  • electromagnetic waves
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3
Q

what are the 2 types of mechanical waves

A
  • transverse wave
  • longitudinal wave
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4
Q

what type of wave is ultrasound

A

longitudinal

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5
Q

what is the concept of transverse wave

A
  • particles of a medium moves perpendicular to the direction of wave propogation
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6
Q

what is the concept of longitudinal wave

A
  • particles of medium moves parallel or antiparallel to the motion of the wave
  • particles oscillate back and fourth about their resting positions, in line with the direction of the wave travel
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7
Q

what is the formula of wave speed

A

c = f x wavelength

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8
Q

what is the avg speed of sound in soft tissue

A

1540m/s

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9
Q

what is speed of sound dependent on

A
  • density
  • compressibility of media
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10
Q

how does density affect speed of sound

A

density = number of particles per unit vol
as density increases, there are more particles to move
so they become harder to move

denser medium = slower sound wave propagation

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11
Q

how does compressibility affect speed of sound

A

compressibility = how squashy is the material
the more compressible, the lower the velocity

stiffer medium = faster wave propagation

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12
Q

so how does both compressibility and density of the same medium affect the speed of sound

A

when there is a higher density medium = harder to compress
but higher density = slower wave
harder to compress = faster wave

when think about velocity, look in terms of compressibility

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13
Q

what is frequency

A
  • number of cycles per sec
  • units: Hertz
  • inverse of frequency is period (T)
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14
Q

what is wavelength

A
  • length of space which one completes wave cycle occurs (dist)
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15
Q

interaction of sound and matters

A
  • attenuation
  • reflection
  • scattering
  • refraction
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16
Q

what is attenuation

A

attenuation = absorption + scattering

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17
Q

what is absorption of beam

A
  • process whereby the US beam gives up energy to the tissue it is travelling in
  • the tissue energy content increases while beam energy decreases
  • 2 effects occur: tissue molecules vibrate with greater amplitude and tissue heats up
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18
Q

factors affecting attenuation

A
  • type of tissue (fluid attenuate less than tissues)
  • frequency
  • depth
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19
Q

what is acoustic impedance

A
  • defined as the resistance of a medium to the transmission of sound
  • if two media have different impedances, they are acoustically different
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20
Q

what is impedance mismatch

A
  • if acoustic impedance is same in 2 medium, sound will readily travel from one to the other
  • if there is a difference in acoustic impedance, causes a portion of the sound to be reflected at the interface
21
Q

explain reflection of beam

A
  • reflection occurs at the boundary/interface between 2 adjacent tissues
  • difference in acoustic impedance cause reflection of sound wave
22
Q

what are the 2 types of reflections

A
  • specular reflection
  • non-specular reflection (scattering)
23
Q

explain specular reflection

A
  • US strikes smooth boundary
  • interface large and relatively smooth
  • maximum reflection at 90deg
24
Q

explain non specular reflection

A
  • US strikes boundary with irregular surfaces with multiple individual interfaces
  • sound deflects in multiple directions
25
Q

what is refraction

A
  • US crosses a boundary where there is a velocity difference, it will be refracted
  • change in the direction of the wave, where is passes obliquely through another medium with different velocity
  • increase difference of velocity = increase refraction
26
Q

what is critical angle

A
  • angle of incidence for which the angle of refraction is 90deg
27
Q

what are some commonly encountered artefacts

A
  • reverberations
  • dirty vs clean acoustic shadowing
  • acoustic enhancement
  • comet tail artifacts
28
Q

when does reverberation occur

A

when beam encounters two highly reflective interfaces

29
Q

what is posterior acoustic shadowing

A
  • reduction in echo strength distal to a highly attenuating or reflective object
30
Q

clean vs dirty shadowing

A

clean shadowing commonly occurs distal to larger calculi and bon, appears as dark anechoic band

dirty shadowing is commonly seen distal to a highly reflective surface such as gas

31
Q

what is posterior acoustic enhancement

A
  • increased intensity echoes relative to surrounding tissues occurring distal to low-attenuating structure
32
Q

what is comet tail artifact

A
  • multiple internal reflections within a small reflective object producing a series of short tapered echoes
33
Q

what is piezo-electric effect

A
  • properties of some crystalline material
  • when force is applied, these materials change shape
  • and develop an electric charge on their surface (electrically polarised - generates voltages)
  • when short electrical burst is applied - material vibrates
34
Q

what is reverse piezo-electric effect

A
  • voltage applied across the surface of crystal, crystal undergoes deformation
  • deformation depends on polarity of voltage
35
Q

relationship between wavelength and frequency

A
  • inverse relationship
  • wavelength decrease, f increase
36
Q

what kind of crystals produce higher resonant frequencies

A

thinner
0.2-1mm

2MHz: 1mm
7.5MHz: 0.3mm

37
Q

formula for distance measurement in US

A

s=ct

38
Q

how do transducers receive signals

A
  • produce US pulse in response to electric stimulus
  • convert electrical energy to mechanical energy in the form of vibration
  • convert sound energy (echoes) into electrical signals to generate sonogram
39
Q

what makes up a simple transducer

A
  • backing material
  • connector
  • matching layer
  • crystal
  • casing and acoustic insulator
40
Q

types of transducer

A
  • curvi-linear
  • linear
  • phased array
  • transvaginal, transrectal
41
Q

describe the curvi-linear trasnducer

A
  • low frequency
  • deep tissue: abdo, pelvis, kidney, liver
42
Q

describe the linear transducer

A
  • superficial tissue - thyroid, testis, breast, vascular
  • higher frequency
  • lower penetration
  • lower resolution
43
Q

use of crystal in the transducer

A
  • acts as both sound source and sound receiver
  • thickness of crystal according to desired frequency of output
44
Q

what is the electrode for in the transducer

A
  • electrodes plated onto 2 faces
  • act as electrical connections to carry electric impulses to the crystal and from it during echo reception
45
Q

what is damping

A
  • when a crystal is pulsed, it will ring
  • but if the crystal is still producing sound then it cannot react to returning sound
  • therefore need to damp the crystal to produce short pulse
  • short pulses allow better better resolution
46
Q

what is absorption

A
  • absorbing sound produced by the back face of the crystal
  • if sound is allowed to reach the front of the crystal, it would destructively interfere with the sound produced by the front face and reduce intensity of the US beam
  • backing material is selected to be as absorbing as possible
47
Q

what are the possible artefacts caused by transducer

A
  • dropped/ handed roughy- “dead” elements
48
Q

parameters for testing

A
  • sensitivity
  • calibration
  • dead space
  • axial resolution
  • lateral resolution
  • cyst resolution
  • contrast resolution

PHYSICS of modalities (4 decks)

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