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Huda > Chapter 17 Ultrasound I > Flashcards

Chapter 17 Ultrasound I Flashcards

1
Q

what are sound waves

A

a pressure disturbance that travel away from their source

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2
Q

how are US waves transmitted through tissue?

A

waves of alternating compression and rarefaction

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3
Q

US velocity

A

product of wavelength and frequency

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4
Q

frequency

A

cycles/s
number of oscillations at a fixed point along the wave in each second
measured in Hz (oscillation/s)

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5
Q

wavelength

A

distance between successive wave crests

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6
Q

period

A

reciprocal of frequency
time between successive oscillations

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7
Q

what are harmonic frequencies

A

integral multiples of a fundamental frequency

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8
Q

frequency of audible sound

A

15 Hz to 20 kHz

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9
Q

what instruments produce high vs low frequency?

A

small instrument = high frequency
large instrument = low frequency

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10
Q

US frequencies

A

> 20 kHz

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11
Q

infrasound frequencies

A

< 20 Hz

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12
Q

frequencies used for US in clinic

A

1-20 MHz

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13
Q

pros and cons of high vs low US frequency

A

low frequency = better penetration
high frequency = axial resolution

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14
Q

when are sound waves formed?

A

electrical energy is converted into mechanical energy to form a wave of varying pressure

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15
Q

what does US wavelength depend on?

A

material compressability

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16
Q

US wavelength in soft tissue at 1.5 MHz

A

1 mm in soft tissue
0.2 mm in air
3 mm in bone

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17
Q

how does frequency affect wavelength?

A

US wavelength decreases with increasing frequency

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18
Q

wavelenght at 15 MHz in soft tissue

A

0.1 mm

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19
Q

how long are the US pulses?

A

about 2 wavelengths long

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20
Q

what determines the axial resolution?

A

length of US pulse
resolution is the wavelength (i.e. half the pulse length)

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21
Q

does sound velocity depend on frequency?

A

NO
-since velocity is fixed, frequency and wavelength are inversely related

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22
Q

what does sound velocity depend on?

A

type of material
materials that are not highlt compressible have high sound velocity (bone)
compressible materials (air) have low sound velocities

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23
Q

average velocity of sound in soft tissue

A

1540 m/s

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24
Q

velocity of sound in fat

A

5% lower than it is in soft tissue
-leads to artifacts

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25
Q

velocity of sound in air

A

~ 343 m/s

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26
Q

how are relative intensities in US expressed?

A

dB

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27
Q

what is dB

A

log scale
negative dB = signal attenuation and vice versa

-10% is -10 dB
-1 % is -20 dB
-0.1 % is - 30 dB

-+20 dB is 100 fold increase
-doubling the intensity is + 3 dB

-halving the intensity is -3 dB

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28
Q

what is acoustic impedance, Z

A

product of density and sound velocity’
expressed in rayls

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29
Q

what has low acoustic impedance

A

air and lung
-low density and low sound velocity

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30
Q

what has high acoustic impedance

A

bone and piezoelectric crystal
-high density and high sound velocity

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31
Q

what does fraction of US reflected at interface depend on?

A

acoustic impedance of the two tissues on both sides of the interface

when there are big differences in impedance, most of the US energy is reflected
when the acoustic impedances are similar, most of the US is transmitted and echoes are weaker

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32
Q

specular vs non-specular reflections

A

specular = occur from large smooth surfaces
-angle of incidence is equal to angle of reflection
non-specular = occur from rough surfaces
-don’t contribute to US image because any echoes reaching the transducer are weak

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33
Q

what is US echo

A

specular reflection travelling back to a transducer
used to create US images

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34
Q

what must transmitted and reflected US intensities add up to?

A

unity

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35
Q

why is gel applied between the skin and the transducer?

A

tissue/air interfaces reflect 100% of the incident beam

gel displaces the air and minimizes the large reflections so US can be transmitted into patient

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36
Q

do bone/tissue interfaces reflect a large amount of the US intensity?

A

yes

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37
Q

can you image through air or bone?

A

No, they reflect too much of the US energy

38
Q

fat/tissue interface- more reflection or transmission

A

mostly transmission

39
Q

when does scattering occur?

A

when US encounters objects that are smaller than the US wavelength

40
Q

what organs contain many scattering sites?

A

kidney
pancreas
spleen
liver

41
Q

reflected intensity for different materials or tissues

A

air > 99%
lung 50 %
bone 40%
fat 0.8%
muscle < 0.1 %

42
Q

what is speckle

A

scattering of US beam from small reflectors characteristic of the tissue structure

43
Q

what is hyperechoic speckle

hypoechoic speckle

A

high scatter amplitude relative to background signal

low scatter intensity relative to background signal

44
Q

what organs contain almost no scatter (show black)

A

bladder
cysts

45
Q

what is refraction

A

change in direction of US beam when passing from one tissue to another
-due to differences in speed of sound in the two tissues

46
Q

what changes with refraction?

A

frequency stays same but wavelength and speed velocity changes

47
Q

angle of refraction

A

when velocity of sound in tissue 2 is > tissue 1, transmission angle is > than angle of incidence

48
Q

why does refraction cause artifacts?

A

US machines assume straight line propagation
refraction results in artifacts

49
Q

what kind of artifacts does refraction cause?

A

spatial distortions

straight sticks partially immersed in water appear bent because of diffraction of light that has different velocities in air and water

refraction can also result in shadows because the beam is deflected from its normal path

50
Q

what is attenuation in US?

A

loss of US energy because of scattering and absorption

-absorbed sound energy is converted to heat

51
Q

how is US energy attenuated?

A

-exponential
-proportional to frequency
-doubling the distance travelled in tissue doubles the attenuation

52
Q

do fluids have low or high attenuation in US?

A

low

53
Q

attenuation coefficient for clinical imaging

A

0.5 dB/cm/MHz

54
Q

what organs have very high attenuation

A

lung and bone

55
Q

what is a transducer?

A

device that converts from one form of energy into another

56
Q

transducers in US

A

piezoelectric
convert electrical enegy into US and vice cersa
typically PZT (lead-zirconate-titanate)

57
Q

explain the mechanics of the transducer

A

high frequency voltage oscillations are produced by scanner electronics and sent to the US transducer over so-axial cables

transducer crystals do not conduct electricity but each side is coated with a thin layer of silver which acts as electrodes

Non-conducting crystals change shape in resoonse to voltages at these electrodes

crystal shape changes increase and decrease the pressure in front of the transducer, producing US waves

when the crystal is subjected to pressure changes by the returning US echoes, the pressure changes are converted back into electrical energy signals

-voltage signals are returning echoes are transferred from the receiver to a computer, which creates US images

58
Q

broad bandwidth transducers

A

generate more than 1 frequency

operators select the exam frequency

59
Q

resonance frequency of transducer

A

frequency at which piezoelectric transducer is most efficient in converting electric energy to acoustic energy

-determined by piezoelectric element thickness

60
Q

transducer crystal thickness

A

-usually 1/2 the wavelength at resonance

-high frequency transducers are thin (ex 0.1 mm at 7.5 MHz)
-low frequency transducers are thick (ex 0.5 mm at 1.5 MHz)

61
Q

what kind of US is used in clinic?

A

pulsed

62
Q

what is used to create the short pulses?

A

blocks of damping material placed behind the transducers reduce vibration to shorten pulses

63
Q

dampened transducer vs non dampened

A

dampened generates broad range of frequencies
transducers without damping generate pure frequency

-pure frequencies produce very long pulses and vice versa

64
Q

what is the impedance of the matching material on front surface of transducer?

A

intermediate between that of transducer and tissue

65
Q

what is thickness of the matching layer?

A

1/4 the wavelength of sound in that material (quarter wave matching)

66
Q

size of piezoelectric element

A

width is usually < 1/2 wavelength
vertical heights are several mm

67
Q

what region is used for US imaging?

A

near field
fresnel zone

68
Q

what is length of near field proportional to?

A

effective transducer size

doubling the transducer effective size will quadruple the length of the near field

69
Q

where does the far field start?

A

where the near field ends

70
Q

what is far field

A

US beam diverges and intensity falls of fast
Fraunhofer zone
-US imaging doesnt extent into far field

71
Q

what are side lobes and what do they do

A

small beams of reduced intensity emitted at angles to the primary beam

-presence of side lobes can give rise to artifacts

-multi-element arrays may also have grating lobes, similar to side lobes, that also cause artifacts

72
Q

what does focusing US beam do?

A

convergence and narrowing of the beam

improves lateral resolution

73
Q

focusing with acoustic lens

A

use a transducer face that is concave
beam will be narrowed at pre-determined distance from transducer

placing a concave acoustic lense on the surface of the transducer will also accomplish this

74
Q

focusing using a phased array

A

focal depth can be varied for each pulse using different patterns of element acivation

generate several images with different focal depths then cherry pick best parts of each pulse to create composite image

75
Q

what is focal depth

A

point at which beam is at its narrowest

76
Q

region over which beam is relatively narrow for focused US

A

focal zone

77
Q

what is high intensity focused US

A

applies US energy to locally heat and destroy diseased tissue through ablation
-uses low frequencies to improve penetration

78
Q

linear array

A

one line of sight’receive
receive echoes along single line
group of elements (rather than single element) increases near field

next line of sight is obtained by firing another group of elements that are displaced by one or two elements

complete frame is obtained by firing groups of elements from one end of the linear array to the other end

-128 to 256 elements

-rectangular FOV

-used in peds to visualize superficial structures

-limited FOV is determined by size of linear array

79
Q

convex arrays

A

operates same way as linear aary; scan lines perpendicular to transducer surface

-naturally creates image with an arc
-trapezoidal FOV

-fits better on abdomen
-wider FOV than linear array

-lateral resolution < 1 mm

-however, beams separate at depth, creating gaps
-increasing gaps decreases lateral resolution

80
Q

annular array

A

crystal is cut into rings like cross section of an onion
-focused by applying electrical pulse to each element in turn
-individual pulses combine to create a composite pulse, focused to a point at a specific depth
-focal point depth depends on time delay between electrical pulses applied to transducer elements
-varying delays permits US beam to be focused to different depths

-annular arrays allow US pulses to be focused in 2 dimensions instead of just one

-US beam produced by annular array transducers is symmetrical: produces thinner scan slice than other types of arrays

81
Q

phased arrays

A

96 elements

have many small transducers: each can be pulsed independently

-varying the time delays to individual elements results in a beam at a set angle (steers beam direction electronically)

-US beam is swept through similar to a search light
-images are generated by detecting echoes along each line of sight

-data from multiple beams are put together to generate slices

-US images obtained with phased arrays originate from a single point

-use phased arrays when there is a limited acoustic window, like ribs

82
Q

is electronic focusing or convex lens better?

A

electronic focusing introduces flexibility that improves image quality

but using electronic focusing will result in reduced frame rate, lower line density, or reduced FOV

83
Q

how much US energy is reflect at fat/tissue interface?

A

< 1 %

84
Q

where can electronic focusing be applied?

A

any array that has multiple elements (linear, convex, phased)

-only phased arrays steer the beam with electronic focusing

85
Q

velocity of sound in air vs tissue

A

330 m/s vs 1540 m/s

86
Q

high acoustic impedance material

A

ex. bone
has high density and high sound velocity

87
Q

transucer thickness

A

half the wavelength

88
Q

what will doubling the transducer element diameter do to the near zone distance?

A

will quadruple near zone distance

89
Q

what does use of focusing in US do?

A

results in narrower beams, thus improving lateral resolution

90
Q

width and height of individual elements in multi-element arrays

A

width of half a wavelength and height of a few mm

91
Q

why are linear arrays used to image infants?

A

large FOV is not required
superficial regions will be more visible

92
Q

where can you apply electronic focusing?

A

to any array that has multiple elements

Huda (20 decks)

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