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utrasound Flashcards

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1
Q

Ultrasound machine and speed of sound?

A

assumes constant in every medium 1540 m/s

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2
Q

Change in wavelength and frequency between media?

A

Frequency doesn’t change

Wavelength changes in media

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3
Q

Interacting waves

A

‘constructive’ or ‘destructive’ effects can be used to shape and steer beam

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4
Q

Relative intensity dB

A

Reducing to 10% = -10dB

to 1% = -20dB

to 0.1% = -30dB

-3dB = 50% loss of signal intensity

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5
Q

US half-value thickness

A

Tissue thickness that reduces US intensity by 3dB

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6
Q

Impedance

A

Z = density x speed of sound

reflection based in differences in impedance

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7
Q

Refraction

depends on ?

clinical example

A

Angle of incidence

change in speed

along edges of tissue/fluid interfaces like the GB you see ‘bending’ of the beam

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8
Q

Specular reflection and frequency

what happens to scatter with frequency?

A

Shorter wavelengths see surfaces as more ‘rough’, non-specular. So higher frequency = more scatter

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9
Q

Absorption =

A

Sound energy turned to heat, increases with frequency

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10
Q

‘attenuation’

A

loss of intensity from both scatter and absorption

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11
Q

rough attenuation calc for ‘soft tissue’

dB and cm

A
  1. 5 dB per cm per MHz
  2. 5 (dB/cm)/MHz
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12
Q

Attenuation and frequency

A

Proportional

a 2 MHz beam will have twice the attenuation of a 1MHz beam

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13
Q

frequency and HVT

A

(3dB reduction)

HVT decreases with increasing frequency

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14
Q

What determines strength of echoes?

A

angle and impedance

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15
Q

Unit for impedance?

A

Rayl

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16
Q

Piezoelectric material

A

Can be quartz, usually PZT (lead-zinc-titanate)

molecular arrangement of electrical dipoles can be compressed, disturbed and measured

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17
Q

Operating frequency of a transducer dependent on?

A

speed of sound in, and thickness of the piezoelectric material

ONLY WAY TO CHANGE FREQUENCY IS TO CHANGE PROBE

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18
Q

Crystal/transducer thickness and frequency

A

thickness of transducer = 1/2 wavelength

lower frequency = thicker

higher frequency = thinner

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19
Q

Dampening block

where?

what?

A

Sits behind the crystal and absorbs backward directed US energy.

Also dampens transducer vibration, shortens spatial pulse length, preserving axial resolution

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20
Q

Thickness of dampening block

Dampening introduces a broadband frequency spectrum

A

Thin (ding)

called high Q, long pulse length

NARROW bandwidth

Thick (thud)

Low Q, short pulse length

BROAD bandwidth

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21
Q

When is good for low or heavy damping?

A

Low damping (thin) = narrow bandwidth, used for DOPPLER, to preserve velocity information

High damping (thick) = High spatial (axial) resolution (fewer interference effects and more uniformity)

Review, thick = LOW Q

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22
Q

Matching layer

what is

made of

optimal thickness??

A

Between crystal and patient to minimize differences in acoustic impedance

made of stuff intermediate in impedance between soft tissue and transducer material

optimal thickness = 1/4 the wavelength

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23
Q

Linear (sequenced) transducers

Width of transducer =

A

individual elements firing and receiving on their own

(no steering or interference)

Width of transducer = width of the sum of individual elements

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24
Q

Linear arrays are good for?

A

Peds, superficial things (carotids, leg veins, testicles, thyroids)

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25
Q

curve-linear

A

still ‘linear’, each element operates on its own

scan lines diverge deeper into image

abdominal imaging

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26
Q

Phased array

good for?

A

hive mind

groups of elements firing in multiples with interference patterns used to steer the beam

CAN BE MADE SMALLER THAN LINEAR

Good for limited windows (between ribs)

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27
Q

near field, far field, focal zone

A

near and far fall on either side of focal zone

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28
Q

Near field (fresnel zone)

Near field length depends on?

A

Near field converges

Higher frequency = LONGER NEAR FIELD

Larger diameter element = LONGER NEAR FIELD

Longer near field = less divergence of far field

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29
Q

“stand off pad”

A

For very superficial things

low impedance barrier to scan superficial things

(moves them from near zone to focal zone, BEST lateral resolution)

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30
Q

Three US spatial dimensions

A

Axial, lateral, elevation (slice thickness)

31
Q

Axial resolution

returning echoes

A

Returning echoes need to be SEPARATE and not overlap

minimim required sparation is 1/2 the spatial pulse length

32
Q

Spatial pulse length =

A

cycles per pulse multiplied by wavelength

length x number

Two objects closer than 1/2 SPL will NOT be resolved as separate objects (axial)

33
Q

Lateral resolution

A

Dependent on skinny beam. Skinniest at focal zone. Shitty lateral resolution both near and far

34
Q

Lateral and axial res vs depth

A

Axial depends on SPL, NOT DEPTH

Lateral worse at increasing depth past focal zone

35
Q

Gain and lateral res

A

Gain widens the beam

WORSE lateral res

36
Q

Elevation res (slice thickness)

A

Similar to lateral res

TYPICALLY WEAKEST

Depends on HEIGHT OF TRANSDUCER ELEMENTS

Source of volume averaging before and after the focal zone

37
Q

IMPROVING AXIAL RES

A

Shorter pulses (smaller SPL)

MORE DAMPING (low Q)

Higher frequency (shorter wavelength)

38
Q

Better lateral res

A

Put in focal zone

narrow beam

minimal gain (GAIN WIDENS BEAM)

Phased array (multiple focal zones)

INCREASE line density (lines per cm)

39
Q

Improving elevation res

A

fixed focal length across surface of array

minimize slice thickness (phase excitation of outer to inner arrays)

40
Q

US Artifacts

Side lobe artifact

example/usual?

Worst with which ‘ducer?

A

When a strong reflector falls out in the side of the beam

Transducer assumes that echoes originate from main beam, places strong reflector out on the side into the central field, moreso when central field is something ANECHOIC

pseudo sludge in GB

WORSE with LINEAR ARRAY TRANSDUCERS

41
Q

US Artifacts

Beam width artifact

usually seen where?

fix?

A

similar to side lobe, strong reflector in far field picked up by highly diverged beam will be placed in main beam

Usually seen as bladder with peripheral echoes

FIX = adjusting focal zone to ROI, placing transducer at center of image

42
Q

US Artifacts

Reverb

A

2 parallel highly reflective surfaces

43
Q

US Artifacts

comet tail

A

kind of reverb

parallel reflectors are closer than 1/2 SPL, therefore not resolved as separate (triangle fading down)

44
Q

US Artifacts

Ring down

A

fluid trapped between air bubbles creates a nearly continuous sound wave back to probe

LINE or Series of parallel bands posterior to a collection of gas

45
Q

US Artifacts

Mirror image

A

Kinda like reverb with bouncing but results in duplication deep to strong reflector

CLASSIC (almost always) = liver parenchyma where lung should be at liver/lung interface

“trapped behind a strong reflector”

46
Q

US Artifacts

Velocity related - machine assumes 1540 m/sec

Speed displacement

A

speed of sound slows down in fat relative to liver

Liver edge imaged behind some fat, beam takes longer to get back, assumed to be further, results in discontinuous, posteriorly displaced liver border

47
Q

US Artifacts

Refraction artifact

A

Returning echo gets refracted right before detection

object can then be displayed as

1 wider than it should be

2 misplaced to the side

3 duplicated

CLASSIC = DUPLICATED SMA deep to rectus muscles and fat (move transducer and it will be single)

48
Q

MODES

A mode

A

Amplitude

historical

used by optho now

processed info from the reciever vs time

gives amplitude as a function of time

49
Q

MODES

B

A

Brightness

conversion of a line of info to brightness-modulated dots on a display.

proportional relationship of brightness to echo signal amplitude

50
Q

MODES

M

A

Motion

B mode info is used to display the echoes from a moving organ from a fixed transducer and beam position

M mode is 4x greater than B mode

Pulsed doppler is 20x greater than B mode

51
Q

Doppler angle

ideal

why

A

30-60

something cosign

zero would be ideal, but less than 20 there’s too much refraction

90 gives no flow and possibly a mirror image

52
Q

pulsed wave (spectral) doppler

transducer

A

utilizes a single transducer for both reception and transmission

flow velocity varies giving a spectrum of doppler shifts instead of a single frequency

53
Q

Color doppler

how?

angle?

A

obtains samples of each pixel multiple times then displays the average shift

doppler angle not as important since info is semi-quantitative

54
Q

Power doppler

aliasing?

angle?

A

VERY sensitive for flow without info on direction

still get color but each pixel registers total number of frequency shifts

NO ALIASING

NO DEPENDENCE ON ANGLE, CAN BE MEASURED PERPENDICULAR

55
Q

Doppler artifacts

Aliasing

doppler shift is greater than ?

A

Greater than Nyquist frequency

1/2 pulse repetition frequency

1/2 PRF > shift to avoid aliasing

Ex. SHIFT = 3.5 kHz, need PRF of 7kHz to avoid aliasing

PRF needs to be double the shift

56
Q

Doppler artifacts

How to limit aliasing

A

Decrease doppler shift

lower frequency transducer doppler angle closer to 90

INCREASE PRF

INCREASE THE SCALE

57
Q

Flash color artifact

A

transducer or patient motion

FETAL KICK

58
Q

Color bleed

A

color extending beyond vessel wall

FIX = decrease color gain

59
Q

Image optimization

Output power (transmit gain)

A

Increases brightness by adjusting strength of sound pulse

LOSE LATERAL RES - WIDENING BEAM

60
Q

Image Optimization

Receiver gain

A

Increases brightness AFTER IT RETURNS

transmit vs receiver gain ?

ALARA says Receiver Gain first

61
Q

Image Optimization

Time Gain Compensation

A

makes top and bottom uniform

now automated

62
Q

Harmonics

A

Receiving at second harmonic frequency

harmonics not produced in near field

IMPROVES lateral resolution

REDUCED reverb

LOSE depth

63
Q

Compound imaging

A

Imaging an object in multiple different directions

electronic steering of the beam

sharpens edges and gets rid of posterior shadowing

(can make a cyst look solid)

64
Q

Harmonics vs compound imaging

example with a breast ‘nodule’

Normal

Harmonics

Compound

A

Normally hypoechoic with blurry margins, reverb and some shadowing

Harmonics- MORE ANECHOIC, WORSE SHADOWING, NO REVERB

Compound- hypoechoic, SHADOWING GONE, Sharp margins

Compound makes cystic look solid

Harmonics makes solid look cystic

65
Q

US Safety

Thermal index

A

max temperature rise

based on a homogenous tissue model with given instrument parameters

66
Q

US Safety

Mechanical index

A

How likely it is that cavitation will occur considering peak rarefaction pressure and frequency.

Most relevant with contrast enhanced US

67
Q

Cavitation

A

Sonically generated activity in compressible bodies composed of gas or vapor

Stable - bubbles already present, they expand and contract

Transient cavitation - bubbles collapse, shock waves ripple causing tissue damage

68
Q

Thermal induced damage

A

worse at higher frequency

rise in temp slows 2/2 conduction and perfusion

Damage has a THRESHOLD, none til a certain temp

69
Q

Cavitation most likely at?

A

Lower frequency and higher pressure

70
Q

Wat deposits most heat?

A

Spectral doppler

71
Q

Bad numbers for MI and TI?

A

risk benefit decision when

TI > 1.0

MI > 0.5

Til MIS

72
Q

Fetus and US

Where is thermal damage most likely?

rules

A

soft tissue - bone interface (brain and spinal cord)

USE different index (BONE INDEX) after 10 weeks

1st trimester

no pulsed doppler (color, spectral, power)

M mode used for HR instead

TI under 1.0

73
Q

General TI guidelines

<0.7

  1. 0-1.5
  2. 5-3.0

>3.0

A

< 0.7 = general rec for OB

  1. 0-1.5 = DONT exceed 30 mins
  2. 5 - 3.0 = NOT exceed 1 minute

Greater than 3.0, NONE ATALL

74
Q

Mirror image artifact

what is wrong assumption

A

all echoes return after a single reflection

physics factoids (8 decks)

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